Stable oxygen isotope analysis (δ18O) of cellulose from tree rings has been widely used to reconstruct changes in source water and atmospheric conditions. Under conditions in which trees tap into a consistent water source, any changes in δ18O of cellulose can be attributed to changes in relative humidity (RH). Conversely, if trees grow in a high relative humidity environment, then changes in δ18O cellulose can be attributed to changes in source water. However, under conditions when both source water and RH are changing, partitioning each individual signal can be difficult. In this study, we coupled field measurements of tree core analyses with a sensitivity analysis to partition δ18O cellulose signal between source water and RH. Fieldwork was conducted in a conifer forest located in western Montana. Across the watershed, cores from Douglas fir and Ponderosa Pine were collected and extracted for α-cellulose. The samples were then analyzed for δ18O at annual time scales to reconstruct changes over the last 100 years. At each location where trees were collected, micrometeorology stations also collected air temperature, RH, and soil moisture data for the last four years, along with δ18O of source water.
Results/Conclusions
The sensitivity analysis modeled δ18O cellulose as a function of source water and RH, using the Craig-Gordon model for leaf water enrichment and the Roden model for cellulose fractionation. The sensitivity analysis was run using a Monte Carlo approach, where 10,000 values were randomly selected within a range observed measurements (e.g. δ18O source water, δ18O leaf water, δ18O of atmosphere, relative humidity). We used the Monte Carlo sensitivity output to estimate the likelihood that a particular annual mean δ18O cellulose value would occur across a range of source water and RH values. For example, an annual mean δ18O value of 22‰ could only occur if source water was highly negative, even if relative humidity ranged from 10-60%, while a δ18O value of 35‰ had a higher probability of occurring if both source-water was highly enriched and RH was low. However, δ18O values of 30‰ could occur across a wide range of source water and RH levels. Thus, this sensitivity analysis allowed us to estimate over the last 100 years periods with deep snowpack (smaller δ18O cellulose values due to depleted source water) as well as periods with increased rain/smaller snowpack and dry growing season atmospheric conditions (higher δ18O cellulose values due to enriched source water values and low relative humidity).