A δ18O and δ2H stable water isotope analysis of subalpine forest water
sources under seasonal and hydrological stress in the Canadian Rocky
Mountains
Abstract
Subalpine forests are hydrologically important to the function and
health of mountain basins. Identifying the specific water sources and
the proportions used by subalpine forests is necessary to understand
potential impacts to these forests under a changing climate. The recent
‘Two Water Worlds’ hypothesis suggests that trees can favour tightly
bound soil water instead of readily available free-flowing soil water.
Little is known about the specific sources of water used by subalpine
trees Abies lasiocarpa (Subalpine fir) and Picea engelmannii (Engelmann
spruce) in the Canadian Rocky Mountains. In this study, stable water
isotope (δ18O and δ2H) samples were obtained from Subalpine fir and
Engelmann spruce trees at three points of the growing season in
combination with water sources available at time of sampling (snow,
bound soil water, saturated soil water, precipitation). Using the
Bayesian Mixing Model, MixSIAR, relative source water proportions were
calculated. In the drought summer examined, there was a net loss of
water via evapotranspiration from the system. Results highlighted the
importance of tightly bound soil water to subalpine forests, providing
insights of future health under sustained years of drought and net loss
in summer growing seasons. This work builds upon concepts from the ‘Two
Water Worlds’ hypothesis, showing that subalpine trees can draw from
different water sources depending on season and availability. In our
case, water use was largely driven by a tension gradient within the soil
allowing trees to utilize tightly bound soil water and saturated soil
water at differing points of the growing season.