Impacts of the 2014 Drought on Vegetation Processes in the Sierra Nevada of California

Abstract:

Sierra Nevada snowpack provides over 60 percent of California’s freshwater supplies. The drought of 2014 has been unprecedented in the state’s history, and followed below-average precipitation for the hydrologic years 2012 and 2013. Record-low precipitation has resulted in minimal Sierra Nevada snow pack and runoff, and massive reductions in reservoir storage, which has triggered widespread drought adaptation measures for one of the world’s largest economies. We assessed the impacts of the 2014 drought on vegetation processes in the headwaters of the Owens River, which is one of the main watersheds for the city of Los Angeles. We monitored water relations, photosynthesis, growth and Leaf Area Index of tree, shrub, herb, and grass species. In order to better understand the effects of drought, we examined responses to watering manipulations, long-term snow fences, elevation gradient analysis, and comparisons to previous wetter years. 1 April 2014 snow pack depth was 330 mm (average for 1928 – 2012 = 1344 mm, CV = 49%). Despite widespread mortality of Pinus jeffreyi saplings (mean 1.5 m tall) at 2300 m, older trees as well as saplings of Pinus contorta showed new growth. There were no significant differences in water potential (Ψ) for the two conifer species in a wet year (2006, 1 April snow depth = 2240 mm) vs. 2014. Water potential for P. contorta in 2014 was higher at 2900 m than at 2300 m but photosynthetic CO₂ assimilation (A) and stomatal conductance (g_s), were not different. By contrast, Ψ, A, g_s, V_cmax and J_max for the widespread shrub Artemisia tridentata increased along a gradient from 2100 m to 2900 m in 2014. Watering only significantly increased these photosynthetic parameters at the lowest, driest elevation. At the middle elevation, Leaf Area Index in 2014 was about 20% of the 2006 value for the N-fixing shrub Purshia tridentata. Results show reductions in photosynthesis and growth for some species but not others in response to the severe drought conditions of 2014. The ability to tolerate drought may be due to utilization of deep water for some species, or an ability to survive and grow on very little precipitation for other species. Incorporation of functional group survival, photosynthesis and growth responses to severe, ongoing drought stress should help to improve global models of carbon cycling.