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 (ToptA) did not significantly increase in warm‐grown trees in either species. Similarly, the thermal optima (ToptV and ToptJ) and activation energies (EaV and EaJ) of maximum Rubisco carboxylation capacity (Vcmax) and maximum electron transport rate (Jmax) were largely unaffected by warming. However, Vcmax, Jmax and foliar dark respiration (Rd) 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 Jmax/Vcmax 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.

• Tropical climates are getting warmer, with pronounced dry periods in large areas. The productivity and climate feedbacks of future tropical forests depend on the ability of trees to acclimate their physiological processes, such as leaf dark respiration (R d), to these new conditions. However, knowledge on this is currently limited due to data scarcity. • We studied the impact of growth temperature on R d and its dependency on net photosynthesis (A n), leaf nitrogen (N) and phosphorus (P) contents, and leaf mass per unit area (LMA) in 16 early-(ES) and late-successional (LS) tropical tree species in multi-species plantations along an elevation gradient. Moreover, we explored the effect of drought on R d in one ES and one LS species. • Leaf R d at 20 °C decreased at warmer sites, regardless if it was expressed per unit leaf area, mass, N or P. This acclimation resulted in 8% and 28% decrease in R d at prevailing nighttime temperatures in trees at the intermediate and warmest sites, respectively. Moreover, drought reduced R d , particularly in the ES species and at the coolest site. • Thermal acclimation of R d is complete or over-compensatory and independent of changes in leaf nutrients or LMA in African 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.

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No abstract

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. They 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 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.

<p><span lang="EN-GB">Tropical montane forests are among the most productive ecosystems within the tropical region and store a significant amount of carbon in live biomass. With ongoing global climate warming, tropical climates are also getting warmer. The productivity and climate feedbacks of future tropical montane forests depend on the ability of trees to acclimate their photosynthetic metabolism to these new, warmer conditions. However, knowledge of acclimation ability of photosynthesis and its underlying biochemical processes to warming in trees grown under natural field conditions is currently limited due to data scarcity. To reduce this knowledge gap, we used two separate field experiments located in Colombia (with 15 species) and Rwanda (16 species), and for each experiment, tree species were grown at three different sites along an elevation gradient differing in ambient air temperature. At all three sites, we measured the responses of net CO<sub>2</sub> assimilation at different CO<sub>2</sub> concentration (50 to 2000 ppm) and at different leaf temperatures (15 to 40 °C) in ≈ 3 years old trees. We used these data to derive key photosynthetic biochemical parameters (maximum Rubisco carboxylation capacity - <em>V</em><sub>cmax</sub> and maximum electron transport rate - <em>J</em><sub>max</sub>) and their temperature sensitivity, as well as the thermal optimum of net photosynthesis (<em>T</em><sub>optA</sub>). We show that tropical montane tree species from the two continents are generally able to acclimate their <em>T</em><sub>optA</sub> by increasing in trees grown in warmer conditions, but the magnitude of change in <em>T</em><sub>optA</sub> differs among species from different successional groups (early- versus late succession) and climate of origin (lowland versus montane). Shifts in <em>T</em><sub>optA</sub> are largely driven by concomitant changes in thermal sensitivity parameters of underlying biochemical processes of photosynthesis (<em>V</em><sub>cmax</sub> and <em>J</em><sub>max</sub>) with warming. We also show that, at a standard temperature of 25 °C, <em>V</em><sub>cmax</sub> is largely constant, while <em>J</em><sub>max</sub> decreases with warming. Our findings indicate tropical montane tree species from Latin America and Africa can thermally acclimate their photosynthetic physiology, but that this thermal acclimation ability is related to species successional group and their climate of origin.</span></p>