Climate change and California’s terrestrial biodiversity

In this review and synthesis, we argue that California is an important test case for the nation and world because terrestrial biodiversity is very high, present and anticipated threats to biodiversity from climate change and other interacting stressors are severe, and innovative approaches to protecting biodiversity in the context of climate change are being developed and tested. We first review salient dimensions of California’s terrestrial physical, biological, and human diversity. Next, we examine four facets of the threat to their sustainability of these dimensions posed by climate change: direct impacts, illustrated by a new analysis of shifting diversity hotspots for plants; interactive effects involving invasive species, land-use change, and other stressors; the impacts of changing fire regimes; and the impacts of land-based renewable energy development. We examine recent policy responses in each of these areas, representing attempts to better protect biodiversity while advancing climate adaptation and mitigation. We conclude that California’s ambitious 30 × 30 Initiative and its efforts to harmonize biodiversity conservation with renewable energy development are important areas of progress. Adapting traditional suppression-oriented fire policies to the reality of new fire regimes is an area in which much progress remains to be made.

range in mean annual precipitation from >300 cm in redwood forests to <5 cm in the southern deserts.California supports the highest interannual variability in precipitation in the United States, with episodic atmospheric rivers (i.e., moisture-laden flowing columns of the atmosphere) contributing 30 to 45% of California's annual precipitation, and multiyear droughtswhich have major influence on wildfire occurrence-occurring with regularity (9).
Relatively recent (late Miocene to Pleistocene) uplift of the state's mountains exposed diverse bedrock types, weathering in variable settings to produce a vast number of named soil types differing in texture, chemistry, and water availability (10).These soils support hundreds of distinct vegetation types (435 Alliances, >1,200 Associations; 11,12).Geologic and topographic diversity alone is not enough to generate high biotic diversity, as can be seen by comparing California with lower-diversity Alaska (four times larger in area and also with dramatic topography of tectonic origins); the Mediterranean climate plays a decisive role in evoking high biodiversity from sharp physical gradients (7).
Fire is the most important natural disturbance regime shaping ecosystem dynamics in California's upland habitats with a Mediterranean climate (13).Most of California is intrinsically fire-prone due to moderate to high plant productivity combined with a lengthy dry season.However, ignitions prior to European settlement only occurred where lightning coincided with dry weather or where Indigenous burning.Fire is critically important in California, yet its historical frequency, severity, and ecological effects are complex and poorly understood (14,15).1.2.Biotic Diversity.California supports 4,266 full species of naturally occurring (native) plants, 1,307 (31%) of which are endemic; these numbers change to 5,006 and 1,846 (37%) when considering the California Floristic Province (CFP, >322,000 km 2 ; 87% in California; 16,17), the biotic region defined by the Mediterranean climate that excludes the desert but includes small parts of Oregon and Baja California.Most CFP endemics or near-endemics are either shrubs with sclerophyllous (hard) evergreen leaves, or herbaceous plants with bulbs or seeds capable of belowground persistence, as is characteristic of the Mediterranean biome.More temperate life forms, such as many broadleaved deciduous trees, include few Californian endemics and are found in wetter locations such as coastal and riparian areas and north-facing slopes (oaks being a notable exception).This flora is believed to have originated via persistence of subtropical lineages from a wetter Eocene, incursion of temperate and desertadapted lineages during and after the drying Miocene, and rapid Plio-Pleistocene radiation by some arid-adapted shrub and herb lineages to produce endemic species (18,19).The Miocene onset of widespread fire also played a critical role in plant speciation and adaptation (20).
The state's diversity of native terrestrial mammals (163 species), birds (>300 breeding species), reptiles and amphibians (171 species), insects (ca.30,000 species), and other animals is high, as expected from its large size and ecological variability (21).However, Californian animal endemism is not high in most groups, except for a few notable for their low mobility (e.g., Ensatina legless salamanders; various flightless arthropods; 7).Even its most famous near-endemic animal, the condor (Gymnogyps californianus), had a broader distribution until recently (22).
Invasive species are more numerous in California than in any other state, including ca. 1,300 plant species, and continue to accumulate.California's invasibility is attributed largely to its island-like evolutionary history of isolation, followed by the influx of humans and their associated biota from climatically similar parts of Eurasia and northern Africa.Invasive grasses have been especially impactful as they have transformed perennial grasslands into annual ones with reduced biodiversity (23).
