Abstract
The overarching question of this dissertation is: how do conifer trees in Northern Idaho use water across long growing seasons with little precipitation? There are four chapters, an introductory chapter first followed by three research projects. All three projects took place on the University of Idaho Experimental Forest. As in the much of the region, the soils on the experimental forest can hold large amounts of water because of their depth and composition (more than a meter-deep silt loam). Paired with a climate that allows for very little summer precipitation, the soils facilitate a slow and consistent dry down. That allowed us to study trees’ responses to drought and how they changed throughout growing seasons. The first research project described here was conducted in 2015 addresses the question: how do six of the most regionally important conifer species function from June to October of the worst drought on record? That study produced an interesting dataset that, in collaboration with Drs. Xiaonan Tai and Scott Mackay (now of University of Utah and SUNY-Buffalo, respectively), was incorporated into the TREES process-based tree physiology model. We can now model those species under different scenarios of climate and substrate. However, that project inspired a question: how do specific parameters, like whole-tree conductance, whole-tree water flux, organ conductivity, and leaf gas exchange interact through a growing season? Chapter three addresses that question by describing intensely measured water use of three individual mature P. ponderosa trees from June to October of 2017. The insights that chapter provided were then scaled up to the fourth chapter, which explores the influence that different densities of pre-commercial thinning treatments have on P. ponderosa tree hydraulic function within the same site in 2018. Each of these three research projects produced some results that were expected and consistent with literature and also some novel findings that add new insights to our understanding of conifers in the inland Northwest.