Water has been applied to several arctic ecosystem types to simulate increased precipitation, with the expectation that the additions would enhance plant production. The production could be stimulated either directly as a response to decreased drought, most likely to occur in dry polar deserts or semideserts (Aleksandrova, 1988; Bliss et it/., 1984), or indirectly by enhancement of nutrient supply to plants. For instance, increase in soil moisture facilitates the transport of nutrients toward the plants' roots (Chapin et al., 1988) and creates favorable environments for N fixation (Gold and Bliss, 1995).
Water additions alone have generally not led to any detectable changes in plant productivity or nutrient cycling, even in dry, high arctic ecosystem types (Press et al., 1998b). The generally low response contrasts with the control of water on community structure, distribution of organic matter, and the turnover of C and mineral nutrients. These differences indicate that water exerts an overall long-term control on ecosystem development and function and that the tundra is buffered against short-term fluctuations in moisture conditions within the levels of the additions, which usually have been within the range of natural between-year variation in precipitation.
However, water addition has in some cases interacted with temperature enhancement or fertilizer addition and increased the productivity of single plant species (Press et al., 1998a). Surprisingly, water even when applied in "moderate" amounts can have negative effect on dry plant communities. Robinson et al. (1998) found that combined water and fertilizer addition to a high arctic semidesert caused high winter injury of plants in some years, probably because winter hardening was delayed (Press et al., 1998b).
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