Implications of global warming for the climate of African rainforests

African rainforests are likely to be vulnerable to changes in temperature and precipitation, yet there has been relatively little research to suggest how the regional climate might respond to global warming. This study presents projections of temperature and precipitation indices of relevance to African rainforests, using global climate model experiments to identify local change as a function of global temperature increase. A multi-model ensemble and two perturbed physics ensembles are used, one with over 100 members. In the east of the Congo Basin, most models (92%) show a wet signal, whereas in west equatorial Africa, the majority (73%) project an increase in dry season water deficits. This drying is amplified as global temperature increases, and in over half of coupled models by greater than 3% per °C of global warming. Analysis of atmospheric dynamics in a subset of models suggests that this could be partly because of a rearrangement of zonal circulation, with enhanced convection in the Indian Ocean and anomalous subsidence over west equatorial Africa, the Atlantic Ocean and, in some seasons, the Amazon Basin. Further research to assess the plausibility of this and other mechanisms is important, given the potential implications of drying in these rainforest regions.     

Spatial patterns and recent trends in the climate of tropical rainforest regions

We present an analysis of the mean climate and climatic trends of tropical rainforest regions over the period 1960-1998, with the aid of explicit maps of forest cover and climatological databases. Until the mid-1970s most regions showed little trend in temperature, and the western Amazon experienced a net cooling probably associated with an interdecadal oscillation. Since the mid-1970s, all tropical rainforest regions have experienced a strong warming at a mean rate of 0.26 +/- 0.05 degrees C per decade, in synchrony with a global rise in temperature that has been attributed to the anthropogenic greenhouse effect. Over the study period, precipitation appears to have declined in tropical rainforest regions at a rate of 1.0 +/- 0.8% per decade (p < 5%), declining sharply in northern tropical Africa (at 3-4% per decade), declining marginally in tropical Asia and showing no significant trend in Amazonia. There is no evidence so far of a decline in precipitation in eastern Amazonia, a region thought vulnerable to climate-change-induced drying. The strong drying trend in Africa suggests that this should be a priority study region for understanding the impact of drought on tropical rainforests. We develop and use a dry-season index to study variations in the length and intensity of the dry season. Only African and Indian tropical rainforests appear to have seen a significant increase in dry-season intensity. In terms of interannual variability, the El Niño-Southern Oscillation (ENSO) is the primary driver of temperature variations across the tropics and of precipitation fluctuations for large areas of the Americas and southeast Asia. The relation between ENSO and tropical African precipitation appears less direct.     

Long-Term Foraging Expeditions (Molongo) among the Baka Hunter-Gatherers in the Northwestern Congo Basin, with Special Reference to the “Wild Yam Question”

While “Pygmy” hunter-gatherers have generally been assumed to be the original inhabitants of the central African rainforest, recent studies dispute the viability of subsistence in tropical rainforests through foraging alone. There are, however, few studies that are based on sound data of actual long-term foraging. This paper examines the viability of “pure” foraging in tropical rainforests, based on data from participant observation of a long-term foraging expedition among the Baka in the northwestern Congo Basin. The group was observed to subsist solely on wild food resources, particularly wild yams, for two-and-half months. The study was carried out during the dry season when food resources are generally thought to be scarce in tropical rainforests.     

Stomatal sensitivity to vapour pressure deficit of temperate and tropical evergreen rainforest trees of Australia

Although rainforests of eastern Australia grow in regions of high precipitation, there is a shift from a summer dry season in the temperate south to a winter dry season in the tropical north. Therefore, rainforest trees of eastern Australia provide an opportunity to investigate stomatal sensitivity of mesic trees to vapour pressure deficit (VPD) along a gradient in seasonality of precipitation. Eight rainforest canopy tree species were selected to cover the latitudinal range of rainforests in eastern Australia. Seedlings of these species were grown for a year in glasshouses under ambient conditions or at low VPD and water vapour exchange of leaves was measured during summer. Tropical species, which experience summer-dominant precipitation, showed higher stomatal sensitivities to VPD than temperate species, which experience winter-dominant precipitation. Growing plants under a low VPD increased stomatal sensitivity to increasing VPD in most species. The high stomatal sensitivity to VPD of the tropical species is consistent with the infrequent water stress experienced during their growing season and suggests a high susceptibility to water deficits. In contrast, temperate species may use other mechanisms to maintain photosynthesis under the relatively drier conditions of the temperate growing season.     

