Rising atmospheric carbon dioxide (CO2) concentration are a primary driver of global warming and are expected to be associated with more frequent droughts, greater temperatures, and increased vapour pressure deficit (VPD). These factors are all key drivers of tree mortality. While elevated CO2 (eCO2) enhances photosynthesis (Anet) and intrinsic water use efficiency (iWUE), its capacity to mitigate heat stress in mature trees under field conditions remains poorly understood. We investigated the effects of eCO2 on the physiological and growth responses of mature Quercus robur (~180 years old) at a forest Free Air Carbon Enrichment (FACE) experiment. We combined measurements of tree growth, canopy conductance and leaf gas exchange, together with leaf morphological traits, collected during a naturally occurring heat events (> 32°C), to assess whether eCO2 buffered the impacts of extreme heat events (>32°C). The leaf-level measurements also enabled us to determine whether the physiological enhancements previously observed during the early years of BIFoR FACE were maintained following prolonged exposure to elevated CO2. After eight years of CO2 enrichment (+ 150 ppm above ambient), eCO2-grown trees showed increased iWUE (+ 33 %), driven by increased Anet (+ 26.1 %) and modest reductions in stomatal conductance (- 11.1 %), with no significant changes in stomatal anatomy. Elevated CO2 increased the sensitivity of canopy-level conductance to VPD during heat events, indicating stronger stomatal regulation under high atmospheric demand in eCO2 compared to aCO2-grown trees. Heat stress reduced tree growth in both conditions, but the reduction was less pronounced under eCO2, suggesting partial mitigation of heat stress effects. These findings indicate that eCO2 can confer partial physiological buffering against heat stress in mature Q. robur, enhancing resilience without compromising structure. Shifts in water use highlight the importance of integrating CO2-climate interactions when predicting forest responses to future climate extremes.

Enhanced ‘woody growth’ (dry matter increments, specifically), averaging 10%, has been sustained in patches of long-established (180+ years old) oak forest through 9 years of treatment with elevated CO2 (eCO2; 150 ppm above ambient). Root exudation of carbon (C) into the rhizosphere increased by 63%, which primed the microbes for nutrient acquisition to meet enhanced tree N demands. A ‘faster-tighter’ nitrogen cycle accelerates the return of nitrogen via ammonification to plant-available forms and suppresses processes such as nitrification. This ecosystem-scale N conservation strategy supports increased net productivity by maintaining the nutritional balance of the trees in the C-rich atmosphere. The faster-tighter N-cycle makes an additional 25 kg N ha-1 yr-1 available to the trees under eCO2. That is, the forest’s N-cycle adjusts to the increased C supply, but whether this capacity to adjust endures may be constrained by soil organic N stocks and anthropogenic N deposition. Further, when considering broader aspects of the forest under eCO2, we find nutritional deficiencies producing a cascade of nascent ecosystem fragility in pollen, seeds, seedlings, and food webs. The clear policy implications are: (i) that enhanced net primary productivity does not, in itself, guarantee forest resilience; (ii) that both C and N emission pathways must be accounted for when forecasting 21st-century C uptake into temperate forests; and (iii) that, when proposing forests as natural climate solutions, understanding C-nutrient interactions is of primary concern.

