Native/non-native trees

California has been steadily losing trees in recent years — 30% Since the turn of the century, some of the tree cover has disappeared. The only way to solve this problem is to plant more trees.

While these forest losses are occurring across the state, trees in California’s urban areas play an important role in the health and quality of life of many. The benefits of an urban tree canopy are well known: better mental health, more shade, a reduced urban heat island effect, not to mention cleaner air. And while Los Angeles the largest urban forest in the countrymany parts of the city are starved for tree coverespecially in low-income neighborhoods.

But while many people may support planting more trees, the question of which tree to plant is often a tricky, complicated one, because the trees we want to see in our region aren’t always the ones that will survive. So to help break it down, we called Bryan Vejar, a training and education expert with the organization TreePeople, which has planted 3 million trees over the course of nearly 50 years.

Native trees come first…

First things first: Vejar said that planting native species is always TreePeople’s preference. In many cases, they’re the right trees to plant, and the benefits of planting native species are well-documented when it comes to promoting healthy ecosystems. Native animals tend to love native trees and shrubs, so if you want to attract more California butterflies to your yard, here’s what you need to know To do.

Certain tree species do best in certain areas of Greater Los Angeles, as the region has distinct microclimates. The Coastal Live Oak is a TreePeople favorite because of its large canopy. The Catalina Ironwood thrives in coastal areas, as it is native to the Channel Islands, although it can also survive in warmer inland climates.

…but native does not mean drought resistant

However, the list of trees actually native to the LA basin is short: about 15 trees — and many of them require more water than can be allocated in the event of a drought.

“Of those 15 or so species, willows and poplars are water-consuming trees,” Vejar said. “And so, if water is not a problem, then yes, sure, let’s plant them, right? But of course we know water is a problem.”

To qualify for planting, a native tree must be drought- and heat-tolerant, and potentially able to withstand further increases in temperature of up to 10 degrees Fahrenheit due to climate change. (Remember, these trees are also being planted for Los Angeles residents 50 or even 100 years from now.) And even in areas where native trees won’t do well, Vejar said other native plants can still provide benefits to the ecosystem.

“We fully support planting native shrubs, annuals and native grasses to complement the available space,” Vejar said.

Sometimes non-native trees survive best

As most ecologically conscious LA residents already know, many of the most iconic trees associated with Southern California — jacarandas, ficuses, not to mention almost all palm trees — are not native to this part of the world.

But the built environment of Southern California, with its concrete canals and higher temperatures due to heat islands, differs significantly from most natural environments. As such, it is not always possible to plant native trees in urban areas.

Then come non-native, non-invasive species to fill the void. However, Vejar stresses that they must be adapted to the climate, such as Tecate cypress, Maverick mesquite or ghost gum.

When planting non-native plants, Vejar said there is a way to determine which trees will thrive in drought, pests and extreme temperatures, especially as California is expected to warm in the coming years due to climate change.

“Those climate-adaptive trees are not always native trees,” says Vejar.

But he said they usually come from “other areas in the southwestern United States, Australia or Africa, or areas that have very, very intense summers and low rainfall.”

A big taboo: invasive trees

Vejar said TreePeople always avoids planting invasive trees — trees that quickly take over the ecosystem and significantly crowd out native species.

An example is the tree of heaven, which has been described as “devilishly invasive” in temperate climates because of its ability to grow quickly and clone itself easily. (In the eastern U.S. it is doubly hated because it helps spread another invasive species, the spotted lanternfly.)

The tricky thing, according to Vejar, is that certain species can adapt so well to climate change that they make it difficult for competing species to grow.

Vejar said that TreePeople will comply with the provisions of the California Invasive Plant Councilwhich monitors the growth of plant species in the state and assesses whether and when certain species have become invasive.

Too much of a good tree

Even if it is not invasive, there is such a thing as too much of a particular tree. The common definition of this has changed over the years, however.

Southern California has a significant diversity of trees — 234 species with a population of more than 1,000, according to TreePeople’s count — but the nonprofit has approved only about a fifth of those species for future plantings. That’s because the organization is trying to increase biodiversity in the urban canopy, and also because many of those species require a significant amount of care and maintenance.

“Maybe they’re very thirsty, or maybe there’s a pest or pathogen that wipes them out very easily, or maybe they have poor root growth characteristics,” Vejar said. “They’re just not suited to our climate anymore. And so we have to rely on adding additional native species, cultivated species of native species, and non-native, non-invasive trees that are adaptable to the climate.”

a “10-20-30” The rule of thumb has long been the prevailing view to limit overreliance on particular types of trees in urban areas: less than 10 percent of tree cover should come from a particular species, 20 percent from a particular genus, and 30 percent from the same family.

“Biodiversity suffers when we make decisions like that, right?” Vejar said. “Those are reasonable, defensible decisions to make, but frankly, we need to increase the number of species in our urban environment.”

For that reason, Vejar said TreePeople now has a stricter target: 5% trees of one species, 7% of one genus and 9% of one family. Diversifying the urban canopy is “the only way we can ensure that we have resilience in our urban forests” against pests, pathogens and diseases, Vejar said.

With all that said, concerns are different in parts of the region that aren’t urbanized, like Angeles National Forest. TreePeople plants only native species in the mountains, where conditions mimic the original climate of hundreds of years ago.