tropics

Wet canopy evaporation from a Puerto Rican lower montane rain forest: the importance of realistically estimated aerodynamic conductance

Holwerda F., Bruijnzeel L.A., Scatena F.N., Vugts H.F., Meesters A.G.C.A 2011. Wet canopy evaporation from a Puerto Rican lower montane rain forest: the importance of realistically estimated aerodynamic conductance. In press Journal of Hydrology

Abstract: 
Rainfall interception (I) was measured in 20 m tall Puerto Rican tropical forest with complex topography for a one-year period using totalizing throughfall (TF) and stemflow (SF) gauges that were measured every 2–3 days. Measured values were then compared to evaporation under saturated canopy conditions (E) determined with the Penman-Monteith (P-M) equation, using (i) measured (eddy covariance) and (ii) calculated (as a function of forest height and wind speed) values for the aerodynamic conductance to momentum flux (ga,M). E was also derived using the energy balance equation and the sensible heat flux measured by a sonic anemometer (Hs). I per sampling occasion was strongly correlated with rainfall (P): I = 0.21P + 0.60 (mm), r2 = 0.82, n = 121. Values for canopy storage capacity (S = 0.37 mm) and the average relative evaporation rate (E/R = 0.20) were derived from data for single events (n = 51). Application of the Gash analytical interception model to 70 multiple-storm sampling events using the above values for S and E/R gave excellent agreement with measured I. For E/R = 0.20 and an average rainfall intensity (R) of 3.16 mm h-1, the TF-based E was 0.63 mm h-1, about four times the value derived with the P-M equation using a conventionally calculated ga,M (0.16 mm h-1). Estimating ga,M using wind data from a nearby but more exposed site yielded a value of E (0.40 mm h-1) that was much closer to the observed rate, whereas E derived using the energy balance equation and Hs was very low (0.13 mm h-1), presumably because Hs was underestimated due to the use of too short a flux-averaging period (5-min). The best agreement with the observed E was obtained when using the measured ga,M in the P-M equation (0.58 mm h-1). The present results show that in areas with complex topography, ga,M, and consequently E, can be strongly underestimated when calculated using equations that were derived originally for use in flat terrain; hence, direct measurement of ga,M using eddy covariance is recommended. The currently measured ga,M (0.31 m s-1) was at least several times, and up to one order of magnitude higher than values reported for forests in areas with flat or gentle topography (0.03–0.08 m s-1, at wind speeds of about 1 m s-1). The importance of ga,M at the study site suggests a negative, downward, sensible heat flux sustains the observed high evaporation rates during rainfall. More work is needed to better quantify Hs during rainfall in tropical forests with complex topography.

Climate is affected more by maritime than by continental land use change: A multiple scale analysis

Van der Molen, M. K., Dolman, A. J.,Waterloo, M. J. and Bruijnzeel, L.
A. 2006. Climate is affected more by maritime than by continental land
use change: A multiple scale analysis. Global and Planetary Change,
54, 128–149.

Abstract: 
Tropical deforestation appears to have larger impacts on local, regional and global climate when it occurs under maritime conditions rather then under continental conditions. At the local scale, we compare results from a field experiment in Puerto Rico with other long-term studies of the changes in surface fluxes after deforestation. Changes in surface fluxes are larger in maritime situations because a number of feedback mechanisms appears less relevant (e.g. the dependency of soil moisture on recycling of water and the larger reduction of net radiation in the wet season due to clouds in continental regions). Pastures may evaporate at similarly high rates as forests when soil moisture is sufficient, which has a strong reducing effect on the sensible heat flux after deforestation. At the regional scale (∼102 km2), model simulations show that the meso-scale sea breeze circulation under maritime conditions is more effective in transporting heat and moisture to the upper troposphere than convection is in the continental case. Thus islands function as triggers of convection, whereas the intensity of the sea breeze-trigger is sensitive to land use change. At the global scale, using satellite-derived latent heating rates of the upper troposphere, it is shown that 40% of the latent heating associated with deep convection takes place in the Maritime Continent (Indonesia and surroundings) and may be produced mostly by small islands. Continents contribute only 20% of the latent heating of the upper troposphere. Thus, sea breeze circulations exert significant influence on the Hadley cell circulation. These results imply that, from a climate perspective, further deforestation studies would do well to focus more on maritime conditions.

