A constraint on historic growth in global photosynthesis due to increasing CO2.

The global terrestrial carbon sink is increasing1-3, offsetting roughly a third of anthropogenic CO2 released into the atmosphere each decade1, and thus serving to slow4 the growth of atmospheric CO2. It has been suggested that a CO2-induced long-term increase in global photosynthesis, a process known as CO2 fertilization, is responsible for a large proportion of the current terrestrial carbon sink4-7. The estimated magnitude of the historic increase in photosynthesis as result of increasing atmospheric CO2 concentrations, however, differs by an order of magnitude between long-term proxies and terrestrial biosphere models7-13. Here we quantify the historic effect of CO2 on global photosynthesis by identifying an emergent constraint14-16 that combines terrestrial biosphere models with global carbon budget estimates. Our analysis suggests that CO2 fertilization increased global annual photosynthesis by 11.85 ± 1.4%, or 13.98 ± 1.63 petagrams carbon (mean ± 95% confidence interval) between 1981 and 2020. Our results help resolve conflicting estimates of the historic sensitivity of global photosynthesis to CO2, and highlight the large impact anthropogenic emissions have had on ecosystems worldwide.

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.

Light interception efficiency explained by two simple variables: a test using a diversity of small- to medium-sized woody plants.

• Plant light interception efficiency is a crucial determinant of carbon uptake by individual plants and by vegetation. Our aim was to identify whole-plant variables that summarize complex crown architecture, which can be used to predict light interception efficiency. • We gathered the largest database of digitized plants to date (1831 plants of 124 species), and estimated a measure of light interception efficiency with a detailed three-dimensional model. Light interception efficiency was defined as the ratio of the hemispherically averaged displayed to total leaf area. A simple model was developed that uses only two variables, crown density (the ratio of leaf area to total crown surface area) and leaf dispersion (a measure of the degree of aggregation of leaves). • The model explained 85% of variation in the observed light interception efficiency across the digitized plants. Both whole-plant variables varied across species, with differences in leaf dispersion related to leaf size. Within species, light interception efficiency decreased with total leaf number. This was a result of changes in leaf dispersion, while crown density remained constant. • These results provide the basis for a more general understanding of the role of plant architecture in determining the efficiency of light harvesting.

Soil [N] modulates soil C cycling in CO2-fumigated tree stands: a meta-analysis.

Under elevated atmospheric CO(2) concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO(2) effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta-analysis to test the hypotheses that: (1) elevated atmospheric CO(2) stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO(2) induces a C allocation shift towards below-ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO(2). Soil N concentration strongly interacted with CO(2) fumigation: the effect of elevated CO(2) on fine root biomass and -production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO(2) are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.

Increased understanding of nutrient immobilization in soil organic matter is critical for predicting the carbon sink strength of forest ecosystems over the next 100 years.

The terrestrial biosphere is currently thought to be a significant sink for atmospheric carbon (C). However, the future course of this sink under rising [CO2] and temperature is uncertain. Some contrasting possibilities that have been suggested are: that the sink is currently increasing through CO2 fertilization of plant growth but will decline over the next few decades because of CO2 saturation and soil nutrient constraints; that the sink will continue to increase over the next century because rising temperature will stimulate the release of plant-available soil nitrogen (N) through increased soil decomposition; that, alternatively, the sink will not be sustained because the additional soil N released will be immobilized in the soil rather than taken up by plants; or that the sink will soon become negative because loss of soil C through temperature stimulation of soil respiration will override any CO2 or temperature stimulation of plant growth. Soil N immobilization is thus a key process; however, it remains poorly understood. In this paper we use a forest ecosystem model of plant-soil C and N dynamics to gauge the importance of this uncertainty for predictions of the future C sink of forests under rising [CO2] and temperature. We characterize soil N immobilization by the degree of variability of soil N:C ratios assumed in the model. We show that the modeled C sink of a stand of Norway spruce (Picea abies (L.) Karst.) in northern Sweden is highly sensitive to this assumption. Under increasing temperature, the model predicts a strong C sink when soil N:C is inflexible, but a greatly reduced C sink when soil N:C is allowed to vary. In complete contrast, increasing atmospheric [CO2] leads to a much stronger C sink when soil N:C is variable. When both temperature and [CO2] increase, the C sink strength is relatively insensitive to variability in soil N:C; significantly, however, with inflexible soil N:C the C sink is primarily a temperature response whereas with variable soil N:C, it is a combined temperature-CO2 response. Simulations with gradual increases of temperature and [CO2] indicate a sustained C sink over the next 100 years, in contrast to recent claims that the C sink will decline over the next few decades. Nevertheless, in using a relatively simple model, our primary aim is not to make precise predictions of the C sink over the next 100 years, but rather to highlight key areas of model uncertainty requiring further experimental clarification. Here we show that improved understanding of the processes underlying soil N immobilization is essential if we are to predict the future course of the forest carbon sink.

