Los Angeles sits squarely within a Mediterranean shrubland biome, one of only five regions on Earth with this climate and vegetation type. The dominant plant communities surrounding the city are chaparral, a dense thicket of tough evergreen shrubs, and coastal sage scrub, a lower-growing mix of drought-deciduous aromatic plants like white sage and California sagebrush. These ecosystems define the hillsides, canyons, and mountain flanks that ring the LA basin, and they share a climate pattern found nowhere else in the Americas outside central Chile: mild, wet winters and long, bone-dry summers.
Chaparral and Coastal Sage Scrub Are Not the Same Thing
People often lump all of LA’s native shrublands together, but chaparral and coastal sage scrub are distinct communities with different structures, different species, and different strategies for surviving drought. Chaparral tends to grow at slightly higher elevations and on north-facing slopes. Its shrubs are evergreen and leathery-leaved, clinging to their foliage year-round. A mature stand of chaparral can pack enormous amounts of woody biomass into a relatively small area. In a well-studied site in the Santa Monica Mountains, a 22-year-old stand of a single chaparral species, bigpod ceanothus, carried roughly 6,500 grams of live aboveground biomass per square meter and produced over 1,000 grams of new growth per square meter each year.1Ecological Monographs. Community Structure and Productivity in Ceanothus Chaparral and Coastal Sage Scrub of Southern California
Coastal sage scrub, by contrast, is a lighter, more open community. Its signature plants, white sage and California sagebrush, shed their leaves during summer to conserve water. At that same Santa Monica Mountains site, the adjacent coastal sage scrub had only about 925 grams of live biomass per square meter, roughly one-seventh the chaparral’s mass, and produced about a third as much new growth annually.1Ecological Monographs. Community Structure and Productivity in Ceanothus Chaparral and Coastal Sage Scrub of Southern California Coastal sage scrub dominates the lower, warmer, drier slopes closer to the coast and the inland valleys, which makes it the community most Angelenos encounter in daily life and also the one most threatened by development.
A Biome Found in Only Five Places
Mediterranean-type ecosystems exist today in just five widely separated regions: California, the Mediterranean Basin, central Chile, southwestern Australia, and the Cape Region of South Africa. All five share the same defining climate quirk, rain concentrated in cool months with a prolonged summer drought, but they evolved independently on different continents with entirely different plant lineages. Despite that independence, they converged on strikingly similar vegetation forms: dense, woody, fire-adapted shrublands.2Annual Review of Ecology, Evolution, and Systematics. Mediterranean Biomes: Evolution of Their Vegetation, Floras, and Climate
What makes these five regions collectively remarkable is their plant diversity. Outside the wet tropics, Mediterranean-type ecosystems harbor the highest levels of plant species richness in the world.2Annual Review of Ecology, Evolution, and Systematics. Mediterranean Biomes: Evolution of Their Vegetation, Floras, and Climate The combination of varied terrain, seasonal drought stress, and periodic fire creates a patchwork of microclimates and disturbance regimes that rewards specialization. Within California specifically, the California Floristic Province, which includes the LA region, harbors more endemic plant and animal species and more identifiable subspecies than any comparably sized area in North America.3Molecular Ecology. Patterns of molecular evolution and diversification in a biodiversity hotspot: the California Floristic Province That biodiversity is concentrated in exactly the kinds of shrublands and woodlands that ring Los Angeles.
Why Fire Is Not Optional for This Biome
Fire is not an occasional misfortune in LA’s shrublands. It is a fundamental ecological process, as essential to the biome’s functioning as seasonal rain. Many chaparral species require fire to trigger seed germination. Others resprout vigorously from root crowns after their aboveground growth is incinerated. The entire system evolved with periodic burning, and without it, old-growth chaparral eventually becomes decadent and less biologically productive.
The most destructive fires in southern California are driven by Santa Ana winds, those hot, dry offshore gusts that funnel through mountain passes and canyons each autumn. Research using regional climate model data has found that wildfire burned area per day is roughly three and a half to four and a half times larger on Santa Ana days than on non-Santa Ana days.4PubMed Central. Santa Ana winds and predictors of wildfire progression in southern California That multiplier is large enough to explain why the same landscape that manages ordinary summer fires without catastrophe can erupt into a city-threatening inferno when Santa Ana conditions arrive.
Not all Santa Ana events are equal, though. Researchers have distinguished between “hot” and “cold” flavors. Cold Santa Ana winds tend to be stronger in raw wind speed, but hot Santa Ana winds last longer and are far more dangerous for fire spread. Historically, about 90% of large Santa Ana-driven wildfires were associated with the hot variety, and those fires accounted for roughly 95% of total burned area.5PubMed Central. Hot and cold flavors of southern California’s Santa Ana winds: their causes, trends, and links with wildfire For residents, the practical distinction matters: a cold Santa Ana day with gusty winds and low humidity is nerve-racking, but a sustained hot Santa Ana event with elevated temperatures is when the most destructive fires tend to run.
