By Jill Thompson
The processes that maintain plant species diversity and the structure of tropical forests 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 forest community. Factors that we believe contribute to tropical forest structure and species composition include the past history of the forest and physical environment, 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 our Environmental Setting). Information from the LFDP contributes to the efforts of the Center for Tropical Forest Science (CTFS) network of large tropical forest plots that was set up 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 investigate 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 because of its history of land use of different intensities, and past hurricane damage (see Disturbance and Recovery). These two disturbance types interact to influence the forest community dynamics and distribution of species across the LFDP. The northern area of the LFDP (approximately two thirds of the plot) was disturbed by tree felling and farming until 1934. Farming ended when the land was purchased by United States Department of Agriculture (USDA) Forest Service. The southern third of the plot was not farmed, and suffered low intensity disturbance from selective logging in the 1940’s. 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. 2002). The distribution of species resulting from 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 1989 and Hurricane Georges in 1998, both hurricanes caused significant damage. In September 2017 Hurricane Maria struck the forest and caused major damage to the forest canopy and initial observations suggest that a greater number of trees were broken off or tipped up than either during Hurricane Hugo or Georges. A damage assessment to trees in the LFDP from Hurricane Maria is currently underway. The understory is rapidly being filled with herbaceous plants and saplings, particularly of pioneer species.
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).
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 DBH (stem diameter at a 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 six full censuses, census 1990-1993 1995, 2000-2002, 2005, 2011 and 2015-20016. In all censuses individual stems were tagged, identified and measured for DBH. In censuses from 2000 all stems were also mapped. For multiple stemmed plants, all stems were separately tagged, and the group of stems that represented 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 1989. This reconstruction showed there were 89 species >= 10 cm D130 in the LFDP. The most common were Prestoea acuminata var. montana (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 with low intensity land use, while Casearia arborea dominated the northern area of the LFDP (Figure 1) which had greater intensity land use prior to 1934. The presence in the northern section of fruit trees commonly used in agriculture, mango and coffee for example, also suggests human land use. This land use history and a multivariate analysis of species distribution and environmental factors showed that the species composition across the LFDP 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., 2002). 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.
The number of stems between 1 and 10 cm DBH was relatively high during the first census of this size class of stems (1992-1993), following the canopy opening caused by Hurricane Hugo in 1989. We tagged 91,152 stems ≥1 <10 cm DBH, representing 135 species (not counting ferns and Musaceae species). The number of stems in this size class had substantially decreased by the second census (1995) as a result of growth into the ≥10 cm DBH size class or mortality, as the forest canopy recovered from the 1989 hurricane damage and the understory became darker. The number of species had also decreased by the second census as pioneer species and individuals of rare species died. A summary of data for the first two censuses are shown in the table below. 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., 2004).
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 DBH showed much change in relative abundance, since those stems ≥ 10 cm DDBH 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. It showed an 8-fold increase in number of stems (≥10 cm DBH) as a result of light reaching the ground after canopy destruction and the subsequent rapid growth of this species. Comparing the relative abundance of tree species recorded in censuses between 1989 and 2000 shows that the forest composition changed relatively little and that the hurricanes did not establish a landscape-wide successional trajectory (Zimmerman et al 2010). The results emphasized that the land use history, through the effects on the canopy structure, cause complex differences in the spatial dynamics of the understory after a hurricane (Zimmerman et al 2010).
The community dynamics recorded through the 6 censuses of the LFDP are summarized in a recent paper by Hogan et al (2016a) which shows the dramatic changes in stem numbers over 21 years. The effect of the past land use history and how this augmented the plant community relationships is described in Hogan et al (2016b see figure 2 below).
To predict the future of this tropical forest we developed 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. The version of the model for the LFDP includes hurricane damage and its affects on survival and mortality (Uriarte et. al. 2009). The model predicted that over the next 200 years a new forest type would develop that would even out the distribution of tree species across the LFDP and that the special distribution of dominant tree species caused by past land use would disappear over time. The time taken for this novel forest composition will be influenced by the temporal distribution of hurricane frequency and intensity.
Recent analyses have involved cross site comparisons with many CTFS forest plots to look at carbon storage and dynamics (Poorter et al 2015), the effect of plant functional traits (Kunstler et al 2016) and the effect of biodiversity climate and soil on the functioning of tropical forest (Poorter et al 2018). Other papers involving the LFDP research can be found in the Luquillo LTER publications on the web site.
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 interaction between land use history and the cumulative effects of Hurricanes Hugo, Georges and Maria. 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. The LFDP research has contributed to many articles that utilize the Centre for Tropical forest network of large forest plots for comparisons, to develop theory and understanding of tropical forest structure and function and the inpact of climate change. See the Luquillo LTER publications for additional articles about the LFDP.
To request permission to conduct research on the LFDP:
The LFDP Committee MUST approve in advance any research in the LFDP. A form to request permission is available. Please contact the LFDP research manager to obtain a form and request permission at email@example.com. Further information on the LFDP can be obtained from Dr Jess Zimmerman (firstname.lastname@example.org)
To download the data for analyses see http://forestgeo.si.edu/