Plant water sources in the cold semiarid ecosystem of the upper Kherlen River catchment in Mongolia: A stable isotope approach

Abstract

In the cold semiarid region of northeastern Mongolia, we used stable isotopes (18O and D) to determine potential plant water sources during the 2003 growing season (June to September) in two habitats: montane forest and an elevation gradient from the forest to Kherlen river bank. The forest is dominated by larch (Larix sibirica) with patches of cinquefoil shrubs (Potentilla fruticosa). The latter also grow throughout the elevation gradient, while the larch grows only on the top slope. Poplar (Populus spp.) and willow (Salix spp.) trees grow only on the river bank. All plant and soil samples showed isotopic signatures similar to summer precipitation, which is isotopically heavier in summer than winter. In July and August, larch trees in the forest tended to shift their water uptake to shallow depths in response to recent rainfall, but during the remaining months (June and September), depths of water uptake were unclear. Further, both the larch trees and cinquefoil shrubs in the forest used water at similar depths, suggesting potential competition for water. Plants along the elevation gradient showed different patterns of water use: (1) in July, larch used recent rainfall only, but in other months, the pattern was unclear; (2) cinquefoil depended on rainfall from recent weeks (as in August), but sometimes used antecedent rainwater from one month prior; and (3) poplar and willow seemed to use water from the river (as in August) or from precipitation that fell a few weeks prior (as in September), but the factors controlling this unusual shift are unknown. This study contributes to our understanding of plant water use strategies in cold semiarid ecosystems, and provides baseline data for models designed to understand large-scale hydrological effects of global climate change.