Spatial pattern of gas exchange for montane moist meadow species

Montane moist meadows of western North America are relatively unique in that they are characterized by annual cycles of spring inundation followed by summer drought. Thus, plants adapted to these ecosystems must cope with a wide range of environmental conditions. We compared gas exchange and water relations of upper canopy leaves and leaf area index (LAI) of three dominant montane meadow graminoids growing at both streamside and mid-meadow locations (2.5 and 20.0 m, respectively from the stream edge). The two locations were environmentally very different in spite of their close proximity. The water table at the streamside location was ca. 40 cm closer to the surface compared to the mid-meadow. When averaged over season and species, photosynthetic rates were 12.5 % (p < 0.05) higher at the streamside compared to the mid-meadow location, but both conductance and transpiration were similar between locations (p > 0.05) The similarity in stomatal response was surprising given that streamside plants had midday xylem potentials that averaged 0.72 MPa less negative (less water-stressed) than that of mid-meadow plants (p < 0.05). There was no clear relationship between xylem potential and conductance for the species studied. Total LAI for the three species combined was nearly three times higher at the streamside compared to mid-meadow location. At the streamside location, Carex nebrascensis had a LAI over twice as high as that of the other species (p < 0.05), but there were no significant differences among species (p > 0.05) at the mid-meadow location. Carex nebrascensis had a more conservative gas exchange strategy (lower photosynthesis and transpiration) than Juncus balticus or Deschampsia cespitosa, yet developed more leaf area than either of these species. Juncus had higher photosynthesis and transpiration compared to the other species, yet maintained less negative midday xylem potential (p < 0.05). Apparently, Juncus balticus maintained a greater supply of moisture to above-ground tissues than the other species, which is probably an indication of higher root length per unit leaf area, or a more efficient water conducting system. The other two species – Carex nebrascensis in particular, maintain relatively conservative gas exchange rates, but under favorable growing conditions (such as occurred at the streamside) can develop high leaf area indices. Wetland species with similar growth forms (graminoid) and distributions can have contrasting patterns of gas exchange and plant water relations.