Structural variability and species diversity of a dwarf Caribbean dry forest

Low stature woody vegetation of the south-west coast of Puerto Rico grows on a rocky calcareous substrate where plants can only root in holes, cracks, and crevices accumulating water and sediments that allow seed germination and seedling development. Being in a coastal location these communities are influenced by steady onshore winds, high solar radiation, and salt spray. We studied dwarfed forest communities located at the southern limit of the Guanica State Forest, Puerto Rico, that have a well known floristic composition, but with little information on their organization and heterogeneity. We quantified the species composition of vegetation plots located along the southern coastline and compared their structure and diversity with those of dry forests on neighboring hills. The hypothesis was that forest structural development was negatively associated with proximity to the coast line and altitude above sea level. Species richness of the coastal dwarf forest area was similar to that of inland semi deciduous forest plots with which it shares at least 15 woody species. In addition, it contains a number of species resistant to salt spray and possibly brackish water, such as Conocarpus erectus, Strumpfia maritima, and Coccoloba uvifera. The average canopy height of the coastal vegetation increases from 0.4 to 2.3 m between 0 to 150 m from the coastline and 7 to 19 m elevation above sea level. Within this spatial range the predominant bearing of the canopy changes from a SE-NW direction to a SW-NE direction revealing the influence of onshore winds in combination with salt spray. The dominant woody species occur as multistemmed individuals, a characteristic probably associated to occasional heavy winds and recurrent drought that impairs shoot apical dominance. Floristic and structure comparisons with mature forest plots located between 25 and 150 m altitude showed that the coastal dwarf vegetation has a more even species dominance distribution associated with the discontinuity of substrate available for establishment.