Amazon Basin climate under global warming: the role of the sea surface temperature

The Hadley Centre coupled climate-carbon cycle model (HadCM3LC) predicts loss of the Amazon rainforest in response to future anthropogenic greenhouse gas emissions. In this study, the atmospheric component of HadCM3LC is used to assess the role of simulated changes in mid-twenty-first century sea surface temperature (SST) in Amazon Basin climate change. When the full HadCM3LC SST anomalies (SSTAs) are used, the atmosphere model reproduces the Amazon Basin climate change exhibited by HadCM3LC, including much of the reduction in Amazon Basin rainfall. This rainfall change is shown to be the combined effect of SSTAs in both the tropical Atlantic and the Pacific, with roughly equal contributions from each basin. The greatest rainfall reduction occurs from May to October, outside of the mature South American monsoon (SAM) season. This dry season response is the combined effect of a more rapid warming of the tropical North Atlantic relative to the south, and warm SSTAs in the tropical east Pacific. Conversely, a weak enhancement of mature SAM season rainfall in response to Atlantic SST change is suppressed by the atmospheric response to Pacific SST. This net wet season response is sufficient to prevent dry season soil moisture deficits from being recharged through the SAM season, leading to a perennial soil moisture reduction and an associated 30% reduction in annual Amazon Basin net primary productivity (NPP). A further 23% NPP reduction occurs in response to a 3.5 degrees C warmer air temperature associated with a global mean SST warming.     

Photosynthetic responses to vapour pressure deficit in temperate and tropical evergreen rainforest trees of Australia

Rainforests occur in high precipitation areas of eastern Australia, along a gradient in seasonality of precipitation, ranging from a summer dry season in the temperate south to a winter dry season in the tropical north. The response of net photosynthesis to increasing vapour pressure deficit (VPD) was measured in a range of Australian rainforest trees from different latitudes to investigate possible differences in their response to atmospheric drought. Plants were grown in glasshouses under ambient or low VPD to determine the effect of growth VPD on the photosynthetic response. Temperate species, which experience low summer precipitation, were found to maintain maximum net photosynthesis over the measurement range of VPD (0.5–1.9 kPa). In contrast, the tropical species from climates with high summer precipitation showed large reductions in net photosynthesis with increasing VPD. Temperate species showed higher intrinsic water-use efficiencies under low VPD than the tropical species, whereas their efficiencies were similar under high VPD. Growing plants under a low VPD had little effect on either the photosynthetic response to VPD or the intrinsic water-use efficiency of the species. These different responses of gas exchange to VPD shown by the tropical and temperate rainforest species may reflect different strategies to maximise productivity in their respective climates.     

Simulation of forest carbon dynamics based on a dry-matter production model

Effects of dry season upon a tropical rainforest ecosystem are analysed by employing the microcomputer model developed in a previous paper (Oikawa, 1985). Surplus production (P _s) illustrated in a three-dimensional figure linearly decreases with the length of dry season (λ). Simulation experiments demonstrate that all the three strata constituting the tropical rainforest ecosystem model are able to exist stably and perpetually unless λ exceeds four months. However, it is suggested that the dry season lasting five months brings about a transition from a tropical rainforest to a subtropical deciduous forest, because the upper story fails in a stable and perpetual occurrence at λ =5 months. Though the total living phytomass gradually decreases with prolonged dry season within four months, supplementary increase of living phytomass is observed in the middle story. A considerable amount of soil organic dead accumulated is also observed with increasing dry season, so that fairly constant ecosystem biomass including soil organic dead is predicted independent of the length of dry season so long as it is less than five months.     

