Tree rings and ecosystem productivity across moisture gradients in the boreal forest of central Saskatchewan

Background/Question/Methods

Rapid increases in global air temperatures have occurred during the last 30 years, and are predicted to continue rising during the next century. Recent drought-related declines in forest productivity and tree survival have been documented in both arid and wet regions, and appear to be related to increasing evaporative demand—independent of mean annual precipitation. Yet it is still unclear how subsurface moisture availability can mitigate these temperature-induced decreases in forest health. Numerous studies have investigated the effects of drought on forest carbon balance, biomass decline, and mortality. However few, if any, studies have used tree rings from multiple species to investigate climate-related changes in multi-decadal ecosystem production along sub-surface moisture gradients. During September/October of 2012, we sampled tree rings from 449 trees from 21 plots across three forest types (jack pine, black spruce, and aspen) and three relative ground moisture regimes (dry, mesic, wet) within central Saskatchewan. Ring-width will be used to estimate net ecosystem production (NEP) using the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) and potential for changes in ecosystem state will be assessed by examining variability and persistence present in the detrended time series of NEP.

Results/Conclusions

Preliminary analyses demonstrate notable correspondence between hot and dry years—as illustrated by a temperature-precipitation climate index (CI)—and low productivity of forested ecosystems. Furthermore, significant increases in variability and autocorrelation present in the CI prior to the 2001-2003 drought across the Canadian prairies suggest potential for prediction of these high magnitude, low frequency events. Our initial results reveal the boreal forest is sensitive to climate change, though do not yet reveal an ecological mechanism for decreased productivity.