Luquillo Experimental Forest

Carbon Sequestration and Plan Community Dynamics Following Reforestation of Tropical Pasture

Silver W.L., Kuppers L.M., Lugo A.E. et al. Carbon Sequestration and Plan Community Dynamics Following Reforestation of Tropical Pasture. Ecological Applications, Vol 14(4), 2004 pp 1115-1127.

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 6 10 Mg/ha [mean 6 1 SE]) than in an adjacent pasture of similar age (69 6 16 Mg/ha). Forest soil C (C3-C) increased at a rate of ;0.9 Mg·ha21·yr21, but residual pasture C (C4-C) was lost at a rate of 0.4 Mg·ha21·yr21, 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 6 0.05 Mg·ha21·yr21, to a total of 80 6 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.

Variation in Susceptibility to Hurricane Damage as a Function of Storm Intensity in Puerto Rican Tree Species

Canham, Charles D.; Thompson, Jill; Zimmerman, Jess K.; Uriarte, Maria. 2010 Variation in Susceptibility to Hurricane Damage as a Function of Storm Intensity in Puerto Rican Tree Species. Biotropica, 42 (1). 87-94. 10.1111/j.1744-7429.2009.00545.x

One of the most significant challenges in developing a predictive understanding of the long-term effects of hurricanes on tropical forests is the development of quantitative models of the relationships between variation in storm intensity and the resulting severity of tree damage and mortality. There have been many comparative studies of interspecific variation in resistance of trees to wind damage based on aggregate responses to individual storms. We use a new approach, based on ordinal logistic regression, to fit quantitative models of the susceptibility of a tree species to different levels of damage across an explicit range of hurricane intensity. Our approach simultaneously estimates both the local intensity of the storm within a plot and the susceptibility to storm damage of different tree species within plots. Using the spatial variation of storm intensity embedded in two hurricanes (Hugo in 1989 and Georges in 1998) that struck the 16 ha Luquillo Forest Dynamics Plot in eastern Puerto Rico, we show that variation in susceptibility to storm damage is an important aspect of life history differentiation. Pioneers such as Cecropia schreberiana are highly susceptible to stem damage, while the late successional species Dacryodes excelsa suffered very little stem damage but significant crown damage. There was a surprisingly weak relationship between tree diameter and the susceptibility to damage for most of the 12 species examined. This may be due to the effects of repeated storms and trade winds on the architecture of trees and forest stands in this Puerto Rican subtropical wet forest.

Impact of experimental drought on greenhouse gas emissions and nutrient availability in a humid tropical forest

We excluded throughfall from humid tropical forests in Puerto Rico for a period of three months to determine how drought affects greenhouse gas emissions from tropical forest soils. We established five 1.24 m2 throughfall exclusion and five control plots of equal size in three sites located on ridges, slopes, and an upland valley dominated by palms (total of 30 plots). We measured weekly changes in carbon dioxide (CO2) and bi-weekly changes in nitrous oxide (N2O) and methane (CH4) in response to manipulation. We additionally measured the effects of throughfall exclusion on soil temperature and moisture, nutrient availability, and pH. Rainout shelters significantly reduced throughfall by 22 to 32 % and decreased soil moisture by 16 to 36% (top 10 cm). Rates of CO2 emissions decreased significantly in the ridge and slope sites (30%, 28%, respectively), but not the palm during the experimental drought. In contrast, the palm site became a significantly stronger sink for CH4 in response to drying (480% decline relative to controls), while CH4 fluxes in the ridge and slope sites did not respond to drought. Both the palm and ridge site became a sink for N2O in response to drought and the slope site followed a similar trend. Soil pH and available P decreased significantly in response to soil drying; however, available N was not affected. Variability in the response of greenhouse gas emissions to drought among the three sites highlights the complexity of biogeochemical cycling in tropical forested ecosystems, as well as the need for research that incorporates the high degree of spatial heterogeneity in experimental designs. Our results show that humid tropical forests are sensitive to climate change and that short-term declines in rainfall could result in a negative feedback to climate change via lowered greenhouse gas emissions and increased greenhouse gas consumption by soils.

Spatial and seasonal dynamics of surface soil carbon in the Luquillo Experimental Forest, Puerto Rico

Wang, Hongqing; Cornell, Joseph D.; Hall, Charles A.S.; Marley, David P. 2002. Spatial and seasonal dynamics of surface soil carbon in the Luquillo Experimental Forest, Puerto Rico.. Ecological Modelling 147 105-122.

