Canadian Forest Service Publications
A simple soil moisture index for representing multi-year drought impacts on aspen productivity in the western Canadian interior. 2013. Hogg, E.H.; Barr, A.G.; Black, T.A. Agricultural and Forest Meteorology Vol. 178-179:173-182.
Issued by: Northern Forestry Centre
Catalog ID: 34829
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Tree ring studies have shown that drought is a major factor governing growth of aspen (Populus tremuloides Michx.) forests in western Canada. Previous analyses showed that interannual variation in aspen radial growth is moderately well-correlated with a climate moisture index (CMI), calculated annually as the difference between precipitation (P) and potential evapotranspiration (PE). However, there are multi-year lags, where current year growth is significantly related to CMI over each of the preceding 5 years. We postulated that such lags arise because of tree growth responses to soil water content, which in deep soils may change slowly in response to interannual variation in P and PE. To address this, a model was developed that simulates changes in a soil moisture index (SMI) from inputs of P and PE only. The SMI represents the quantity of available soil water (mm) for aspen forest evapotranspiration and growth, and also provides a measure of relative soil water content (θr). Model performance was tested using measurements made at an intensively instrumented boreal aspen stand in Saskatchewan, Canada, over a 9-year period that included an exceptionally severe drought (2001–2003). Following optimization of the equations describing soil water limitations on evapotranspiration, the model was successful in simulating the observed, monthly variation in θr (r2 = 0.86–0.88). The model was then used to estimate historic variation in the SMI across a regional network of aspen stands where historical variation in growth was reconstructed from tree-rings. Subsequent analyses showed that average SMI during the current growing season was comparable to the CMI in its ability to explain temporal variation in aspen growth. However, the multi-year lags associated with the CMI were no longer statistically significant when the SMI was used as the independent moisture variable. In a case study of aspen stands that had been free of significant defoliation by insects, tree-ring analysis showed that growth was significantly related to CMI in each of the preceding 5 years, but was significantly related to SMI only in the current year and the preceding year. Thus, hydrological lags can explain much of the apparent delay in aspen growth responses to moisture, and future tree-ring studies may benefit from using modeled SMI as a more realistic index for assessing drought impacts on the productivity of aspen and other forest types.
Plain Language Summary
In our research, we are studying how drought affects the productivity and health of aspen (poplar) forests across western Canada. From looking at aspen tree-rings and weather station records, we have found that aspen growth is stimulated for up to 5 years following periods of much rain and snow. We thought that the reason for this could be that aspen trees get their moisture from deep soil layers that change slowly in response to weather conditions. We tested this idea by building a computer model that predicts how changes in temperature and rain and snow affect soil moisture conditions within aspen forests. The model performed well in simulating the changes in soil moisture that were measured using instruments over 9 years at one of our aspen research sites. We then used the model to estimate historical changes in soil moisture at other locations where we have conducted tree ring studies on aspen. The soil moisture model was remarkably successful in explaining the past year-to-year variation in aspen growth based on tree-rings. We expect that this model will provide a useful tool for assessing and forecasting the impacts of drought on the productivity of aspen and other forest types under a changing climate.