by William Hallstrom (Arlington Garden volunteer) Aug 27, 2021
For the past few months, most of the volunteer crew at Arlington Garden have spent at least some of their time wrapping the trunks of each of the garden’s trees with the kind of soft tape measure you might use for sewing, looking up to the highest branches and pacing underneath them while jotting down notes. It’s all part of the tree survey, one of the recent volunteer projects at the garden, whose goal is to determine how much carbon is being sequestered by the trees in Arlington Garden.
Arlington Garden Communications & Volunteer Manager Andrew Jewell says the idea for the tree survey began from a need for some real data to help bolster a proposal. “We set out to quantify the effect the garden has on various environmental measures including atmospheric carbon, the urban heat island effect, rainwater capture,” he says. This would require human effort, which was facilitated by the growth of the volunteer program in recent months.
Oak trees are highly diverse and widespread, and they are keystone species in the forests they inhabit.
Advances in genomics have allowed researchers to reconstruct the evolutionary history of oaks.
The findings will have implications for managing oaks to ensure their survival as the planet warms.
AUTHORS
Andrew L. Hipp is a senior scientist and herbarium director at the Morton Arboretum in Lisle, Ill. His research addresses the evolution, maintenance and implications of plant diversity, with a focus on the phylogenomics of oaks. Credit: Nick Higgins
Paul S. Manos is a professor at Duke University. He studies the systematics and biogeography of the flowering plants, with a particular focus on the evolution of oaks, hickories and walnuts. Credit: Nick Higgins
Jeannine Cavender-Bares is a professor at the University of Minnesota. She studies the origins, physiological function, and organization of plant biodiversity and their consequences, with an emphasis on oaks. Credit: Nick Higgins
If you were dropped into virtually any region of North America 56 million years ago, you probably would not recognize where you had landed. Back then, at the dawn of the Eocene epoch, the earth was warmer and wetter than it is today. A sea had just closed up in the middle of the Great Plains, and the Rocky Mountains had not yet attained their full height. The continent’s plant and animal communities were dramatically different. In the Canadian High Arctic, which today harbors relatively few tundra plant species, year-round temperatures above freezing nurtured a rich and diverse flora; Ellesmere Island in far northern Canada, across from the northwestern coast of Greenland, was home to alligators and giant tortoises. What is now the southeastern U.S. was dominated by tropical rain forest, complete with primates. The northeastern U.S., for its part, ranged from broad-leaved (as opposed to needle-leaved) evergreen forest to deciduous forests of ginkgo, viburnum, birch and elm, among other species. The deciduous broad-leaved forests that now cover 11 percent of North America north of Mexico were in their infancy. But that was about to change, with the spread and extraordinary diversification of what would eventually become some of the most ecologically and economically significant woody plants in the world: the acorn-bearing, wind-pollinated trees we call oaks.
Over the course of some 56 million years, oaks, which all belong to the genus Quercus, evolved from a single undifferentiated population into the roughly 435 species found today on five continents, ranging from Canada to Colombia and from Norway to Borneo. Oaks are keystone species, foundational to the functioning of the forests they form across the Northern Hemisphere. They foster diversity of organisms across the tree of life, from fungi to wasps, birds and mammals. They help clean the air, sequestering carbon dioxide and absorbing atmospheric pollutants. And they have shaped human culture, feeding us with their acorns and providing wood to build our homes, furniture and ships. Indeed, oaks have proved so valuable to people that we have immortalized them in legends and myths for centuries.
Oaks are especially prominent in the Americas. Approximately 60 percent of all Quercus species live here. This astounding variety, along with the fact that the oaks in this region account for more forest tree biomass than any other woody plant genus in North America and Mexico, makes them the single most important group of trees in the continent’s forests. To understand forests, then—their biodiversity, food webs and contributions to human well-being—one must understand how oaks came to rule them. For decades scientists could only speculate about much of the evolutionary history of oaks because of gaps in their fossil record and limitations of the biomolecular techniques used to infer evolutionary events from the DNA of living organisms. But recent advances in genome sequencing and analysis have allowed us and our colleagues to reconstruct a detailed picture of the origin, diversification and dispersal of oaks. It is a remarkable evolutionary success story, one that will have important implications for predicting how these essential trees will fare in the face of climate change—and for developing management plans to ensure their survival.
Red and White
The differences between major groups of oaks are readily apparent to even a casual observer. In the Americas, oaks are dominated by two evolutionary lineages that you may already know. One of these, the red oak group, is composed of species with bristle-tipped leaves. In most red oak group species, pollen takes a full year from the time it lands on the female flower to fertilize the seed, so that acorns—the fruits of these trees—pollinated in one year only ripen in the next. Species in the other major lineage, the white oak group, have no bristles on their leaves, and the leaves generally contain more soil-enriching nutrients when they fall than those of red oaks do. Also, white oak acorns almost all ripen the same year they are pollinated, sometimes germinating before they even fall. Gray squirrels preferentially cache red oak acorns to eat at a later date because they are less likely than white oak acorns to go bad before the squirrels can get back to them.
White oaks are also able to efficiently plug the water-conducting, tubelike cells called vessels in their wood with tyloses, balloonlike structures that seal the vessels as a barrier against deadly fungal diseases such as oak wilt. Red oaks are slower and sloppy in their defense. Consequently, white oaks have long served as wood for ships and wine barrels because the plugged vessels of the white oak species hold water more effectively than those of the red oaks. Chewing insects recognize the differences between red and white oaks, and most are adapted to favor either one or the other of these two groups. Even mycorrhizal fungi, which connect plant roots to soil nutrients, appear to recognize the differences between the two types of oaks: many favor symbiotic relationships with species in one lineage over the other.
When we get to the species level, however, closely related oaks are often difficult to tell apart. The variation within species, the result of both plastic responses of the trees to their environment and genetic variation between individuals, often appears to be as great as the variation between species. And oaks hybridize commonly within their group, be it the white or red lineages or any of the six other major lineages of oaks worldwide. These two factors—high variation within species and ongoing hybridization between species—complicate classification.
Hybridization can also make it difficult to reconstruct the evolutionary history of oaks using traditional biomolecular techniques, which involve sequencing one or a few genes, because individual genes often trace different histories. Moreover, a single oak species may have hybridized with numerous different species, so that different genes record different aspects of this history across the geographical range of the species. The oak genome is thus a mosaic shaped by speciation and hybridization. The sequences of only one or a few genes cannot reveal the full history of speciation in oaks.
Two decades ago researchers had only the sequences of DNA from chloroplasts—the cell organelles that carry out photosynthesis—and a few nuclear genes to go on. It was enough to discern the overall branching structure of the oak tree of life, but we could not see the arrangement of its endmost branches. In 2008 the three of us realized that new molecular techniques we were already using to study hybridization and the limits of species in the red oak group might also enable us to infer oak evolutionary history. Since then, we, in collaboration with colleagues around the world, have employed an approach called restriction-site associated DNA sequencing to read short regions of DNA from across the genome. We analyze these data using statistical methods that reconstruct the order in which species have branched from common ancestors and which ones have hybridized since that divergence. By marrying these analyses to fossil data, we can estimate the maximum ages of key events in oak evolutionary history. Despite the complex genetic history of oaks, we have been able to deduce much of the history of speciation in this group going back to the root of the oak tree of life.
Red oaks have bristle-tipped leaves (top); the leaves of white oaks lack bristles (bottom). Credit: John Seiler Getty Images (top); Getty Images (bottom)
Southward Bound
We may never know precisely when or where the very first oaks arose, but roughly 56 million years ago a population of oaks growing near what is now Salzburg, Austria, left in the mud a bit of the massive amount of pollen they produced each spring. These pollen grains, which are shaped like a rugby ball with three grooves running lengthwise and with surface textures that vary by lineage, are the earliest unambiguous fossil evidence of oaks on record. Throughout the early Eocene, land bridges spanning the Atlantic and Pacific Oceans connected North America and Eurasia. Plants and animals freely crossed between the two continents. Oaks were most likely part of a vast forest that spread across the continents of North America, Europe and Asia. This makes it difficult to say with any confidence whether oaks originated in Eurasia and sent a branch off to the Americas, or vice versa. The better answer to where modern oaks arose may simply be “in the north.”
