Kelvin S.-H. Peh

Increasing carbon storage in intact African tropical forests

The response of terrestrial vegetation to a globally changing environment is central to predictions of future levels of atmospheric carbon dioxide. The role of tropical forests is critical because they are carbon-dense and highly productive. Inventory plots across Amazonia show that old-growth forests have increased in carbon storage over recent decades, but the response of one-third of the world’s tropical forests in Africa is largely unknown owing to an absence of spatially extensive observation networks. Here we report data from a ten-country network of long-term monitoring plots in African tropical forests. We find that across 79 plots (163 ha) above-ground carbon storage in live trees increased by 0.63 Mg C ha-1 yr-1 between 1968 and 2007 (95% confidence interval (CI), 0.22–0.94; mean interval, 1987–96). Extrapolation to unmeasured forest components (live roots, small trees, necromass) and scaling to the continent implies a total increase in carbon storage in African tropical forest trees of 0.34 Pg C yr-1 (CI, 0.15–0.43). These reported changes in carbon storage are similar to those reported for Amazonian forests per unit area, providing evidence that increasing carbon storage in old-growth forests is a pan-tropical phenomenon. Indeed, combining all standardized inventory data from this study and from tropical America and Asia together yields a comparable figure of 0.49 Mg C ha-1 yr-1 (n = 156; 562 ha; CI, 0.29–0.66; mean interval, 1987–97). This indicates a carbon sink of 1.3 Pg C yr-1 (CI, 0.8–1.6) across all tropical forests during recent decades. Taxon-specific analyses of African inventory and other data suggest that widespread changes in resource availability, such as increasing atmospheric carbon dioxide concentrations, may be the cause of the increase in carbon stocks, as some theory and models predict.

Drought–mortality relationships for tropical forests

*The rich ecology of tropical forests is intimately tied to their moisture status. Multi-site syntheses can provide a macro-scale view of these linkages and their susceptibility to changing climates. Here, we report pan-tropical and regional-scale analyses of tree vulnerability to drought. *We assembled available data on tropical forest tree stem mortality before, during, and after recent drought events, from 119 monitoring plots in 10 countries concentrated in Amazonia and Borneo. *In most sites, larger trees are disproportionately at risk. At least within Amazonia, low wood density trees are also at greater risk of drought-associated mortality, independent of size. For comparable drought intensities, trees in Borneo are more vulnerable than trees in the Amazon. There is some evidence for lagged impacts of drought, with mortality rates remaining elevated 2 yr after the meteorological event is over. *These findings indicate that repeated droughts would shift the functional composition of tropical forests toward smaller, denser-wooded trees. At very high drought intensities, the linear relationship between tree mortality and moisture stress apparently breaks down, suggesting the existence of moisture stress thresholds beyond which some tropical forests would suffer catastrophic tree mortality.

Residence times of woody biomass in tropical forests

Background: The woody biomass residence time (τw) of an ecosystem is an important variable for accurately simulating its biomass stocks. Methods and results: We reviewed published data from 177 forest plots across the tropics and found a six-fold variation (23–129 years) in τw across our dataset, with a median τw of ca. 50 years. This value is similar to the median default value across 21 vegetation models for tropical forests, although the range of values used in models is large (20 to 200 years). Conclusions: The notion of a constant τw across all tropical forests may be of limited utility, given the large observed variation in τw. We found that while there was little relationship between climate variables and τw, there was evidence that edaphic factors exerted a strong influence on τw. In both the Neotropics and the Paleotropics, τw was highest in heavily weathered soils, suggesting that low soil fertility and/or non-limiting soil physical conditions exert a critical influence on τw. There is considerable uncertainty in how τw will be affected by global environmental change, especially by increased atmospheric CO2. Even small changes in τw could significantly reduce the future tropical forest carbon sink predicted by many vegetation models.

POTENTIAL EFFECTS OF CLIMATE CHANGE ON ELEVATIONAL DISTRIBUTIONS OF TROPICAL BIRDS IN SOUTHEAST ASIA

Abstract An analysis of the elevational distributions of Southeast Asian birds over a 28-year period provides evidence for a potential upward shift for 94 common resident species. These species might have shifted their lower, upper, or both lower and upper boundaries toward a higher elevation in response to climate warming. These upward shifts occurred regardless of habitat specificity, further implicating climate warming, in addition to habitat loss, as a potentially important factor affecting the already imperiled biotas of Southeast Asia.

