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Background and aimsTropical forests exchange more carbon dioxide (CO2) with the atmosphere than any other terrestrial biome. Yet, uncertainty in the projected carbon balance over the next century is roughly three-times greater for the tropics than other ecosystems. Our limited knowledge of tropical plant physiological responses, including photosynthetic, to climate change is a substantial source of uncertainty in our ability to forecast the global terrestrial carbon sink.MethodsWe used a meta-analytic approach, focusing on tropical photosynthetic temperature responses, to address this knowledge gap. Our dataset, gleaned from 18 independent studies, included leaf-level light saturated photosynthetic (Asat) temperature responses from 108 woody species, with additional temperature parameters (35 species) and rates (250 species) of both maximum rates of electron transport (Jmax) and Rubisco carboxylation (Vcmax). We investigated how these parameters responded to mean annual temperature (MAT), temperature variability, aridity, and elevation, as well as also how responses differed among successional strategy, leaf habit, and light environment.Key resultsOptimum temperatures for Asat (ToptA) and Jmax (ToptJ) increased with MAT but not for Vcmax (ToptV). Although photosynthetic rates were higher for "light" than "shaded" leaves, light conditions did not generate differences in temperature response parameters. ToptA did not differ with successional strategy, but early successional species had ~4 °C wider thermal niches than mid/late species. Semi-deciduous species had ~1 °C higher ToptA than broadleaf evergreen. Most global modeling efforts consider all tropical forests as a single "broadleaf evergreen" functional type, but our data show that tropical species with different leaf habits display distinct temperature responses that should be included in modeling efforts.ConclusionsThis novel research will inform modeling efforts to quantify tropical ecosystem carbon cycling and provide more accurate representations of how these key ecosystems will respond to altered temperature patterns in the face of climate warming.

As Earth's climate has varied strongly through geological time, studying the impacts of past climate change on biodiversity helps to understand the risks from future climate change. However, it remains unclear how paleoclimate shapes spatial variation in biodiversity. Here, we assessed the influence of Quaternary climate change on spatial dissimilarity in taxonomic, phylogenetic, and functional composition among neighboring 200-kilometer cells (beta-diversity) for angiosperm trees worldwide. We found that larger glacial-interglacial temperature change was strongly associated with lower spatial turnover (species replacements) and higher nestedness (richness changes) components of beta-diversity across all three biodiversity facets. Moreover, phylogenetic and functional turnover was lower and nestedness higher than random expectations based on taxonomic beta-diversity in regions that experienced large temperature change, reflecting phylogenetically and functionally selective processes in species replacement, extinction, and colonization during glacial-interglacial oscillations. Our results suggest that future human-driven climate change could cause local homogenization and reduction in taxonomic, phylogenetic, and functional diversity of angiosperm trees worldwide.

Rising temperatures are influencing forests on many scales, with potentially strong variation vertically across forest strata. Using published research and new analyses, we evaluate how microclimate and leaf temperatures, traits, and gas exchange vary vertically in forests, shaping tree, and ecosystem ecology. In closed-canopy forests, upper canopy leaves are exposed to the highest solar radiation and evaporative demand, which can elevate leaf temperature (Tleaf ), particularly when transpirational cooling is curtailed by limited stomatal conductance. However, foliar traits also vary across height or light gradients, partially mitigating and protecting against the elevation of upper canopy Tleaf . Leaf metabolism generally increases with height across the vertical gradient, yet differences in thermal sensitivity across the gradient appear modest. Scaling from leaves to trees, canopy trees have higher absolute metabolic capacity and growth, yet are more vulnerable to drought and damaging Tleaf than their smaller counterparts, particularly under climate change. By contrast, understory trees experience fewer extreme high Tleaf 's but have fewer cooling mechanisms and thus may be strongly impacted by warming under some conditions, particularly when exposed to a harsher microenvironment through canopy disturbance. As the climate changes, integrating the patterns and mechanisms reviewed here into models will be critical to forecasting forest-climate feedback.

