Forests under elevated atmospheric CO₂ concentration as a result of climate change are expected to require more available nitrogen (N) to sustain the enhanced CO₂ uptake for photosynthesis and C storage. Therefore, it is essential to evaluate how CO₂ fumigation of forests will affect availability of N to trees. Main pathways to sustain the high N demand are increasing biological N fixation (BNF), increasing N turn-over and reducing N losses. The purpose of this research is to explore the effects of elevated CO₂ on soil N cycling in a temperate forest under the Birmingham Institute of Forest Research (BIFoR) Free Air Carbon Dioxide Enrichment (FACE) facility. We hypothesize that under CO₂ fertilization, trees will allocate more carbon belowground to enhance microbial activity resulting in an increase of all nitrogen fluxes (i.e. N mineralisation, N fixation and N gas emissions). Net mineralisation rates were measured in-situ every month over 10 months and gross mineralisation rates were measured in-situ in spring, summer and autumn using the ¹⁵N pool dilution method. BNF by free-living organisms was investigated using the ¹⁵N assimilation method and N₂O production rates were measured using the ¹⁵N-Gas flux method. Net mineralisation was increased on average by 30% under elevated CO₂, delivering an extra 24 kgN.ha^-1.y^-1, and by 80% during the budburst period (April). Gross ammonification and NH₄⁺ immobilisation rates were also slightly higher under elevated CO₂ by respectively 33% and 19%. Yet, nitrification, NO₃ immobilisation and N₂O emission rates were not affected, demonstrating that, ammonium and nitrate cycling responded differently to elevated CO₂. In addition, under elevated CO₂, soils were more concentrated in DOC, DON and ammonium (p<0.05). Soil respiration and fine root biomass were also significantly higher by respectively 25% and 40%. Together, these findings suggest that trees allocate more C belowground through a higher root production and exudation enhancing N mineralisation under elevated CO₂. Yet, the effects of elevated CO₂ on N cycling are focused on increasing soil NH₄⁺ availability as NO₃^- availability, N₂O emission and N₂ fixation were not responsive to eCO₂. Collectively, our results suggest that trees are able to control and shape the nitrogen cycling communities to cope with N limitation. Nonetheless, it is difficult to predict if that will be sufficient to delay or alleviate progressive nitrogen limitation under future climate.

How to cite: Rumeau, M., Mackenzie, R., Carillo, Y., Sgouridis, F., Reay, M., and Ullah, S.: Nitrogen cycling in forest soils under elevated CO2: response of a key soil nutrient to global change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11920, https://doi.org/10.5194/egusphere-egu23-11920, 2023.

Carbon sequestration, particularly in forests, is assumed to increase with increasing atmospheric CO₂ concentrations due to the CO₂ fertilization effect on photosynthesis (CFE). Estimating the contemporary effect of increasing atmospheric CO₂ on continuous measurements of gross primary production (GPP) in forest stands is still lacking as it is challenging to disentangle the CFE from other effects on GPP acting on long time scales such as climate variability, succession, land-cover change, and nutrient deposition. Here we introduce a statistical method, i.e., the “GPP residual” method, to estimate the effect of climate on GPP based on short-term variability, to remove it from the long-term signal, yielding the GPP residual that can be, at least partly, attributed to CFE.

We validate the applicability of this “GPP residual” method by testing whether it can accurately identify the CFE in simulations of the process-based land surface model QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system). We compare (i) the difference in the simulated GPP between historical simulations forced with transient and constant CO₂ concentrations with (ii) the non-climatic GPP variations determined when applying the “GPP residual” method to the transient-CO₂ simulation, and find encouraging agreement.

We next apply our approach to eddy-covariance derived GPP at 32 forested sites located in Europe and the US to quantify the CFE for each site and month-of-year in growing seasons. The median CFE across all site-months is 22 ± 6 % per 100 ppm change in CO₂. We note that other effects, such as nitrogen deposition and land management, also influence the GPP residual and could be incorrectly attributed to CFE. Assuming that these more site-specific effects may partially cancel out across sites as random effects, the estimated median value still reflects the strength of CFE. However, causal research will be needed to disentangle these long-term effects which cannot be separated by time scale.

In summary, our study derives for the first time an observation-based estimation of the CO₂ fertilization effect across forested ecosystems based on the eddy covariance record. Our results encourage future work to reconcile the uncertainties of the effect of increasing CO₂ on the global carbon cycle as determined from models, experiments and observations.

How to cite: Zhan, C., Orth, R., Yang, H., Reichstein, M., Zaehle, S., Rammig, A., and Winkler, A.: The strength of CO2 fertilization in forests inferred from the eddy-covariance record, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8437, https://doi.org/10.5194/egusphere-egu23-8437, 2023.

