Photosynthetic capacity and leaf nitrogen decline along a controlled climate gradient in provenances of two widely distributed Eucalyptus species

Pushing the envelope: do narrowly and widely distributed Eucalyptus species differ in response to climate warming?

Contemporary climate change will push many tree species into conditions that are outside their current climate envelopes. Using the Eucalyptus genus as a model, we addressed whether species with narrower geographical distributions show constrained ability to cope with warming relative to species with wider distributions, and whether this ability differs among species from tropical and temperate climates. We grew seedlings of widely and narrowly distributed Eucalyptus species from temperate and tropical Australia in a glasshouse under two temperature regimes: the summer temperature at seed origin and +3.5°C. We measured physical traits and leaf-level gas exchange to assess warming influences on growth rates, allocation patterns, and physiological acclimation capacity. Warming generally stimulated growth, such that higher relative growth rates early in development placed seedlings on a trajectory of greater mass accumulation. The growth enhancement under warming was larger among widely than narrowly distributed species and among temperate rather than tropical provenances. The differential growth enhancement was primarily attributable to leaf area production and adjustments of specific leaf area. Our results suggest that tree species, including those with climate envelopes that will be exceeded by contemporary climate warming, possess capacity to physiologically acclimate but may have varying ability to adjust morphology.

Variable thermal plasticity of leaf functional traits in Andean tropical montane forests

Tropical montane forests (TMFs) are biodiversity hotspots and provide vital ecosystem services, but they are disproportionately vulnerable to climate warming. In the Andes, cold-affiliated species from high elevations are being displaced at the hot end of their thermal distributions by warm-affiliated species migrating upwards from lower elevations, leading to compositional shifts. Leaf functional traits are strong indicators of plant performance and at the community level have been shown to vary along elevation gradients, reflecting plant adaptations to different environmental niches. However, the plastic response of such traits to relatively rapid temperature change in Andean TMF species remains unknown. We used three common garden plantations within a thermosequence in the Colombian Andes to investigate the warming and cooling responses of key leaf functional traits in eight cold- and warm-affiliated species with variable thermal niches. Cold-affiliated species shifted their foliar nutrient concentrations when exposed to warming, while all other traits did not significantly change; contrastingly, warm-affiliated species were able to adjust structural, nutrient and water-use efficiency traits from acquisitive to conservative strategies in response to cooling. Our findings suggest that cold-affiliated species will struggle to acclimate functional traits to warming, conferring warm-affiliated species a competitive advantage under climate change.

Climatic Warming-Induced Drought Stress Has Resulted in the Transition of Tree Growth Sensitivity from Temperature to Precipitation in the Loess Plateau of China

Ongoing climate warming poses significant threats to forest ecosystems, particularly in drylands. Here, we assess the intricate responses of tree growth to climate change across two warming phases (1910-1940 and 1970-2000) of the 20th century in the Loess Plateau of China. To achieve this, we analyzed a dataset encompassing 53 ring-width chronologies extracted from 13 diverse tree species, enabling us to discern and characterize the prevailing trends in tree growth over these warming phases. The difference in the primary contributors over two warming phases was compared to investigate the association of tree growth with climatic drivers. We found that the first warming phase exerted a stimulating effect on tree growth, with climate warming correlating to heightened growth rates. However, a contrasting pattern emerged in the second phase as accelerated drought conditions emerged as a predominant limiting factor, dampening tree growth rates. The response of tree growth to climate changed markedly during the two warming phases. Initially, temperature assumed a dominant role in driving the tree growth of growth season during the first warming phase. Instead, precipitation and drought stress became the main factors affecting tree growth in the second phase. This drought stress manifested predominantly during the early and late growing seasons. Our findings confirm the discernible transition of warming-induced tree growth in water-limited regions and highlight the vulnerability of dryland forests to the escalating dual challenges of heightened warming and drying. If the warming trend continues unabated in the Loess Plateau, further deterioration in tree growth and heightened mortality rates are foreseeable outcomes. Some adaptive forest managements should be encouraged to sustain the integrity and resilience of these vital ecosystems in the Loess Plateau and similar regions.

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.

The effects of spring versus summer heat events on two arid zone plant species under field conditions

Heatwaves are increasingly occurring out-of-season, which may affect plants not primed for the event. Further, heat stress often coincides with water and/or nutrient stress, impairing short-term physiological function and potentially causing downstream effects on reproductive fitness. We investigated the response of water-stressed arid-zone Solanum oligacanthum and Solanum orbiculatum to spring vs summer heat stress under differing nutrient conditions. Heat stress events were imposed in open-topped chambers under in situ desert conditions. To assess short-term impacts, we measured leaf photosystem responses (F v /F m ) and membrane stability; long-term effects were compared via biomass allocation, visible damage, flowering and fruiting. Plants generally fared more poorly following summer than spring heat stress, with the exception of F v /F m . Summer heat stress caused greater membrane damage, reduced growth and survival compared with spring. Nutrient availability had a strong influence on downstream effects of heat stress, including species-specific outcomes for reproductive fitness. Overall, high temperatures during spring posed a lower threat to fitness than in severe arid summer conditions of high temperature and low water availability, which were more detrimental to plants in both the short and longer term. Our study highlights the importance of considering ecologically relevant, multiple-stressor events to understand different species responses to extreme heat.