In any case, remarkably soon after they arose, oaks started to separate into two major branches: one limited to Europe, Asia and North Africa and the other largely limited to the Americas. The separation between continents was imperfect at first. For example, the oldest fossil attributable to the ring-cupped oaks, based on the concentric rings formed by the woody scales on its acorn cap, was deposited in Oregon around 48 million years ago. Today this lineage is restricted to Southeast Asia. And red oaks, which today are an American group, have been reported from fossil sites in Europe dating to some 35 million years ago. But when global temperatures started their long descent about 52 million years ago, oaks were gradually pushed southward, away from the land bridges that have connected Eurasia and North America intermittently over the past 50 million years. As cooling drove northern oak populations extinct, the divisions between the two continents became very clean, with no species from the Eurasian clade showing up in the Americas and only two branches of the American clade showing up in Eurasia.
Before they could be pushed too far to the south, oaks were further subdivided into the eight major lineages we recognize in modern forests. Three of them are restricted to the Americas: the red, golden cup and southern live oaks. One lineage, that of the white oaks, originated and diversified in the Americas but sent an offshoot back to Eurasia. We know these major lineages arose early in oak evolution because one of the oldest American oak fossils is a 45-million-year-old white oak from Axel Heiberg Island in Nunavut, Canada, that can be distinguished from the red oaks and all other major lineages of oaks. But fossils from this initial phase of diversification are hard to assign to any one lineage, so we rely on molecular data to estimate when the other oaks separated into independent lineages. The integration of molecular data with selected fossils indicates that the world’s eight lineages split early on. It is an important part of the story because it explains what happened next as the North American oaks underwent their own burst of diversification.
Lands of Opportunity
As temperatures cooled worldwide, the North American climate also became more seasonal. The Rocky Mountains were continuing to rise, and their rain shadow dried out the Great Plains. The tropical forests and broad-leaved evergreen forests that had flourished across North America were gradually restricted in range and driven to extinction by around 40 million years ago. Oak pollen and leaf impressions became more common in the North American fossil record 35 million years ago, by which time decreased temperatures and increased seasonality had converted North America north of Mexico from a mostly tropical to a mostly temperate continental landscape. As climate change extirpated tropical forests from North America, ecological opportunity arose for the oaks.
The red and white oaks moved south into this newly opened territory, each splitting into a lineage on the western side of the Rocky Mountains that gave rise to the modern-day oaks of California and the Pacific Northwest and into a lineage on the eastern side of the Rockies that gave rise to the oaks of eastern North America. Within the latter region, each of these major oak groups subdivided into a predominantly northeastern lineage, a predominantly southeastern lineage and a primarily Texan lineage. From eastern North America, perhaps by way of Texas, the red and white oaks then moved into Mexico between 10 million and 20 million years ago.
In all these areas, palms and broad-leaved evergreen trees had been pushed south or driven partially or wholly extinct by the cooling and increasingly fluctuating climate. The resulting abundance of open habitat enabled oaks to diversify. Increased ecological opportunity allowed oaks to undergo an adaptive radiation, in which nascent species rapidly fill spaces that other species are not occupying. In doing so, these young populations became more ecologically distinct from one another, thereby limiting the movement of genes between them. They became reproductively isolated, so that genes moved less between separated populations than among trees within populations. Subsequently, new genetic mutations and rearrangements could accumulate that distinguished the populations from one another. Through this process, they became new species.
This adaptive radiation played out most dramatically in Mexico and Central America, where about 40 percent of all the world’s oaks reside. Recall that oaks were a largely cold-adapted lineage that spread across the continent as temperatures dropped and seasonality increased. As they migrated south into Mexico, oaks climbed to higher elevations that more closely resembled the temperate biome in which they had evolved, and they encountered high topographic variation that readily separated them into reproductively isolated populations. Oaks also evolved more rapidly along the continuum from low water availability to high water availability as they moved into Mexico. Tacking up and down the mountains, different populations adapted to different levels of drought. This ecological differentiation most likely worked hand in hand with increased physical separation to promote reproductive isolation between populations.
Thus, the reason for the high oak diversity in Mexico appears not to be warmer temperatures. And because Mexican oaks are relatively young, their high diversity has not accrued over comparatively long periods of evolutionary time. Rather adaptive radiation led to higher speciation rates in these evolutionarily young Mexican oaks as they moved into the mountains. This change suggests that if oaks had been suited to climb into the Rockies and flourish there—that is, if they could have survived the combination of short growing seasons and cold winters of the northern mountains—they might have developed high diversity in this region as well. Their evolutionary heritage simply did not equip them for these extremely harsh environments. Only a lone white oak species, the Gambel oak (Quercus gambelii), even comes close, and that species is limited to the southern Rockies.
The oaks were finally stopped in their march southward, perhaps by dramatic reduction in seasonality or strong competition from tropical forest species, only barely making it across the Isthmus of Panama into the north of South America. Yet this is not the whole story. The oaks’ southward journey actually played out twice, simultaneously and in the same places. Because white and red oaks had already separated from each other by the time they started moving south, this diversification history happened in parallel in both the red and white oaks. Two distinct but very closely related lineages, not one, traced the biogeographical history we just described: moving south, splitting around the Rocky Mountains, heading into Mexico from an eastern North American ancestor. This history may explain part of the species richness and abundance of oaks in the Americas. They essentially double-dipped as they ventured south.
Fossil acorn from Oregon dates to the Eocene epoch. Credit: Thomas J. Bones
Good Neighbors
One of the most exciting areas of our research has been the integration of a genome-level understanding of the oak tree of life with physiological studies of oak adaptation to climate and habitat and community studies of oak forest structure. As oaks spread south and diversified in different regions, the white and the red oaks encountered similar habitats and repeatedly solved the same ecological problems in novel ways. As a result, we often find red and white oaks growing together in the same habitats. For example, on poor rocky soils and bluffs in the eastern U.S., you can find the white oak Quercus stellata, also known as the post oak, growing next to the red oak Quercus marilandica, commonly called the blackjack oak. In the mountains of southern Arizona, the iconic white oak Quercus arizonica often grows beside the red oak Quercus emoryi.
This pattern of oak co-occurrence is found in wooded plant communities across much of the country, and it has another intriguing feature. Whereas distantly related oaks tend to grow together, closely related oaks within lineages tend not to be found together. Along an elevational gradient in the Chiricahua Mountains of southern Arizona, for example, white oak species pass the baton as you walk upslope, transitioning broadly from one to the next as you hike uphill, and red oak species do as well. In the lowlands of Florida, white oak species separate across the sandhill, scrub and ravine habitats shaped by karst topography and fire. Red oaks do the same.
What shapes this pattern of oak co-occurrence? Ecological differentiation within the red and white oaks is influenced in part by the fact that no single species is able to master every habitat. Instead species tend to specialize on a limited part of the available ecological space. In oaks, physiological trade-offs within each lineage subdivide habitat and climatic space so that close relatives are less likely to co-occur. In the Chiricahua Mountains, for instance, drought adaptation separates close relatives along the elevation gradient. Species living near the bottom of the mountain are particularly good at avoiding drought, dropping their leaves during dry seasons. Species living at higher elevation, where there is more overall moisture, focus on surviving daily fluctuations in water availability by allowing leaf water content to drop lower before they suffer damage.
In contrast, in Florida, which is comparatively flat, soil moisture availability and fire intensity structure oak communities. Closely related species in these communities show trade-offs between growth rate and drought tolerance along moisture gradients and between bark thickness and the ability to reproduce via underground stems along gradients of fire intensity. In both regions, and indeed across the country, parallel trade-offs are found in both red and white oaks, and trees with convergent traits from the two lineages tend to grow together.
Members of different lineages may co-exist well with one another in each habitat because they differ in their susceptibility to disease: proximity to a more distantly related neighbor may be less likely to result in an epidemic because red and white oaks tend not to spread the same diseases. There is even evidence that oaks help one another get established and persist by creating a soil environment that benefits the mycorrhizal fungi they need to acquire nutrients. Then, once a forest has become established, oaks become dominant and prevent other kinds of trees from setting up shop. Our work makes clear that the evolutionary origins of oaks shape the complex ecological interactions that help to explain why the trees are so abundant and diverse in North America. The tree of life casts its shadow across the structure of our oak forests.
