Leaf heat tolerance of 147 tropical forest species varies with elevation and leaf functional traits, but not with phylogeny

Tropical Forest Soil Microbiome Modulates Leaf Heat Tolerance More Strongly Under Warming Than Ambient Conditions

It is unclear how plants respond to increasing temperatures. Leaf heat tolerance (LHT) is often at its upper limit in tropical forests, suggesting that climate change might negatively impact these forests. We hypothesized that intraspecific variation in LHT might be associated with changes in the soil microbiome, which might also respond to climate. We hypothesized that warming would increase LHT through changes in the soil microbiome: we combined an in situ tropical warming experiment with a shade house experiment in Puerto Rico. The shade house experiment consisted of growing seedlings of Guarea guidonia, a dominant forest species, under different soil microbiome treatments (reduced arbuscular mycorrhizal fungi, reduced plant pathogens, reduced microbes, and unaltered) and soil inoculum from the field experiment. Heat tolerance was determined using chlorophyll fluorescence (F V /F m ) on individual seedlings in the field and on groups of seedlings (per pot) in the shade house. We sequenced soil fungal DNA to analyze the impacts of the treatments on the soil microbiome. In the field, seedlings from ambient temperature plots showed higher F V /F m values under high temperatures (0.648 at 46°C and 0.067 at 52°C) than seedlings from the warming plots (0.535 at 46°C and 0.031 at 52°C). In the shade house, the soil microbiome treatments significantly influenced the fungal community composition and LHT (T crit and F V /F m ). Reduction in fungal pathogen abundance and diversity altered F V /F m before T 50 for seedlings grown with soil inoculum from the warming plots but after T 50 for seedlings grown with soil inoculum from the ambient plots. Our findings emphasize that the soil microbiome plays an important role in modulating the impacts of climate change on plants. Understanding and harnessing this relationship might be vital for mitigating the effects of warming on forests, emphasizing the need for further research on microbial responses to climate change.

Leaf drought and heat tolerance are integrated across three temperate biome types

Leaf-scale heat and drought tolerance provide direct measures of the ability to withstand environmental stress and can be used to evaluate plant susceptibility to emerging climatic extremes. However, recent droughts increasingly occur with heatwaves, causing plants to withstand two simultaneous environmental stresses. Tolerance of leaf-level processes to heat and drought stress have mostly been studied independently, preventing an understanding of whether tolerance co-occurs for these two environmental stresses. To address this, we measured leaf photosynthetic heat tolerance as the critical temperatures at which photosystem II efficiency starts to decrease (Tcrit) and shows a decrease of 50% (T50) or 95% (T95) in three temperate biomes (desert, oak-pine forest, and mediterranean-type shrubland). We also characterized drought tolerance as the water potential at leaf turgor loss point (πtlp) and cellular membrane stability in response to simulated drought. We found coordination of heat and drought tolerance through a significant relationship of πtlp with T50 and Tcrit that varied with season, whereas T95 showed no relation to πtlp. Species with greater drought tolerance also showed greater membrane stability, implicating membrane leakiness as a potential mechanism of physiological decline during stress. Despite local variation in temperature and precipitation extremes, leaf heat and drought tolerance converged to common cross-biome relationships, providing evidence of interdependence that spanned distinct climates.

Growth of Brasenia schreberi requries good water quality and appropriate sediment nitrogen content

Background

Stem tissue structures are the basis of stem function and are essential for maintaining the normal physiological metabolism of aquatic plants. Water and sediment conditions are important factors affecting the functional characteristics and physiological metabolism of Brassenia schreberi. Due to pollution and other water and sediment issues caused by human activities, the natural habitat and population size of B. schreberi have dramatically decreased. Understanding the responses of the functional characteristics of B. schreberi to water and sediment conditions is the key to its scientific conservation and management.

Objectives and methods

This study selected Beihai Wetland in Tengchong, China, which boasts the largest natural habitat of B. schreberi, as the research site. To detect the response strategies of B. schreberi to water and sediment conditions, the photosynthetic parameters and stem structural characteristics of this species at 17 locations, as well as the water and sediment nutrient parameters at these locations were measured. We examined the relationships between the trait characteristics of B. schreberi and the water and sediment parameters by using correlation analysis. The aim was to explore the effects of sediment nutrients and water quality on the photosynthetic and stem structural characteristics of B. schreberi.

Results and conclusions

B. schreberi with higher coverage exhibited higher stomatal conductance (Gs ) and transpiration rate (Tr ), but lower vascular bundle area and ventilation hole area (P<0.05), while the net photosynthetic rate (Pn ) maintained content, indicating lower utilization efficiency of water and CO2. Water temperature (WT), sediment nitrogen content (ω(N)) and water dissolved oxygen (DO) were the main parameters affecting the characters of B. schreberi. The Pn , was significantly negatively correlated with ω(N), while it was positively correlated with DO and sediment phosphorus content (P<0.05). The findings indicate that B. schreberi requires good water quality to maintain a high photosynthetic rate and is prone to phosphorus limitation, but it has low requirements for sediment nitrogen content. The findings of this study provide a scientific basis for the habitat restoration and species-specific management of B. schreberi in degraded wetlands.

Leaf Transpirational Cooling and Thermal Tolerance Vary Along the Spectrum of Iso-Anisohydric Stomatal Regulation in Sand-Fixing Shrubs

Transpirational cooling is crucial for plant thermal regulation to avoid overheating; however, during prolonged and/or acute heat stress it often necessitates stomatal closure to reduce the risk of hydraulic failure due to dehydration. The intricate interplay between thermal regulation, water transport and use may govern plant performance in water-limited and simultaneously heat-stressed environments, yet this remains inadequately understood. Here, in a common garden, we evaluated the functional associations among physiological characteristics related to leaf thermoregulation, heat tolerance, xylem water transport, and stomatal regulation in eight shrub species commonly used for fixing active sand dunes in northern China. Our study showed that traits associated with heat adaptation and xylem hydraulics were closely related to stomatal regulation. More isohydric shrub species with higher water transport efficiency possessed stronger transpirational cooling capacity; whereas the more anisohydric species demonstrated greater tolerance to overheating. Moreover, leaf heat tolerance was strongly coordinated with drought tolerance reflected by leaf turgor loss point. These results underscore the importance of stomatal regulation in shaping plant thermal adaptive strategies and provide valuable insights into the coupling of water and heat-related physiological processes in plants adapted to sandy land environments prone to combined drought and heat stresses.

Heat-induced F0-fluorescence rise is not an indicator of severe tissue necrosis in thermotolerance assays of young and mature leaves of a tropical tree species, Calophyllum inophyllum

In heating experiments with leaves, the temperature at which dark-level F0 chlorophyll a fluorescence begins to rise, Tcrit, is widely used as an indicator of photosystem II thermotolerance. However, little is known about how Tcrit correlates with irreversible leaf tissue damage. Young and mature leaves of the tropical tree species Calophyllum inophyllum were heated stepwise from 30 to 55°C, at 1°C min-1. Tcrit was 47°C in young leaves and 49°C in mature leaves. Contrary to the higher Tcrit in mature leaves, heating to 55°C elicited greater tissue damage in mature than in young leaves. Young and mature leaves heated to their respective Tcrit or Tcrit + 2°C exhibited no or little tissue necrosis after 14 d of post-culture. It is concluded that measurements of the temperature-dependent F0 fluorescence rise underestimate the thermal thresholds above which significant irreversible leaf damage occurs.