Land ecosystems absorb ~29% of the total CO2 emissions from anthropogenic sources. Global forests contributes ~62% to the total land ecosystem atmospheric CO2 sinks. The carbon (C) sink in forests is predicted to increase with increasing atmospheric CO2 concentration, called the “CO2 fertilization effect”. However, the projections of the land C sink by the end of the 21st Century based on simulations of state-of-the-art Earth System Models (ESM) is relatively uncertain where a 25 to 50% reduction in the C sink is predicted when nutrient availability including nitrogen (N) is accounted for. This uncertainty emanates from poor representation of key ecosystem types, particularly mature forests, to changing nutrient supplies under eCO2. To elucidate the feedbacks between elevated CO2 (eCO2), C capture and nutrient availability, the Birmingham Institute of Forest Research (BIFoR) established a Free-Air CO2 Enrichment (FACE) facility in a mature temperate forest in the UK, where three FACE arrays (30 m dia) are exposed to elevated CO2 (+150 ppm above the ambient) during the growing season.1 The FACE enrichment started in 2017 and continues to date. In response to the CO2 enrichment, photosynthetic CO2 uptake increased by an average of 23% in the first three years and this enhanced uptake was sustained by the seventh year of CO2 enrichment.2 The enhanced CO2 uptake resulted in an overall significant increase in tree dry matter (+10.5%) and a 28% increase in tree basal area increments. Belowground C allocation via litter fall (+9.5 %), root exudates (+40%) and fine root biomass and specific root length in organic and mineral soil layers were increased as well. The overall net primary productivity calculated for years 2021 and 2022 was higher by ~2 tons of dry matter under eCO2 compared to ambient arrays confirming and quantifying the extent of the CO2 fertilization effect. Whilst the litter fall increased under elevated CO2, the N content of the litter decreased significantly pointing towards N conservation via resorption by trees before senescence. Similarly, root C exudation increased; however, exudation of N was not affected, thus leading to a shift in the C:N ratio from an average of 13 to 18 under eCO2. Thus N was conserved via resorption and low root N exudation by trees to sustain enhanced photosynthesis and growth. Gross N mineralization rates were 20% higher under eCO2.3 Enhanced N cycling processes sustained larger soil mineral N supply (~25 kg N ha-1 y-1) under eCO2. Root uptake of N increased by 26% and potential uptake rates of amino acids was larger than mineral N. Tree N conservation and faster N cycling in soils appear to have sustained enhanced tree N uptake and demands. The implications of nutrient availability for C sequestration will depend on how long upregulation of soil N availability via soil organic matter decomposition will last before manifestation of nutrient limitation, if any. References 1 Hart, K. M. et al. 2020. Global Change Biology 26, 1023-1037. https://doi.org:10.1111/gcb.14786 2 Gardner, A., et al. 2022. Tree Physiology 42, 130-144. https://doi.org:10.1093/treephys/tpab090 3 Sgouridis, F. et al. 2023. Soil Biology & Biochemistry 184. https://doi.org:10.1016/j.soilbio.2023.109072

The free-air CO2 enrichment (FACE) site at the Birmingham Institute of Forest Research (BIFoR) is situated in a long-established mature, temperate oak-dominated forest. Over the past 8 years, these tree species have been exposed to elevated CO2 (eCO2 target: 150 ppm above ambient). The observed eCO2 fertilization effect is significant in the mature oak trees, with sustained increases in photosynthesis and stem dry matter production. The fine root standing crop was greater in the eCO2 plots, and carbon exudation flux was greatly increased, stimulating soil gross nitrogen (N) mineralization, enhanced microbial activity, and enzyme functions. Nitrification was suppressed, particularly in the rhizosphere, pointing towards ecosystem N conservation strategies. Alongside the positive forest productivity response, however, comes evidence of nascent ecosystem fragility, including susceptibility to powdery mildew and insect herbivory that is unchanged in the mature trees but increased in seedlings. Changes to invertebrate food webs were observed, including flower-pollinator phenologies and detritivore abundances. Pollen and flower mineral contents were significantly reduced under eCO2, and acorns contained significantly less protein but more phytic acid, suggesting a carbon-rich, nutrient-poor, future diet for animals and insects in natural forest ecosystems.

Experimentally elevated CO2 does not significantly alter the overall leaf reflectance of mature Quercus robur L., but increases Plant Senescence Reflectance Index (PSRI) suggesting a change in the ratio of chlorophyll to carotene content. Rising atmospheric CO2 concentrations, driven by anthropogenic emissions, are projected to reach 550 ppm by 2050. Elevated CO2 (eCO2) is expected to have a fertilisation effect on forests, influencing productivity, water relations, and phenology. However, the impact of eCO2 on leaf reflectance in mature forests remains poorly understood, despite its critical role in radiative transfer processes and remote sensing of forest health. Utilising the Birmingham Institute of Forest Research (BIFoR) Free-Air CO2 Enrichment (FACE) experiment, we investigated the hyperspectral leaf reflectance of 180-year-old Quercus robur L. trees exposed to eCO2 for 7 years. Our results demonstrate that overall leaf reflectance under eCO2 is similar to that of leaves exposed to ambient CO2, but the Plant Senescence Reflectance Index (PSRI) is significantly higher under eCO2. This index relates to the ratio of foliar chlorophyll and carotene pigments. These findings suggest that Q. robur reflectance will not significantly shift under future CO2 conditions, but the relative content of pigments will change, altering the reflectance of specific wavelengths and providing insights into the leaf level physiological and phenological responses of mature trees to eCO2.