1.3.Human and Institutional Diversity.California was one of the most linguistically diverse places on earth, with at least 78 languages among its hundreds of thousands of Indigenous inhabitants.Small-scale communities predominated, and human-environment relationships were highly diverse owing to varied habitats and resources (24).Today, the state's Native American population is the most urbanized in the country, with relatively few Tribal communities living on their traditional lands (24), and those who do live on these lands rarely possess them.A recent focus on recovering and understanding traditional ecological knowledge is improving clarity about historical and contemporary Indigenous influences, respectively, on California ecosystems, which promote viable populations of culturally important plants and animals through active land tending (pruning, propagation, cultural burning; see ref. 25), (24) but much uncertainty still exists as to the intensity and geographic extent of those influences (15).The present-day human population of California is heavily coastal and urban (26), and has the greatest ethnic diversity and percentage of immigrants in the nation.Interior valleys have been transformed by the most diverse agriculture in the world (25), with >400 cash crops, but the farming population is sparse.
California has a history and diversity of environmental institutions, agencies, policies, and laws addressing nature conservation as well as climate change.The state is home to some of the nation's first National Parks (Yosemite, 1864), State Parks (Big Basin, 1927), forest reserves (San Gabriel and San Bernardino, 1892 to 1893), environmental organizations (Sierra Club, 1892), and Land Trusts (Marin Agricultural Land Trust, 1980).An astonishing 46% of the state's area is park or wilderness land, mainly in mountains and deserts (27,28 We next turn to four of the most acute aspects of climate change's impact on California's terrestrial biodiversity: direct impacts, which we summarize with a new analysis of projected shifts in biodiversity hotspots; interactions of climate change with other global change drivers; impacts driven by altered fire regimes; and impacts driven by climate change mitigation efforts, particularly land-intensive solar energy development.

Direct Impacts of Climate on Terrestrial Biodiversity
California's climate has become warmer, effectively drier, and more variable since the mid-20th century and is projected to change further in the future with increasing extreme weather events (29).Climate change impacts are already observable in plant and animal species distributions in California (30,31), including widespread tree mortality from a combination of drought, wildfire, and expansion of pests and pathogens (32), upslope shifts in plant ranges (33), retraction of lowerelevation coniferous forests (34), shifts toward smaller trees and a higher predominance of broadleaved trees relative to conifers (35), and a wide variety of changes in plant and animal distributions and abundances (36)(37)(38)(39)(40).Most of California's ecosystems are water-limited rather than energy-limited (41), and the decreased soil moisture availability caused by warming in such a context may generally drive decreases in the diversity of plant communities (42,43).Tracking their climatedriven range shifts in California's highly complex physiography could require movement in different directions by individual species (44).
One way to seek generalities when forecasting the potential future impacts of 21st-century anthropogenic climate warming on California's biodiversity is to consider the state's geographic concentrations of high numbers of native and endemic plant species, which we term regional hotspots.By identifying general patterns of change in the locations of these regional hotspots, it is possible to reduce the complexity of anticipated future change, the potential limitations of current conservation approaches, and the variety of strategies that will likely be needed (36).
We present an example of such a forecast to illustrate anticipated impacts on regional hotspots (rather than individual species).We analyzed projected floristic change in the Mediterranean climate (California Floristic Province, CFP) part of California, which comprises 70% of the state and does not include the deserts; the CFP is characterized by its globally outstanding plant diversity and endemism and is well documented with georeferenced floristic data.
Briefly, we used species distribution modeling of 6,418 plant taxa (species, subspecies, and varieties) native to the Californian part of the CFP to identify current (1960 to 1990) regional biodiversity hotspots (SI Appendix, Materials and Methods).For each 20.25 km 2 grid cell, we combined species richness with range-size-weighted rarity value for each species present to create an index of conservation value.We selected clusters of 11 or more grid cells with the top 2% of this index to define current regional hotspots in California.We next projected species' ranges with 2061 to 2080 climate predictions from 14 global climate models (GCM)-all under the Representative Concentration Pathway (RCP) 8.5 greenhouse gas emissions scenario-using the species distribution models and again calculated conservation value.Spatial changes in regional biodiversity hotspot locations over time were based on the mean values of the conservation indices from 14 projections.