Leaf warming in the canopy of mature tropical trees reduced photosynthesis due to downregulation of photosynthetic capacity and reduced stomatal conductance

Tropical forests play a large role in the global carbon cycle by annually absorbing 30% of our annual carbon emissions. However, these forests have evolved under relatively stable temperature conditions and may be sensitive to current climate warming. Few experiments have investigated the effects of warming on large, mature trees to better understand how higher temperatures affect these forests in situ. We targeted four tree species (Endiandra microneura, Castanospermum australe, Cleistanthus myrianthus and Myristica globosa) of the Australian tropical rainforest and warmed leaves in the canopy by 4°C for 8 months. We measured temperature response curves of photosynthesis and respiration, and determined the critical temperatures for chloroplast function based on Chl fluorescence. Both stomatal conductance and photosynthesis were strongly reduced by 48 and 35%, respectively, with warming. While reduced stomatal conductance was likely in response to higher vapour pressure deficit, the biochemistry of photosynthesis responded to higher temperatures via reduced Vcmax25 (-28%) and Jmax25 (-29%). There was no shift of the Topt of photosynthesis. Concurrently, respiration rates at a common temperature did not change in response to warming, suggesting limited respiratory thermal acclimation. This combination of physiological responses to leaf warming in mature tropical trees may suggest a reduced carbon sink with future warming in tropical forests.

Leaf Temperatures in an Indian Tropical Forest Exceed Physiological Limits but Durations of Exposures Are Currently Not Sufficient to Cause Lasting Damage

Increasing temperatures in the tropics will reduce performance of trees and agroforestry species and may lead to lasting damage and leaf death. One criterion to determine future forest resilience is to evaluate damage caused by temperature on Photosystem-II (PSII), a particularly sensitive component of photosynthesis. The temperature at which 50% of PSII function is lost (T50) is a widely used measure of irreversible damage to leaves. To assess vulnerability to high temperatures, studies have measured T50 or leaf temperatures, but rarely both. Further, because extant leaf temperature records are short, duration of exposure above thresholds like T50 has not been considered. Finally, these studies do not directly assess the effect of threshold exceedance on leaves. To understand how often, and how long, leaf temperatures exceed critical thresholds, we measured leaf temperatures of forest and agroforestry species in a tropical forest in the Western Ghats of India where air temperatures are high. We quantified species-specific physiological thresholds and assessed leaf damage after high-temperature exposure. We found that leaf temperatures already exceed T50. However, continuous exposure durations above critical thresholds are very skewed with most events lasting for much less than 30 min. As T50 was measured after a 30-min exposure, our results suggest that threshold exceedances and exposure durations for lasting damage are currently not reached and will rarely be reached if maximum air temperatures increase by 4°C. Consistent with this, we found only minor indications of heat damage in the forest species. However, there were indications of heat-induced reduction in PSII function and damage in the agroforestry leaves which have lower T50. Our findings suggest that, for forest species, while high-temperature thresholds may be surpassed, durations of exposure above thresholds remain short, and therefore, are unlikely to lead to irreversible damage and leaf death, even under 4°C warming.

Thermal acclimation of tree species in a tropical Andean city: Exploring the role of species origin and thermal niche

PREMISE

The warmer and drier atmospheric conditions of urban environments challenge plant performance to different extents based on a species' ability to acclimate to the conditions. We evaluated the influence of species origin and thermal niche on the acclimation of leaf traits and shifts in the occupation of the functional trait space of 10 tree species growing in two environmentally contrasting sites in Bogotá, Colombia.

METHODS

We measured six leaf traits per species in both sites and used generalized linear models to evaluate the influence of origin and thermal niche on acclimation of leaf traits and t-tests to analyze shifts in the occupation of the functional trait space.

RESULTS

Species origin predicted thermal tolerance and morphological trait acclimation to warmer conditions. Although exotic species decreased thermal tolerance at the warmer site, species from both origins acclimated traits consistently. Shifts in the occupation of the functional trait space varied between origins; warmer conditions reduced the size of the functional trait space of exotics and increased the phenotypic similarity of natives. Thermal tolerance acclimation and changes in functional trait space varied across species. Finally, thermal niche metrics were uncoupled from species origin and failed to explain the acclimation capacity of the studied species.

CONCLUSIONS

Although species origin influenced acclimation to warmer conditions, the effect of origin was not related to species' thermal niches. Our results provide crucial information for decision-makers involved in designing urban and peri-urban green spaces that can withstand climate change.

Symbiotic nitrogen fixation in trees: patterns, controls and ecosystem consequences

Symbiotic nitrogen fixation (SNF) represents the largest natural input of bioavailable nitrogen into the biosphere, impacting key processes spanning from local community dynamics to global patterns of nutrient limitation and primary productivity. While research on SNF historically focused largely on herbaceous and agricultural species, the past two decades have seen major advances in our understanding of SNF by tree species in forest and savanna communities. This has included important developments in the mathematical theory of SNF in forest ecosystems, experimental work on the regulators of tree SNF, broad observational analyses of tree N-fixer abundance patterns and increasingly process-based incorporation of tree SNF into ecosystem models. This review synthesizes recent work on the local and global patterns, environmental drivers and community and ecosystem effects of nitrogen-fixing trees in natural ecosystems. By better understanding the drivers and consequences of SNF in forests, this review aims to shed light on the future of this critical process and its role in forest functioning under changing climate, nutrient cycling and land use.

Clustered warming tolerances and the nonlinear risks of biodiversity loss on a warming planet

Anthropogenic climate change is projected to become a major driver of biodiversity loss, destabilizing the ecosystems on which human society depends. As the planet rapidly warms, the disruption of ecological interactions among populations, species and their environment, will likely drive positive feedback loops, accelerating the pace and magnitude of biodiversity losses. We propose that, even without invoking such amplifying feedback, biodiversity loss should increase nonlinearly with warming because of the non-uniform distribution of biodiversity. Whether these non-uniformities are the uneven distribution of populations across a species' thermal niche, or the uneven distribution of thermal niche limits among species within an ecological community, we show that in both cases, the resulting clustering in population warming tolerances drives nonlinear increases in the risk to biodiversity. We discuss how fundamental constraints on species' physiologies and geographical distributions give rise to clustered warming tolerances, and how population responses to changing climates could variously temper, delay or intensify nonlinear dynamics. We argue that nonlinear increases in risks to biodiversity should be the null expectation under warming, and highlight the empirical research needed to understand the causes, commonness and consequences of clustered warming tolerances to better predict where, when and why nonlinear biodiversity losses will occur.This article is part of the discussion meeting issue 'Bending the curve towards nature recovery: building on Georgina Mace's legacy for a biodiverse future'.