Leaves form the primary interface between plant physiology, CO2 in the atmosphere and energy (light). Plant productivity (i.e. carbon (C) gain) is primarily determined by the amount of leaf area, leaf orientation and distribution in space. Not much attention has been paid to possible changes in leaf orientation and distribution with elevated CO2 (eCO2), but its effect on plant growth could alter the proportions of sunlit and shaded leaf areas and feedback on carbohydrate available for further growth. We report on first measurements of leaf inclination angle distribution, foliage clumping in a native evergreen Eucalyptus woodland - the EucFACE experiment in Western Sydney, New South Wales, Australia - in ambient CO2 and exposed to +150 ppm elevated CO2 (eCO2). We found that a spherical leaf angle distribution, a common assumption in ecosystem modeling, was not an appropriate supposition for present species (Eucalyptus tereticornis Sm.; Eucalyptus amplifolia Naudin) at this site. Our measurements of leaf inclination angles from imagery indicated an erectophile, highly skewed unimodal leaf inclination angle distribution function. We conclude that despite the measured steeper angles under eCO2 concentrations, the leaf angle change is not significant and falls within the expected natural variability and uncertainties connected with the measurement method. The lack of a clear response of leaf orientation and foliage clumping to eCO2 concentration indicates that the previously produced datasets of leaf inclination angles and foliage clumping maps with Earth observation data may be suitable while modelling carbon and water cycles under climate change.

How to cite: Pisek, J., Řezníčková, L., Adamson, K., and Ellsworth, D.: How can elevated CO2 (eCO2) affect the vegetation structure of a mature evergreen Eucalyptus woodland – results from the Eucalyptus Free-Air CO2 Enrichment (EucFACE) experiment in Australia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5024, https://doi.org/10.5194/egusphere-egu23-5024, 2023.

Tropical montane forests are among the most productive ecosystems within the tropical region and store a significant amount of carbon in live biomass. With ongoing global climate warming, tropical climates are also getting warmer. The productivity and climate feedbacks of future tropical montane forests depend on the ability of trees to acclimate their photosynthetic metabolism to these new, warmer conditions. However, knowledge of acclimation ability of photosynthesis and its underlying biochemical processes to warming in trees grown under natural field conditions is currently limited due to data scarcity. To reduce this knowledge gap, we used two separate field experiments located in Colombia (with 15 species) and Rwanda (16 species), and for each experiment, tree species were grown at three different sites along an elevation gradient differing in ambient air temperature. At all three sites, we measured the responses of net CO₂ assimilation at different CO₂ concentration (50 to 2000 ppm) and at different leaf temperatures (15 to 40 °C) in ≈ 3 years old trees. We used these data to derive key photosynthetic biochemical parameters (maximum Rubisco carboxylation capacity - V_cmax and maximum electron transport rate - J_max) and their temperature sensitivity, as well as the thermal optimum of net photosynthesis (T_optA). We show that tropical montane tree species from the two continents are generally able to acclimate their T_optA by increasing in trees grown in warmer conditions, but the magnitude of change in T_optA differs among species from different successional groups (early- versus late succession) and climate of origin (lowland versus montane). Shifts in T_optA are largely driven by concomitant changes in thermal sensitivity parameters of underlying biochemical processes of photosynthesis (V_cmax and J_max) with warming. We also show that, at a standard temperature of 25 °C, V_cmax is largely constant, while J_max decreases with warming. Our findings indicate tropical montane tree species from Latin America and Africa can thermally acclimate their photosynthetic physiology, but that this thermal acclimation ability is related to species successional group and their climate of origin.

How to cite: Dusenge, M. E., Wittemann, M., Mujawamariya, M., Tarvainen, L., Ntawuhiganayo Bahati, E., Zibera, E., Ntirugulirwa, B., Way, D., Nsabimana, D., Uddling, J., Wallin, G., Restrepo Correa, Z., Gonzalez-Carro, S., Meir, P., and Sanchez, A.: Photosynthetic thermal acclimation capacity of tropical montane rainforest trees in Rwanda and in the Colombian Andes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-586, https://doi.org/10.5194/egusphere-egu23-586, 2023.

The responses of tropical forests to climate change depends on the ability of trees to acclimate to warming, as well as how interspecific variation in these responses affect tree community composition. In a unique tropical elevation gradient experiment in Rwanda, Rwanda TREE, we examine the sensitivity of tropical trees and forest stands to warming and altered water supply. Mixed multi-species plantations (20 tree species, 1800 trees per site) have been established at three sites with large variation in elevation (1300-2400 m) and climate (17-24 °C mean daytime temperature), with additional water and nutrient manipulation treatments being applied at each site. Here we present an overview of results obtained this far regarding: (1) leaf gas exchange physiology; (2) photosynthetic heat tolerance; (3) water-use traits; (4) tree growth and mortality; (5) stand-level tree community composition. We also discuss the potential implications of our findings for the biodiversity and carbon storage of tropical forests in a changing climate.

How to cite: Uddling, J., Dusenge, M. E., Manishimwe, A., Manzi, O. J. L., Mujawamariya, M., Ntirugulirwa, B., Tarvainen, L., Wittemann, M., Zibera, E., Nsabimana, D., and Wallin, G.: Warming responses of tropical trees and forest stands explored in an elevation gradient experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14858, https://doi.org/10.5194/egusphere-egu23-14858, 2023.

