Ecophysiological responses of three riparian graminoids to changes in the soil water table

We examined the effect of changes in soil water table on the water relations, leaf gas exchange, and aboveground biomass of three riparian graminoids native to the semiarid western United States: Carex lanuginosa, Juncus balticus, and Carex nebrascensis. All three species co-occur at the wettest microhabitats within riparian corridors, but J. balticus and C. nebrascensis extend into drier areas. Lowering the water table 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 affect gas exchange rates or water potential in any of the three species. Lowering the water table when plants had one fully mature leaf resulted in limited differences between plants grown under high and low water table in J. balticus and C. lanuginosa. Further, these differences were only apparent after long periods of depressed water table (19 wk). In the field, rates of leaf gas exchange did not differ between plants growing near the creek from those occurring distant from the creek. Three factors contribute to the ability of these riparian graminoids to maintain favorable gas exchange and water relations across a range of water table depths. Each species appears to adjust rooting depth to, or just above, the shallow saturated zone. In the held, C. nebrascensis 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 help cells of C. nebrascensis and J. balticus maintain turgor as water table drops during the season. The limited distribution of C. lanuginosa 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 growth stages.