Biophysical impacts of climate change on Australia's forests. Contribution of Work Package 2 to the Forest Vulnerability Assessment

The assessment of the vulnerability of Australian forests to climate change is an initiative of the Natural Resource Management Ministerial Council (NRMMC). The National Climate Change Adaptation Research Facility (NCCARF) was approached to carry out a comprehensive Forest Vulnerability Assessment (FVA). NCCARF engaged four research groups to investigate distinct aspects in relation to the vulnerability of forests, each of which has produced a report. In addition a fifth group was engaged to create a summary and synthesis report of the project. This report – Biophysical impacts of climate change on Australia's forests - is the second in the series. It presents a review of the primary literature on evidence of impacts of climate change on Australian forests. Existing evidence for climate change impacts in relation to direct stresses (CO2, temperature and rainfall), indirect stresses (fire, pests, pathogens and weeds) and plant processes (growth, transpiration and phenology) is discussed. The report concludes with a discussion of the overall impact of climate change on vegetation and the ecosystem services provided by forests. It should be noted that there have been several excellent reviews of climate change impacts on Australian forests as well as reports on climate change impacts on natural heritage and biodiversity. Conclusions drawn from these earlier reviews are not repeated. Instead, the report focuses on drawing evidence from the primary literature, including grey literature. Relevant literature was identified by bibliographic searches and in consultation with experts across Australia. This review highlighted a number of uncertainties involved in assessing forest vulnerability to climate change. These include uncertainty over changes in the climate, the ecosystem-scale responses to climate change, and interactions of climate change impacts with other global change processes. There is, however, clear evidence of the impact of some individual factors

Incorporating non-stomatal limitation improves the performance of leaf and canopy models at high vapour pressure deficit

Vapour pressure deficit (D) is projected to increase in the future as temperature rises. In response to increased D, stomatal conductance (gs) and photosynthesis (A) are reduced, which may result in significant reductions in terrestrial carbon, water and energy fluxes. It is thus important for gas exchange models to capture the observed responses of gs and A with increasing D. We tested a series of coupled A-gs models against leaf gas exchange measurements from the Cumberland Plain Woodland (Australia), where D regularly exceeds 2 kPa and can reach 8 kPa in summer. Two commonly used A-gs models were not able to capture the observed decrease in A and gs with increasing D at the leaf scale. To explain this decrease in A and gs, two alternative hypotheses were tested: hydraulic limitation (i.e., plants reduce gs and/or A due to insufficient water supply) and non-stomatal limitation (i.e., downregulation of photosynthetic capacity). We found that the model that incorporated a non-stomatal limitation captured the observations with high fidelity and required the fewest number of parameters. Whilst the model incorporating hydraulic limitation captured the observed A and gs, it did so via a physical mechanism that is incorrect. We then incorporated a non-stomatal limitation into the stand model, MAESPA, to examine its impact on canopy transpiration and gross primary production. Accounting for a non-stomatal limitation reduced the predicted transpiration by ~19%, improving the correspondence with sap flow measurements, and gross primary production by ~14%. Given the projected global increases in D associated with future warming, these findings suggest that models may need to incorporate non-stomatal limitation to accurately simulate A and gs in the future with high D. Further data on non-stomatal limitation at high D should be a priority, in order to determine the generality of our results and develop a widely applicable model. © The Author(s) 2019. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected]. was supported by a PhD scholarship from Hawkesbury Institute for the Environment, Western Sydney University. M.G.D.K. acknowledges funding from the Australian Research Council (ARC) Centre of Excellence for Climate Extremes (CE170100023), the ARC Discovery Grant (DP190101823) and support from the NSW Research Attraction and Acceleration Program. EucFACE was built as an initiative of the Australian Government as part of the Nation-building Economic Stimulus Package and is supported by the Australian Commonwealth in collaboration with Western Sydney University. It is also part of a Terrestrial Ecosystem Research Network Super-site facility

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. © 2020, The Author(s), under exclusive licence to Springer Nature Limited

Stomatal responses of Eucalyptus species to elevated CO2 concentration and drought stress