We developed a spatially-explicit version of the CENTURY soil model to characterize the storage and flux of soil organic carbon (SOC, 0–30 cm depth) in the Luquillo Experimental Forest (LEF), Puerto Rico as a function of climate, vegetation, and soils. The model was driven by monthly estimates of average air temperature, precipitation, and potential evapotranspiration (PET), which in turn were simulated as a function of elevation, slope, and aspect using a spatially-explicit and validated model (TOPOCLIM) of solar insolation/microclimate in mountainous areas. We simulated forest gross primary productivity (GPP) and distribution of above- and below-ground biomass production using a forest productivity model (TOPOPROD). Output from TOPOCLIM and TOPOPROD models was used to run the CENTURY soil model for 1200 months under current climate conditions and in response to potential global warming. We validated our version of CENTURY soil model using 69 soil samples taken throughout the LEF. Simulated SOC storage agrees reasonably well with the observed storage (R2=0.71). The simulated SOC storage in the top 30 cm within the LEF is highly variable, ranging from approximately 20–230 Mg/ha. The rates of decomposition were especially sensitive to changes in elevation. Carbon release rates due to decomposition were close to carbon assimilation rates and ranged from 0.6–0.96 Mg/ha per year at high elevations to 1.2–1.68 Mg/ha per year at lower elevations. Our simulations indicated that differences in elevation affect decomposition and SOC content primarily by changing microclimate. Finally, we found that a projected warming of 2.0 °C is likely to result in losses of SOC in the lower and higher elevation, but increased storage in the middle elevations in the LEF.

Land Use History, Environment, and Tree Composition in a Tropical Forest

Thompson, Jill; Brokaw, Nicholas; Zimmerman, Jess K.; Waide, Robert B.; Everham, Edwin M. III; Lodge, D. Jean; Taylor, Charlotte M.; Garcia-Montiel, Diana; Fluet, Marcheterre 2002. Land use history, environment, and tree composition in a tropical forest. Ecological applications. Vol. 12, no. 5 (2002): pages 1344-1363.

The effects of historical land use on tropical forest must be examined to understand present forest characteristics and to plan conservation strategies. We compared the effects of past land use, topography, soil type, and other environmental variables on tree species composition in a subtropical wet forest in the Luquillo Mountains, Puerto Rico. The study involved stems > 10 cm diameter measured at 130 cm above the ground, within the 16-ha Luquillo Forest Dynamics Plot (LFDP), and represents the forest at the time Hurricane Hugo struck in 1989. Topography in the plot is rugged, and soils are variable. Historical documents and local residents described past land uses such as clear-felling and selective logging followed by farming, fruit and coffee production, and timber stand improvement in the forest area that now includes the LFDP. These uses ceased 40-60 yr before the study, but their impacts could be differentiated by percent canopy cover seen in aerial photographs from 1936. Using these photographs, we defined four historic cover classes within the LFDP. These ranged from cover class 1, the least tree-covered area in 1936, to cover class 4, with the least intensive historic land use (selective logging and timber stand improvement). In 1989, cover class 1 had the lowest stem density and proportion of large stems, whereas cover class 4 had the highest basal area, species richness, and number of rare and endemic species. Ordination of tree species composition (89 species, 13 167 stems) produced arrays that primarily corresponded to the four cover classes (i.e., historic land uses). The ordination arrays corresponded secondarily to soil characteristics and topography. Natural disturbances (hurricanes, landslides, and local treefalls) affected tree composition, but these effects did not correlate with the major patterns of species distributions on the plot. Thus, it appears that forest development and natural disturbance have not masked the effects of historical land use in this tropical forest, and that past land use was the major influence on the patterns of tree composition in the plot in 1989. The least disturbed stand harbors more rare and endemic species, and such stands should be protected.

Ca/Sr and 87Sr/86Sr ratios as tracers of Ca and Sr cycling in the Rio Icacos watershed, Luquillo Mountains, Puerto Rico

Julie C. Pett-Ridge, Louis A. Derry, Jenna K. Barrows
Ca/Sr and 87Sr/86Sr ratios as tracers of Ca and Sr cycling in the Rio Icacos watershed, Luquillo Mountains, Puerto Rico
Chemical Geology (2009)
Volume: 267, Issue: 1-2, Publisher: Elsevier B.V., Pages: 32-45

