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Transcript
Luquillo LTER 2001-2002 ACTIVTIES AND FINDINGS Project Activities and Findings LUQ-LTER began in 1988. This report covers the period 2001-2002. Literature cited are LUQ-LTER publications during the 2001-2002 period. The Luquillo Long-Term Ecological Research (LUQ-LTER) program takes place in the Luquillo Mountains of Puerto Rico. The major question of LUQ-LTER has been how disturbance, the physical environment, and the biota interact to shape the structure and function of a tropical ecosystem. Results help with management of ecosystem services such as conservation of biodiversity, sequestration of carbon, and supply of fresh water (J. Clark et al. 2001, Scatena 2001, Lugo 2002). LUQ-LTER is multi-disciplinary and involves many researchers and students, using the tools of long-term field experiments, short-term mechanistic experiments, monitoring of populations and processes, and simulation modeling. Primary research locations are in 'tabonuco' forest at El Verde Research Area and the Bisley Experimental Watersheds, in higher elevation forests near Pico del Este, and in several streams in the Luquillo Mountains. LUQ-LTER also participates in cross-site studies, comparing Luquillo with other LTER and tropical forest sites. From 1988 the focus of LUQ-LTER has been on natural disturbance and recovery of forest and stream ecosystems. Disturbances are 'experiments', or large-scale manipulations that can reveal mechanisms of recovery which ultimately control the structure and function of an ecosystem. Natural disturbances during LUQ-LTER have included two major hurricanes, landslides, droughts, and floods. Hurricane Hugo struck in the second year of study and Hurricane Georges struck in the ninth year, providing before-and-after perspectives on forest dynamics (changes in tree species composition and forest structure), animal population dynamics and food webs, and ecosystem function (energy flow and nutrient cycling). The main question of LUQ-LTER studies on forest dynamics are: what factors determine the current and changing distributions and abundances of tree species, the overall diversity of tree species, and the three-dimensional vegetation structure in the tabonuco forest? We study forest dynamics mainly in the 16-ha Luquillo Forest Dynamics Plot (LFDP) at El Verde (Thompson et al. 2002) and in plots at Bisley. The LFDP is an especially intensive project: all non-liana, woody stems 1.0 DBH on the plot are identified, mapped, measured for DBH, and permanently tagged, totaling about 70,000 stems. We are conducted the third census of this plot in 2001-2002, with the help of many students and financial support from the Andrew Mellon Foundation. This longterm, large-scale, and detailed study is needed to understand the patterns and dynamics of tree species in a species-rich, heterogeneous tropical forest. Starting in 2001-2002, forest dynamics on the plot are being modeled with the forest model SORTIE, as a separately funded NSF project. A study on the effects of hurricane debris on seedling communities showed that, broadly speaking, removing hurricane debris increased diversity and density of woody seedlings and decreased mortality (Guzmán-Grajales, in prep.). We have also continued in the past year to monitor vegetation changes on landslides and to carry out experiments that explore the mechanisms of those changes (Myster 2001, Walker 2001, Frizano et al. 2002). One experiment concerned the effects of nutrient additions. Expectations about the responses of species in functional groups such as pioneer and nonpioneer species were not met. These results support the concept that individualistic responses of species contribute to the promotion and maintenance of species diversity in the tropics. In a study of alien species in different forest types in the Luquillo Mountains, preliminary results showed that the average number of species within the 10 x 10 plots in the different forest types varies between 3 species in the mangrove forest to 35.44 in the tabonuco forest (G. González, unpublished data). All forest types had high numbers (between 88% and 100%) of native species. The amount of non-natives species are represented by less than 3% in all forests. The number of native species fluctuates between 22 and 35 for all the forests except the Pterocarpus and the mangrove forests which have less than 8. The highest percentage of introduced species occurs in the lowland moist forest with 8.5%. There were no introduced species in the palm, Pterocarpus and mangrove forests. The main question of LUQ-LTER animal studies is similar to that for forest dynamics: What factors control the distributions and abundances of species and the diversity of species? However, animals are more explicitly studied as part of terrestrial and stream food webs, including both consumer and detritus pathways. We continue to census animals regularly to reveal short- and long-term population changes, especially in response to disturbance. Groups