Forest Dynamics by Jill Thompson

The processes that maintain 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 or the Luquillo long-term ecological research grid) is a 16-hectare forest plot located near El Verde Field Station, in the Luquillo Mountains of northeastern Puerto Rico (see article on Environmental Setting in this web site). Information from the LFDP contributes to the efforts of the Center for Tropical Forest Science (CTFS) network of large tropical forest plots in order to improve our understanding of tropical forest and to predict its future (CTFS: http://www.forestgeo.si.edu/). 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 due to its history of intensive land use and hurricane damage (see article Disturbance and Recovery in this web site). 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 (USDA) 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. b, 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.

LFDP topography and soil

Topography on the LFDP has northwest-running drainages producing steep northeast and southwest-facing slopes, with an elevation across the plot which ranges from 333 to 428 m asl. The mean slope of the plot is 17% but ranges from 3 to 60%. Soils were formed in residual volcanic ash that fell in the ocean to form volcaniclastic sandstones and siltstones, which were subsequently uplifted (see Geology article). Soils are dominated by old, deeply weathered kaolinitic Oxisols (Zarzal) and Ultisols (Cristal), and young, less-developed Entisols (Coloso and Fluvaquents) and Inceptisols (Prieto) in stream channels. Zarzal, Cristal and Prieto are deep clay soils, while Coloso and Fluvequents are formed from alluvium in the stream channels (Soil Survey Staff 1995).

LFDP censusing

The LFDP was established in 1990. It is 500 m N-S and 320 m E-W and is divided into 400 20 x 20 m quadrats, with each quadrat subdivided into sixteen 5 x 5 m sub-quadrats. Censuses of all self-supporting, woody stems ≥ 1.0 cm D130 (diameter at height of 130 cm from the ground) are carried out in accordance with CTFS protocol (Condit 1998) with only minor variations. We have carried out three major censuses, in the early-1990s, mid-1990s, and during 2000-2002. In all censuses individual stems were tagged, identified and measured for D130. In the most recent census all stems have been mapped. For multiple stemmed plants, all stems were individually tagged and the group of stems representing an individual plant was recorded.

Summary of LFDP census results

Land use history

We investigated the land use history of the area, including; interpretation of canopy cover in aerial photographs taken in 1936; historical records; and interviews with local people (García-Montiel, in press). The assessment of damaged trees and the first census enabled us to reconstruct the forest as it appeared before Hurricane Hugo struck in 1998. This reconstruction showed there were 89 species >= 10 cm D130 in the LFDP. The most common were Prestoea acuminata (a palm), Dacryodes excelsa, Casearia arborea, Manilkara bidentata, Inga laurina, and Sloanea berteriana.The species composition varied markedly across the plot with Dacryodes excelsa characterizing the southern part, while Casearia arborea dominated the northern area of the LFDP (Figure 1). The presence in the northern section of fruit trees commonly used in agriculture, mango and coffee for example, also suggests human use. This land use history and a multivariate analysis of species distribution and environmental factors showed that the species composition was largely determined by the extent of human disturbance prior to 1934, when the forest tract was purchased by the USDA Forest Service (Thompson et al., in press b). Topography and soil type also influenced distribution of some species, for example, Prestoea acuminata is more common in poorly drained areas, including stream channels, and on slopes.

Hurricane impacts

The number of stems between 1 and 10 cm D130 was relatively high during the first census, as a result of the canopy opening caused by Hurricane Hugo. We tagged 91,152 stems ≥1 <10 cm D130, representing 135 species (not counting ferns and Musaceae species). The number of stems in this size class had substantially decreased by the second census as a result of growth into the ≥10 cm D130 size class or mortality as the canopy recovered from the hurricane damage. The number of species had also decreased by the second census as a result of losing individuals of some rare species and the death of pioneer species as the canopy closed. A summary of data for the first two censuses are shown in the following table. The table includes the number of individual trees (not counting multiple stems) and species for two size classes of trees (see also Thompson et al., in press a).

