Reduced Rainfall Increases Metabolic Rates in Upper Mixed Layers of Tropical Lakes

Ecosystems - Ecosystem-level metabolism is a good sentinel for human and natural disturbances in freshwater systems, responding from local changes (for example, land use) to regional and global...     

Growth of Juvenile Centropomus undecimalis in a Tropical Island

Age, growth rates and hatching dates were determined for juvenile common snook, Centropomus undecimalis, collected in several Puerto Rican estuarine systems. Two growth models (von Bertalanffy and power curve) were used and compared based on the fitting, the residual distribution and homogeneity of variances. A multiple linear regression was also fit to predict age from otolith weight, fish weight and standard length. Hatching dates back-calculated from age estimates, suggested that spawning occurs throughout the year but peaks from May to October. Early juveniles (age < 100d) recruited during the wet season (August to November) showed a significantly higher growth rate (0.67mmd^–1) than the dry season (December to July) recruits (0.41mmd^–1). These results are compared with snook collected in west Florida.     

Low Redox Decreases Potential Phosphorus Limitation on Soil Biogeochemical Cycling Along a Tropical Rainfall Gradient

Ecosystems - Humid tropical forests on highly weathered soils are often characterized by low bioavailable phosphorus (P) concentrations. These ecosystems also often experience low and fluctuating...     

Occurrence of Plant Growth Inhibitors in Tropical and Subtropical Vegetation

Acetone, ethanol and water extracts of mature fruits of yaupon (llex vomitoria Ait.) inhibited germination of mesquite (Prosopis juliflora Swartz DC. var. glandulosa (Torr.) Cockerell and sorghum (Sorghum bicolor (L.) Moench). Extracts of guava fruit (Psidium guajava L.) inhibited cucumber (Cucumis sativus L.) seed germination. Water soluble inhibitors were found in fruits, leaves, roots and bark of several tropical species representing 10 different families. Strong inhibition of cucumber seed germination and growth did not occur in sand when water extracts containing inhibitors were applied. Growth of corn, sorghum, cucumber and bean was reduced in soils collected beneath Malay apple (Eugenia malaccensis L.) trees. Plant growth-inhibitors occurred in all species studied in various plant parts, and some apparently affect the growth and ecology of other plant species.     

Tracking Human Disturbance in Mangroves: Estimating Harvest Rates on a Micronesian Island

Disturbance is an integral component in mangrove forest dynamics, influencing forest structure, composition, and function. The impacts of human disturbance, however, threaten mangrove forests throughout the world. Small-scale wood harvesting on the small Pacific island of Kosrae, Federated States of Micronesia, provided an instructive scenario for exploring the dynamics of human disturbance. Natural disturbances on the island are rare, but the growing island population harvests mangrove trees for firewood and construction materials, placing pressure on the forest. In order to determine recent harvest rates, we estimated gap ages by developing a time scale for mangrove wood decomposition and by quantifying growth rates for Rhizophora apiculata and Bruguiera gymnorhiza seedlings. Stump and log decomposition patterns were useful in aging gaps, although some patterns were more reliable than others. Seedlings of both species added approximately 5 nodes/year depending on light conditions. The island-wide harvest rate was 10% over the last 10 years, but the rates varied widely among different parts of the island. Rhizophora apiculata has been harvested preferentially, and a dearth of young trees where harvesting has been heaviest portends a decline of this highly desired species in the forest. Socio-economic data substantiated some but not all of the trends we observed. Even on a small island, local differences in both natural and anthropogenic factors are important to understanding forest dynamics.     

Soil Bacterial Community Shift Correlated with Change from Forest to Pasture Vegetation in a Tropical Soil

The change in vegetative cover of a Hawaiian soil from forest to pasture led to significant changes in the composition of the soil bacterial community. DNAs were extracted from both soil habitats and compared for the abundance of guanine-plus-cytosine (G+C) content, by analysis of abundance of phylotypes of small-subunit ribosomal DNA (SSU rDNA) amplified from fractions with 63 and 35% G+C contents, and by phylogenetic analysis of the dominant rDNA clones in the 63% G+C content fraction. All three methods showed differences between the forest and pasture habitats, providing evidence that vegetation had a strong influence on microbial community composition at three levels of taxon resolution. The forest soil DNA had a peak in G+C content of 61%, while the DNA of the pasture soil had a peak in G+C content of 67%. None of the dominant phylotypes found in the forest soil were detected in the pasture soil. For the 63% G+C fraction SSU rDNA sequence analysis of the three most dominant members revealed that their phyla changed from Fibrobacter and Syntrophomonas assemblages in the forest soil to Burkholderia and Rhizobium–Agrobacterium assemblages in the pasture soil.     

