Testing for the Influence of Light Availability on Tree-Ring Reconstructed Temperature at Sonora Pass, CA

Abstract:

Tree-ring width and density near treeline tend to covary with local interannual temperature, motivating the use of such records to reconstruct past temperature variability. However, recent work has introduced the possibility of multiple environmental factors contributing to tree growth in cold environments. We investigate the influence of small-scale light variability on tree-ring based temperature reconstructions from a treeline ecotone. We establish an experimental plot near Sonora Pass in the California Sierra Nevada (38.32N, 119.64W; elev. 3130 m). This treeline environment is dominated by whitebark pine (Pinus albicaulis) growing as individuals and in stands, providing an opportunity to test the sensitivity of mean growth rate and interannual variability to light availability. For each tree we quantify the local light environment using three approaches: (i.) geometrical estimates of shading from neighboring trees, (ii.) photographic estimates of shading from neighboring trees, and (iii.) geometric estimates of direct light availability resulting from aspect and local topography. Geometrical estimates of shading are made by mapping the relative position and crown dimensions of each tree in the plot in order to calculate a shading index that will be used to test hypotheses about the influence of shading on tree growth. Photographic estimates of tree-level shading are created by taking hemispheric photographs at the crown edge of each tree and calculating the effects of neighboring trees on direct and diffuse light availability using the Gap Light Analyzer software. To quantify tree growth, increment cores are collected from all trees in the plot to develop sub-chronologies of ring records grouped by different light environments. We hypothesize that trees growing in open areas or at edge of stands, which experience little inter-tree competition, would likely produce ring records more closely correlated with the temperature record; whereas trees growing in middle of stands may produce ring records resembling both temperature and light variability.