Our analysis identified 15 well-known current regional hotspots (SI Appendix, Table S1 and Fig. 1A), ranging from small ones such as the Channel Islands to very large areas including the Southern Sierra Nevada and the Central and Southern Coast Ranges.Under projected future climates, the 15 regional hotspots lose 18.8% of their component native species (SI Appendix, Table S1).We identified five major spatial modes of change in hotspots under future climates: i) Four current hotspots in the Channel Islands, Transverse Ranges, and Southern Coast Ranges remain relatively stable in location and were projected to lose less than half their area.ii) Three current hotspots contract by more than half their area.For example, the Central and Southern Coastal Ranges hotspot (Fig. 1) contracts and forms seven smaller coastal hotspots, possibly tracking a diminishing maritime influence.The Inner Coast Range hotspot disappears completely.iii) Three current hotspots shrink so much that they are no longer detected with this approach.iv) Five hotspots shift gradually to track suitable climate.For example, Sierra Nevadan regional hotspots tend to lose lower elevation areas and either contract or advance toward higher elevations (Fig. 1).v) Nine new hotspots emerge in northerly and/or high-elevation locations.Most of these are small, newly climatically suitable areas appearing by 2080 not far from their current locations (SI Appendix, Table S2 and Fig. 1).
The various spatial modes of regional hotspot dynamics suggest different general strategies for climate-adaptive conservation planning.For the hotspots that shift gradually upslope, preserving connecting habitats may be a suitable conservation strategy (45).For contracting hotspots such as those in the central coast, protection of remnant populations and refugia (46)(47)(48) could be the only available conservation strategy, especially given the additional challenges urbanization poses.For novel hotspots, a combination of land protection and assisted migration (e.g., ref. 49) could help enhance the value of these locations to protect future biodiversity.
In summary, recently observed and predicted climatedriven biotic changes in California, including those predicted by our simple model, highlight a much higher degree of complexity than the canonical poleward and upward shifts.Of course, all models have limitations, and ours did not include details found in many models of individual species (e.g., demography, dispersal capacity, functional traits, soils; 50-53).The greatest limitation of any predictive species distribution model arises from the complex interactions between climate and other factors (54).We consider some of the most critical of these interactions in the next section.

Interactive and Ecosystem-Level Impacts of Climate on Biodiversity
Climate change interacts with other land-use and land-cover changes, biotic disruptions (invasive species, pests, and pathogens), disturbances (fire, floods), and pollution, to affect terrestrial biodiversity (54).The same complex physiography that shapes California's biodiversity also powerfully influences these additional stressors (55,56).Urban development is concentrated in coastal areas, intensive agriculture in the Central Valley, and ranching in the foothill oak savannas; montane conifer forests have been extensively altered by logging and fire suppression (57,58).Even desert regions have experienced urban and agricultural expansion, novel fire regimes driven by invasive plants (59), and large-scale renewable energy development (Section 5).One demonstration of interactive effects of multiple stressors comes from >40 y of surveys indicating that both land-use and land-cover change and climate change have altered butterfly distributions and diversity (60,61); low-elevation species have declined, consistent with intensive land use, while montane species have shifted to higher elevations, consistent with warming Over the past century in California's Central Valley, many bird species declined, and some more generalized species increased under the combined impacts of altered water availability, wetland loss and agricultural conversion (62).
Models of expected land-use change, based on population growth projections and the socioeconomic pathways used in climate forecasting, forecast severe negative impacts on the wildlife of California's oak woodlands, savannas, and grasslands (63).When these models also consider climate change, they show climate and land-use impacts can be of the same order of magnitude, although not always in the same places (64)(65)(66)(67)(68)(69).These conclusions are based on species distribution modeling that projects shifts in the distribution of climatically suitable habitat, both separately and combined with expected land-use change.Projected habitat expansion due to climate change is sometimes partially or entirely canceled by land-use change, for example for the plants in southern California (67) and the birds of oak woodlands in the Sierra Nevada foothills (66).In highly urbanized southern California, expansion of urban development poses a more immediate threat than climate change, and it also exacerbates the threat of climate change by reducing habitat connectivity (64,65).The most effective solution to the threat of interacting urban growth and climate change, as identified by a statewide modeling study, is urban infill-pushing new urban development into the existing footprint of current cities.This strategy outperforms alternatives of protecting agricultural lands least at risk to climate change, or protecting major dispersal corridors for plant species from urban development, "Business as usual" urban growth, exemplified by sprawl, was the least favorable scenario for biodiversity (69).
