Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees

We investigated how water transport capacity, wood density and wood anatomy were related to leaf photosynthetic traits in two lowland forests in Panama. Leaf-specific hydraulic conductivity (k_L) of upper branches was positively correlated with maximum rates of net CO₂ assimilation per unit leaf area (A_area) and stomatal conductance (g_s) across 20 species of canopy trees. Maximum k_L showed stronger correlation with A_area than initial k_L suggesting that allocation to photosynthetic potential is proportional to maximum water transport capacity. Terminal branch k_L was negatively correlated with A_area/g_s and positively correlated with photosynthesis per unit N, indicating a trade-off of efficient use of water against efficient use of N in photosynthesis as water transport efficiency varied. Specific hydraulic conductivity calculated from xylem anatomical characteristics (k_theoretical) was positively related to A_area and k_L, consistent with relationships among physiological measurements. Branch wood density was negatively correlated with wood water storage at saturation, k_L, A_area, net CO₂ assimilation per unit leaf mass (A_mass), and minimum leaf water potential measured on covered leaves, suggesting that wood density constrains physiological function to specific operating ranges. Kinetic and static indices of branch water transport capacity thus exhibit considerable co-ordination with allocation to potential carbon gain. Our results indicate that understanding tree hydraulic architecture provides added insights to comparisons of leaf level measurements among species, and links photosynthetic allocation patterns with branch hydraulic processes.     

Photosynthetic acclimation of the liana Stigmaphyllon lindenianum to light changes in a tropical dry forest canopy

Tropical plant canopies show abrupt changes in light conditions across small differences in spatial and temporal scales. Given the canopy light heterogeneity, plants in this stratum should express a high degree of plasticity, both in space (allocation to plant modules as a function of opportunity for resource access) and time (photosynthetic adjustment to temporal changes in the local environment). Using a construction crane for canopy access, we studied light acclimation of the liana Stigmaphyllon lindenianum to sun and shade environments in a tropical dry forest in Panama during the wet season. Measured branches were randomly distributed in one of four light sequences: high- to low-light branches started the experiment under sun and were transferred to shade during the second part of the experiment; low- to high-light branches (LH) were exposed to the opposite sequence of light treatments; and high-light and low-light controls , which were exposed only to sun and shade environments, respectively, throughout the experiment. Shade branches were set inside enclosures wrapped in 63% greenhouse shade cloth. After 2 months, we transferred experimental branches to opposite light conditions by relocating the enclosures. Leaf mortality was considerably higher under shade, both before and after the transfer. LH branches reversed the pattern of mortality by increasing new leaf production after the transfer. Rates of photosynthesis at light saturation, light compensation points, and dark respiration rates of transferred branches matched those of controls for the new light treatment, indicating rapid photochemical acclimation. The post-expansion acclimation of sun and shade foliage occurred with little modification of leaf structure. High photosynthetic plasticity was reflected in an almost immediate ability to respond to significant changes in light. This response did not depend on the initial light environment, but was determined by exposure to new light conditions. Stigmaphyllon responded rapidly to light changes through the functional adjustment of already expanded foliage and an increase in leaf production in places with high opportunity for carbon gain. Received: 24 April 1998 / Accepted: 11 May 1999     

Potential impacts of climate change on the environmental services of humid tropical alpine regions

Aim Humid tropical alpine environments are crucial ecosystems that sustain biodiversity, biological processes, carbon storage and surface water provision. They are identified as one of the terrestrial ecosystems most vulnerable to global environmental change. Despite their vulnerability, and the importance for regional biodiversity conservation and socio‐economic development, they are among the least studied and described ecosystems in the world. This paper reviews the state of knowledge about tropical alpine environments, and provides an integrated assessment of the potential threats of global climate change on the major ecosystem processes. Location Humid tropical alpine regions occur between the upper forest line and the perennial snow border in the upper regions of the Andes, the Afroalpine belt and Indonesia and Papua New Guinea. Results and main conclusions Climate change will displace ecosystem boundaries and strongly reduce the total area of tropical alpine regions. Displacement and increased isolation of the remaining patches will induce species extinction and biodiversity loss. Drier and warmer soil conditions will cause a faster organic carbon turnover, decreasing the below‐ground organic carbon storage. Since most of the organic carbon is currently stored in the soils, it is unlikely that an increase in above‐ground biomass will be able to offset soil carbon loss at an ecosystem level. Therefore a net release of carbon to the atmosphere is expected. Changes in precipitation patterns, increased evapotranspiration and alterations of the soil properties will have a major impact on water supply. Many regions are in danger of a significantly reduced or less reliable stream flow. The magnitude and even the trend of most of these effects depend strongly on local climatic, hydrological and ecological conditions. The extreme spatial gradients in these conditions put the sustainability of ecosystem management at risk.     