Creative Hybridization
Now that we can delineate the branching history of the oak tree of life in some detail, the trees’ propensity to hybridize has become all the more interesting. People often think of hybridization as a destructive force, eroding genetic differences between species. Yet oaks form what is called a syngameon, in which ecologically and physically distinctive species persist in spite of ongoing gene flow. It has long been hypothesized that genes migrating between species of the syngameon might help oaks adapt to novel environments. Could, for example, genes that contribute to drought adaptation in the post oak migrate into the bur oak (Quercus macrocarpa) in the southern regions, where they co-occur, and help the bur oak adapt to the drying conditions it is expected to encounter under global warming? We know already that there is localized gene flow between oak species and that species differ in what genes they exchange depending on where on the landscape they are, what species they co-occur with, and the climate and habitat in which the trees are growing. We also know that after genes move from one species into the other, they can move beyond the range of the species in which they arose, apparently propelled by environmental selection. These examples suggest that adaptive gene flow may play an important role in oak evolution. We are on the cusp of the integrative genomic and ecological studies needed to understand this process in depth.
We would still like to know what genes and attributes—flowering time, habitat preference, geographical distance—drive speciation in oaks and whether ecological differences evolve while populations are growing together or only when they are separated. We are close to understanding what genes shape differentiation. Recent work in European oaks shows that genes influencing both their ability to cross-pollinate and their ecological preferences (for instance, tolerance of drought, cold and disease) are involved in species differentiation. Yet these findings only tell us that ecological differences evolve in species, not that they drive species differences. Statistical analyses that simulate alternative speciation histories suggest that in a group of four widespread European white oaks that hybridize today, the genomic differences between the species arose when the species were born in different geographical areas, with opportunities for gene flow arising only after the fully formed species migrated back into contact with each other. Still, the high degree of species co-occurrence in the American oaks raises the question of whether hybridization contributed to their diversity.
A firm grasp of when, where and how oaks came to be so diverse is crucial to understanding how oaks will resist and adapt to rapidly changing environments. Oaks migrated rapidly as continental glaciers receded starting around 20,000 years ago, and hybridization between species appears to have been key to their rapid response. The insights we can gain from elucidating the adaptive benefits of gene flow are critical to predicting how resilient oaks may be as climate change exposes them to fungal and insect diseases with which they did not evolve. As insects that transport pathogenic fungi increase their ranges and change their patterns of reproduction with earlier springs, oaks may have trouble holding their ground unless they can evolve quickly enough to resist diseases they have never before encountered. Our challenge for the coming decade as plant biodiversity scientists will be to figure out how differentiation between species and movement of genes between those species will influence the trajectory of oak evolution and population persistence. If we understand these processes well enough, we stand a chance of using that knowledge to predict what our forests will look like a century or more from now. Perhaps it can guide our plans to manage longer-term survival of the vital oaks.
This article was originally published with the title “Ascent of the Oaks” in Scientific American 323, 2, 42-49 (August 2020)
By Amy Chillag, CNN Updated 12:44 PM ET, Wed September 18, 2019
If you’re looking for a reason to care about tree loss, this summer’s record-breaking heat waves might be it. Trees can lower summer daytime temperatures by as much as 10 degrees Fahrenheit, according to a recent study.
But tree cover in US cities is shrinking. A study published last year by the US Forest Service found that we lost 36 million trees annually from urban and rural communities over a five-year period. That’s a 1% drop from 2009 to 2014.
If we continue on this path, “cities will become warmer, more polluted and generally more unhealthy for inhabitants,” said David Nowak, a senior US Forest Service scientist and co-author of the study.
Nowak says there are many reasons our tree canopy is declining, including hurricanes, tornadoes, fires, insects and disease. But the one reason for tree loss that humans can control is sensible development.
“We see the tree cover being swapped out for impervious cover, which means when we look at the photographs, what was there is now replaced with a parking lot or a building,” Nowak said.
More than 80% of the US population lives in urban areas, and most Americans live in forested regions along the East and West coasts, Nowak says.
“Every time we put a road down, we put a building and we cut a tree or add a tree, it not only affects that site, it affects the region.”
The study placed a value on tree loss based on trees’ role in air pollution removal and energy conservation.
Heat reduction: Trees provide shade for homes, office buildings, parks and roadways, cooling surface temperatures. They also take in and evaporate water, cooling the air around them. “Just walk in the shade of a tree on a hot day. You can’t get that from grass,” Nowak said. To get the full temperature benefit, tree canopy cover should exceed 40% of the area to be cooled, according to a recent study in the Proceedings of the National Academy of Sciences. “A single city block would need to be nearly half-covered by a leafy green network of branches and leaves,” the authors wrote.
Air pollution reduction: Trees absorb carbon and remove pollutants from the atmosphere.
Energy emissions reduction: Trees reduce energy costs by $4 billion a year, according to Nowak’s study. “The shading of those trees on buildings reduce your air conditioning costs. Take those trees away; now your buildings are heating up, you’re running your air conditioning more, and you’re burning more fuel from the power plants, so the pollution and emissions go up.”
Water quality improvement: Trees act as water filters, taking in dirty surface water and absorbing nitrogen and phosphorus into the soil.
Flooding reduction: Trees reduce flooding by absorbing water and reducing runoff into streams.
Noise reduction: Trees can deflect sound, one reason you’ll see them lining highways, along fences and between roads and neighborhoods. They can also add sound through birds chirping and wind blowing through leaves, noises that have shown psychological benefits.
Protection from UV radiation: Trees absorb 96% of ultraviolet radiation, Nowak says.
Improved aesthetics: Ask any real estate agent, architect or city planner: Trees and leaf cover improve the looks and value of any property.
Nowak says there’s a downside to trees too, such as pollen allergies or large falling branches in storms, “and people don’t like raking leaves.” But, he says, there are ways cities and counties can manage trees to help communities thrive. “You can’t just say ‘we’re not going to have forests.’ We might as well manage and work with the trees.”
“You don’t want a tree in the middle of a baseball field. It’s very difficult to play sports if you have trees in the way. Or trees in the middle of freeways.”
Nowak says we can design and manage tree canopies in our cities to help “affect the air, to affect the water, to affect our well-being.”
Urban forests especially need our help to replace fallen trees. Unlike rural areas, it is very difficult for trees to repopulate themselves in a city environment with so much pavement and asphalt.
“A lot of our native trees can’t actually find a place to drop an acorn so they can regenerate,” explains Greg Levine, co-executive director for Trees Atlanta.
“That’s why the community has to go in and actually plant a tree because the areas just aren’t natural anymore.”
The job is not complete when the saplings take root. Organizations like Trees Atlanta and their volunteers plan most of their year to care for these young trees until they’re mature enough to thrive on their own.
“We try to prune trees for 10 years to make sure they get a good healthy structure.” Levine adds. “We also add mulch around trees to help keep the moisture in the ground so the tree doesn’t dry up. We have to have a lot of patience with planting trees around pavement, making sure that they can rise to the challenge. “
How you can help stop tree loss
Protect what you have: Nowak says the first step is caring for the trees on your own property. “We think we pay for our house, and so we must maintain it. But because we don’t pay for nature, we don’t need to. And that’s not necessarily true.”
Prune the dead limbs out of your trees: If they’re small enough, do it yourself or hire a company. The risk of limbs damaging your house is significantly lowered when there’s tree upkeep, Nowak said.
Notice where your trees may be in trouble: Often, you can observe when something’s wrong, such as when branches are losing leaves and breaking or when mushrooms are growing at the base or on the trees. You can also hire an arborist or tree canopy expert to assess the health of your trees on an annual basis. Or you can contact your local agricultural extension office for advice.
Don’t remove old trees if it’s not necessary: Instead, try taking smaller actions like removing branches. “It takes a long time for these big trees to get big: 50 to 100 years. And once they’re established, they can live a long time. But taking a big tree out and saying ‘we’ll replant,’ there’s no guarantee small trees will make it, and it will take a very long time to grow.”
Allow trees to grow on your property: Although everyone’s aesthetic is different, it’s the cheap way to get cooler yards and lower energy bills. It’s also an inexpensive approach to flood and noise control.
Nowak says he laughs when his neighbors wonder why their property doesn’t have more trees, because “I hear people running their lawn mowers.” Fallen seeds need a chance to implant, and constant mowing prevents that. If you don’t like where a seedling is growing, you can dig it up and plant it or a new tree where you like.
Educate yourself about trees and get involved: Many cities have tree ordinances that seek to protect very old, significant trees. You can get involved by attending city council meetings. You can also help your city plant trees by joining local nonprofit groups.