Acclimation of photosynthetic capacity and foliar respiration in Andean tree species to temperature change

Climate warming is causing compositional changes in Andean tropical montane forests (TMFs). These shifts are hypothesised to result from differential responses to warming of cold- and warm-affiliated species, with the former experiencing mortality and the latter migrating upslope. The thermal acclimation potential of Andean TMFs remains unknown. Along a 2000 m Andean altitudinal gradient, we planted individuals of cold- and warm-affiliated species (under common soil and irrigation), exposing them to the hot and cold extremes of their thermal niches, respectively. We measured the response of net photosynthesis (A_net ), photosynthetic capacity and leaf dark respiration (R_dark ) to warming/cooling, 5 months after planting. In all species, A_net and photosynthetic capacity at 25°C were highest when growing at growth temperatures (T_g ) closest to their thermal means, declining with warming and cooling in cold-affiliated and warm-affiliated species, respectively. When expressed at T_g , photosynthetic capacity and R_dark remained unchanged in cold-affiliated species, but the latter decreased in warm-affiliated counterparts. R_dark at 25°C increased with temperature in all species, but remained unchanged when expressed at T_g . Both species groups acclimated to temperature, but only warm-affiliated species decreased R_dark to photosynthetic capacity ratio at T_g as temperature increased. This could confer them a competitive advantage under future warming.

Thermal acclimation of leaf respiration is consistent in tropical and subtropical populations of two mangrove species

Populations from different climates often show unique growth responses to temperature, reflecting temperature adaptation. Yet, whether populations from different climates differ in physiological temperature acclimation remains unclear. Here, we test whether populations from differing thermal environments exhibit different growth responses to temperature and differences in temperature acclimation of leaf respiration. We grew tropical and subtropical populations of two mangrove species (Avicennia germinans and Rhizophora mangle) under ambient and experimentally warmed conditions in a common garden at the species' northern range limit. We quantified growth and temperature responses of leaf respiration (R) at seven time points over ~10 months. Warming increased productivity of tropical populations more than subtropical populations, reflecting a higher temperature optimum for growth. In both species, R measured at 25 °C declined as seasonal temperatures increased, demonstrating thermal acclimation. Contrary to our expectations, acclimation of R was consistent across populations and temperature treatments. However, populations differed in adjusting the temperature sensitivity of R (Q10) to seasonal temperatures. Following a freeze event, tropical Avicennia showed greater freeze damage than subtropical Avicennia, while both Rhizophora populations appeared equally susceptible. We found evidence of temperature adaptation at the whole-plant scale but little evidence for population differences in thermal acclimation of leaf physiology. Studies that examine potential costs and benefits of thermal acclimation in an evolutionary context may provide new insights into limits of thermal acclimation.

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.

Contrasting warming responses of photosynthesis in early- and late-successional tropical trees

The productivity and climate feedbacks of tropical forests depend on tree physiological responses to warmer and, over large areas, seasonally drier conditions. However, knowledge regarding such responses is limited due to data scarcity. We studied the impact of growth temperature on net photosynthesis (An), maximum rates of Rubisco carboxylation at 25°C (Vcmax25), stomatal conductance (gs) and the slope parameter of the stomatal conductance-photosynthesis model (g1), in ten early- (ES) and eight late-successional (LS) tropical tree species grown at three sites along an elevation gradient in Rwanda, differing by 6.8°C in daytime ambient air temperature. The effect of seasonal drought on An was also investigated. We found that warm climate decreased wet-season An in LS species, but not in ES species. Values of Vcmax25 were lower at the warmest site across both successional groups, and An and Vcmax25 were higher in ES compared to LS species. Stomatal conductance exhibited no significant site differences and g1 was similar across both sites and successional groups. Drought strongly reduced An at warmer sites but not at the coolest montane site and this response was similar in both ES and LS species. Our results suggest that warming has negative effects on leaf-level photosynthesis in LS species, while both LS and ES species suffer photosynthesis declines in a warmer climate with more pronounced droughts. The contrasting responses of An between successional groups may lead to shifts in species' competitive balance in a warmer world, to the disadvantage of LS trees.

Thermal optimum of photosynthesis is controlled by stomatal conductance and does not acclimate across an urban thermal gradient in six subtropical tree species

Modelling the response of plants to climate change is limited by our incomplete understanding of the component processes of photosynthesis and their temperature responses within and among species. For ≥20 individuals, each of six common subtropical tree species occurring across steep urban thermal gradients in Miami, Florida, USA, we determined rates of net photosynthesis (A_net ), maximum RuBP carboxylation, maximum RuBP regeneration and stomatal conductance, and modelled the optimum temperature (T_opt ) and process rate of each parameter to address two questions: (1) Do the T_opt of A_net (T_optA ) and the maximum A_net (A_opt ) of subtropical trees reflect acclimation to elevated growth temperatures? And (2) What limits A_net in subtropical trees? Against expectations, we did not find significant acclimation of T_optA , A_opt  or the T_opt of any of the underlying photosynthetic parameters to growth temperature in any of the focal species. Model selection for the single best predictor of A_net both across leaf temperatures and at T_optA revealed that the A_net of most trees was best predicted by stomatal conductance. Our findings are in accord with those of previous studies, especially in the tropics, that have identified stomatal conductance to be the most important factor limiting A_net , rather than biochemical thermal responses.

Simulated climate warming decreases fruit number but increases seed mass

Climate warming is changing plant sexual reproduction, having consequences for species distribution and community dynamics. However, the magnitude and direction of plant reproductive efforts (e.g., number of flowers) and success (e.g., number and mass of fruits or seeds) in response to warming have not been well-characterized. Here, we generated a global dataset of simulated warming experiments, consisting of 477 pairwise comparisons for 164 terrestrial species. We found evidence that warming overall decreased fruit number and increased seed mass, but little evidence that warming influenced flower number, fruit mass, or seed number. The warming effects on seed mass were regulated by the pollination type, and insect-pollinated plants exhibited a stronger response to warming than wind-pollinated plants. We found strong evidence that warming increased the mass of seeds for the nondominant species but no evidence of this for the dominant species. There was no evidence that phylogenetic relatedness explained the effects of warming on plant reproductive effort and success. In addition, the effects of warming on flowering onset negatively related to the responses in terms of the number of fruits and seeds to warming, revealing a cascading effect of plant reproductive development. These findings provide the first quantification of the response of terrestrial plant sexual reproduction to warming and suggest that plants may increase their fitness by producing heavier seeds under a warming climate.