Rates of Dry-matter Production in Some Tropical Forest-tree Seedlings

An analysis of growth in full daylight of four tropical forest-treeseedlings and Helianthus was made with the view of ascertainingwhether woody plants in the tropics have a lower net assimilationrate than herbaceous plants as has been demonstrated in otherenvironments. Significant differences in net assimilation ratewere found between Helianthus and three of the woody speciestested: Chlorophora excelsa, Musanga cecropioides, and Terminaliaivorensis; the fourth woody plant, Ceiba pentandra, had an unusuallyhigh net assimilation rate which, in one experiment, did notdiffer significantly from the value found for Helianthus. The data are discussed in relation to the general problems ofcomparative growth-rates of herbaceous and woody plants andof factors contributing to differences in forest productivitybetween climates.     

Redox Fluctuations Frame Microbial Community Impacts on N-cycling Rates in a Humid Tropical Forest Soil

Fluctuating soil redox regimes may facilitate the co-occurrence of microbial nitrogen transformations with significantly different sensitivities to soil oxygen availability. In an upland humid tropical forest, we explored the impact of fluctuating redox regimes on gross nitrogen cycling rates and microbial community composition. Our results suggest that the rapidly fluctuating redox conditions that characterize these upland soils allow anoxic and oxic N processing to co-occur. Gross nitrogen mineralization was insensitive to soil redox fluctuations. In contrast, nitrifiers in this soil were directly affected by low redox periods, yet retained some activity even after 3–6 weeks of anoxia. Dissimilatory nitrate reduction to ammonium (DNRA) was less sensitive to oxygen exposure than expected, indicating that the organisms mediating this reductive process were also tolerant of unfavorable (oxic) conditions. Denitrification was a stronger sink for NO₃⁻ in consistently anoxic soils than in variable redox soils. Microbial biomass and community composition were maintained with redox fluctuation, but biomass decreased and composition changed under static oxic and anoxic soil regimes. Bacterial community structure was significantly correlated with rates of nitrification, denitrification and DNRA, suggesting that redox-control of soil microbial community structure was an important determinant of soil N-cycling rates. Specific nitrogen cycling functional groups in this environment (such as nitrifiers, DNRA organisms, and denitrifiers) appear to have adapted to nutrient resources that are spatially and temporally variable. In soils where oxygen is frequently depleted and re-supplied, characteristics of microbial tolerance and resilience can frame N cycling patterns.     

Causal Connections between Water Quality and Land Use in a Rural Tropical Island Watershed

We examined associations between riparian canopy cover, presence or absence of cattle, rainfall, solar radiation, month of year, dissolved oxygen, turbidity, salinity, and Enterococcus concentrations in riparian surface soils with Enterococcus geometric mean in-stream water concentrations at Waipā watershed on the north side of the Hawaiian island Kaua’i. Each 1% decrease in riparian canopy cover was associated with a 4.6 most probable number (MPN)/100 ml increase of the geometric mean of Enterococcus in stream water (P < 0.05). Each unit decrease in salinity (ppt) was associated with an increase of Enterococcus by 68.2 MPN/100 ml in-stream water geometric mean concentrations (P < 0.05). Month of year was also associated with increases in stream water Enterococcus geometric mean concentrations (P < 0.05). Reducing riparian canopy cover is associated with Enterococcus increases in stream water, suggesting that decreasing riparian vegetation density could increase fecal bacteria surface runoff.     

Microbial Composition and Wood Decomposition Rates Vary with Microclimate From the Ground to the Canopy in a Tropical Forest

Ecosystems - Wood decomposition is a key component of carbon cycling. However, our understanding of decomposition is limited by the absence of information regarding wood separated from the forest...     

A Meta-analysis of Climatic and Chemical Controls on Leaf Litter Decay Rates in Tropical Forests

Although tropical forests occupy a small fraction of the earth’s total land area, they play a disproportionately large role in regulating the global carbon cycle. Yet controls on both primary productivity and decomposition in tropical forests are not well-studied in comparison with temperate forests and grasslands, despite their extreme biogeochemical heterogeneity. To evaluate the relative importance of climate and foliar chemical variables in driving decomposition in tropical forests, I performed a meta-analysis of reported leaf litter decay rates throughout tropical forest ecosystems. Using a model selection procedure based on Akaike’s Information Criterion, I found that temperature and precipitation played little direct role in regulating decomposition rates, except in montane forests where cool temperatures slowed decay. Foliar concentrations of calcium, magnesium, nitrogen, phosphorus, and potassium were important predictors of mass loss rates, although each of these factors explained a very small amount of variance when considered in isolation. The large amount of unexplained variation in decomposition rates observed both within and across tropical forest sites may be due to other factors not explored here, such as soil biota or complex plant secondary chemistry. Carbon cycling in tropical forests seems to be modulated by the availability of multiple nutrients, underscoring the need for additional manipulative experiments to explore patterns of belowground nutrient limitation across the biome. Because models of decomposition developed in temperate ecosystems do not appear to be generalizable to wet tropical forests, new biogeochemical paradigms should be developed to accommodate their unique combination of climatic, edaphic, and biotic factors.     