Volunteer or donate to tree planting and research organizations:
California Native Plant Society’s Riverside-San Bernardino Chapter presents Rebecca Latta on Oak Stewardship
Oaks are the iconic tree of California, providing timeless beauty, shade and a sense of place. Each oak can support thousands of associated species. Now with the stresses of climate change and new pests, oaks need our attention more than ever. Rebecca Latta will discuss how and when to water, when and whether to prune, and all aspects of the care of oaks.
FIRE ON CALIFORNIA LANDSCAPES by Jon E. Keeley
The California wildfire problem involves wildfire hazards to the urban environment and protection of natural resources. Managing this risk involves fire suppression, changing home construction standards, reducing landscaping fuels, and more serious attention to zoning decisions.
HOW CNPS DEVELOPED A POLICY ON NATIVE PLANTS AND FIRE SAFETY by Betsey Landis
Extensive inquiry and discussion has resulted in a CNPS policy for the state of California that protects native plants, ensures fire safety, and suggests ways to implement it.
THE WILDLAND-URBAN INTERFACE FIRE PROBLEM by Jack Cohen
Homes can survive wildfires without the necessity of wildfire control. But this will require changing our mindset from wildfire exclusion to wildfire compatibility.
INTERPRETING FIRE AND LIFE HISTORY INFORMATION IN THE MANUAL OF CALIFORNIA VEGETATION by Todd Keeler-Wolf, Julie M. Evens, and John O. Sawyer
The natural history and fire information in the new edition of A Manual of California Vegetation provides land managers and others with better ways to preserve the state’s natural heritage.
INVASIVE SPECIES AND FIRE IN CALIFORNIA ECOSYSTEMS by Adam M. Lambert, Carla M.
D’Antonio, and Tom L. Dudley
Although fire is a natural disturbance in many California plant communities, invasive species can alter fire dynamics in ways that are transforming some of these communities.
SUSTAINABLE AND FIRE-SAFE LANDSCAPES: ACHIEVING WILDFIRE RESISTANCE AND ENVIRONMENTAL HEALTH IN THE WILDLAND-URBAN INTERFACE by Sabrina L. Drill
UC Cooperative Extension, Los Angeles, has developed an educational program to help homeowners create and maintain fire-safe landscaping, while also being good stewards of the land and avoiding the use of invasive plants.
THE ROLE OF FIRE SAFE COUNCILS IN CALIFORNIA by Yvonne Everett
All over California, where communities meet wildlands, citizen groups are working together to help protect their neighborhoods from wildfire. They call themselves Fire Safe Councils.
FIRE-RESISTANT LANDSCAPING: A GENERAL APPROACH AND CENTRAL COAST PERSPECTIVE by Suzanne Schettler
Wildfires are capricious and yet there are steps we can take to reduce the risk to a home when designing or retrofitting a landscape setting.
WILDFIRE SAFETY: LESSONS LEARNED FROM SOUTHERN CALIFORNIA by Greg Rubin
It is possible to create defensible space around your home while still embracing the natural ecology that surrounds it. Some pragmatic tips for enhancing fire safety without wholesale environmental destruction.
THE MENDOCINO COUNTY FIRE SAFE COUNCIL by Julie Rogers
From humble beginnings, Fire Safe Councils such as this one in Mendocino County are helping communities “survive and thrive” in wildfire-prone environments.
The Woolsey Fire, California. US Forest Service photo courtesy of Peter Buschmann
HEAT, DROUGHT and CLIMATE CHANGE Rebecca Latta and her team serve as trusted advisors to owners of trees and landscapes stressed by heat, drought, fire and the effects of climate change. Her team reviews properties to determine strategies to protect trees and plants by planning and modifying the landscaped environment. Property owners learn how to encourage landscapes to heal by such techniques as post-fire seeding, allowing damaged plants to sprout from the base, and selecting alternative heat- and drought-tolerant varieties. Rebecca’s team can assist with resource information about landscape restoration and planting new sustainable gardens that are better suited to the coming changes in the climate.
FIRE DAMAGE If you have fire-damaged trees or landscaping, our professional evaluation provides you with guidance to transition your garden to health. Rebecca and her crew survey and assess trees and landscapes, and work with local government agencies to coordinate any permits required to remove or restore damaged trees and vegetation. Our assessment can determine if fire damaged trees should remain or be removed. Fire damaged trees can provide value to wildlife, serving as as roosts for birds and homes for animals. Downed trees and burned plants also decompose, enriching the soil to allow new trees and plants to thrive.
It is late in the day and still very hot—approaching and possible reaching 100 degrees—and, besides that, it’s a Wednesday, so I think there’s no way anyone’s going to be out looking at trees. But I have underestimated the human ability to disregard nature when trying to commune with it. There are people everywhere. In our golf cart we dodge around them, standing still on the paths and enduring the heat to spend time with trees that are standing still off the paths and enduring the heat. “We’re all in this together,” I think as Kathy Musial drives us around the Huntington Library, Art Collections, and Botanical Gardens in San Marino. We’re also looking at trees and, specifically, at how they’re adapting to an ecosystem that’s getting away from all of us.
Musial, the Huntington’s collections manager, takes us past the sprawling desert garden with its otherworldly cacti and succulents, one vision for a definitively more arid Los Angeles. The art gallery, once the founder’s home, opens onto a great lawn. At the far end, beyond a fountain, we see a view of the San Gabriel Mountains that Musial says simply didn’t exist when she started working here because the smog was so bad. We weave through the 12 gardens representing this region’s historical ability to support just about anything that wanted to take root.
A research institute, a library of art and rare books, and a science museum, this is the kind of place where I overhear one person say to another, “I do know a snail wrangler.” But the Huntington is also a horticultural laboratory, testing for more than a century the limits of what the environment can sustain. Giant figs and hundred-foot bamboo in the jungle garden; young Pasadena and cork oak in the California garden; camellias and roses, herbs and lilies, and 200 species of palm in one of the oldest gardens. “We’re always trying new things,” Musial says, animated and lithe as a reed. “I’m not a big believer that we should just be choosing plants from similar climates.” We pass through cooler microclimates created by the gardens. The bridges and houses of the Chinese and Japanese gardens sparkle in the heat like paradise, or at least a good enough rendition of paradise that NBC’s The Good Place uses the gardens for its afterlife scenes. Most of the plants we pass on our drive seem so happy it’s hard to imagine that they weren’t always here.
In the Australia garden, Musial points out bright red leaves on a brush cherry tree. “That’s been sunburned,” she says. Look closer and you’ll see trees ailing, because what’s happening in here is happening out there.
What’s happening out there you know about: heat waves, wildfires, a massive die-off of trees in California caused by a multiyear drought and parasitic insects and disease and high temperatures. That’s not just in forests: Urban trees, too, are dying, in part because the ecosystem, particularly in Los Angeles, is hitting a bunch of limits—social, economic, ecological—at once. What to do? It would be nice if all our standing around looking at trees would yield some answers. But right now all the trees are showing us is how we, too, might die in uncomfortably large numbers if we don’t figure something out. This is exactly the sort of thing we tend to disregard.
photo: Millicent Harvey
By just about any metric, trees are good. You should see the research. Here’s just a sampling: air purification, noise reduction, “traffic calming,” attracting shoppers. Wildlife habitat, carbon storage, city cooling, energy-use reduction. Trees cut crime and improve feelings of community. Just standing among them reduces stress, confusion, depression. They make children more active. Walking a kid with ADHD through a park does the work of peak-effect meds. Read the reports. Trees are ace at stormwater capture, each potentially storing thousands of gallons in its root system. Tree canopy can offset the heat-island effect that makes cities 10 degrees hotter than surrounding environments. Trees can block UV rays; trees can keep you out of the hospital. Trees can put a dent in the $22 billion in annual health-care costs from air pollution in the South Coast Basin, can reduce the more than 2,000 premature deaths a year from vehicle air pollution in Greater Los Angeles, can cut pollution in the city by 2,000 tons every year.
A study by the USDA Forest Service used a satellite imagery program called i-Tree to quantify some of the math—the dollar value of sequestered carbon, or the effects of tree canopy on water quality—for L.A. Every square kilometer of tree cover is worth $142 per person a year in health-care savings from air pollution, water runoff, and energy costs, and a total annual savings of $2 billion in air pollution, $40 million in water savings, and $1 billion in carbon storage. You may not think about all this, but stand in the deep shade and you’ll feel it; you can’t help it, you’ve got 50 million years of primate evolution to thank. Trees are nature that protects us from other nature. But to work with them for mutual benefit flies in the face of 150 years of development in which trees were used to adorn unchecked growth.
