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ABSTRACT We studied near‐instantaneous temperature responses of light‐saturated photosynthesis (A_sat) and the operating efficiency of photosystem II (ΦPSII) in relation to acclimation of photosynthetic biochemistry to temperature at the timescale of 1–8 days. We grew two closely relatedBrassica oleraceaL. accessions under diurnal temperature ranges of 15°C–25°C and 5°C–15°C, representing the warm and cool native environments of the two accessions. Near‐instantaneous temperature responses ofA_satand ΦPSII were measured on acclimated plants exposed to a temperature ramp from 10°C to 45°C. Biochemical acclimation was quantified by switching plants between warm and cool regimes before measuring photosynthetic variables at Day 1 and Day 8 after the switch. Near‐instantaneous temperature optima ofA_satand ΦPSII functionally shifted towards the acclimated growth temperatures in both accessions, and both accessions achieved their highest instantaneous maximum values ofA_satand ΦPSII when grown under representative native regimes. Differences in maximum carboxylation capacity were evident by Day 1; no further change was detected by Day 8. Our results highlight functional coordination between adaptation, acclimation and near‐instantaneous temperature responses of photochemistry and biochemistry. Moreover, our results suggest photosynthetic biochemistry can acclimate to temperature changes within a day. 

Abstract Background and AimsDynamic global vegetation models (DGVMs) are essential for quantifying the role of terrestrial ecosystems in the Earth’s climate system, but struggle with uncertainty and complexity. Eco-evolutionary optimality (EEO) theory provides a promising approach to improve DGVMs based on the premise that leaf carbon gain is optimized with resource costs. However, the timescales at which plant traits can adjust to environmental changes have not yet been systematically incorporated in EEO-based models. Our aims were to identify temporal constraints on key leaf photosynthetic and leaf functional traits, and develop a conceptual framework for incorporation of temporal leaf trait dynamics in EEO-based models. MethodsWe reviewed the scientific literature on temporal responses of leaf traits associated with stomata and hydraulics, photosynthetic biochemistry, and morphology and lifespan. Subsequent response times were categorized from fast to slow considering physiological, phenotypic (acclimation) and evolutionary (adaptation) mechanisms. We constructed a conceptual framework including several key leaf traits identified from the literature review. We considered temporal separation of dynamics in the leaf interior to atmospheric CO2 concentration (ci:ca) from the optimal ci:ca ratio [χ(optimal)] and dynamics in stomatal conductance within the constraint of the anatomical maximum stomatal conductance (gsmax). A proof-of-concept was provided by modelling temporally separated responses in these trait combinations to CO2 and humidity. Key ResultsWe identified 17 leaf traits crucial for EEO-based modelling and determined their response mechanisms and timescales. Physiological and phenotypic response mechanisms were considered most relevant for modelling EEO-based trait dynamics, while evolutionary constraints limit response ranges. Our conceptual framework demonstrated an approach to separate near-instantaneous physiological responses in ci:ca from week-scale phenotypic responses in χ(optimal), and to separate minute-scale physiological responses in stomatal conductance from annual-scale phenotypic responses in gsmax. ConclusionsWe highlight an opportunity to constrain leaf trait dynamics in EEO-based models based on physiological, phenotypic and evolutionary response mechanisms. 

Abstract Grasslands cover approximately a third of the Earth’s land surface and account for about a third of terrestrial carbon storage. Yet, we lack strong predictive models of grassland plant biomass, the primary source of carbon in grasslands. This lack of predictive ability may arise from the assumption of linear relationships between plant biomass and the environment and an underestimation of interactions of environmental variables. Using data from 116 grasslands on six continents, we show unimodal relationships between plant biomass and ecosystem characteristics, such as mean annual precipitation and soil nitrogen. Further, we found that soil nitrogen and plant diversity interacted in their relationships with plant biomass, such that plant diversity and biomass were positively related at low levels of nitrogen and negatively at elevated levels of nitrogen. Our results show that it is critical to account for the interactive and unimodal relationships between plant biomass and several environmental variables to accurately include plant biomass in global vegetation and carbon models. 

Abstract Forbs (“wildflowers”) are important contributors to grassland biodiversity but are vulnerable to environmental changes. In a factorial experiment at 94 sites on 6 continents, we test the global generality of several broad predictions: (1) Forb cover and richness decline under nutrient enrichment, particularly nitrogen enrichment. (2) Forb cover and richness increase under herbivory by large mammals. (3) Forb richness and cover are less affected by nutrient enrichment and herbivory in more arid climates, because water limitation reduces the impacts of competition with grasses. (4) Forb families will respond differently to nutrient enrichment and mammalian herbivory due to differences in nutrient requirements. We find strong evidence for the first, partial support for the second, no support for the third, and support for the fourth prediction. Our results underscore that anthropogenic nitrogen addition is a major threat to grassland forbs, but grazing under high herbivore intensity can offset these nutrient effects. 

