Master of Science degree with a major in Natural Resources, option Forestry, Watershed, & Wildland Sciences
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In the western US, aspen forests tend to be small and rare, but have great ecological importance. There is much interest and concern over how aspen in the Sierra Nevada Mountains of the western USA will respond to a changing climate and future disturbances. Impacts from climate change create stress on aspen trees that further compound threats to aspen communities. This analysis assessed the radial growth response of aspen under previously recorded climate conditions to better understand how measurable climate variables affect aspen growth. Along with aspen’s growth response to climate, this analysis also assessed the growth response of aspen within the vicinity of a wildfire by measuring growth from aspen stands above and below the 2002 Showers fire footprint. Increment cores were collected from aspen trees in 20 stands around Lake Tahoe, California and Nevada, USA, spanning different aspects, elevations, and species compositions. Tree ring widths were measured using WinDENDRO and the data were visually cross-dated through microscopic comparison. The relationship between aspen growth, climate, disturbance, and stand conditions were analyzed using linear mixed effects regression. The models incorporated random effects for time and space since the data exhibited temporal and spatial autocorrelation. The data were separated into northeast (NE) and southwest (SW) regions of the Lake Tahoe Basin based on the similarities of the stands’ climate values revealed by a dendrogram. In both regions, the most influential climate variables were annual maximum temperature and annual precipitation. In the NE region, the highest aspen tree basal area increment (BAI) was measured in previously recorded years with a low temperature/high precipitation climate regime. For the SW region of the Lake Tahoe Basin, aspen tree BAI was higher under a low temperature/low precipitation climate regime. Along with climate, stand level variables such as canopy stratum (overstory/understory), elevation, and species composition (percent aspen presence) also influenced growth of aspen trees. The regression analysis indicated that aspen BAI was greater in areas with a higher proportion of aspen composition and for dominant trees in the canopy. However, aspen BAI declined with increases in elevation.
The post-wildfire analysis modeled how aspen responded when downstream of a wildfire compared to unaffected stands upstream, where downstream aspen could be influenced by added availability of water and nutrients, due to increased runoff and erosion from the fire. However, only the stand closest to the burned area exhibited a significant increase in aspen tree growth downstream from the wildfire. A response was not detected when stands further downstream were included in the analysis. Therefore, a wildfire could produce increases in aspen growth post-disturbance depending on proximity to the fire. In terms of growth response longevity, increased growth was detected in ratios of growth over a 3- and 5-year period from when the fire occurred.
Sakihara, Keath K., "Aspen growth response in the presence of inter-annual climate fluctuation and disturbance in the Lake Tahoe Basin" (2020). Cal Poly Humboldt theses and projects. 433.