Extensive drought‐associated plant mortality as an agent of type‐conversion in chaparral shrublands

Abstract

Contents Summary 498 I. Introduction 498 II. Ecological drought and vegetation type-conversion 499 III. Chaparral mortality during extreme drought events 501 IV. Some species survive drought and others do not 501 V. Recovery potential 502 VI. Conclusions 503 Acknowledgements 503 References 503 SUMMARY: California experienced an intense drought from 2012 to 2015, with southern California remaining in drought to the present. Widespread chaparral shrub mortality was observed during the peak of the drought in 2014. Some species were more impacted than others and shallow-rooted shrub species were the most vulnerable to drought-associated mortality. This type of drought represents what is termed an 'ecological drought' during which an ecosystem is driven beyond thresholds of vulnerability, triggering impairment of ecosystem services and feedbacks that may result in long-term type-conversion of natural communities. The ability of shrublands to recover will depend on the timing, intensity and seasonality of future extreme climate events, post-fire recruitment potential of species with obligate fire-associated recruitment, and interactions with other stresses.

Extent and drivers of vegetation type conversion in Southern California chaparral

“…Both in the Santa Monica Mountains (via AWCL) and here in San Diego County (via AET), the parts of the landscape most susceptible to drought or low soil moisture availability were also more likely to experience woody conversion or decline, which was also the case in Park et al (2018). Extreme drought conditions have been shown to increase post-fire mortality of resprouts or seedlings, in addition to adult shrub mortality (Paddock et al 2013, Ventura et al 2016, Jacobsen and Pratt 2018, which could provide a competitive advantage to exotic annuals in the postfire environment. Given that many of the plots in San Diego County do not receive the moisturizing benefits of coastal fog and that San Diego County is generally warmer and drier than the Santa Monica Mountains, this may explain why chaparral overall takes longer to recover here.…”

“…, Ventura et al. , Jacobsen and Pratt ), which could provide a competitive advantage to exotic annuals in the post‐fire environment. Given that many of the plots in San Diego County do not receive the moisturizing benefits of coastal fog and that San Diego County is generally warmer and drier than the Santa Monica Mountains, this may explain why chaparral overall takes longer to recover here.…”

Shrub persistence and increased grass mortality in response to drought in dryland systems

“…Drylands cover more than 40% of the terrestrial Earth surface (Prăvălie, 2016) and have experienced large morality events related to drought in recent decades (e.g., Breshears et al, 2005;Ehleringer & Sandquist, 2018;Jacobsen & Pratt, 2018). Although episodic mortality has been observed in shrubs and grasses, a majority of the dieoffs have been observed and studied in tree species, leaving drought thresholds for grasses and shrubs relatively poorly understood.…”

Jarmila Pittermann

“…Water deficit and heatwaves are common in California but the 2012–2015 drought surpassed historic records of climate extremes. Jacobsen & Pratt's () Tansley insight nicely examines the combined impacts of physiology, water deficit, fire and natural regeneration on change in California's chaparral communities but the thinking behind it will apply to many terrestrial systems. Finally, Larter et al .…”