Abstract Photosynthesis links terrestrial carbon, water and nutrient cycles. Photosynthetic least-cost theory suggests that plants optimize photosynthesis at the lowest summed investments in nutrient and water use. The theory predicts that increasing nutrient availability should increase nutrient allocation toward photosynthetic enzymes and reduce stomatal conductance, allowing similar photosynthetic rates achieved at a lower ratio of leaf intercellular to atmospheric CO2 concentration (χ) and reduced water loss. The theory suggests similar responses to increasing soil pH in acidic soils due to common correlations between soil pH and nutrient availability. However, empirical tests of the theory outside of environmental gradients are rare. To test this theory experimentally, we measured photosynthetic traits in mature Acer saccharum Marshall trees growing in a 9-year, nitrogen-by-pH manipulation in the northeastern USA. Increasing soil nitrogen availability did not affect net photosynthesis (Anet) or stomatal conductance (gs) rates, but was associated with increased area-based leaf nitrogen content (Narea), increased photosynthetic capacity (Vcmax, Jmax) and decreased χ (i.e, increased water-use efficiency). These patterns strengthened the tradeoff between nitrogen and water use, indicated by steeper slopes of Narea–χ and Vcmax–χ with increasing soil nitrogen availability. When examined across all plots, soil pH had no effect on any traits. However, in plots without nitrogen additions, increasing soil pH increased the slopes of Narea–χ and Vcmax–χ, though did not modify χ. Supporting the theory, A. saccharum maintained Anet across the soil nitrogen availability gradient by trading less efficient nitrogen use for more efficient water use. Additionally, the effects of soil pH on nitrogen–water use tradeoffs appear to occur through indirect pH effects on soil nitrogen availability. These results indicate that elevated nitrogen deposition could stimulate photosynthesis less than commonly expected and instead reduce water losses, and conversely, that reductions in photosynthesis expected from increasing nitrogen limitation in some regions could be lessened if accompanied by increased transpiration. 

ABSTRACT Leaf intrinsic water use efficiency (iWUE) quantifies the trade‐off between carbon assimilation and water loss in plants, and is constrained by leaf traits such as maximum carboxylation capacity (V_c,max) and stomatal conductance. Yet, the potential links of iWUE with leaf elementomes across different forest types remain unclear. Here, we analyzed iWUE (estimated by leaf carbon isotopes) variability and its associations withV_c,max, stomatal conductance (estimated by¹⁸O enrichment in leaf dry matter above source water, Δ¹⁸O), and leaf elementomes across 82 tree species from temperate, subtropical and tropical forests, and evaluated the effectiveness of leaf reflectance spectroscopy as an indicator of iWUE variability and trait–iWUE associations. Across species,V_c,max, Δ¹⁸O, leaf mass per area (LMA) and leaf iron, nitrogen, sodium and manganese concentrations were the traits most strongly associated with cross‐site iWUE variability. Furthermore, climatic factors (mean annual precipitation, mean annual temperature and climate moisture index) shaped trait–iWUE covariation by negatively linking leaf elements and positively with LMA, which affected iWUE more directly than indirectly viaV_c,maxand Δ¹⁸O. Leaf reflectance spectroscopy accurately predicted iWUE (R² = 0.83), and the trait–iWUE relationships derived from spectral modelling were consistent with those obtained through field measurements. These findings reveal strong linkages between the leaf elementomes and iWUE, and highlight the potential of reflectance spectroscopy for characterizing iWUE variability and trait–iWUE relationships, thereby improving process modelling of forest carbon and water cycles. 

Summary It has been 60 years since the discovery of C₄photosynthesis, an event that rewrote our understanding of plant adaptation, ecosystem responses to global change, and global food security. Despite six decades of research, one aspect of C₄photosynthesis that remains poorly understood is how the pathway fits into the broader context of adaptive trait spectra, which form our modern view of functional trait ecology. The C₄CO₂‐concentrating mechanism supports a general C₄plant phenotype capable of fast growth and high resource‐use efficiencies. The fast‐efficient C₄phenotype has the potential to operate at high productivity rates, while allowing for less biomass allocation to root production and nutrient acquisition, thereby providing opportunities for the evolution of novel trait covariances and the exploitation of new ecological niches. We propose the placement of the C₄fast‐efficient phenotype near the acquisitive pole of the world‐wide leaf economic spectrum, but with a pathway‐specific span of trait space, wherein selection shapes both acquisitive and conservative adaptive strategies. A trait‐based perspective of C₄photosynthesis will open new paths to crop improvement, global biogeochemical modeling, the management of invasive species, and the restoration of disturbed ecosystems, particularly in grasslands. 

