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

This paper reports on the design and evaluation of Field Studies in Functional Ecology (FSFE), a two-week intensive residential field course that enables students to master core content in functional ecology alongside skills that facilitate their transition from "student" to "scientist." We provide an overview of the course structure, showing how the constituent elements have been designed and refined over successive iterations of the course. We detail how FSFE students: (1) Work closely with discipline specialists to develop a small group project that tests an hypothesis to answer a genuine scientific question in the field; (2) Learn critical skills of data management and communication; and (3) Analyze, interpret, and present their results in the format of a scientific symposium. This process is repeated in an iterative "cognitive apprenticeship" model, supported by a series of workshops that name and explicitly instruct the students in "hard" and "soft" skills (e.g., statistics and teamwork, respectively) critically relevant for research and other careers. FSFE students develop a coherent and nuanced understanding of how to approach and execute ecological studies. The sophisticated knowledge and ecological research skills that they develop during the course is demonstrated through high-quality presentations and peer-reviewed publications in an open-access, student-led journal. We outline our course structure and evaluate its efficacy to show how this novel combination of field course elements allows students to gain maximum value from their educational journey, and to develop cognitive, affective, and reflective tools to help apply their skills as scientists.

Many forest communities in Sarawak, and globally, practise some form of smallholder management system including agroforestry. Yet, agroforestry practitioners predominantly focus on non-timber crops as the major source of income while the timber output remains underutilized and understudied. Smallholder timber agroforestry systems, therefore, have huge potential to equitably meet the increasing demand for both timber and food while reducing the pressure on natural forests. We investigate the readiness of agroforestry smallholders in Sarawak to supply timber commercially, and the opportunities and challenges they face. Semi-structured interviews were conducted with Sarawak forestry stakeholders including agroforestry farmers, timber industry actors, civil society organizations, academia, and government agencies. Descriptive statistics are used to analyse the data. Our results demonstrate that smallholders in Sarawak lack technical silvicultural and horticultural knowledge and formal timber market access, despite majority smallholders expressing interest in timber production. The underutilization of smallholder timber is linked with conservation and protection objectives. Legal constraints, limited capacity building and lack of market information were cited as major barriers for smallholders. Key recommendations to lower barriers for smallholder participation include provision of resources and an enabling legal framework that harmonizes agriculture and forestry demands while being inclusive of smallholders’ perspectives and realities.

This paper reports on the design and evaluation of Field Studies in Functional Ecology (FSFE), a two-week intensive residential field course that engages students to master core content in functional ecology alongside skills that facilitate their transition from ‘student’ to ‘scientist’. This paper provides an overview of the course structure, showing how the constituent elements have been designed and refined over successive iterations of the course. We detail how FSFE students are guided first to develop their own research questions (Field Problems), and then through a structured reflective approach of rapid prototyping and iterative refinement of each stage of the research process. Using extensive evaluation data, we show how, coached by experts within a cognitive apprenticeship framework, FSFE students develop a coherent and nuanced understanding of how to approach and execute ecological studies. The sophisticated knowledge and skills as ecology researchers that they develop during the course is highlighted through high quality presentations and peer-reviewed publications in a student-led journal. We illustrate how field course elements can be used to provide cognitive, affective and reflective tools that allow students to gain maximum value from their educational journey, and the confidence to consider research in their future careers.

•Majority forecasts for feeding the future global population predict an increase in a croplands at the expense of forests and pastures.•A third of the forecasts shows that it is feasible to halt further agricultural expansion and mitigate climate change.•Policy that provides economic incentives for carbon stock conservation and enhancement is the only effective option to reverse the trend of forest loss.

Pressure to increase food production grows with population growth. Agriculture dominates the global landscape, and more food is being produced than ever before. Yet, a large part of the population is undernourished. Concomitantly, much of the agricultural expansion related to achieving global food security is at the expense of forests ecosystems, critical for biodiversity and the provision of ecosystem services. SDG 2: Zero Hunger seeks to 'End hunger, achieve food security and nutrition and promote sustainable agriculture'. This chapter explores the potential impacts of achieving SDG 2 on forests and forest-dependent people. It focuses on five of the SDG 2 targets that are closely entwined with forests and forest related livelihoods. It discusses how the current food system polarises food production and forest conservation, when in fact they should, and can, be harmonised. We conclude with observations on the potential trade-offs and synergies between SDG 2 and the other SDGs, emphasizing the need for integrated land use management.

