Climatic Drivers of Growth in Mixed Conifer Forests of the Sierra Nevada for Different Tree Sizes and Thinning Treatments

The Mediterranean region of northern California is projected to get increasingly warmer under all Intergovernmental Panel on Climate Change (IPCC) emission scenarios, with future precipitation projections not showing much of a trend. This poses a problem to the already dry summers that are experienced in the Mediterranean region of California. If precipitation does not increase alongside temperatures, the dry seasons will likely only get drier. The use of dendroclimatology to assess how mixed conifer species in the Sierra Nevada responded to past climate is a key resource that can be used to infer how trees may respond to a future changing climate. In this study, I assessed and compared responses of small, medium, and large diameter mixed conifer species to different climate variables (temperature, precipitation, and climate moisture index (CMI)). One of the most coherent responses from all diameter groups and across all species was the positive response to increasing minimum winter temperatures. All diameter groups and species also responded positively to precipitation and CMI at some point in the analysis period. Perhaps the most notable difference when comparing the three diameter groups to climate was the higher occurrence of negative responses to temperature of the previous year from the largest diameter group—as well as the higher number of negative responses to temperature in general. These results suggest that larger trees may be more sensitive to future climate projections compared to smaller trees, and they may carry those effects into the next year. Due to the multiple ecosystem benefits that these iconic large, old growth trees provide, forest managers are applying radial thinning treatments around these legacy trees to improve their vigor and reduce mortality. However, there is limited information on the effectiveness of these treatments. One objective of this current study was to analyze sub-hourly stem fluctuations of legacy ponderosa and sugar pines in multiple different radius thinning treatments to assess the short-term effects of these treatments. Thinning treatments applied were: R30C0 (9.1 m radius), R30C2 (9.1 m radius leaving 2 competitors), and RD1.2 (radius equaling DBH multiplied by 1 ft/in multiplied by 1.25). The other objective of this study was to assess climatic drivers of hourly stem fluctuations. Using the dendrometeR package in the program R, I gathered daily statistics (i.e. daily amplitude) of the stem fluctuations, as well as stem cycle statistics such as duration and magnitude of contraction, expansion, and stem radial increment. I then performed correlation analyses between those statistics and the environmental variables to assess the climatic drivers of stem fluctuations as well as try to determine which radial thinning treatment was most effective at increasing growth and vigor. The findings from this study highlighted the important role that mean solar radiation, air temperature, and relative humidity play in stem variations of sugar and ponderosa pine. One of the main findings from a management perspective was that RD1.2 was the only treatment group for sugar pine that contracted less on warmer, higher solar radiation days and put on more stem radial increment on higher solar radiation days. For ponderosa pine, treatment RD1.2 also contracted less on warmer, higher solar radiation days. These findings suggest that the extended radius RD1.2 treatment may be the most effective at releasing legacy sugar and ponderosa pine trees compared to the other treatments applied