Safeguarding Earth's tree diversity is a conservation priority due to the importance of trees for biodiversity and ecosystem functions and services such as carbon sequestration. Here, we improve the foundation for effective conservation of global tree diversity by analyzing a recently developed database of tree species covering 46,752 species. We quantify range protection and anthropogenic pressures for each species and develop conservation priorities across taxonomic, phylogenetic, and functional diversity dimensions. We also assess the effectiveness of several influential proposed conservation prioritization frameworks to protect the top 17% and top 50% of tree priority areas. We find that an average of 50.2% of a tree species' range occurs in 110-km grid cells without any protected areas (PAs), with 6,377 small-range tree species fully unprotected, and that 83% of tree species experience nonnegligible human pressure across their range on average. Protecting high-priority areas for the top 17% and 50% priority thresholds would increase the average protected proportion of each tree species' range to 65.5% and 82.6%, respectively, leaving many fewer species (2,151 and 2,010) completely unprotected. The priority areas identified for trees match well to the Global 200 Ecoregions framework, revealing that priority areas for trees would in large part also optimize protection for terrestrial biodiversity overall. Based on range estimates for >46,000 tree species, our findings show that a large proportion of tree species receive limited protection by current PAs and are under substantial human pressure. Improved protection of biodiversity overall would also strongly benefit global tree diversity.

Leaf-level gas exchange data support the mechanistic understanding of plant fluxes of carbon and water. These fluxes inform our understanding of ecosystem function, are an important constraint on parameterization of terrestrial biosphere models, are necessary to understand the response of plants to global environmental change, and are integral to efforts to improve crop production. Collection of these data using gas analyzers can be both technically challenging and time consuming, and individual studies generally focus on a small range of species, restricted time periods, or limited geographic regions. The high value of these data is exemplified by the many publications that reuse and synthesize gas exchange data, however the lack of metadata and data reporting conventions make full and efficient use of these data difficult. Here we propose a reporting format for leaf-level gas exchange data and metadata to provide guidance to data contributors on how to store data in repositories to maximize their discoverability, facilitate their efficient reuse, and add value to individual datasets. For data users, the reporting format will better allow data repositories to optimize data search and extraction, and more readily integrate similar data into harmonized synthesis products. The reporting format specifies data table variable naming and unit conventions, as well as metadata characterizing experimental conditions and protocols. For common data types that were the focus of this initial version of the reporting format, i.e., survey measurements, dark respiration, carbon dioxide and light response curves, and parameters derived from those measurements, we took a further step of defining required additional data and metadata that would maximize the potential reuse of those data types. To aid data contributors and the development of data ingest tools by data repositories we provided a translation table comparing the outputs of common gas exchange instruments. Extensive consultation with data collectors, data users, instrument manufacturers, and data scientists was undertaken in order to ensure that the reporting format met community needs. The reporting format presented here is intended to form a foundation for future development that will incorporate additional data types and variables as gas exchange systems and measurement approaches advance in the future. The reporting format is published in the U.S. Department of Energy's ESS-DIVE data repository, with documentation and future development efforts being maintained in a version control system.

Despite their low contribution to forest carbon stocks, lianas (woody vines) play an important role in the carbon dynamics of tropical forests. As structural parasites, they hinder tree survival, growth and fecundity; hence, they negatively impact net ecosystem productivity and long-term carbon sequestration.Competition (for water and light) drives various forest processes and depends on the local abundance of resources over time. However, evaluating the relative role of resource availability on the interactions between lianas and trees from empirical observations is particularly challenging. Previous approaches have used labour-intensive and ecosystem-scale manipulation experiments, which are infeasible in most situations.We propose to circumvent this challenge by evaluating the uncertainty of water and light capture processes of a process-based vegetation model (ED2) including the liana growth form. We further developed the liana plant functional type in ED2 to mechanistically simulate water uptake and transport from roots to leaves, and start the model from prescribed initial conditions. We then used the PEcAn bioinformatics platform to constrain liana parameters and run uncertainty analyses.Baseline runs successfully reproduced ecosystem gas exchange fluxes (gross primary productivity and latent heat) and forest structural features (leaf area index, aboveground biomass) in two sites (Barro Colorado Island, Panama and Paracou, French Guiana) characterized by different rainfall regimes and levels of liana abundance.Model uncertainty analyses revealed that water limitation was the factor driving the competition between trees and lianas at the drier site (BCI), and during the relatively short dry season of the wetter site (Paracou). In young patches, light competition dominated in Paracou but alternated with water competition between the wet and the dry season on BCI according to the model simulations.The modelling workflow also identified key liana traits (photosynthetic quantum efficiency, stomatal regulation parameters, allometric relationships) and processes (water use, respiration, climbing) driving the model uncertainty. They should be considered as priorities for future data acquisition and model development to improve predictions of the carbon dynamics of liana-infested forests. Synthesis. Competition for water plays a larger role in the interaction between lianas and trees than previously hypothesized, as demonstrated by simulations from a process-based vegetation model.