Savanna–forest hysteresis in the tropics

A simple dynamic model relating forest area in a region, its contribution to dry season precipitation and the effect on its own establishment was developed. The model equation shows hysteresis between forest and savannas as a function of imported dry season precipitation. Regions are either dominated by forests or savannas, with each ecosystem showing stability despite changes in imported dry season precipitation. Deforestation beyond a certain threshold value, however, could cause a collapse of forest ecosystems and replacement by savannas in marginal areas. The predictions of this model corroborate pollen core analysis in the Amazon basin, where historical stability of tropical forest cover has been shown despite global climate change.     

A study on the relationship between Atlantic sea surface temperature and Amazonian greenness

The growth of tropical rainforest in Amazon is critically vulnerable to the change in rainfall and radiation than in temperature, and that amount of rainfall and cloudiness in the northeast region of South American is strongly affected by the Atlantic sea surface temperature (SST). Results from recent model experiments for future climate projection have indicated a reduction of Amazonian greenness by a weakening of tropical vapor circulation system related with the change in SST. Therefore, the observational investigation of the relations between the Amazon greenness and Atlantic SST is fundamental to understand the response of Amazonian tropical forest to climate change. In this study, the effect of Atlantic SST on the spatial and temporal change of the Normalized Difference Vegetation Index (NDVI) in the Amazonian region is examined by using satellite remote sensing data for the period of 1981–2001. A strong correlation between NDVI and SST is found for certain regions in Amazon during the periods of 1980s and 1990s, respectively. In addition, strong correlations with NDVI lagging behind SST for two months and one year, respectively, are also identified from the interannual December-to-February (rain season) variations during 1981–2001. Despite these findings, the mechanisms behind the identified correlation remain unclear. Further analyses using observed precipitation and radiation data are required to understand the potential changes of Amazonian rainforest in the context of global warming.     

Evidence for strong seasonality in the carbon storage and carbon use efficiency of an Amazonian forest

The relative contribution of gross primary production and ecosystem respiration to seasonal changes in the net carbon flux of tropical forests remains poorly quantified by both modelling and field studies. We use data assimilation to combine nine ecological time series from an eastern Amazonian forest, with mass balance constraints from an ecosystem carbon cycle model. The resulting analysis quantifies, with uncertainty estimates, the seasonal changes in the net carbon flux of a tropical rainforest which experiences a pronounced dry season. We show that the carbon accumulation in this forest was four times greater in the dry season than in the wet season and that this was accompanied by a 5% increase in the carbon use efficiency. This seasonal response was caused by a dry season increase in gross primary productivity, in response to radiation and a similar magnitude decrease in heterotrophic respiration, in response to drying soils. The analysis also predicts increased carbon allocation to leaves and wood in the wet season, and greater allocation to fine roots in the dry season. This study demonstrates implementation of seasonal variations in parameters better enables models to simulate observed patterns in data. In particular, we highlight the necessity to simulate the seasonal patterns of heterotrophic respiration to accurately simulate the net carbon flux seasonal tropical forest.     

High-resolution time series of vessel density in Kenyan mangrove trees reveal a link with climate

Tropical trees are often excluded from dendrochronological investigations because of a lack of distinct growth ring boundaries, causing a gap in paleoclimate reconstructions from tropical regions. The potential use of time series of vessel features (density, diameter, surface area and hydraulic conductivity) combined with spectral analysis as a proxy for environmental conditions in the mangrove Rhizophora mucronata was investigated. Intra-annual differences in the vessel features revealed a trade-off between hydraulic efficiency (large vessels) during the rainy season and hydraulic safety (small, more numerous vessels) during the dry season. In addition to the earlywood-latewood variations, a semiannual signal was discovered in the vessel density and diameters after Fourier transformation. The similarity in the Fourier spectra of the vessel features and the climate data, in particular mean relative humidity and precipitation, provides strong evidence for a climatic driving force for the intra-annual variability of the vessel features. The high-resolution approach used in this study, in combination with spectral analysis, may have great potential for the study of climate variability in tropical regions.     

