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Determination of water sources of plant species in a community is critical for understanding the hydrological processes and their importance in ecosystem functions. Such partitioning of plant xylem water into specific sources (i.e. precipitation, groundwater) can be achieved by analysing deuterium isotopic composition (delta D) values for source waters. A subalpine dark coniferous forest in southwestern China was selected to examine water use strategies of three key species. Our objectives are to: (1) determine contributions of rainfall and groundwater to soil and xylem water and (2) examine effects of rain, days after rain, soil depth, and species on such contributions. We found that the three species tend to have different, but complementary water use patterns. The dominant canopy tree of Abies faxoniana relies primarily on groundwater (i.e. 66-96% of its water sources) and does not change its dependency on groundwater between seasons. In contrast, the midstory Betula utilis and the understory Bashania fangiana depend predominantly on rainwater (i.e. 13-94% and 32-93%, respectively), and tend to use water opportunistically, switching to groundwater as the main source under stressful conditions during the dry season. The complementary and, for some species, shifting water use strategies and the lack of dependency on rainwater by the foundation species of the subalpine coniferous forest ecosystem should act together to promote species co-existence and maintain community resiliency under potentially increasing water stress caused by climate change. Copyright (C) 2010 John Wiley & Sons, Ltd.