Forest Regeneration in a chronosequence of tropical abandoned pastures: implications for restoration ecology

Forest Regeneration in a chronosequence of tropical abandoned pastures: implications for restoration ecology

Abstract: 
During the mid-1900s, most of the island of Puerto Rico was deforested, but a shift in the economy from agriculture to small industry beginning in the 1950s resulted in the abandonment of agricultural lands and recovery of secondary forest. This unique history provides an excellent opportunity to study secondary forest succession and suggest strategies for tropical forest restoration. To determine the pattern of secondary succession, we describe the woody vegetation in 71 abandoned pastures and forest sites in four regions of Puerto Rico. The density, basal area, aboveground biomass, and species richness of the secondary forest sites were similar to those of the old growth forest sites (>80 yr) after approximately 40 years. The dominant species that colonized recently abandoned pastures occurred over a broad elevational range and are widespread in the neotropics. The species richness of Puerto Rican secondary forests recovered rapidly, but the species composition was quite different in comparison with old growth forest sites, suggesting that enrichment planting will be necessary to restore the original composition. Exotic species were some of the most abundant species in the secondary forest, but their long-term impact depended on life history characteristics of each species. These data demonstrate that one restoration strategy for tropical forest in abandoned pastures is simply to protect the areas from fire, and allow natural regeneration to produce secondary forest. This strategy will be most effective if remnant forest (i.e., seed sources) still exist in the landscape and soils have not been highly degraded. Patterns of forest recovery also suggest strategies for accelerating natural recovery by planting a suite of generalist species that are common in recently abandoned pastures in Puerto Rico and throughout much of the neotropics.

Ecosystem Development and Plant Succession on Landslides in the Caribbean

Ecosystem Development and Plant Succession on Landslides in the Caribbean
Lawrence R. Walker, Daniel J. Zarin, Ned Fetcher, Randall W. Myster and Arthur H. Johnson
Biotropica
Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances (Dec., 1996), pp. 566-576

Abstract: 
Landslides are common in mountainous regions of the Caribbean and are triggered by heavy rains and earthquakes, and often occur in association with human disturbances (e.g., roads). Spatially heterogeneous removal of both substrate and vegetation is responsible for a variety of patterns of ecosystem development and plant successional trajectories within Caribbean landslides. Soil nutrient pools in exposed mineral soils reach levels comparable to mature forest soils within 55 yr but soil organic matter recovers more slowly. Plant colonization of landslides depends on the availability of propagules and suitable sites for germination, soil stability, and the presence of residual or newly deposited soil organic matter and associated nutrients. Once initial colonization occurs, the rate and trajectory of plant succession on landslides is strongly affected by plant/plant interactions. We present two conceptual models of landslide succession that summarize the major processes and pathways of ecosystem development and plant succession on landslides. Additional work is needed to characterize interactions between spatially heterogeneous zones, controls over soil development, impacts of key plant species, and the role of animals on Caribbean landslides.