Neglecting acclimation of photosynthesis under drought can cause significant errors in predicting leaf photosynthesis in wheat

Extreme climatic events, such as heat waves, cold snaps and drought spells, related to global climate change, have become more frequent and intense in recent years. Acclimation of plant physiological processes to changes in environmental conditions is a key component of plant adaptation to climate change. We assessed the temperature response of leaf photosynthetic parameters in wheat grown under contrasting water regimes and growth temperatures (T_growth ). Two independent experiments were conducted under controlled conditions. In Experiment 1, two wheat genotypes were subjected to well-watered or drought-stressed treatments; in Experiment 2, the two water regimes combined with high, medium and low T_growth were imposed on one genotype. Parameters of a biochemical C₃ -photosynthesis model were estimated at six leaf temperatures for each factor combination. Photosynthesis acclimated more to drought than to T_growth . Drought affected photosynthesis by lowering its optimum temperature (T_opt ) and the values at T_opt of light-saturated net photosynthesis, stomatal conductance, mesophyll conductance, the maximum rate of electron transport (J_max ) and the maximum rate of carboxylation by Rubisco (V_cmax ). T_opt for V_cmax was up to 40°C under well-watered conditions but 24-34°C under drought. The decrease in photosynthesis under drought varied among T_growth but was similar between genotypes. The temperature response of photosynthetic quantum yield under drought was partly attributed to photorespiration but more to alternative electron transport. All these changes in biochemical parameters could not be fully explained by the changed leaf nitrogen content. Further model analysis showed that both diffusional and biochemical parameters of photosynthesis and their thermal sensitivity acclimate little to T_growth , but acclimate considerably to drought and the combination of drought and T_growth . The commonly used modelling approaches, which typically consider the response of diffusional parameters, but ignore acclimation responses of biochemical parameters to drought and T_growth , strongly overestimate leaf photosynthesis under variable temperature and drought.

Transplanting larch trees into warmer areas increases the photosynthesis and its temperature sensitivity

To investigate the effects of climate warming on photosynthesis, Dahurian larch (Larix gmelinii Rupr.) trees from four sites (spanning ~ 5.5° in latitude and ~4 °C of warming) within the geographic range in China were transplanted into a common garden close to the warmer border in 2004. Throughout the growing season of 2018, the CO2- and temperature-response curves of the photosynthesis in the common garden and at the original sites were measured. It was discovered that warming treatment considerably increased the maximum net photosynthetic rate (Amax) by 23.4-35.3% depending on the sites, signifying that warming upregulated Amax with respect to the degree of warming. At 25 °C, warming enhanced the maximum Rubisco carboxylation rate (Vcmax), maximum electron transport rate (Jmax), and mass-based leaf nitrogen concentration (Nmass). The climate warming effect (CWE) on Amax was positively associated with the CWEs on Vcmax, Jmax and Nmass, which indicated that warming promoted Amax primarily via increasing carboxylation and photosynthetic electron transport rates and leaf nitrogen supply. The CWE in optimal photosynthetic temperature (Topt) was significant for the trees from the northern sites rather than the southern sites; however, the effect vanished for the trees transplanted to the common garden; this implied that Topt exhibited limited local thermal acclimation. Nevertheless, warming narrowed the temperature-response curve, the effect of which was positively associated with the warming magnitude. These findings implied that trees transplanted into warmer areas changed the photosynthetic optimum temperature and sensitivity. In summary, our results deepen the understanding of the underlying mechanisms of intraspecific responses of photosynthesis to temperature changes, including which of the modeling would improve the prediction of tree growth and forest carbon cycling under climate warming.

The roles of divergent and parallel molecular evolution contributing to thermal adaptive strategies in trees

Local adaptation is a driver of biological diversity, and species may develop analogous (parallel evolution) or alternative (divergent evolution) solutions to similar ecological challenges. We expect these adaptive solutions would culminate in both phenotypic and genotypic signals. Using two Eucalyptus species (Eucalyptus grandis and Eucalyptus tereticornis) with overlapping distributions grown under contrasting 'local' temperature conditions to investigate the independent contribution of adaptation and plasticity at molecular, physiological and morphological levels. The link between gene expression and traits markedly differed between species. Divergent evolution was the dominant pattern driving adaptation (91% of all significant genes); but overlapping gene (homologous) responses were dependent on the determining factor (plastic, adaptive or genotype by environment interaction). Ninety-eight percent of the plastic homologs were similarly regulated, while 50% of the adaptive homologs and 100% of the interaction homologs were antagonistical. Parallel evolution for the adaptive effect in homologous genes was greater than expected but not in favour of divergent evolution. Heat shock proteins for E. grandis were almost entirely driven by adaptation, and plasticity in E. tereticornis. These results suggest divergent molecular evolutionary solutions dominated the adaptive mechanisms among species, even in similar ecological circumstances. Suggesting that tree species with overlapping distributions are unlikely to equally persist in the future.

