Evapotranspiration of two vegetation communities in a high-elevation riparian meadow at Hart Prairie, Arizona

Hart Prairie, Arizona, has the largest Bebb willow (Salix bebbiana) community in the United States; however, greater than 95% of willows are older than 80 years and regeneration by seed is not occurring. This study examined the evapotranspiration of two herbaceous communities that dominate the Hart Prairie watershed: (1) a mixture of bracken fern (Pteridium aquilinum) and graminoids and (2) dominantly graminoids. Transpiration during premonsoon and postmonsoon dry periods of 2000, 2001, and 2002 was estimated for each community by the difference in volumetric soil-water content (0-30 cm soil depth) between replicated plots that were clipped of all vegetation and control, unclipped plots. Transpiration rates estimated under conditions of minimal soil drainage varied between 0.63 and 2.4 mm/d for the fern-graminoid community and 0.57 and 1.1 mm/d for the graminoid community over the study. The fern-graminoid community produced more biomass than the graminoid community in all years, but generally had lower transpiration rates. Severe drought in year 2002 reduced growth and transpiration of the fern-graminoid community more than the graminoid community. Evaporation rates were estimated by temporal changes in soil-water content in clipped plots during dry periods, and were 54 to 474% of transpiration rates estimated under conditions of minimal soil drainage because of the dry and windy conditions that occur at the study site. Based on this study and a study of transpiration of scattered trees invading the meadow, transpiration by the herbaceous understory was higher than transpiration by trees during similar seasons.