Dissimilatory Nitrate Reduction to Ammonium in Upland Tropical Forest Soils

Dissimilatory Nitrate Reduction to Ammonium in Upland Tropical Forest Soils
Whendee L. Silver, Donald J. Herman and Mary K. Firestone
Ecology
Vol. 82, No. 9 (Sep., 2001), pp. 2410-2416

Abstract: 
The internal transformations of nitrogen in terrestrial ecosystems exert strong controls over nitrogen availability to net primary productivity, nitrate leaching into groundwater, and emissions of nitrogen-based greenhouse gas. Here we report a reductive pathway for nitrogen cycling in upland tropical forest soils that decreases the amount of nitrate susceptible to leaching and denitrification, thus conserving nitrogen in the ecosystem. Using 15N tracers we measured rates of dissimilatory nitrate reduction to ammonium (DNRA) in upland humid tropical forest soils averaging ;0.6 mg·g21·d21. Rates of DNRA were three times greater than the combined N2O and N2 fluxes from nitrification and denitrification and accounted for 75% of the turnover of the nitrate pool. To determine the relative importance of ambient C, O2, and NO3 concentrations on rates of DNRA, we estimated rates of DNRA in laboratory assays using soils from three tropical forests (cloud forest, palm forest, and wet tropical forest) that differed in ambient C and O2 concentrations. Rates of DNRA measured in laboratory assays ranged from 0.5 to 9 mg·g21·d21 in soils from the three different forests and appeared to be primarily limited by the availability of NO3, as opposed to C or O2. Tests of sterile soils indicated that the dominant reductive pathway for both NO2 and NO3 was biotic and not abiotic. Because NH4 is the form of N generally favored for assimilation by plants and microbes, and NO3 is easily lost from the ecosystem, the rapid and direct transformation of NO3 to NH4 via DNRA has the potential to play an important role in ecosystem N conservation.

Earthworm communities along an elevation gradient in Northeastern Puerto Rico

Gonzalez, Grizelle; Garcia, Emerita; Cruz, Veronica; Borges, Sonia; Zalamea, Marcela; Rivera, Maria M. 2007. Earthworm communities along an elevation gradient in Northeastern Puerto Rico.. European Journal of Soil Biology 43 .

Abstract: 
In this study, we describe earthworm communities along an elevation gradient of eight forest types in Northeastern Puerto Rico, and determine whether their abundance, biomass and/or diversity is related to climatic, soil physical/chemical and/or biotic characteristics. We found that the density, biomass, and diversity of worms varied significantly among forest types. The density of earthworms was highest in the Pterocarpus forest. In terms of biomass, both elfin and the Pterocarpus forests had the highest values. The number of earthworm species significantly increased as elevation and annual rainfall increased and air temperature decreased. We conclude that differences in earthworm species richness along this elevation gradient may be due to a combination of biotic and soil physical and chemical factors. Soil pH and root length density are important predictors of number of worm species along this elevation gradient.

Wet canopy evaporation from a Puerto Rican lower montane rain forest: the importance of realistically estimated aerodynamic conductance

Abstract: 
Rainfall interception (I) was measured in 20 m tall Puerto Rican tropical forest with 4 complex topography for a one-year period using totalizing throughfall (TF) and stemflow 5 (SF) gauges that were measured every 23 days. Measured values were then compared to 6 evaporation under saturated canopy conditions (E) determined with the Penman-Monteith 7 (P-M) equation, using (i) measured (eddy covariance) and (ii) calculated (as a function of 8 forest height and wind speed) values for the aerodynamic conductance to momentum flux 9 (ga,M). E was also derived using the energy balance equation and the sensible heat flux 10 measured by a sonic anemometer (Hs). I per sampling occasion was strongly correlated with rainfall (P): I = 0.21P + 0.60 (mm), r2 11 = 0.82, n = 121. Values for canopy storage 12 capacity (S = 0.37 mm) and the average relative evaporation rate (E/R = 0.20) were 13 derived from data for single events (n = 51). Application of the Gash analytical 14 interception model to 70 multiple-storm sampling events using the above values for S and 15 E/R gave excellent agreement with measured I. For E/R = 0.20 and an average rainfall intensity (R) of 3.16 mm h-1, the TF-based E was 0.63 mm h-116 , about four times the value derived with the P-M equation using a conventionally calculated ga,M (0.16 mm h-117 ). 18 Estimating ga,M using wind data from a nearby but more exposed site yielded a value of E (0.40 mm h-119 ) that was much closer to the observed rate, whereas E derived using the energy balance equation and Hs was very low (0.13 mm h-120 ), presumably because Hs was 21 underestimated due to the use of too short a flux-averaging period (5-min). The best 22 agreement with the observed E was obtained when using the measured ga,M in the P-M equation (0.58 mm h-123 ). The present results show that in areas with complex topography, 1 strongly underestimated when calculated using 2 equations that were derived originally for use in flat terrain; hence, direct measurement of ga,M using eddy covariance is recommended. The currently measured ga,M (0.31 m s-13 ) 4 was at least several times, and up to one order of magnitude higher than values reported for forests in areas with flat or gentle topography (0.03–0.08 m s-15 , at wind speeds of about 1 m s-16 ). The importance of ga,M at the study site suggests a negative, downward, 7 sensible heat flux sustains the observed high evaporation rates during rainfall. More work 8 is needed to better quantify Hs during rainfall in tropical forests with complex 9 topography.

