Small understorey trees have greater capacity than canopy trees to adjust hydraulic traits following prolonged experimental drought in a tropical forest

Future climate change predictions for tropical forests highlight increased frequency and intensity of extreme drought events. However, it remains unclear whether large and small trees have differential strategies to tolerate drought due to the different niches they occupy. The future of tropical forests is ultimately dependent on the capacity of small trees (<10 cm in diameter) to adjust their hydraulic system to tolerate drought. To address this question, we evaluated whether the drought tolerance of neotropical small trees can adjust to experimental water stress and was different from tall trees. We measured multiple drought resistance-related hydraulic traits across nine common neotropical genera at the world's longest-running tropical forest throughfall-exclusion experiment and compared their responses with surviving large canopy trees. Small understorey trees in both the control and the throughfall-exclusion treatment had lower minimum stomatal conductance and maximum hydraulic leaf-specific conductivity relative to large trees of the same genera, as well as a greater hydraulic safety margin (HSM), percentage loss of conductivity and embolism resistance, demonstrating that they occupy a distinct hydraulic niche. Surprisingly, in response to the drought treatment, small trees increased specific hydraulic conductivity by 56.3% and leaf:sapwood area ratio by 45.6%. The greater HSM of small understorey trees relative to large canopy trees likely enabled them to adjust other aspects of their hydraulic systems to increase hydraulic conductivity and take advantage of increases in light availability in the understorey resulting from the drought-induced mortality of canopy trees. Our results demonstrate that differences in hydraulic strategies between small understorey and large canopy trees drive hydraulic niche segregation. Small understorey trees can adjust their hydraulic systems in response to changes in water and light availability, indicating that natural regeneration of tropical forests following long-term drought may be possible.

The sensitivity of wood production to seasonal and interannual variations in climate in a lowland Amazonian rainforest

Understanding climatic controls on tropical forest productivity is key to developing more reliable models for predicting how tropical biomes may respond to climate change. Currently there is no consensus on which factors control seasonal changes in tropical forest tree growth. This study reports the first comprehensive plot-level description of the seasonality of growth in a Peruvian tropical forest. We test whether seasonal and interannual variations in climate are correlated with changes in biomass increment, and whether such relationships differ among trees with different functional traits. We found that biomass increments, measured every 3 months on the two plots, were reduced by between 40 and 55% in the peak dry season (July-September) relative to peak wet season (January-March). The seasonal patterns of biomass accumulation are significantly (p < 0.01) associated with seasonal patterns of rainfall and soil water content; however, this may reflect a synchrony of seasonal cycles rather than direct physiological controls on tree growth rates. The strength of the growth seasonality response among trees is significantly correlated to functional traits: consistent with a hypothesised trade-off between maximum potential growth rate and hydraulic safety, tall and fast-growing trees with broad stems had the most strongly seasonal biomass accumulation, suggesting that they are more productive in the wet season, but more vulnerable to water limitation in the dry season.

The sensitivity of tropical leaf litter decomposition to temperature: results from a large-scale leaf translocation experiment along an elevation gradient in Peruvian forests

• We present the results from a litter translocation experiment along a 2800-m elevation gradient in Peruvian tropical forests. The understanding of the environmental factors controlling litter decomposition is important in the description of the carbon and nutrient cycles of tropical ecosystems, and in predicting their response to long-term increases in temperature. • Samples of litter from 15 species were transplanted across all five sites in the study, and decomposition was tracked over 448 d. • Species' type had a large influence on the decomposition rate (k), most probably through its influence on leaf quality and morphology. When samples were pooled across species and elevations, soil temperature explained 95% of the variation in the decomposition rate, but no direct relationship was observed with either soil moisture or rainfall. The sensitivity of the decay rate to temperature (κ(T)) varied seven-fold across species, between 0.024 and 0.169 °C⁻¹, with a mean value of 0.118 ± 0.009 °C⁻¹ (SE). This is equivalent to a temperature sensitivity parameter (Q₁₀) for litter decay of 3.06 ± 0.28, higher than that frequently assumed for heterotrophic processes. • Our results suggest that the warming of approx. 0.9 °C experienced in the region in recent decades may have increased decomposition and nutrient mineralization rates by c. 10%.