Temperature acclimation of net photosynthesis and its underlying component processes in four tropical tree species

The effect of temperature change on leaf physiology has been extensively studied in temperate trees and to some extent in boreal and tropical tree species. While increased temperature typically stimulates leaf CO2 assimilation and tree growth in high-altitude ecosystems, tropical species are often negatively affected. These trees may operate close to their temperature optima and have a limited thermal acclimation capacity due to low seasonal and historical variation in temperature. To test this hypothesis, we studied the extent to which the temperature sensitivities of leaf photosynthesis and respiration acclimate to growth temperature in four common African tropical tree species. Tree seedlings native to different altitudes and therefore adapted to different growth temperatures were cultivated at three different temperatures in climate-controlled chambers. We estimated the acclimation capacity of the temperature sensitivities of light-saturated net photosynthesis, the maximum rates of Rubisco carboxylation (Vcmax) and thylakoid electron transport (J), and dark respiration. Leaf thylakoid membrane lipid composition, nitrogen content and leaf mass per area were also analyzed. Our results showed that photosynthesis in tropical tree species acclimated to higher growth temperatures, but that this was weakest in the species originating from the coolest climate. The temperature optimum of J acclimated significantly in three species and variation in J was linked to changes in the thylakoid membrane lipid composition. For Vcmax, there was only evidence of significant acclimation of optimal temperature in the lowest elevation species. Respiration acclimated to maintain homeostasis at growth temperature in all four species. Our results suggest that the lowest elevation species is better physiologically adapted to acclimate to high growth temperatures than the highest elevation species, indicating a potential shift in competitive balance and tree community composition to the disadvantage of montane tree species in a warmer world.

The Morpho-Physio-Biochemical Attributes of Urban Trees for Resilience in Regional Ecosystems in Cities: A Mini-Review

Increased urbanization means human beings become the dominant species and reduction in canopy cover. Globally, urban trees grow under challenging and complex circumstances with urbanization trends of increasing anthropogenic carbon dioxide (CO2) emissions, high temperature and drought stress. This study aims to provide a better understanding of urban trees’ morpho-physio-biochemical attributes that can support sustainable urban greening programs and urban climate change mitigation policies. Globally, urban dwellers’ population is on the rise and spreading to suburban areas over time with an increase in domestic CO2 emissions. Uncertainty and less information on urban tree diversification and resistance to abiotic stress may create deterioration of ecosystem resilience over time. This review uses general parameters for urban tree physiology studies and employs three approaches for evaluating ecosystem resilience based on urban stress resistance in relation to trees’ morphological, physiological and biochemical attributes. Due to the lack of a research model of ecosystem resilience and urban stress resistance of trees, this review demonstrates that the model concept supports future urban tree physiology research needs. In particular, it is necessary to develop integral methodologies and an urban tree research concept to assess how main and combined effects of drought and/or climate changes affect indigenous and exotic trees that are commonly grown in cities.

Tropical rainforest species have larger increases in temperature optima with warming than warm-temperate rainforest trees

While trees can acclimate to warming, there is concern that tropical rainforest species may be less able to acclimate because they have adapted to a relatively stable thermal environment. Here we tested whether the physiological adjustments to warming differed among Australian tropical, subtropical and warm-temperate rainforest trees. Photosynthesis and respiration temperature responses were quantified in six Australian rainforest seedlings of tropical, subtropical and warm-temperate climates grown across four growth temperatures in a glasshouse. Temperature-response models were fitted to identify mechanisms underpinning the response to warming. Tropical and subtropical species had higher temperature optima for photosynthesis (T_optA ) than temperate species. There was acclimation of T_optA to warmer growth temperatures. The rate of acclimation (0.35-0.78°C °C^-1 ) was higher in tropical and subtropical than in warm-temperate trees and attributed to differences in underlying biochemical parameters, particularly increased temperature optima of V_cmax25 and J_max25 . The temperature sensitivity of respiration (Q₁₀ ) was 24% lower in tropical and subtropical compared with warm-temperate species. Overall, tropical and subtropical species had a similar capacity to acclimate to changes in growth temperature as warm-temperate species, despite being grown at higher temperatures. Quantifying the physiological acclimation in rainforests can improve accuracy of future climate predictions and assess their potential vulnerability to warming.

Temperature responses of photosynthesis and respiration in evergreen trees from boreal to tropical latitudes

Evergreen species are widespread across the globe, representing two major plant functional forms in terrestrial models. We reviewed and analysed the responses of photosynthesis and respiration to warming in 101 evergreen species from boreal to tropical biomes. Summertime temperatures affected both latitudinal gas exchange rates and the degree of responsiveness to experimental warming. The decrease in net photosynthesis at 25°C (A_net25 ) was larger with warming in tropical climates than cooler ones. Respiration at 25°C (R₂₅ ) was reduced by 14% in response to warming across species and biomes. Gymnosperms were more sensitive to greater amounts of warming than broadleaved evergreens, with A_net25 and R₂₅ reduced c. 30-40% with > 10°C warming. While standardised rates of carboxylation (V_cmax25 ) and electron transport (J_max25 ) adjusted to warming, the magnitude of this adjustment was not related to warming amount (range 0.6-16°C). The temperature optimum of photosynthesis (T_optA ) increased on average 0.34°C per °C warming. The combination of more constrained acclimation of photosynthesis and increasing respiration rates with warming could possibly result in a reduced carbon sink in future warmer climates. The predictable patterns of thermal acclimation across biomes provide a strong basis to improve modelling predictions of the future terrestrial carbon sink with warming.

Divergent Abiotic Stressors Drive Grassland Community Assembly of Tibet and Mongolia Plateau

Multiple ecological processes simultaneously govern community assembly, but it remains unclear how abiotic stressors regulate the relative importance of these processes among different biogeographic regions. Therefore, we conducted a comprehensive study on the responses of community assembly to varying environmental gradients, using the mean, variance, skewness, and kurtosis of plant height (height), specific leaf area (SLA) and leaf dry matter content (LDMC) distributions on the Tibetan Plateau (TP) and the Mongolian Plateau (MP). Our results showed that the prevalence of trait convergence across all grasslands in both TP and MP seem to be the result of abiotic filtering or weaker competitive exclusion etc. These trait-convergence assembly processes decrease the functional dispersion but increase the evenness of the trait frequency distribution. The mean, variance, skewness, and kurtosis responses of grassland communities to abiotic stress varied between the TP and MP. On average, plant trait distribution was mainly driven by temperature on the TP, and low-temperature stress altered the community assembly rules. In contrast, water availability shaped plant trait frequency distributions on the MP, and drought stress mediated the balance between different assembly processes. Our results provide empirical evidence that divergent abiotic stressors regulate the grassland community assembly on the TP and MP. Together, our study speculates that different aspects of future climate change, such as climate warming and changing precipitation patterns, on community assembly are dependent on regional climatic regimes.