The temperature response of photosynthesis is one of the key factors determining predicted responses to warming in global vegetation models (GVMs). The response may vary geographically, owing to genetic adaptation to climate, and temporally, as a result of acclimation to changes in ambient temperature. Our goal was to develop a robust quantitative global model representing acclimation and adaptation of photosynthetic temperature responses. We quantified and modelled key mechanisms responsible for photosynthetic temperature acclimation and adaptation using a global dataset of photosynthetic CO2 response curves, including data from 141 C3 species from tropical rainforest to Arctic tundra. We separated temperature acclimation and adaptation processes by considering seasonal and common-garden datasets, respectively. The observed global variation in the temperature optimum of photosynthesis was primarily explained by biochemical limitations to photosynthesis, rather than stomatal conductance or respiration. We found acclimation to growth temperature to be a stronger driver of this variation than adaptation to temperature at climate of origin. We developed a summary model to represent photosynthetic temperature responses and showed that it predicted the observed global variation in optimal temperatures with high accuracy. This novel algorithm should enable improved prediction of the function of global ecosystems in a warming climate.

Summary: As temperature rises, net carbon uptake in tropical forests decreases, but the underlying mechanisms are not well understood. High temperatures can limit photosynthesis directly, for example by reducing biochemical capacity, or indirectly through rising vapor pressure deficit (VPD) causing stomatal closure.To explore the independent effects of temperature and VPD on photosynthesis we analyzed photosynthesis data from the upper canopies of two tropical forests in Panama with Generalized Additive Models.Stomatal conductance and photosynthesis consistently decreased with increasing VPD, and statistically accounting for VPD increased the optimum temperature of photosynthesis (Topt) of trees from a VPD‐confounded apparent Topt of c. 30–31°C to a VPD‐independent Topt of c. 33–36°C, while for lianas no VPD‐independent Topt was reached within the measured temperature range. Trees and lianas exhibited similar temperature and VPD responses in both forests, despite 1500 mm difference in mean annual rainfall.Over ecologically relevant temperature ranges, photosynthesis in tropical forests is largely limited by indirect effects of warming, through changes in VPD, not by direct warming effects of photosynthetic biochemistry. Failing to account for VPD when determining Topt misattributes the underlying causal mechanism and thereby hinders the advancement of mechanistic understanding of global warming effects on tropical forest carbon dynamics. See also the Commentary on this article by Gauthey & Gardner, 244: 1119–1121. [ABSTRACT FROM AUTHOR]

Leaf respiratory carbon loss decreases independent of temperature as the night progresses. Detailed nighttime measurements needed to quantify cumulative respiratory carbon loss at night are challenging under both lab and field conditions. We provide a simple yet accurate approach to represent variation in nighttime temperature‐independent leaf respiratory CO2 efflux in environments with both stable and fluctuating temperatures, which requires no detailed measurements throughout the night. We demonstrate that the inter‐ and intraspecific variation in the cumulative leaf respiratory CO2 efflux at constant temperature, at any length of night, scales linearly with the inter‐ and intraspecific variation in initial measurement of leaf respiratory CO2 efflux at the same temperature at the beginning of the night. This approach informs large‐scale predictions of cumulative leaf respiratory CO2 efflux, which is needed to understand plant carbon economy in global change studies as well as in global modeling and eddy covariance monitoring of the land–atmosphere exchange of CO2. [ABSTRACT FROM AUTHOR]

Patterns in functional trait variation associated with the ecological strategies of lianas and trees in subtropical montane forests remain poorly understood due to lack of trait comparisons. Here, we filled this gap by investigating trait divergence between lianas and trees with different leaf habits for 13 traits of 33 species (7 deciduous and 4 evergreen liana species, and 10 deciduous and 12 evergreen tree species) in a subtropical montane forest in southwestern China. We found that lianas had significantly larger stem xylem vessel diameter, higher theoretical hydraulic conductivity, specific leaf area, leaf nitrogen, and phosphorus concentrations, but lower leaf dry-matter content and N/P ratio than sympatric trees, indicating that lianas employ an acquisitive ecological strategy, with a more efficient stem hydraulic system and more productive leaves. In contrast to trees, lianas exhibited a larger variation in vessel diameter, with a few wide and many narrow vessels in the xylem, indicating a greater vessel dimorphism. Growth form explained 48.7% of the total trait variation, while leaf habit only explained 3.8% of trait variation, without significant interaction between growth form and leaf habit. In addition, significant stem–leaf trait relationships were only found in trees, but not in lianas, indicating decoupling of stem hydraulic function and leaf traits in subtropical lianas. These results suggest that subtropical montane lianas and trees differ strikingly in stem and leaf functional traits. Further studies are warranted to strengthen our understanding of the mechanisms underlying the strong divergence in ecological strategies between lianas and trees in subtropical forest ecosystems. [ABSTRACT FROM AUTHOR]

