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).