forest succesion or change

Synthesis: Land Transitions in the Tropics

Uriarte, M., L. Schneider, and T. K. Rudel. 2010b. Synthesis:
land use transitions in the tropics. Biotropica 42:59–62.

Land cover transformations in the tropics are not limited to deforestation; they include other complex transitions such as agricultural and urban expansion, pasture development, and secondary vegetation regrowth. Understanding the causes and extent of these highly variable and complex transitions requires close collaboration between biological, physical, and social scientists. Here we address three critical issues in the study of land transitions: (1) What methodological and socioecological criteria should be used for characterizing land cover categories and transformations? Results from case studies presented here call for the creation of continuous land cover classes that allow for detection of disturbance and human use dynamics and consideration of socioeconomic and biophysical criteria in characterizing and monitoring land transitions. (2) What are the most promising theoretical frameworks? Successful theoretical frameworks must bridge disciplinary boundaries, and encompass multiple spatial, temporal, and political scales. (3) Are regime shifts, constraints, and resilience of land transformations in the tropics predictable? Resilience of land use systems requires a feedback loop between ecological constraints and management decisions. This loop may be broken by policies, migration, and flow of capital from global commodity markets. In addition, land transformations may lead to novel interactions between land-use and natural disturbance leading to unpredictable regime shifts in ecosystems. Planning for sustainable patterns of land use requires some understanding of these regime transformations.

A Canopy Trimming Experiment in Puerto Rico: The Response of Litter Invertebrate Communities to Canopy Loss and Debris Deposition in a Tropical Forest Subject to Hurricanes

Richardson, Barbara A.; Richardson, Michael J.; Gonzalez, Grizelle; Shiels, Aaron B.; Srivastava, Diane S. 2010. A canopy trimming experiment in Puerto Rico: the response of litter invertebrate communities to canopy loss and debris deposition in a tropical forest subject to hurricanes. Ecosystems. 13: 286-301.

Hurricanes cause canopy removal and deposition of pulses of litter to the forest floor. A Canopy Trimming Experiment (CTE) was designed to decouple these two factors, and to investigate the separate abiotic and biotic consequences of hurricane-type damage and monitor recovery processes. As part of this experiment, effects on forest floor invertebrate communities were studied using litterbags. Canopy opening resulted in increased throughfall, soil moisture and light levels, but decreased litter moisture. Of these, only throughfall and soil moisture had returned to control levels 9 months after trimming. Canopy opening was the major determinant of adverse changes in forest floor invertebrate litter communities, by reducing diversity and biomass, irrespective of debris deposition, which played a secondary role. Plots subjected to the most disturbance, with canopy removed and debris added, had the lowest diversity and biomass. These two parameters were higher than control levels when debris was added to plots with an intact canopy, demonstrating that increased nutrient potential or habitat complexity can have a beneficial effect, but only if the abiotic conditions are suitable. Animal abundance remained similar over all treatments, because individual taxa responded differentlyto canopy trimming. Mites, Collembola, and Psocoptera, all microbiovores feeding mainly on fungal hyphae and spores, responded positively, with higher abundance in trimmed plots, whereas all other taxa, particularly predators and larger detritivores, declined in relative abundance. Litterbag mesh size and litter type had only minor effects on communities, and canopy trimming and debris deposition explained most variation between sites. Effects of trimming on diversity, biomass, and abundance of some invertebrate taxa were still seen when observations finished and canopy closure was complete at 19 months. This suggests that disturbance has a long-lasting effect on litter communities and may, therefore, delay detrital processing, depending on the severity of canopy damage and rate of regrowth.

The Effects of Natural and Human Disturbances on Soil Nitrogen Dynamics and Trace Gas Fluxes in a Puerto Rican Wet Forest

The Effects of Natural and Human Disturbances on Soil Nitrogen Dynamics and Trace Gas Fluxes in a Puerto Rican Wet Forest
P. A. Steudler, J. M. Melillo, R. D. Bowden, M. S. Castro and A. E. Lugo
Vol. 23, No. 4, Part A. Special Issue: Ecosystem, Plant, and Animal Responses to Hurricanes in the Caribbean (Dec., 1991), pp. 356-363

