David S. Ellsworth

· Earth's forests are increasingly exposed to hotter drought events, causing unprecedented dieback of trees and forests. Evidence of stomatal decoupling at high leaf temperatures (T _leaf ) is mounting, but the driving mechanisms and variability across species are unknown. · We experimentally imposed heat and drought stress onto two rainforest and two semi-arid tree species in a glasshouse study to better understand accelerated water loss at high T _leaf by measuring plant thermal and hydraulic balance. · Stomatal decoupling at high T _leaf occurred in two species, including the most anisohydric species, but did not occur in a vulnerable rainforest species. Accelerated leaf water loss occurred predominantly through open stomata, rather than the cuticle, but some species prioritised thermal acclimation over leaf thermoregulation during heat stress. This suggests decoupling occurs only in species with high capacity for transpirational cooling (>5°C), or alternatively, that full sunlight is required for stomatal decoupling. · The maximum rate of light-induced stomatal opening was up to two times faster, while dark-induced stomatal closing was 14 minutes slower on average in leaves at 34°C, relative to 25°C. Stomatal kinetics are an overlooked trait that may help to better predict accelerated water loss under high T _leaf across species.

Summary: In theory, nutrient resorption from leaves should be higher in sites with lower soil nutrients, increasing the retention and reuse of nutrients that are limited in supply. However, evidence for this relationship has remained inconsistent for decades, particularly where the percent of nutrient resorbed is concerned. Furthermore, plant resorption responses to soil phosphorus (P) concentration remain poorly understood. Leaf nitrogen (N) and P concentrations and resorption percentages (efficiencies) were measured for 68 Australian plant species sampled across four nutrient‐rich vs nutrient‐poor site‐pairs representing contrasting soil parent materials. N and P concentrations in senesced leaves were significantly lower on nutrient‐poor than on nutrient‐richer sites, enabling greater internal conservation of these nutrients on nutrient‐poor sites. Resorption efficiencies averaged 45% for N and 69% for P and did not differ between contrasting soils. Though not sensitive to site, P resorption efficiency (P _RE ) was positively related to the proportion of labile P in leaves ( R ² = 0.54). We demonstrated for the first time that senesced leaf P concentrations and the corresponding P _RE are distinctly shaped by plant adaptation to soil nutrient scarcity, and internal labile P pools, respectively. The high P _RE of Australian species informs an upper bound for this process in global vegetation models.

Summary: Phosphorus (P) limitation may constrain the capacity of forests to remove CO ₂ from the atmosphere under elevated CO ₂ (eC _a ). One key mechanism is that limited P availability requires plants to invest additional fixed carbon belowground, which then returns to the atmosphere via soil respiration. However, the extent and pathways of this response remain unclear. We quantified total soil respiration and inferred its component fluxes before and after alleviating P limitation via fertilization in an established eC _a experiment in mature Eucalyptus woodland. eC _a increased soil respiration by 22.5% regardless of P availability. Dependence of the stimulation on soil moisture indicated it was driven primarily by root‐derived respiration rather than soil organic matter‐derived heterotrophic respiration. Fine‐root production was stimulated by eC _a only after P addition, suggesting that P fertilization triggered a shift from rhizomicrobial respiration, associated with microbial use of root exudates and residues, to autotrophic respiration from root growth and metabolism. These findings indicate that within 9 months of P addition, EucFACE plants may have shifted P‐acquisition strategies under eC _a . As the first ecosystem‐scale study on the interactive effect of eC _a and P limitation, this research improves mechanistic understanding of short‐term belowground responses and provides insight to motivate follow‐up studies.

The capacity of nutrient-limited forests to enhance carbon (C) sequestration under elevated CO ₂ (eCO ₂ ) remains a critical uncertainty in C cycle modeling. While existing evidence suggests that low phosphorus (P) bioavailability may constrain CO ₂ fertilization effects on plant growth, the extent to which this limitation modulates ecosystem responses to eCO ₂ in forests adapted to P-deficient soils remains poorly understood. Here, using eight P-enabled models, we simulated the magnitudes and mechanisms through which P bioavailability interacts with eCO ₂ , emulating an ecosystem-scale P enrichment experiment at a P-limited Eucalyptus forest undergoing long-term Free-Air CO ₂ Enrichment. While models predicted pronounced P effects on tree growth, P enrichment unexpectedly did not increase the CO ₂ effects on tree growth and ecosystem C sequestration. Models prioritized either CO ₂ -driven or P-driven growth, but rarely both. This tradeoff emerged due to model-specific assumptions on 1) partitioning of the extra P in soil labile versus nonlabile pools; 2) plant photosynthetic acclimation to P deficiency; 3) C and nutrient use strategies regulating plant size and allocation; and 4) microbial-driven soil decomposition processes. By generating divergent yet biologically plausible outcomes, these predictions establish critical testable hypotheses for empirical research and highlight multiple P-related pathways that may influence the future land C sink.