Forests sequester ~18% of the global carbon dioxide (CO 2 ) emissions from anthropogenic sources (Canadell et al. 2021, Jia et al. 2019). This “CO 2 fertilization effect” results in a predicted 67% increase in the land carbon (C) sink through enhanced CO 2 capture (Friedlingstein et al. 2020, Terrer et al. 2020). The projections of the land C sink by the end of the 21 st Century based on simulations of state-of-the-art Earth System Models (ESM) is highly uncertain, with a 25 to 50% reduction in the predicted land C sink when nutrient availability including nitrogen (N) is accounted for (Fleischer et al. 2019). This uncertainty emanates from poor representation of key ecosystem types, particularly mature forests, to changing nutrient supplies under eCO 2 . The Birmingham Institute of Forest Research (BIFoR) established a Free-Air CO 2 Enrichment (FACE) facility in a mature temperate forest in the UK, where fumigation of three forest plots with elevated CO 2 (+150 ppm above the ambient) was started in 2017 and continues to date. In response to CO 2 fumigation, trees CO 2 uptake increased by an average of 23%, and the tree basal area increments increased by ~ 28%. Belowground C allocation via litterfall (+14%), root exudates (+40%) and fine root biomass increased. However, the litterfall N content decreased by 12% pointing towards N resorption by trees before senescence. Similarly, the soluble N to C ratios in exudates decreased corroborating the observation that trees conserved N for sustaining the growth response. Soil gross N mineralization rates were 20% higher under eCO 2 . These processes sustained larger soil mineral N supply (~25 kg N ha -1 y -1 ) under eCO 2 showing that the trees invested captured C for N acquisition. Whilst microbial biomass phosphorus increased in soils, a consistent treatment effect was not observed. Overall, the forest sustained high C capture and allocation into biomass and soils. The net implications of nutrient availability for C sequestration will depend on how long upregulation of soil nutrient availability will last in meeting plant nutrient demands before manifestation of nutrient limitation, if any. The ongoing eCO 2 fumigation lasting at least until 2030 will help resolve the feedbacks between C capture and nutrient availability to enable a realistic assessment of the role of forests in climate change mitigation.

Tropical forests play a significant role in global carbon sequestration. However, our understanding of how tropical tree species adjust to climate warming remains limited to studies on seedlings grown in pots and highly controlled growth conditions. To reduce this knowledge gap, we used a field experiment with 5-year-old juvenile trees of 12 naturally co-occurring dominant tropical Andean montane and lowland species growing in three common gardens established along a natural thermosequence in the tropical Andes. Based on a few previous studies, we hypothesized that montane species would exhibit a weaker photosynthetic thermal acclimation capacity compared to lowland counterparts. Our results showed that montane tree species can thermally acclimate net photosynthesis by shifting their thermal optimum (Topt) by 0.6°C per 1°C of warming. This strong shift in Topt was correlated to simultaneous strong shifts in Topt of apparent photosynthetic capacity parameters (Vcmax and Jmax), which increased by 0.7°C per 1°C of warming. This strong thermal acclimation resulted in similar rates of net CO2 assimilation between montane and lowland species across different thermal environments. At last, rates of net photosynthesis at growth temperature explained 30% of the variation in the relative tree growth rates across the two species groups and thermal environments. Our results suggest that the strong physiological acclimation of photosynthesis to warming among montane Andean tree species should be considered when predicting future impacts of warming on Andean plant communities.

Climate warming is affecting the composition and distribution of Andean tropical montane forests (TMFs), resulting in varied growth responses among tree species. However, the underlying mechanisms driving these growth responses to climate warming remain largely unexplored. To address this knowledge gap, we investigated the role of leaf functional traits in mediating growth responses to temperature using a common garden experiment across a 2000 m thermosequence in the Colombian Andes. Fifteen dominant Andean tree species—originating from lowland and montane habitats—were grown under common soil and water conditions at three elevations. This experimental design exposed montane and lowland species to their native thermal environments as well as to warming and cooling respectively, thereby simulating upward migration consistent with documented shifts along elevation gradients. We measured 19 traits related to photosynthesis and its temperature response, thermotolerance, leaf structure and water use and assessed their associations with relative growth rates under warming and cooling conditions. Trait-growth relationships varied with thermal context. Thermal tolerance, photosynthesis and its temperature response, water use and leaf structural traits were consistently linked to growth explaining 88-91% of its variability across most thermal environments. However, for montane species under warming conditions, only thermal tolerance and photosynthetic traits remained significant, explaining 48% of the observed growth variability. Thermal acclimation of traits played an important role in mediating growth responses to temperature change. Traits associated with growth did not differ between species groups under native conditions but diverged under altered thermal environments. Collectively, these findings suggest that traits related to alternative physiological processes become increasingly relevant for montane tree species under climate warming. Our findings demonstrate that climate warming alters trait-growth relationships and highlights key functional traits that mediate growth responses to temperature.

