Document: MAR-3-32-14

Effects of added water on monoculture performance and competitive relations of Bromus tectorum and Stipa hymenoides in southeastern Utah.

MILLER, M.E.* ^1,3, J.BELNAP ² and S.W.BEATTY ³

Bureau of Land Management, Kanab, UT USA ¹
U.S. Geological Survey, Moab, UT USA ²
University of Colorado, Boulder, CO USA ³

Abstract:
We conducted a field experiment to assess effects of simulated above-average fall and spring precipitation on monoculture performance and competitive relations of the exotic annual grass Bromus tectorum and the native bunchgrass Stipa hymenoides in southeastern Utah. Between 1 October 1997 and 31 May 1998, "wet" plots received 239 mm and "dry" plots received 119 mm of water representing 175 and 87 percent of average precipitation, respectively. In monoculture, added water increased fall Bromus establishment by a factor of 1.6 but did not affect fall or winter growth. Added water adversely affected Bromus growth during early spring, but enhanced growth as soils dried during mid-spring. Overall, Bromus shoot production was 1.4 times greater in wet plots than in dry plots, with most of this difference attributable to fall establishment. During the same period, added water did not affect culm recruitment or fall growth of Stipa in monoculture. Similar to Bromus findings, added water adversely affected Stipa growth during early spring. There were no effects of added water on growth in mid- or late-spring despite enhancement of late-spring pre-dawn water potentials. Overall, added water had no effect on Stipa shoot production. In plots where the two species were grown together, there were no Bromus responses to Stipa in fall, and responses to Stipa in spring tended to be more negative in wet plots than in dry plots. Conversely, Stipa responses to Bromus in spring tended to be more negative in dry plots than in wet plots. Effects of water on competitive responses were variable and statistically insignificant overall, but these results offer insights concerning likely impacts of precipitation variations on competition between these species. For Bromus, a single fall of above-average precipitation should increase establishment during both the current and the subsequent fall seasons due to effects of litter production on microsite conditions favoring recruitment. Although above-average precipitation should enhance current-season competitive responses of Stipa, the net impact of a single wet year could be negative for Stipa due to greater Bromus competition in subsequent seasons.

Keywords: Bromus tectorum, Stipa hymenoides, exotic species, water relations, competition

This abstract is being presented at: 8:30 AM in session:
Oral Session #45: Water Relations in Shrubs and Annuals.