As the world’s population continues to grow, agricultural expansion is expected to increase to meet future food demand often at the expense of other land uses. However, there are limited studies examining the degree to which forest cover will change and the underlying assumptions driving these projections. Focusing on food and forest scenarios for the middle to the end of the current century, we review 63 main scenarios and 28 global modelling studies to address variations in land use projections and evaluate the potential outcomes on forest cover. Further, their potential impacts on greenhouse gases (GHG) emission/sequestration and global temperature are explored. A majority (59%) of scenarios expected a reduction in both forests and pasturelands to make way for agricultural expansion (particularly reference and no mitigation scenarios). In most scenarios, the extent of forest loss is proportional to that of crop gain, which is associated with higher GHG emission and global temperature, loss of carbon sequestration potential and increase in soil erosion. However, 32% of scenarios predicted that meeting food security objectives is possible without leading to further deforestation if there is a global reduction in the demand for energy intensive foods, and improvements in crop yields. Forest gain and lower rates of deforestation are needed to achieve ambitious climate targets over the next decade. Our analysis also highlights carbon taxes (prices), reforestation/afforestation and bioenergy as important variables that can contribute to maintaining or increasing global forest area in the future.

Mesophyll conductance (gm ) is known to affect plant photosynthesis. However, gm is rarely explicitly considered in land surface models (LSMs), with the consequence that its role in ecosystem and large-scale carbon and water fluxes is poorly understood. In particular, the different magnitudes of gm across plant functional types (PFTs) are expected to cause spatially divergent vegetation responses to elevated CO2 concentrations. Here, an extensive literature compilation of gm across major vegetation types is used to parameterize an empirical model of gm in the LSM JSBACH and to adjust photosynthetic parameters based on simulated An  - Ci curves. We demonstrate that an explicit representation of gm changes the response of photosynthesis to environmental factors, which cannot be entirely compensated by adjusting photosynthetic parameters. These altered responses lead to changes in the photosynthetic sensitivity to atmospheric CO2 concentrations which depend both on the magnitude of gm and the climatic conditions, particularly temperature. We then conducted simulations under ambient and elevated (ambient + 200 μmol/mol) CO2 concentrations for contrasting ecosystems and for historical and anticipated future climate conditions (representative concentration pathways; RCPs) globally. The gm -explicit simulations using the RCP8.5 scenario resulted in significantly higher increases in gross primary productivity (GPP) in high latitudes (+10% to + 25%), intermediate increases in temperate regions (+5% to + 15%), and slightly lower to moderately higher responses in tropical regions (-2% to +5%), which summed up to moderate GPP increases globally. Similar patterns were found for transpiration, but with a lower magnitude. Our results suggest that the effect of an explicit representation of gm is most important for simulated carbon and water fluxes in the boreal zone, where a cold climate coincides with evergreen vegetation.

Globally, trees originating from high-rainfall tropical regions typically exhibit lower rates of light-saturated net CO2 assimilation (A) compared with those from high-rainfall temperate environments, when measured at a common temperature. One factor that has been suggested to contribute towards lower rates of A is lower mesophyll conductance. Using a combination of leaf gas exchange and carbon isotope discrimination measurements, we estimated mesophyll conductance (g(m)) of several Australian tropical and temperate wet-forest trees, grown in a common environment. Maximum Rubisco carboxylation capacity, V-cmax, was obtained from CO2 response curves.g(m) and the drawdown of CO2 across the mesophyll were both relatively constant. V-cmax estimated on the basis of intercellular CO2 partial pressure, C-i, was equivalent to that estimated using chloroplastic CO2 partial pressure, C-c, using apparent' and true' Rubisco Michaelis-Menten constants, respectively Having ruled out g(m) as a possible factor in distorting variations in A between these tropical and temperate trees, attention now needs to be focused on obtaining more detailed information about Rubisco in these species.