The temperature sensitivity (e.g. Q10) of night-time leaf respiratory CO2 efflux (RCO2) is a fundamental aspect of leaf physiology. The Q10 typically exhibits a dependence on measurement temperature, and it is speculated that this is due to temperature-dependent shifts in the relative control of leaf RCO2. Two decades ago, a review hypothesized that this mechanistically caused change in values of Q10 is predictable across plant taxa and biomes. Here, we discuss the most appropriate measuring protocol among existing data and for future data collection, to form the foundation of a future mechanistic understanding of Q10 of leaf RCO2 at different temperature ranges. We do this primarily via a review of existing literature on Q10 of night-time RCO2 and only supplement this to a lesser degree with our own original data. Based on mechanistic considerations, we encourage that instantaneous Q10 of leaf RCO2 to represent night-time should be measured: only at night-time; only in response to short-term narrow temperature variation (e.g. max. 10°C) to represent a given midpoint temperature at a time; in response to as many temperatures as possible within the chosen temperature range; and on still attached leaves.

Land ecosystems absorb ~29% of the total CO2 emissions from anthropogenic sources. Global forests contributes ~62% to the total land ecosystem atmospheric CO2 sinks. The carbon (C) sink in forests is predicted to increase with increasing atmospheric CO2 concentration, called the “CO2 fertilization effect”. However, the projections of the land C sink by the end of the 21st Century based on simulations of state-of-the-art Earth System Models (ESM) is relatively uncertain where a 25 to 50% reduction in the C sink is predicted when nutrient  availability including nitrogen (N)  is accounted for. This uncertainty emanates from poor representation of key ecosystem types, particularly mature forests, to changing nutrient supplies under eCO2. To elucidate the feedbacks between elevated CO2 (eCO2), C capture and nutrient availability, the Birmingham Institute of Forest Research (BIFoR) established a Free-Air CO2 Enrichment (FACE) facility in a mature temperate forest in the UK, where three FACE arrays (30 m dia) are exposed to elevated CO2 (+150 ppm above the ambient) during the growing season.1 The FACE enrichment started in 2017 and continues to date. In response to the CO2 enrichment, photosynthetic CO2 uptake increased by an average of 23% in the first three years and this enhanced uptake was sustained by the seventh year of CO2 enrichment.2 The enhanced CO2 uptake resulted in an overall significant increase in tree dry matter (+10.5%) and a 28% increase in tree basal area increments.  Belowground C allocation via litter fall (+9.5 %), root exudates (+40%) and fine root biomass and specific root length in organic and mineral soil layers were increased as well. The overall net primary productivity calculated for years 2021 and 2022 was higher by ~2 tons of dry matter under eCO2 compared to ambient arrays confirming and quantifying the extent of the CO2 fertilization effect. Whilst the litter fall increased under elevated CO2, the N content of the litter decreased significantly pointing towards N conservation via resorption by trees before senescence. Similarly, root C exudation increased; however, exudation of N was not affected, thus leading to a shift in the C:N ratio from an average of 13 to 18 under eCO2. Thus N was conserved via resorption and low root N exudation by trees to sustain enhanced photosynthesis and growth. Gross N mineralization rates were 20% higher under eCO2.3 Enhanced N cycling processes sustained larger soil mineral N supply (~25 kg N ha-1 y-1) under eCO2. Root uptake of N increased by 26% and potential uptake rates of amino acids was larger than mineral N. Tree N conservation and faster N cycling in soils appear to have sustained enhanced tree N uptake and demands. The implications of nutrient availability for C sequestration will depend on how long upregulation of soil N availability via soil organic matter decomposition will last before manifestation of nutrient limitation, if any. References 1 Hart, K. M. et al. 2020. Global Change Biology 26, 1023-1037. https://doi.org:10.1111/gcb.14786 2 Gardner, A., et al. 2022. Tree Physiology 42, 130-144. https://doi.org:10.1093/treephys/tpab090 3 Sgouridis, F. et al. 2023. Soil Biology & Biochemistry 184. https://doi.org:10.1016/j.soilbio.2023.109072

This article is a Commentary on Slot et al. (2024), 244: 1238–1249.