Los Angeles is a big experiment in tolerances. Its early hypothesis could be described as, roughly, “What the hell can we get away with?” It’s an attitude embodied by the Huntington’s founder, Henry E. Huntington, who helped grow the fledgling Los Angeles into a metropolis in the late 19th and early 20th centuries with stiff doses of imported water, public transit, and real-estate speculation. In pictures, he’s sporting a mustache, alternately trimmed neat or full walrus. Caught in the posed act of removing a book from a shelf, he has a look that we, a century later, instinctively associate with civilization-shaping wealth. He inherited a railroad from his uncle. He inherited the man’s widow, too. Both he and his wife loved art, books, and trees.
Huntington believed something the modern mind would find ludicrous: that all of this was limitless and eternal. You get the sense he built these gardens just to see if he could. They reflected and informed what would grow elsewhere. Consequently the Huntington’s botany is now a microcosm of Southern California, where people and trees and buildings and streets are all, in one way or another, having to adapt or die.
The Japanese Garden at Huntington Botanical Gardens
The Huntington has lost 250 trees since 2016. “Many conifers are suffering,” Musial says. “The heat just cooked them. Redwoods, pines, junipers.” Other trees are infested by a beetle called the polyphagous shot hole borer. The insect spreads a fungal disease called fusarium dieback, which, along with the drought, has killed 129 million trees statewide and could kill 27 million in the Southland alone. (That’s about 38 percent of the trees in the region.) The shot hole borer, plus other species of borers and pests like the glassy-winged sharpshooter—not to mention long-standing ailments like sudden oak death—collectively threaten more than a hundred species of trees in urban and rural forests. That includes natives like coast live oak and sycamore, popular immigrants like avocado, sweet gum, and olive, and drought-resistant species like palo verde that we may come to rely on as the climate changes to what we now shorthand as “hotter hots, drier dries, and wetter wets.”
As of this writing, California is on track to see a record number of acres burned by wildfire in 2018—more than 1.5 million. That includes the Mendocino Complex, the largest fire in recorded state history, that burned 459,000 acres over more than a month. Before that, the Carr fire killed eight and burned more than 1,000 homes. Fourteen-thousand firefighters worked across the state against blazes that seemed to engage in an escalating arms race, introducing the public to tornadoes made of flames that not only pursue but inhale. Nationwide more than seven million acres have burned.
Mostly, it’s hot. NASA and the National Oceanic and Atmospheric Administration report that 17 of the last 18 years have been the hottest on record. There have been record temperatures in Europe, readings of 122 degrees in Pakistan, people in Japan watching outdoor movies in baby pools. Mortality rates are so high for the old, young, and poor that the CDC ranks heat as a severe health threat. Downtown L.A. hit a record high of 108 degrees in July. With extremes of weather and temperature, climate change is warping existing patterns to civilization-stressing levels. How to describe it? We’re entering an era of extraordinary climate variability.
The heat kills trees, too, by destroying their ability to move water from stem to leaves and through “carbon starvation,” wherein the photosynthetic process shuts down. Heat-stressed trees are more susceptible to insects and disease. Since a dead tree can’t cool the environment through evapotranspiration, it contributes to climate change, which in turn contributes to hotter temperatures, and so on. In forests, a dead tree also becomes kindling for ever-bigger fires, perpetrator and victim both.
“Unfortunately, the way Los Angeles and almost every city in this country was built was not understanding the role that the trees have” —Andy Lipkis
None of this is news to Andy Lipkis, who is Los Angeles’s professional Lorax. He runs a nonprofit called TreePeople that he started in 1972 when he was a 15-year-old kid who just liked trees, like Huntington, and over time he has seen where Huntington’s endless city may end. TreePeople teaches people about trees, it plants trees in poor neighborhoods, it studies how to capture rainwater and runoff so we don’t have to spend billions importing it. TreePeople’s roots are so intertwined with the city’s now that it acts as the idealistic, tree-loving conscience of L.A.
For Lipkis, the tree is central to the conversation about what Los Angeles will become. Trees, he tells me, are “a tentpole of the ecosystem, and as we pull that apart, disintegrate it, remove the trees, it triggers a collapse—a breakdown of services we rely on and that urban humans have not had to think about as long as they could afford it, as long as they could pay for the energy, they could pay for the flood-control system, they could pay for the insurance of having vulnerable houses.” In cities like Los Angeles, the death of trees does the opposite of terraforming: It turns the Earth into Mars.
So this is kind of a science-fiction story: Los Angeles has become its own hostile planet, and it has to figure out how to become habitable long-term. One answer is to plant more trees. The urban canopy protects us, after all, from the “urban.” But it’s not so easy. Plant- ing trees does tend to interfere with some of the fundamentals of society, like streets and sidewalks and real estate. Historically humans would rather have streets and houses even though these contribute to the environmental effects that kill trees.
“Unfortunately, the way Los Angeles and almost every city in this country was built was not understanding the role that the trees have,” Lipkis tells me. “[They] are vitally needed now to moderate the climate—to make it possible for us to continue living in a healthy, safe way, [protected] from heat, from air pollution, from water pollution, from all these things.” Unfortunately, somebody’s got to water the things.
The modern guide to urban tree planting would start like this: “Step 1: Dig a hole. Step 2: Uproot dominant economic and governmental paradigms. Step 3: Replace with….” And then it would descend into a fevered argument with itself about hundreds of years of social philosophy and eventually retire to a cabin in the woods to write a manifesto.
Point being: The most delicate part of a tree in Los Angeles County is the 4,700 square miles of people, houses, and infrastructure that surround it. For trees, there is a terrifying verticality of bureaucracy: three local, three state, and two federal agencies. The L.A. Department of Public Works and Department of Recreation and Parks handle most of the urban trees on nonresidential property. City Planning oversees private trees. Sanitation gets in there, too, when it comes to water issues.
There are now about 6 million trees in the city. Of those, 700,000 are street trees. Historically L.A. has not prioritized the urban forest when compared to human-centric needs like sidewalks. Budget cuts hit tree-watering trucks or trimmed the urban forestry staff. But money is being shifted toward the forestry division again. The Department of Public Works, responsible for the street trees, was budgeted $18,695,000 for tree maintenance: planting, watering, trimming, and removal. Its five crews can trim 38,000 trees a year. With 700,000 trees, that means each one gets a trim every 18 years. This is, perhaps, why the default has been the flattop, or the “psych-ward” buzz cut, that leaves the tree stressed and prone to growing weirdly.
Tim Tyson is the chief forester of the city’s Urban Forestry Division. He says the department waters new trees for three to five years to get them established. “Once they’re older trees, they’ve found their own watering source,” he says. There’s no guarantee there, though. “As water tables drop there’s less water for them to get, and they start showing signs of decline,” he says. Even if they do become established, trees have to then grow up in, you know, a city. Street trees become the responsibility of the homeowner, whether he likes it or not, and that homeowner may already have his hands full completely ignoring the needs of the trees on his own property.
It isn’t just the L.A. infrastructure that sidelines trees; it’s the citizens themselves. Or to put it another way, inside is more valuable than outside. At USC, Travis Longcore studies satellite imagery of tree cover and rages calmly against square footage. Longcore, tall with an unpruned thatch of brown hair and a salt-and-pepper beard concealing a boyish face, looks at the mansionization effect—how homes are getting bigger even as the plots they’re built on stay the same size. Who loses out? Trees.
It isn’t just the L.A. infrastructure that sidelines trees; it’s the citizens themselves.
Across Los Angeles County, tree cover on the lots of single-family homes declined 14 to 55 percent as house sizes doubled and tripled. “Real estate is the place where excess capital goes,” he says. “The trees ultimately have no value in that.”
Early on, the value of the land was the coin of the realm. The trees upon it were always a distant second. This holds true across neighborhoods, races, and classes. Middle- and upper-class areas like Tarzana and Encino get McMansioned, while homeowners in lower- income Baldwin Hills and Compton add legal and illegal conversions to create second dwellings while replacing trees with cheaper-to-maintain hardscape.
To combat the McMansionizers, there’s a city “in-lieu” tree program: a fee ($2,000-plus for developers, $267 for homeowners) paid when trees are pulled up during construction projects. Longcore thinks that’s an insignificant fee if you really want to deter canopy loss, even if the money is used to plant more trees. He and others point out that it takes years for a young tree to grow enough to provide “ecosystem services” like canopy cover.