Phosphorus (P) limitation is known to have substantial impacts on leaf metabolism. However, uncertainty remains around whether P deficiency alters scaling functions linking leaf metabolism to associated traits. We investigated the effect of P deficiency on leaf gas exchange and related leaf traits in 17 contrasting Eucalyptus species that exhibit inherent differences in leaf traits. Saplings were grown under controlled-environment conditions in a glasshouse, where they were subjected to minus and plus P treatments for 15 weeks. P deficiency decreased P concentrations and increased leaf mass per area (LMA) of newly-developed leaves. Rates of photosynthesis (A) and respiration (R) were also reduced in P-deficient plants compared with P-fertilised plants. By contrast, P deficiency had little effect on the temperature sensitivity of R. Irrespective of P treatment, on a log-log basis A and R scaled positively with increasing leaf nitrogen concentration [N] and negatively with increasing LMA. Although P deficiency had limited impact on A-R-LMA relationships, rates of CO2 exchange per unit N were consistently lower in P-deficient plants. Our results highlight the importance of P supply for leaf carbon metabolism and show how P deficiencies (i.e. when excluding confounding genotypic and environmental effects) can have a direct effect on commonly used leaf trait scaling relationships.

We examined whether variations in photosynthetic capacity are linked to variations in the environment and/or associated leaf traits for tropical moist forests (TMFs) in the Andes/western Amazon regions of Peru. We compared photosynthetic capacity (maximal rate of carboxylation of Rubisco (Vcmax ), and the maximum rate of electron transport (Jmax )), leaf mass, nitrogen (N) and phosphorus (P) per unit leaf area (Ma , Na and Pa , respectively), and chlorophyll from 210 species at 18 field sites along a 3300-m elevation gradient. Western blots were used to quantify the abundance of the CO2 -fixing enzyme Rubisco. Area- and N-based rates of photosynthetic capacity at 25°C were higher in upland than lowland TMFs, underpinned by greater investment of N in photosynthesis in high-elevation trees. Soil [P] and leaf Pa were key explanatory factors for models of area-based Vcmax and Jmax but did not account for variations in photosynthetic N-use efficiency. At any given Na and Pa , the fraction of N allocated to photosynthesis was higher in upland than lowland species. For a small subset of lowland TMF trees examined, a substantial fraction of Rubisco was inactive. These results highlight the importance of soil- and leaf-P in defining the photosynthetic capacity of TMFs, with variations in N allocation and Rubisco activation state further influencing photosynthetic rates and N-use efficiency of these critically important forests.

Why do forest productivity and biomass decline with elevation? To address this question, research to date generally has focused on correlative approaches describing changes in woody growth and biomass with elevation. We present a novel, mechanistic approach to this question by quantifying the autotrophic carbon budget in 16 forest plots along a 3300 m elevation transect in Peru. Low growth rates at high elevations appear primarily driven by low gross primary productivity (GPP), with little shift in either carbon use efficiency (CUE) or allocation of net primary productivity (NPP) between wood, fine roots and canopy. The lack of trend in CUE implies that the proportion of photosynthate allocated to autotrophic respiration is not sensitive to temperature. Rather than a gradual linear decline in productivity, there is some limited but nonconclusive evidence of a sharp transition in NPP between submontane and montane forests, which may be caused by cloud immersion effects within the cloud forest zone. Leaf-level photosynthetic parameters do not decline with elevation, implying that nutrient limitation does not restrict photosynthesis at high elevations. Our data demonstrate the potential of whole carbon budget perspectives to provide a deeper understanding of controls on ecosystem functioning and carbon cycling.