Optimal stomatal theory predicts that stomata operate to maximise photosynthesis (Anet ) and minimise transpirational water loss to achieve optimal intrinsic water-use efficiency (iWUE). We tested whether this theory can predict stomatal responses to elevated atmospheric CO2 (eCO2 ), and whether it can capture differences in responsiveness among woody plant functional types (PFTs). We conducted a meta-analysis of tree studies of the effect of eCO2 on iWUE and its components Anet and stomatal conductance (gs ). We compared three PFTs, using the unified stomatal optimisation (USO) model to account for confounding effects of leaf-air vapour pressure difference (D). We expected smaller gs , but greater Anet , responses to eCO2 in gymnosperms compared with angiosperm PFTs. We found that iWUE increased in proportion to increasing eCO2 in all PFTs, and that increases in Anet had stronger effects than reductions in gs . The USO model correctly captured stomatal behaviour with eCO2 across most datasets. The chief difference among PFTs was a lower stomatal slope parameter (g1 ) for the gymnosperm, compared with angiosperm, species. Land surface models can use the USO model to describe stomatal behaviour under changing atmospheric CO2 conditions.

Current carbon cycle models attribute rising atmospheric CO2 as the major driver of the increased terrestrial carbon sink, but with substantial uncertainties. The photosynthetic response of trees to elevated atmospheric CO2 is a necessary step, but not the only one, for sustaining the terrestrial carbon uptake, but can vary diurnally, seasonally and with duration of CO2 exposure. Hence, we sought to quantify the photosynthetic response of the canopy-dominant species, Quercus robur, in a mature deciduous forest to elevated CO2 (eCO(2)) (+150 mu mol mol(-1) CO2) over the first 3 years of a long-term free air CO2 enrichment facility at the Birmingham Institute of Forest Research in central England (BIFoR FACE). Over 3000 measurements of leaf gas exchange and related biochemical parameters were conducted in the upper canopy to assess the diurnal and seasonal responses of photosynthesis during the 2nd and 3rd year of eCO(2) exposure. Measurements of photosynthetic capacity via biochemical parameters, derived from CO2 response curves, (V-cmax and J(max)) together with leaf nitrogen concentrations from the pre-treatment year to the 3rd year of eCO(2) exposure, were examined. We hypothesized an initial enhancement in light-saturated net photosynthetic rates (A(sat)) with CO2 enrichment of approximate to 37% based on theory but also expected photosynthetic capacity would fall over the duration of the study. Over the 3-year period, A(sat) of upper-canopy leaves was 33 +/- 8% higher (mean and standard error) in trees grown in eCO(2) compared with ambient CO2 (aCO(2)), and photosynthetic enhancement decreased with decreasing light. There were no significant effects of CO2 treatment on V-cmax or J(max), nor leaf nitrogen. Our results suggest that mature Q. robur may exhibit a sustained, positive response to eCO(2) without photosynthetic downregulation, suggesting that, with adequate nutrients, there will be sustained enhancement in C assimilated by these mature trees. Further research will be required to understand the location and role of the additionally assimilated carbon.

Key message In mature Q. robur, chlorophyll varied with season and canopy height, whilst eCO 2 -driven changes were consistent with M area, highlighting key factors for consideration when scaling photosynthetic processes and canopy N-use. Nitrogen-rich chlorophyll and carotenoid pigments are important in photosynthetic functioning. Photosynthetic pigments have been found to decrease with elevated CO 2 (eCO 2 ), but few such studies have been done in aged forest trees. This study aimed to assess the effects of eCO 2 (150 μmol mol −1 above ambient) and canopy position on chlorophyll content in mature Quercus robur ( Q. robur ). Over 5000 in situ chlorophyll absorbance measurements, alongside laboratory chlorophyll extractions, were collected on canopy-dominant Q. robur in the 3rd and 4th season of CO 2 fumigation of a free-air CO 2 enrichment (FACE) study in central England. Mass-based chlorophyll concentration (Chl mass , mg g −1 ) was significantly higher in the lower canopy compared to upper canopy foliage ( P < 0.05). In contrast, significantly higher chlorophyll content (Chl area , mg m −2 ) was observed in the upper canopy. ECO 2 did not affect Chl mass but Chl area significantly increased, attributable to increased leaf mass per unit area (M area , g m −2 ). We found no effect of eCO 2 on mass-based or area-based nitrogen ( N mass , mg g −1 or N area g m −2 ); however, N area significantly increased with canopy height, again attributable to M area . The parallel relationships between M area , N area and Chl area suggest the allocation of N to light harvesting is maintained with eCO 2 exposure as well as in the upper canopy, and that increased photosynthetic mass may help regulate the eCO 2 variation. An understanding of changes in the light-harvesting machinery with eCO 2 will be useful to assess canopy processes and, at larger scales, changes in biogeochemical cycles in future climate scenarios.