Five species of Eucalyptus (E. grandis, E. urophylla, E. camaldulensis, E. torelliana, and E. phaeotrica), among the ten species most commonly used in large scale plantations, were selected for studies on the effects of elevated CO2 concentration [CO2] and drought stress on stomatal responses of 2.5-month old seedlings. The first three species belong to the subgenus Smphyomyrtus, whereas the fourth species belongs to the subgenus Corymbia and E. phaeotrica is from the subgenus Monocalyptus. Seedlings were grown in four pairs of open-top chambers, arranged to have 2 plants of each species in each chamber, with four replications in each of two CO2 concentrations: 350 ± 30 mumol mol-1 and 700 ± 30 mumol mol-1. After 100 days in the chambers, a series of gas exchange measurements were made. Half the plants in each chamber, one plant per species per chamber, were drought-stressed by withholding irrigation, while the remaining plants continued to be watered daily. Drought stress decreased stomatal conductance, photosynthesis and transpiration rates in all the species. The effect of drought stress on stomatal closure was similar in both [CO2]. The positive effects of elevated [CO2] on photosynthesis and water use efficiency were maintained longer during the stress period than under well-watered conditions. The photosynthetic rate of E. phaeotrica was higher even in the fourth day of the drought stress. Drought stress increased photoinhibition of photosynthesis, as measured by chlorophyll fluorescence, which varied among the species, as well as in relation to [CO2]. The results are in agreement with observed differences in stomatal responses between some eucalyptus species of the subgenera Symphyomyrtus and Monocalyptus

A novel optimization approach incorporating non-stomatal limitations predicts stomatal behaviour in species from six plant functional types

The primary function of stomata is to minimize plant water loss while maintaining CO 2 assimilation. Stomatal water loss incurs an indirect cost to photosynthesis in the form of non-stomatal limitations (NSL) via reduced carboxylation capacity (CAP) and/or mesophyll conductance (MES). Two optimal formulations for stomatal conductance (g s) arise from the assumption of each type of NSL. In reality, both NSL could coexist, but one may prevail for a given leaf ontogenetic stage or plant functional type, depending on leaf morphology. We tested the suitability of two g s formulations (CAP versus MES) on species from six plant functional types (C 4 crop, C 3 grass, fern, conifer, evergreen, and deciduous angiosperm trees). MES and CAP parameters (the latter proportional to the marginal water cost to carbon gain) decreased with water availability only in deciduous angiosperm trees, while there were no clear differences between leaf ontogenetic stages. Both CAP and MES formulations fit our data in most cases, particularly under low water availability. For ferns, stomata appeared to operate optimally only when subjected to water stress. Overall, the CAP formulation provided a better fit across all species, suggesting that sub-daily stomatal responses minimize NSL by reducing carboxylation capacity predominantly, regardless of leaf morphology and ontogenetic stage. © 2019 The Author(s).This work was funded by the IdEx programme of the Université de Bordeaux (project USIFlux) and a Marie Skłodowska-Curie individual fellowship (grant no. 653223). Additional support was provided by the French Agence National de la Recherche (grant agreements ANR-13-BS06-0005-01 and ANR-13-BS06-0005-01) and the European Research Council, under the EU FP7 framework programme (grant agreement 338264)

Thirty-eight years of CO2 fertilization has outpaced growing aridity to drive greening of Australian woody ecosystems

Climate change is projected to increase the imbalance between the supply (precipitation) and atmospheric demand for water (i.e., increased potential evapotranspiration), stressing plants in water-limited environments. Plants may be able to offset increasing aridity because rising CO2 increases water use efficiency. CO2 fertilization has also been cited as one of the drivers of the widespread “greening” phenomenon. However, attributing the size of this CO2 fertilization effect is complicated, due in part to a lack of long-term vegetation monitoring and interannual- to decadal-scale climate variability. In this study we asked the question of how much CO2 has contributed towards greening. We focused our analysis on a broad aridity gradient spanning eastern Australia's woody ecosystems. Next we analyzed 38 years of satellite remote sensing estimates of vegetation greenness (normalized difference vegetation index, NDVI) to examine the role of CO2 in ameliorating climate change impacts. Multiple statistical techniques were applied to separate the CO2-attributable effects on greening from the changes in water supply and atmospheric aridity. Widespread vegetation greening occurred despite a warming climate, increases in vapor pressure deficit, and repeated record-breaking droughts and heat waves. Between 1982–2019 we found that NDVI increased (median 11.3 %) across 90.5 % of the woody regions. After masking disturbance effects (e.g., fire), we statistically estimated an 11.7 % increase in NDVI attributable to CO2, broadly consistent with a hypothesized theoretical expectation of an 8.6 % increase in water use efficiency due to rising CO2. In contrast to reports of a weakening CO2 fertilization effect, we found no consistent temporal change in the CO2 effect. We conclude rising CO2 has mitigated the effects of increasing aridity, repeated record-breaking droughts, and record-breaking heat waves in eastern Australia. However, we were unable to determine whether trees or grasses were the primary beneficiary of the CO2-induced change in water use efficiency, which has implications for projecting future ecosystem resilience. A more complete understanding of how CO2-induced changes in water use efficiency affect trees and non-tree vegetation is needed.Sami W. Rifai, Martin G. De Kauwe, Anna M. Ukkola, Lucas A. Cernusak, Patrick Meir, Belinda E. Medlyn, and Andy J. Pitma

