The fate of carbon in a mature forest under carbon dioxide enrichment.

Atmospheric carbon dioxide enrichment (eCO2) can enhance plant carbon uptake and growth1-5, thereby providing an important negative feedback to climate change by slowing the rate of increase of the atmospheric CO2 concentration6. Although evidence gathered from young aggrading forests has generally indicated a strong CO2 fertilization effect on biomass growth3-5, it is unclear whether mature forests respond to eCO2 in a similar way. In mature trees and forest stands7-10, photosynthetic uptake has been found to increase under eCO2 without any apparent accompanying growth response, leaving the fate of additional carbon fixed under eCO2 unclear4,5,7-11. Here using data from the first ecosystem-scale Free-Air CO2 Enrichment (FACE) experiment in a mature forest, we constructed a comprehensive ecosystem carbon budget to track the fate of carbon as the forest responded to four years of eCO2 exposure. We show that, although the eCO2 treatment of +150 parts per million (+38 per cent) above ambient levels induced a 12 per cent (+247 grams of carbon per square metre per year) increase in carbon uptake through gross primary production, this additional carbon uptake did not lead to increased carbon sequestration at the ecosystem level. Instead, the majority of the extra carbon was emitted back into the atmosphere via several respiratory fluxes, with increased soil respiration alone accounting for half of the total uptake surplus. Our results call into question the predominant thinking that the capacity of forests to act as carbon sinks will be generally enhanced under eCO2, and challenge the efficacy of climate mitigation strategies that rely on ubiquitous CO2 fertilization as a driver of increased carbon sinks in global forests.

Tropical rainforest species have larger increases in temperature optima with warming than warm-temperate rainforest trees.

While trees can acclimate to warming, there is concern that tropical rainforest species may be less able to acclimate because they have adapted to a relatively stable thermal environment. Here we tested whether the physiological adjustments to warming differed among Australian tropical, subtropical and warm-temperate rainforest trees. Photosynthesis and respiration temperature responses were quantified in six Australian rainforest seedlings of tropical, subtropical and warm-temperate climates grown across four growth temperatures in a glasshouse. Temperature-response models were fitted to identify mechanisms underpinning the response to warming. Tropical and subtropical species had higher temperature optima for photosynthesis (ToptA ) than temperate species. There was acclimation of ToptA to warmer growth temperatures. The rate of acclimation (0.35-0.78°C °C-1 ) was higher in tropical and subtropical than in warm-temperate trees and attributed to differences in underlying biochemical parameters, particularly increased temperature optima of Vcmax25 and Jmax25 . The temperature sensitivity of respiration (Q10 ) was 24% lower in tropical and subtropical compared with warm-temperate species. Overall, tropical and subtropical species had a similar capacity to acclimate to changes in growth temperature as warm-temperate species, despite being grown at higher temperatures. Quantifying the physiological acclimation in rainforests can improve accuracy of future climate predictions and assess their potential vulnerability to warming.

Predicting resilience through the lens of competing adjustments to vegetation function.

There is a pressing need to better understand ecosystem resilience to droughts and heatwaves. Eco-evolutionary optimization approaches have been proposed as means to build this understanding in land surface models and improve their predictive capability, but competing approaches are yet to be tested together. Here, we coupled approaches that optimize canopy gas exchange and leaf nitrogen investment, respectively, extending both approaches to account for hydraulic impairment. We assessed model predictions using observations from a native Eucalyptus woodland that experienced repeated droughts and heatwaves between 2013 and 2020, whilst exposed to an elevated [CO2 ] treatment. Our combined approaches improved predictions of transpiration and enhanced the simulated magnitude of the CO2 fertilization effect on gross primary productivity. The competing approaches also worked consistently along axes of change in soil moisture, leaf area, and [CO2 ]. Despite predictions of a significant percentage loss of hydraulic conductivity due to embolism (PLC) in 2013, 2014, 2016, and 2017 (99th percentile PLC > 45%), simulated hydraulic legacy effects were small and short-lived (2 months). Our analysis suggests that leaf shedding and/or suppressed foliage growth formed a strategy to mitigate drought risk. Accounting for foliage responses to water availability has the potential to improve model predictions of ecosystem resilience.

Lower photorespiration in elevated CO2 reduces leaf N concentrations in mature Eucalyptus trees in the field.