西双版纳不同热带森林下土壤铵态氮和硝态氮动态研究

用顶盖埋管法(Close-Top Tube Incubations)就西双版纳3种热带森林（热带季节雨林、片断热带雨林、橡胶林）研究了土壤铵态氮（NH４-N）和硝态氮（NO３-N）以及土壤氮素矿化速率的季节动态情况。结果表明：西双版纳3种不同林型土壤NH４-N、NO３-N和氮素矿化速率均具有明显的季节性变化。NH４-N以干热季（4月末）最高（平均为26.92 mg*kg－1）和干季（2月末）最低(平均为12.01 mg*kg－1);NO３-N则以雨季中期（7月中旬）最高(平均为8.9 mg*kg－1）和干季（2月末）最低(平均为4.04 mg*kg－1）;矿化速率则以干热季（(2月末～4月末)最高(平均为0.496 mg*kg－1*d－1）,以雨季（7月中旬～11月初）最低(平均为0.0037 mg*kg－1*d－1）。就不同林型而言,季节雨林年均氮矿化速率(0.319 mg*kg－1*d－1)>片断热带雨林(0.25 mg*kg－1*d－1)>橡胶林(0.074 mg*kg－1*d－1)。     

Distributional patterns of herpetofauna in monsoon rainforests of the Northern Territory,Australia

Abstract The herpetofauna of 50 monsoon rainforest patches in the Top End of the Northern Territory was surveyed during the dry season of 1990. This fauna contains few obligate monsoon rainforest species, many species which favour this habitat as part of a broad habitat range and a large number of species (indeed most of the regional species pool) that occasionally occur within monsoon rainforests. The taxonomic composition of species favouring monsoon rainforests is a non-random selection from the regional pool, with relatively few species in the families Agamidae and Scincidae occurring commonly in monsoon rainforests. Environmental variation among the rainforest patches sampled was portrayed by ordination, with the first axis corresponding to an environmental gradient from coastal sites to inland rocky rainforests and the second a gradient from relatively dry thickets to tall dense rainforests close to water. The distributions of herpetofauna species were depicted on this ordination space. Most frog species occurred in relatively wet rainforests and most gecko species were relatively restricted to drier rainforests. A substantial component of the herpetofauna was associated with rainforests on rocky substrate. In contrast to this relatively good association with these defined gradients, there was little apparent influence of patch size or level of disturbance on the distribution of individual species of herpetofauna. Sampling month was related to the abundance of many species, with many frog species and some snake and skink species declining (but some skink and one frog species increasing) in abundance in rainforest patches during the late dry season. This seasonal change in abundance is not due to movements from rainforest patches to adjacent vegetation types (or vice versa) but rather to total landscape (cross-habitat) changes in abundance (or detectability). The species composition of patches tended to be idiosyncratic, with substantial variability in composition, even between nearby patches of like environment. Hence it is not possible to nominate a representative rainforest herpetofauna, and indeed a classification of all quadrats (including those from rainforests, rainforest edges and adjacent habitats) based on herpetofauna species composition grouped many non-rainforest quadrats with those from rainforests. There was no rainforest edge herpetofauna assemblage. The herpetofauna from rainforests of the Northern Territory was similar to but somewhat richer than that recorded from the even more attenuated monsoon rainforest area of the Kimberley of northwestern Australia, but shared relatively few species with a sampling from monsoon rainforests from western Cape York. Frog species were more likely to be recorded across these three regions than were snake species. The number of herpetofaunal species per patch was low compared to tropical forests in northeastern Australia, Asia and central America. The long dry season of the Top End may contribute to this impoverishment. However, the small total area of monsoon rainforests in this region, the current scattered network of patches and historical fluctuations in extent and distribution of this habitat are probably at least as important.     

