Understanding Climate Driven Drought Mortality Across Biomes

As the earth's climate continues to change, forests within a diverse set of biomes are experiencing increasingly frequent, severe, and longer periods of drought stress. The continued rise of global temperatures could radically alter the composition, structure, and biogeography of forests in many regions (Van Mantgem, 2007). Recent reviews of climate-induced mortality events include cases of large scale die-offs that span broad gradients of woody ecosystems, from monsoonal savannas with mean precipitation <400 mm/year, to tropical rainforests with mean precipitation >3000 mm/year (Allen, 2010). The diversity of ecosystems, from coastal tide pools to redwood forests and inland deserts, located within the UC reserve system, have been similarly recently impacted by a wide variety of drought frequencies and intensities. As local and global climate continues to change and drought recurrence increases, understanding subsequent changes within and between these systems is crucial in evaluating how ecosystem services and dynamics will be altered. Aside from the economic value associated with such stands of forest, the loss of biomass has ramifications that are closely tied to major climate and hydrological feedbacks, through loss of stored carbon and altered water cycling. Studying plant hydraulics bridges physiological regulation of transpiration to the environmental drivers of climate. However, the current parameters used to model plant drought responses are unresolved (Sperry, 2015). After a thorough review of the literature and preliminary evaluation, we believe that using sapwood capacitance as the primary functional trait in drought risk analysis will provide a more accurate prediction of mortality risk across species and plant functional types. By further understanding how climate drives mechanistic processes involved in hydraulic regulation, we can better predict changes in species composition and potentially mitigate loss of ecosystem function. Previous studies have parameterized tree mortality associated with drought, using the value of water potential where 50% of water transport capacity is lost (P50). However, observations of tree mortality in the field, in combination with direct measurements of P50 are weakly correlated, revealing a critical inconsistency in what physiologists believe to be the main mechanism in plant-water regulation under stress (Breshears, 2009). Based on accumulating evidence that functional tradeoffs play a significant role in hydraulic performance, we hypothesize that water movement in sapwood, described in terms of sapwood capacitance, is a better predictor of mortality and will provide a new axis to measure drought resistance.