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 (δ 18 O and δ 2 H) samples were obtained from S. fir and Engelmann spruce trees at three points of the growing season in combination with water sources available at time of sampling (snow, vadose zone water, saturated zone 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 vadose zone, or 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 vadose zone water and saturated zone water at differing points of the growing season.

Using stable isotopes to investigate differences of plant water sources in subalpine habitats

“…Second, the soil depths of these two aspects were different. The root structure of subalpine forest species is affected by bedrock, limiting the root depth (Langs et al, 2020; Liu et al, 2011). In the study area, soil depth until the bedrock was greater on the semi‐sunny aspect (0–80 cm) than on the semi‐shaded aspect (0–60 cm); thus, the roots of subalpine shrubs could reach deeper soil to absorb soil water on the semi‐sunny aspect.…”

“…Langs et al (2020) found differences in the water sources of subalpine forests under seasonal and hydrological pressures in the Canadian Rockies, and emphasized the importance of soil moisture reserves for the health of subalpine forests. Consistent with Langs' findings, our study found that soil water is essential for the growth and development of subalpine shrubs.…”

“…In the study area, changes in water use strategies of Potentilla fruticosa, Salix sclerophylla, Salix cupularis, Salix oritrepha, Rhododendron anthopogonoides, and Rhododendron przewalskii allowed these shrubs to tolerate drought stress, promoted species coexistence and stability (Chen et al, 2020;Hu et al, 2021;Tiemuerbieke et al, 2018). Langs et al (2020) found differences in the water sources of subalpine forests under seasonal and hydrological pressures in the Canadian Rockies, and emphasized the importance of soil moisture reserves for the health of subalpine forests. Consistent with Langs' findings, our study found that soil water is essential for the growth and development of subalpine shrubs.…”

“…In subalpine habitats, soil depth and soil water storage vary due to glacial and complex terrain, which affects the water sources of plants (Christensen et al, 2016). Langs et al (2020) found that water source allocation differed according to the species and age of subalpine trees in the Canadian Rockies. However, water sources of plants on slopes with different aspects in subalpine habitats have rarely been explored, although this study is essential for the restoration of mountain ecological environment.…”

Flushing or mixing? Stable water isotopes reveal differences in arctic forest and peatland soil water seasonality

“…The macropore findings of the study by (Renée Brooks et al, 2010) have been confirmed to some extent in many biomes across the globe (Evaristo et al, 2016; Javaux et al, 2016). Other studies revealed that under a rainy temperate (Herve‐Fernandez et al, 2016), humid subtropical (Luo et al, 2019), tropical savanna (Mahindawansha et al, 2020) or subalpine (Langs et al, 2020) climate these interactions may be seasonally limited and dependent on soil moisture conditions. Bowling et al (2017) examined whether snowmelt water can create a unique pool of tightly bound water that is used by plants in seasonally snow‐covered areas during the growing season.…”