include insects (terrestrial and aquatic) (Bloch & Weiss 2002), birds, snails, frogs, bats (Jones et al. 2001) and shrimp in streams. For several species we have exceptionally long-term records. Several terrestrial groups are repeatedly censused at designated points on the LFDP, which puts the animal data into a data-rich habitat context. Food web analysis is based on observation and experiment. In 2000-2001 we conducted terrestrial food web experiments by adding or excluding animals to microcosms created with large cages in the forest (Beard 2001). Similarly, stream food webs were studied using removal experiments in natural streams, and in artificial streams in work funded by a separate NSF grant. Another grant is funded work on herbivory by canopy insects and the effects of resulting frass on soil processes. Food webs are paths of energy flow and nutrient cycling, linking LUQ-LTER plant, animal, and ecosystem studies (Kilham & Pringle 2001, Covich & Crowl 2002). The general question of LUQ-LTER ecosystem studies is: how do disturbance, physical environment, and biota interact to control energy flow and nutrient cycling? The varied disturbance regime; gradients in climate, soil, and topography; and high species diversity make the Luquillo Mountains an appropriate setting for this question. A foundation of this ecosystem work is long-term monitoring in LUQ-LTER of vegetation biomass, climate, and biogeochemical flux in rain, throughfall, and stream water along environmental gradients in the mountains. Building on this foundation, in 2000-2001 LUQ-LTER studied long-term patterns in litterfall productivity, tree growth, and decomposition in relation to climate (González and Seastedt 2001, González et al. 2001) and nutrient and plant community characteristics at sites along an elevational gradient (Cox et al. 2002, Pett-Ridge & silver 2002f). Variables of special interest were soil oxygen availability (typically low at high elevations) and deposition of 'green' litter (leaves stripped from trees during a storm). We studied the effects of green litter using reciprocal litter bag experiments to distinguish litter versus site effects. A new topic in our research is the newly understood nitrogen cycling pathway: dissimilatory nitrate reduction to ammonium (Silver et al. 2001). Continuing topics were fungi and decomposition (Cantrell et al. 2001 a,b, Groffman et al. 2001, Lodge 2001a, b, Myster 2002); earthworms (Zou & González 2001, Lachnicht et al. 2002, Liu & Zou 2002) and shrimps, important components of the decomposer guilds in terrestrial and steam systems (Crowl et al. 2001, March et al. 2001, Peterson et al. 2001); controls on dissolved organic carbon and nitrogen fluxes in streams (McDowell 2001, Mulholland et al. 2001,); flood effects on litter inputs and stream organisms; and residence time of coarse woody debris in streams (Crowl et al. 2002). Detrital processing rates and export were compared among four streams within the LUQ that differ in decapod abundance and community composition. These analyses show that long-term detrital dynamics are significantly affected by the species composition and abundance of the dominant shrimp species in these streams (Crowl et al. 2001). For a comparative perspective on how biodiversity and climate affect decomposition and soil processes LUQ-LTER participated in the LIDET cross-site studies (McDowell 2002. LUQ-LTER also participated in global syntheses of primary production (D. Clark et al. 2001a, b). Another project, sponsored by the both USFS and LUQ-LTER program, made extensive use of the Luquillo LTER data base and demonstrated how deforestation of the coastal plain can affect cloud and rainfall inputs to the upper Luquillo Mountains (Van der Molem, in prep.). During 2001-2002 LTER-LUQ progressed toward synthesis of ecosystem studies at a landscape scale by developing models of spatial and temporal variation in forest productivity, soil carbon, and nitrogen. We used geo-spatial statistical techniques to examine the spatial variation and relation of soil organic carbon and soil moisture, and we used TOPOPROD, a spatially explicit forest productivity model, to simulate spatial patterns of gross primary production, net primary production, and respiration (Wang et al. 2002a, b). Beginning in 1994 LUQ-LTER focused more on human disturbance, including, among other things, logging, clearing for pasture, farming, and water diversion from streams (López del Mar et al. 2001, Lugo et al. 2001, Zimmerman 2001). Some the resulting impacts are apparent in a recent analysis of tree species composition in the LFDP, where most land uses ceased c. 70 yr ago (Thompson et al. 2002). This past year we also studied patterns in above and belowground C pools over 65 yr of forest regrowth, the impacts of N saturation on tropical ecosystems, and the effects of water diversion on stream organisms (Pringle 2001 a,b,c). The multiplicity of disturbance types and frequencies, natural and human, in the Luquillo Mountains has increased appreciation of