Conversion of abandoned cattle pastures to secondary forests and plantations in the tropics has been proposed as a means to increase rates of carbon (C) sequestration from the atmosphere and enhance local biodiversity. We used a long‐term tropical reforestation project (55–61 yr) to estimate rates of above‐ and belowground C sequestration and to investigate the impact of planted species on overall plant community structure. Thirteen tree species (nine native and four nonnative species) were planted as part of the reforestation effort in the mid to late 1930s. In 1992, there were 75 tree species (>9.1 cm dbh) in the forest. Overall, planted species accounted for 40% of the importance value of the forest; planted nonnative species contributed only 5% of the importance value. In the reforested ecosystem, the total soil C pool (0–60 cm depth) was larger than the aboveground C pool, and there was more soil C in the forest (102 ± 10 Mg/ha [mean ± 1 se ]) than in an adjacent pasture of similar age (69 ± 16 Mg/ha). Forest soil C (C 3 ‐C) increased at a rate of ∼0.9 Mg·ha −1 ·yr −1 , but residual pasture C (C 4 ‐C) was lost at a rate of 0.4 Mg·ha −1 ·yr −1 , yielding a net gain of 33 Mg/ha as a result of 61 years of forest regrowth. Aboveground C accumulated at a rate of 1.4 ± 0.05 Mg·ha −1 ·yr −1 , to a total of 80 ± 3 Mg/ha. A survey of 426 merchantable trees in 1959 and 1992 showed that they grew faster in the second 33 years of forest development than in the first 22 years, indicating that later stages of forest development can play an important role in C sequestration. Few indices of C cycling were correlated with plant community composition or structure. Our results indicate that significant soil C can accumulate with reforestation and that there are strong legacies of pasture use and reforestation in plant community structure and rates of plant C sequestration.
The philosophy of research in the Long-Term Ecological Research (LTER) program expanded what I learned in graduate school from H. T. Odum by providing an approach for a holistic understanding of ecological processes in the tropics. Participation in the LTER program enabled collaborations with many talented people from many parts of the world and enabled the mentoring and education of a new cadre of tropical natural and social sciences students. By expanding the opportunities for research and analysis at larger scales, the LTER program allowed me to address tropical ecosystem responses to such phenomena as hurricanes, floods, landslides, and past land uses and to do so at the appropriate scales of time and space. Paradigms of tropical forest resilience and adaptability in the Anthropocene emerged from research at the Luquillo (LUQ) LTER site. I first became aware of the LTER program in 1978 as I walked by the White House in Washington, DC, with Sandra Brown, then an intern on the President’s Council on Environmental Quality (CEQ), and Wayne Swank, a US Forest Service employee on detail with the National Science Foundation (NSF). I was a staff member at CEQ, and W. Swank explained to us a new long-term ecological research program that he was helping develop at the NSF. Although the first cadre of sites appeared to have been selected, I was immediately captured by the concept and expressed my interest in developing a proposal for a tropical site in Puerto Rico. Little did I know at the time that my whole scientific career was about to change, in part because of the LTER program, but also because I was to become a US Forest Service scientist. The first 30 years of my US Forest Service career would be heavily influenced by the LTER program and the people I worked with while developing a new way of thinking about tropical forest ecosystems. I am an ecologist trained at the Universities of Puerto Rico and North Carolina at Chapel Hill. My experience before becoming part of the LTER program involved (1) teaching at the University of Florida at Gainesville and (2) government work at the Commonwealth (Puerto Rico Department of Natural Resources) and federal (President’s Council on Environmental Quality) levels.
Given that ecological effects of disturbance have been extensively studied in many ecosystems, it is surprising that few quantitative syntheses across diverse ecosystems have been conducted. Multi-system studies tend to be qualitative because they focus on disturbance types that are difficult to measure in an ecologically relevant way. In addition, synthesis of existing studies across systems or disturbance types is challenging because sufficient information needed for analysis is not easily available. Theoretical advances and improved predictions can be advanced by generalizations obtained from synthesis activities that include multiple sites, ecosystems, and disturbance events. Building on existing research, we present a conceptual framework and an operational analog to integrate this rich body of knowledge and to promote quantitative comparisons of disturbance effects across different types of ecosystems and disturbances. This framework recognizes individual disturbance events that consist of three quantifiable components: (1) environmental drivers, (2) initial system properties, and (3) physical and biological mechanisms of effect, such as deposition, compaction, and combustion. These components result in biotic and abiotic legacies that can interact with subsequent drivers and successional processes to influence system response. Through time, a coarse-scale quasi-equilibrial state can be reached where variation in drivers interacting with biotic processes and feedbacks internal to the system results in variability in dynamics. At any time, a driver of sufficient magnitude can push the system beyond its realm of natural variability to initiate a new kind of event. We use long-term data from diverse terrestrial ecosystems to illustrate how our approach can facilitate cross-system comparisons, and provide new insights to the role of disturbance in ecological systems. We also provide key disturbance characteristics and measurements needed to promote future quantitative comparisons across ecosystems.
Abstract Background Categorization of topographical features into landform type is a long-standing method for understanding physiographic patterns in the environment. Differences in forest composition between landform types are driven primarily by concurrent differences in soil composition and moisture, but also disturbance regime. Many studies have focused on the interaction between fire disturbance, succession, and landforms, but the effects of hurricane disturbance on compositional differences between landforms are poorly understood. In the study presented here, we assess compositional and structural differences between landform types in the tree community of a young sub-tropical forest that is frequently subjected to hurricanes. Specifically, we ask whether the tree community (1) changed structurally over the study period, (2) experienced compositional change over the study period, (3) is compositionally different between landform types, and (4) exhibits compositional change mediated by landform type. Results The tree community experienced significant structural change over the course of our study, but compositional change was only significant for some landforms. Conclusion Despite large-scale, intense, and frequent hurricane disturbance to our study system, compositional change in the tree community was localized and only significant for some landform types.
Urban activities and related infrastructure alter the natural patterns of stream physical and chemical conditions. According to the Urban Stream Syndrome, streams draining urban landscapes are characterized by high concentrations of nutrients and ions, and might have elevated water temperatures and variable oxygen concentrations. Here, we report temporal and spatial variability in stream physicochemistry in a highly urbanized watershed in Puerto Rico. The main objective of the study was to describe stream physicochemical characteristics and relate them to urban intensity, e.g., percent impervious surface cover, and watershed infrastructure, e.g., road and pipe densities. The Rio Piedras Watershed in the San Juan Metropolitan Area, Puerto Rico, is one of the most urbanized regions on the island. The Rio Piedras presented high solute concentrations that were related to watershed factors, such as percent impervious cover. Temporal variability in ion concentrations lacked seasonality, as did all other parameters measured except water temperature, which was lower during winter and highest during summer, as expected based on latitude. Spatially, stream physicochemistry was strongly related to watershed percent impervious cover and also to the density of urban infrastructure, e.g., roads, pipe, and building densities. Although the watershed is serviced by a sewage collection system, illegal discharges and leaky infrastructure are probably responsible for the elevated ion concentration found. Overall, the Rio Piedras is an example of the response of a tropical urban watershed after major sewage inputs are removed, thus highlighting the importance of proper infrastructure maintenance and management of runoff to control ion concentrations in tropical streams.