Background and aims: There is considerable interest in how plants allocate phosphorus (P) and how this varies by environment. How leaf P fractions change with soil P supplies and soil type is essential to understand the roles of P in storage, structure and biochemistry including photosynthesis. Here, we contrasted the P allocation patterns for native woody plants in south-eastern Australia on soils derived from either P-poor sedimentary or P-rich igneous parent materials. Methods: We measured total leaf P and four leaf P fractions: inorganic phosphate (P _i ), metabolite P, nucleic acid P, and lipid P for 33 native species. We also measured photosynthetic capacity ( A _sat ) for species across four sites with contrasting soil P and parent material types: high total soil P from basalt versus low total soil P in sands. Results: The leaf P _i fraction scaled consistently with total leaf P within and across the high-P versus low-P sites. Species growing on high P soils from basalt tended to have similar P fractions of total P to those of species on low P soils, except for the lipid P fraction which was greater for species at high P sites. There was a substantial reduction in P allocated to the lipid fraction for species on low P soils, especially in Proteaceae species. Leaf photosynthetic P-use efficiency was significantly higher with lower leaf P concentration. Conclusion: The adaptive strategy of reducing phospholipids in leaves occurs in multiple species in low-P environments, including non-Proteaceae. This resulted in species from more than one plant family maintaining lower leaf P concentrations on sedimentary-derived soils than on igneous soils, thus achieving more efficient P use in photosynthesis at low P.

Global environmental changes significantly impact nitrogen (N) and phosphorus (P) availability in desert steppes, thereby reshaping plant species interactions and ultimately influencing ecosystem structure and functioning. This study investigated how these nutrients affect the competitive interactions between legumes and grasses by altering their adaptive strategies. Pot experiments were conducted using the dominant grass species, Stipa breviflora, and the leguminous species, Melissitus ruthenica, under different treatments involving control without nutrient inputs, N input alone, P input alone, and combined N and P inputs. Plant growth, nutrient uptake, and root traits were evaluated in monocultures and mixed plantings. In the relatively N-enriched desert steppes, P addition increased grass biomass by 76% in monocultures; however, this effect was not observed when they were planted alongside leguminous species. Legumes exhibited a more pronounced response to P supplementation, with biomass increasing by up to 106%. The relative total biomass (RBT) remained below 1 across all treatments, indicating the presence of interspecific competition. In simultaneous mixed planting, grass species were dominant under N-only treatments, whereas legumes exhibited a competitive advantage under P-only treatment. The concentrations of N and P in shoots of grasses remained unchanged following nutrient inputs and coexisting with legumes. In contrast, the N and P concentrations in legume shoots demonstrated the contrary trends, and were negatively and positively correlated with biomass, respectively. Both grasses and legumes increased total root length and reduced root diameter when coexisting. The priority effect (i.e., first seeding) enhanced the secretion of acid phosphatase and carboxylates by roots of legumes, and coexistence stimulated those root exudates in grasses. Nitrogen inputs moderately enhance grass growth, whereas P inputs benefited legumes by modifying rhizosphere processes, thereby mitigating their competitive disadvantage under N enrichment. These findings highlight the potential for global change-induced reduced P availability to shift plant dominance in grasslands.

Global vegetation growth is thought to be limited by nitrogen (N) more than by other nutrients. Here we document a stronger phosphorus (P) limitation on global photosynthesis compared with N over the last four decades. On the basis of more than 80,000 field observations of foliar nutrients and a machine learning method, we generated a long-term global dataset of foliar N and P concentrations for the period 1980–2017. We show a larger declining rate of foliar P concentration (−0.80 ± 0.008% yr⁻¹) than of N concentration (−0.31 ± 0.002% yr⁻¹). This decline has led to an increase in terrestrial areas limited by foliar P and a widespread constraint on vegetation photosynthesis, more than 1.5 times stronger than the constraint by foliar N. The increasing trend in global photosynthesis over the past 4 decades has been reduced by approximately 17.2% and 6.7% as a result of the decline in foliar P and N, respectively. This stronger P limitation on global photosynthesis implies a weakening of terrestrial carbon sinks due to an emerging P constraint and calls for stricter strategies for reducing anthropogenic emissions to mitigate climatic warming.

