Chaparral Shrub Hydraulic Traits, Size, and Life History Types Relate to Species Mortality during California’s Historic Drought of 2014

Abstract

Chaparral is the most abundant vegetation type in California and current climate change models predict more frequent and severe droughts that could impact plant community structure. Understanding the factors related to species-specific drought mortality is essential to predict such changes. We predicted that life history type, hydraulic traits, and plant size would be related to the ability of species to survive drought. We evaluated the impact of these factors in a mature chaparral stand during the drought of 2014, which has been reported as the most severe in California in the last 1,200 years. We measured tissue water potential, native xylem specific conductivity, leaf specific conductivity, percentage loss in conductivity, and chlorophyll fluorescence for 11 species in February 2014, which was exceptionally dry following protracted drought. Mortality among the 11 dominant species ranged from 0 to 93%. Total stand density was reduced 63.4% and relative dominance of species shifted after the drought. Mortality was negatively correlated with water potential, native xylem specific conductivity, and chlorophyll fluorescence, but not with percent loss in hydraulic conductivity and leaf specific conductivity. The model that best explained mortality included species and plant size as main factors and indicated that larger plants had greater survival for 2 of the species. In general, species with greater resistance to water-stress induced cavitation showed greater mortality levels. Despite adult resprouters typically being more vulnerable to cavitation, results suggest that their more extensive root systems enable them to better access soil moisture and avoid harmful levels of dehydration. These results are consistent with the hypothesis that short-term high intensity droughts have the strongest effect on mature plants of shallow-rooted dehydration tolerant species, whereas deep-rooted dehydration avoiding species fare better in the short-term. Severe droughts can drive changes in chaparral structure as a result of the differential mortality among species.

Stomatal behaviour and stem xylem traits are coordinated for woody plant species under exceptional drought conditions

“…Theoretically, they can continue to photosynthesize and gain carbon for longer time periods or during less favourable conditions than less drought resistant species, though this is debated (Martínez‐Vilalta & Garcia‐Forner, ). However, a recent study also conducted in the California chaparral during the exceptional 2014 drought found that chaparral species with high cavitation resistance experienced high mortality rates, whereas deeply rooted species had high survival rates (Venturas et al, ). Obviously, although coordination between the P 50 and P close may vary depending on the vegetation‐type, disentangling rooting depth and stomatal behaviour is of value (Feng, Dawson, Ackerly, Santiago, & Thompson, ).…”

“…In fact, the 2012–2014 drought in California was the most intense drought in the past 1,200 years (Griffin & Anchukaitis, ). A recent study conducted in a southern California native shrubland found that shallow‐rooted species with cavitation resistant xylem experienced the highest mortality levels due to drought, whereas deeply rooted species with xylem vulnerable to cavitation had lower mortality levels (Venturas et al, ). This differential mortality among co‐occurring species due to varying drought survival strategies (Pivovaroff, Pasquini, et al, ) may result in changes in community composition and ultimately biodiversity loss.…”

Leaf rehydration capacity: Associations with other indices of drought tolerance and environment

“…By contrast, leaves of three of our study species ( C. spinosus , Q. agrifolia , and Q. berberidifolia ) recovered from extreme water deficits (<10% RWC) to 48–66% of their initial pre‐dehydration SWC. These species are evergreens native to chaparral, adapted to maintaining leaves through severe drought events (Venturas et al, ). In this way, they show a degree of “resurrection” type behaviour, such as that found in species that can recover from virtually complete dehydration (>300 pteridophytes and angiosperms [Rascio & Rocca, ];e.g., Boea hygrometrica [Xiao et al, ], Craterostigma wilmsii [Cooper & Farrant, ], Haberlea rhodopensis [Moyankova et al, ], and Selaginella lepidophylla [Eickmeier, ]).…”

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

“…By contrast, the most recent drought (2012–present) triggered significant dieback and mortality of adult shrubs in many regions throughout southern California, including areas in the center of chaparral distribution. At one chaparral site, stand density was reduced by > 60%, primarily due to mortality that occurred during winter 2014, and some species exhibited > 90% mortality (Venturas et al ., ). The same patterns were also apparent at broader scales (Coates et al ., ).…”

“…Peak mortality events were associated with very low PDSI values, with high chaparral mortality during winter 2014 during an especially intense period (Fig. ; Venturas et al ., ). If drought duration was the key driver of chaparral mortality, one would predict mortality events to occur repeatedly as the drought continued, rather than the observed episodes of mortality during the periods of highest drought intensity.…”