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Extent of chaparral vegetation within the 4 December 2017 Thomas Fire perimeter (114,078 ha) in Santa Barbara and Ventura counties and the 8 November 2018 Woolsey Fire perimeter (39,234 ha) in Los Angeles and Ventura counties, California, USA.
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Drought contributed to extensive dieback of southern California chaparral, and normalized difference vegetation index before drought and near the end of the drought was used to estimate this dieback, after accounting for other disturbances recorded in aerial photographs. Within the perimeters of two megafires that occurred after the drought, the 20...
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... 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. ...
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.
... For example, tree mortality from drought and bark beetles impacts vegetation burn severity, and the Sierra Nevada had been in a multi-year drought with millions of dead trees leading up to the Creek Fire (Fettig et al., 2019(Fettig et al., , 2021Littell et al., 2016). Longer droughts lead to more fuels with lower moisture, more fuels in contact with the soil, and increased vegetation burn severity when ignited (Huang et al., 2020;Keeley et al., 2022). Covariates related to drought impacts could improve prediction of SBS, as has been reported for vegetation burn severity (Huang et al., 2020;Keeley et al., 2022). ...
... Longer droughts lead to more fuels with lower moisture, more fuels in contact with the soil, and increased vegetation burn severity when ignited (Huang et al., 2020;Keeley et al., 2022). Covariates related to drought impacts could improve prediction of SBS, as has been reported for vegetation burn severity (Huang et al., 2020;Keeley et al., 2022). We attempted to capture some drought impact to vegetation by utilizing pre-fire NDVI and NDWI, but a different NDVI (i.e., dNDVI) from a pre-drought period to a post-drought period would be a better metric of drought impact, as opposed to the general vegetation health captured with a single pre-fire NDVI/EVI/EVI image (Keeley et al., 2022). ...
... Covariates related to drought impacts could improve prediction of SBS, as has been reported for vegetation burn severity (Huang et al., 2020;Keeley et al., 2022). We attempted to capture some drought impact to vegetation by utilizing pre-fire NDVI and NDWI, but a different NDVI (i.e., dNDVI) from a pre-drought period to a post-drought period would be a better metric of drought impact, as opposed to the general vegetation health captured with a single pre-fire NDVI/EVI/EVI image (Keeley et al., 2022). ...
Fire alters soil hydrologic properties leading to increased risk of catastrophic debris flows and post‐fire flooding. As a result, US federal agencies map soil burn severity (SBS) via direct soil observation and adjustment of rasters of burned area reflectance. We developed a unique application of digital soil mapping (DSM) to map SBS in the Creek Fire which burned 154,000 ha in the Sierra Nevada. We utilized 169 ground‐based observations of SBS in combination with raster proxies of soil forming factors, pre‐fire fuel conditions, and fire effects to vegetation to build a digital soil mapping model of soil burn severity (DSMSBS) using a random forest algorithm and compared the DSMSBS map to the established SBS map. The DSMSBS model had a cross‐validation accuracy of 48%. The established technique had 46% agreement between field observations and pixels. However, since the established technique is manual, it could not be compared to the DSMSBS model via cross‐validation. We produced SBS class uncertainty maps, which showed high prediction probabilities around field observations, and low probabilities away from field observations. SBS prediction probabilities could aid post‐fire assessment teams with sample prioritization. We report 107 km² more area classified as high and moderate SBS compared to the established technique. We conclude that blending soil forming factors based mapping and vegetation burn severity mapping can improve SBS mapping. This represents a shift in SBS mapping away from validating remotely sensed reflectance imagery and toward a quantitative soil landscape model, which incorporates both fire and soils information to directly predict SBS.
... With wind speeds reaching up to 97 km/h (60 mph), the fire crossed the multi-lane 101 Freeway overnight and reached the ocean 6.5 h later, with most of the area burned and buildings lost on November 9th. The Woolsey Fire was massive in size, likely due to extensive dieback after a prolonged drought, resulting in rapid fire spread via large-size ember generation, long-distance transport, and high ignition potential (Keeley et al., 2022). Like most Santa Ana wind-driven fires in chaparral-dominated systems, it burned at high intensity with moderate-to high-severity throughout most of the perimeter (Jin et al., 2015;Keeley et al., 2022) (Jin et al., 2015;Keeley et al., 2022). ...
... The Woolsey Fire was massive in size, likely due to extensive dieback after a prolonged drought, resulting in rapid fire spread via large-size ember generation, long-distance transport, and high ignition potential (Keeley et al., 2022). Like most Santa Ana wind-driven fires in chaparral-dominated systems, it burned at high intensity with moderate-to high-severity throughout most of the perimeter (Jin et al., 2015;Keeley et al., 2022) (Jin et al., 2015;Keeley et al., 2022). By November 21, 2018, the fire had caused the evacuation of more than 295,000 people, burned 392 km 2 (151 sq mi), led to three fatalities, destroyed 1643 buildings and damaged 364 buildings (totals include Ventura and Los Angeles Counties) (Los Angeles County, 2019a,b) ( Fig. 1). ...