Climate change also interacts with biological invasions to erode biodiversity.Trait-based modeling indicates that future climate change will favor non-native over native grass species in already heavily invaded California grasslands (70).Field experiments have shown that nitrogen deposition, climate change (increasing/decreasing aridity), and plant invasions (71) or elevated CO 2 and fire (72) interact in complex ways to alter ecosystem processes (e.g., primary productivity, nutrient acquisition by plants) and their drivers (e.g., arbuscular mycorrhizal fungi) in California.Invasive annual grasses have promoted novel fire regimes in California's deserts with negative ecological consequences, and climate change is expected to accelerate and amplify this grass-fire cycle (73).
There is increasing evidence of secondary climate change effects, in which pests and pathogens respond to changing environmental conditions, as well as to stress in their host species (74).This is particularly evident in tree-dominated ecosystems.The effects of hotter drought over the past decade in California (75) have led cumulatively to mass tree mortality in the Sierra Nevada Mountains.The unprecedented scale of tree mortality, with associated increase in fuel loads and vulnerability to bark beetles, presents an increased risk of large, severe fires in the coming decades (76).Sudden Oak Death (SOD; Phytophthora ramorum) is estimated to have killed 48 million trees in coastal and northern California and infected 150 million more since 1995, with 1.8 billion remaining at risk (77).The SOD pathogen benefits from warmer rainy temperatures, and although a direct connection has not been established, historical warming of air temperature in the wet winter months of California's north coast ecoregion has increased, with mean air temperature warming of 1.33 ± 0.29°F from 1951 to 1980 (33.63°F) to 1981 to 2020 (34.95°F) (78).
In summary, climate change impacts on California's biodiversity will be exacerbated by the loss of newly suitable habitats to land-use changes; lack of connectivity for reaching newly suitable habitats; and increased pressure from invasive species, pests, and pathogens.Changing fire regimes is an additional stressor that may become even more severe under climate change, and we turn to them in the next section.

Fire-Driven Impacts of Climate on Biodiversity
Fire has long been a dominant ecological process in California, and California's diverse fire regimes are strongly shaped by the state's physical and biotic diversity (79,80).The term "fire regime" denotes the characteristic frequency, seasonality, extent, and severity of fire at any given place in the landscape.Fire regimes broadly depend on climate, fuels, and ignitions, and over the long term, fire regimes also shape the vegetation that provides the fuel supply.Five major fire regimes (called Fire Regime Groups, or FRG) are recognized by the US Federal Land Management Agencies, ranging in frequency from low (200+ years return interval) to high (1 to 35 y return interval), and in severity from low (<25% of overstory vegetation killed) to high (>75% of overstory vegetation killed).Emblematic of its diversity, the CFP supports all five of these major US fire regimes (Fig. 2).Two fire regimes were historically most prevalent in the state: FRG-I, dominated by high frequency (average of one fire every 5 to 15 y) and lowseverity fire, in pine-and oak-dominated lower montane forests and foothill woodlands; and FRG-IV, dominated by moderate frequency (average of one fire every 40 to 90 y) and high-severity fire, in lower elevation chaparral and other shrubland ecosystems.FRG-II, frequent high-severity fire, was once widespread in the state's valley grasslands, but these have been largely converted to agriculture or urbanization (80).
Recent drastic alterations to long-reigning fire regimes, which have been driven by climate change as well as by more direct human agency, are major drivers of change in terrestrial ecosystem function, structure, and composition in California landscapes (81).The fire season is lengthening at high elevations due to lower snow accumulation, earlier spring snowmelt, and higher summer temperatures, and wildfires have been burning into higher elevations (82).In northern California montane forests, the influence of climate on fire size and annual burned area has increased by twofold to fourfold over the last century (83,84).At lower elevations, increased temperatures and decreased precipitation are propelling an increase in highly destructive wind-driven autumn wildfires (85).Forest cover has declined by nearly 7% since 1985, almost entirely because of fire (86), and many ecosystems are undergoing type conversion (i.e., conversion to different dominant species or life form) under the influence of unusually frequent and/or severe fire (87).
