Evidence of water conservation mechanisms in several subarctic wetland species.
The hypothesis that subarctic hygrophilous plant species show poorly developed mechanisms for water conservation was tested by measuring the stomatal conductance, transpiration rate and xylem pressure potential of the ubiquitous subarctic wetland shrubby species Salix planifolia, S. reticulata, S. candida, Betula glandulosa, Myrica gale and Carex aquatilis, on a coastal site near Churchill, Manitoba. Measurements were made during 3 warm, clear-sky days in June-July 1991, when evidence of these mechanisms would be most prevalent. Mid-day stomatal depressions were experienced by all species except S. candida, with the frequency and magnitude of the depression increasing as the atmospheric humidity decreased. The mid-day stomatal depression resulted in a decrease in the transpiration rate when air temperatures were high and atmospheric humidity was low. The soil-to-leaf water potential (resistance) showed a dependence on the transpiration rate, whereby asymptotic values of the soil-to-leaf pressure potential differences were reached at high transpiration rates for all species. The implication is that these shrubs decrease their internal resistance to the movement of water at high transpiration rates in order to reduce physiological stress. In view of the sensitivity of the Salix/Betula community to the water supply provided by the shallow water table and root network, the implications for wetland management are discussed.