A comparative analysis of simulated and observed photosynthetic CO2 uptake in two coniferous forest canopies

Gross canopy photosynthesis (Pg) can be simulated with canopy models or retrieved from turbulent carbon dioxide (CO2) flux measurements above the forest canopy. We compare the two estimates and illustrate our findings with two case studies. We used the three-dimensional canopy model MAESTRA to simulate Pg of two spruce forests differing in age and structure. Model parameter acquisition and model sensitivity to selected model parameters are described, and modeled results are compared with independent flux estimates. Despite higher photon fluxes at the site, an older German Norway spruce (Picea abies L. (Karst.)) canopy took up 25% less CO2 from the atmosphere than a young Scottish Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation. The average magnitudes of Pg and the differences between the two canopies were satisfactorily represented by the model. The main reasons for the different uptake rates were a slightly smaller quantum yield and lower absorptance of the Norway spruce stand because of a more clumped canopy structure. The model did not represent the scatter in the turbulent CO2 flux densities, which was of the same order of magnitude as the non-photosynthetically-active-radiation-induced biophysical variability in the simulated Pg. Analysis of residuals identified only small systematic differences between the modeled flux estimates and turbulent flux measurements at high vapor pressure saturation deficits. The merits and limitations of comparative analysis for quality evaluation of both methods are discussed. From this analysis, we recommend use of both parameter sets and model structure as a basis for future applications and model development.20 page(s

Mechanisms of recovery from drought stress in Eucalyptus saligna: refilling or growth of new xylem?

Póster presentado en el XIM4 4th Xylem International Meeting Padua, 25-27 September 2019N

Low sensitivity of gross primary production to elevated CO2 in a mature eucalypt woodland

The response of mature forest ecosystems to a rising atmospheric carbon dioxide concentration (span classCombining double low line"inline-formula"iC/ia/span) is a major uncertainty in projecting the future trajectory of the Earth's climate. Although leaf-level net photosynthesis is typically stimulated by exposure to elevated span classCombining double low line"inline-formula"iC/ia/span (espan classCombining double low line"inline-formula"iC/ia/span), it is unclear how this stimulation translates into carbon cycle responses at the ecosystem scale. Here we estimate a key component of the carbon cycle, the gross primary productivity (GPP), of a mature native eucalypt forest exposed to free-air span classCombining double low line"inline-formula"CO2/span enrichment (the EucFACE experiment). In this experiment, light-saturated leaf photosynthesis increased by 19andthinsp;% in response to a 38andthinsp;% increase in span classCombining double low line"inline-formula"iC/ia/span. We used the process-based forest canopy model, MAESPA, to upscale these leaf-level measurements of photosynthesis with canopy structure to estimate the GPP and its response to espan classCombining double low line"inline-formula"iC/ia/span. We assessed the direct impact of espan classCombining double low line"inline-formula"iC/ia/span, as well as the indirect effect of photosynthetic acclimation to espan classCombining double low line"inline-formula"iC/ia/span and variability among treatment plots using different model scenarios./p At the canopy scale, MAESPA estimated a GPP of 1574andthinsp;gandthinsp;Candthinsp;mspan classCombining double low line"inline-formula"-2/spanandthinsp;yrspan classCombining double low line"inline-formula"-1/span under ambient conditions across 4 years and a direct increase in the GPP of span classCombining double low line"inline-formula"+/span11andthinsp;% in response to espan classCombining double low line"inline-formula"iC/ia/span. The smaller canopy-scale response simulated by the model, as compared with the leaf-level response, could be attributed to the prevalence of RuBP regeneration limitation of leaf photosynthesis within the canopy. Photosynthetic acclimation reduced this estimated response to 10andthinsp;%. After taking the baseline variability in the leaf area index across plots in account, we estimated a field GPP response to espan classCombining double low line"inline-formula"iC/ia/span of 6andthinsp;% with a 95andthinsp;% confidence interval (span classCombining double low line"inline-formula"-/span2andthinsp;%, 14andthinsp;%). These findings highlight that the GPP response of mature forests to espan classCombining double low line"inline-formula"iC/ia/span is likely to be considerably lower than the response of light-saturated leaf photosynthesis. Our results provide an important context for interpreting the espan classCombining double low line"inline-formula"iC/ia/span responses of other components of the ecosystem carbon cycle. © Author(s) 2020.Martin G. De Kauwe was supported by the NSW Research Attraction and Acceleration Program (RAAP). Euc-FACE was built as an initiative of the Australian Government as part of the Nation Building Economic Stimulus Plan and is supported by the Australian Commonwealth in collaboration with Western Sydney University