Masters Thesis

Physical controls on Sierra Nevada treeline growth and implications for climate reconstruction

Tree-ring width (TRW) from the Sierra Nevada treeline has been widely sampled for reconstruction of past local temperature variability. We identified two challenges with the use of such records: (1) most collections from the region are decades-old, leaving a knowledge gap in tree growth response to recent climatic changes; (2) strong covariance between temperature and solar radiation permit ambiguity in attribution of growth response. We developed an updated TRW chronology using whitebark pine (Pinus albicaulis) from Sonora Pass, CA (38.32N, 119.64W; elev. 3130 m), which exhibits a strong correlation with May-Jun temperature (r=0.36, p0.001). Moreover, an increasing correlation with previous winter (pOct-pNov) temperature in recent decades suggests the possibility of growth response shift due to climate warming. We also mapped all trees in the 80m x 70m study site to test for an influence of inter-tree shading on growth. We show that differences in growth rate between trees are predicted by competition (r=-0.46, p0.001). We developed a directional competition index and show that competition effects on growth are strongly anisotropic and scale with average intensity of direct solar radiation, indicating that competition for incident radiation exists and light availability likely poses a control on growth at treeline environments.

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