Longer droughts in California may be a particularly disruptive outcome of global climate change. The ecological consequences of longer droughts are numerous, and extend from individuals to communities and ecosystems. Severe drought reduces plant water content and decreases photosynthetic ability, which can reduce fitness and productivity. The goal of this study was to determine the sensitivity of desert shrub physiology to dramatic inter-annual fluctuations of annual precipitation. Measurements were conducted between 2011, the last year before the start of California’s 1-in-1200 year drought, and 2019, two years after the wet winter of 2017. I monitored plant water potential, stomatal conductance to water vapor, photosynthetic CO2 assimilation, and aboveground Net Primary Productivity for two widespread Great Basin Desert shrub species. Concurrent experiments that tested the impacts of within-year snow depth variation on physiology utilized decades-old roadside snow fences near Mammoth Lakes, CA, USA. Measurements were conducted for plants exposed to ambient, increased (+100%), or decreased (-20%) snow depth, based on the equilibrium drift created by the snow fences. Annual measurements were made in late June each year, when soil moisture generally reflects the winter’s snow pack and conditions result in maximal seasonal photosynthesis and growth.
Results/Conclusions
Winter 2011 was the last wet season before California’s historic drought of 2012-2016. Snow depth decreased 90% from 2011 to 2012, and was well below the long-term mean between 2013 and 2015. Snowfall in winter 2017 was ~1.5 SDs above the 88-year mean. Between 2011 and 2014, mean stem water potential decreased from -1.1 MPa to -1.8 MPa for Artemisia tridentata, and from -1.2 MPa to -2.3 MPa for Purshia tridentata. For both species, mean annual stomatal conductance to water vapor (gs) and photosynthetic CO2 assimilation (A) showed a pronounced decline between 2011 and 2012. This was followed by an incremental increase towards the average levels of gs and A measured before the drought. Above-ground net primary productivity (ANPP) showed patterns that reflected precipitation patterns during the drought of 2012-2016, and the wet winter of 2017. ANPP for A. tridentata and P. tridentata appears to be most sensitive to annual precipitation in the range between 500 - 700 mm. Within-year snow depth effects on shrub physiology were species-specific and small compared to the inter-annual differences. Overall, photosynthetic physiology and growth for these two shrubs were very resilient to dramatic swings between historic drought and deluge precipitation years.