The physiological acclimation and growth response of Populus trichocarpa to warming

Plant metabolic acclimation to thermal stress remains underrepresented in current global climate models. Gaps exist in our understanding of how metabolic processes (i.e., photosynthesis, respiration) acclimate over time and how aboveground versus belowground acclimation differs. We measured the thermal acclimation of Populus trichocarpa, comparing aboveground versus belowground physiology over time. Ninety genetically identical ramets were propagated in mesocosms that separated root and microbial components. After establishment at 25°C for 6 weeks, 60 clones were warmed +4 or +8°C and monitored for 10 weeks, measuring photosynthesis (A), leaf respiration (R), soil respiration (R_s ), root plus soil respiration (R_s+r ), and root respiration (R_r ). We observed thermal acclimation in both A and R, with rates initially increasing, then declining as the thermal photosynthetic optimum (T_opt ) and the temperature-sensitivity (Q₁₀ ) of respiration adjusted to warmer conditions. Photosynthetic acclimation was constructive, based on an increase in both T_opt and peak A. Belowground, R_s+r decreased linearly with warming, while R_s rates declined abruptly, then remained constant with additional warming. Plant biomass was greatest at +4°C, with 30% allocated belowground. Rates of mass-based R_r were similar among treatments; however, root nitrogen declined at +8°C leading to less mass nitrogen-based R_r in that treatment. The Q₁₀ -temperature relationship of R_r was affected by warming, leading to differing values among treatments. Aboveground acclimation exceeded belowground acclimation, and plant nitrogen-use mediated the acclimatory response. Results suggest that moderate climate warming (+4°C) may lead to acclimation and increased plant biomass production but increases in production could be limited with severe warming (+8°C).

Repeated extreme heatwaves result in higher leaf thermal tolerances and greater safety margins

The frequency and severity of heatwave events are increasing, exposing species to conditions beyond their physiological limits. Species respond to heatwaves in different ways, however it remains unclear if plants have the adaptive capacity to successfully respond to hotter and more frequent heatwaves. We exposed eight tree populations from two climate regions grown under cool and warm temperatures to repeated heatwave events of moderate (40°C) and extreme (46°C) severity to assess adaptive capacity to heatwaves. Leaf damage and maximum quantum efficiency of photosystem II (F_v /F_m ) were significantly impacted by heatwave severity and growth temperatures, respectively; populations from a warm-origin avoided damage under moderate heatwaves compared to those from a cool-origin, indicating a degree of local adaptation. We found that plasticity to heatwave severity and repeated heatwaves contributed to enhanced thermal tolerance and lower leaf temperatures_, leading to greater thermal safety margins (thermal tolerance minus leaf temperature) in a second heatwave. Notably, while we show that adaptation and physiological plasticity are important factors affecting plant adaptive capacity to thermal stress, plasticity of thermal tolerances and thermal safety margins provides the opportunity for trees to persist among fluctuating heatwave exposures.

Limited thermal acclimation of photosynthesis in tropical montane tree species

The temperature sensitivity of physiological processes and growth of tropical trees remains a key uncertainty in predicting how tropical forests will adjust to future climates. In particular, our knowledge regarding warming responses of photosynthesis, and its underlying biochemical mechanisms, is very limited. We grew seedlings of two tropical montane rainforest tree species, the early-successional species Harungana montana and the late-successional species Syzygium guineense, at three different sites along an elevation gradient, differing by 6.8℃ in daytime ambient air temperature. Their physiological and growth performance was investigated at each site. The optimum temperature of net photosynthesis (T_optA ) did not significantly increase in warm-grown trees in either species. Similarly, the thermal optima (T_optV and T_optJ ) and activation energies (E_aV and E_aJ ) of maximum Rubisco carboxylation capacity (V_cmax ) and maximum electron transport rate (J_max ) were largely unaffected by warming. However, V_cmax , J_max and foliar dark respiration (R_d ) at 25℃ were significantly reduced by warming in both species, and this decline was partly associated with concomitant reduction in total leaf nitrogen content. The ratio of J_max /V_cmax decreased with increasing leaf temperature for both species, but the ratio at 25℃ was constant across sites. Furthermore, in H. montana, stomatal conductance at 25℃ remained constant across the different temperature treatments, while in S. guineense it increased with warming. Total dry biomass increased with warming in H. montana but remained constant in S. guineense. The biomass allocated to roots, stem and leaves was not affected by warming in H. montana, whereas the biomass allocated to roots significantly increased in S. guineense. Overall, our findings show that in these two tropical montane rainforest tree species, the capacity to acclimate the thermal optimum of photosynthesis is limited while warming-induced reductions in respiration and photosynthetic capacity rates are tightly coupled and linked to responses of leaf nitrogen.

Experimental warming across a tropical forest canopy height gradient reveals minimal photosynthetic and respiratory acclimation

Tropical forest canopies cycle vast amounts of carbon, yet we still have a limited understanding of how these critical ecosystems will respond to climate warming. We implemented in situ leaf-level + 3°C experimental warming from the understory to the upper canopy of two Puerto Rican tropical tree species, Guarea guidonia and Ocotea sintenisii. After approximately 1 month of continuous warming, we assessed adjustments in photosynthesis, chlorophyll fluorescence, stomatal conductance, leaf traits and foliar respiration. Warming did not alter net photosynthetic temperature response for either species; however, the optimum temperature of Ocotea understory leaf photosynthetic electron transport shifted upward. There was no Ocotea respiratory treatment effect, while Guarea respiratory temperature sensitivity (Q₁₀ ) was down-regulated in heated leaves. The optimum temperatures for photosynthesis (T_opt ) decreased 3-5°C from understory to the highest canopy position, perhaps due to upper canopy stomatal conductance limitations. Guarea upper canopy T_opt was similar to the mean daytime temperatures, while Ocotea canopy leaves often operated above T_opt . With minimal acclimation to warmer temperatures in the upper canopy, further warming could put these forests at risk of reduced CO₂ uptake, which could weaken the overall carbon sink strength of this tropical forest.

