Submitted to: Oecologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/20/2008
Publication Date: 3/15/2008
Citation: James, J.J., Davies, K.W., Sheley, R.L., Aanderud, Z.T. 2008. Linking nitrogen partitioning and species abundance to invasion resistance in the great basin. Oecologia. 156:637-648. Interpretive Summary: Species diverse plant communities may inhibit the establishment of invasive plants more than less diverse communities if species differ in how they capture resources. Alternatively, invasion resistance may be driven by only the dominant species. We quantified the degree to which species partition resources and evaluated the contribution of resource partitioning to invasion resistance. We found strong evidence that species differed in pattern of resource capture but no evidence that this increased invasion resistance. Establishing and maintaining dominant species appears to be the critical mechanism for invasion resistance.
Technical Abstract: Resource partitioning among species has been suggested as an important mechanism of invasion resistance. Research to date, however, has not directly linked resource partitioning among species to the ability of a species to quantitatively sequester a limiting resource and contribute to invasion resistance. This study had two objectives. First, we quantified the degree to which nitrogen (N) is partitioned by time, depth and chemical form among coexisting species from different functional groups by injecting 15N into soils around the study species three times during the growing season (April, May, June), at two soil depths (2-7 cm and 17-22 cm) and as two chemical forms (NH4+ and NO3-). A watering treatment also was applied in May and June to evaluate the impact of soil water content on N partitioning. Second, we examined the degree to which native functional groups contributed to invasion resistance by seeding a non-native annual grass into plots where bunchgrasses, perennial forbs or annual forbs had been removed. Dominant bunchgrasses and subdominant forbs were differentiated in timing, depth and chemical form of N capture and these patterns of N partitioning among species were not affected by soil water content. However, when we incorporated abundance (biomass) with these relative measures of N capture to determine N sequestration by the community there was no evidence suggesting functional groups partitioned different soil N pools. Instead, dominant bunchgrasses acquired the most N from all soil N pools. Consistent with these findings we also found that bunchgrasses were the only functional group that inhibited annual grass establishment. These results suggest that at natural levels of species abundance N partitioning may facilitate coexistence but species and functional group identity is the main driver of N sequestration and invasion resistance by the plant community. Taken together, our results indicate a general mechanism of invasion resistance may not be expected across systems. Instead, the key mechanism of invasion resistance within a system may depend on trait variation among coexisting species and on how species abundance is distributed in the system.