Impacts of long-term precipitation manipulation on hydraulic architecture, xylem function, and canopy status in a piñon-juniper woodland

The Southwestern US is predicted to become hotter and drier, as global climate change forces increasing temperatures and variability in timing and size of precipitation inputs. Drought stress has become more frequent in recent decades, and resulted in massive forest mortality in piñon-juniper woodlands. During recent severe droughts (2000-2003, 2009-2012), piñon pine (Pinus edulis Englem.) suffered disproportionately high mortality compared to co-occurring one-seed juniper (Juniperus monosperma [Engelm.] Sarg.). A large-scale precipitation manipulation experiment was established in a piñon-juniper woodland in central New Mexico to test hypotheses regarding tree survival and mortality with respect to altered water regimes. Our treatments consisted of water addition (+20% ambient precipitation), partial precipitation-exclusion (-45% ambient precipitation), and an exclusion-structure control. The research presented in this dissertation seeks to identify differences in hydraulic architecture between piñon and juniper to provide new insight into how these species survive extreme climate events. Hydraulic architecture is a fundamental control on water transport, which underpins plant productivity and survival. The extent to which mature trees can adjust hydraulic architecture as a response to drought is unknown. In Chapter 1, we measured seasonal stem hydraulic performance for multiple years to assess the acclimation of hydraulic architecture in mature piñon and juniper after 3+ years of precipitation manipulation. In Chapter 2, we focused on defining the relative roles of abiotic and biotic stressors in piñon mortality. We established two additional treatment plots (an ambient control and the same drought treatment) in the same experiment to test the impact of drought while bark beetle attack was prevented by insecticide treatment. We measured piñon hydraulic architecture for five years following the start of experimental drought. In Chapter 3, we used the continuum of foliar health furnished by the precipitation manipulation experiment to investigate the links between foliar spectral reflectance signals and direct measurements of water stress and hydraulic function. Our findings suggest that 1) mature trees are unlikely to adjust hydraulic transport safety or efficiency in response to climate change, 2) bark beetles are critical mediators of piñon mortality during drought, and 3) spectral signals from foliage accurately predict water status and hydraulic function