Vogt, DanielFeng, Tuo2019-08-142019-08-142019Feng_washington_0250O_20389.pdfhttp://hdl.handle.net/1773/44264Thesis (Master's)--University of Washington, 2019Understanding the trends and variations of total ecosystem net primary productivity (NPP) is critical for simulating and predicting changes in the terrestrial carbon cycle across a range of spatial and temporal scales. A unique location to assist with enhancing our understanding of NPP is found in one of the largest temperate rainforests in the United States (US). It is a forest with one of the biggest carbon sinks in the US and is located in the Olympic Peninsula. In this study, four different models (Biome-BGC, Carnegie Ames Stanford Approach, Thornthwaite Memorial model and Miami model) were used to estimate mean annual NPP between the years 1986 to 2017 in two watersheds (Hoh and Queets) located on the Olympic Peninsula of Washington State. The spatial-temporal changes in NPP were correlated with climatic factors, topography and forest stand age. Spatially, annual NPP was estimated to be higher in the northwestern Hoh watershed versus that found in the southeastern Queets watershed. This was likely due to differences in forest structure and species composition. In the two watersheds, the models suggested a general trend of NPP levels increasing with stand age. During that time period, two potential Turning Points (significant changes in NPP) were detected in the years 1997 and 2007. These NPP Turning Points (i.e., NPP increases) occurred after severe droughts. Results from this study showed predicted NPP levels being highly correlated to climatic factors. In fact, three of the models predicting NPP were very sensitive to changes in mean annual temperatures. Topographic effects on NPP were analyzed using the Topographic Solar Radiation Index (TSRI) acquired from the national digital elevation model (DEM) and airborne light detection and ranging (LiDAR) data. Changes in NPP estimates using LiDAR-based TSRI resulted in significant R2 values of 0.59 for deciduous forests, 0.38 for evergreen forests, and 0.64 for mixed-forest composition. In this study region, forest stand successional stage was explained by forest disturbance and recovery history. Finally, three empirical functions were developed to describe the relationship between forest stand age and NPP levels, in terms of the different forest types. This study provides valuable information to assess the potential drivers of changes in carbon sequestration rates under dynamic climatic fluctuations in the Olympic Peninsula. It also highlights the significance of using solar radiation energy as a tool to estimate changes in forest productivity at a watershed scale.application/pdfen-USnoneRemote sensingForestryForestryThesis