Abstract: 

The processes that determine the plant species diversity and structure of tropical forest are still uncertain despite many years of investigation. A tropical forest characteristically has few common species and many rare species, with conspecifics often widely distributed throughout the forest. The potential for inter and intra-specific interactions are numerous, and it is difficult to determine which factors maintain species diversity or how rare species survive in the population. Factors that we believe contribute to tropical forest structure and composition include the physical environment and past history of the forest, species-specific physiological requirements for light, nutrients and water necessary for growth, survival and reproduction, and pathogens and herbivores. Despite the substantial amount of information available about these factors, we are still unable to accurately model the current composition of tropical forest or to predict its future response to human and natural disturbances.

The Luquillo Forest Dynamics Plot (LFDP), previously known as the Hurricane Recovery Plot (Zimmerman et. al. 1994) and the Luquillo long-term ecological research grid (Soil Survey 1995), is a 16-ha forest plot (SW corner 18° 20' N, 65° 49' W) located near El Verde Field Station. The plot is 500 m N-S and 320 m E-W and is divided into 400 20 x 20 m quadrats, with each quadrat sub divided into 16 5 x 5 m sub-quadrats. The field station and LFDP are in the Luquillo Mountains of northeastern Puerto Rico, approximately 35 km southeast of San Juan. Information from the LFDP contributes to the efforts of the Center for Tropical Forest Science (CTFS, Smithsonian) network of large tropical forest plots in order to improve our understanding of tropical forest and to predict its future. Large plots (typically 50 ha) are required to cover local environmental variation, include sufficient numbers of individuals of both the common and rare species, and to determine plant spatial relationships. Population monitoring is required over many years to elucidate tree life histories, species interactions and population changes in order to determine the forest response to environmental changes and disturbance.

The LFDP is unique among the CTFS sites as it has a history of land use disturbance and also hurricane damage. These two disturbance types interact and influence the community dynamics and species composition in the LFDP. The Northern area (approximately two thirds of the plot) was disturbed by tree felling and farming until 1934 when the land was purchased by United States Department of Agriculture, Forest Service. The Southern third was not farmed and only suffered relatively light disturbance from selective logging. Aerial photographs taken in 1936 show differences in canopy cover caused by the patterns of land use history. Clear patterns in the distribution of some species reflect this land use history (Thompson et. al. in press). . The distribution of species as a result of the land use history interacts with hurricane disturbance, as those species colonizing the northern part of the LFDP are more susceptible to hurricane damage (Zimmerman et. al. 1994). Major hurricanes struck the forest area of the LFDP in 1928 and 1932 and after a 66 year period with relatively little hurricane damage, the forest was struck by Hurricane Hugo struck in 1998 and Hurricane Georges in 1989, both hurricanes causing significant damage.