Anthropogenic climate change is expected to increase the frequency, intensity, and duration of extreme drought events around the world, with one of the consequences being widespread episodes of tree mortality. Drought-induced tree mortality is now widely documented across diverse ecosystems. Consequently, understanding the physiological causes and ecological consequences of drought-induced tree mortality is critically important to effectively predict and manage the consequences of future drought events around the world. However, accurately predicting drought-induced mortality risk for different species across broad climatic, topographic, and environmental conditions is challenging, because drought exposure can vary substantially both within and across species’ geographic ranges. For plant communities facing increased likelihoods of exposure to extreme drought, acclimation and survival of individual trees will require morphological and physiological modifications that offset the negative impacts of drought. Therefore, investigating such responses when historic, multi-year drought conditions arise is essential to advance our understanding of the physiological capacities of different plant species, and will improve modelling efforts geared towards predicting vulnerability to future extreme drought events. This work examines how variation in climate, topography, and drought exposure affected the in-situ responses of two coexisting tree species to a historic five-year drought event at several sites spanning a climatic gradient.
This work focuses on two long-lived, drought-adapted tree species in California: blue oak (Quercus douglasii, Hook. & Arn., Fagaceae) and valley oak (Quercus lobata, Née, Fagaceae). These oaks are iconic endemic tree species that coexist with each other in deciduous woodlands in the Mediterranean-climate region of California. In response to the onset of California’s historic 2012 - 2016 drought (the region’s most severe drought in over 160 years of meteorological records and 1200 years of dendro-climatological reconstructions), I established three study sites across a climate gradient that captured a wide range of variation in historical climate and overall drought exposure. Within these sites, I quantified the differential extent of seasonal water stress and water status recovery until the drought subsided in spring 2016. I found that water stress though each growing season was associated with the restriction of roots to seasonal precipitation in shallow soil zones compared to deeper soil moisture reserves. Seasonal water status recovery was driven by the extent of water stress experienced during the previous growing season, likely due to promotion of xylem embolism. This lack of recovery was most pronounced in the blue oaks at the most xeric and drought exposed study site, where the greatest extent of canopy damage and mortality was also documented.
To place these results involving water status, canopy damage, and mortality into more of an ecophysiological context, I quantified seasonal changes in the stable isotope composition of leaves and xylem water, and how these changes reflect differential photosynthetic performance of the two species as a function of their capacity to uptake water through the drought. I found that these species had different water acquisition strategies within and across my study sites, likely reflecting differences in root architecture or placement in relation to soil water profiles spread throughout the landscape. These differences translated into differing degrees of carbon fixation through the drought, such that seasonal gas exchange was most inhibited in blue oaks at the most xeric and drought exposed site. Across sites, seasonal reductions in transpiration and water potential in both species were all directly associated with a lack of root access to stable groundwater resources in deeper soils.
To investigate how morphological modifications of the canopy architectures and of the leaves of each species may have allowed for acclimation to the high evaporative demands imposed by the drought, I quantified seasonal changes in leaf size, leaf mass per area, and leaf area-to-sapwood area ratios. I found that large reductions in the number and size of leaves were exhibited by blue oaks at the most xeric and drought exposed site that had roots which were restricted to shallow rooting zones. These reductions consequently reduced the effective leaf area per unit of sapwood area throughout their canopies, which acts to reduce vulnerability to hydraulic failure and drought-induced mortality. Conversely, valley oaks maintained larger leaves and larger leaf area-to-sapwood area ratios through the drought in response to more stable access to deeper groundwater resources. Seasonal changes in leaf mass per area were not observed in either species.
Taken together, these results indicate that the maintenance of physiological water-carbon balance for these species under multiple years of extreme drought is driven by variation in functional rooting depth and its effects on the seasonal trajectories of plant water status, seasonal water status recovery, and canopy architecture modifications, which control the efficiency of gas exchange through growing seasons. Despite the resilience of these oak species to such extreme drought conditions demonstrated in this work under natural, in-situ field conditions, their inability to sufficiently acclimate and maintain water-carbon balance can still lead to significant degrees of canopy damage and mortality. With the greatest drought impacts observed in individuals of both species that had more restricted capacities for water acquisition through this historic drought event, this work demonstrates the ecophysiological conditions under which these species are most vulnerable to damage and mortality.