Gas exchange of four arctic and alpine tundra plant species in relation to atmospheric and soil moisture stress.

Gas exchange measurements of two arctic tundra plants, Dupontia fischeri and Carex aquatilis, and two alpine tundra species, Deschampsia caespitosa and Geum rossii, were conducted under a range of atmospheric and soil moisture stress conditions to determine if photosynthetic adaptations to water stress may play a role in the local distributions of these species. Under low soil moisture stress, the species which are normally restricted to wet sites, Dupontia and Deschampsia, exhibited higher net photosynthetic rates than Carex and Geum which are more widely distributed. However, photosynthetic of the wider ranging species was not so abruptly curtailed as that of the wet site species when the plants were exposed to increasing atmospheric or soil moisure stress. Although the depression of photosynthesis with water stress in these tundra species could be partially attributed to reduced stomatal aperture, with decreased soil water potential most of the decline of photosynthesis was due to a greater non-stomatal or residual resistance, indicating a direct impact of water stress on the photosynthetic apparatus. Dark respiration did not increase with enhanced water stress. Stomata of the wet site species did not appear to undergo a closing response until the bulk leaf water potential decreased. However, at high soil water potential reduced stomatal aperture of the more widely distributed species was noted before leaf water potential dropped. Therefore, stomata of these species may respond directly to the vapor pressure gradient between the leaf and the atmosphere when soil moisture potential is high. The wet site species typically exhibited higher photosynthesis/transpiration ratios at low soil moisture stress; however, as soil moisture stress increased, the photosynthesis/transpiration ratios of the wider ranging species generally exceeded those of the wet site species.