Effects of ENSO and Temporal Rainfall Variation on the Dynamics of Successional Communities in Old-Field Succession of a Tropical Dry Forest

The effects of temporal variation of rainfall on secondary succession of tropical dry ecosystems are poorly understood. We studied effects of inter-seasonal and inter-year rainfall variation on the dynamics of regenerative successional communities of a tropical dry forest in Mexico. We emphasized the effects caused by the severe El Niño Southern Oscillation (ENSO) occurred in 2005. We established permanent plots in sites representing a chronosequence of Pasture (abandoned pastures, 0–1 years fallow age), Early (3–5), Intermediate (8–12), and Old-Growth Forest categories (n = 3 per category). In total, 8210 shrubs and trees 10 to 100-cm height were identified, measured, and monitored over four years. Rates of plant recruitment, growth and mortality, and gain and loss of species were quantified per season (dry vs. rainy), year, and successional category, considering whole communities and separating seedlings from sprouts and shrubs from trees. Community rates changed with rainfall variation without almost any effect of successional stage. Mortality and species loss rates peaked during the ENSO year and the following year; however, after two rainy years mortality peaked in the rainy season. Such changes could result from the severe drought in the ENSO year, and of the outbreak of biotic agents during the following rainy years. Growth, recruitment and species gain rates were higher in the rainy season but they were significantly reduced after the ENSO year. Seedlings exhibited higher recruitment and mortality rate than sprouts, and shrubs showed higher recruitment than trees. ENSO strongly impacted both the dynamics and trajectory of succession, creating transient fluctuations in the abundance and species richness of the communities. Overall, there was a net decline in plant and species density in most successional stages along the years. Therefore, strong drought events have critical consequences for regeneration dynamics, delaying the successional process and modifying the resilience of these systems.     

Vegetation mapping and nature conservation: a case study in Terceira Island (Azores)

In order to emphasize the importance of vegetation mapping for nature conservation purposes a case study in Terceira island (Azores) is presented, in which the importance of the natural vegetation of the eastern slope of Santa Bárbara volcano (which is part of the Site of Community Importance of Santa Bárbara–Pico Alto) is evaluated through the elaboration of its vegetation map. Fourteen (14) different natural vegetation types were identified: grasslands (1 type), peat bogs (2 types), scrubs (2), forests (5), successional vegetation (3) and vegetation of rocky slopes (1). All communities are protected under the Habitat and Species Directive (EC/92/43) and most of them are endemic to the Azores Islands. This fact, together with the significant number of Azorean endemic taxa (18), Macaronesian endemic taxa (5) and species protected under the Habitat and Species Directive (7), gives this area an important conservation value that justifies future protection actions. Vegetation mapping is an important tool for the characterization, evaluation and implementation of managing plans of natural areas of the Azores islands. The use of a floristic-based classification, supported by multivariate analysis and structural data, is an efficient methodology for the construction of these maps. The data collected comprise an important set of information about the distribution and abundance of natural vegetation types and endemic and rare species. This information was not available until now and is indispensable for the elaboration of management plans of Special Zones for Conservation that will be part of the NATURA 2000 network.     

香港长洲岛野生植物物种多样性与植被的研究

长洲岛共有野生维管植物237种,隶属于73科184属.植被类型以次生常绿阔叶林为主,其次为灌丛群落.植物区系以热带亚热带植物地理成分占优势,热带性质明显.该岛的植物区系与邻近岛屿植物区系具有极大的相似性,在香港植物区系组成中占有一定地位.由于人为活动和外来植物的影响,其植物多样性及植被的保护应引起注意.     