Southern California’s fire season also has a less-discussed summer component. These summer fires occur without Santa Ana conditions, driven instead by hot, dry weather and weak onshore winds. Both types contribute meaningfully to the annual burned area and to economic losses, and they respond to different meteorological triggers, which complicates any single prediction model.6Journal of Geophysical Research: Biogeosciences. Contrasting controls on wildland fires in Southern California during periods with and without Santa Ana winds
What Fire Does to the Ground Itself
The effects of fire on LA’s shrublands go deeper than the visible destruction of plant canopy. Intense burns change the physical properties of the soil in ways that make the landscape temporarily hostile to its own recovery. In a steep chaparral watershed in southern California, prescribed burning significantly increased soil water repellency at the surface and at shallow depths within the first week after the fire. The soil essentially develops a waxy, hydrophobic layer from vaporized plant oils that condenses in the ground. This repellency largely returned to near pre-fire values within about two and a half months, but during that window, rain runs off the surface instead of soaking in, dramatically raising the risk of flash flooding and debris flows.7Geoderma. Prescribed burning effects on soil physical properties and soil water repellency in a steep chaparral watershed, southern California, USA
The debris flow problem is real and specific. After a fire strips hillslopes of vegetation, dry ravel, the gravity-driven movement of loose soil and rock fragments, can strip much of the soil mantle before rain even arrives. In one bedrock landscape study, the sandy sediment that accumulated in the channel network after a fire was then evacuated by debris flows during just a one-year recurrence rainstorm. About 95% of the material deposited in the debris flow fan came from material already sitting in the channel, not from fresh hillslope erosion during the storm itself.8Geological Society of America (GSA Bulletin). Debris flow initiation from ravel-filled channel bed failure following wildfire in a bedrock landscape with limited sediment supply For LA residents living below burned slopes, this means the danger from a fire does not end when the flames are out. It lingers through the first rainy season.
How Shrubland Plants Survive Drought
Summer drought in LA can stretch five or six months without significant rain. The native shrubs handle this with an array of strategies that go well beyond simply having deep roots. Coastal sage scrub species like white sage take the approach of going partially dormant, dropping most of their leaves in summer to slash water loss. Chaparral evergreens take a different route, retaining their tough, small leaves but relying on extremely low leaf water potentials to keep pulling moisture from increasingly dry soil.
Some chaparral species push this to extremes. During a severe drought, researchers measured water potentials as low as negative 74 bars in manzanita shrubs, a level of internal water stress that would kill most plants outright. Only three of 90 shrubs in the study died. The rest survived by sacrificing certain branches. Shrubs with 90% or greater branch mortality actually produced significantly more new twig growth on their surviving branches in the years after the drought ended than shrubs that had lost fewer branches.9American Journal of Botany. Survival of Severe Drought by a Non-Sprouting Chaparral Shrub The mechanism appears to be a form of deliberate self-pruning: the plant concentrates its dwindling water supply into fewer branches, letting the rest die, so that at least some part of the organism makes it through. New growth occurred only on branches that survived, never from the dead wood.
Animals in a Fire-Prone Landscape
Fire creates an obvious problem for animals that live in a shrubland: the vegetation they depend on for food and cover is periodically incinerated. The fauna of LA’s chaparral and coastal sage scrub have evolved three broad strategies for coping. Some survive fire endogenously, riding it out in a dormant or inactive stage underground. Others shelter in place within unburned patches, small refugia within the burn perimeter where the fire did not reach. And many simply flee the fire front and recolonize afterward as vegetation recovers.10Fire Ecology. Faunal Responses to Fire in Chaparral and Sage Scrub in California, USA
The recolonization strategy depends on connectivity, the ability of animals to move between habitat patches. In a fragmented urban landscape like greater Los Angeles, highways and development can sever those movement corridors. Studies of wildlife undercrossings beneath southern California highways found that many native mammals used highway passages, but their use depended heavily on the presence of suitable habitat on both sides. For larger animals like deer and coyotes, the physical dimensions of the passage also mattered.11Biological Conservation. Use of undercrossings by wildlife in southern California A culvert connecting two patches of native shrubland gets used; the same culvert connecting a parking lot to a strip mall does not.
The Invasion Problem
One of the most pressing threats to LA’s native shrublands is not fire itself but what happens when fires come too often. Chaparral and coastal sage scrub can recover from a fire that occurs on a natural return interval of several decades. When fires burn the same area every few years, though, the native shrubs never have time to mature and set seed, and non-native annual grasses from the Mediterranean Basin fill the gaps. These grasses then cure into fine, flashy fuels each summer, making the next fire more likely and easier to start. The result is a self-reinforcing cycle called type conversion: native shrubland permanently shifts to invasive grassland.