... The Woolsey Fire was massive in size, likely due to extensive dieback after a prolonged drought, resulting in rapid fire spread via large-size ember generation, long-distance transport, and high ignition potential (Keeley et al., 2022). Like most Santa Ana wind-driven fires in chaparral-dominated systems, it burned at high intensity with moderate-to high-severity throughout most of the perimeter (Jin et al., 2015;Keeley et al., 2022) (Jin et al., 2015;Keeley et al., 2022). By November 21, 2018, the fire had caused the evacuation of more than 295,000 people, burned 392 km 2 (151 sq mi), led to three fatalities, destroyed 1643 buildings and damaged 364 buildings (totals include Ventura and Los Angeles Counties) (Los Angeles County, 2019a,b) ( Fig. 1). ...
There are growing concerns about increases in the size, frequency, and destructiveness of wildfire events. One commonly used mitigation strategy is the creation and maintenance of defensible space, a zone around buildings where vegetation is managed to increase potential for structures to survive during wildfires. Despite widespread acceptance and advocacy of defensible space, few studies provide empirical evidence documenting the efficacy of different fuel modification practices under real wildfire conditions. The 2018 Woolsey Fire in Los Angeles County, California, occurred a short time after high-resolution (0.07 m 2) land cover data were created, providing a unique opportunity to quantify vegetation before the fire. We integrated measurements from this high-resolution land cover data with parcel data, building attributes, and environmental context. We then used Random Forests models to analyze the extent to which these factors predicted structure loss in the wildfire. Variable importance scores showed vegetation around buildings was not a strong predictor of building-level damage outcomes compared to building materials and landscape features such as paved land cover per parcel, elevation, building density, and distance to road networks. Among building materials, multi-paned windows and enclosed eaves were most highly associated with building survival. These results are consistent with other studies that conclude building materials and environmental context are more related to survivorship than defensible space.
... (3) How do the relationships between water status, water content and flammability vary across species and dry versus wet sampling seasons? (Keeley, 1992;Keeley et al., 2022). Soils at the two sites are primarily Maymen series of Typic Dystroxerepts and the Lodo series of Lithic Haploxerolls, with significant rock outcropping (SoilWeb, UC Davis). ...
In semi‐arid regions where drought and wildfire events often co‐occur, such as in Southern California chaparral, relationships between plant hydration, drought‐ and fire‐adapted traits may explain landscape‐scale wildfire dynamics. To examine these patterns, fire scientists and plant physiologists quantify hydration in plants via mass‐based metrics of water content, including live fuel moisture, or pressure‐based metrics of physiological status, such as xylem water potential; however, relationships across these metrics, plant traits and flammability remain unresolved.
To determine the impact of hydration on tissue‐level flammability (leaves and stems), we conducted laboratory dehydration tests across wet and dry seasons in which we simultaneously measured xylem water potential, live fuel moisture and flammability. We tested two widespread chaparral shrubs, Adenostoma fasciculatum and Ceanothus megacarpus.
Live fuel moisture showed a threshold‐type relationship with tissue flammability (increased ignitability and combustibility at specific hydration levels) that aligned with drought‐response traits (turgor loss point) and fire behaviour (increased fire likelihood and spread) identified at the landscape scale. Water potential was the better predictor of flammability in linear statistical models.
A. fasciculatum was more flammable than C. megacarpus, and both species were more flammable during the wet growing season, suggesting seasonal growth or drought‐related tissue characteristics other than moisture content, such as lignin or chemical content, are critical for determining flammability.
Our results suggest a mechanism for landscape‐scale increases in flammability at specific levels of drought stress. Integration of drought‐related traits, such as the turgor loss point, might improve models of wildfire risk in drought‐ and fire‐prone systems.
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... Other issues are different areas and different phenomena within different natural zones in the world. We collected and reviewed a few studies in different study areas including Siberia, Russia [18,20,21], Indonesia [22], Canada [23,24], Australia [13,[25][26][27], Spain [28], Portugal [8], Mediterranean [2, [29][30][31][32], China [5,[33][34][35][36], California and Alaska [37][38][39], US [40][41][42][43][44], Peru [9], Iran [45], Bolivia [46], Amazon of Brazil [47] and India [48]. The wildfire studies of each country had their characteristics. ...