In thermotolerance tests of tropical tree leaves, the chlorophyll fluorescence parameter Fv/Fm measured soon after heat exposure is not a reliable predictor of tissue necrosis

Tropical rainforests are hot and may be particularly sensitive to ongoing anthropogenic global warming. This has led to increased interest in the thermotolerance of tropical trees. Thermotolerance of leaves of two tropical tree species, Terminalia catappa and Coccoloba uvifera, was determined by exposing leaf samples to 15-min heat treatments, followed by measurements of potential photosystem II quantum yield (dark-adapted value of variable/maximum chlorophyll a fluorescence, Fv/Fm) after 24 h and 14 days, and visible damage (necrosis) after 14 days. T50 (24 h), the temperature at which Fv/Fm declined by 50% 24 h after heat treatments, was associated with only ~10% leaf area damage in C. uvifera and no damage in T. catappa. In neither species was leaf necrosis observed at T5 (24 h), the temperature at which Fv/Fm declined by 5%. In both species, temperatures significantly higher than T50 (24 h) were required for 50% leaf area necrosis to occur. T50 (14 days) was a better proxy of visible leaf damage than T50 (24 h). The relationship between heat-induced Fv/Fm decline and tissue necrosis varies among species. In species surveys of leaf thermal tolerances, calibration of the Fv/Fm assay against the necrosis test is recommended for each species under investigation. Fv/Fm measurements soon after heat exposure do not reliably predict irreversible heat damage and may thus not be suitable to model and predict the thermostability of tropical forest trees.

Leaf Photosynthetic and Respiratory Thermal Acclimation in Terrestrial Plants in Response to Warming: A Global Synthesis

Leaf photosynthesis and respiration are two of the largest carbon fluxes between the atmosphere and biosphere. Although experiments examining the warming effects on photosynthetic and respiratory thermal acclimation have been widely conducted, the sensitivity of various ecosystem and vegetation types to warming remains uncertain. Here we conducted a meta-analysis on experimental observations of thermal acclimation worldwide. We found that the optimum temperature for photosynthetic rate (Topt) and the maximum rate of carboxylation of Rubisco (ToptV) in tropical forest plants increased by 0.51°C and 2.12°C per 1°C of warming, respectively. Similarly, Topt and the optimum temperature for maximum electron transport rate for RuBP regeneration (ToptJ) in temperate forest plants increased by 0.91°C and 0.15°C per 1°C of warming, respectively. However, reduced photosynthetic rates at optimum temperature (Aopt) were observed in tropical forest (17.2%) and grassland (16.5%) plants, indicating that they exhibited limited photosynthetic thermal acclimation to warming. Warming reduced respiration rate (R25) in boreal forest plants by 6.2%, suggesting that respiration can acclimate to warming. Photosynthesis and respiration of broadleaved deciduous trees may adapt to warming, as indicated by higher Aopt (7.5%) and Topt (1.08°C per 1°C of warming), but lower R25 (7.7%). We found limited photosynthetic thermal acclimation in needleleaved evergreen trees (-14.1%) and herbs (-16.3%), both associated with reduced Aopt. Respiration of needleleaved deciduous trees acclimated to warming (reduced R25 and temperature sensitivity of respiration (Q10)); however, broadleaved evergreen trees did not acclimate (increased R25). Plants in grasslands and herbaceous species displayed the weakest photosynthetic acclimation to warming, primarily due to the significant reductions in Aopt. Our global synthesis provides a comprehensive analysis of the divergent effects of warming on thermal acclimation across ecosystem and vegetation types, and provides a framework for modeling responses of vegetation carbon cycling to warming.

Ecotypic Variation in Leaf Thermoregulation and Heat Tolerance but Not Thermal Safety Margins in Tropical Trees

To avoid reaching lethal temperatures during periods of heat stress, plants may acclimate either their biochemical thermal tolerance or leaf morphological and physiological characteristics to reduce leaf temperature (Tleaf). While plants from warmer environments may have a greater capacity to regulate Tleaf, the extent of intraspecific variation and contribution of provenance is relatively unexplored. We tested whether upland and lowland provenances of four tropical tree species grown in a common garden differed in their thermal safety margins by measuring leaf thermal traits, midday leaf-to-air temperature differences (∆Tleaf) and critical leaf temperatures defined by chlorophyll fluorescence (Tcrit). Provenance variation was species- and trait-specific. Higher ∆Tleaf and Tcrit were observed in the lowland provenance for Terminalia microcarpa, and in the upland provenance for Castanospermum australe, with no provenance effects in the other two species. Within-species covariation of Tcrit and ∆Tleaf led to a convergence of thermal safety margins across provenances. While future studies should expand the number of provenances and species investigated, our findings suggest that lowland and upland provenances may not differ substantially in their vulnerability to heat stress, as determined by thermal safety margins, despite differences in operating temperatures and Tcrit.

High heat tolerance and thermal safety margins in mangroves from the southwestern coast of India

Mangroves are key components of productive ecosystems that provide a multitude of ecosystem goods and services. How these species will respond to future climates with more frequent and severe extreme temperatures has not received much attention. To understand how vulnerable mangroves are to future warming, we quantified photosynthetic heat tolerance and estimated thermal safety margins for thirteen mangrove species from the southwestern Indian coast. We quantified heat tolerance as temperatures that resulted in a 5 % (T5) and 50 % (T50) decline in photosystem II function, and thermal safety margins (TSM) as the difference between T50 and maximum leaf temperatures. T50 ranged from 48.9 °C in Avicennia Marina to 55.3 °C in Bruguiera gymnorhiza, with a mean of 53.3 °C for the thirteen species. Heat tolerance was higher for species with bigger leaves which experience higher leaf temperatures, but was not related to the other leaf traits examined. Heat tolerance was exceptionally high in these mangroves compared to other woody species. With their high tolerance and large safety margins these mangroves may be relatively less vulnerable to future climates with higher temperatures.

Intensive leaf cooling promotes tree survival during a record heatwave

Increasing heatwaves are threatening forest ecosystems globally. Leaf thermal regulation and tolerance are important for plant survival during heatwaves, though the interaction between these processes and water availability is unclear. Genotypes of the widely distributed foundation tree species Populus fremontii were studied in a controlled common garden during a record summer heatwave-where air temperature exceeded 48 °C. When water was not limiting, all genotypes cooled leaves 2 to 5 °C below air temperatures. Homeothermic cooling was disrupted for weeks following a 72-h reduction in soil water, resulting in leaf temperatures rising 3 °C above air temperature and 1.3 °C above leaf thresholds for physiological damage, despite the water stress having little effect on leaf water potentials. Tradeoffs between leaf thermal safety and hydraulic safety emerged but, regardless of water use strategy, all genotypes experienced significant leaf mortality following water stress. Genotypes from warmer climates showed greater leaf cooling and less leaf mortality after water stress in comparison with genotypes from cooler climates. These results illustrate how brief soil water limitation disrupts leaf thermal regulation and potentially compromises plant survival during extreme heatwaves, thus providing insight into future scenarios in which ecosystems will be challenged with extreme heat and unreliable soil water access.