Rising atmospheric CO2 concentrations is expected to stimulate photosynthesis and carbohydrate production, while inhibiting photorespiration. By contrast, nitrogen (N) concentrations in leaves generally tend to decline under elevated CO2 (eCO2 ), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO2 : (a) A "dilution effect" caused by increased concentration of leaf carbohydrates; and (b) inhibited nitrate assimilation caused by reduced supply of reductant from photorespiration under eCO2 . This second hypothesis is fully tested in the field for the first time here, using tall trees of a mature Eucalyptus forest exposed to Free-Air CO2 Enrichment (EucFACE) for five years. Fully expanded young and mature leaves were both measured for net photosynthesis, photorespiration, total leaf N, nitrate ( N O 3 - ) concentrations, carbohydrates and N O 3 - reductase activity to test these hypotheses. Foliar N concentrations declined by 8% under eCO2 in new leaves, while the N O 3 - fraction and total carbohydrate concentrations remained unchanged by CO2 treatment for either new or mature leaves. Photorespiration decreased 31% under eCO2 supplying less reductant, and in situ N O 3 - reductase activity was concurrently reduced (-34%) in eCO2 , especially in new leaves during summer periods. Hence, N O 3 - assimilation was inhibited in leaves of E. tereticornis and the evidence did not support a significant dilution effect as a contributor to the observed reductions in leaf N concentration. This finding suggests that the reduction of N O 3 - reductase activity due to lower photorespiration in eCO2 can contribute to understanding how eCO2 -induced photosynthetic enhancement may be lower than previously expected. We suggest that large-scale vegetation models simulating effects of eCO2 on N biogeochemistry include both mechanisms, especially where N O 3 - is major N source to the dominant vegetation and where leaf flushing and emergence occur in temperatures that promote high photorespiration rates.

Nitrogen and Phosphorus Retranslocation of Leaves and Stemwood in a Mature Eucalyptus Forest Exposed to 5 Years of Elevated CO2.

Elevated CO2 affects C cycling processes which in turn can influence the nitrogen (N) and phosphorus (P) concentrations of plant tissues. Given differences in how N and P are used by plants, we asked if their stoichiometry in leaves and wood was maintained or altered in a long-term elevated CO2 experiment in a mature Eucalyptus forest on a low P soil (EucFACE). We measured N and P concentrations in green leaves at different ages at the top of mature trees across 6 years including 5 years in elevated CO2. N and P concentrations in green and senesced leaves and wood were determined to evaluate both spatial and temporal variation of leaf N and P concentrations, including the N and P retranslocation in leaves and wood. Leaf P concentrations were 32% lower in old mature leaves compared to newly flushed leaves with no effect of elevated CO2 on leaf P. By contrast, elevated CO2 significantly decreased leaf N concentrations in newly flushed leaves but this effect disappeared as leaves matured. As such, newly flushed leaves had 9% lower N:P ratios in elevated CO2 and N:P ratios were not different in mature green leaves (CO2 by Age effect, P = 0.02). Over time, leaf N and P concentrations in the upper canopy slightly declined in both CO2 treatments compared to before the start of the experiment. P retranslocation in leaves was 50%, almost double that of N retranslocation (29%), indicating that this site was P-limited and that P retranslocation was an important mechanism in this ecosystem to retain P in plants. As P-limited trees tend to store relatively more N than P, we found an increased N:P ratio in sapwood in response to elevated CO2 (P < 0.01), implying N accumulation in live wood. The flexible stoichiometric ratios we observed can have important implications for how plants adjust to variable environmental conditions including climate change. Hence, variable nutrient stoichiometry should be accounted for in large-scale Earth Systems models invoking biogeochemical processes.

The concentration of ascorbic acid and glutathione in 13 provenances of Acacia melanoxylon.

Climate change can negatively affect sensitive tree species, affecting their acclimation and adaptation strategies. A common garden experiment provides an opportunity to test whether responses of trees from different provenances are genetically driven and if this response is related to factors at the site of origin. We hypothesized that antioxidative defence systems and leaf mass area ofAcacia melanoxylonR. Br. samples collected from different provenances will vary depending on local rainfall. Thirteen provenances ofA. melanoxylonoriginating from different rainfall habitats (500-2000 mm) were grown for 5 years in a common garden. For 2 years, phyllode samples were collected during winter and summer, for measurements of leaf mass area and concentrations of glutathione and ascorbic acid. Leaf mass area varied between seasons, years and provenances ofA. melanoxylon, and an increase was associated with decreasing rainfall at the site of origin. Ascorbic acid and glutathione concentrations varied between seasons, years (i.e., environmental factors) and among provenances ofA. melanoxylon In general, glutathione and ascorbic acid concentrations were higher in winter compared with summer. Ascorbic acid and glutathione were different among provenances, but this was not associated with rainfall at the site of origin.