Disturbances can change species relative abundance and may modify strategies for species persistence. In response to drought, the relative competitive ability of seedlings that germinate under facilitator canopies may be an important filter determining future species composition in arid environments. We conducted a surface response experiment to determine the effects of density and frequency on growth of common shrub seedlings from an extensively monitored plot in the Colorado Desert. Ambrosia dumosa, Larrea tridentata and Eriogonum fasciculatum experienced differential mortality in response to extensive drought that decimated Eriogonum in the monitored area, and reduced canopy cover of Ambrosia, which is the most common facilitator at this site. Given the pulsed recruitment of seedlings in deserts, we were interested in the outcomes of interactions among different combinations of the focal species. Seedlings were grown in 1, 2, and 3 species combinations at densities of 8, 16 and 32 seedlings. Height gain (final –initial) is the response variable. Linear regression was used to model growth responses to variable densities of focal species. ANOVA was used to distinguish the effects of species density and frequency on growth. Experimental outcomes are used to assess the competitive landscape among these species.
Results/Conclusions
Linear regression showed that there was a significant, negative 3-way interaction for Larrea (p=0.044) and for Eriogonum (p=0.0016). These results suggest that Larrea and Eriogonum experience synergistic costs from three-species competition beyond the additive effects of each competitor. Ambrosia and Eriogonum showed larger reductions in growth from addition of a seedling of the other species than a conspecific seedling in two species pots (Ambrosia: -0.10 cm per Eriogonum versus no significant conspecific effect; for Eriogonum, -0.18 cm per Ambrosia versus -0.03 cm per conspecific). These two species limited the other more strongly than themselves.
ANOVA showed that density and frequency both had significant effects on the height gain of Ambrosia (p<0.0001; p<0.0001) and Larrea (p<0.0001; p<0.0001); however, only frequency affected the height gain of Eriogonum (p=0.07; p<0.0001). There were no significant interactions between density and frequency.
The three species had differing responses to competition. Ambrosia and Eriogonum had ~3x faster growth rates than Larrea, but also experienced larger reductions in growth from competition. In contrast, Larrea exhibits stress-tolerator characteristics of slower growth but less impact from competition. Although the importance of facilitated establishment may increase with drought, different seedling competitive strategies may promote coexistence in a fluctuating environment.