Investigating the Relationship Between Sapwood Density and Sapwood Capacitance Within the Hydraulic Pipeline Paradigm

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 and structure of forests in many regions (Van Mantgem, 2007). As local and global climate continues to change and drought recurrence increases, understanding subsequent changes within and between these systems is crucial for evaluating how ecosystem services and dynamics will be altered. 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). 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 combined 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 an important component of maintaining hydraulic efficiency under increased water stress. In this study I determine xylem pressure-volume relationships across a variety of woody plant functional types and across global biome types to increase our understanding of the dynamics of water movement in sapwood. First, I evaluated the relationship between sapwood density and sapwood capacitance. Then I looked at the relationships that these parameters have with other important hydraulic traits at the global and biome level. And finally, I evaluated the role that other morphological or environmental aspects have in influencing these traits and their subsequent sequential tradeoffs