Reconstructing Snowpack Using Sierra Nevada Conifer Tree Rings In The Midst Of Changing Climate

Abstract

Snowpack in the Sierra Nevada Mountains accounts for around one third of California’s water supply. Melting snow provides water into dry summer months characteristic of the region’s Mediterranean climate. As climate changes, understanding patterns of snowpack, snowmelt, and biological response is critical in this region of agricultural, recreational, and ecological value. Tree rings can be used as proxy records to inform scientists and resource managers of past climate variability where instrumental data are unavailable. Here we investigate relationships of tree rings of high-elevation conifer trees (Tsuga mertensiana, Abies magnifica, Abies concolor, Calocedrus decurrens, Juniperus occidentalis, and Pinus ponderosa) and regional climate indices with the goal of reconstructing April 1st snow-water equivalent (SWE) in the North Fork American River watershed of the Sierra Nevada Mountains. Chronologies are significantly positively correlated with April 1 SWE of the year prior to ring formation. Tsuga mertensiana ring growth is correlated negatively with April 1 SWE of the year of ring formation. Additionally, temporal trends in correlation between tree-ring chronologies and climate indices indicate strengthening tree-growth response to climate over time. We developed a skillful, nested reconstruction for April 1 SWE, 1661 – 2013. Variability of the reconstruction is within the envelope of 20th and 21st century variability; however, the 2015 record low snowpack is unprecedented in the tree-ring record, as in results from previous studies. We further explored the impacts of climate change on these conifers using seasonal correlation analysis to describe the change in climate signal evident in these tree-ring chronologies pre and post-climate change conditions. Significant rise in temperatures, reduced snowpack, and increased precipitation variability resulted in stronger climate signals in these tree rings since 1956. A subset of snow- sensitive, high-elevation conifers also exhibit signs of moderating environmental climate effects over multiple years. Future research should focus on integrating modern snow-sensor data into paleoclimate research and determining mechanistic linkages between climate and tree growth response.