Insect herbivory is one of the most important ecological processes affecting plant–soil feedbacks and overall forest ecosystem health. In this study, we assess how elevated carbon dioxide (eCO2) impacts (i) leaf level insect herbivory and (ii) the stand-level herbivore-mediated transfer of carbon (C) and nitrogen (N) from the canopy to the ground in a natural mature oak temperate forest community in central England at the Birmingham Institute of Forest Research Free Air CO2 Enrichment (BIFoR FACE) site. Recently abscised leaves were collected every two weeks through the growing season in August to December from 2017–2019, with the identification of four dominant species: Quercus robur (pedunculate oak), Acer pseudoplatanus (sycamore), Crataegus monogyna (common hawthorn) and Corylus avellana (hazel). The selected leaves were scanned and visually analyzed to quantify the leaf area loss from folivory monthly. Additionally, the herbivore-mediated transfer of C and N fluxes from the dominant tree species Q. robur was calculated from these leaf-level folivory estimates, the total foliar production and the foliar C and N contents. This study finds that the leaf-level herbivory at the BIFoR FACE has not changed significantly across the first 3 years of eCO2 treatment when assessed across all dominant tree species, although we detected significant changes under the eCO2 treatment for individual tree species and years. Despite the lack of any strong leaf-level herbivory response, the estimated stand-level foliar C and N transferred to the ground via herbivory was substantially higher under eCO2, mainly because there was a ~50% increase in the foliar production of Q. robur under eCO2. This result cautions against concluding much from either the presence or absence of leaf-level herbivory responses to any environmental effect, because their actual ecosystem effects are filtered through so many (usually unmeasured) factors.

The timings of phenological events play an important role in determining the annual carbon uptake in key terrestrial carbon sinks, such as mature forests. With increases in atmospheric CO2 expected to change physiological processes in plants, it is becoming increasingly important to monitor the changes in plant traits and subsequent phenological changes that may occur. Changes in photosynthetic pigments, such as chlorophyll, can be used as a proxy for physiological changes in leaves and can therefore be useful to monitor potential phenological change, such as autumnal leaf senescence. Non-destructive techniques allow for measurements of photosynthetic pigments without destructive sampling that would disturb the canopy. These methods are particularly useful in logistically difficult environments, such as high forest, or remote environments where traditional chlorophyll extractions are problematic and serve as ground-truthing for remote sensing of greenness. In the present study, we aimed to assess the effects of elevated CO2 (150 mmol mol-1 above ambient) and canopy position on chlorophyll concentrations of a common canopy-dominant species to identify potential implications on phenology. The study was conducted in a mature temperate forest situated at a Free Air Carbon Enrichment (FACE) experiment in the UK. Over 5,000 in-situ chlorophyll measurements were collected, across the 3rd and 4th season of CO2 fumigation, in the canopy-dominant species Quercus robur (Q. robur). Additionally, 100 leaves were destructively sampled to verify chlorophyll concentrations using traditional chlorophyll extraction techniques. The established relationship between chlorophyll absorptance readings and leaf chlorophyll content allowed robust species-specific calibration equations to be calculated. Consistent with previous work, this study observed significantly higher chlorophyll concentrations at lower positions in the canopy in both sampling years (P < 0.001). Additionally, a reduction in foliar chlorophyll concentrations (-2 to -9%) when exposed to eCO2 in both sampling years was observed, but this was only significant for the upper canopy (-7 to -9%, P < 0.05). This study found a marginally significant effect of CO2 treatment on reducing the effective season length, with larger eCO2-induced reductions in chlorophyll occurred through autumn. Overall, the research highlights a simple non-invasive method for monitoring changes in leaf traits of mature trees under eCO2. The results suggest that leaves may be able to reallocate their resources away from light-harvesting apparatus in response to eCO2, particularly in the upper canopy. Furthermore, the findings suggest direct consequences of rising atmospheric CO2 to potential alterations of phenological events, such as leaf senescence, that may have implications for forest productivity and adaptation in a future high CO2 world. Additionally, the research has shown the need to monitor potential changes in resource allocation to photosynthetic apparatus across the season as atmospheric CO2 continues to rise. The information obtained in this study can be used to increase accuracy in the modelling of climate-carbon scenarios.