When Public Works has to remove a street tree, the policy is to replace it with two others if it’s not a “protected” native tree, and four trees if it is. Which is great, but the trees are still going into small wells in narrow strips between homes and businesses on one side and roads on the other. “All these trees that are being planted in the parkways are being planted in very unfavorable environments,” says Tyson. “What we’ve asked them to do is quite amazing, but there’s a certain time when a tree has outgrown the environment in which it was planted or it declines before that, and we lose it.”
Even the city knows it isn’t the ideal setting for trees. Homeowners, meanwhile, may not know they’re responsible for trees in their parkway or may be actively trying to shirk that duty if they do. “There’s a disincentive to pay for trees,” Longcore says. Trees aren’t cheap; chuck them and save money. But, he points out, without trees you have less cooling, and energy costs go up.
“Trees really are our front-line adaptation to a warming climate,” he says. “To adapt, that means having tree canopy, having shade, having the ability to mitigate the effects of a warming city.” That few people in Los Angeles think this way now is because we worked hard as a city to bring a lot of trees here so we could totally misunderstand them.
photo: Millicent Harvey
It’s important to remind ourselves that this whole situation is one big seat-of-the-pants engineering project. The impulse is to think that this is a story of humans once again screwing up nature. And it is, certainly. But Los Angeles is an Eden we created in the first place. Though we’re seeing a loss of greenery in urban spaces, in fact, the diversity and abundance of plant and animal life here is a result of human tinkering.
Way before Europeans showed up, there were 16 native trees in what is now L.A. County. Coast live oak clustered among the low chaparral in savannas, California sycamore lined waterways, California Bay poked up from rocks in the hills. Settlers brought European trees and Midwestern trees and big lusty East Coast trees, a migration of trees westward that paralleled human expansion. The majority of what you see in California’s urban spaces came from overseas: most from Australia and East Asia, China, and Japan, as well as Europe and the rest of the Americas. Only six percent of the state’s urban trees now hail from California. I am told that though there are 400 native species of trees in North America, in L.A. County, there are 569 species (obviously many non-native). We exceed what contains us.
Unique for its semiarid Mediterranean climate, California lured us to its golden bosom. Everyone saw land they could shape. What we didn’t know was that our view was too small. We hadn’t seen the rings from thousand-year-old trees showing that the formative years of Southern California—from about the mid-1800s to the mid-1900s—were uncommonly wet, that droughts lasting hundreds of years were common, that we were building upon a fluke.
In a November 1905 interview in the Los Angeles Herald, a reporter asked Huntington: “Why is Los Angeles?” Huntington answered “climate” and then repeated it eight times, chanting like a hypnotist or someone selling something. In the climate he saw the ability to plant anything, or build anything, and extend outward forever. To do so, he sang the praises of business opportunities and affordable lots for homes and his trolleys to connect them all up.
Trees, and especially palms, have always been linked to transit, sprawl, and the civic and profit motives connected to each. In the late 1800s, railroads and civic boosters cast Los Angeles as a “semitropical” paradise and miracle of wellness, guest-starring oases of palms like the native California fan. People flooded in. At the time, other trees were more popular: eucalyptus and acacias were, as now, favorites. But in another early iteration of L.A., South American pepper trees, not palms, were the iconic vegetation. They ran along Sunset Boulevard in Hollywood in 1900, when the street was dirt. Peppers were among the first victims of L.A.’s ambitions. They carried black scale, which threatened the immensely valuable citrus population. And they were in the way when roads needed widening as cars became essential. So out they went. As L.A. grew alongside and because of the technology that enslaved it, any tree that wasn’t convenient to orderly, human-first growth was removed.
In his book Trees in Paradise, Jared Farmer writes about how a confluence of adaptive pressures introduced L.A.’s most iconic tree, Washingtonia robusta, the Southland sky- duster—the Mexican fan palm. In the 1920s the birth of the city grid, the popularity of automobiles, a forthcoming Olympics, and, later, a Depression-era works project meant the city needed neat, attractive, easy-to-maintain trees that would know their place. When streetcars, and then automobiles, came on the scene, the road became a central piece of infrastructure, and palms—first the native California fan palm, then the Canary Island date palm and others—became “municipal trees,” the city’s choice for urban furniture. Hollywood started filming them in column-lined avenues that looked like open-air palaces, and they became, as Farmer writes, “a metasymbol for the desirable city itself.” No symbol was more enduring.
Over the years cars multiplied and roads expanded under palms that grew tall, grew famous, and grew old. Some are nearing a hundred feet and a hundred years. From that height, anyone can see the choice the city made. L.A.’s 6,500 miles of streets are 28 per- cent of its total developed land. Compare that to the 20 percent that is tree canopy. To look at L.A., with its tree coverage, its clear preference for impermeable pavement that gets so hot it basically throbs, you’d think nobody understood the effect of that choice. The city understands. The need for change was symbolized by a 2006 policy that very gently suggested that the city replace Mexican fan palms as they die with something that casts shade and offsets the heat-island effect. Controversy ensued. “Is Los Angeles over?” the media asked; such was the connection between tree and town. Certainly one long-running version of Los Angeles—the impermeable one, the internally combusting one—is nearing an end of one sort or another.
Hello!! Let me introduce myself…. Botanists call me Kigelia africana, but you can refer to me as the African sausage tree. You may have overlooked my presence in the past, but I am here to inform you of what a unique, interesting, and fascinating tree I am.”
One of the most charming things the City of Los Angeles does is these trifold flyers detailing some of L.A.’s heritage trees. What’s so adorable is that they are written from the perspective of the tree. We learn that all trees are childlike, fond of multiple exclamation points, and occasionally braggy. Still, the city is to be commended for tackling a basically ancient question, which is: How do we relate to nature? One clue. In bold all-caps at the top of each brochure: PLEASE PROTECT ME.
This plea might be bitterly received by the 18 Ficus microcarpa that the city is going to yank out along North Cherokee Avenue in Hollywood. You know ficus? Aka figs? Good shade, bad roots. There are a couple of different species in the city, and they were popular trees for streets and parks. (Ficus are the bathroom walls of the urban canopy: Carved initials or “I Heart whoever” show up dark against the trees’ pale skin.) The shade they create is deep black. You don’t so much drive through it as plunge into it. All of which will vanish, along with its attendant benefits, when the City of L.A. pulls these trees out to replace the sidewalks, buckled by obstreperous roots in too-small wells. The city has to. There was a lawsuit, plus Americans with Disabilities Act violations, and $1.4 billion to repair the walkways around town. It’s a controversial move.
While trees may not know it, they live at the intersection of many competing priorities. “We’re pretty densely populated, and it’s only getting more densely populated,” says Kevin James, president of L.A.’s Board of Public Works. Add community groups, developers, and state and federal regulations, and “it just gets harder to [create a robust tree canopy] in a more densely populated urban area when you have ADA requirements and road-width requirements and sidewalk requirements and tree-well limitations. You’re limited to some trees that we now know don’t buckle the sidewalks like trees that may have been planted 50 years ago have done.”
Rich neighborhoods have trees and the associated benefits of air quality, cooling, better health, greater biodiversity; poor neighborhoods do not.
I’m standing in the shade with Stephanie Pincetl, an environment and sustainability researcher at UCLA. Short-haired and wry, she shares a quality of awe and cynicism common to the tree researchers I meet, which is probably typical of most scientists whose work takes them near the junction of nature and human imperative. The trees are also here, for now, screwing up the city’s best-laid sidewalks. I’m looking for solutions, or at least reassurance that trees are compatible with modern cities. Pincetl’s solutions are not modest. She scoffs at the parked cars. Why pull out sidewalks when you could reduce the size of streets? Why do we need so many cars? E-scooters like Birds and Limes are migrating eastward from Santa Monica; we’re in a meta-ecological debate about whether they are an invasive species or add to transit biodiversity.
Pincetl and I are here to drive around L.A. You should do this. Drive around and filter the city through the lens of what trees are where, and what’s there when there aren’t trees. Hollywood, Inglewood, the oil fields, South L.A. You start to see the truth behind the maxim uttered by researchers who study the connection between wealth and canopy cover: “Trees grow on money.”
