Thesis (Ph.D.)--University of Washington, 2019Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is an important tree species in the Pacific Northwest (PNW), ranging from central British Columbia to Mexico. Douglas-fir is widely used for wood production and silvicultural applications such as thinning, and fertilization have led to increases in productivity. Climate change in the PNW is predicted to increase temperatures, and less precipitation in the summer is expected to increase drought effects and reduce Douglas-fir plantation growth and productivity. Therefore, better understanding the impacts of climate change on Douglas-fir growth and physiology, the interactive effects of silviculture and reduced water availability (i.e., drought) on Douglas-fir plantations in PNW is needed. This study was conducted at UW’s Pack Forest to evaluate the effects of thinning and drought stress on tree physiology and growth. Throughfall exclusion panels were used to create an artificial drought on the study site, reducing 40 % of throughfall precipitation. The first and second studies examined the effects of thinning and drought stress on tree physiological responses, and stem growth. Tree physiological traits such as sapflow density, photosynthesis, leaf water potential and stem growth increased with thinning and decreased with drought. Thinning increased physiological measures and stem growth, and drought stress reduced physiological measures and stem growth. The physiological responses and stem growth on the combination plot (thinning and throughfall exclusion) were similar to the control plot, which indicated that thinning mitigated the drought effects on tree physiology and growth. The third study examined the impacts of thinning and drought on soil respiration. Soil respiration decreased on the throughfall exclusion plot, but soil respiration on the thinned plot was similar to the control plot. Q10 which is the temperature sensitivity of soil respiration, is also negatively affected by the two treatments. Drought treatment lowered the soil respiration by decreasing soil moisture which directly affected root production and breakdown of the organic matter forming the substrate for heterotrophic respiration. Thinning initially reduced soil respiration due to a reduction in tree root respiration, then the soil respiration reached a similar level as the control, likely due to decomposition of dead roots. Overall results indicate that thinning can be beneficial in Douglas-fir plantations experiencing drought stress by increasing soil water availability and serve as a mitigating tool to climate change
