Analysis of the stable isotope composition of tree rings holds promise to understand forest-climate relations in the face of regional climate cycles. The recognition of earlywood, false latewood and latewood portions of single annual rings provides the capability to expand this understanding to even finer temporal scales, including intra-seasonal patterns of precipitation use. Here, we consider the topic of how montane Ponderosa pine forests in the Western US utilize winter and summer precipitation, and persist in the face of intra- and inter-annual drought using results from a network of sites within the spatial domain of the North American Monsoon (NAM) climate system. We used: (1) the δ13C and δ18O ratios of extracted tree-ring cellulose in earlywood and latewood to study patterns of spatial variation in forest responses to precipitation distribution, and (2) analyses of isotope ratios and light microscopy images of xylogenesis in earlywood, false latewood and latewood at a single site to study lags in the coupling of isotope fractionation to seasonal drought. In this presentation, we will address the use of intra-ring stable isotope composition to study forest-climate relations, and interpret isotope fractionation during seasonal and inter-annual drought within the spatial domain of this major regional climate system.
Results/Conclusions
Cellulose δ18O and δ13C values in earlywood and latewood revealed that photoassimilate production in trees within the NAM domain is less sensitive to interannual variation in summer drought and occurs during periods of lower atmospheric vapor pressure deficit, compared to trees beyond the NAM boundary. Statistical analysis of a multi-year isotope time series from earlywood and latewood revealed that: (1) most of the variance among sites was explained by decadal-scale variation in drought with a trend toward higher water-use efficiency at all sites over the last thirty years, and (2) the intra-annual difference between winter and summer drought was greater in sites outside the domain of the NAM system. Values of cellulose δ18O and observations of the phenology of xylogenesis in trees at a single site within the NAM domain revealed a lag of several weeks in responses to seasonal drought. Simulations using a model with mechanisms of isotope fractionation coupled to climate and cellulose biosynthesis provided an accurate reproduction of the lagged response of intra-ring isotope values to seasonal drought. We conclude that intra-annual time series analysis of tree-ring isotopes has the potential to reveal the impact, timing and within-season interactions of forest responses to regional and seasonal drought.