Linking above- and belowground dynamics in tropical urban forests

Background/Question/Methods

Urban ecosystems have a distinct and complex ecology, possessing unique biophysical characteristics while hosting a majority of the global human population. Increasing human pressure makes these stressed systems research priorities as we seek to understand how their structure and function can have significant consequences for ecosystem processes and services. Our research explores how alternative successional trajectories following intensive agricultural affect linkages between the establishment of novel plant communities, soil microbial community composition and function, and soil carbon chemistry and turnover. We are conducting this research in urban ecosystems in St. Croix, U.S. Virgin Islands where island-wide abandonment of sugarcane has resulted in a mosaic of sites in different stages of forest succession. We identified replicate sites with the following post-sugarcane trajectories: 1) natural forest regeneration, 2) low intensity pasture use, followed by reforestation with timber plantation, which are no longer being managed, 3) high intensity pasture use and recent natural forest regeneration, and 4) high intensity pasture use and current active grazing. 

Results/Conclusions

The first two trajectories yielded 40-year old mixed-species secondary forest, dominated by dry forest tree species such as Melicoccus bijugatas, Cordia alba, Capparis indica, and Guapira fragrans. The third trajectory yielded young (10-year) secondary forest that is comprised of 3 tree species, but is primarily dominated by the nitrogen-fixing species Leucaena leucocephala. These young secondary forests also had greater soil microbial biomass and had greater N-cycling microbial enzyme activity when compared with older, mixed-species secondary forest. In older forests that rapidly regenerated to secondary forest without an intermediate land use (i.e. tree plantation or high intensity pasture), we found increased microbial enzyme activity in the forest floor, particularly for enzymes involved in the breakdown of cellulose, hemi-cellulose, and soluble saccharides. Since enzymes are directly linked to the turnover rates of different organic functional groups in soils, there is great potential for differences in enzyme activity to alter pathways of soil organic matter turnover.