We examined the effects of two disturbances (Hurricane Hugo and forest clearcutting) on soil nitrogen dynamics and on the exchanges of N20, CO,, and CH, between soils and the atmosphere of a subtropical wet forest in Puerto Rico. The disturbances resulted in prolonged increases in ammonium pools and short-term increases in rates of net N-mineralization and net nitrification. Nitrous oxide emissions increased following both disturbances. The most dramatic increase was observed 4 mo after clearcutting; N 2 0 emissions (109.49 pg N/m2-hr) from the cut plot were about two orders of magnitude higher than emissions from the reference plot (1.71 pg N/m2-hr). Carbon dioxide emissions from both disturbed plots (mean 102.47 mg C/m2-hr) were about 30 percent lower than the reference (mean 15 1.28 mg C/m2-hr). Soils at all sites were generally sinks for CH,. Methane uptake, however, was suppressed by both disturbances. This suppression may be related to disturbance-induced changes in the nitrogen cycle, as we have previously observed in temperate zone forests.

Geomorphology, disturbance, and the soil and vegetation of two subtropical wet steepland watersheds of Puerto Rico

F.N. Scatena, Ariel E. Lugo 1995. Geomorphology, disturbance, and the soil and vegetation of two subtropical wet steepland watersheds of Puerto Rico.. Geomorphology 13 :199-213.

Relationships between landforms, soil nutrients, forest structure, and the relative importance of different disturbances were quantified in two subtropical wet steepland watersheds in Puerto Rico. Ridges had fewer landslides and treefall gaps, more above-ground biomass, older aged stands, and greater species richness than other landscape positions. Ridge soils had relatively low quantities of exchangeable bases but high soil organic matter, acidity and exchangeable iron. Valley sites had higher frequencies of disturbance, less biomass, younger aged stands, lower species richness and soils with more exchangeable bases.Soil N, P, and K were distributed relatively independently of geomorphic setting, but were significantly related to the composition and age of vegetation. On a watershed basis, hurricanes were the dominant natural disturbance in the turnover of individuals, biomass, and forest canopy. However, turnover by the mortality of individuals that die without creating canopy openings was faster than the turnover by any natural disturbance. Only in riparian areas was forest turnover by treefall gaps faster than turnover by hurricanes. The same downslope mass transfer that links soil forming processes across the landscape also influences the distribution of landslides, treefall gaps, and the structure and composition of the forest. One consequence of these interactions is that the greatest aboveground biomass occurs on ridges where the soil nutrient pools are the smallest. Geomorphic stability, edaphic conditions, and biotic adaptations apparently override the importance of spatial variations in soil nutrients in the accumulation of above-ground biomass at this site.

Effects of an invasive tree on community structure and diversity in a tropical forest in Puerto Rico

Brown, K. A. ; Scatena, F. N., and Gurevitch, J. 2006. Effects of an invasive tree on community structure and diversity in a tropical forest in Puerto Rico. . Forest Ecology and Management . 2006; 226:145-152.

We report the effects of an invasive tree (Syzygium jambos, Myrtaceace) on species composition, plant diversity patterns, and forest regeneration in primary and secondary forest in the Luquillo Mountains of northeastern Puerto Rico, including the area in and around the Caribbean National Forest (CNF) and the Luquillo Long Term Ecological Research site (Luquillo LTER). Land use history was reconstructed using aerial photographs from 1936 to 1989 and study sites were categorized into four groups that corresponded to their status in 1936: unforested, young secondary, mature secondary, and primary forests. In randomly selected forest stands in each forest type, we measured the abundance of invasive and native tree species, seedling recruitment for S. jambos as well as soil nutrient pools and tested for the effects of land use history on S. jambos density and diversity. A partial Mantel test was used to control for historical and elevational differences across study sites. The results indicate that S. jambos density was highest in habitats classified in 1936 as unforested, young, or mature secondary forests. Compared to all other forest classes, species diversity was significantly higher in primary forests. However, there was no statistically significant difference between observed and estimated species richness across the four forest types. S. jambos density and species diversity were strongly negatively correlated, even after controlling for land use history and elevation. There was significantly higher S. jambos seedling recruitment in areas that were either unforested or had young secondary forests in 1936. The results also indicate that S. jambos is able to establish viable populations in habitats with different soil nutrient status. S. jambos has also altered vegetation composition and diversity patterns in habitats where it is the dominant tree species. After nearly 185 years since its introduction to the island, S. jambos is not only well established within 30 m of stream channels, its presence does not appear to be limited by topographic, soil nutrient, or elevational conditions. This study suggests that land use change and subsequent plant invasions have produced a new vegetation assemblage that has led to potentially long-term changes in community structure, species composition, and successional trajectory in regenerating secondary forests in the Luquillo Mountains.
Syndicate content