Search

Plant canopies are wet for substantial amounts of time and this influences physiological performance and fluxes of energy, carbon and water at the ecosystem level. Leaf wetness sensors enable us to quantify the duration of leaf wetness and spatially map this to canopy structure. However, manually analysing leaf wetness data from plot-level experiments can be time-consuming, and requires a degree of subjective judgement in delineating wetness events which can lead to inconsistencies in the analysis. Here we: • Describe how to set up an array of leaf wetness sensors (Phytos 31, Meter) enabling the measurement of leaf wetness duration through the profile of a forest canopy, • Present a method and R script to objectively identify and distinguish periods of rain and dew from the output of leaf wetness sensors, • Provide a criteria for separating the leaf wetness sensor output into dew and rain events which may form a reference standard, or be modified for use, in future studies.

The continued functioning of tropical forests under climate change depends on their resilience to drought and heat. However, there is little understanding of how tropical forests will respond to combinations of these stresses, and no field studies to date have explicitly evaluated whether sustained drought alters sensitivity to temperature. We measured the temperature response of net photosynthesis, foliar respiration and the maximum quantum efficiency of photosystem II (Fv/Fm) of eight hyper‐dominant Amazonian tree species at the world's longest‐running tropical forest drought experiment, to investigate the effect of drought on forest thermal sensitivity. Despite a 0.6°C–2°C increase in canopy air temperatures following long‐term drought, no change in overall thermal sensitivity of net photosynthesis or respiration was observed. However, photosystem II tolerance to extreme‐heat damage (T50) was reduced from 50.0 ± 0.3°C to 48.5 ± 0.3°C under drought. Our results suggest that long‐term reductions in precipitation, as projected across much of Amazonia by climate models, are unlikely to greatly alter the response of tropical forests to rising mean temperatures but may increase the risk of leaf thermal damage during heatwaves. [ABSTRACT FROM AUTHOR]

Summary: Soil–leaf hydraulic conductance determines canopy–atmosphere coupling in vegetation models, but it is typically derived from ex‐situ measurements of stem segments and soil samples. Using a novel approach, we derive robust in‐situ estimates for whole‐tree conductance (ktree), 'functional' soil conductance (ksoil), and 'system' conductance (ksystem, water table to canopy), at two climatically different tropical rainforest sites.Hydraulic 'functional rooting depth', determined for each tree using profiles of soil water potential (Ψsoil) and sap flux data, enabled a robust determination of ktree and ksoil. ktree was compared across species, size classes, seasons, height above nearest drainage (HAND), two field sites, and to alternative representations of ktree; ksoil was analysed with respect to variations in site, season and HAND.ktree was lower and changed seasonally at the site with higher vapour pressure deficit (VPD) and rainfall; ktree differed little across species but scaled with tree circumference; rsoil (1/ksoil) ranged from 0 in the wet season to 10× less than rtree (1/ktree) in the dry season.VPD and not rainfall may influence plot‐level k; leaf water potentials and sap flux can be used to determine ktree, ksoil and ksystem; Ψsoil profiles can provide mechanistic insights into ecosystem‐level water fluxes. [ABSTRACT FROM AUTHOR]

Summary: Plant traits are increasingly being used to improve prediction of plant function, including plant demography. However, the capability of plant traits to predict demographic rates remains uncertain, particularly in the context of trees experiencing a changing climate.Here we present data combining 17 plant traits associated with plant structure, metabolism and hydraulic status, with measurements of long‐term mean, maximum and relative growth rates for 176 trees from the world's longest running tropical forest drought experiment.We demonstrate that plant traits can predict mean annual tree growth rates with moderate explanatory power. However, only combinations of traits associated more directly with plant functional processes, rather than more commonly employed traits like wood density or leaf mass per area, yield the power to predict growth. Critically, we observe a shift from growth being controlled by traits related to carbon cycling (assimilation and respiration) in well‐watered trees, to traits relating to plant hydraulic stress in drought‐stressed trees.We also demonstrate that even with a very comprehensive set of plant traits and growth data on large numbers of tropical trees, considerable uncertainty remains in directly interpreting the mechanisms through which traits influence performance in tropical forests. [ABSTRACT FROM AUTHOR]

Whether tropical trees acclimate to long‐term drought stress remains unclear. This uncertainty is amplified if drought stress is accompanied by changes in other drivers such as the increases in canopy light exposure that might be induced by tree mortality or other disturbances.Photosynthetic capacity, leaf respiration, non‐structural carbohydrate (NSC) storage and stomatal conductance were measured on 162 trees at the world's longest running (15 years) tropical forest drought experiment. We test whether surviving trees have altered strategies for carbon storage and carbon use in the drier and elevated light conditions present following drought‐related tree mortality.Relative to control trees, the surviving trees experiencing the drought treatment showed functional responses including: (a) moderately reduced photosynthetic capacity; (b) increased total leaf NSC; and (c) a switch from starch to soluble sugars as the main store of branch NSC. This contrasts with earlier findings at this experiment of no change in photosynthetic capacity or NSC storage. The changes detected here only occurred in the subset of drought‐stressed trees with canopies exposed to high radiation and were absent in trees with less‐exposed canopies and also in the community average. In contrast to previous results acquired through less intensive species sampling from this experiment, we also observe no species‐average drought‐induced change in leaf respiration.Our results suggest that long‐term responses to drought stress are strongly influenced by a tree's full‐canopy light environment and therefore that disturbance‐induced changes in stand density and dynamics are likely to substantially impact tropical forest responses to climate change. We also demonstrate that, while challenging, intensive sampling is essential in tropical forests to avoid sampling biases caused by limited taxonomic coverage. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Foliar water uptake, FWU, the uptake of atmospheric water directly into leaves, has been reported to occur in nearly 200 species spanning a wide range of ecosystems distributed globally. In order to represent FWU in land‐surface models, a conductance term is required to scale the process to the canopy level. Here we show that conductance to FWU is theoretically equivalent to stomatal conductance and that under commonly occurring conditions vapour could diffuse into leaves at rates equivalent to those reported as FWU. We therefore conclude that such 'reverse transpiration' could partially, or even wholly, account for FWU in some plants. [ABSTRACT FROM AUTHOR]

Summary: CO2 efflux from stems (CO2_stem) accounts for a substantial fraction of tropical forest gross primary productivity, but the climate sensitivity of this flux remains poorly understood. We present a study of tropical forest CO2_stem from 215 trees across wet and dry seasons, at the world's longest running tropical forest drought experiment site. We show a 27% increase in wet season CO2_stem in the droughted forest relative to a control forest. This was driven by increasing CO2_stem in trees 10–40 cm diameter. Furthermore, we show that drought increases the proportion of maintenance to growth respiration in trees > 20 cm diameter, including large increases in maintenance respiration in the largest droughted trees, > 40 cm diameter. However, we found no clear taxonomic influence on CO2_stem and were unable to accurately predict how drought sensitivity altered ecosystem scale CO2_stem, due to substantial uncertainty introduced by contrasting methods previously employed to scale CO2_stem fluxes. Our findings indicate that under future scenarios of elevated drought, increases in CO2_stem may augment carbon losses, weakening or potentially reversing the tropical forest carbon sink. However, due to substantial uncertainties in scaling CO2_stem fluxes, stand‐scale future estimates of changes in stem CO2 emissions remain highly uncertain. [ABSTRACT FROM AUTHOR]