Forests and woodlands with historically frequent lowseverity fire (FRG-I) have been hit by a wave of very large (up to 400,000 ha) and increasingly severe wildfires in the last 20 y, with both climate change and the past century of fire suppression playing critical roles.Forest recovery is imperiled by habitat loss, vegetation type conversion, and low levels of tree regeneration driven partly by increasing climatic moisture deficits (88).Recent studies in California have identified strong negative impacts of high-severity burning and/or large high-severity burn patches in these forests on the diversity of a variety of taxa, including lichens (89), vascular plants (90), large and mesocarnivores (91), and birds (92,93).Various ecosystem properties and processes are also degraded under high severity burning in these forest types, including old-growth forest, forest structural heterogeneity, carbon storage, and soil biogeochemical cycles (94)(95)(96).Essentially, all published models project increasing fire size and severity in these forest types as the 21st century progresses (97).
Chaparral, sage scrub, and sagebrush shrublands with infrequent high-severity fires (FRG-V) are threatened more by increasing fire frequency than severity.Unlike FR-I forests, long-term fire exclusion has not had deleterious effects on these shrublands, and indeed fire suppression may be the only factor preventing large areas of these ecosystems from being irretrievably lost (98).Major threats include frequent human-caused ignitions, nitrogen deposition in and near urban areas, and non-native annual grasses, which enhance fuel loads (98).Climate change impacts on these mostly arid ecosystems and their fire regimes are not as profound as those in more mesic FRG-I forests (99).Nonetheless, rapidly warming temperatures, later arrival of rains, and longer and more severe drought cycles are increasing shrub mortality rates, in turn augmenting fuels and feeding more rapid fire growth (100).In many parts of California, fire frequencies today are so high that the CFP's iconic and diverse chaparral is hard pressed to persist, while exotic annual grasses have become prevalent.
Fire also interacts with other disturbances to impact ecosystems and biodiversity.The 2012 to 2016 drought and subsequent water stress have provoked major forest dieback in the Sierra Nevada primarily due to pest outbreaks (Dendroctonus brevicomis, Scolytus ventralis, among others) in dense, fire-suppressed forest stands.At the same time, the overall trend in California is for much higher variability in annual precipitation, and some of the wettest months and years in history have also occurred in the last decade.Numerous ecologically and economically important forest diseases-e.g., Phytophthora spp., Cronartium ribicola-are positively linked to precipitation, leading to higher tree mortality in the absence of drought.In either case, increased dead fuel loading from extensive woody plant mortality can increase ember cast, fire spread, and fire intensity, leading to larger fires and more severe fire effects (101,102).
In summary, under the combined impacts of climate change and other stresses, California's diverse fire regimes are changing toward larger, more frequent, and/or more severe fires that threaten the resilience of natural vegetation and native species.In the next section, we consider how Californian biodiversity is affected by a key climate change mitigation response: landintensive renewable energy development.

Impacts of Renewable Energy Development on Biodiversity
California already hosts more renewable energy than any other state (103), and this capacity is expanding rapidly on land and water to meet the state's climate change goal of 100% carbon neutrality by 2045 (104,105).While the substitution of fossil fuels with low-carbon sources of energy is a critical aspect of climate change mitigation, area-based conservation is the primary means for conserving terrestrial biodiversity (106), introducing the potential for conflict between these goals.However, the rapid buildout of new energy infrastructure also creates beneficial opportunities for certain species and ecosystem services (107)(108)(109)(110).
For example, at least 72 GW of solar energy capacity (with 37 GW of storage) is anticipated by the state Air Resources Board to be necessary to decarbonize the state's energy system by 2045.To date, the development of solar energy in California has emphasized large, ground-mounted installations sited predominantly in California's deserts and the CFP (104).There has been less emphasis on solar energy development within the built environment and other land-sparing recipient environments (110).To support grid integration of these new solar facilities, as well as a quadrupling of wind capacity and 1 GW of new geothermal, state utility experts recommend approximately 50 new transmission projects.Impacts of new energy transmission infrastructure on biodiversity occur during both site preparation and operation and notably include collision-and electrocution-related mortality in birds.Large, ground-mounted solar energy development can affect biodiversity especially strongly during siting and site preparation, which typically includes removing all aboveground vegetation via bulldozing, grading the soil surface, and sometimes constructing large basins to divert and capture runoff (111).Environmental impacts of solar energy development are better documented in California than in most other regions, and include (but are not limited to) effects on land resources (111), land surface temperature (112), soils and hydrology (113), plants (114), insects (115), birds (116), reptiles (117), and mammals (118), In contrast, the impacts of wind energy occur more in during operation; for example, thousands of bird and bat fatalities per year have been documented at the Altamont Pass Wind Resource Area (119,120).Energy development that seeks to reduce impacts on biodiversity prioritizes meeting energy demands locally, where generation occurs as close as possible to demand loads (121).The greater the spatial separation between where energy is being generated and where it is being consumed, the greater the risks for adverse consequences on biodiversity and the communities that value it vis-a-vis environmental justice.Hoffacker and Hernandez (122) coined the term "outsiting" as the process by which large, centralized energy infrastructure is relegated to locations beyond the responsible decision-maker's jurisdictional footprint.