Including leaf trait information helps empirical estimation of jmax from vcmax in cool-temperate deciduous forests

Understanding the uncertainty in the parameterization of the two photosynthetic capacity parameters, leaf maximum carboxylation rate (Vcmax), and maximum electron transport rate (Jmax), is crucial for modeling and predicting carbon fluxes in terrestrial ecosystems. In gas exchange models, to date, Jmax is typically estimated from Vcmax based on a linear regression. However, recent studies have revealed that this relationship varies, dependent upon species, leaf groups, and time, so it is doubtful that the regression applies universally. Furthermore, far less is known regarding how other leaf traits affect the regression. In this study we analyzed the two key photosynthetic parameters and popularly measurable leaf traits, leaf chlorophyll concentration and leaf mass per area (LMA), of cool-temperate forest stands in Japan, aiming to construct a simple regression applicable to temperate deciduous forests, at least. The analysis was based on a long-term field dataset covering years of data for both sunlit and shaded leaves at different altitudes. Results showed that the best-fitted slope of the regression differed markedly from those previously reported, which were typically acquired from sunlit leaves. LMA had a significant effect on the regression, producing the lowest root mean square errors and the highest ratio of performance to deviation values (RPD = 2.017). Although more data are needed to validate in other ecosystems, our approach at least provides a promising way to substantially improve photosynthesis model predictions, by introducing leaf traits into the popular empirical regression of Jmax against Vcmax, and ultimately to better understand the functioning of the photosynthetic machinery.

Warming induces divergent stomatal dynamics in co-occurring boreal trees

Climate warming will alter photosynthesis and respiration not only via direct temperature effects on leaf biochemistry but also by increasing atmospheric dryness, thereby reducing stomatal conductance and suppressing photosynthesis. Our knowledge on how climate warming affects these processes is mainly derived from seedlings grown under highly controlled conditions. However, little is known regarding temperature responses of trees growing under field settings. We exposed mature tamarack and black spruce trees growing in a peatland ecosystem to whole-ecosystem warming of up to +9°C above ambient air temperatures in an ongoing long-term experiment (SPRUCE: Spruce and Peatland Responses Under Changing Environments). Here, we report the responses of leaf gas exchange after the first two years of warming. We show that the two species exhibit divergent stomatal responses to warming and vapor pressure deficit. Warming of up to 9°C increased leaf N in both spruce and tamarack. However, higher leaf N in the warmer plots translate into higher photosynthesis in tamarack but not in spruce, with photosynthesis being more constrained by stomatal limitations in spruce than in tamarack under warm conditions. Surprisingly, dark respiration did not acclimate to warming in spruce, and thermal acclimation of respiration was only seen in tamarack once changes in leaf N were considered. Our results highlight how warming can lead to differing stomatal responses to warming in co-occurring species, with consequent effects on both vegetation carbon and water dynamics.

Photosynthetic plasticity of a tropical tree species, Tabebuia rosea, in response to elevated temperature and [CO2 ]

Atmospheric and climate change will expose tropical forests to conditions they have not experienced in millions of years. To better understand the consequences of this change, we studied photosynthetic acclimation of the neotropical tree species Tabebuia rosea to combined 4°C warming and twice-ambient (800 ppm) CO₂ . We measured temperature responses of the maximum rates of ribulose 1,5-bisphosphate carboxylation (V_CMax ), photosynthetic electron transport (J_Max ), net photosynthesis (P_Net ), and stomatal conductance (g_s ), and fitted the data using a probabilistic Bayesian approach. To evaluate short-term acclimation plants were then switched between treatment and control conditions and re-measured after 1-2 weeks. Consistent with acclimation, the optimum temperatures (T_Opt ) for V_CMax , J_Max and P_Net were 1-5°C higher in treatment than in control plants, while photosynthetic capacity (V_CMax , J_Max , and P_Net at T_Opt ) was 8-25% lower. Likewise, moving control plants to treatment conditions moderately increased temperature optima and decreased photosynthetic capacity. Stomatal density and sensitivity to leaf-to-air vapour pressure deficit were not affected by growth conditions, and treatment plants did not exhibit stronger stomatal limitations. Collectively, these results illustrate the strong photosynthetic plasticity of this tropical tree species as even fully developed leaves of saplings transferred to extreme conditions partially acclimated.

Reduced photosynthetic thermal acclimation capacity under elevated ozone in poplar (Populus tremula) saplings

The sensitivity of photosynthesis to temperature has been identified as a key uncertainty for projecting the magnitude of the terrestrial carbon cycle response to future climate change. Although thermal acclimation of photosynthesis under rising temperature has been reported in many tree species, whether tropospheric ozone (O₃ ) affects the acclimation capacity remains unknown. In this study, temperature responses of photosynthesis (light-saturated rate of photosynthesis (A_sat ), maximum rates of RuBP carboxylation (V_cmax ), and electron transport (J_max ) and dark respiration (R_dark ) of Populus tremula exposed to ambient O₃ (AO₃ , maximum of 30 ppb) or elevated O₃ (EO₃ , maximum of 110 ppb) and ambient or elevated temperature (ambient +5°C) were investigated in solardomes. We found that the optimum temperature of A_sat (T_optA ) significantly increased in response to warming. However, the thermal acclimation capacity was reduced by O₃ exposure, as indicated by decreased T_optA , and temperature optima of V_cmax (T_optV ) and J_max (T_optJ ) under EO₃ . Changes in both stomatal conductance (g_s ) and photosynthetic capacity (V_cmax and J_max ) contributed to the shift of T_optA by warming and EO₃ . Neither R_dark measured at 25°C ( R dark 25 ) nor the temperature response of R_dark was affected by warming, EO₃ , or their combination. The responses of A_sat , V_cmax , and J_max to warming and EO₃ were closely correlated with changes in leaf nitrogen (N) content and N use efficiency. Overall, warming stimulated growth (leaf biomass and tree height), whereas EO₃ reduced growth (leaf and woody biomass). The findings indicate that thermal acclimation of A_sat may be overestimated if the impact of O₃ pollution is not taken into account.