A 485-million-year history of Earth's surface temperature

A long-term record of global mean surface temperature (GMST) provides critical insight into the dynamical limits of Earth's climate and the complex feedbacks between temperature and the broader Earth system. Here, we present PhanDA, a reconstruction of GMST over the past 485 million years, generated by statistically integrating proxy data with climate model simulations. PhanDA exhibits a large range of GMST, spanning 11° to 36°C. Partitioning the reconstruction into climate states indicates that more time was spent in warmer rather than colder climates and reveals consistent latitudinal temperature gradients within each state. There is a strong correlation between atmospheric carbon dioxide (CO2) concentrations and GMST, identifying CO2 as the dominant control on variations in Phanerozoic global climate and suggesting an apparent Earth system sensitivity of ~8°C.

Canopy temperatures strongly overestimate leaf thermal safety margins of tropical trees

Current estimates of temperature effects on plants mostly rely on air temperature, although it can significantly deviate from leaf temperature (Tleaf). To address this, some studies have used canopy temperature (Tcan). However, Tcan fails to capture the fine-scale variation in Tleaf among leaves and species in diverse canopies. We used infrared radiometers to study Tleaf and Tcan and how they deviate from air temperature (ΔTleaf and ΔTcan) in multispecies tropical tree plantations at three sites along an elevation and temperature gradient in Rwanda. Our results showed high Tleaf (up to c. 50°C) and ΔTleaf (on average 8-10°C and up to c. 20°C) of sun-exposed leaves during 10:00 h-15:00 h, being close to or exceeding photosynthetic heat tolerance thresholds. These values greatly exceeded simultaneously measured values of Tcan and ΔTcan, respectively, leading to strongly overestimated leaf thermal safety margins if basing those on Tcan data. Stomatal conductance and leaf size affected Tleaf and Tcan in line with their expected influences on leaf energy balance. Our findings highlight the importance of leaf traits for leaf thermoregulation and show that monitoring Tcan is not enough to capture the peak temperatures and heat stress experienced by individual leaves of different species in tropical forest canopies.

High heat tolerance, evaporative cooling, and stomatal decoupling regulate canopy temperature and their safety margins in three European oak species

Heatwaves and soil droughts are increasing in frequency and intensity, leading many tree species to exceed their thermal thresholds, and driving wide-scale forest mortality. Therefore, investigating heat tolerance and canopy temperature regulation mechanisms is essential to understanding and predicting tree vulnerability to hot droughts. We measured the diurnal and seasonal variation in leaf water potential (Ψ), gas exchange (photosynthesis Anet and stomatal conductance gs), canopy temperature (Tcan), and heat tolerance (leaf critical temperature Tcrit and thermal safety margins TSM, i.e., the difference between maximum Tcan and Tcrit) in three oak species in forests along a latitudinal gradient (Quercus petraea in Switzerland, Quercus ilex in France, and Quercus coccifera in Spain) throughout the growing season. Gas exchange and Ψ of all species were strongly reduced by increased air temperature (Tair) and soil drying, resulting in stomatal closure and inhibition of photosynthesis in Q. ilex and Q. coccifera when Tair surpassed 30°C and soil moisture dropped below 14%. Across all seasons, Tcan was mainly above Tair but increased strongly (up to 10°C > Tair) when Anet was null or negative. Although trees endured extreme Tair (up to 42°C), positive TSM were maintained during the growing season due to high Tcrit in all species (average Tcrit of 54.7°C) and possibly stomatal decoupling (i.e., Anet ≤0 while gs >0). Indeed, Q. ilex and Q. coccifera trees maintained low but positive gs (despite null Anet), decreasing Ψ passed embolism thresholds. This may have prevented Tcan from rising above Tcrit during extreme heat. Overall, our work highlighted that the mechanisms behind heat tolerance and leaf temperature regulation in oak trees include a combination of high evaporative cooling, large heat tolerance limits, and stomatal decoupling. These processes must be considered to accurately predict plant damages, survival, and mortality during extreme heatwaves.

Are tropical forests approaching critical temperature thresholds?

There is growing concern about the fate of tropical forests in the face of rising global temperatures. Doughty et al. (2023) suggest that an increase in air temperature beyond ∼4 °C will result in massive death of tropical forest leaves and potentially tree death. However, this prediction relies on assumptions that likely underestimate the heat tolerance of tropical leaves.

Environmental and Biogeographic Drivers behind Alpine Plant Thermal Tolerance and Genetic Variation

In alpine ecosystems, elevation broadly functions as a steep thermal gradient, with plant communities exposed to regular fluctuations in hot and cold temperatures. These conditions lead to selective filtering, potentially contributing to species-level variation in thermal tolerance and population-level genetic divergence. Few studies have explored the breadth of alpine plant thermal tolerances across a thermal gradient or the underlying genetic variation thereof. We measured photosystem heat (Tcrit-hot) and cold (Tcrit-cold) thresholds of ten Australian alpine species across elevation gradients and characterised their neutral genetic variation. To reveal the biogeographical drivers of present-day genetic signatures, we also reconstructed temporal changes in habitat suitability across potential distributional ranges. We found intraspecific variation in thermal thresholds, but this was not associated with elevation, nor underpinned by genetic differentiation on a local scale. Instead, regional population differentiation and considerable homozygosity within populations may, in part, be driven by distributional contractions, long-term persistence, and migrations following habitat suitability. Our habitat suitability models suggest that cool-climate-distributed alpine plants may be threatened by a warming climate. Yet, the observed wide thermal tolerances did not reflect this vulnerability. Conservation efforts should seek to understand variations in species-level thermal tolerance across alpine microclimates.

Plasticity and implications of water-use traits in contrasting tropical tree species under climate change

Plants face a trade-off between hydraulic safety and growth, leading to a range of water-use strategies in different species. However, little is known about such strategies in tropical trees and whether different water-use traits can acclimate to warming. We studied five water-use traits in 20 tropical tree species grown at three different altitudes in Rwanda (RwandaTREE): stomatal conductance (gs), leaf minimum conductance (gmin), plant hydraulic conductance (Kplant), leaf osmotic potential (ψo) and net defoliation during drought. We also explored the links between these traits and growth and mortality data. Late successional (LS) species had low Kplant, gs and gmin and, thus, low water loss, while low ψo helped improve leaf water status during drought. Early successional (ES) species, on the contrary, used more water during both moist and dry conditions and exhibited pronounced drought defoliation. The ES strategy was associated with lower mortality and more pronounced growth enhancement at the warmer sites compared to LS species. While Kplant and gmin showed downward acclimation in warmer climates, ψo did not acclimate and gs measured at prevailing temperature did not change. Due to distinctly different water use strategies between successional groups, ES species may be better equipped for a warmer climate as long as defoliation can bridge drought periods.