In summary, the potential for adverse effects of renewable energy development on biological conservation is substantial, but there are also opportunities to mitigate these impacts through intentional planning and partnership.The next section considers California's recent policy commitments aimed at protecting biodiversity in a changing climate.

Recent Policy Responses and Opportunities
Protecting the unique biological diversity of California's terrestrial ecosystems requires policies that accommodate the complex interactions of climate change with land-use change, species invasions, changing fire regimes, renewable energy development, and other competing pressures and stressors.Here, we highlight two areas in which California is advancing its climate change adaptation and mitigation plans while maintaining its commitment to protecting biodiversity: its 30 × 30 Initiative and its efforts to mitigate the adverse impacts of renewable energy on biodiversity.We also highlight a third area, fire management, in which policy progress is greatly needed.The goals of California's 30 × 30 Initiative most relevant here include: conserving habitats representing the full diversity of California's ecosystems, especially rare or remnant ones; restoring degraded habitats, especially rare ecosystems; targeting areas for conservation with high species richness, endemism, and rarity; conserving locations that will persist under future climate conditions, serve as refugia for plants and animals, and accommodate habitat range shifts; and, improving habitat connectivity and other actions that build the resilience of species and habitats by facilitating plant and animal migration and gene flow.Actual implementation of the 30 × 30 goals relies on a coordinated effort that began in 2022, led by the California Natural Resources Agency and involving a large array of Federal, State, local, and nonprofit partners.The 30 × 30 plan is strongly information-based, and relies on CA Nature, a set of interactive virtual mapping and visualization tools available to the public, for identifying conservation opportunities and tracking progress.

Reducing Biodiversity Impacts of Renewable Energy.
Aligning a rapid, renewable energy transition with biodiversity conservation requires acknowledging the land resource requirements of various energy infrastructure types (109,123); understanding and anticipating outcomes of energyrelated development on biodiversity; prioritizing land-sparing options and biodiversity-friendly mitigation activities that maximize benefits for biodiversity; and establishing policies and incentives that align these goals.Over a dozen technoecological synergies have already been identified (104), such as development of solar energy in the built environment.For example, recent legislation (SB 49) calls for the Department of Transportation to evaluate the potential for using road rights-of-way for renewable energy generation.California is also the site of the first land-sparing floating photovoltaic solar energy (FPV) system in the world, as well as some of the largest existing and proposed FPVs (108).
Another enormous opportunity for the state lies in the potential to stack ecosystem services and restoration with renewable energy development.The California prairie biome, which once characterized the Central Valley, has been reduced in area by 95%.In 2000, California's nonprofit utility, Sacramento Municipal Utilities District (SMUD), commissioned what was once the largest photovoltaic solar energy parking lot in the world.SMUD is now working alongside UC Davis scientists, the Xerces Society, and the Electric Power Research Institute (EPRI) to restore ecosystem services and prairie habitat at two large, ground-mounted solar energy facilities in the Central Valley.This community-based project seeks to create a model for irrigation-less California prairie restoration under photovoltaic solar energy panels that developers can adopt across the region, especially as some estimate almost 500,000 acres of farmland will be abandoned in the Central Valley to maintain goals for water conservation.Diverse stakeholders are now collaborating to promote the use of abandoned and fallowed cropland for ecological restoration and solar energy across the state (see California Agriculture-Pollinator-Solar Working Group).