Linking Biodiversity,the Environment and Ecosystem Functioning: Ecological Functions of Dung Beetles Along a Tropical Elevational Gradient

Biodiversity loss and anthropogenic environmental changes are known to impact ecosystem functions and services. However, there are still some uncertainties such as confounding environmental factors other than community attributes that affect ecosystem functioning. Our goal was to understand what factors influence the performance of Scarabaeinae dung beetle functions, testing the hypothesis that both community attributes and environmental variables influence the performance. Toward this aim, we collected dung beetles along an elevational gradient (800–1400 m a.s.l.) in the Espinhaço mountain range (Brazil) and quantified dung beetle functions, that is, dung removal, soil excavation and secondary seed dispersal. We recorded data on environmental factors related to climate, soil and vegetation and evaluated their effects on dung beetle functions. Dung beetle ecological functions declined with elevation and the decrease was more pronounced than richness, indicating that there are other factors involved in functions performance besides diversity of beetles. Indeed, we found that the ecological functions measured were dependent on both dung beetle community attributes and environmental factors. Climate, soil and vegetation influenced dung beetle function performance as much as richness, abundance and body size. Dung beetle functional diversity did not explain any of the functions measured. Our study demonstrates that ecological functions are directly influenced by both community attributes and environmental variables and confirms the link between biodiversity, environment and ecosystem functioning.     

Home Range Use and Movement Patterns of Non-Native Feral Goats in a Tropical Island Montane Dry Landscape

Advances in wildlife telemetry and remote sensing technology facilitate studies of broad-scale movements of ungulates in relation to phenological shifts in vegetation. In tropical island dry landscapes, home range use and movements of non-native feral goats (Capra hircus) are largely unknown, yet this information is important to help guide the conservation and restoration of some of the world’s most critically endangered ecosystems. We hypothesized that feral goats would respond to resource pulses in vegetation by traveling to areas of recent green-up. To address this hypothesis, we fitted six male and seven female feral goats with Global Positioning System (GPS) collars equipped with an Argos satellite upload link to examine goat movements in relation to the plant phenology using the Normalized Difference Vegetation Index (NDVI). Movement patterns of 50% of males and 40% of females suggested conditional movement between non-overlapping home ranges throughout the year. A shift in NDVI values corresponded with movement between primary and secondary ranges of goats that exhibited long-distance movement, suggesting that vegetation phenology as captured by NDVI is a good indicator of the habitat and movement patterns of feral goats in tropical island dry landscapes. In the context of conservation and restoration of tropical island landscapes, the results of our study identify how non-native feral goats use resources across a broad landscape to sustain their populations and facilitate invasion of native plant communities.     

Vegetation structure in relation to micro-landform in an evergreen broad-leaved forest on Amami Ohshima Island,south-west Japan

The relationship between micro-landform and vegetation structure was studied in a plot that was established on a slope from the ridge to the valley bottom in an evergreen broad-leaved forest on Amami Ohshima Island, south-west Japan. Five micro-landform units recognized in the plot were grouped into the upper and lower slopes by their location in relation to the erosion front, and plant distributions and vegetation structure were compared between them. Although small trees with d.b.h. less than 10 cm occurred almost all over the plot, the occurrence of larger trees was limited mostly to the upper slope. Based on the spatial distribution pattern in the plot, woody species are classified into three groups: group A mostly or strictly confined to the upper slope; group B confined to the lower slope; and group C associated statistically with neither slope. A well-developed forest stand, whose dominant species wereCastanopsis sieboldii ssp.lutchuensis andSchima wallichii, was seen on the upper slope, whereas only a poorly developed stand was seen on the lower slope. The difference in stability of the land surface is likely to be the major cause of the observed differences in stand structure between the upper and lower slopes. It is suggested that the erosion front is important as a line demarcating different habitats within a slope from the ridge to the valley bottom.     

Tree Biomass Allocation and Its Model Additivity for Casuarina equisetifolia in a Tropical Forest of Hainan Island,China

Casuarina equisetifolia is commonly planted and used in the construction of coastal shelterbelt protection in Hainan Island. Thus, it is critical to accurately estimate the tree biomass of Casuarina equisetifolia L. for forest managers to evaluate the biomass stock in Hainan. The data for this work consisted of 72 trees, which were divided into three age groups: young forest, middle-aged forest, and mature forest. The proportion of biomass from the trunk significantly increased with age (P<0.05). However, the biomass of the branch and leaf decreased, and the biomass of the root did not change. To test whether the crown radius (CR) can improve biomass estimates of C. equisetifolia, we introduced CR into the biomass models. Here, six models were used to estimate the biomass of each component, including the trunk, the branch, the leaf, and the root. In each group, we selected one model among these six models for each component. The results showed that including the CR greatly improved the model performance and reduced the error, especially for the young and mature forests. In addition, to ensure biomass additivity, the selected equation for each component was fitted as a system of equations using seemingly unrelated regression (SUR). The SUR method not only gave efficient and accurate estimates but also achieved the logical additivity. The results in this study provide a robust estimation of tree biomass components and total biomass over three groups of C. equisetifolia.