Invasive species in Southeast Asia: the knowledge so far.

This review deals with alien species invasion in Southeast Asia, an important conservation and management concern in the region. I report on the current and potential future impacts of biological invasions on biodiversity in Southeast Asia. Current knowledge of the invasive species in Southeast Asia is mostly based on anecdotal observations. Nevertheless, I attempt to compile existing empirical evidence on the negative effects of the biological invaders found in the region. These impacts include displacement of native biota, modification of ecosystems, hybridization, environmental disturbance, and economic loss. Any effective counter-measure will need to involve a multi-national strategy, yet such measure is challenging due to a broad spectrum of political and economic development models among the Southeast Asian countries. An overview of the taxonomic structure of the invasive species in Southeast Asia shows that the invasive plant and fish are the most represented taxonomic groups in all countries. The current research effort in invasion ecology from Southeast Asia is not being up to international standard in comparison to other regions, and the absence of recent international journal articles on invasive plant species reveals the biases in biological invasion-related research. The lack of research capacity and financial support from governments, and the inability to disseminate scholarly data in international journals are the possible reasons for the dearth of research literature on biological invasions from the region. Finally, a forward-looking agenda for the region should include improving the quality and quantity of biological invasion research; adopting a tough approach to the illegal release of wildlife; and applying multi-national strategies that integrate data sharing, prioritization, public awareness, policy work, capacity building, conservation actions and surveillance.

Soil Does Not Explain Monodominance in a Central African Tropical Forest

Background Soil characteristics have been hypothesised as one of the possible mechanisms leading to monodominance of Gilbertiodendron dewerei in some areas of Central Africa where higher-diversity forest would be expected. However, the differences in soil characteristics between the G. dewevrei-dominated forest and its adjacent mixed forest are still poorly understood. Here we present the soil characteristics of the G. dewevrei forest and quantify whether soil physical and chemical properties in this monodominant forest are significantly different from the adjacent mixed forest. Methodology/Principal Findings We sampled top soil (0–5, 5–10, 10–20, 20–30 cm) and subsoil (150–200 cm) using an augur in 6 × 1 ha areas of intact central Africa forest in SE Cameroon, three independent patches of G. dewevrei-dominated forest and three adjacent areas (450–800 m apart), all chosen to be topographically homogeneous. Analysis – subjected to Bonferroni correction procedure – revealed no significant differences between the monodominant and mixed forests in terms of soil texture, median particle size, bulk density, pH, carbon (C) content, nitrogen (N) content, C:N ratio, C:total NaOH-extractable P ratio and concentrations of labile phosphorous (P), inorganic NaOH-extractable P, total NaOH-extractable P, aluminium, barium, calcium, copper, iron, magnesium, manganese, molybdenum, nickel, potassium, selenium, silicon, sodium and zinc. Prior to Bonferroni correction procedure, there was a significant lower level of silicon concentration found in the monodominant than mixed forest deep soil; and a significant lower level of nickel concentration in the monodominant than mixed forest top soil. Nevertheless, these were likely to be the results of multiple tests of significance. Conclusions/Significance Our results do not provide clear evidence of soil mediation for the location of monodominant forests in relation to adjacent mixed forests. It is also likely that G. dewevrei does not influence soil chemistry in the monodominant forests.

Do insectivorous bird communities decline on land-bridge forest islands in Peninsular Malaysia?

To assess the impact of habitat fragmentation on tropical avian communities, we sampled lowland forest birds on six land-bridge islands and two mainland forest sites in Lake Kenyir, Peninsular Malaysia using timed point counts, hypothesizing that insectivorous birds are the worst affected guild. We used an information-theoretic approach to evaluate the effects of area, isolation, primary dietary guild (omnivore, frugivore and insectivore) and their interactions in predicting species richness, abundance and diversity. Our analysis showed that a model that considered the effects of area, dietary guild and their interaction best explained observed patterns of species richness. But a model considering both area and dietary guild best explained the variation in abundance. Notably, insectivorous birds were singled out as the dietary guild most sensitive to fragmentation, followed by frugivorous and omnivorous birds and hence provide support for our hypothesis. Assemblages of insectivorous birds were clearly depauperate on anthropogenic forest islands in Lake Kenyir and are consistent with forest fragmentation studies in the Neotropics. Given their specialized foraging ecology and diversity, conservation of intact communities of insectivorous bird guilds in Malaysia will be critical for maintaining predator–prey interactions in lowland tropical forests.

