Abstract: Facilitative interactions between plants are prominent processes during primary succession in harsh environments such as abandoned industrial waste dumps. Novel urban communities are often heavily invaded by non-native plants, and some evidence suggests that positive interactions between non-native plants, such as nurse trees and shrubs, can lead to further invasion and result in communities dominated by exotic species. We tested three hypotheses: 1) facilitation increases non-native richness with negative or no impact on native richness; 2) facilitation increases richness for both native and non-native species; 3) facilitation has no effect on species richness.
Field sampling took place on the top of an abandoned slag dump in Pittsburgh, PA, USA. Ten scattered non-native trees growing in the slag savanna were selected for analysis. All species present were recorded in two 1-meter plots placed directly north and south of the trees’ trunks. Companion plots were placed in the savannah matrix north and south of the nurse tree plots at least two meters beyond the driplines of the nurse trees. Microtopography, soil depth, herb, and shrub cover were recorded for each plot.
Native species richness was significantly higher in the savannah matrix than under nurse trees; neither overall nor non-native richness differed significantly between groups, which indicates that nurse trees are not increasing species richness through facilitation. The proportion of non-native species in the community was significantly higher under nurse trees. The response to nurse trees also differed between growth forms: non-native shrub richness was significantly higher under the trees, while herbaceous richness was significantly higher in the matrix. Our results suggest that non-native nurse trees lead to communities dominated by non-native species indirectly, by facilitating development of dense clusters of non-native shrubs that compete with shade-intolerant native species found in the slag savannah matrix. We conclude that positive interactions between non-native species can facilitate further invasion and suppress native species in novel successional habitats.
