The effects of prolonged drought on vegetation dieback and megafires in southern California chaparral

Abstract

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 2017 Thomas Fire and the 2018 Woolsey Fire, there had been extensive areas of dieback. Comparing dieback with Monitoring Trends in Burn Severity measures of fire severity, there was a highly significant negative relationship between drought-caused shrub dieback and fire-caused dieback as measured by fire severity. We interpret this as further support for our remote sensing methodology for prefire dieback. Models of fire behavior suggest that one means by which dieback contributes to fire size is through increasing the density and distance of spot fires, particularly under extreme wind conditions. Lower elevation chaparral associations appear to be most vulnerable and are closer to urban environments, which should be a concern to fire managers in regions subjected to extended droughts.

Live fuel moisture and water potential exhibit differing relationships with leaf‐level flammability thresholds

“…Plant tissue samples were collected from two sites in the foothills of the Santa Ynez mountains north of Santa Barbara, California (Site 1: 34.461298 N, −119.693419 W, elevation: 354 m; Site 2: 34.493111 N, −119.791046 W, elevation 565 m). Vegetation primarily consists of co‐occurring dominant chaparral species, Adenostoma fasciculatum and Ceanothus megacarpus (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).…”

Atlantic Multi-decadal Oscillation Modulates the Relationship Between El Niño-Southern Oscillation and Fire Weather in Australia

“…El Niño events (anomalously positive Niño 3.4 index) induce warmer-than-average temperatures and reduced precipitation across most of Australia in SON. These meteorological anomalies create favorable conditions for igniting and spreading wildfires (Keeley et al, 2022;Littell et al, 2016). In addition to temperature and precipitation, Australia is dominated by a high-pressure center and enhanced northwest winds, which also contribute to wildfires by expanding the burnt area (Clements et al, 2008;Koo et al, 2010).…”

Xylem structure and hydraulic function in roots and stems of chaparral shrub species from high and low elevation in the Sierra Nevada, California

“…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).…”

“…When combined, freezing and drought can be particularly potent stressors (Ewers et al, 2003) and winter drought, combining water stress with freeze–thaw events, may partially explain some of the recent extreme levels of mortality in chaparral communities (Jacobsen & Pratt, 2018; 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).…”