Carbon isotope discrimination by Picea glauca and Populus tremuloides is related to the topographic depth to water index and rainfall

Carbon isotope ratio (delta C-13) has been used as an indicator of water stress because plants discriminate less against C-13 when under stress. The depth-to-water (DTW) topographic index provides an estimation of soil moisture based on topographic position and other characteristics of a site. To evaluate whether DTW and carbon isotope discrimination were related and to determine if these relationships are influenced by climate, we sampled three time periods, which differed in the amount of annual precipitation (MAP), from tree cores collected from 42 trembling aspen and 43 white spruce trees growing along DTW gradients at two locations in central Alberta, Canada. Increasing MAP led to lower delta C-13, indicating less drought stress as water availability increases, while delta C-13 increased with DTW up to a threshold value, after which the relationship levelled off, suggesting that higher DTW values represent stress-inducing soil conditions. DTW and MAP were then combined into models (aspen, R-2 = 0.72; spruce, R-2 = 0.44) that could be used to delineate drought-prone areas during periods of low MAP. Tree height and diameter were also related to DTW, suggesting a functional relationship between an index capturing soil properties and tree size. Our results demonstrate the potential to use the DTW index as a measure of site conditions and to predict stand-level responses.