Variation in leaf carbon economics, energy balance, and heat tolerance traits highlights differing timescales of adaptation and acclimation

Multivariate leaf trait correlations are hypothesized to originate from natural selection on carbon economics traits that control lifetime leaf carbon gain, and energy balance traits governing leaf temperatures, physiological rates, and heat injury. However, it is unclear whether macroevolution of leaf traits primarily reflects selection for lifetime carbon gain or energy balance, and whether photosynthetic heat tolerance is coordinated along these axes. To evaluate these hypotheses, we measured carbon economics, energy balance, and photosynthetic heat tolerance traits for 177 species (157 families) in a common garden that minimizes co-variation of taxa and climate. We observed wide variation in carbon economics, energy balance, and heat tolerance traits. Carbon economics and energy balance (but not heat tolerance) traits were phylogenetically structured, suggesting macroevolution of leaf mass per area and leaf dry matter content reflects selection on carbon gain rather than energy balance. Carbon economics and energy balance traits varied along a common axis orthogonal to heat tolerance traits. Our results highlight a fundamental mismatch in the timescales over which morphological and heat tolerance traits respond to environmental variation. Whereas carbon economics and energy balance traits are constrained by species' evolutionary histories, photosynthetic heat tolerance traits are not and can acclimate readily to leaf microclimates.

Weak correlations among leaf thermal metrics, economic traits and damages under natural heatwaves

The frequency and intensity of heatwaves are increasing around the world, causing severe damages to plants, but whether leaf thermal metrics is in line with leaf economic spectrum is still controversial. Here, we measured leaf damage ratio, leaf thermal metrics (tolerance and sensitivity) and economic traits of 131 woody species across five cities along the Yangtze River after a two-month natural extreme temperature event. We found that leaf thermal sensitivity but not thermal tolerance was correlated with leaf damage ratio, and the relationships between leaf thermal metrics and economic traits were weak, indicating that leaf thermal adaptation may be independent from leaf carbon construction. This study suggests a potential indicator for predicting plant survival under heatwaves, urging future research to explore more physiological traits to comprehensively understand plant heat responses and adaptations.

Leaf thermal safety margins decline at hotter temperatures in a natural warming 'experiment' in the Amazon

The threat of rising global temperatures may be especially pronounced for low-latitude, lowland plant species that have evolved under stable climatic conditions. However, little is known about how these species may acclimate to elevated temperatures. Here, we leveraged a strong, steep thermal gradient along a natural geothermal river to assess the ability of woody plants in the Amazon to acclimate to elevated air temperatures. We measured leaf traits in six common tropical woody species along the thermal gradient to investigate whether individuals of these species: acclimate their thermoregulatory traits to maintain stable leaf temperatures despite higher ambient temperatures; acclimate their photosynthetic thermal tolerances to withstand hotter leaf temperatures; and whether acclimation is sufficient to maintain stable leaf thermal safety margins (TSMs) across different growth temperatures. Individuals of three species acclimated their thermoregulatory traits, and three species increased their thermal tolerances with growth temperature. However, acclimation was generally insufficient to maintain constant TSMs. Notwithstanding, leaf health was generally consistent across growth temperatures. Acclimation in woody Amazonian plants is generally too weak to maintain TSMs at high growth temperatures, supporting previous findings that Amazonian plants will be increasingly vulnerable to thermal stress as temperatures rise.

How a boiling river is helping to highlight the risks of warming for tropical forests

This article is a Commentary on Kullberg et al. (2024), 241: 1447–1463.

Using warming tolerances to predict understory plant responses to climate change

Climate change is pushing species towards and potentially beyond their critical thermal limits. The extent to which species can cope with temperatures exceeding their critical thermal limits is still uncertain. To better assess species' responses to warming, we compute the warming tolerance (ΔTniche ) as a thermal vulnerability index, using species' upper thermal limits (the temperature at the warm limit of their distribution range) minus the local habitat temperature actually experienced at a given location. This metric is useful to predict how much more warming species can tolerate before negative impacts are expected to occur. Here we set up a cross-continental transplant experiment involving five regions distributed along a latitudinal gradient across Europe (43° N-61° N). Transplant sites were located in dense and open forests stands, and at forest edges and in interiors. We estimated the warming tolerance for 12 understory plant species common in European temperate forests. During 3 years, we examined the effects of the warming tolerance of each species across all transplanted locations on local plant performance, in terms of survival, height, ground cover, flowering probabilities and flower number. We found that the warming tolerance (ΔTniche ) of the 12 studied understory species was significantly different across Europe and varied by up to 8°C. In general, ΔTniche were smaller (less positive) towards the forest edge and in open stands. Plant performance (growth and reproduction) increased with increasing ΔTniche across all 12 species. Our study demonstrated that ΔTniche of understory plant species varied with macroclimatic differences among regions across Europe, as well as in response to forest microclimates, albeit to a lesser extent. Our findings support the hypothesis that plant performance across species decreases in terms of growth and reproduction as local temperature conditions reach or exceed the warm limit of the focal species.

Limits of thermal and hydrological tolerance in a foundation tree species (Populus fremontii) in the desert southwestern United States

Populus fremontii is among the most dominant, and ecologically important riparian tree species in the western United States and can thrive in hyper-arid riparian corridors. Yet, P. fremontii forests have rapidly declined over the last decade, particularly in places where temperatures sometimes exceed 50°C. We evaluated high temperature tolerance of leaf metabolism, leaf thermoregulation, and leaf hydraulic function in eight P. fremontii populations spanning a 5.3°C mean annual temperature gradient in a well-watered common garden, and at source locations throughout the lower Colorado River Basin. Two major results emerged. First, despite having an exceptionally high Tcrit (the temperature at which Photosystem II is disrupted) relative to other tree taxa, recent heat waves exceeded Tcrit , requiring evaporative leaf cooling to maintain leaf-to-air thermal safety margins. Second, in midsummer, genotypes from the warmest locations maintained lower midday leaf temperatures, a higher midday stomatal conductance, and maintained turgor pressure at lower water potentials than genotypes from more temperate locations. Taken together, results suggest that under well-watered conditions, P. fremontii can regulate leaf temperature below Tcrit along the warm edge of its distribution. Nevertheless, reduced Colorado River flows threaten to lower water tables below levels needed for evaporative cooling during episodic heat waves.

Klaus Winter - the indefatigable CAM experimentalist

BACKGROUND

In January 1972, Klaus Winter submitted his first paper on crassulacean acid metabolism (CAM) whilst still an undergraduate student in Darmstadt. During the subsequent half-century, he passed his Staatsexamensarbeit, obtained his Dr. rer. nat. summa cum laude and Dr. rer. nat. habil., won a Heinz Maier-Leibnitz Prize and a Heisenberg Fellowship, and has occupied positions in Germany, Australia, the USA and Panama. Now a doyen in CAM circles, and a Senior Staff Scientist at the Smithsonian Tropical Research Institute (STRI), he has published over 300 articles, of which about 44 % are about CAM.