Research has shown that this type conversion from native shrubland to invasive annual grassland is increasing due to prolonged drought and rising fire frequency.12Restoration Ecology. Invasive grass density negatively impacts chaparral seedling establishment And the conversion is hard to reverse. Invasive grass density actively suppresses chaparral seedling establishment, meaning that even if fire frequency decreases, the grasses are already outcompeting the native shrubs for light and moisture during the critical post-fire regeneration window.
Restoration efforts have had mixed success. In one coastal sage scrub restoration project in southern California, researchers tested whether adding mycorrhizal fungi, the underground fungal networks that help plants absorb nutrients, could boost native plant recovery after seeding. Neither commercial nor native fungal inocula significantly improved root colonization, shoot biomass, or shrub density compared to control plots that received no inocula. In fact, plots treated with commercial inoculum produced shorter plants than the untreated controls.13EPA HERO. Arbuscular mycorrhizal inoculation in coastal sage scrub restoration Results like these suggest that restoring type-converted grassland to native shrubland is not simply a matter of adding seeds and soil amendments. The problem is more structural.
LA’s Urban Forest and Its Biome Roots
Inside the city limits, the native biome has been largely replaced by an engineered landscape of lawns, street trees, and ornamental plantings, but traces of the original shrubland persist in surprising ways. An assessment of LA’s urban forest found roughly 6 million trees covering about 25% of the city’s area. The most common species are a revealing mix: Italian cypress and Mexican fan palm, both non-native ornamentals, sit alongside scrub oak and laurel sumac, which are native shrubland species hanging on in parks, hillsides, and unmaintained lots.14Treesearch (USFS). Assessing urban forest effects and values, Los Angeles’ urban forest Those 6 million trees store about 1.3 million tons of carbon, a service whose value depends on the tree canopy remaining intact.
The distribution of that urban canopy is far from even, and the consequences track with income. Satellite-based measurements of daytime land surface temperature across the LA metro area from 2018 to 2021 showed persistent negative correlations between surface temperature and median household income. In practical terms, poorer neighborhoods are hotter, and the disparity holds up across multiple heatwave events.15PubMed Central. Unequal exposure to heatwaves in Los Angeles: Impact of uneven green spaces Wealthier areas tend to have more tree cover, which shades pavement and cools the air through evapotranspiration. Lower-income areas get more concrete and less canopy. The native biome’s cooling function has been preserved where money replanted it and erased where it was never replaced.
What Climate Change Means for LA’s Shrublands
The future of LA’s Mediterranean shrublands depends heavily on how precipitation patterns shift. Researchers have modeled the potential distribution of 44 focal plant species in southern California’s scrub and shrubland habitats under baseline conditions (1951–1980) and projected midcentury conditions (2040–2069) across five different climate scenarios. The results varied widely depending on the species and the scenario, but one pattern stood out: climate futures with the most extreme changes in precipitation, whether much wetter or much drier, produced the greatest projected losses of suitable habitat for most taxa.16Treesearch. Using species distribution models with climate change scenarios to aid ecological restoration decisionmaking for southern California shrublands
That finding is counterintuitive in one direction. You might expect more rain to help drought-adapted shrubs, but many of these species are finely tuned to specific moisture and temperature windows. A wetter future could favor trees that would shade out the shrubs, or promote the growth of invasive grasses that outcompete native seedlings after fire. A drier future, meanwhile, could push drought stress past the survival limits even of species with extraordinary tolerance, like the self-pruning manzanita described earlier. The shrublands’ fate is not a simple “hotter equals worse” story. It depends on whether precipitation shifts are moderate or extreme, and in which direction.
Speciation and Why Mediterranean Regions Are Evolutionary Hotspots
The exceptional plant diversity around Los Angeles is not an accident of geography alone. Mediterranean-type ecosystems appear to have genuinely elevated rates of species formation. Analyses of the Mediterranean Basin and the Cape Region of South Africa found that speciation rates in those Mediterranean-climate zones were higher than in surrounding non-Mediterranean areas.17PLoS ONE. Contrasting Biogeographic and Diversification Patterns in Two Mediterranean-Type Ecosystems Extinction rates were also higher in those regions, suggesting a fast evolutionary turnover where new species form rapidly but also disappear at higher-than-average rates. The net result is a constantly refreshing pool of species, high diversity maintained not through stability but through dynamism.
California’s version of this story involves additional complexity. The California Floristic Province’s diversity has been shaped by tectonic uplift creating mountain ranges, the rain shadow effects those mountains produce, and the presence of fog along the coast that creates narrow bands of moisture in otherwise arid terrain.18Annual Review of Ecology, Evolution, and Systematics. Origins of Plant Diversity in the California Floristic Province For the plants around Los Angeles, the Santa Monica Mountains, the San Gabriel Mountains, and the coastline each create different microenvironments. A species adapted to a north-facing canyon might never successfully colonize a south-facing ridge a mile away. Over thousands of generations, that kind of fine-grained habitat variation drives the formation of new species and subspecies, which is one reason why a seemingly monotonous hillside of chaparral, if you look closely, holds dozens of species rather than two or three.