We investigated Ca and Sr cycling in a humid tropical forest by analyzing Ca/Sr ratios and 87Sr/86Sr ratios in soil minerals, soil exchangeable cations, soil porewater, and plant roots, wood and leaves, and calculating the relative contributions of Sr from atmospheric inputs and weathering of local bedrock. An unexpectedly large contribution of bedrock-derived Sr and presumably Ca is cycled through the vegetation, re!ecting the important role of geological processes in controlling the cycling of base cation nutrients even in a system with intensely weathered soil. This is surprising because over 99% of the Ca and Sr that was originally in the bedrock is leached out of the soil and saprolite during early stages of weathering at this site, and because there are large atmospheric inputs to the site of both sea salt and Saharan dust. Substantial differences in Ca and Sr cycling are seen on small spatial scales between a ridgetop and an adjacent steep hillslope site. Measured Ca/Sr ratios re!ect fractionation between these elements during biogeochemical cycling. Fractionation was particularly evident between wood and foliar tissue, but fractionation during soil exchange processes is also likely. In comparing the Ca/Sr ratios of plants, exchangeable cations, and bulk soils, we found that foliar Ca/Sr ratios were greater than exchangeable cation Ca/Sr ratios, which in turn were greater than soil Ca/Sr ratios, similar to patterns observed at other highly weathered tropical sites.

Leaffall Phenology in a Subtropical Wet Forest in Puerto Rico: From Species to Community Patterns

Zalamea, M. & González, G. (2008) Leaffall phenology in a subtropical wet forest in Puerto
Rico: from species to community patterns. Biotropica, 40, 295-304.

leaffall periodicity has been related to rainfall regime and dry season length. In weakly seasonal forests, where there is no marked dry season, other climatic factors could trigger leaf shed. In this study, we aimed to determine if other climatic variables (wind speed, solar radiation, photosynthetic photon flux density [PPFD], day length, temperature, and relative humidity) could be better correlated with patterns of litter and leaffall in a weakly seasonal subtropical wet forest in Puerto Rico. Leaffall patterns were correlated mainly with solar radiation, PPFD, day length, and temperature; and secondarily with rainfall. Two main peaks of leaffall were observed: April–June and August–September, coinciding with the periods of major solar radiation at this latitude. Community leaffall patterns were the result of overlapping peaks of individual species. Of the 32 species analyzed, 21 showed phenological patterns, either unimodal (16 species), bimodal (three species), or multimodal (two species). Lianas also presented leaffall seasonality, suggesting that they are subject to the same constraints and triggering factors affecting trees. In addition to solar radiation as a main determinant of leaffall timing in tropical forests, our findings highlight the importance of interannual variation and asynchrony, suggesting that leaffall is the result of a complex interaction between environmental and physiological factors.

Multi-scale analysis of species introductions: combining landscape and demographic models to improve management decisions about non-native species

Brown, K.A., Spector, S.& Wu, W. (2008)Multi-scale analysis of species introductions:
combining landscape and demographic models to improve management
decisions about non-native species. Journal of Applied Ecology, 45,

1. Non-native, invasive species can affect biological patterns and processes at multiple ecological scales. The multi-scalar effects of invasions can influence community structure, ecosystem processes and function, and the nature and intensity of ecological interactions. Consequently, efforts to assess the spread of invasive species may benefit from a multi-scale analytic approach. 2. We analysed results from landscape- and population-scale models for Syzygium jambos , a nonnative tree in the Luquillo Mountains of Puerto Rico, to demonstrate a multi-scale approach that can be used to inform management decisions about invasive plants. At the landscape-level, we used an Ecological Niche Modelling approach to predict environmentally suitable habitats for the target plant. At the population-level, we constructed matrix projection models to determine the finite rate of population increase ( λ ) for S. jambos . We then extrapolated λ values to the landscape-scale to obtain a distribution map of λ values for the Luquillo forest. 3. The landscape analyses suggested that the most environmentally suitable habitats were those most similar to where S. jambos had already been observed. The population-level analyses showed that four of the seven populations had λ values less than 1, indicating that they were projected to be below replacement. The λ distribution map showed that S. jambos growth was highest in areas where it was most common and lowest in areas where it was most rare. 4. Our analyses further suggested that the importance of different drivers of invasion and the environmental variables that mediate them appear to be strongly scale-dependent. Past disturbances seemed most important for controlling invasions at fine-spatial scales; while abiotic environmental variables modulated coarse-scale invasion dynamics. 5. Synthesis and applications. We have shown that a multi-scale analytic approach can be used to manage invasive species by simultaneously targeting susceptible life stages and rapidly growing populations in a landscape. The utility of this approach stems from an ability to: (i) map the distribution of habitats that can potentially sustain λ values above replacement; (ii) identify populations to manage or monitor during selected stages of an invasion; (iii) forecast the probability for a target species to increase above a critical threshold abundance; and (iv) set priorities for control and monitoring actions.

