Tesis
Revisiting the Relative Robes Roles of Land Use and the Environment in Subtropical Wet Forest: 21-Years of Dynamics from the Luquillo Forest Dynamics Plot, Puerto Rico
Autor
Hogan, James Aaron
Zimmerman, Jess K. (Consejero)
Institución
Resumen
The Luquillo Forest Dynamics Plot (LFDP) has played a critical role in the initial
discovery and subsequent investigation of many processes that govern tropical island wet
forest dynamics. Previous work has identified past land use as the main factor in creating
forest community compositional and structural differences across the plot. The
responses of different species to past land-use intensity and to hurricane disturbances
have created an evolving forest mosaic ideal for studying tropical forest successional
dynamics. I revisited the interaction of land-use legacies and natural disturbance in the
LFDP with new data and new approaches, with the motivation to reveal new information
about the relative roles of anthropogenic disturbance and environmental-niche
partitioning on tropical plant communities over time.
In the context of tropical forests and their successional dynamics, I asked how
succession resulting from a history of human land use and more recent hurricanes
interacts with background environmental variation to effect community structure and
diversity. Community dynamics, in terms of forest structure and composition, were
summarized over a twenty-one year period, noticing a decreasing trend in species
richness over time and structural maturation of the forest, shown by a decline in small stems (trees < 10 cm diameter) as it recovered from the compound effect of two major
hurricane disturbances – Hugo, 1989 and Georges, 1998. We evaluate the magnitude of
past human land use effects over time and define indicator species for areas of differing
land-use pressure within the 16-Ha permanent LFDP. Using redundancy analysis, plant
community-environmental relationships with respect to soils and topography are
quantified. Spatial variables, computed using a principle coordinates of neighborhood
matrix, explained the majority of the variability in plant community composition between
areas of high and low past land-use within the LFDP, meaning environmental differences
(e.g. niche differentiation among tree species) were found to be secondary to land-use
legacies in determining forest community composition.
Over two decades, the effect of past land-use peaked about 15-years following the
first of two hurricanes, and remained relative stable over time. Despite damaging the
forest, hurricanes preserved community differences in species composition and reinforced
structural asymmetries due primarily to two species; Dacryodes excels Vahl., a dominant
primary forest tree species, and Casearia arborea (Rich.) Urb., an abundant secondary
forest species. Abiotic environmental factors (e.g. soil resources and topographic
variation) were weak at explaining differences in forest community composition. Plant
community-environmental relationships were stronger in more anthropogenicallydisturbed
areas, suggesting long-term effects of land use on tropical forest communities
on current community dynamics.