Invasive tree species in tropical forests along an urban-rural gradient

Background/Question/Methods

Urbanization and anthropogenic activities can have major effects on the spread of invasive species in the tropics. Because of human activity, Puerto Rico has only 6% of original forest cover and only 1% is mature forest with only native species. In the summer of 2011, we surveyed seedling diversity along an urban-rural-remote gradient outside of San Juan, Puerto Rico. Woody species were identified to the genus or species level in three 2m by 5m plots in nine forest stands along the gradient. Canopy tree species composition was also assessed for invasive species. We hypothesized that urban expansion and land use change are linked to domination by exotic plant species in understories nearer to the urban center and major roads, indicative of new successional pathways for these forests.  

Results/Conclusions

Preliminary statistical results show that rural and remote sites had significantly higher species diversity (p < 0.05), whereas stem abundance was not significantly different across the watershed. Remote sensing analyses using ENVI 4.8 revealed that lower Normalized Difference Vegetation Index (NDVI) values were indicative of more invasive legumes, and lower NDVI values were positively associated with distance to the urban center. Areas closest to the urban center had NDVI values averaging -0.05 whereas areas farther away averaged 0.25. Forests more than 6.5 kilometers from the urban center had positive NDVI values. These data suggest a positive effect of urbanization on invasive plant species in the canopy as well as the understory for these Puerto Rican forests, with distance to the urban center playing the strongest role in determining the extent of invasive species.