SCOPE

I document Winter's career, attempting to place his CAM-related scientific output and evolution in the context of factors that have influenced him as he and his science progressed from the 1970s to the 2020s.

Global change impacts on cacti (Cactaceae): current threats, challenges and conservation solutions

BACKGROUND

The plant family Cactaceae provides some of the most striking examples of adaptive evolution, expressing undeniably the most spectacular New World radiation of succulent plants distributed across arid and semi-arid regions of the Americas. Cacti are widely regarded for their cultural, economic and ecological value, yet they are also recognized as one of the most threatened and endangered taxonomic groups on the planet.

SCOPE

This paper reviews current threats to species of cacti that have distributions in arid to semi-arid subtropical regions. Our review focuses primarily on four global change forces: (1) increases in atmospheric CO2 concentrations; (2) increases in mean annual temperatures and heat waves; (3) increases in the duration, frequency and intensity of droughts; and (4) and increases in competition and wildfire frequency from invasion by non-native species. We provide a broad range of potential priorities and solutions for stemming the extinction risk of cacti species and populations.

CONCLUSIONS

Mitigating ongoing and emerging threats to cacti will require not only strong policy initiatives and international cooperation, but also new and creative approaches to conservation. These approaches include determining species at risk from climate extremes, enhancing habitat quality after disturbance, approaches and opportunities for ex situ conservation and restoration, and the potential use of forensic tools for identifying plants that have been removed illegally from the wild and sold on open markets.

Functional traits and drought strategy predict leaf thermal tolerance

Heat stress imposes an important physiological constraint on native plant species-one that will only worsen with human-caused climate change. Indeed, rising temperatures have already contributed to large-scale plant mortality events across the globe. These impacts may be especially severe in cities, where the urban heat island effect amplifies climate warming. Understanding how plant species will respond physiologically to rising temperatures and how these responses differ among plant functional groups is critical for predicting future biodiversity scenarios and making informed land management decisions. In this study, we evaluated the effects of elevated temperatures on a functionally and taxonomically diverse group of woody native plant species in a restored urban nature preserve in southern California using measurements of chlorophyll fluorescence as an indicator of leaf thermotolerance. Our aim was to determine if species' traits and drought strategies could serve as useful predictors of thermotolerance. We found that leaf thermotolerance differed among species with contrasting drought strategies, and several leaf-level functional traits were significant predictors of thermotolerance thresholds. Drought deciduous species with high specific leaf area, high rates of transpiration and low water use efficiency were the most susceptible to heat damage, while evergreen species with sclerophyllous leaves, high relative water content and high water use efficiency maintained photosynthetic function at higher temperatures. While these native shrubs and trees are physiologically equipped to withstand relatively high temperatures in this Mediterranean-type climate, hotter conditions imposed by climate change and urbanization may exceed the tolerance thresholds of many species. We show that leaf functional traits and plant drought strategies may serve as useful indicators of species' vulnerabilities to climate change, and this information can be used to guide restoration and conservation in a warmer world.

Detailed in situ leaf energy budget permits the assessment of leaf aerodynamic resistance as a key to enhance non-evaporative cooling under drought

The modulation of the leaf energy budget components to maintain optimal leaf temperature are fundamental aspects of plant functioning and survival. Better understanding these aspects becomes increasingly important under a drying and warming climate when cooling through evapotranspiration (E) is suppressed. Combining novel measurements and theoretical estimates, we obtained unusually comprehensive twig-scale leaf energy budgets under extreme field conditions in droughted (suppressed E) and non-droughted (enhanced E) plots of a semi-arid pine forest. Under the same high mid-summer radiative load, leaf cooling shifted from relying on nearly equal contributions of sensible (H) and latent (LE) energy fluxes in non-droughted trees to relying almost exclusively on H in droughted ones, with no change in leaf temperature. Relying on our detailed leaf energy budget, we could demonstrate that this is due to a 2× reduction in leaf aerodynamic resistance. This capability for LE-to-H shift in leaves of mature Aleppo pine trees under droughted field conditions without increasing leaf temperature is likely a critical factor in the resilience and relatively high productivity of this important Mediterranean tree species under drying conditions.

Tropical forests are approaching critical temperature thresholds

The critical temperature beyond which photosynthetic machinery in tropical trees begins to fail averages approximately 46.7 °C (Tcrit)1. However, it remains unclear whether leaf temperatures experienced by tropical vegetation approach this threshold or soon will under climate change. Here we found that pantropical canopy temperatures independently triangulated from individual leaf thermocouples, pyrgeometers and remote sensing (ECOSTRESS) have midday peak temperatures of approximately 34 °C during dry periods, with a long high-temperature tail that can exceed 40 °C. Leaf thermocouple data from multiple sites across the tropics suggest that even within pixels of moderate temperatures, upper canopy leaves exceed Tcrit 0.01% of the time. Furthermore, upper canopy leaf warming experiments (+2, 3 and 4 °C in Brazil, Puerto Rico and Australia, respectively) increased leaf temperatures non-linearly, with peak leaf temperatures exceeding Tcrit 1.3% of the time (11% for more than 43.5 °C, and 0.3% for more than 49.9 °C). Using an empirical model incorporating these dynamics (validated with warming experiment data), we found that tropical forests can withstand up to a 3.9 ± 0.5 °C increase in air temperatures before a potential tipping point in metabolic function, but remaining uncertainty in the plasticity and range of Tcrit in tropical trees and the effect of leaf death on tree death could drastically change this prediction. The 4.0 °C estimate is within the 'worst-case scenario' (representative concentration pathway (RCP) 8.5) of climate change predictions2 for tropical forests and therefore it is still within our power to decide (for example, by not taking the RCP 6.0 or 8.5 route) the fate of these critical realms of carbon, water and biodiversity3,4.

Tropical Trees Will Need to Acclimate to Rising Temperatures-But Can They?

For tropical forests to survive anthropogenic global warming, trees will need to avoid rising temperatures through range shifts and "species migrations" or tolerate the newly emerging conditions through adaptation and/or acclimation. In this literature review, we synthesize the available knowledge to show that although many tropical tree species are shifting their distributions to higher, cooler elevations, the rates of these migrations are too slow to offset ongoing changes in temperatures, especially in lowland tropical rainforests where thermal gradients are shallow or nonexistent. We also show that the rapidity and severity of global warming make it unlikely that tropical tree species can adapt (with some possible exceptions). We argue that the best hope for tropical tree species to avoid becoming "committed to extinction" is individual-level acclimation. Although several new methods are being used to test for acclimation, we unfortunately still do not know if tropical tree species can acclimate, how acclimation abilities vary between species, or what factors may prevent or facilitate acclimation. Until all of these questions are answered, our ability to predict the fate of tropical species and tropical forests-and the many services that they provide to humanity-remains critically impaired.