Effects of Hurricane Disturbance on Stream Water Concentrations and Fluxes in Eight Tropical Forest Watersheds of the Luquillo Experimental Forest, Puerto Rico

SCHAEFER, DOUGLAS. A.; McDOWELL, WILLIAM H.; SCATENA, FREDRICK N.; ASBURY,CLYDE E. 2000. Effects of hurricane disturbance on stream water concentrations and fluxes in eight tropical forest watersheds of the Luquillo Experimental Forest, Puerto Rico. . Journal of Tropical Ecology 16:189-207

Abstract: 
Stream water chemistry responds substantially to watershed disturbances, but hurricane effects have not been extensively investigated in tropical regions. This study presents a long-term (2.5-1 1 y) weekly record of stream water chemistry on eight forested watersheds (catchment basins) in the Luquillo Mountains of Puerto Rico. This includes a period before and at least 2 y after the disturbance caused by the 1989 Hurricane Hugo. Nitrate, potassium and ammonium concentrations increased after the hurricane and remained elevated for up to 2 y. Sulphate, chloride, sodium, magnesium and calcium showed smaller relative significant changes. Average stream water exports of potassium, nitrate and ammonium increased by 13.1, 3.6 and 0.54 kg ha-' y-' in the first post-hurricane year across all watersheds. These represent increases of 119, 182 and 102% respectively, compared to the other years of record. The increased stream outputs of potassium and nitrogen in the first 2 y post-hurricane are equivalent to 3% (potassium) and 1% (nitrogen) of the hurricanederived plant litter. Effects of hurricanes on tropical stream water potassium and nitrogen can be greater than those caused by canopy gaps or limited forest cutting, but less than those following large-scale deforestation or fire.

STRUCTURAL AND FUNCTIONAL RESPONSES OF A SUBTROPICAL FOREST TO 10 YEARS OF HURRICANES AND DROUGHTS

Beard, Karen H., Kristiina A. Vogt, Daniel J. Vogt, Frederick N. Scatena, Alan P. Covich, Ragnhildur Sigurdardottir, Thomas G. Siccama, and Todd A. Crowl. 2005. STRUCTURAL AND FUNCTIONAL RESPONSES OF A SUBTROPICAL FOREST TO 10 YEARS OF HURRICANES AND DROUGHTS. Ecological Monographs 75:345–361. [doi:10.1890/04-1114]