The 'human revolution' in lowland tropical Southeast Asia: the antiquity and behavior of anatomically modern humans at Niah Cave (Sarawak, Borneo)

Recent research in Europe, Africa, and Southeast Asia suggests that we can no longer assume a direct and exclusive link between anatomically modern humans and behavioral modernity (the 'human revolution'), and assume that the presence of either one implies the presence of the other: discussions of the emergence of cultural complexity have to proceed with greater scrutiny of the evidence on a site-by-site basis to establish secure associations between the archaeology present there and the hominins who created it. This paper presents one such case study: Niah Cave in Sarawak on the island of Borneo, famous for the discovery in 1958 in the West Mouth of the Great Cave of a modern human skull, the 'Deep Skull,' controversially associated with radiocarbon dates of ca. 40,000 years before the present. A new chronostratigraphy has been developed through a re-investigation of the lithostratigraphy left by the earlier excavations, AMS-dating using three different comparative pre-treatments including ABOX of charcoal, and U-series using the Diffusion-Absorption model applied to fragments of bones from the Deep Skull itself. Stratigraphic reasons for earlier uncertainties about the antiquity of the skull are examined, and it is shown not to be an 'intrusive' artifact. It was probably excavated from fluvial-pond-desiccation deposits that accumulated episodically in a shallow basin immediately behind the cave entrance lip, in a climate that ranged from times of comparative aridity with complete desiccation, to episodes of greater surface wetness, changes attributed to regional climatic fluctuations. Vegetation outside the cave varied significantly over time, including wet lowland forest, montane forest, savannah, and grassland. The new dates and the lithostratigraphy relate the Deep Skull to evidence of episodes of human activity that range in date from ca. 46,000 to ca. 34,000 years ago. Initial investigations of sediment scorching, pollen, palynomorphs, phytoliths, plant macrofossils, and starch grains recovered from existing exposures, and of vertebrates from the current and the earlier excavations, suggest that human foraging during these times was marked by habitat-tailored hunting technologies, the collection and processing of toxic plants for consumption, and, perhaps, the use of fire at some forest-edges. The Niah evidence demonstrates the sophisticated nature of the subsistence behavior developed by modern humans to exploit the tropical environments that they encountered in Southeast Asia, including rainforest.     

Dissolved organic carbon concentration of a natural water body and its relationship to water color in Central Kalimantan, Indonesia

In Central Kalimantan, Indonesia, the water of lakes and rivers showed high dissolved organic carbon (DOC) concentration, 5-50mgC/l. There was a clear relationship between DOC concentration and pH. DOC also contributed to low water transparency in the studied lakes. Water colors measured by spectrophotometer showed a strong relationship with DOC concentration, indicating high applicability of water color measurement for estimation of DOC. The development of a simple but quick estimation of DOC will contribute to understanding the seasonal dynamics of DOC, which might regulate both abiotic and biotic conditions in aquatic ecosystems in this area.     

Competition between laboratory populations of green leafhoppers,Nephotettix spp. (Homoptera: Cicadellidae)

Intra- and interspecific competition between laboratory populations of four green leafhoppers, Nephotettix spp. was studied in the laboratory under three different temperature regimes of 24°C, 27°C and 30°C. For the single-species population of the three tropical species, the equilibrium density increased as the temperature increased. On the other hand, for the temperature species N. cincticeps, the highest equilibrium density was at the intermediate temperature and the lowest at high temperature. Interspecific interactions between two tropical (N. virescens vs. N. nigropictus), a tropical and a temperature (N. virescens vs. N. cincticeps) and a rice-feeding and a grass-inhabiting (N. virescens vs. N. malayanus) Nephotettix species were also studied in the laboratory at the three temperature regimes. Temperature differentially affected the outcome of competition between two Nephotettix species. Between N. virescens and N. nigropictus, the latter was more successful over the former at low and intermediate temperatures, while the former was more successful at high temperature. Between N. virescens and N. cincticeps, the temperate species inhibited the growth of the tropical species at low temperature while the tropical species inhibited the growth of the temperate species at high temperature. At intermediate temperature, the population of N. virescens persisted at a slightly higher density over the population of N. cincticeps. Between the rice-feeding N. virescens and the grass-inhabiting N. malayanus, regardless of temperature the population density of the latter was greatly reduced and later became extinct while the population of the former continued its growth. These consequences of competition between two Nephotettix species conformed fairly well to those predicted by theLotka-Volterra model using demographic parameters specified for each species.     

