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Let's clear up misconceptions first: Los Angeles is not a desert. Los Angeles is a semi-arid zone with a Mediterranean climate. The distinction may sound small, almost semantic, but when you look to the hills or the undeveloped lands north of the Valley you don't see sand dunes, you see oak trees and chaparral. Los Angeles and the rest of Southern California has spent the better part of a century building up its vegetation (a drive down the tree-canopied streets of Bel Air or Pasadena's Oak Knoll will make this readily apparent), but all that may be changing. The Southland is in the midst of a massive tree die-off.
“We’re witnessing a transition to a post-oasis landscape in Southern California,” Greg McPherson, supervisory research forester at the U.S. Forest Service, said, notes the Los Angeles Times. “Many of the trees we grow evolved in temperate climates and can’t tolerate the stress of drought, water restrictions, higher salinity levels in recycled water, wind and new pests that arrive almost daily via global trade and tourism, local transportation systems, nurseries and the movement of infected firewood.”
One of these pests is the polyphagous shot hole borer beetle, already found in parts of the Southland. McPherson conducted a recent survey that concluded that the effects of this single beetle could kill off 38% (27 million) of trees throughout Los Angeles, Orange, San Bernardino, and Riverside counties, causing irrecoverable losses to the ecosystem and some $36 billion in economic damages.
“Catastrophic loss of our canopy would have consequences for human health and well-being, property values, air-conditioning savings, carbon storage, the removal of pollutants from the air we breathe, and wildlife habitat,” McPherson added.
Meanwhile, according to the Orange County Register, Southern California's famed citrus groves are under threat by another invasive insect: Asian citrus psyllid.
“A few great plagues have hit citrus over the years,” Jay Van Rein, a spokesman for the California Department of Food and Agriculture, said. “This is right up there.”
A 2016 study conducted by Jon Christensen at UCLA took a look at native California species adapting to climate change and competition from invasive species.
“We see different kinds of species moving at different rates, and that raises the concern that California’s ecosystems are unraveling,” Christensen said of his findings. “Native species may face not only a changing climate, but also competition from invasive species which are moving more quickly.”
“So many of the trees we grow don’t belong here and aren’t sustainable without plentiful supplies of imported water,” Frank McDonough, a botanist with the Los Angeles County Arboretum, explained, reports the Times. “Historic photos of the region show coastal shrubs, oaks on the foothills and sycamores along streams and rivers. Yet, we planted way too many trees from areas that get two to three times as much rain as we do.”
This suggestion of a return to native species and more drought-tolerant ones is a suggestion McPherson agrees with. He hopes cities will begin replacing dying trees with “a new, diverse palette of well-adapted species that may not be currently available in nurseries...It may be, for example, that trees that grow well in, say, Phoenix...are the ones that will grow well in Los Angeles in decades to come.”