L.A.’s 20 percent urban canopy cover is not evenly spread. Maps of tree canopy superimpose neatly over class divisions. Rich neighborhoods have trees and the associated benefits of air quality, cooling, better health, greater biodiversity; poor neighborhoods do not.
Why? Maintenance costs, for one. Research shows that the affluent have more resources to plant and water and trim trees than lower-income homeowners. (Although as drier climate pushes the water table lower, “well-monied” trees will suffer, too. Visit well-forested neighborhoods and you’ll start to see thirsty trees.) Meanwhile, landlords of rental properties, where incomes are lower, yank the trees to avoid the cost of upkeep. The spike in the electric bill from running the AC gets passed on to renters. But everyone wants a free ride: Pincetl’s research shows the best tree for boosting property value is one that’s near the property but not on it. All the benefits of a tree with none of the costs.
Pincetl, like others, sees a reckoning ahead for the whole ecosystem, regardless of class. “L.A. was an artifact of wealth, of material abundance. Lots of fossil fuels, cheap; cheap building materials; lots of space; abundant water. Abundance. So what do you do when you have abundance? Well, you’re not very careful with managing scarce resources be- cause they’re not scarce,” she says. “We are entering into a transition period where that abundance no longer exists.” People who pay attention to trees all talk this way, a style of speech I’d call “Reasonable Apocalyptic.” It’s a perspective that suggests, humbly, that maybe it’s time for a new city. One that doesn’t just dole out trees and hope for the best.
In 2006 then-mayor Antonio Villaraigosa launched a program to increase canopy cover by putting free trees in people’s hands. By the official end of the program, 400,000 trees had been planted. Of those, many suffered from lack of general care. What was learned? The city could not support the massive expenditure of resources required to distribute trees, to integrate them into sometimes hostile infrastructure, to correct for social and economic inequality, and then to lavish years of attention needed for them to establish themselves. Nor was it reasonable to expect the average resident to do it, despite the zeal of volunteers.
Pincetl thinks that trees are often used as a token of environmental intention. “Trees cannot be implemented as a one-off policy,” she says as we drive by a long, narrow wetlands park that used to be a Superfund site. “We don’t want to do the hard work. We don’t want to really think about actually changing things. If we plant enough trees, everything will be good.” Instead, recognize that a city is a huge and unwieldy ecosystem. Its many competing priorities should not lead to more treeless patches when everyone agrees we’re trying to build a forest here. We need to accept it: For all its pavement, the city is nature. Or at least nature-adjacent.
photo: Millicent Harvey
Changes are afoot. The city favors drought-resistant trees and in a wide variety of ages and species so that a street’s worth of trees doesn’t all die at the same time. By the time you read this, L.A. will have voted on a measure to charge property owners a fee based on their impermeable square footage. The proceeds, estimated at $300 million, will fund a system to prevent stormwater from becoming runoff to the ocean and be used to plant trees and create bioswales—those foliated trenches you see in parking lots. (Critics call it a rain tax, proving that no element can go unpoliticized.) To reduce the heat-island effect, the city is already deploying “cool pavement,” a light-colored asphalt coating.
Meanwhile, importing water and controlling runoff are major concerns. The City and County of Los Angeles spend billions trying to figure it out. TreePeople’s Andy Lipkis tells me about a study the organization conducted that found that 1.2 million of the county’s single-family homes could support a rainwater-capture system. That would make up for some 10 percent of residential demand, reduce polluted runoff, and create emergency supplies. Add trees into that equation, and it becomes more sustainable still. City and county departments are wise to these findings and are working, in fits and starts, to in- corporate them into planning. “The mindset that we were a desert was only amplified by our actions,” says Lipkis. “We were making it a desert by throwing that water away.”
Lipkis and others see Santa Monica as a good model for L.A.’s future urban forest. It is perhaps obnoxious for L.A. to be compared to its rich neighbor. Still, Santa Monica has responsibility for representing the finish line for everyone heading west. On a map, Santa Monica is a seaside cube of wealth, the gold tooth in L.A. County’s westward grin, visible from Asia. What the West is, what California is, what Los Angeles means, culminates and terminates here: ocean, beach, houses, cliffs, and palm trees.
Matthew Wells is Santa Monica’s head of Urban Forestry. He, too, migrated west. Before Santa Monica, he was in New York, and before that, London. Tall and long-limbed, he is a able, precise, and winningly English. His parents were pharmacists and spare-time horticulturalists who raised Wells in a town called Nailsworth in Gloucestershire. Gardening was approximately the number two pastime there. “The only thing we’re really famous for is cheese-rolling,” he says. “There’s nothing else to do there but chase cheese down a hill.”
In maintaining the urban forest, Wells also tends to the identity of Santa Monica—and, by extension, the whole region, since outsiders don’t distinguish “L.A.” as a collection of 88 cities so much as a great breathing organism, the Gaia hypothesis with the top down and Ray-Bans glinting in forever sun. Replacing old and sick palms at Palisades Park with tidy, xeriscaped succulents isn’t an option for city fathers, residents, or tourists. So he proposes replacing them with other palms, an elegant sleight of palm, which, like magic, isn’t really magic but gives the eye what it thinks it wants.
Wells does it with data. Every current and planned tree plotted on maps, classified by Wells and his team for once and future size, longevity, water needs, variety. Each species broken down by gallons of stormwater retained, kilowatt hours of electricity saved, pounds of atmospheric carbon removed, cost of environmental benefits. This is relatively novel for city planning (and city budgets focused on the fiscal Now) to think in terms of how the urban is an extension of, and an interaction with, the natural world.
What the data show is that some trees don’t comport with the new climate. Magnolias, for example. Early arborists brought in the Southern favorite for its beauty, size, and the biggest and most uncomfortably lush flowers of all the region’s trees. (According to a 2011 study, it’s Santa Monica’s third-favorite tree, behind the jacaranda and palm.) Magnolias are not a part of Santa Monica’s long term. As they go, they’ll be replaced by, among others, Quercus suber, the good old cork oak. So if you drive through the city and see a magnolia, recognize that you are witnessing the end of a historical moment. Blow it a kiss, or draw your finger slowly across your throat, depending on your sympathies for the overly demanding, underly adaptable genus Magnolia.
How do you do it, Santa Monica? A small, rich city with controversial futurist aspirations to restricted development, low carbon production, and single-family homes that produce rather than consume energy. The affluence affords a vision of how a tree-forward city operates: the ability to prune lightly every three years rather than L.A.’s 18; to address buckling sidewalks with minor surgery to tree or infrastructure; to recycle 550 million gallons of water for landscape and irrigation and use trees to capture stormwater before it runs into the Pacific, all toward meeting a 2020 goal of “water self-sufficiency.”
The City and County of Los Angeles are creeping bureaucratically toward similar solutions. “Planting the trees is not the hard part,” says Lipkis. “It’s setting up the system so trees survive and you get the desired set of outcomes.”
It’s a squishy thing, talking about how trees improve mental health and reduce crime and clean the air. How to feel about that, really? Driving around with Kathy Musial and Stephanie Pincetl, I was hoping to learn the secret to loving trees and to share it with you so you’d love them, too, not just for their beauty, but in an evolved way that includes social equality, government efficiency, and futuristic infrastructure. It’s got to do with the value of the data, to be sure. But even Travis Longcore—a high priest of urban forest data collection, a man whom I have seen patiently review blurry satellite images of the city, crown by leafy crown, to determine the carbon savings of trees in a particular area—even he has a moment of public emotional evangelism. In a scholarly paper, he essentially throws his hands up in the air and says, “the public is most strongly motivated to action on biodiversity and environmental action based on personal emotional satisfaction.” He adds, “Surveys of property owners show that by far the top reason for planting trees is their beauty, not direct appreciation for the economic value trees provide or an abstract sense of environmental services. One way that beauty or love motivates landscape choices is through wildlife.”
Where is the beauty in all this? I could tell you that, at the Huntington, experimentation continues. After my visit, Musial sends me an email saying, “I want to say just one word to you. Just one word. (Cue The Graduate.) Brachychiton,” and then includes a picture of a big, new, leafy tree—very green, very happy in the heat. “Planted in March, the Australian genus were three feet tall. They’ve since quadrupled in height.”
Or I could tell you to forget beauty and instead let fear drive you. For Lipkis, after half a century of doing this, he worries Los Angeles has hit its limits. “There are cities around the world that understand that trees are part of the watershed,” he says. “Now we’re at a tipping point. We have to bring all this into focus because lives are at stake.”