Summary Natural selection favors growth by selecting a combination of plant traits that maximize photosynthetic CO₂assimilation at the lowest combined carbon costs of resource acquisition and use. We quantified how soil nutrient availability, plant nutrient acquisition strategies, and aridity modulate the variability in plant costs of nutrient acquisition relative to water acquisition (β).We used an eco‐evolutionary optimality framework and a global carbon isotope dataset to quantify β.Under low soil nitrogen‐to‐carbon (N : C) ratios, a mining strategy (symbioses with ectomycorrhizal and ericoid mycorrhizal fungi) reduced β by mining organic nitrogen, compared with a scavenging strategy (symbioses with arbuscular mycorrhizal fungi). Conversely, under high N : C ratios, scavenging strategies reduced β by effectively scavenging soluble nitrogen, compared with mining strategies. N₂‐fixing plants did not exhibit reduced β under low N : C ratios compared with non‐N₂‐fixing plants. Moisture increased β only in plants using a scavenging strategy, reflecting direct impacts of aridity on the carbon costs of maintaining transpiration in these plants. Nitrogen and phosphorus colimitation further modulated β.Our findings provide a framework for simulating the variability of plant economics due to plant nutrient acquisition strategies in earth system models. 

Summary Allocation of leaf phosphorus (P) among different functional fractions represents a crucial adaptive strategy for optimizing P use. However, it remains challenging to monitor the variability in leaf P fractions and, ultimately, to understand P‐use strategies across diverse plant communities.We explored relationships between five leaf P fractions (orthophosphate P, P_i; lipid P, P_L; nucleic acid P, P_N; metabolite P, P_M; and residual P, P_R) and 11 leaf economic traits of 58 woody species from three biomes in China, including temperate, subtropical and tropical forests. Then, we developed trait‐based models and spectral models for leaf P fractions and compared their predictive abilities.We found that plants exhibiting conservative strategies increased the proportions of P_Nand P_M, but decreased the proportions of P_iand P_L, thus enhancing photosynthetic P‐use efficiency, especially under P limitation. Spectral models outperformed trait‐based models in predicting cross‐site leaf P fractions, regardless of concentrations (R² = 0.50–0.88 vs 0.34–0.74) or proportions (R² = 0.43–0.70 vs 0.06–0.45).These findings enhance our understanding of leaf P‐allocation strategies and highlight reflectance spectroscopy as a promising alternative for characterizing large‐scale leaf P fractions and plant P‐use strategies, which could ultimately improve the physiological representation of the plant P cycle in land surface models. 

Abstract Plants respond to increasing atmospheric CO2 concentrations by reducing leaf nitrogen content and photosynthetic capacity—patterns that correspond with increased net photosynthesis and growth. Despite the longstanding notion that nitrogen availability regulates these responses, eco-evolutionary optimality theory posits that leaf-level responses to elevated CO2 are driven by leaf nitrogen demand for building and maintaining photosynthetic enzymes and are independent of nitrogen availability. In this study, we examined leaf and whole-plant responses of Glycine max L. (Merr) subjected to full-factorial combinations of two CO2, two inoculation, and nine nitrogen fertilization treatments. Nitrogen fertilization and inoculation did not alter leaf photosynthetic responses to elevated CO2. Instead, elevated CO2 decreased the maximum rate of ribulose-1,5-bisophosphate oxygenase/carboxylase (Rubisco) carboxylation more strongly than it decreased the maximum rate of electron transport for ribulose-1,5-bisphosphate (RuBP) regeneration, increasing net photosynthesis by allowing rate-limiting steps to approach optimal coordination. Increasing fertilization enhanced positive whole-plant responses to elevated CO2 due to increased below-ground carbon allocation and nitrogen uptake. Inoculation with nitrogen-fixing bacteria did not influence plant responses to elevated CO2. These results reconcile the role of nitrogen availability in plant responses to elevated CO2, showing that leaf photosynthetic responses are regulated by leaf nitrogen demand while whole-plant responses are constrained by nitrogen availability.