Internal nutrient recycling, such as leaf nutrient resorption, serves as an important strategy for plants to optimise their growth and survival on nutrient‐poor soils. Phosphorus resorption efficiency (P _RE ) varies widely (20%–90%) among species living on P‐poor soils. However, the key drivers behind this local variation are poorly understood. We hypothesised that two traits would drive variation in P _RE among species at a site characterised by chronically low soil P (total soil P of 84 ppm): leaf lifespan (LL) and the proportion of leaf P in ‘labile’ fractions that are easily resorbed. Labile P concentration, P _labile , is comprised of inorganic phosphates and soluble phosphorylated metabolites. To test this hypothesis and gain a wider understanding of how leaf nutrient resorption varies locally, we quantified a set of related traits for 14 common woody species in a species‐rich but nutrient‐poor sclerophyll woodland community at Davies Park in the Blue Mountains, NSW, Australia. These traits were LL, P _labile , green and senesced leaf N and P concentrations and P resorption efficiency (P _RE ). Supporting our hypothesis, LL explained >50% variation in leaf P _RE . Similarly, P _RE was strongly and positively associated ( R ² > 60%) with the allocation of green leaf P to P _labile . The LL‐P _RE relationship was mainly driven by lower senesced leaf P than green leaf P. The local soil P availability explained 60% variation in green leaf P. Overall, this study highlights the combination of traits leading to greater internal recycling of P, including high P _RE , long LL and high allocation of leaf P to P _labile , in species‐rich, nutrient‐poor ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

Extreme heat can push plants beyond their thermal safety margin ( TSM ) if maximum leaf temperature ( T _{leaf_max} ) exceeds leaf critical temperature ( T _crit ). The TSM is potentially useful for assessing heat vulnerability across species but needs further validation, so we exposed 50 tree/shrub species in controlled glasshouses to a 6‐day heatwave (peak air temperature = 41°C). Many species increased their mean T _crit during the heatwave (42%), with Δ T _crit ranging from +1°C to 4°C, but other species did not acclimate or were impaired by heat stress (58%). Species T _{leaf_max} explained ~55% of the variation in species T _crit and was a key correlate of the plasticity of T _crit among species. Species with high Δ T _crit also had higher Δ T _{leaf_max} , with leaves being 7°‒12°C hotter during the heatwave than under baseline conditions. Both T _{leaf_max} and TSMs were correlated with heatwave damage across diverse species from contrasting climate zones. Species differences in TSMs were stable across measurement temperatures, correctly identified the most vulnerable species, and were strongly associated with T _{leaf_max} . Our results suggest that (1) T _{leaf_max} alone is more informative than T _crit for ranking species heat tolerance, and (2) species vulnerability to heatwaves is most reliably assessed by using TSMs that integrate T _{leaf_max} with T _crit across species.

Background and Aims: Phosphorus (P)-impoverished soils shape plant adaptation in biodiverse ecosystems worldwide, from Australian heathlands to Amazonian rainforests to southern China's karst regions. While non-mycorrhizal lineages like Proteaceae and Cyperaceae use carboxylate exudation that mobilise P, and are celebrated for such strategies, the mechanisms allowing mycorrhizal Myrtaceae—especially eucalypts—to thrive in these soils without fungal assistance remain unclear. Given Myrtaceae's dominance in P-impoverished Australian ecosystems, a key question arises: How do mycorrhizal plants succeed in P-impoverished environments without relying on fungal symbiosis? We challenge the paradigm that carboxylate-driven P acquisition is exclusive to non-mycorrhizal species. Methods: Using leaf manganese concentrations ([Mn]) as a proxy for carboxylate exudation, we assessed trait diversification across Myrtaceae genera. We collected leaf and soil samples from 34 species of eucalypt (Angophora, Blakella, Corymbia, Eucalyptus) and other Myrtaceae from 18 sites in south-eastern Australia. Key Results: Our findings reveal consistently high leaf [Mn] in many Myrtaceae, comparable to that in known carboxylate-releasing species, indicating intensive P mining. This suggests convergent evolution of carboxylate exudation in mycorrhizal Myrtaceae, fundamentally reshaping our understanding of nutrient acquisition in symbiotic plants. Significant interspecific variation was observed, with Angophora showing markedly higher [Mn] than Eucalyptus, suggesting divergent P-acquisition strategies within Myrtaceae. Weak phylogenetic signals for leaf [Mn] and [P] in eucalypts imply repeated evolutionary change in these traits, similar to what is known in other Australian species adapted to P scarcity. Conclusions: By demonstrating carboxylate-driven P mining in mycorrhizal Myrtaceae, we redefine the mechanisms behind their dominance in low-P environments. Trait diversity—linked to variation in carboxylate-mediated P acquisition and plant-soil feedbacks—likely drives niche differentiation and genus-level distribution across south-eastern Australia. Connecting leaf [Mn] to carboxylate-driven P mining advances our understanding of trait evolution in Myrtaceae and provides a framework for predicting plant-soil interactions in P-impoverished ecosystems globally.