Community invasibility in old fields: From establishment to distribution across time and space

Background/Question/Methods Because biological invasions threaten the integrity of natural ecosystems, much ecological research on community invasibility has focused on the controls on establishment and whether those controls vary with spatial scale. Our study investigated the role of both biotic and abiotic factors associated with the initial establishment of Lespedeza cuneata and its abundance at several spatial scales. We particularly asked: (1) Does resource availability affect community structure and the establishment of Lespedeza in local old-field communities?, and (2) Are resource availability and community structure associated with patterns of Lespedeza abundance from local communities to landscapes? To answer our first question, we manipulated soil nitrogen (N) availability at three levels in existing old-field communities and tracked emergence and survivorship of two Lespedeza seedling cohorts over two years. To address our second question, we performed surveys of Lespedeza cover across 17 old fields at three spatial scales: 1 m2 quadrats, 50 m2 plots and entire old-fields. Results/Conclusions Lespedeza seedling density was reduced in nitrogen added plots (5.2 + 0.5) when compared to ambient (100.6 + 9.8) and nitrogen-reduced plots (153 + 20.9). Total biomass increased in N-elevated plots when compared to N-reduced plots (p<0.05) while mature Lespedeza cover decreased. Both light availability and soil moisture were reduced in soils with elevated N compared to N-reduced plots only early in the growing season (date×nutrients: p<0.001). Finally, increases in resource availability negatively affected Lespedeza establishment via direct effects on community structure and function. The broad-scale surveys indicated that similar biotic factors predicted Lespedeza foliar cover across multiple spatial scales. In fact, dominant species biomass (Solidago altissima, Verbesina virginica & occidentalis), as well as the biomass of other N-fixing species were negatively associated with Lespedeza cover at local and landscape scales. However, resource availability was not related to Lespedeza abundance at any spatial scale. In sum, these results suggest that biotic and abiotic factors associated with establishment of an invasive plant species at local scales are not necessarily related to its distribution at landscape scales.