Shifts in plant respiration and carbon use efficiency at a large-scale drought experiment in the eastern Amazon

*The effects of drought on the Amazon rainforest are potentially large but remain poorly understood. Here, carbon (C) cycling after 5 yr of a large-scale through-fall exclusion (TFE) experiment excluding about 50% of incident rainfall from an eastern Amazon rainforest was compared with a nearby control plot. *Principal C stocks and fluxes were intensively measured in 2005. Additional minor components were either quantified in later site measurements or derived from the available literature. *Total ecosystem respiration (R(eco)) and total plant C expenditure (PCE, the sum of net primary productivity (NPP) and autotrophic respiration (R(auto))), were elevated on the TFE plot relative to the control. The increase in PCE and R(eco) was mainly caused by a rise in R(auto) from foliage and roots. Heterotrophic respiration did not differ substantially between plots. NPP was 2.4 +/- 1.4 t C ha(-1) yr(-1) lower on the TFE than the control. Ecosystem carbon use efficiency, the proportion of PCE invested in NPP, was lower in the TFE plot (0.24 +/- 0.04) than in the control (0.32 +/- 0.04). *Drought caused by the TFE treatment appeared to drive fundamental shifts in ecosystem C cycling with potentially important consequences for long-term forest C storage.

Estimates and relationships between aboveground and belowground resource exchange surface areas in a Sitka spruce managed forest

Our knowledge of the nature of belowground competition for moisture and nutrients is limited. In this study, we used an earth impedance method to determine the root absorbing area of Sitka spruce (Picea sitchensis (Bong.) Carr.) trees, making measurements in stands of differing density (2-, 4- and 6-m inter-tree spacing). We compared absorbing root area index (RAI(absorbing); based on the impedance measure) with fine root area index (RAI(fine); based on estimates of total surface area of fine roots) and related these results to investment in conductive roots. Root absorbing area was a near-linear function of tree stem diameter at 1.3 m height. At the stand level, RAI(absorbing), which is analogous to and scaled with transpiring leaf area index (maximum stomatal pore area per unit ground area; LAI(transpiring)), increased proportionally with basal area across the three stands. In contrast, RAI(fine) was inversely propotional to basal area. The ratio of RAI(absorbing) to LAI(transpiring) ranged from 7.7 to 17.1, giving an estimate of the relative aboveground versus belowground resource exchange areas. RAI(absorbing) provides a way of characterizing ecosystem functioning as a physiologically meaningful index of belowground absorbing area.

The fate of assimilated carbon during drought: impacts on respiration in Amazon rainforests

Interannual variations in CO2 exchange across Amazonia, as deduced from atmospheric inversions, correlate with El Niño occurrence. They are thought to result from changes in net ecosystem exchange and fire incidence that are both related to drought intensity. Alterations to net ecosystem production (NEP) are caused by changes in gross primary production (GPP) and ecosystem respiration (Reco). Here, we analyse observations of the components of Reco (leaves, live and dead woody tissue, and soil) to provide first estimates of changes in Reco during short-term (seasonal to interannual) moisture limitation. Although photosynthesis declines if moisture availability is limiting, leaf dark respiration is generally maintained, potentially acclimating upwards in the longer term. If leaf area is lost, then short-term canopy-scale respiratory effluxes from wood and leaves are likely to decline. Using a moderate short-term drying scenario where soil moisture limitation leads to a loss of 0.5m2m-2yr-1 in leaf area index, we estimate a reduction in respiratory CO2 efflux from leaves and live woody tissue of 1.0 (+/-0.4) tCha-1yr-1. Necromass decomposition declines during drought, but mortality increases; the median mortality increase following a strong El Niño is 1.1% (n=46 tropical rainforest plots) and yields an estimated net short-term increase in necromass CO2 efflux of 0.13-0.18tCha-1yr-1. Soil respiration is strongly sensitive to moisture limitation over the short term, but not to associated temperature increases. This effect is underestimated in many models but can lead to estimated reductions in CO2 efflux of 2.0 (+/-0.5) tCha-1yr-1. Thus, the majority of short-term respiratory responses to drought point to a decline in Reco, an outcome that contradicts recent regional-scale modelling of NEP. NEP varies with both GPP and Reco but robust moisture response functions are clearly needed to improve quantification of the role of Reco in influencing regional-scale CO2 emissions from Amazonia.