Biotic and abiotic controls on the ecosystem significance of consumer excretion in two contrasting tropical streams

Biotic and abiotic controls on the ecosystem significance of consumer excretion in two contrasting tropical streams


Article first published online: 14 JUN 2010

DOI: 10.1111/j.1365-2427.2010.02461.x

1. Excretion of nitrogen (N) and phosphorus (P) is a direct and potentially important role for aquatic consumers in nutrient cycling that has recently garnered increased attention. The ecosystem-level significance of excreted nutrients depends on a suite of abiotic and biotic factors, however, and few studies have coupled measurements of excretion with consideration of its likely importance for whole-system nutrient fluxes. 2. We measured rates and ratios of N and P excretion by shrimps (Xiphocaris elongata and Atya spp.) in two tropical streams that differed strongly in shrimp biomass because a waterfall excluded predatory fish from one site. We also made measurements of shrimp and basal resource carbon (C), N and P content and estimated shrimp densities and ecosystem-level N and P excretion and uptake. Finally, we used a 3-year record of discharge and NH4-N concentration in the high-biomass stream to estimate temporal variation in the distance required for excretion to turn over the ambient NH4-N pool. 3. Per cent C, N, and P body content of Xiphocaris was significantly higher than that of Atya. Only per cent P body content showed significant negative relationships with body mass. C:N of Atya increased significantly with body mass and was higher than that of Xiphocaris. N : P of Xiphocaris was significantly higher than that of Atya. 4. Excretion rates ranged from 0.16–3.80 lmol NH4-N shrimp)1 h)1, 0.23–5.76 lmol total dissolved nitrogen (TDN) shrimp)1 h)1 and 0.002–0.186 lmol total dissolved phosphorus (TDP) shrimp)1 h)1. Body size was generally a strong predictor of excretion rates in both taxa, differing between Xiphocaris and Atya for TDP but not NH4-N and TDN. Excretion rates showed statistically significant but weak relationships with body content stoichiometry. 5. Large between-stream differences in shrimp biomass drove differences in total excretion by the two shrimp communities (22.3 versus 0.20 lmol NH4-N m)2 h)1, 37.5 versus 0.26 lmol TDN m)2 h)1 and 1.1 versus 0.015 lmol TDP m)2 h)1), equivalent to 21% and 0.5% of NH4-N uptake and 5% and <0.1% of P uptake measured in the high- and lowbiomass stream, respectively. Distances required for excretion to turn over the ambient NH4-N pool varied more than a hundredfold over the 3-year record in the high-shrimp stream, driven by variability in discharge and NH4-N concentration. 6. Our results underscore the importance of both biotic and abiotic factors in controlling consumer excretion and its significance for nutrient cycling in aquatic ecosystems. Differences in community-level excretion rates were related to spatial patterns in shrimp biomass dictated by geomorphology and the presence of predators. Abiotic factors also had important effects through temporal patterns in discharge and nutrient concentrations. Future excretion studies that focus on nutrient cycling should consider both biotic and abiotic factors in assessing the significance of consumer excretion in aquatic ecosystems.


Larsen, M.C., Torres-Sánchez, A.J., and Concepción, I.M., 1998, Slopewash, surface runoff, and fine-litter transport in forest and landslide scars in humid-tropical steeplands, Luquillo Experimental Forest, Puerto Rico [abs] EOS, Transactions American Geophysical Union, vol. 80.

Rainfall, slopewash (the erosion of soil particles), surface runoff, and fine-litter transport at humid-tropical steepland sites in the Luquillo Experimental Forest, Puerto Rico (18? 20' N, 65? 45' W) were measured from 1991 to 1995. Hillslopes underlain by: 1) Cretaceous tuffaceous sandstone and siltstone in subtropical rain (tabonuco) forest with vegetation recovering from Hurricane Hugo (1989); and underlain by 2) Tertiary quartz diorite in subtropical lower montane wet (colorado and dwarf) forest with undisturbed forest canopy were compared to recent landslide scars. Monthly surface runoff on these very steep hillslopes (24? to 43?) was only 0.2 to 0.5 percent of monthly rainfall. Slopewash was higher in sandy loam soils whose parent material is quartz diorite (averaging 46 g m-2 a-1) than in silty-clay loam soils derived from tuffaceous sandstone and siltstone where the average was 9 g m-2 a-1. Annual slopewash of 100 to 349 g m-2 on the surfaces of two recent, small landslide scars was measured initially but slopewash decreased to only 3 to 4 g m-2 a-1 by the end of the study. The mean annual mass of fine litter (mainly leaves and twigs) transported downslope at the forested sites ranged from 5 to 8 g m-2 and was lower at the tabonuco forest site, where post-Hurricane Hugo recovery is still in progress. Mean annual fine-litter transport was 2.5 g m-2 on the two landslide scars.
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