Natural vegetation groups and canopy chemical markers in a dry subtropical forest on calcareous substrate: the vegetation of Mona Island, Puerto Rico
Mona Island is the third largest island in the archipelago of Puerto Rico located about 70 km west of the main island. Presently it is a wilderness refuge that contains well-preserved arboreal and shrubby vegetation, and distinct cactus forests, covering the calcareous, elevated plateau. During a forest inventory conducted by the US Forest Service, we obtained leaves of 53 species constituting the vegetation canopy on the plateau of Mona Island. We conducted a biochemical characterization of these leaves based on analyses of carbon (C), nitrogen (N), acid detergent fiber (cellulose, hemicellulose and lignin), and ash content with emphasis on the most abundant species. Four clusters of species were characterized by (1) relative high % N and low lignin, (2) high % C, low % ash, and cellulose + hemicellulose, (3) low % C and N, and high % ash, and (4) low % ash and high % lignin. These clusters overlapped partially with the characteristic species of physiognomic vegetation types previously described for the island. Cluster 2 species dominated the forest on the calcareous plateau, whereas cluster 3 species dominated forests on depressions. Shrublands were dominated by species in clusters 1 and 2. The data set of Mona Island species showed substantially higher average C/N ratios (probably indicating N limitation), and lower % lignin than species of tropical dry and humid forests. In addition, a large fraction of species had leaf traits associated with herbivore deterrence. The species in clusters 2 and 4 showed % C at approximately 55%, indicating the accumulation of carbon-rich compounds such as lignin and lipids. This project was part of a larger one seeking to study tropical dry vegetation and understand functional types as well as their relationships with climate, canopy leaf chemistry, and remotely sensed imagery. The data set assembled and our findings regarding the association with vegetation types may serve as a baseline for evaluating climate-change processes.