Spatial Patterns of Soil Moisture and the Relative Importance of Atmospheric Water Demand Versus Soil Moisture Availability in Limiting Late-summer Growth of Douglas-fir Under a Changing Climate

The rapid pace of climate change is increasing tree mortality and highlighting the need to improve the mechanistic understanding of plant function under increased water stress. However, the processes that control soil moisture availability in steep mountainous terrain are poorly understood, as are the relative effects of atmospheric and soil conditions that regulate tree water stress. This dissertation describes the factors controlling the spatial patterns and temporal persistence in soil moisture distributions at hillslope scales and how interactions between climate, soils, and hydrology influenced Douglas-fir (Psuedotsuga menziesii) water stress in the Pacific Northwest’s seasonally dry climate. In Chapter 2, I tested the influence of topography on soil moisture across steep, highly dissected terrain of an even-aged Douglas-fir forest in the western Cascade Mountains of Oregon. I found that terrain-based metrics were poor predictors of shallow soil moisture whereas soil properties were stronger predictors. These results suggest that the horizontal redistribution of water along topographic gradients is unlikely to influence shallow soil moisture in areas with steep, quick-draining, and deep (> 2 m) soils. Chapter 3 examined how interannual climate variability affects seasonal growth of Douglas-fir. We found that latewood growth decreased with increasing vapor pressure deficit (VPD) in early summer. In contrast, latewood growth did not appear to be sensitive to the timing of rainfall. Chapter 4 complements Chapter 3 by disentangling the relative effects of VPD and precipitation on Douglas-fir gross primarily productivity and transpiration. I found that gross primary productivity showed greater decline with increased VPD than decreased rainfall when varying VPD and rainfall over ranges expected from climate projections for the Pacific Northwest. However, transpiration over the growing season declined significantly with decreased rainfall while cumulative transpiration increased with elevated VPD. My work has important implications for forest management practices aimed at improving drought resilience in forests. Management choices such as thinning need to be evaluated for their impact both to increased atmospheric aridity and to soil moisture availability. Simply put, thinning forests to reduce competition for soil moisture may not increase drought resilience if either soil moisture is not limiting or if thinning increases canopy exposure to a hotter and drier atmosphere.