Due to the intensification of climate change in the world, the incidence of natural disasters is increasing year by year, and monitoring, forecasting, and detecting evolution using satellite imaging technology is an important guide for remote sensing. This study aims to monitor the occurrence of fire disasters using Sentinel-2 satellite imaging technology, to determine the burned severity area with its classification and the recovery process for determining the extraordinary natural phenomena. The study area was sampled in the southeastern part of Mongolia, where have most wildfires in each year, near the Shiliin Bogd mountain in the natural steppe zone and in Bayan-Uul soum in the forest-steppe natural zone. For the methods, the NBR was used to map the area of the fire site and the classification of the burned area into 5 categories: unburned, low, moderate-low, moderate-high, and high, which are process-defined works. NDVI index was used to determine the recovery process in a timely series in the summer from April to October. In result, the burned areas were mapped from the satellite images, and the total burned area of steppe natural zone was 1164.27 km2, of which 757.34 km2 (65.00 percent) was low, 404.57 km2 (34.70 percent) was moderate-low, and remaining 2.36 km2 (0.30 percent) was moderate-high, and the total burned area of forest-steppe natural zone was 588,35 km2, of which 158.75 km2 (26.90 percent) was low, 297.75 km2 (50.61 percent) was moderate-low, 131.25 km2 (22.31 percent) was moderate-high and the remaining 0.60 km2 (0.10 percent) was high-medium. Finally, we believe that this research is most important to helpful for emergency workers, researchers, and environmental specialists.
... ENSO events, characterized by anomalously positive Niño 3.4 indices, typically result in warmer-than-average temperatures and reduced precipitation across most of Australia during the SON season. These meteorological anomalies create favorable conditions for the ignition and spread of wildfires (Keeley et al., 2022;Littell et al., 2016). Additionally, Australia is influenced by a highpressure center and enhanced northwesterly winds, which contribute to the expansion of burned areas (Clements et al., 2008;Koo et al., 2010). ...
The El Niño–Southern Oscillation (ENSO) is a crucial driver of fire weather in Australia, with the correlation between ENSO and Australian fire weather having intensified over the past 2 decades. However, the underlying causes for this change have not been thoroughly investigated. In this study, we utilize reanalysis datasets and numerical model simulations to demonstrate that the Atlantic Multidecadal Oscillation (AMO) could potentially modulate the ENSO–Australian fire weather relationship. The correlation between ENSO and the Australian Fire Weather Index (FWI) increases from 0.17 to 0.70 as the AMO transitions from its negative to positive phase. This strengthening effect can be attributed to atmospheric teleconnection mechanisms. Specifically, the positive AMO phase, characterized by warming in the northern and tropical Atlantic, generates Rossby wave trains, leading to high-pressure systems over Australia. Consequently, local temperature and wind speed increase, while precipitation decreases. This signal, superimposed on ENSO, serves to amplify the ENSO effect on Australian fire weather.
... When combined, freezing and drought can be particularly potent stressors (Ewers et al., 2003) and winter drought, combining water stress with freezethaw events, may partially explain some of the recent extreme levels of mortality in chaparral communities Langan et al., 1997;Venturas et al., 2016). Increased plant dieback and mortality also leave MTCR species vulnerable to other types of disturbances, including increased fire, pathogens, and invasive species (Esler et al., 2018;Jacobsen et al., 2012;Keeley et al., 2022;Venturas et al., 2016). ...
... In the present study, xylem structural and hydraulic traits of five chaparral shrub species were compared between two sites representing the lower and upper elevation distribution limits for species along a steep transect in the southern Sierra Nevada, California, USA (Table 1). We hypothesized that xylem traits would differ within species (intra-specifically) between the upper and lower elevation extremes and that these traits would correlate with the local environmental conditions (hotter and drier low and cooler and wetter high; Keeley et al., 2022). Regular winter freeze-thaw events at the upper site were of particular interest because freezing may be an important and understudied driver of species distributions in MTCR (Davis & Matusick, 2018). ...
Xylem structure and hydraulics were compared between individuals at lower and upper elevation distribution limits for five chaparral shrub species along a steep transect in the southern Sierra Nevada, California, USA. Higher-elevation plants experienced frequent winter freeze-thaw events and increased precipitation. We hypothesized that environmental differences would lead to xylem trait differences between high and low elevations, but predictions were complicated because both water stress (low elevation) and freeze-thaw events (high elevation) may select for similar traits, such as narrow vessel diameter. We found significant changes in the ratio of stem xylem area to leaf area (Huber value) between elevations, with more xylem area required to support leaves at low elevations. Co-occurring species significantly differed in their xylem traits, suggesting diverse strategies to cope with the highly seasonal environment of this Mediterranean-type climate region. Roots were more hydraulically efficient and more vulnerable to embolism relative to stems, potentially due to roots being buffered from freeze-thaw stress, which allows them to maintain wider diameter vessels. Knowledge of the structure and function of both roots and stems is likely important in understanding whole-plant response to environmental gradients. This article is protected by copyright. All rights reserved.