Contrasting acclimation responses to elevated CO2 and warming between an evergreen and a deciduous boreal conifer

Rising atmospheric carbon dioxide (CO₂ ) concentrations may warm northern latitudes up to 8°C by the end of the century. Boreal forests play a large role in the global carbon cycle, and the responses of northern trees to climate change will thus impact the trajectory of future CO₂ increases. We grew two North American boreal tree species at a range of future climate conditions to assess how growth and carbon fluxes were altered by high CO₂ and warming. Black spruce (Picea mariana, an evergreen conifer) and tamarack (Larix laricina, a deciduous conifer) were grown under ambient (407 ppm) or elevated CO₂ (750 ppm) and either ambient temperatures, a 4°C warming, or an 8°C warming. In both species, the thermal optimum of net photosynthesis (T_optA ) increased and maximum photosynthetic rates declined in warm-grown seedlings, but the strength of these changes varied between species. Photosynthetic capacity (maximum rates of Rubisco carboxylation, V_cmax , and of electron transport, J_max ) was reduced in warm-grown seedlings, correlating with reductions in leaf N and chlorophyll concentrations. Warming increased the activation energy for V_cmax and J_max (E_aV and E_aJ , respectively) and the thermal optimum for J_max . In both species, the T_optA was positively correlated with both E_aV and E_aJ , but negatively correlated with the ratio of J_max /V_cmax . Respiration acclimated to elevated temperatures, but there were no treatment effects on the Q₁₀ of respiration (the increase in respiration for a 10°C increase in leaf temperature). A warming of 4°C increased biomass in tamarack, while warming reduced biomass in spruce. We show that climate change is likely to negatively affect photosynthesis and growth in black spruce more than in tamarack, and that parameters used to model photosynthesis in dynamic global vegetation models (E_aV and E_aJ ) show no response to elevated CO₂ .

Diverging Responses of Two Subtropical Tree Species (Schima superba and Cunninghamia lanceolata) to Heat Waves

The frequency and intensity of heat waves (HWs) has increased in subtropical regions in recent years. The mechanism underlying the HW response of subtropical trees remains unclear. In this study, we conducted an experiment with broad-leaved Schima superba (S. superba) and coniferous Cunninghamia lanceolata (C. lanceolata) seedlings to examine HW (5-day long) effects on stem water transport, leaf water use efficiency (WUE), morphology and growth, and to elucidate differences in the responses of both species. Our results indicated that HWs can significantly reduce hydraulic conductivity in both species. C. lanceolata experienced significant xylem embolism, with the percentage loss of conductivity (PLC) increasing by 40%, while S. superba showed a non-significant increase in PLC (+25%). Furthermore, HW also caused a reduction in photosynthesis rates (An), but transpiration rates (Tr) increased on the 5th day of the HW, together leading to a significant decrease in leaf WUE. From diurnal dynamics, we observed that the HW caused significant decrease of S. superba An only in the morning, but nearly the all day for C. lanceolata. During the morning, with a high vapor pressure deficit (VPD) environment, the HW increased Tr, which contributed a lot to latently cooling the foliage. In comparing the two tree species, we found that HW effects on S. superba were mostly short-term, with leaf senescence but limited or no xylem embolism. The surviving S. superba recovered rapidly, forming new branches and leaves, aided by their extensive root systems. For C. lanceolata, continued seedling growth initially but with subsequent xylem embolism and withering of shoots, led to stunted recovery and regrowth. In conclusion, apart from the direct thermal impacts caused by HW, drought stress was the main cause of significant negative effects on plant water transport and the photosynthetic system. Furthermore, S. superba and C. lanceolata showed clearly different responses to HW, which implies that the response mechanisms of broad-leaved and coniferous tree species to climate change can differ.

Effect of elevated ozone, nitrogen availability and mesophyll conductance on the temperature responses of leaf photosynthetic parameters in poplar

Although ozone (O3) concentration and nitrogen (N) availability are well known to affect plant physiology, their impacts on the photosynthetic temperature response are poorly understood. We addressed this knowledge gap by exposing seedlings of hybrid poplar clone '107' (Populous euramericana cv. '74/76') to elevated O3 (E-O3) and N availability variation in a factorial experiment. E-O3 decreased light-saturated net photosynthesis (Asat), mesophyll conductance (gm) and apparent maximum rate of carboxylation (Vcmax, based on intercellular CO2 concentration) but not actual Vcmax (based on chloroplast CO2 concentration) and increased respiration in light (Rd) at 25 °C. Nitrogen fertilization increased Asat, gm, Vcmax and the maximum rate of electron transport (Jmax) and reduced Rd at 25 °C and the activation energy of actual Vcmax. No E-O3 or E-O3 x N interaction effects on the temperature response parameters were detected, simplifying the inclusion of O3 impacts on photosynthesis in vegetation models. gm peaked at 30 °C, apparent Vcmax and Jmax at 32-33 °C, while the optimum temperatures of actual Vcmax and Jmax exceeded the measured temperature range (15-35 °C). Ignoring gm would, thus, have resulted in mistakenly attributing the decrease in Asat at high temperatures to reduced biochemical capacity rather than to greater diffusion limitation.