A fundamental aspect of the carbon cycle is the exchange of carbon between plants and the atmosphere. It is, therefore, important to understand factors that affect differences in gas exchange and carbon balance within and among species. Concentrations of nonstructural carbohydrates are often used as a proxy for carbon balance. We determined diel patterns of leaf carbohydrate concentrations in relation to irradiance (sun vs. shade) in seedlings and mature trees of two sympatric oak species (Quercus virginiana Mill. and Quercus hemisphaerica Bartram ex Willd.). For seedlings, we also measured leaf gas exchange. Higher sun exposure significantly increased photosynthesis and carbohydrate concentrations in both species. Carbohydrate concentrations of seedling leaves showed strong diel fluctuations, whereas concentrations in mature tree leaves did not. This contrast might be attributed to faster carbohydrate export from leaves of mature trees. The difference in sink strength between seedlings and adults may be related to the decreasing ratio of leaf mass to plant mass with ontogeny, increasing the demand for carbohydrates per unit leaf mass. Seedlings and mature trees are clearly functionally different and care must be taken when extrapolating results from seedling experiments to mature trees. Key words: carbohydrate dynamics, gas exchange, Quercus virginiana, Quercus hemisphaerica, plant ontogeny. L'echange de carbone entre les vegetaux et l'atmosphere constitue un aspect fondamental du cycle du carbone. Il est consequemment important de comprendre les facteurs qui affectent les differences sur le plan des echanges gazeux et de la balance de carbone a l'interieur et parmi les especes. Les concentrations de glucides non-structurels sont souvent utilisees comme indicateur de la balance de carbone. Les auteurs ont determine les patrons sur 24 h des concentrations de glucides des feuilles en fonction de l'eclairementenergetique(soleilvs. ombre) de juveniles et d'arbres matures de deux especes sympatriques de chenes (Quercus virginiana Mill. et Quercus hemisphaerica Bartram ex Willd.). En ce qui concerne les juveniles, les auteurs ont aussi mesure l'echange gazeux foliaire. Une exposition au soleil plus forte accroissait significativement la photosynthese et les concentrations de glucides chez les deux especes. Les concentrations de glucides des feuilles des juveniles fluctuaient fortement au cours des 24 h, contrairement aux feuilles des arbres matures. Ce contraste pourrait etre attribuable a un export de glucides plus rapide chez les feuilles d'arbres matures. La difference sur le plan de la force du puits entre les juveniles et les adultes peut etre reliee a un ratio diminue de la masse foliaire sur la masse de la plante en fonction de l'ontogenie, accroissant la demande en glucides par unite de masse foliaire. Les juveniles et les arbres matures sont clairement fonctionnellement differents et une attention doit etre portee lorsque des resultats sont extrapoles des juveniles aux arbres matures. [Traduit par la Redaction] Mots-cles: dynamique des glucides, echange gazeux, Quercus virginiana, Quercus hemisphaerica, ontogenie vegetale., Introduction Nonstructural carbohydrates are key components of plant carbon balance (Chapin et al. 1990; Korner 2003), and concentrations of nonstructural carbohydrates are often used in ecophysiological studies to compare carbon [...]