Or I could tell you about being stuck in traffic with Pincetl on the 405, where Western civilization spends a lot of its day staring blankly ahead. Why are we forced to change so much stuff all at once, I ask? It’s a new ecosystem, she says.
“If we want people to live decent lives, which I would hope most people would, we’ve got to conserve those resources so that they can go around more. There’s more for everybody rather than just a whole lot for a very few, which is what we have today, a whole lot for very few. That’s a recipe for revolution, frankly…. I’m not saying that that’s going to happen here, or anywhere, particularly, but it’s just so skewed. Am I—?” She looks over at me. “Do I sound crazy to you?”
She doesn’t. I get off the highway. I’m dropping her off near the Hammer Museum in Westwood, where she’s meeting a colleague. They’re beginning work on an L.A. County sustainability plan. “I’m not saying we shouldn’t be rethinking things or reforming things or having more oversight and scrutiny,” she says. “I’m not saying any of that. I’m just saying that it’s a complicated, dense world out there.”
Across the street, pruners are working on a row of magnolias. It is a rare thing to witness. The workers might as well have been unicorns, with chainsaws and a delicate touch. “They did a nice job on that tree,” she says as she gets out. And they really did. It is a beautiful thing to see, a system of systems all working together. A well-shaped crown, plenty leafy, cut to satisfy the needs of a city for at least a generation, and then turned back over to its wildness.
Brandon R. Reynolds is a contributing writer for Los Angeles. His article on Noches con Platanito appeared in the July 2018 issue.
The worst plant-damaging high heat event I’ve seen in my career happened in early July of this year, affecting plants in the San Gabriel Valley and throughout the Southern California region. Many of you have asked for help on how to manage your trees and gardens in such a severe event. I’m responding with some tips gained from my 25 years of experience promoting healthy trees and landscaping. If you have questions, please do call on me to assist you, 626-272-8444.
In the record-busting heat wave in July, mature trees dropped green leaves, buds and fruit, plants wilted, sunburned and scorched. Shoots, seedlings and potted plants wilted and died. What caused it? Climate change is promoting heat events and drought conditions here and around the world. More extreme weather events of this nature are expected, and will continue. We need to learn strategies to protect our trees and landscaping, which I outline below.
This garden, protected by a top layer of mulch, features drought-tolerant native oaks and sycamores.
Choose the right species
Climate change presents significant challenges to the future of our urban landscape. As our once-mild Mediterranean climate heats up, we have to choose species that can adapt. Species such as redwood, birch, saucer magnolia, Victorian box and avocado tend to struggle in extreme heat events, and may need to be protected from heat and intense reflected light, or replaced with local natives, such as oaks and sycamores. Heat tolerant species from South America and the Sonoran desert, such as tipu, mesquite, desert willow, velvet ash, pinyon pine, California juniper, red willow, desert apricot and cypress may also be good choices.
Plant for both drought and heat resistance
Some plants from coastal climates can take dry soil but are not genetically adapted to high temperatures. Plants that thrive in hot conditions can feature waxy leaves, smaller leaf surface area, reflective surfaces or blue-gray color to reduce heat absorption, or can be drought deciduous, dropping leaves during dry parts of the year. Some plants store water below ground in roots or above ground in stems.
Heat damage can make plants more susceptible to opportunistic diseases and wilt, chlorosis and fruit drop. Excessive heat and sunlight can speed up disease issues. Reflected heat can do damage. Plants near hardscape, artificial turf and on sunny walls with a southern or eastern exposure can be scorched or burned. High soil temperature damages seedlings and may cause root and tissue damage that shows up later in trees and shrubs.
Burlap shade cloth protects this tomato plant.
Implement cooling strategies during the event
Provide shade. Plan ahead and plant tall annuals like sunflowers to shade smaller plants. During the heat event, use shade cloth, cardboard and patio umbrellas to protect plants from heat. Protect mature shade trees that are shading other plants in the landscape, and keep them well hydrated prior to high heat events. Wrap burlap on exposed trunks, or use a foliar spray, whitewash or latex paint on trunks for sunburn protection. Shade can also be sprayed on plants and trees in the form of a clay product called Kaolin that reflects sunlight, much as we use zinc oxide as a sun block. Apply with an inexpensive sprayer. Be sure to follow package directions and wear a mask and goggles.
Water strategically
Water to establish deeper roots – less frequent deep irrigation in the spring and fall. During the summer, watering may need to be based on the weather—weekly or monthly—even for established trees. Get ahead of any high heat events and water deeply several days in advance of the event.
Soak plants to 1-2 feet deep, and allow the top one or two inches to dry out before watering again. Make sure water is reaching the roots at and beyond the drip lines of plants. Check moisture with an inexpensive moisture meter, with a small shovel or long screwdriver.
Mulching, blowing and pruning
Mulching is the best way to protect soil and plant roots from heat. It’s also key to conserving water. Mulches hold moisture, encourage soil microbial activity, suppress weeds and improve soil structure.
As you phase out heat-and drought-sensitive plants and plant new trees and shrubs that can tolerate drier and hotter environments, be sure to chip and spread them on-site as valuable mulch. Your old plants can help protect your new plants and provide food for their growth.
Apply bark chip mulch or mulch/compost mix about 4 inches deep around plant roots. Don’t use rocks or inorganic mulch—it can retain heat in the soil at night when roots need to cool down. Artificial turf should not be used—it can overheat soil, damaging roots.
Keep your mulch in place, don’t blow it away. Plants need this organic cover to protect the roots from heat and drying. Remind gardeners that blowers are for paved surfaces only. Keep dropped leaves in planter beds—leaf litter is good plant food.
Remove understory competition such as ivy, creeping fig and vinca. These plants can take water and nutrients away from trees and shrubs. Mulch these areas or plant sparsely.
Resist the urge to prune. Keep dead leaves on plants and trees until cooler fall weather. Those dead leaves will offer some sun protection for the rest of the summer.
Patio umbrellas provide protection for squash and pumpkin vines
Wait until fall to plant and fertilize
Plant new shrubs and trees in the fall, after the weather starts to cool off. Encourage new root and shoot growth by using products with potassium, that strengthens cell walls.
Don’t fertilize during a heat event. When you do fertilize, use products that strengthen cell walls. If soil is so dry it repels water, buy a wetting agent to penetrate the soil.
The Ficus microcarpa trees along Hollywood’s Cherokee Street create a majestic arch. Walking beneath them is an almost otherworldly experience. In the impenetrable shade, as birds chirp high in the deep green canopy above, the air is unmistakably cooler.
The shade that trees produce can cool surfaces like soil and pavement. But trees can also lower the surrounding daytime summer air temperature up to 10 degrees, thanks to water evaporating from their leaves.
That’s why preserving mature trees that form a canopy should be LA’s priority, says Glynn Hulley, a scientist in the carbon cycle and ecosystems group at NASA’s Jet Propulsion Laboratory.
“It’s a pretty precious resource in cities, and you don’t want to take them down—you want to be adding to them,” he says.
LA’s palm trees are iconic, but they require a lot of water, and don’t create a great deal of shade. Getty Images/Collection Mix: Subjects RF
Instead, since 2000, many neighborhoods in the LA region have seen a tree canopy reduction of 14 to 55 percent, according to a University of Southern California study published in 2017.
In recent years, the city’s street trees have taken a hit. According to permits filed with the city of Los Angeles’s street services bureau, 263 street trees—including the 18 on the 1200 block of North Cherokee—are slated to be ripped out in the first five months of this year alone for sidewalk repairs and street widening.
Those numbers are for removals of three or more trees at a time and do not include instances where one or two trees are removed for repairs, which do not require a public hearing. They also do not include permits by developers to remove one or two street trees.
“People should be climbing into these trees to stop them from being cut down,” says Hulley.
Hulley is publishing a major study this summer looking at heatwave trends in the region, which he recently presented to a Los Angeles County sustainability task force.
“Heatwaves are not only increasing in frequency and intensity, but also their seasonality is changing, with more heatwaves earlier and later in the year,” says Hulley. “Trees are the most cost-effective way to cool down the urban environment.”
Since 2010, the region has experienced extreme drought conditions, which not only kills trees but also makes them more susceptible to disease. But the drought is only partly to blame for LA’s recent tree loss.
:format(webp)/cdn.vox-cdn.com/uploads/chorus_asset/file/11488971/GettyImages_84543459.jpg)