Invasive annual grasses dominate large areas in the arid American West. In the southern San Joaquin Valley of California, invasive grasses widely invade upland habitats, with native saltbush shrub communities existing only in remnant patches. We tested the hypothesis that invasive annual grasses limit saltbush recruitment and survival, leading to persistently invaded grasslands. One way this could happen is through competition for belowground resources between the grasses and saltbush seedlings, whereas another possibility is that the dense cover of residual dry matter (RDM) produced by the annual grasses alters the habitat for saltbush seedlings.
An experiment manipulating competition through weeding, RDM presence, and shade cast by the RDM (utilizing shade cloth) was carried out over a 12-month period. We tested the effect of competition and RDM presence on seeds and seedlings of the most common upland saltbush shrub in the southern San Joaquin Valley, Atriplex polycarpa S. Watson. Equal numbers of seeds were sown in plots to assess germination and emergence, seedling density, and percent vegetation cover in each of six treatments (n = 15 plots/ treatment). Parametric (ANOVA) and non-parametric (Kruskal-Wallis) analyses were conducted as appropriate to assess treatment effects. Soil moisture and temperature were measured in 18 of the plots (n = 3/ treatment). Measurements were taken monthly over the course of the study.
Results/Conclusions
We found both competitive interactions and +RDM significantly adversely affected germination and survival of A. polycarpa seedlings (competition: F1,29 = 5.57, P = 0.033, RDM: F1,29 = 19.72, P < 0.001) with no interaction between the treatments. A. polycarpa coverage was significantly lower in the unweeded (high competition) and +RDM treatments compared to controls (H1 = 11.89, P < 0.001). However, we did not find a significant difference in soil moisture or temperature between the treatments (P > 0.05), indicating that the grasses may reduce germination and survival in ways independent of soil moisture or temperature. In shaded treatments, weeding had no effect on A. polycarpa density (F1, 14 = 0.60, P= 0.452) or coverage (H1 = 0.01, P = 0.929), indicating the competitive edge held by the grasses is reduced in shade. In summary, RDM presence was the primary factor governing A. polycarpa germination, emergence, and seedling survival, with competition serving as a secondary significant factor. Land management activities (e.g., grazing) which reduce RDM and minimize the competitive ability of invasive grasses may benefit saltbush seedling recruitment and ultimately shrubland restoration.