Climatic seasonality, resource bottlenecks, and abundance of rainforest birds: implications for global climate change

We demonstrate that within-year climatic variability, particularly rainfall seasonality, is the most significant variable explaining spatial patterns of bird abundance in Australian tropical rainforest. The likely mechanism causing this pattern is a resource bottleneck (insects, nectar, and fruit) during the dry season that limits the population size of many species. The patterns support both the diversity–climatic–stability hypothesis and the species–energy hypothesis but clearly show that seasonality in energy availability may be a more significant factor than annual totals or means. An index of dry season severity is proposed that quantifies the combined effect of the degree of dryness and the duration of the dry season. We suggest that the predicted increases in seasonality due to global climate change could produce significant declines in bird abundance, further exacerbating the impacts of decreased range size, increased fragmentation, and decreased population size likely to occur as a result of increasing temperature. We suggest that increasing climatic seasonality due to global climate change has the potential to have significant negative impacts on tropical biodiversity.     

Wet- vs. Dry-Season Transpiration in an Amazonian Rain Forest Palm Iriartea deltoidea

The wet and dry seasons in tropical rain forests can differ in precipitation, soil moisture and irradiance more significantly than often assumed. This could potentially affect the water relations of many tree species that may exhibit either increased transpiration in the dry season as a response to the increased irradiance or decreased transpiration as a result of decreases in soil moisture and increases in atmospheric vapor pressure deficit (VPD). Atmospheric data, soil moisture data and sap fluxes in Iriartea deltoidea palms were measured in eastern Ecuador during the wet and dry seasons. There were no differences between total daily sap fluxes in I. deltoidea palms during the wet and dry seasons; however, evaporative demand was significantly higher in the dry season and therefore, transpiration was more restricted by stomatal closure during the dry season than the wet season. This is likely the result of larger atmospheric VPD during the dry season compared with the wet season and possibly the result of reduced soil moisture availability. Additionally, based on published tree abundances in this area, measured sap fluxes in I. deltoidea were scaled up to the hectare level. Transpiration from I. deltoidea palms was estimated to be around 0.03 mm/d, which could represent about 1 percent of total transpiration in this area of the Amazon rain forest. If climate change predictions for more lengthy tropical dry periods are realized, greater stomatal control of dry-season sap flux has the potential to become even more prevalent in tropical species.     

Global climate cycles and cyclones: consequences for rainfall patterns and lemur reproduction in southeastern Madagascar

Most studies that examine the influence of climatic change on flora and fauna have focused on northern latitudes; however, there is increasing recognition that tropical regions are also being affected. Despite this, regions such as Madagascar, which are rich in endemic biodiversity but may have low adaptive capacity to climatic change, are poorly represented in studies examining the effects of climate variability on biota. We investigated how El Niño Southern Oscillations (ENSO) influence precipitation patterns in the rainforest region of southeastern Madagascar (1962–2006) and then constructed models to assess the potential contribution of climatic variables on the reproductive parameters of the Milne Edward's sifaka, a threatened lemur species (Propithecus edwardsi), over a 20‐year period. The Southern Oscillation Index of sea surface temperature (SST) anomalies in the tropical Pacific was associated with precipitation patterns including wetter wet seasons during warmer phases and drier dry seasons following cooler phases. The best‐supported models of lemur fecundity (female offspring per female that survive to 1 year of age per year) included cyclone presence during gestation and ENSO phase before conception and during the first 6 months of life. Models also suggested that heavy rains during gestation may limit birth rates and that prolonged drought during female lactation may limit first year offspring survival; although these variables were given little importance for predicting overall fecundity relative to ENSO phases and cyclone presence. Our results linking lemur reproduction with climatic variability suggest that climatic changes may be an additional threat to Madagascar's unique and already endangered flora and fauna. The association between precipitation in southeastern Madagascar and SST anomalies in the tropical Pacific suggests that dynamics of wildlife populations even in tropical areas such as Madagascar can be affected by global climate cycles making them potentially vulnerable to global climate change.     