How leaf respiration (R-d) is represented in leading terrestrial biosphere models (TBMs) is reviewed, followed by an overview of how emerging global datasets provide opportunities to improve parameterization of leaf Rd in large-scale models. We first outline how TBMs have historically accounted for variations in respiratory CO2 release in mature leaves, using assumed relationships between leaf nitrogen, photosynthetic capacity and Rd. The need for TBMs to account for light inhibition of Rd in mature leaves is highlighted, followed by a discussion on how Rd of upper canopy leaves is used to predict maintenance respiration in whole plants. We then outline how respiratory energy requirements of growth are accounted for in TBMs, pointing out that current assumptions on the costs of biosynthesis are based on theoretical calculations that may not be valid for all plant species and environments. The chapter then considers how improvements might be made to TBMs with respect to the parameterization of leaf Rd. We show how recently compiled datasets provide improved capacity to predict global variations in baseline Rd measured at a standard temperature, and how baseline Rd likely acclimates to sustained changes in growth temperature. Application of this dataset reveals markedly higher rates of leaf Rd than currently predicted by TBMs, suggesting that TBMs may be underestimating global plant respiratory CO2 release. The availability of a new, global dataset on short--term temperature responses of leaf Rd is highlighted. Analysis of this dataset reveals that leaf Rd does not exhibit the exponential response assumed by most TBMs; rather, the temperature-sensitivity declines as leaves warm, with convergence in the temperature--response across biomes and plant functional types. We show how equations derived from these datasets may provide the TBM community with a new framework to improve representation of mature leaf respiration in TBMs.

High‐temperature tolerance in plants is important in a warming world, with extreme heat waves predicted to increase in frequency and duration, potentially leading to lethal heating of leaves. Global patterns of high‐temperature tolerance are documented in animals, but generally not in plants, limiting our ability to assess risks associated with climate warming. To assess whether there are global patterns in high‐temperature tolerance of leaf metabolism, we quantified T crit (high temperature where minimal chlorophyll a fluorescence rises rapidly and thus photosystem II is disrupted) and T max (temperature where leaf respiration in darkness is maximal, beyond which respiratory function rapidly declines) in upper canopy leaves of 218 plant species spanning seven biomes. Mean site‐based T crit values ranged from 41.5 °C in the Alaskan arctic to 50.8 °C in lowland tropical rainforests of Peruvian Amazon. For T max , the equivalent values were 51.0 and 60.6 °C in the Arctic and Amazon, respectively. T crit and T max followed similar biogeographic patterns, increasing linearly ( ˜ 8 °C) from polar to equatorial regions. Such increases in high‐temperature tolerance are much less than expected based on the 20 °C span in high‐temperature extremes across the globe. Moreover, with only modest high‐temperature tolerance despite high summer temperature extremes, species in mid‐latitude (~20–50°) regions have the narrowest thermal safety margins in upper canopy leaves; these regions are at the greatest risk of damage due to extreme heat‐wave events, especially under conditions when leaf temperatures are further elevated by a lack of transpirational cooling. Using predicted heat‐wave events for 2050 and accounting for possible thermal acclimation of T crit and T max , we also found that these safety margins could shrink in a warmer world, as rising temperatures are likely to exceed thermal tolerance limits. Thus, increasing numbers of species in many biomes may be at risk as heat‐wave events become more severe with climate change.

High-temperature tolerance in plants is important in a warming world, with extreme heatwaves predicted to increase in frequency and duration, potentially leading to lethal heating of leaves. Global patterns of high-temperature tolerance are documented in animals, but generally not plants, limiting our ability to assess risks associated with climate warming. To assess whether there are global patterns in high-temperature tolerance of leaf metabolism, we quantified Tcrit (high temperature where minimal chlorophyll a fluorescence rises rapidly, and thus where photosystem II is disrupted) and Tmax (temperature where leaf respiration in darkness is maximal, beyond which respiratory function rapidly declines) in upper-canopy leaves of 218 plant species spanning seven biomes. Mean site-based Tcrit values ranged from 41.5 °C in the Alaskan arctic to 50.8 °C in lowland tropical rainforests of Peruvian Amazon. For Tmax, the equivalent values were 51.0 and 60.6 °C in the Arctic and Amazon, respectively. Tcrit and Tmax followed similar biogeographic patterns, increasing linearly (~8 °C) from polar to equatorial regions. Such increases in high temperature tolerance are much less than expected based on the 20 °C span in high temperature extremes across the globe. Moreover, with only modest high-temperature tolerance despite high summer temperature extremes, species in mid-latitude (~20°-50°) regions have the narrowest thermal safety margins in upper-canopy leaves; these regions are at the greatest risk of damage due to extreme heat-wave events, especially under conditions when leaf temperatures are further elevated by a lack of transpirational cooling. Using predicted heat-wave events for 2050 and accounting for possible thermal acclimation of Tcrit and Tmax, we also found that these safety margins could shrink in a warmer world, as rising temperatures are likely to exceed thermal A cc ep te d A rt ic le This article is protected by copyright. All rights reserved. tolerance limits. Thus, increasing numbers of species in many biomes may be at risk as heatwave events become more severe with climate change.

