Catchment vegetation dynamics
Vegetation in the Luquillo Mountains shows strong variation along a complex elevation gradient (Gould et al. 2006, Barone et al. 2008) with forest types typically described as those occurring either above or below the cloud condensation level. Lower elevation riparian wetlands and tabonuco forest (dominated by Dacryodes excelsa ) occur below the cloud levels. At higher elevations, forest community structure includes an increase in epiphytes, bromeliads on the forest floor, and a higher density of shorter and smaller trees. The upper cloud forests comprise palo colorado communities (Cyrilla racemiflora ) and sierra palm (Prestoea montana ) in both floodplain and palm break forests. At the highest elevations, elfin forests are the dominant vegetative community (Tabebuia rigida) , and include herbaceous and Sphagnum bogs (Harris et al. 2012). Palms are found throughout all elevations in the Luquillo Mountains. Hurricanes disturbances cause significant stem breakage, leading to changes in forest structure and shifts in species composition during succession (Heartsill-Scalley 2017, Uriarte et al. 2019). Cecropia schreberiana is an early successional species that contributes to re-establish canopy cover following hurricanes (Thompson et al. 2002). Export of stream coarse particulate organic matter (CPOM) has been measured biweekly since 1993, and response to hurricanes indicates that total CPOM export is strongly associated with the level of maturity of watershed vegetation (Heartsill-Scalley et al. 2012). Litterfall is measured biweekly at multiple sites and is altered by both hurricanes and experiments that simulate a portion of hurricane effects (Silver et al. 2014; Silver 2018). Seasonal patterns in leaf fall are correlated mainly with solar radiation, photosynthetic photon flux density (PPFD), day length, and temperature; and secondarily with rainfall (Zalamea and González 2008). Vegetation is not limited to any significant extent by nitrogen availability. A long-term N fertilization experiment resulted in no change in biomass increment or litterfall, and only modest impacts on litter chemistry (Cusack et al. 2011). Soil organisms and hurricanes have significant effects on litter decay and associated nutrient release (González et al. 2014).