Vegetation plays a key role in the global carbon cycle and thus is an important component within Earth system models (ESMs) that project future climate. Many ESMs are adopting methods to trace the size and succession-stage-structure of plants within demographic models. These models make it feasible to conduct more realistic simulation of processes that control vegetation dynamics. Separately, increasing understanding of the ecophysiological processes governing plant water use, and the need to understand ecosystem responses to drought in particular, has led to the adoption of physical plant hydrodynamic schemes within ESMs. In this study, we report on a new hydrodynamics (HYDRO) model incorporated in the Functionally Assembled Terrestrial Ecosystem simulator (FATES). The size and canopy structured representation within FATES is able to simulate how plant size and hydraulic traits affect vegetation dynamics and carbon/water fluxes. To better understand this new model system and its functionality in tropical forest systems in particular, we conducted a global parameter sensitivity analysis at Barro Colorado Island, Panama. We assembled observations of plant hydraulic traits for stomata, leaves, stems, and roots, and determined the best-fit statistical distribution for each trait. Our model analysis showed that the taper component determining hydraulic conductivity tapering from trunk to branch, the water potential leading to 50 % loss (P50) of stomatal conductance, the maximum hydraulic conductivity for the stem, and the fraction of total hydraulic resistance in the above ground section are the top 5 traits determining the simulated water potential and loss of conductivity for different plant organs. For the risk of hydraulic failure and potential tree mortality, we found that ensemble members with high risk of mortality generally have a higher taper exponent and a higher xylem conductivity, less negative P50 for stomata conductance, and more negative P50 for stem and transporting roots. We expect that our results will provide guidance on future modeling studies using plant hydrodynamic models to predict the forest responses to droughts, and future field campaigns that aim to better parameterize the plant hydrodynamic model. [ABSTRACT FROM AUTHOR]

Regulation of the uptake of silicon (Si) varies among plant species; some species may passively transport Si, through transpiration, from soils to shoots, while others actively transport silica and deposit it in leaf tissues at high concentrations. Cucumber (Cucumis sativus L.) accumulates moderate amounts of silica in leaves, but the relative importance of passive and active processes pertaining to silica accumulation is poorly understood. In a factorial experiment with cucumber seedlings, we manipulated transpiration rates by changing humidity and air movements around pot-grown plants receiving a daily supply of solutions containing 0, 1, 1.5, and 2 mmol Si x [L.sup.-1]. Higher transpiration rates resulted in significantly greater Si per unit leaf mass after 4 days, suggesting that passive processes affect the rate of silica accumulation. Actual silica accumulation during the 4-day period was significantly higher than the expected accumulation attributable to passive transport alone in 1 and 1.5 mmol Si x [L.sup.-1]treatments, while passive processes alone could account for the actual silica accumulation at 2 mmol Si x [L.sup.-1]. We conclude that the relative importance of active and passive processes in silica deposition in cucumber leaves depends on transpiration rates and the balance between soil Si availability and plants' demands for Si. Key words: active uptake, cucumber, plant silica, silicon, transpiration. La regulation de l'absorption de la silice (Si) varie selon les especes de plantes; certaines peuvent transporter passivement la Si, via la transpiration, du sol aux tiges alors que d'autres transportent la silice activement et la deposent dans les tissus foliaires en fortes concentrations. Le concombre (Cucumis sativus L.) accumule des quantites moderees de silice dans ses feuilles, mais on comprend mal l'importance relative des processus passifs et actifs intervenant dans l'accumulation de la silice. Les auteurs ont conduit une experience factorielle avec des plantules de concombre, en modulant les taux de transpiration par une modification de l'humidite et des mouvements de l'air autour de plants cultives en pots recevant quotidiennement des solutions contenant 0, 1, 1,5 et 2 mmol Si x [L.sup.-1]. Les taux de transpirations les plus forts ont engendre une teneur significativement plus grande en Si par unite de masse foliaire apres 4 jours, suggerant que les processus passifs affectent le taux d'accumulation de la silice. L'accumulation reelle de Si au cours de la periode de 4 jours etait significativement plus grande que l'accumulation attendue attribuable au transport passif seul, en presence de traitement avec 1 et 1,5 mmol Si x [L.sup.-1], alors que les processus passifs seuls pouvaient expliquer l'accumulation reelle de silice en presence de 2 mmol Si x [L.sup.-1]. Les auteurs concluent que l'importance relative des processus actifs et passifs de la deposition de silice dans les feuilles de concombre depend des taux de transpiration et de la balance entre la disponibilite de la Si dans le sol et les demandes en Si de la plante. Mots-cles : absorption active, concombre, silice des plantes, silice, transpiration. [Traduit par la Redaction], Introduction Silicon (Si) is a beneficial element for many higher plants, including important agricultural crops such as rice, maize, and cucumber (Takahashi et al. 1990; Epstein 1999; Ma and Takahashi [...]