Climate change in tropical fresh waters (comment on the paper ‘Plankton dynamics under different climatic conditions in space and time’ by de Senerpont Domis et al., )

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Foraging ecology in the lizard Anolis oculatus (Iguanidae) from Dominica, West Indies

In general, populations of Anolis lizards on West Indian islands face few predators, are at high density and are thought to be limited by food. This paper describes how the foraging ecology of Anolis oculatus , a solitary species confined to the island of Dominica, Lesser Antilles, varies with habitat and season in relation to the quantity and quality of available food.
Availability of invertebrate food (determined using pitfall traps and sticky traps) was greater in a dry scrub woodland site, Cabrits National Park (CNP), compared with a montane rainforest site, Palmiste Ridge. In the former, there were general increases in abundance, volumes of softbodied prey and sizes of invertebrates from dry season to wet season. Concomitant dietary changes, as determined principally by stomach flushing, included an increase (by volume) in the proportion of soft-bodied prey. Dietary analyses confirmed the importance of ants (Formicidae) in the diet of A. oculatus , although for large individuals (mainly adult males) at the rainforest site, soft-bodied prey such as Oligochaeta and Orthoptera were, in volumetric terms, more important. Prey capture observations showed that large A. oculatus fed mainly above ground. Anolis oculatus at the montane rainforest site used higher perch heights than those in dry scrub woodland, although in both habitats, small individuals (mainly juveniles) fed mainly at ground level on ants. In the dry season in CNP, the diet (in volumetric terms) of smaller Anolis was dominated by hardbodied prey such as ants, springtails (Collembola), barklice (Psocoptera) and beetles (Coleoptera). large Anolis used springtails and barklice to a lesser extent, resulting in relatively low food niche overlap values between size classes and a reduced potential for intraspecific competition compared with the wet season.     

Seasonality of insect abundance in an Australian upland tropical rainforest

Monthly light and Malaise trap catches, taken over 31 months, were used to examine seasonal and annual changes in the abundance of predominant orders of upland tropical rainforest insects. Insect numbers and biomass increased during the late dry season, reached a peak during the wetter months, and declined during the early dry period. Fluctuations in insect abundance appeared to relate to (1) climatic factors such as length and severity of the dry season, or amount and period of rainfall; and (2) food availability such as an increase in the production of new leaves, or flowering and fruiting periodicity.     

Regional atmospheric CO2 inversion reveals seasonal and geographic differences in Amazon net biome exchange

Understanding tropical rainforest carbon exchange and its response to heat and drought is critical for quantifying the effects of climate change on tropical ecosystems, including global climate–carbon feedbacks. Of particular importance for the global carbon budget is net biome exchange of CO₂ with the atmosphere (NBE), which represents nonfire carbon fluxes into and out of biomass and soils. Subannual and sub‐Basin Amazon NBE estimates have relied heavily on process‐based biosphere models, despite lack of model agreement with plot‐scale observations. We present a new analysis of airborne measurements that reveals monthly, regional‐scale (~1–8 × 10⁶ km²) NBE variations. We develop a regional atmospheric CO₂ inversion that provides the first analysis of geographic and temporal variability in Amazon biosphere–atmosphere carbon exchange and that is minimally influenced by biosphere model‐based first guesses of seasonal and annual mean fluxes. We find little evidence for a clear seasonal cycle in Amazon NBE but do find NBE sensitivity to aberrations from long‐term mean climate. In particular, we observe increased NBE (more carbon emitted to the atmosphere) associated with heat and drought in 2010, and correlations between wet season NBE and precipitation (negative correlation) and temperature (positive correlation). In the eastern Amazon, pulses of increased NBE persisted through 2011, suggesting legacy effects of 2010 heat and drought. We also identify regional differences in postdrought NBE that appear related to long‐term water availability. We examine satellite proxies and find evidence for higher gross primary productivity (GPP) during a pulse of increased carbon uptake in 2011, and lower GPP during a period of increased NBE in the 2010 dry season drought, but links between GPP and NBE changes are not conclusive. These results provide novel evidence of NBE sensitivity to short‐term temperature and moisture extremes in the Amazon, where monthly and sub‐Basin estimates have not been previously available.