Development of a drought risk assessment tool for British Columbia forests using a stand-level water-balance approach.
We used an annual water-balance approach to assess the relative risk of current and future drought-induced stress and mortality at the stand level for tree species in British Columbia, Canada. The aim was to develop a drought risk-mapping tool that can be used by forest managers to inform harvest and silvicultural decisions at the stand level. We used the concept of absolute soil moisture regime (ASMR), which equates to the ratio of actual evapotranspiration (AET) to potential evapotranspiration (PET), to compare estimates of ASMR class based on expert opinion with ASMR class calculated by a water-balance equation using long-term climate data and reference site and soil conditions for different site types. The quantitative estimates of ASMR class generally agreed with those based on expert opinion. Current tree distribution on ecologically classified units for which we could calculate AET/PET was used to determine the AET/PET limits for 10 common tree species in British Columbia. With climate warming we estimate that seven of the tree species examined may be at risk of drought-induced stress and/or mortality. Risk varied for these species across different climate and edaphic conditions. Under future climate, moist to wet site types were never projected to be in a moisture-deficit situation, suggesting that these sites are the most stable sites from a drought perspective under a changing climate and therefore should warrant extra consideration for forest conservation. We describe a variety of ways in which this research can be used to make forest management decisions.