Urban development and expansion are accelerating in many tropical regions, and may promote the spread of non-native plant species into urban-proximate forests and degraded lands. In addition to increased seed sources of introduced plants from urban centers, human activity can increase forest fragmentation, canopy disturbance, and soil degradation, which may contribute to the spread of introduced species better adapted to these conditions than native species. For example, soil disturbance may deplete naturally high soil nitrogen (N) pools in wet tropical soils, favoring introduced species with N-fixing capabilities. Here we measured understory woody diversity and soil conditions in urban, suburban, and rural secondary forests in Puerto Rico with canopies dominated by native species, by non-native Fabaceae (potential N-fixers), and by non-native non-Fabaceae species. A suite of biological and physical forest characteristics, as well as landscape-scale urban features, were assessed as predictors. We hypothesized that forest structure and distance to the urban center correspond most strongly with understory plant diversity. We also predicted that forest stands with canopy Fabaceae would have improved soil conditions and higher understory diversity compared to stands with other introduced species in the canopy.
Results/Conclusions
We found that canopy openness and forest fragment size were strongly associated with understory plant species richness and abundance, with the highest diversity in the rural forest. In contrast, the abundance of introduced N-fixing plants in the understory was most strongly associated with distance to the urban center. We found that forest stands with Fabaceae in the canopy had higher soil N levels than stands with other introduced species. These stands also had intermediate levels of understory diversity, approaching levels under all-native canopies, and the lowest diversity was under introduced non-Fabaceae canopies. These results suggest that disturbances to aboveground forest characteristics are most likely to affect overall understory plant diversity in urban settings, whereas proximity to the urban center promotes the spread of non-native plants into urban forests. However, some introduced Fabaceae species may have the potential to improve soil conditions and promote the restoration of understory native plant diversity in disturbed urban sites.