Characteristics of nocturnal roosts of house crows in Singapore

Roosts of house crows (Corvus splendens) in urban Singapore are perceived as a nuisance. Public complaints primarily focus on the fouling of gardens, pedestrian paths, buildings, and vehicles beneath or near roosts. We investigated their roost-site-selection patterns to help develop management recommendations for controlling urban crows. Roost sites (n = 30) were better illuminated by 89%, closer to food centers by 672.5 m, 33.1% more enclosed by tall buildings, and located in areas of greater human activity (measured using variables such as pedestrian traffic) by 47.0% more than randomly selected non-roost sites (n = 30). Roost trees, which primarily consisted of angsana (Pterocarpus indicus), were taller by 66.7%, spaced farther apart by 4.1 m, had larger crown volume by 275.9%, and had denser crowns by 15.4% more than non-roost trees. Crow roosts were surrounded by 76.3% more built-up urban environments with 90.3% less vegetated areas and 89.8% fewer water bodies than non-roost trees. However, discriminant analysis (DA) identified only 4 variables that clearly distinguished roost from non-roost sites: average distance to surrounding buildings, clear bole height (the visible portion of tree trunk), distance between adjacent roost trees, and percentage of the area around the roost tree covered with vegetation. Discriminanat analysis correctly classified 100% of roost trees and 80% of non-roost trees. We recommend measures designed to avoid the creation of new roost sites (e.g., planting short [<18 m tall] trees), making existing roost sites less attractive (e.g., tree pruning), and establishing alternate roosting sites.

PHENOLOGY OF TROPICAL BIRDS IN PENINSULAR MALAYSIA: EFFECTS OF SELECTIVE LOGGING AND FOOD RESOURCES

Abstract The increasing prevalence of selective logging in Southeast Asian tropical rainforests compels much-needed studies to examine its effects on the vital life-cycle events of their resident understory birds, which are particularly sensitive to such degradation. Food abundance, which can be an important factor in avian phenology, may be affected by selective logging. Therefore, studies that compare food abundance and breeding and molting occurrence simultaneously at the same sites are important for the ecological monitoring of such logging regimes. Using bimonthly mist netting in two rainforest areas in Peninsular Malaysia, we assessed the breeding and molting occurrence and diets of understory birds and compared the abundance of food resources in unlogged forests and forests that had been selectively logged 30 years before. Our study revealed no differences between forest types in overall understory-resident bird abundance; comparative species richness; feeding-guild composition; breeding and molting occurrence and temporal variation; or arthropod, fruit, and flower abundance. The similarity in food resources could account for the similarity in avian phenological characteristics between forest types. Increased breeding corresponded with increased food abundance at the feeding-guild level, and dietary data supported these findings. Breeding cycles of sensitive indicator groups such as understory resident birds are likely to depend on food abundance and forest structure and, thus, on selective logging. Our observation of undiminished avian breeding and molting occurrence in selectively logged forests is an encouraging indication of their conservation potential. However, we emphasize that differences in selective logging practices could affect the forest structure and avifauna differently. Setting thresholds for extraction and regeneration time and subscribing to minimum-impact methods that reduce collateral damage are essential if selectively logged areas are to maintain most of their initial biodiversity. Fenología de Aves Tropicales en Malasia Peninsular: Efectos de la Tala Selectiva y los Recursos Alimenticios

Artificial nest and seed predation experiments on tropical southeast Asian islands

Southeast Asia is rapidly losing native habitats and the consequences of this are poorly understood. Because habitat loss and disturbance can affect avian and seed survivorship, we conducted artificial nest and seed predation experiments on tropical southeast Asian islands. Data among islands and fragments or different forest types (e.g. primary versus exotic forest) within the islands are compared. On Singapore Island, predation among different forest types (primary, secondary and woodland) did not differ. Only at one of the sites, nest predation was higher at 75 m from the forest edge than at 25 m. In other sites, predation did not differ in relation to the distance from the forest edge. Predation among 10 small (0.8–1026 ha) Singaporean islands differed. However, none of the environmental variables (e.g. island area) could explain the predation differences. The lowest predation of both nests and seeds was recorded in the primary forest areas of a contiguous forest (25 500 ha) in central Java (Linggoasri). Small mammals were the main predators on Singapore and other surrounding islands. However, the index of potential predator abundance, overall, did not correlate with predation. While larger and more pristine forests may be better for avian and seed survivorship, pinpointing variables affecting both artificial nest and seed predation may be difficult.