Warming Treatment Methodology Affected the Response of Plant Ecophysiological Traits to Temperature Increases: A Quantitive Meta-Analysis

Global mean temperature is expected to significantly increase by the end of the twenty-first century and could have dramatic impacts on a plant's growth, physiology, and ecosystem processes. Temperature manipulative experiments have been conducted to understand the responsive pattern of plant ecophysiology to climate warming. However, it remains unknown how different methodology used in these experiments will affect plants ecophysiological responses to warming. We conducted a comprehensive meta-analysis of the warming manipulative studies to synthesize the ecophysiological traits responses to warming treatment of different intensities, durations, and conducted for different species and under different experimental settings. The results indicated that warming enhanced leaf dark respiration (Rd) and specific leaf area (SLA) but decreased net photosynthetic rate (Anet) and leaf nitrogen content (LN). The positive and negative effects of warming on Rd and Anet were greater for C4 species than C3 species, respectively. The negative effect of warming treatment on Anet and LN and the positive effect on Rd were more evident under >1 year warming treatment. Negative effects of warming were more evident for plants grown at <10 L pots when experiment duration was longer than 1 year. The magnitude of warming treatment had a significant impact on most of the parameters that were investigated in the study. Overall, the results showed that warming effects on plant ecophysiological traits varied among different response variables and PFTs and affected by the magnitude of temperature change and experimental methodology. The results highlight the need for cautiously selecting the values of plant ecophysiological parameters in forecasting ecosystem function changes in future climate regimes and designing controlled experiments to realistically reflecting ecosystems responses to future global warming.

Optimization of photosynthesis and stomatal conductance in the date palm Phoenix dactylifera during acclimation to heat and drought

We studied acclimation of leaf gas exchange to differing seasonal climate and soil water availability in slow-growing date palm (Phoenix dactylifera) seedlings. We used an extended Arrhenius equation to describe instantaneous temperature responses of leaf net photosynthesis (A) and stomatal conductance (G), and derived physiological parameters suitable for characterization of acclimation (T_opt , A_opt and T_equ ). Optimum temperature of A (T_opt ) ranged between 20-33°C in winter and 28-45°C in summer. Growth temperature (T_growth ) explained c. 50% of the variation in T_opt , which additionally depended on leaf water status at the time of measurement. During water stress, light-saturated rates of A at T_opt (i.e. A_opt ) were reduced to 30-80% of control levels, albeit not limited by CO₂ supply per se. Equilibrium temperature (T_equ ), around which A/G and substomatal [CO₂ ] are constant, remained tightly coupled with T_opt . Our results suggest that acclimatory shifts in T_opt and A_opt reflect a balance between maximization of photosynthesis and minimization of the risk of metabolic perturbations caused by imbalances in cellular [CO₂ ]. This novel perspective on acclimation of leaf gas exchange is compatible with optimization theory, and might help to elucidate other acclimation and growth strategies in species adapted to differing climates.

Climate warming and tree carbon use efficiency in a whole-tree 13 CO2 tracer study

Autotrophic respiration is a major driver of the global C cycle and may contribute a positive climate warming feedback through increased atmospheric concentrations of CO₂ . The extent of this feedback depends on plants' ability to acclimate respiration to maintain a constant carbon use efficiency (CUE). We quantified respiratory partitioning of gross primary production (GPP) and CUE of field-grown trees in a long-term warming experiment (+3°C). We delivered a ¹³ C-CO₂ pulse to whole tree crowns and chased that pulse in the respiration of leaves, whole crowns, roots, and soil. We also measured the isotopic composition of soil microbial biomass and the respiration rates of leaves and whole crowns. We documented homeostatic respiratory acclimation of foliar and whole-crown respiration rates; the trees adjusted to experimental warming such that leaf-level respiration rates were not increased. Experimental warming had no detectable impact on respiratory partitioning or mean residence times. Of the ¹³ C label acquired by the trees, aboveground respiration consumed 10%, belowground respiration consumed 40%, and the remaining 50% was retained. Experimental warming of +3°C did not alter respiratory partitioning at the scale of entire trees, suggesting that complete acclimation of respiration to warming is likely to dampen a positive climate warming feedback.

Acclimation and adaptation components of the temperature dependence of plant photosynthesis at the global scale

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.

Thermal acclimation of photosynthetic activity and RuBisCO content in two hybrid poplar clones

The mechanistic bases of thermal acclimation of net photosynthetic rate (An) are still difficult to discern, and the data sets available are scarce, particularly for hybrid poplar. In the present study, we examined the contribution of a number of biochemical and biophysical traits on thermal acclimation of An for two hybrid poplar clones. We grew cuttings of Populus maximowiczii × Populus nigra (M×N) and Populus maximowiczii × Populus balsamifera (M×B) clones under two day/night temperature of 23°C/18°C and 33°C /27°C and under low and high soil nitrogen level. After ten weeks, we measured leaf RuBisCO (RAR) and RuBisCO activase (RARCA) amounts and the temperature response of An, dark respiration (Rd), stomatal conductance, (gs), apparent maximum carboxylation rate of CO2 (Vcmax) and apparent photosynthetic electron transport rate (J). Results showed that a 10°C increase in growth temperature resulted in a shift in thermal optimum (Topt) of An of 6.2±1.6°C and 8.0±1.2°C for clone M×B and M×N respectively, and an increased An and gs at the growth temperature for clone M×B but not M×N. RuBisCO amount was increased by N level but was insensitive to growth temperature while RARCA amount and the ratio of its short to long isoform was stimulated by the warm condition for clone M×N and at low N for clone M×B. The activation energy of apparent Vcmax and apparent J decreased under the warm condition for clone M×B and remained unchanged for clone M×N. Our study demonstrated the involvement of both RARCA, the activation energy of apparent Vcmax and stomatal conductance in thermal acclimation of An.