In this study, we hypothesized that the primary variable determining species responses and interactions within wet/mesic riparian meadows in central Nevada, USA was the water table but that the direct and indirect effects of livestock grazing modified both species responses and interactions. We tested this hypothesis for two widespread riparian species, Carex nebrascensis and Poa pratensis ssp. pratensis, that co-occur but have maximum expression at different water tables. Carex nebrascensis has widely spread tillers typical of ‘guerilla’ plant architecture, while P. pratensis has closely spaced and compact tillers typical of ‘phalanx’ plant architecture. Individuals of both species were grown at mid- and low water tables with or without neighbors and were either clipped or not clipped at the end of the first growing season. For the study site with the most complete record, mean water-table depth during the growing season (May through August) on the mid sites was −32 cm in year 1 and −7cm in year 2. Water-table depth on the low sites was −69 cm in year 1 and −31 cm in year 2. Plant survival, tillering, biomass, and seed production over a 2-yr period were used to quantify the species responses. Water table had no effect on tillering or biomass of C. nebrascensis, indicating that it is adapted for growth and persistence over the range of water tables examined. In contrast, growth and tillering of P. pratensis was severely restricted at more shallow water tables. Poa pratensis had about 50% fewer tillers and lower biomass (9.0 vs 46.5 g) for neighbor-removed plots on mid- than low-water-table sites at one of the study meadows. Further, tiller numbers of P. pratensis increased over time on the low-water-table plots, but decreased on the mid-water-table plots. Clipping had no effect on the survival, tillering, or growth of either species. The clipping treatment may not have removed sufficient leaf mass or may have been applied too near the end of the growing season to elicit a response. Neighborhood removal resulted in a 3- to 10-fold increase in tillering and higher plant mass (1.3 vs. 9.5 g) for C. nebrascensis. Poa pratensis showed an even greater response to neighbor removal. Tillering was 6- to 100-fold greater and mass was 15 to 50 times greater on neighbor removed than neighbor-intact plots. Comparisons between single species and mixed species plots indicated that there was an interaction between the species that limited tiller production in C. nebrascensis. These results indicate that P. pratensis, the ‘phalanx’ species, is capable of more rapidly responding to disturbances that remove neighbors and increase available space than C. nebrascensis, the ‘guerilla’ species. Contrary to previous studies, the interactions between the two species do not seem to be related to plant architecture and can be best attributed to generally greater growth rates and increased competitive ability for P. pratensis at lower water tables. Grazing may further alter the relative competitive ability of the two species in favor of P. pratensis.
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Vol. 21 • No. 3