interactions between disturbances. Training and Development One post-doctoral associate, John Barone, was supported by this project. He conducted a study of plant community variation along the elevational gradient in the Luquillo Mountains. Eight graduate students at the University of Puerto Rico and mainland institutions were supported by the project. During the summers, 10 students conducted research in the Research Experience for Undergraduate Program. Ten teachers from six high schools participated in the Schoolyard LTER program. During the year they attended workshops on measuring forest dynamics and sampling and analysis of soils. Outreach In both 2001 and 2002 LUQ-LTER organized a symposium at the University of Puerto Rico entitled “Long-Term Ecological Research in Puerto Rico”. About 120 scientists and students interested in ecological issues attended each year. This year, the publication of the Science Teachers Association of Puerto Rico, Acta Cientifica, published a special issue of research results from high schools in the Schoolyard LTER Program. The special issue included seven articles with research results ranging from stand nutrient cycling in secondary forests to stand dynamics in mangrove forests. People from John Thomlinson's lab have led a reforestation project on the island of Vieques. This project, Sembrando Paz, had a major accomplishment in April when the initial planting of over 100 trees was conducted on Monte Carmelo during the Earth Day weekend. The activity was attended by local citizens, a state senator, Viequense environmental activists, university and high school students, representatives of municipal and state agencies, and members of the press. The project continues, having gained the support of the PR Department of Natural and Environmental Resources. With funding from USDA Forest Service (State and Private Forestry), we made two environmental outreach posters on water quality and watershed protection for the Luquillo Experimental Forest that are passed out to school children and sold at the Forest Service's Visitors Center. CONTRIBUTIONS Contributions within discipline The principal discipline of the Luquillo Long-Term Ecological Research (LUQLTER) program, in the Luquillo Mountains of Puerto Rico, is life sciences. More specifically, the disciplines are: population biology and ecosystem ecology. Population biology is broadly defined as the study of the distribution and abundance of organisms. Ecosystem ecology is defined as study of the flow of energy and cycling of nutrients in an environment. Our work in these areas has developed new information as well as new methods in these fields. Most of these contributions result from work over many years. In this report we describe this long-term progress, while highlighting contributions from 2000-2001. Population Biology: Contributions LUQ-LTER has made many contributions to population biology, in two main areas. First, we have demonstrated the pervasive effects of disturbance (a discrete event that disrupts ecosystem, community, or population structure and alters the environment) on the distribution and abundance of organisms. Second, we have described and experimented on the food web (array of feeding relationships among organisms) of an ecosystem. Understanding the effects of disturbance is a key to managing any ecosystem in the face of human impacts. Understanding food web relations is a key to managing any species population. The disturbance regime refers to the size, intensity, and frequency of different types of disturbance that occur over a particular landscape. At our site we have developed an unusually good record of this regime, which includes: hurricanes and lesser windstorms, treefalls not associated with storms, floods, droughts, and landslides, as well as several types of human-caused disturbance. In the past year we have increased our understanding of the spatial variation of human impacts on streams, including dams, water diversion, and fishing. LUQ-LTER has amassed a detailed record of the population biology response to disturbance, based on both observation and experiment. Among other things, we have described mechanisms of primary succession (development of vegetation in a completely unvegetated site) on landslides, leading to a better understanding of the many factors that control this process and that could be used for forest restoration. In 2000-2001 we experimentally removed some colonizing plant species from landslides, to determine if those species were facilitating or inhibiting colonization by other species and affecting overall vegetation development. To test methods for enhancing vegetation development we added artificial bird perches to landslides, to see if perches increase the number of seeds dispersed to landslides by birds, and we added various kinds of organic matter to possibly stimulate growth. In other studies of response to disturbance LUQ-LTER has helped pioneer methods and models that can be used elsewhere. To describe changes in tree populations, we