ECOPHYSIOLOGICAL RESPONSES OF 3 RIPARIAN GRAMINOIDS TO CHANGES IN THESOIL-WATER TABLE

We examined the effect of changes in soil water table on the water rel ations, leaf gas exchange, and aboveground biomass of three riparian g raminoids native to the semiarid western United States: Carex lanugino sa, Juncus balticus, and Carex nebrascensis. All three species co-occu r at the wettest microhabitats within riparian corridors, but J. balti cus and C. nebrascensis extend into drier areas. Lowering the water ta ble to 1 m had little effect on the leaf gas exchange characteristics of the three graminoids. In the greenhouse, experimental reductions of the water table when plants had three fully mature leaves did not aff ect gas exchange rates or water potential in any of the three species. Lowering the water table when plants had one fully mature leaf result ed in limited differences between plants grown under high and low wate r table in J. balticus and C. lanuginosa. Further, these differences w ere only apparent after long periods of depressed water table (19 wk). In the field, rates of leaf gas exchange did not differ between plant s growing near the creek from those occurring distant from the creek. Three factors contribute to the ability of these riparian graminoids t o maintain favorable gas exchange and water relations across a range o f water table depths. Each species appears to adjust rooting depth to, or just above, the shallow saturated zone. In the held, C. nebrascens is and J. balticus exhibited reductions of aboveground live biomass at locations far from the creek compared to those near the creek. Small adjustments of osmotic potential and the bulk modulus of elasticity he lp cells of C. nebrascensis and J. balticus maintain turgor as water t able drops during the season. The limited distribution of C. lanuginos a near the creek may result, in part, from a higher biomass allocation to leaves and a less efficient water transport from roots to leaves, particularly when depressions of the water table occur during early gr owth stages.