Abstract: 
Little is known about ecosystem-level responses to multiple, climatic disturbance events. In the subtropical forests of Puerto Rico, the major natural disturbances are hurricanes and droughts. We tested the ecosystem-level effects of these disturbances in sites with different land use histories. From 1989 to 1992, data were collected to determine the effects of Hurricane Hugo and two droughts on litterfall inputs, fine-root biomass, and decomposition rates in three topographic locations (stream, riparian, upslope) within two watersheds. From 1994 to 1998, we added a third watershed and an experiment in which coarse-wood levels were manipulated to simulate hurricane inputs. Data were collected on tree and palm growth rates, litterfall inputs, fine-root biomass, and decomposition rates. From 1994 to 1998, four hurricanes and three droughts were recorded. Measured parameters had unique responses and recovery rates to hurricanes and droughts. Litterfall inputs returned to long-term mean rates within one month following droughts and small-to-moderate hurricanes but required five years to recover after an intense hurricane. In contrast, fine-root biomass recovered seven months after an intense hurricane but failed to recover after five years following a severe drought. Despite the dramatic effects of these weather events on some ecosystem parameters, we found that aboveground measures of tree and palm growth were more affected by preexisting site conditions (e.g., nitrogen availability due to past land use activities) than hurricanes or droughts. The addition of coarse woody debris increased tree and palm growth, fine-root biomass, and litter production; however, in the case of tree and palm growth, this effect was least measurable in the sites with the highest productivity. We found that decomposition rates were more controlled by litter quality than weather conditions. In conclusion, we found that certain ecosystem structures (e.g., canopy structure and fine-root biomass) generally recovered more slowly from disturbance events than certain ecosystem processes (e.g., plant growth rates, decomposition rates). We also found that past land use activities and disturbance legacies were important in determining the responses and recovery rates of the ecosystem to disturbance.

STRUCTURAL AND FUNCTIONAL RESPONSES OF A SUBTROPICAL FOREST TO 10 YEARS OF HURRICANES AND DROUGHTS

Beard, Karen H., Kristiina A. Vogt, Daniel J. Vogt, Frederick N. Scatena, Alan P. Covich, Ragnhildur Sigurdardottir, Thomas G. Siccama, and Todd A. Crowl. 2005. STRUCTURAL AND FUNCTIONAL RESPONSES OF A SUBTROPICAL FOREST TO 10 YEARS OF HURRICANES AND DROUGHTS. Ecological Monographs 75:345–361. [doi:10.1890/04-1114]

Abstract: 
Little is known about ecosystem-level responses to multiple, climatic disturbance events. In the subtropical forests of Puerto Rico, the major natural disturbances are hurricanes and droughts. We tested the ecosystem-level effects of these disturbances in sites with different land use histories. From 1989 to 1992, data were collected to determine the effects of Hurricane Hugo and two droughts on litterfall inputs, fine-root biomass, and decomposition rates in three topographic locations (stream, riparian, upslope) within two watersheds. From 1994 to 1998, we added a third watershed and an experiment in which coarse-wood levels were manipulated to simulate hurricane inputs. Data were collected on tree and palm growth rates, litterfall inputs, fine-root biomass, and decomposition rates. From 1994 to 1998, four hurricanes and three droughts were recorded. Measured parameters had unique responses and recovery rates to hurricanes and droughts. Litterfall inputs returned to long-term mean rates within one month following droughts and small-to-moderate hurricanes but required five years to recover after an intense hurricane. In contrast, fine-root biomass recovered seven months after an intense hurricane but failed to recover after five years following a severe drought. Despite the dramatic effects of these weather events on some ecosystem parameters, we found that aboveground measures of tree and palm growth were more affected by preexisting site conditions (e.g., nitrogen availability due to past land use activities) than hurricanes or droughts. The addition of coarse woody debris increased tree and palm growth, fine-root biomass, and litter production; however, in the case of tree and palm growth, this effect was least measurable in the sites with the highest productivity. We found that decomposition rates were more controlled by litter quality than weather conditions. In conclusion, we found that certain ecosystem structures (e.g., canopy structure and fine-root biomass) generally recovered more slowly from disturbance events than certain ecosystem processes (e.g., plant growth rates, decomposition rates). We also found that past land use activities and disturbance legacies were important in determining the responses and recovery rates of the ecosystem to disturbance.
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