established a large-scale (16 ha), long-term, intensive study plot in the forest, linking our work to a network of such plots in the tropics coordinated by the Smithsonian Institution. Our plot is the largest, long-term, intensive forest dynamics plot in the Caribbean, and its disturbance regime, including hurricanes and well-known historical land uses, make it unique in the Smithsonian plot network. In 2001 we continued our third inventory of the plot, adding to our understanding of impacts from both natural and human disturbance. This inventory is of special interest because it follows the second hurricane in 10 years on the site and thus may help us predict the possible consequences of the predicted increase of hurricane frequency with global warming. To understand mechanisms of changes in tree populations, in the past year we began using the SORTIE forest dynamics model. This promotes comparisons with other forests, since the model has also be used in Connecticut and British Columbia. The food web of an ecosystem is the total array of feeding relationships among organisms. Since feeding has an obvious impact on survival and reproduction of organisms, understanding feeding patterns gives insight into control of a species' population, and, taking all species into account, insight into control of the distribution and abundance of species in an ecological community. Last year saw completion partly under LUQ-LTER auspices of a landscape-level analysis of fruit availability for the endangered Puerto Rican Parrot. This array of feeding relationships making up a food web can be complex, especially in species-rich tropical systems. The web is composed of producers (plants), consumers (animals that consume live or freshly killed organisms), and detritivores (consumers of dead organisms), all in cross-linked chains. Based on long-term and detailed field studies LUQ-LTER has constructed the most nearly complete picture of a food web for any tropical forest (The Food Web of a Tropical Rain Forest, Douglas Reagan & Robert Waide, editors) and thus provided a model and standard for such studies elsewhere. We are building on this description of food web patterns with experimental approaches to elucidate mechanisms that produce the patterns. One set of experiments completed in the past year used cages to exclude or increase the density of different consumers, thereby revealing their impact on the ecosystem. Another set of experiments being planned in 2001 involves additions and subtractions of detritivores and organic matter to reveal their respective roles in nutrient cycling, a subject of ecosystem ecology. Indeed, both disturbance and food web dynamics, important in population biology, relate strongly to ecosystem ecology, because organisms, nutrients, and energy all interact. LUQ-LTER scientists also presented overviews and syntheses of biological diversity (Willig 2002a, b). Ecosystem Ecology: Contributions An ecosystem is all the interacting parts of the physical and biological worlds; however, ecosystem ecology is usually considered to be the study of energy flow and nutrient cycling in an ecosystem. Rates of ecosystem processes and storage of nutrients in different parts of an ecosystem depend on physical conditions (e.g., climate, soil), species composition (because species differ in nutrient and energy use), and disturbance (which redistributes nutrients and energy). The results of ecosystem ecology help us maintain 'ecological services' that are valued by society, such as conservation of biodiversity, sequestration of carbon, and supply of fresh water. As mentioned, the LUQ-LTER site has suffered two severe hurricanes, allowing us to compare ecosystem states before and after a large-scale, major disturbance. Hurricanes are a large-scale experiment that reveals the function of an ecosystem and that would be impossible to carry out by design. Hurricanes redistribute large amounts of biomass and nutrients (volumes of downed leaves and wood) and energy (light penetrating the damaged forest). A striking previous result was how efficiently some plants responded to the changes with growth that restored above-ground biomass and prevented loss of nutrients. The third inventory of the forest dynamics plot, underway in 2001, as mentioned above, will allow us to compare some of these patterns through two hurricanes. Moving over the landscape, physical conditions, species composition, and disturbance regime all change along the elevational gradient in the Luquillo Mountains. LUQ-LTER studies along this gradient in the past year have begun to reveal how these factors control ecosystem processes. We found, for example, that litterfall (production of dead plant material), and thus productivity, decreases with elevation and is correlated with a small decrease in temperature. Control of productivity by temperature has clear implications for ecosystems that may be affected by global warming . An important topic of LUQ-LTER research has been the flux of carbon (C) and soil organic matter (SOM). Work in the past year in a secondary forest at our site showed that soil C accumulated rapidly and was not affected by species composition, while aboveground C accumulation was highly variable. Previously we used the CENTURY model to describe disturbance effects on soil organic matter (SOM). SOM affects forest recovery and productivity and has implications for the global carbon budget and thus global warming. We recently developed a spatial version of CENTURY for the entire Luquillo Mountains area to test how variation in climate, vegetation, and soils affects SOM. Further, we then used the TOPOPROD to model gross and net primary productivity on the gradient (Wang et al. 2002a, b). Thus, climate appears to strongly influence, directly or indirectly, the way different communities in the LEF are able to respond to a disturbance such as a hurricane. Another issue in global change is elevated nitrogen (N) addition to ecosystems, which can drastically alter productivity. Ecosystems process N in several ways. This past year LUQ-LTER began work on dissimilatory nitrate reduction to ammonium (DNRA) in upland forest soils, a process that is only recently been understood. We found that DNRA in Puerto Rico is about five times greater than the better understood process of denitrification. Also in the past year we continued participation in comparative studies of N cycling in streams among LTER sites. This has shown that the smallest streams are the most effective at retaining and processing N inputs. Work on stream biota in 2000-2001 showed that nutrient losses in streams are related to the presence of certain kinds of shrimp, which consumers detrital input from the forest. A continuing contribution of LUQ-LTER are studies linking terrestrial and stream environments. Among LUQ-LTER ecosystem studies another area of comparative work that advanced over the past year was review and synthesis of studies on the relationship between biodiversity and net primary productivity. Given declines in biodiversity in some habitats, this relationship is important to the human economy. Contributions to other disciplines Outside the life sciences LUQ-LTER has contributed in the past year to an understanding of the economics and ecosystem services of the Luquillo Mountains (Scatena et al. 2002), ethnobotany (Vogt et al. 2002), and the development of methods of data acquisition and management(Meléndez-Colom & Baker. 2002). At our study site we established and maintained, on an experimental basis, climatic monitoring devices that transmit data by radio to central data banks. This wireless technology could greatly improve quality and quantity of available field data in environmental biology. There is also a need to develop ways of managing and synthesizing the increased amount of data being collected (Meléndez-Colom & Baker. 2002). The cross-site database on biodiversity and net primary productivity mentioned above is being used to test new ways to manage ecological data. This work is LUQ-LTER's contribution to a project undertaken by the LTER Network Office, the National Center for Ecological Analysis and Synthesis, and the San Diego Supercomputer Center. Contribution to human resource development The University of Puerto Rico is the largest Hispanic institution of higher learning in the U.S. system. Our project has created a magnet drawing Hispanic and undergraduate students to study and conduct research in environmental sciences. Through the Schoolyard LTER program, we expose local high school teachers and students to ecological research. In collaboration with Wheeling Jesuit University we continued developing software that allows middle school students to learn tropical ecology using our research program as an example &&& web site). Initial development of this project was conducted using SGER funding of which the final report can be found in this reporting system (proposal ESI-9908438). Contributions to Resources for Research and Education El Verde Field Station is one of the principal field sites for the project. The staff and facilities maintenance are supported by matching funds to the LUQ-LTER grant from the University of Puerto Rico. In addition to LUQ-LTER research, the station supports student learning (field trips) as well as a location for research by investigators and students from both Puerto Rican and mainland institutions. Our web site is frequently used by educational institutions as a source of information on tropical ecology. Our site in included as a link on many other educationally-oriented web pages. The analytical laboratory of the International Institute for Tropical Forestry (USDA Forest Service) provides free analyses to undergraduate and graduate students performing research projects in the Luquillo Experimental Forest. Contributions Beyond Science and Engineering LUQ-LTER studies of dams and water withdrawals on stream organisms by this project caused the Puerto Rico Water Authority to modify the practices they use in acquiring drinking water from local streams.