Similar patterns of leaf temperatures and thermal acclimation to warming in temperate and tropical tree canopies

As the global climate warms, a key question is how increased leaf temperatures will affect tree physiology and the coupling between leaf and air temperatures in forests. To explore the impact of increasing temperatures on plant performance in open air, we warmed leaves in the canopy of two mature evergreen forests, a temperate Eucalyptus woodland and a tropical rainforest. The leaf heaters consistently maintained leaves at a target of 4 °C above ambient leaf temperatures. Ambient leaf temperatures (Tleaf) were mostly coupled to air temperatures (Tair), but at times, leaves could be 8-10 °C warmer than ambient air temperatures, especially in full sun. At both sites, Tleaf was warmer at higher air temperatures (Tair > 25 °C), but was cooler at lower Tair, contrary to the 'leaf homeothermy hypothesis'. Warmed leaves showed significantly lower stomatal conductance (-0.05 mol m-2 s-1 or -43% across species) and net photosynthesis (-3.91 μmol m-2 s-1 or -39%), with similar rates in leaf respiration rates at a common temperature (no acclimation). Increased canopy leaf temperatures due to future warming could reduce carbon assimilation via reduced photosynthesis in these forests, potentially weakening the land carbon sink in tropical and temperate forests.

Determining the ecophysiological limits of a narrow niche tropical conifer tree (Podocarpus trinitensis)

Many tropical species live close to their thermal limits within a narrow niche. Here, we investigate the ecophysiological limits of the tropical tree Podocarpus trinitensis, which is endemic to Trinidad and Tobago where most populations exist as isolated stands on hilltops. Five wild stands from a range of elevations were compared in the field with measurements of leaf temperature, canopy cover, stomatal conductance (gs), chlorophyll content and several chlorophyll fluorescence parameters. A parallel greenhouse experiment was used to acclimate seedlings to 'CONTROL' and 'HEAT' treatments (with mid-day air temperatures of 34.5 and 37 °C respectively), after which the above parameters were measured along with photosynthetic light and temperature response curves, leaf morphology and in vitro Fv/Fm thermostability. There was a positive association between improved physiological performance and elevation. In the high elevation sites, leaf temperatures were significantly lower while most of the physiological parameters were higher (gs, chlorophyll content, ɸ PSII, ETRmax and Isat90). In the greenhouse, HEAT and CONTROL plants were similar for most parameters, except leaf temperature (which was coupled with air temperature) and leaf mass per unit area (which was higher in HEAT plants). Temperature response curves showed an optimum temperature for photosynthesis of 30 ± 0.5 °C (TOpt) and in vitro Fv/Fm indicated a critical temperature of 47.4 ± 0.38 °C for HEAT and 48.2 ± 0.24 °C for CONTROL (T50), with no indication of heat acclimation. Podocarpus trinitensis was found to be shade tolerant. In the field, seedlings established under a close canopy (>95% canopy cover) and had a low light saturation point (LCP). In the greenhouse, where more light was available, seedlings retained a low light compensation point, light saturation point (LSP) and maximum photosynthetic rate (Amax). The results suggest that P. trinitensis is moderately heat tolerant with the higher elevation sites being more habitable, but stands are also able to survive near sea level under a closed canopy. The narrow niche, along with the 30 ± 0.5 °C optimum temperature for photosynthesis and the lack of thermal plasticity in critical temperature, suggests that P. trinitensis has little room to acclimate to temperatures higher than those currently experienced.

Thermal sensitivity across forest vertical profiles: patterns, mechanisms, and ecological implications

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 (T_leaf ), 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 T_leaf . 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 T_leaf than their smaller counterparts, particularly under climate change. By contrast, understory trees experience fewer extreme high T_leaf '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.

Consistently high heat tolerance acclimation in response to a simulated heatwave across species from the broadly distributed Acacia genus

When leaves exceed their thermal threshold during heatwaves, irreversible damage to the leaf can accumulate. However, few studies have explored short-term acclimation of leaves to heatwaves that could help plants to prevent heat damage with increasing heatwave intensity. Here, we studied the heat tolerance of PSII (PHT) in response to a heatwave in Acacia species from across a strong environmental gradient in Australia. We compared PHT metrics derived from temperature-dependent chlorophyll fluorescence response curves (T-F 0 ) before and during a 4-day 38°C heatwave in a controlled glasshouse experiment. We found that the 15 Acacia species displayed surprisingly large and consistent PHT acclimation responses with a mean tolerance increase of 12°C (range, 7.7-19.1°C). Despite species originating from diverse climatic regions, neither maximum temperature of the warmest month nor mean annual precipitation at origin were clear predictors of PHT. To our knowledge, these are some of the largest measured acclimation responses of PHT from a controlled heatwave experiment. This remarkable capacity could partially explain why this genus has become more diverse and common as the Australian continent became more arid and suggests that the presence of Acacia in Australian ecosystems will remain ubiquitous with climate change.

Summer temperatures reach the thermal tolerance threshold of photosynthetic decline in temperate conifers

Climate change-related environmental stress has been recognized as a driving force in accelerating forest mortality over the last decades in Central Europe. Here, we aim to elucidate the thermal sensitivity of three native conifer species, namely Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and silver fir (Abies alba), and three non-native species, namely Austrian pine (Pinus nigra), Douglas fir (Pseudotsuga menziesii) and Atlas cedar (Cedrus atlantica). Thermal sensitivity, defined here as a decline of the maximum quantum yield of photosystem II (F_v /F_m ) with increasing temperature, was measured under varying levels of heat stress and compared with the turgor loss point (π_tlp ) as a drought resistance trait. We calculated three different leaf thermotolerance traits: the temperature at the onset (5%) of the F_v /F_m decline (T5), the temperature at which F_v /F_m was half the maximum value (T50) and the temperature at which only 5% F_v /F_m remained (T95). T5 ranged from 38.5 ± 0.8 °C to 43.1 ± 0.6 °C across all species, while T50 values were at least 9 to 11 degrees above the maximum air temperatures on record for all species. Only Austrian pine had a notably higher T5 value than recorded maximum air temperatures. Species with higher T5 values were characterized by a less negative π_tlp compared to species with lower T5. The six species could be divided into 'drought-tolerant heat-sensitive' and 'drought-sensitive heat-tolerant' groups. Exposure to short-term high temperatures thus exhibits a considerable threat to conifer species in Central European forest production systems.

Correlations between photosynthetic heat tolerance and leaf anatomy and climatic niche in Asian mangrove trees

Photosynthetic heat tolerance (P_HT ) is a key predictor of plant response to climate change. Mangroves are an ecologically and economically important coastal plant community comprised of trees growing at their physiological limits. Mangroves are currently impacted by global warming, yet the P_HT of mangrove trees is poorly understood. In this study, we provide the first assessment of P_HT in 13 Asian mangrove species, based on the critical temperature that causes the initial damage (T_Crit ) and the temperature that causes 50% damage (T₅₀ ) to photosystem II. We tested the hypotheses that the P_HT in mangroves is: (i) correlated with climatic niche and leaf traits, and (ii) higher than in plants from other tropical ecosystems. Our results demonstrated correlations between P_HT and multiple key climate variables, the palisade to spongy mesophyll ratio and the leaf area. The two most heat-sensitive species were Kandelia obovata and Avicennia marina. Our study also revealed that mangrove trees show high heat tolerance compared to plants from other tropical ecosystems. The high P_HT of mangroves thus demonstrated a conservative evolutionary strategy in heat tolerance, and highlights the need for integrative and comparative studies on thermoregulatory traits and climatic niche in order to understand the physiological response of mangrove trees to climate change-driven heatwaves and rising global temperatures.