... Local precipitation was also shown to be the single best predictor of full invasion following disturbance in the nearby Angeles National Forest (Park et al., 2018). Most recently, Keeley et al. (2022) revealed prolonged drought to be a significant driver of fire size and severity of the Woolsey Fire, which burned 88% of the Santa Monica Mountains National Recreation Area (SMMNRA) Park Service land. The fire and prolonged drought may be supporting non-native plant invasions in the Santa Monica Mountains, yet there are no recent studies providing insight into the current status of native and non-native cover in this region. ...
Abstract Mediterranean‐type ecosystems are under chronic ecological stress. By assessing changes in plant species and functional groups across the landscape, identifying ecosystem degradation is possible. The Santa Monica Mountains are in close proximity to a densely populated urban area where non‐native species invasions are being driven by changing fire regimes, climate change, and anthropogenic disturbances. Non‐native growth impacts biodiversity levels and vegetation distributions of native plant communities that are critical for ecosystem health. This study uses analyses of line‐point data from the National Park Service Inventory and Monitoring Program to assess ecosystem health in the Santa Monica Mountains from 2014 to 2020 and, in particular, to assess the effects of the 2018 Woolsey Fire on the balance of native and non‐native species in the region. Results of this analysis show an increase in non‐native cover since 2014 and rapid regrowth of non‐native annual grasses and herbaceous cover after the 2018 Woolsey Fire while native communities regrew more slowly. A hotspot of non‐native cover was identified in the Northern Simi Hills region, and rapid regrowth was seen after the Woolsey Fire. The hotspot is dominated by non‐native annual grasses and annual herbaceous species, some of which returned to prefire populations within a year after the Woolsey Fire. These results raise concerns for the future of native vegetation composition and function throughout the park and highlight the damage densely populated non‐native plant communities accumulate in the wake of disturbance events.
... Reading Carlquist's ideas on tracheids and drought, we surmise that he viewed embolism occurrence as common (at least during drought), and that xylem has evolved to cope with it. Working in semi-arid and arid southern California, as we do, it is not difficult to understand this point of view: drought-induced plant mortality (partial or complete) is widespread (Coates et al. 2015;Keeley et al. 2022), and plants regularly reach their hydraulic limits (Frazer & Davis 1988;Davis et al. 2002;Paddock et al. 2013;Pratt et al. 2014;Venturas et al. 2016;Jacobsen & Pratt 2018a). This is a radical view for some, with many arguing that embolism is rare and that embolism avoidance is the norm in all but extreme circumstances. ...
One of Sherwin Carlquist's greatest scientific legacies is how far he advanced our understanding of the structural and functional relationships within the xylem. His research was critical in illuminating the occurrence and importance of tracheids co-occurring with vessels in the xylem of some eudicots. Here, we highlight key concepts and findings from his work on these topics, and how testing has advanced with the advent of recent methods that produce spatially explicit data on embolism formation. There are three classifications of tracheids within vessel-bearing xylem: vasicentric, true, and vascular. Carlquist viewed tracheids as critical in safely transporting water. We review the literature on tracheid function, focusing on transport safety and efficiency, and discuss how the presence of tracheids might affect other xylem traits in the context of tradeoffs. Available data indicate that Carlquist was correct in his view that tracheids are resistant to embolism, and they contribute to the safe transport of sap. Tracheids provide connections (bridges) among vessels that remain sap-filled within highly embolized vessel networks and, in this role, they promote embolism tolerance. There is still much to learn, and integrating tracheids into models of sap flow, understanding their pit traits and connections, and how they affect xylem functional tradeoffs will transform our understanding of plant sap transport. Diverse xylem structural arrangements suggest many potential ways for xylem networks to function. In species containing both vessels and tracheids, the hydraulic network appears to be able to tolerate high levels of vessel embolism within seasonal environments.
Southern California experienced unprecedented megadrought between 2012 and 2018. During this time, Malosma laurina , a chaparral species normally resilient to single‐year intense drought, developed extensive mortality exceeding 60% throughout low‐elevation coastal populations of the Santa Monica Mountains. We assessed the physiological mechanisms by which the advent of megadrought predisposed M. laurina to extensive shoot dieback and whole‐plant death. We found that hydraulic conductance of stem xylem (K s, native ) was reduced seven to 11‐fold in dieback adult and resprout branches, respectively. Staining of stem xylem vessels revealed that dieback plants experienced 68% solid‐blockage, explaining the reduction in water transport. Following Koch's postulates, persistent isolation of a microorganism in stem xylem of dieback plants but not healthy controls indicated that the causative agent of xylem blockage was an opportunistic endophytic fungus, Botryosphaeria dothidea . We inoculated healthy M. laurina saplings with fungal isolates and compared hyphal elongation rates under well‐watered, water‐deficit, and carbon‐deficit treatments. Relative to controls, we found that both water deficit and carbon‐deficit increased hyphal extension rates and the incidence of shoot dieback.