TABLE CENSUS

The resistance and resilience of the forest to hurricane damage was demonstrated by comparing our reconstruction of the forest before Hurricane Hugo with results of the second census that described the forest approximately six years after Hugo. Few tree species with stems ≥ 10 cm D130 showed much change in relative abundance, since those stems ≥ 10 cm D130 that died as a result of Hurricane Hugo were generally replaced in the population by in-growth from smaller size classes. It also appears that hurricane damage perpetuates the effect of land use history, as the storm damaged tree species such as C. arborea, characteristic of human disturbed areas, more so than the old growth species such as D. excelsa in the less disturbed area (Zimmerman et al. 1994). The only species to show a major change in population size after Hurricane Hugo was Cecropia schreberiana.This species requires high light levels for seed germination and growth, and showed an 8-fold increase in number of stems (≥10 cm D130) as a result of the canopy destruction and the consequent increase in light availability.Figure 1 compares the abundance of tree species (stems ≥10 cm) in the forest at the time of Hurricane Hugo versus their abundance about six years after Hugo.

Future research in the LFDP

In the future we will continue to use the unique history of this tropical forest, represented by the LFDP, to continue our investigations into effects of human and hurricane disturbance. This includes analyzing the effects of Hurricane Georges that damaged the forest between the second and third censuses. We will focus on the resistance and resilience of forest structure and species composition, and the effect of the redistribution of biomass during hurricanes on the recovery of biomass and nutrient capital in the years following hurricanes. We will also continue our contributions to studies on the forest vegetation as habitat structure and a food source for animal populations, and the processes of decomposition and carbon cycling. Through our collaboration with CTFS we will contribute to the efforts to investigate processes that determine the number and diversity of plant species, and the complex structure of tropical forest.

To predict the future of this tropical forest we are developing the SORTIE model in collaboration with Charles Canham, of the Institute of Ecosystem Studies (Millbrook, New York). SORTIE is a spatially explicit population model that has been used successfully in temperate zone forests. The model incorporates tree growth and survival and the light experienced by individual trees. We are developing this model to include hurricane damage and its affects on survival and mortality.

See the CTFS website (http://www.ctfs.si.edu/doc/index.php) for additional articles about the LFDP.

Asking for permission to work on the LFDP

The LFDP Committee MUST approve in advance any research in the LFDP.  form to request permission. You can contact Jill Thompson for further information.

Literature cited

Brokaw, N. V. L. 1998.Cecropia schreberiana in the Luquillo Mountains of Puerto Rico.The Botanical Review 64:91-120.

Condit, R. 1998.Tropical Forest Census Plots. Springer, Berlin.

García-Montiel, D. C. In press. La presencia humana en los bosques neotropicales húmedos. In M. Guariguata and G. Kattan (eds. ).Ecología y Conservación de Bosques Neotropicales. Libro Universitario Regional del Instituto Tecnológico, San José, Costa Rica.

Soil Survey Staff. 1995. Order 1 Soil Survey of the Luquillo Long-Term Ecological Research Grid, Puerto Rico. United States Department of Agriculture, Natural Resources Conservation Service, Lincoln, Nebraska, USA.

Thompson, J., Brokaw, N., Zimmerman, J. K, Waide, R. B., Everham, III, E. M., and Schaefer, D.A. In press a. Luquillo Forest Dynamics Plot. In E. Losos, R. Condit, and J. LaFrankie (eds.).Tropical Forest Diversity and Dynamism: Results from a Long-Term Tropical Forest Network.Smithsonian Institution.

Thompson, J., N. Brokaw, J. K. Zimmerman, R. B. Waide, E. M. Everham, III, D. J. Lodge, C. M. Taylor, D. García-Montiel, and M. Fluet. 2002. Land use history, environment, and tree composition in a tropical forest.Ecological Applications 12(5):1344-1363..

Zimmerman, J. K., E. M. Everham, III, R. B. Waide, D. J. Lodge, C. M. Taylor, and N. V. L. Brokaw. 1994. Responses of tree species to hurricane winds in subtropical wet forest in Puerto Rico: implications for tropical tree life histories.Journal of Ecology 82:911-922.

Results from the 16-ha Luquillo Forest Dynamics Plot

MISSING

Figure 1 - Results from the 16-ha Luquillo Forest Dynamics Plot showing (lefthand graph) the stability of species composition in response to Hurricane Hugo (1989). Note the exceptional increase in the abundance of the pioneer Cecropia schreberiana (Brokaw 1998). The temporal response to hurricane damage is in contrast to the legacy of human disturbance evident in species distributions (right-hand graph). The northern (upper) portion of the plot was clear-cut in the 1920s and the present-day forest is dominated by Casearia arborea, a successional species. The lowermost portion of the plot was subject to selective logging and maintains a near native composition of species, including tabonuco (Dacryodes excelsa; Thompson et al., in press b).