6.3.Fire Management.New approaches to managing fire are a critical priority for California.Current policies remain focused on fire suppression despite the inevitability of fire and the ecological necessity for periodic fire in many California ecosystems.In forests and woodlands (FRG-I), stand thinning can alleviate fire hazards and impacts, but it is only practical near human settlements and road infrastructure (124).Greater use of fire as a management and restoration tool may be the only way to mitigate the risks of increasingly catastrophic, stand-replacing fires as the climate warms and fire weather becomes more severe.In these ecosystems, fire has always been a major driver of natural selection and species pool definition, and its longterm absence is leading to dense and vulnerable forests.In chaparral and sage scrub ecosystems (FRG-IV), in contrast, fire suppression is a conservation necessity because of the escalating frequency of anthropogenic fires (98).
Fire management incorporating traditional Indigenous burning practices has been widely discussed as a tool for ecocultural restoration (125)(126)(127)(128).For example, the North Fork Mono tribe in northern California have been working with public lands managers to restore montane meadows and oak groves within conifer forests (129), using burning, digging, and other practices that promote traditional foods, materials, and lifeways.However, in comparison with the immense scale of California's interrelated challenges of fire, climate, and biodiversity protection, the movement toward greater use of Indigenous land-tending practices is likely to have effects that are relatively local in scale.
Fire management policies in California and the western United States, in general, are notably less progressive than policies in the areas of climate and conservation.As in the world's other Mediterranean climate regions (130), focus in California has been on fire suppression for more than a century, leading to massive accumulations of live and dead biomass in FRG-I forests and woodlands, and ecosystem damage due to, for example, widespread and intensive use of heavy mechanized equipment, large-scale burnout operations, and aerial retardant drops (131).More than 50 y ago, the federal wildfire management agencies made a much-advertised switch from fire suppression to "ecological fire management," but retrenchment was already underway by the early 2000s (132,133), and the split between fire management and ecosystem management is arguably as wide today as it has ever been (134,135).
Recent policy and regulatory efforts have attempted to heal this split.In 2015, State and Federal agencies agreed to increase the use of fire in management, and in 2019, they set ambitious targets for vegetation management and restoration; these targets have gone unmet, however, in part because of frequent bans on the use of prescribed fire due to safety and air quality concerns.National Forests in California are increasingly being zoned for ecological fire use (136).Funding has been made available for vegetation management and fuel reduction through the California Carbon Cap and Trade Market (as of May 2023, cap-and-trade provided $792 million to the California Department of Forestry and Fire Protection for its forest health, forest health research, and forest carbon programs; see California Climate Investments).The 2021 California Wildfire and Forest Resilience Action Plan (137) strongly emphasizes using fire for ecosystem management.Still, economic and political incentives have engendered a deeply entrenched preference for fire suppression that is not easily overcome (130,132).
We conclude in the sincere hope that California's current willingness to experiment with bold, knowledge-based solutions to sustaining biodiversity in a changing climate will continue, will be recognized and emulated elsewhere, and will pay off in a better future for natural ecosystems and for humans.
Data, Materials, and Software Availability.All study data are included in the article and/or SI Appendix.

Fig. 1 .
Fig.1.Locations of current and future regional hotspots, represented as those identified by an ensemble from 14 GCMs.(A) Locations of regional hotspots, labeled 1 to 15, identified under the baseline time period of 1960 to 1990.(B) Regional hotspot locations identified in the 2061 to 2080 time period, including those remaining from baseline, which retain their numbers from panel (A), and nine new locations, labeled N1 to N9. (C) Change in regional hotspot locations.

Fig. 2 .
Fig. 2. Fire regimes in California, as they existed at the time of the beginning of mass Euro-American settlement in 1850.I-high frequency, low severity; IIhigh frequency, high severity (not shown); III-moderate frequency, mixed severity; IV-moderate frequency, high severity; V-847 low frequency, high severity.*Redwood (Sequoia sempervirens).Figure modified from ref. 80.

6.1. California's 30 × 30 Plan.
Conservation of biodiversity under climate change is the centerpiece of the state's new "30 × 30 Initiative" or Pathways to 30 × 30: Accelerating Conservation of California's Nature.California was the first state to commit to the international 30 × 30 movement, under which many countries and now a dozen more US states have committed to "protect biodiversity, advance equitable access to nature and combat climate change" by conserving 30% of their lands and coastal waters by 2030.The California strategy is intended to complement the state's Natural and Working Lands Climate Smart Strategy and newly launched equity-focused California Outdoors for All Initiative.