Limits in the resilience of chaparral to severe drought
Chaparral shrubs species of California are known for their resilience to environmental stress, such as recurrent summer drought, periodic wildfire, and winter freezing events. However, there are limits to survival that differ among life history types in response to wildfire (non-sprouters after fire, facultative sprouters after fire, and obligate sprouters after fire,) life form (seedlings, post-fire resprouts, and adults) and the possible interactions among stress factors, such as drought and freezing, drought and wildfire, and drought/wildfire/deer browse. We examine not only the limits of resilience among diverse species of chaparral shrubs to severe stress but also how observed differences are influenced by life history type, life form, and interactions among multiple stress factors.
Recent studies on the interaction of severe drought with wildfire document maximum resprout mortalities of 75 to 80% for some chaparral species and nearly 90% resprout mortality due to the interactions of severe drought, wildfire, and deer browse. For adult shrubs, we found adult mortalities to range between 67 to 75% at some sites during a record-breaking drought, and most recently, in February 2014, after the worst drought on record in southern California, we observed adult mortality for some species exceeded 90%. We were surprised to find that adult species with the highest mortality were also species known to have high resistance of their stem xylem to water stress-induced cavitation. These same species are frequently shallow rooted, in part because of their non-sprouting life history type. We conclude that although species of chaparral shrubs are resilient to severe drought, some species are more so than others, and it appears that for adult shrubs, rooting depth is a key determinant of final mortality. Furthermore, interactions of drought with wildfire, freezing, and deer browsing can enhance overall mortality, impacting plant community structure.