tropical

Diversity and composition of tropical secondary forests recovering from large-scale clearing: results from the 1990 inventory in Puerto Rico

Chinea, J. Danilo; Helmer, Eileen H. 2003. Diversity and composition of tropical secondary forests recovering from large-scale clearing: results from the 1990 inventory in Puerto Rico.. Forest Ecology and Management 180 :227-240.

Abstract: 
The extensive recovery from agricultural clearing of Puerto Rican forests over the past half-century provides a good opportunity to study tropical forest recovery on a landscape scale. Using ordination and regression techniques, we analyzed forest inventory data from across Puerto Rico’s moist and wet secondary forests to evaluate their species composition and whether the landscape structure of older forest affected tree species composition of recovering forests at this scale. Our results support conclusions from studies conducted in Puerto Rico at smaller scales and temperate forests at larger scales that timing of abandonment and land use history are of overwhelming importance in determining the species composition of recovering forests. Forest recovery is recent enough in Puerto Rico that previous land use is clearly evident in current species composition, and creates new forest communities. As demonstrated in other work, physical factors such as elevation and substrate co-vary with land use history, so that the species composition of the forest landscape results from the interplay between biophysical and socioeconomic forces over time. Our results also indicate that increasing the distance to the largest forest patches occurring in the landscape 12 years previous had a small negative impact on species richness but not species diversity or community composition.We conclude that land use history has as much influence in species composition as biophysical variables and that, at the scale of this study, there is no large influence of forest landscape structure on species diversity or composition.

ECOLOGICAL CONSEQUENCES OF HYDROPOWER DEVELOPMENT IN CENTRAL AMERICA: IMPACTS OF SMALL DAMS AND WATER DIVERSION ON NEOTROPICAL STREAM FISH ASSEMBLAGES

Anderson EP, Freeman MC, Pringle CM. 2006a. Ecological consequences of hydropower development in Central America: impacts of small
dams and water diversion on neotropical stream fish assemblages. River Research and Applications 22: 397–411.

Abstract: 
Small dams for hydropower have caused widespread alteration of Central American rivers, yet much of recent development has gone undocumented by scientists and conservationists. We examined the ecological effects of a small hydropower plant (Don˜a Julia Hydroelectric Center) on two low-order streams (the Puerto Viejo River and Quebradon stream) draining a mountainous area of Costa Rica. Operation of the Don˜a Julia plant has dewatered these streams, reducing discharge to 10% of average annual flow. This study compared fish assemblage composition and aquatic habitat upstream and downstream of diversion dams on two streams and along a 4 km dewatered reach of the Puerto Viejo River in an attempt to evaluate current instream flow recommendations for regulated Costa Rican streams. Our results indicated that fish assemblages directly upstream and downstream of the dam on the third order Puerto Viejo River were dissimilar, suggesting that the small dam (<15m high) hindered movement of fishes. Along the 4 km dewatered reach of the Puerto Viejo River, species count increased with downstream distance from the dam. However, estimated species richness and overall fish abundance were not significantly correlated with downstream distance from the dam. Our results suggested that effects of stream dewatering may be most pronounced for a subset of species with more complex reproductive requirements, classified as equilibrium-type species based on their life-history. In the absence of changes to current operations, we expect that fish assemblages in the Puerto Viejo River will be increasingly dominated by opportunistic-type, colonizing fish species. Operations of many other small hydropower plants in Costa Rica and other parts of Central America mirror those of Don˜a Julia; the methods and results of this study may be applicable to some of those projects. Copyright # 2006 John Wiley & Sons, Ltd.

A Comparison of Two Sampling Strategies to Assess Discomycete Diversity in Wet Tropical Forests

CANTRELL, SHARON A. 2004. A Comparison of Two Sampling Strategies to Assess Discomycete Diversity in Wet Tropical Forests. Caribbean Journal of Science, Vol. 40, No. 1, 8-16, .

Abstract: 
Most of the fungal diversity studies that have used a systematic collecting scheme have not included the discomycetes, so optimal sampling methods are not available for this group. In this study, I tested two sampling methods at each sites in the Caribbean National Forest, Puerto Rico and Ebano Verde Reserve, Dominican Republic. For a plot-based sampling method, 10 × 10 m plots were established and divided into one hundred 1 × 1 m subplots. For each sample, 12 subplots were selected at random with replacement. For a transect-based sampling method, 60 m long transects were established with twelve 1 × 1 m subplots randomly placed on either side of the transect line at 5 m intervals at the beginning of the study. The study was conducted from October 2001 to September 2002. For Puerto Rico, 46 and 51 morpho-species were identified in the transects and plots, respectively. There was a 32% overlap (68% complementarity) between sites. The Sorensen Similarity Coefficient between sites was 0.50 for both methods, and 0.55-0.63 between methods within sites. For the Dominican Republic, 25 and 26 morpho-species were identified in the transects and plots, respectively. There was a 24-31% overlap (69-76% complementarity) between sites. The Sørensen Similarity Coefficient between sites was 0.40-0.47 for transects and plots, respectively, and 0.40-0.70 between methods within sites. The species accumulation curve indicates that the minimum number of subplots needed is 10 per transect and 60-70 per plot to obtain between 70-80% of the species. In terms of sampling effort, I concluded that at least 12 samples distributed throughout a year but with shorter intervals during the rainy season are needed. There was no difference between using transects or plots based on the number of species and similarity indexes. Based on a Chi-Square analysis using the frequencies of species, however, transects were better that plots because the distribution of species is more homogeneous.

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Nitrous Oxide Fluxes

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Nitrous Oxide Fluxes
William B. Bowden, William H. McDowell, Clyde E. Asbury and Amy M. Finley
Biogeochemistry
Vol. 18, No. 2 (1992), pp. 77-99

Abstract: 
Fluxes of N<sub>2</sub>O at the soil surface, dissolved N<sub>2</sub>O in near-surface groundwater, and potential N<sub>2</sub>O production rates were measured across riparian catenas in two rain forest watersheds in Puerto Rico. In the Icacos watershed, mean N<sub>2</sub>O fluxes were highest at topographic breaks in the landscape (∼40-300 μg N<sub>2</sub>O-N m<sup>-2</sup> h<sup>-1</sup>). At other locations in the riparian zone and hillslope, fluxes were lower (≤ 2 μg N<sub>2</sub>O-N m<sup>-2</sup> h<sup>-1</sup>). This pattern of surface N<sub>2</sub>O fluxes was persistent. In the Bisley watershed, mean suface N<sub>2</sub>O fluxes were lower (<40 μg N<sub>2</sub>O-N m<sup>-2</sup> h<sup>-1</sup>) and no identifiable spatial or temporal pattern. Although the spatial patterns and intensities of N<sub>2</sub>O emissions differed between the two watersheds, surface soils from both sites had a high potential to reduce NO<sub>3</sub> to N<sub>2</sub>O (and perhaps N<sub>2</sub>). This potential declined sharply with depth as did soil %C, %N, and potential N-mineralization. Simple controls on denitrification (i.e. aeration, nitrate, and carbon) explained characteristics of potential N<sub>2</sub>O production in surface and deep soils from riparian and upslope locations. In the field, spatial patterns in these controlling variables were defined by geomorphological differences between the two watersheds, which then explained the spatial patterns of observed N<sub>2</sub>O flux.
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