Differential mortality in chaparral species during California’s 2014 historic drought is related to life history and hydraulic traits

Background/Question/Methods

Chaparral is the most abundant plant community in California and the shrubs that dominate this community are adapted to summer aridity and periodic fires. Current climate change models predict more frequent and severe droughts in southern California that could affect the structure and species composition of this important community. Understanding the factors related to species-specific patterns of drought mortality are essential to predict the impacts of these changes. We hypothesized that life history type (non-resprouter, facultative resprouter, and obligate resprouter after fire) and hydraulic traits affect species survival. We evaluated hydraulic traits, community structure, and mortality in a mature chaparral stand containing 11 species. We used a point-quarter sampling technique during 2014 drought, which has been reported as the worst in California in the last 1,200 years. A systematic sampling was also performed to determine mortality of the least frequent species. We evaluated several parameters to characterize tissue dehydration, including water potential, native xylem specific conductivity, leaf specific conductivity, and percentage loss in conductivity for all 11 species in February 2014, which was a particularly dry period. We performed correlation analyses and used generalized linear models (GLMs) to determine the factors that better explained plant mortality.

Results/Conclusions

Mortality among the 11 species ranged from 0 to 93%. Total stand density (plants/ha) was reduced 63.4% due to plant mortality and relative dominance of species shifted after the 2014 drought. Mortality was negatively correlated with predawn water potential (r²=0.50; p=0.015) and native xylem specific conductivity (r²=0.40; p=0.038). However, percent loss in hydraulic conductivity and leaf specific conductivity were not correlated with mortality (p>0.05). The GLM that best explained mortality had species and crown area nested within species as main factors, the outcome of which was that larger plants had greater rates of survival for 2 of the species. Generally, non-resprouter species had greater mortality rates than resprouters despite their xylem being more resistant to water-stress induced cavitation. Resprouters, especially ones that do not recruit seedlings after fire, usually have larger and deeper root systems, which enables them to access moisture from deeper soil layers, mitigating drought effects and ultimately increasing their survival during drought. These results are consistent with the hypothesis that short-term high intensity droughts have the strongest effect on shallow-rooted anisohydric species. We conclude that severe droughts can drive changes in plant community structure as a result of the differential mortality of chaparral species.