Studying global change through investigation of the plastic responses of xylem anatomy in tree rings

Variability in xylem anatomy is of interest to plant scientists because of the role water transport plays in plant performance and survival. Insights into plant adjustments to changing environmental conditions have mainly been obtained through structural and functional comparative studies between taxa or within taxa on contrasting sites or along environmental gradients. Yet, a gap exists regarding the study of hydraulic adjustments in response to environmental changes over the lifetimes of plants. In trees, dated tree-ring series are often exploited to reconstruct dynamics in ecological conditions, and recent work in which wood-anatomical variables have been used in dendrochronology has produced promising results. Environmental signals identified in water-conducting cells carry novel information reflecting changes in regional conditions and are mostly related to short, sub-annual intervals. Although the idea of investigating environmental signals through wood anatomical time series goes back to the 1960s, it is only recently that low-cost computerized image-analysis systems have enabled increased scientific output in this field. We believe that the study of tree-ring anatomy is emerging as a promising approach in tree biology and climate change research, particularly if complemented by physiological and ecological studies. This contribution presents the rationale, the potential, and the methodological challenges of this innovative approach.     

Ecophysiology of six selected shrub species in different plant communities at the periphery of the Atlantic Forest of SE-Brazil

The Atlantic rain forest of Brazil is one of the 25 biodiversity hotspots of the world. It is surrounded by distinct marginal plant communities which are floristically related to the rain forest. We performed an in situ ecophysiological study of six shrub and tree species which are abundant in one or more of these marginal ecosystems. They formed three pairs of taxonomically related species, which, however, differed in geographic distribution and/or habitat preference. We aimed to answer the following questions: (1) Do habitat generalists and specialists differ in ecophysiological behaviour? (2) Are there general trends responsive to site characteristics, irrespective of taxonomic affiliation? (3) In which variables between-site intraspecific variation is more often detected? Plants chosen were the Clusiaceae Calophyllum brasiliense Cambess (swamp forest specialist) and Rheedia brasiliensis (Mart.) Planch &; Triana (a generalist plant, occurring in swamp forests and dry, sandy coastal vegetation); the Myrsinaceae Myrsine parvifolia A.DC. (widespread in sandy plains) and Myrsine gardneriana A.DC. (widespread in high-altitude habitats); and the Euphorbiaceae Styllingia dichotoma Muell. Arg. (a coastal inselberg specialist) and Croton compressus Lam. (a generalist found in coastal inselbergs and dry forests). We compared measurements of photosynthesis (chlorophyll fluorescence parameters), stable isotope (δ¹³C, δ¹⁵N) and metabolic carbon and nitrogen compounds. Generalists and specialists did not always differ in ecophysiological behaviour: for instance, while widespread generalists such as M. parvifolia and C. compressus were ecophysiologically versatile, R. brasiliense appeared to perform below optimum in all sampled habitats. Light and water were abiotic factors which apparently explained most of the variation found. Overall, ecophysiological responses were often more clearly related to individual species rather than to taxonomic affinities in higher hierarchies (family, genus) or habitat. These results are discussed as a further indication of high biodiversity even in habitats marginal to the rain forest hotspot.     

Vegetation survey of the Wasgomuwa National Park,Sri Lanka: analysis of the Wasgomuwa Oya Forest

Three types of forests were recognised (high, disturbed and open) based on the openings in the canopy in a Tropical Monsoonal Forest at the Wasgomuwa National Park, Sri Lanka. The analysis of these forests showed that the species composition varied between forest types. The shrub vegetation, not only was very characteristic but was also a major component in each forest, unlike in the tropical rain forest. The exact role of the shrubs in the dynamics of the tropical monsoon forests is not established, but is suggested that it may act as a buffer during the dry periods under a semi deciduous canopy. The sapling composition differed from the tree vegetation and indicated that the species composition may change with time in this forest. Mosaic theory or the patch dynamics may best explain the dynamics of this tropical monsoon forest which is a mixture of forest types. More studies are required before generalisations can be made of tropical monsoon forests.