Planting non-native trees isn’t ideal — but sometimes it makes sense in LA

California has been steadily losing trees over the last few years — 30% of its tree cover has been lost since the turn of the century — and the only way to solve that problem is to get more trees in the ground.

Though these losses are happening across forests in the whole state, trees in California’s urban regions play a key role in health and quality of life for many. The benefits of an urban tree canopy are well-established: better mental health, more shade, a lower heat island effect, not to mention cleaner air. And even though Los Angeles has the largest urban forest in the nation, many parts of town have been starved for tree cover, especially in low-income neighborhoods.

But while many people can get behind planting more trees, the question of which tree to plant is often a thorny one complicated by the fact that the trees that we want to see in the region aren’t always the ones that will survive. So to help break it down, we called up Bryan Vejar, a training and educational expert at 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 planting native species is always TreePeople’s preference. In many cases, they’re the right tree to plant, and the benefits of planting natives are well-documented when it comes to fostering healthy ecosystems. Native wildlife tends to really like native trees and shrubs, so if you’re looking to bring more California butterflies into your yard, you know what to do.

Certain tree species tend to do best in certain areas of greater Los Angeles, since the region has several microclimates. The coastal live oak is a TreePeople favorite for its large canopy. The Catalina ironwood thrives in coastal regions, being native to the Channel Islands, though it can also survive in warmer inland climates.

… but native doesn’t equal drought tolerant

However, the list of trees that are actually native to the L.A. basin is short — approximately 15 trees — and many of them require more water than can be allocated in drought conditions.

“Of those 15 or so species, willows and cottonwoods are high-water usage trees,” Vejar said. “And so, if water is not an issue, then yeah, sure, let's plant it, right? But of course we know water is an issue.”

In order for a native tree to be considered for planting, it needs to be drought tolerant and heat tolerant, and able to potentially sustain further rise in temperatures of up to 10 degrees Fahrenheit with climate change. (Don’t forget — these trees are also being planted for Angelenos 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 endorse planting native shrubs, annuals, and native grasses as well to sort of supplement that available space,” Vejar said.

Sometimes, non-native trees survive best

As most ecologically minded L.A. residents already know, many of the most iconic trees associated with Southern California — jacarandas, ficuses, not to mention almost every palm tree — are not native to this part of the world.

But the built environment of Southern California, with its concretized channels and higher temperatures due to heat island, is substantially different from most of the natural environment. As such, it’s not always possible to plant native trees in urban areas.

That’s when non-native, non-invasive species come in to fill the void. However, Vejar stresses that they need to be climate-adaptable, like the Tecate cypress, Maverick mesquite, or ghost gum.

When non-native plants are brought in, Vejar said there’s a method to determine which trees will do best in the face of droughts, pests and extreme temperatures, especially as California is expected to heat up due to climate change in the coming years.

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

But he said they tend to be from “other areas in the southwestern United States, Australia, or Africa, or areas that have very, very intense summers and low precipitation.”

A big no-no: Invasive trees

Vejar said TreePeople always avoids planting invasive trees — trees that take over the ecosystem rapidly and substantially displace native species.

One example is the tree of heaven, which has been described as “devilishly invasive” in temperate climates for its abilities to grow fast and clone itself easily. (It’s doubly hated in the eastern U.S. for helping to spread another invasive species, the spotted lanternfly.)

The tricky part, according to Vejar, is that as the climate changes, certain species might become so well-adapted to the point that they make it difficult for competing species to grow.

Vejar said that TreePeople follows the determinations made by the California Invasive Plant Council, which monitors the growth of plant species across the state and assesses if and when certain species have crossed over to becoming invasive.

Too much of a good tree

Even when it’s not invasive, there’s such a thing as too much of a certain tree — but the conventional wisdom on that definition has changed over the years.

Southern California does have a significant variety of trees — 234 species with a population of over 1,000, by TreePeople’s count — but the nonprofit has approved only about a fifth of those species for their future plantings. That’s because the organization is trying to increase biodiversity in the urban canopy, and also because many of these species need a significant amount of care and upkeep.

“Maybe they're very thirsty, or there's a pest or pathogen that's wiping them out very easily, or they have poor root growing characteristics,” Vejar said. “They're just no longer appropriate for our climate. And so we have to rely on the incorporation of additional natives, cultivated varieties of natives, and non-native, non-invasive trees that are climate adaptive.”

A “10-20-30” rule of thumb was long the conventional wisdom to limit overreliance on certain kinds of trees in urban canopies: less than 10 percent of the tree cover coming from a certain species, 20 percent coming from a certain genus, and 30 percent from the same family.

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

To that end, Vejar said that TreePeople is currently using a stricter target: 5% of trees from one species, 7% from one genus and 9% from one family. Diversifying the urban canopy is “the only way that we can ensure that we have resilience in our urban forestry” from pests, pathogens, and diseases, Vejar said.

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