Patterns of leaf trait variation underlie ecological differences among sympatric tree species of Damburneya in a tropical rainforest

PREMISE

Although ecological differentiation driven by altitude and soil is hypothesized to promote coexistence of sympatric tree species of Damburneya (Lauraceae), the mechanistic role of leaf functional variation on ecological differentiation among co-occurring species remains unexplored. We aimed to determine whether the patterns of leaf trait variation reflect ecological differences among sympatric Damburneya species. We tested whether trait correlations underlying functional strategies and average species traits vary in response to local soil heterogeneity along an altitudinal gradient, potentially affecting species distributions.

METHODS

At two contrasting altitudes (100, 1100 m a.s.l.) in a Mexican tropical rainforest, we characterized soil chemical and physical properties and sampled four Damburneya species to quantify five leaf functional traits. We used linear models to analyze paired and multivariate trait correlations, spatial and interspecific effects on trait variation, and trait response to local soil heterogeneity. Relative contributions of intra- and interspecific variation to local trait variability were quantified with an ANOVA.

RESULTS

Soil nutrient availability was higher at low altitude, but all species had a high leaf N:P ratio across altitudes suggesting a limited P supply for plants. Species distribution differed altitudinally, with some species constrained to low or high altitude, potentially reflecting soil nutrient availability. Leaf traits responded to altitude and local soil properties, suggesting interspecific differences in functional strategies according to the leaf economics spectrum (conservative vs. acquisitive).

CONCLUSIONS

The interspecific divergence in functional strategies in response to local environmental conditions suggests that trait variation could underlie ecological differentiation among Damburneya sympatric species.

Wheat photosystem II heat tolerance responds dynamically to short- and long-term warming

Wheat photosynthetic heat tolerance can be characterized using minimal chlorophyll fluorescence to quantify the critical temperature (Tcrit) above which incipient damage to the photosynthetic machinery occurs. We investigated intraspecies variation and plasticity of wheat Tcrit under elevated temperature in field and controlled-environment experiments, and assessed whether intraspecies variation mirrored interspecific patterns of global heat tolerance. In the field, wheat Tcrit varied diurnally-declining from noon through to sunrise-and increased with phenological development. Under controlled conditions, heat stress (36 °C) drove a rapid (within 2 h) rise in Tcrit that peaked after 3-4 d. The peak in Tcrit indicated an upper limit to PSII heat tolerance. A global dataset [comprising 183 Triticum and wild wheat (Aegilops) species] generated from the current study and a systematic literature review showed that wheat leaf Tcrit varied by up to 20 °C (roughly two-thirds of reported global plant interspecies variation). However, unlike global patterns of interspecies Tcrit variation that have been linked to latitude of genotype origin, intraspecific variation in wheat Tcrit was unrelated to that. Overall, the observed genotypic variation and plasticity of wheat Tcrit suggest that this trait could be useful in high-throughput phenotyping of wheat photosynthetic heat tolerance.

Handling the heat - photosynthetic thermal stress in tropical trees

Warming climate increases the risk for harmful leaf temperatures in terrestrial plants, causing heat stress and loss of productivity. The heat sensitivity may be particularly high in equatorial tropical tree species adapted to a thermally stable climate. Thermal thresholds of the photosynthetic system of sun-exposed leaves were investigated in three tropical montane tree species native to Rwanda with different growth and water use strategies (Harungana montana, Syzygium guineense and Entandrophragma exselsum). Measurements of chlorophyll fluorescence, leaf gas exchange, morphology, chemistry and temperature were made at three common gardens along an elevation/temperature gradient. Heat tolerance acclimated to maximum leaf temperature (T_leaf ) across the species. At the warmest sites, the thermal threshold for normal function of photosystem II was exceeded in the species with the highest T_leaf despite their higher heat tolerance. This was not the case in the species with the highest transpiration rates and lowest T_leaf . The results point to two differently effective strategies for managing thermal stress: tolerance through physiological adjustment of leaf osmolality and thylakoid membrane lipid composition, or avoidance through morphological adaptation and transpiratory cooling. More severe photosynthetic heat stress in low-transpiring montane climax species may result in a competitive disadvantage compared to high-transpiring pioneer species with more efficient leaf cooling.

Short-term warming does not affect intrinsic thermotolerance but induces strong sustaining photoprotection in tropical evergreen citrus genotypes

Consequences of warming and postwarming events on photosynthetic thermotolerance (P_T ) and photoprotective responses in tropical evergreen species remain elusive. We chose Citrus to answer some of the emerging questions related to tropical evergreen species' P_T behaviour including (i) how wide is the genotypic variation in P_T ? (ii) how does P_T respond to short-term warming and (iii) how do photosynthesis and photoprotective functions respond over short-term warming and postwarming events? A study on 21 genotypes revealed significant genotypic differences in P_T , though these were not large. We selected five genotypes with divergent P_T and simulated warming events: T_max 26/20°C (day-time highest maximum/night-time lowest maximum) (Week 1) < T_max 33/30°C (Week 2) < T_max 36/32°C (Week 3) followed by T_max 26/16°C (Week 4, recovery). The P_T of all genotypes remained unaltered despite strong leaf megathermy (leaf temperature > air temperature) during warming events. Though moderate warming showed genotype-specific stimulation in photosynthesis, higher warming unequivocally led to severe loss in net photosynthesis and induced higher nonphotochemical quenching. Even after a week of postwarming, photoprotective mechanisms strongly persisted. Our study points towards a conservative P_T in evergreen citrus genotypes and their need for sustaining higher photoprotection during warming as well as postwarming recovery conditions.

Large differences in leaf cuticle conductance and its temperature response among 24 tropical tree species from across a rainfall gradient

More frequent droughts and rising temperatures pose serious threats to tropical forests. When stomata are closed under dry and hot conditions, plants lose water through leaf cuticles, but little is known about cuticle conductance (gmin ) of tropical trees, how it varies among species and environments, and how it is affected by temperature. We determined gmin in relation to temperature for 24 tropical tree species across a steep rainfall gradient in Panama, by recording leaf drying curves at different temperatures in the laboratory. In contrast with our hypotheses, gmin did not differ systematically across the rainfall gradient; species differences did not reflect phylogenetic patterns; and in most species gmin did not significantly increase between 25 and 50°C. gmin was higher in deciduous than in evergreen species, in species with leaf trichomes than in species without, in sun leaves than in shade leaves, and tended to decrease with increasing leaf mass per area across species. There was no relationship between stomatal and cuticle conductance. Large species differences in gmin and its temperature response suggest that more frequent hot droughts may lead to differential survival among tropical tree species, regardless of species' position on the rainfall gradient.