A method for extracting plant roots from soil which facilitates rapid sample processing without compromising measurement accuracy

This study evaluates a novel method for extracting roots from soil samples and applies it to estimate standing crop root mass (+/- confidence intervals) in an eastern Amazon rainforest. Roots were manually extracted from soil cores over a period of 40 min, which was split into 10 min time intervals. The pattern of cumulative extraction over time was used to predict root extraction beyond 40 min. A maximum-likelihood approach was used to calculate confidence intervals. The temporal prediction method added 21-32% to initial estimates of standing crop root mass. According to predictions, complete manual root extraction from 18 samples would have taken c. 239 h, compared with 12 h using the prediction method. Uncertainties (percentage difference between mean, and 10th and 90th percentiles) introduced by the prediction method were small (12-15%), compared with uncertainties caused by spatial variation in root mass (72-191%, for nine samples per plot surveyed). This method provides a way of increasing the number of root samples processed per unit time, without compromising measurement accuracy.

Photosynthetic capacity in a central Amazonian rain forest

The vertical profile in leaf photosynthetic capacity was investigated in a terra firme rain forest in central Amazonia. Measurements of photosynthesis were made on leaves at five levels in the canopy, and a model was fitted to describe photosynthetic capacity for each level. In addition, vertical profiles of photosynthetic photon flux density, leaf nitrogen concentration and specific leaf area were measured. The derived parameters for maximum rate of electron transport (J(max)) and maximum rate of carboxylation by Rubisco (V(cmax)) increased significantly with canopy height (P < 0.05). The highest J(max) for a single canopy level was measured at the penultimate canopy level (20 m) and was 103.9 &mgr;mol m(-2) s(-1) +/- 24.2 (SE). The highest V(cmax) per canopy height was recorded at the top canopy level (24 m) and was 42.8 +/- 5.9 &mgr;mol m(-2) s(-1). Values of J(max) and V(cmax) at ground level were 35.8 +/- 3.3 and 20.5 +/- 1.3 &mgr;mol m(-2) s(-1), espectively. The increase in photosynthetic capacity with increasing canopy height was strongly correlated with leaf nitrogen concentration when examined on a leaf area basis, but was only weakly correlated on a mass basis. The correlation on an area basis can be largely explained by the concomitant decrease in specific leaf area with increasing height. Apparent daytime leaf respiration, on an area basis, also increased significantly with canopy height (P < 0.05). We conclude that canopy photosynthetic capacity can be represented as an average vertical profile, perturbations of which may be explained by variations in the environmental variables driving photosynthesis.

The effect of aqueous transport of CO(2) in xylem sap on gas exchange in woody plants

The influence of CO(2) transported in the transpiration stream on measurements of leaf photosynthesis and stem respiration was investigated. Measurements were made on trees in a temperate forest in Scotland and in a tropical rain forest in Cameroon, and on shrubs in the Sahelian zone in Niger. A chamber was designed to measure the CO(2) partial pressure in the gas phase within the woody stems of trees. High CO(2) partial pressures were found, ranging from 3000 to 9200 Pa. Henry's Law was used to estimate the CO(2) concentration of xylem sap, assuming that it was in equilibrium with the measured gas phase partial pressures. The transport of CO(2) in the xylem sap was calculated by multiplying sap CO(2) concentration by transpiration rate. The magnitude of aqueous transport in the studied species ranged from 0.03 to 0.35 &mgr;mol CO(2) m(-2) s(-1), representing 0.5 to 7.1% of typical leaf photosynthetic rates. These values strongly depend on sap pH. To examine the influence of aqueous transport of CO(2) on stem gas exchange, we made simultaneous measurements of stem CO(2) efflux and sap flow on the same stem. After removing the effect of temperature, stem CO(2) efflux was positively related to sap flow. The apparent effect on measurements of stem respiration was up to 0.7 &mgr;mol m(-2) s(-1), representing ~12% of peak stem respiration rates.

An in vitro Study on the Effect of Momordica charantia on Glucose Uptake and Glucose Metabolism in Rats

The effect of extracts of MOMORDICA CHARANTIA L. fruits on glucose metabolism and glucose uptake was investigated. The fruit of momordica, at all ripening stages, was found to contain two inhibitory compounds: one against hexokinase activity and the other against glucose uptake by rat intestinal fragments. Both inhibitions exhibit competitive profiles VERSUS glucose. The inhibitory factors are extractable in hot water, hot alcohol and hot acetone. A partial purification procedure by TLC for both inhibitors is described.