Site: Luquillo LTER
Sawyers in the Luquillo Mountains in the early 20th Century.
USDA Forest Service, International Institute of Tropical Forestry

Ecosystems are increasingly subjected to combined natural and human disturbances. Since 1990 in Puerto Rico we have been studying human and hurricane disturbance and their interactions in a tropical forest. The work takes place on the 16-ha Luquillo Forest Dynamics Plot (LFDP) in the Luquillo Mountains. Part of the plot was logged or farmed up until the 1930s (Fig. 1). On the rest of the plot just a few trees were removed for timber. There has been no human disturbance since the 1950s. All parts of the LFDP were severely affected by hurricanes in 1989 and 1998.

We censused trees on the LFDP in 1990, 1995, 2000, and 2005. Our first notable result was that past human disturbance is the most important influence on tree species composition, more important that soil type or topography, which vary greatly on the plot. Our second notable result was that the severe impacts of hurricanes caused little change in species composition. This is because, although damage to trees was great, most damaged trees recovered and mortality (about 17%) rates were similar among species. Thus the relative abundances of abundant and dominant species did not change. This is not surprising, considering that Puerto Rico has been subjected to hurricanes for millions of years and any species especially sensitive to this type of disturbance would have been eliminated long ago.

Our third notable result was that the combination of past human and natural disturbance maintains forests with new tree species combinations. We discovered this using a simulator of forest dynamics called SORTIE. SORTIE is an individual tree-based, spatially-explicit model of the eleven most common tree species on the LFDP (accounting for 75% of stems >10 cm diameter). SORTIE includes the history of human land use (logging and agriculture), which strongly influences variation in the current tree species composition in favor of secondary species that are more common in the disturbed portion of the LFDP. Succession theory predicts that, over time, secondary species that colonize after disturbance will be replaced by primary species, whose seedlings survive in shade. However, SORTIE simulations predicted that following hurricane disturbance, and contrary to expectations of succession theory, secondary species associated with human land use would not be lost from the forest and replaced by primary species. Rather, the entire plot will become a homogenized mixture of primary and secondary species. This persistent composition of tree species is a novel tree community unlike any past or present communities of trees. Lower seedling survival in the high light produced by hurricane damage to the forest canopy in human-impacted areas is mainly responsible for this pattern.

Research on long-term changes in tree species on the LFDP illuminates our understanding of the potential interactions between natural disturbance and human land use. Humans are having a greater and greater impact on the planet’s ecosystems. Future ecosystems will differ from those we know today. Luquillo LTER science helps us foresee and adapt to those new ecosystems.

Graph for
Land use history and current distribution of tree species on the Luquillo Forest Dynamics Plot, Puerto Rico. Cover Classes 1, 2, and 3 are areas on the plot where there was intensive land use until the 1930s. Cover Class 4 is an area that was used only lightly. The spatial distributions today of Dacryodes exclesa (D. excelsa) and Casearia arborea (C. arborea) correspond to those past land used, as do the distributions of other tree species.
Jill Thompson