Stomatal conductance of forest species after long-term exposure to elevated CO2 concentration: a synthesis.

• Data from 13 long-term (> 1 yr), field-based studies of the effects of elevated CO2 concentration ([CO2 ]) on European forest tree species were analysed using meta-analysis and modelling. Meta-analysis was used to determine mean responses across the data sets, and data were fitted to two commonly used models of stomatal conductance in order to explore response to environmental conditions and the relationship with assimilation. • Meta-analysis indicated a significant decrease (21%) in stomatal conductance in response to growth in elevated [CO2 ] across all studies. The response to [CO2 ] was significantly stronger in young trees than old trees, in deciduous compared to coniferous trees, and in water stressed compared to nutrient stressed trees. No evidence of acclimation of stomatal conductance to elevated [CO2 ] was found. • Fits of data to the first model showed that growth in elevated [CO2 ] did not alter the response of stomatal conductance to vapour pressure deficit, soil water content or atmospheric [CO2 ]. Fits of data to the second model indicated that conductance and assimilation responded in parallel to elevated [CO2 ] except when water was limiting. • Data were compared to a previous meta-analysis and it was found that the response of gs to elevated [CO2 ] was much more consistent in long-term (> 1 yr) studies, emphasising the need for long-term elevated [CO2 ] studies. By interpreting data in terms of models, the synthesis will aid future modelling studies of responses of forest trees to elevated [CO2 ].

Interactive effects of atmospheric carbon dioxide and leaf nitrogen concentration on canopy light use efficiency: a modeling analysis.

Potential increases in plant productivity in response to increasing atmospheric CO(2) concentration are likely to be constrained by nutrient limitations. However, the interactive effects of nitrogen nutrition and CO(2) concentration on growth are difficult to define because both factors affect several aspects of growth, including photosynthesis, respiration, and leaf area. By expressing growth as a product of light intercepted and light use efficiency (epsilon), it is possible to decouple the effects of nutrient availability and CO(2) concentration on photosynthetic rates from their effects on other aspects of plant growth. I used measured responses of leaf photosynthesis to leaf nitrogen (N) content and CO(2) concentration to parameterize a model of canopy radiation absorption and photosynthesis, and then used the model to estimate the response of epsilon to elevated CO(2) concentration for Pinus radiata D. Don, Nothofagus fusca (Hook. f.) Ørst. and Eucalyptus grandis W. Hill ex Maiden. Down-regulation of photosynthesis at elevated CO(2) was represented as a reduction in either leaf N content or leaf Rubisco activity. The response of epsilon to elevated CO(2), which differed among the three species, was analyzed in terms of the underlying relationships between leaf photosynthesis and leaf N content. The response was independent of leaf N content when photosynthesis was down-regulated to the same extent at low and high leaf N content. Interactive effects of N availability and CO(2) on growth are thus likely to be the result of either differences in down-regulation of photosynthesis at low and high N availability or interactive effects of CO(2) and N availability on other aspects of plant growth.