Understanding of the extent of acclimation of light-saturated net photosynthesis (A(n)) to temperature (T), and associated underlying mechanisms, remains limited. This is a key knowledge gap given the importance of thermal acclimation for plant functioning, both under current and future higher temperatures, limiting the accuracy and realism of Earth system model (ESM) predictions. Given this, we analysed and modelled T-dependent changes in photosynthetic capacity in 10 wet-forest tree species: six from temperate forests and four from tropical forests. Temperate and tropical species were each acclimated to three daytime growth temperatures (T-growth): temperate -15, 20 and 25 degrees C; tropical -25, 30 and 35 degrees C. CO2 response curves of An were used to model maximal rates of RuBP (ribulose-1,5-bisphosphate) carboxylation (V-cmax) and electron transport (J(max)) at each treatment's respective T-growth and at a common measurement T (25 degrees C). SDS-PAGE gels were used to determine abundance of the CO2-fixing enzyme, Rubisco. Leaf chlorophyll, nitrogen (N) and mass per unit leaf area (LMA) were also determined. For all species and T-growth, An at current atmospheric CO2 partial pressure was Rubisco-limited. Across all species, LMA decreased with increasing T-growth. Similarly, area-based rates of Vcmax at a measurement T of 25 degrees C (V-cmax(25)) linearly declined with increasing T-growth, linked to a concomitant decline in total leaf protein per unit leaf area and Rubisco as a percentage of leaf N. The decline in Rubisco constrained Vcmax and An for leaves developed at higher T-growth and resulted in poor predictions of photosynthesis by currently widely used models that do not account for T-growth-mediated changes in Rubisco abundance that underpin the thermal acclimation response of photosynthesis in wet-forest tree species. A new model is proposed that accounts for the effect of T-growth-mediated declines in V-cmax(25) on An, complementing current photosynthetic thermal acclimation models that do not account for T sensitivity of V-cmax(25).

In intact leaves, mitochondrial populations are highly heterogeneous among contrasting cell types; how such contrasting populations respond to sustained changes in the environment remains, however, unclear. Here, we examined respiratory rates, mitochondrial protein composition and response to growth temperature in photosynthetic (mesophyll) and non-photosynthetic (epidermal) cells from fully expanded leaves of warm-developed (WD) and cold-developed (CD) broad bean (Vicia faba L.). Rates of respiration were significantly higher in mesophyll cell protoplasts (MCPs) than epidermal cell protoplasts (ECPs), with both protoplast types exhibiting capacity for cytochrome and alternative oxidase activity. Compared with ECPs, MCPs contained greater relative quantities of porin, suggesting higher mitochondrial surface area in mesophyll cells. Nevertheless, the relative quantities of respiratory proteins (normalized to porin) were similar in MCPs and ECPs, suggesting that ECPs have lower numbers of mitochondria yet similar protein complement to MCP mitochondria (albeit with lower abundance serine hydroxymethyltransferase). Several mitochondrial proteins (both non-photorespiratory and photorespiratory) exhibited an increased abundance in response to cold in both protoplast types. Based on estimates of individual protoplast respiration rates, combined with leaf cell abundance data, epidermal cells make a small but significant (2%) contribution to overall leaf respiration which increases twofold in the cold. Taken together, our data highlight the heterogeneous nature of mitochondrial populations in leaves, both among contrasting cell types and in how those populations respond to growth temperature.