COMPARING THE EFFECTS OF CLIMATE VARIABILITY AND ENSO ON TREE GROWTH AT GLACIAL AND MODERN TIMES

Our research compares the physiological response of North American Juniper trees to climate variability and El Niño-Southern Oscillation (ENSO) history between glacial (17,000-50,000 yr BP) and modern (100 yr BP) times. Glacial samples are collected from the La Brea tarpits in Los Angeles, CA and modern equivalents are constrained within 150 km of the tarpits, allowing for powerful comparisons of temperature, climate variability and atmospheric CO₂ concentration between glacial and modern trees. This project has two objectives: (1) to quantify interannual variation in tree productivity resulting from ENSO events between glacial and modern times, and (2) to compare modern and glacial samples from the same location to determine if the ratio of intercellular to atmospheric [CO₂] (c_i/c_a) within the plant leaves has changed. We are able to reliably relate climate variability to Juniper responses by coupling the occurrence of ENSO events (with ¹⁸O), growth analysis (tree ring width), and physiological functioning (with ¹³C) within a given tree ring. Our results show a strong correlation between α–cellulose δ¹⁸O and tree ring width with corresponding fluctuations during ENSO years. Changes in α-cellulose δ¹⁸O reflect environmental changes in the source-water (including timing of rainfall) and humidity experienced by the tree. The response of these species to ENSO events is not only valuable for predicting future plant responses to climate variability, but also has important implications for the dynamics of ecosystem productivity and carbon sequestration that have varied between glacial and modern times.