Current estimates of temperature effects on plants are usually based on air temperature (T_air), although it is well known that leaf temperature (T_leaf) can deviate considerably from T_air. In some studies, to overcome the problem of T_air often being a poor proxy of T_leaf, measurements of canopy temperature (T_can) have been used instead. However, T_can data do not capture the spatial variation in T_leaf among leaves with different thermoregulatory traits. This may be particularly problematic for highly diverse and heterogeneous tropical forest canopies. In this study, we used infrared thermometers to study T_leaf and T_can in multispecies tropical tree plantations established at three sites along an elevation gradient from 2,400 m a.s.l. (17.1°C mean daytime temperature) to 1,300 m a.s.l. (24.0°C) in Rwanda.  Measurements of chlorophyll fluorescence were also conducted to study the photosynthetic heat tolerance of these species. Our results showed high T_leaf (up to ~50°C) and leaf-to-air temperature differences (ΔT_leaf; on average 8-10°C and up to 24°C) of sun-exposed leaves. Both leaf size and stomatal conductance were important traits in controlling T_leaf. The T_leaf (and thus ΔT_leaf) of sun-exposed leaves greatly exceeded the simultaneously measured values of T_can (and ΔT_can). Photosynthetic heat tolerance partially acclimated to increased growth temperature; on average 0.31°C increase in heat tolerance per 1°C increase in growth temperature. Consequently, thermal safety margins were narrower for species at the warmer, lower-elevation sites. Our findings highlight the importance of leaf traits for leaf thermoregulation and show that monitoring of canopy temperature is not enough to capture the peak temperatures and heat stress experienced by individual leaves in diverse tropical forest canopies. They also suggest that tropical trees have limited abilities to thermally acclimate to increasing temperatures.

Keywords: Canopy temperature, elevation gradient, fluorescence, heat tolerance, leaf area, leaf temperature, stomatal conductance, thermoregulation, tropical forest.

How to cite: Manzi, O. J. L., Wittemann, M., Mujawamariya, M., Manishimwe, A., Habimana, J., Dusenge, E. M., Zibera, E., Tarvainen, L., Nsabimana, D., Wallin, G., and Uddling, J.: Trait-based, spatial, and temporal variation in leaf temperature of tropical trees., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11921, https://doi.org/10.5194/egusphere-egu23-11921, 2023.

How to cite: Schneider, P. D., Gessler, A., and Stocker, B. D.: Photosynthesis acclimates to warming through predictable changes in photosynthetic capacities, stomatal sensitivity and enzyme kinetics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12108, https://doi.org/10.5194/egusphere-egu23-12108, 2023.

To what extent plants thermoregulate their canopy temperature (T_c) in response to environmental variability is a fundamental question in ecology, and influences accurate projections of plants' metabolic response and resilience to climate change. However, debate remains, with opinions ranging from no to moderate plant thermoregulation capacities. Traditionally, it has been hypothesized that if plant thermoregulation occurs (i.e. ‘limited homeothermy’ hypothesis holds): 1) T_c will change more slowly than T_a over time, leading the T_cvs. T_a regression slope < 1; 2) T_c is cooler than T_a when T_a exceeds some threshold, typically during high net radiation conditions (e.g. at midday). Here, with global datasets of T_c, air temperature (T_a), and other environmental and biotic variables from FLUXNET and satellites, we tested the ‘limited homeothermy’ hypothesis across global extratropics, including temporal and spatial dimensions.

Our results demonstrate that across daily to monthly timescales, over 80% of sites/ecosystems have T_cvs. T_a regression slopes≥1 or T_c>T_a around midday, which rejects the ‘limited homeothermy’ hypothesis. For those sites unsupporting the hypothesis, their T_c-T_a difference (ΔT) still exhibits considerable seasonality that is negatively, partially correlated with their canopy structure seasonality (as indicated by leaf area index), implying a certain degree of thermoregulation capability. Across global sites, both site-mean ΔT and slope indicator exhibit considerable spatial variability, with ΔT having greater variability than the slope indicator. Furthermore, this large spatial ΔT variation (0-6°C) can be mainly explained by environmental variables (38%) and, to a lesser extent, by biological factors (15%). Our results suggested that plant thermoregulation patterns are diverse across global extratropics, with most ecosystems rejecting the ‘limited homeothermy’ hypothesis, but their thermoregulation still occurs, implying that slope<1 or T_c<T_a are not necessary conditions for plant thermoregulation.

How to cite: Guo, Z., Still, C., Lee, C., Ryu, Y., Blonder, B., Wang, J., Bonebrake, T., Hughes, A., Li, Y., Yeung, H., Zhang, K., Law, Y., Lin, Z., and Wu, J.: Does plant ecosystem thermoregulation occur?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4761, https://doi.org/10.5194/egusphere-egu23-4761, 2023.

The emission of mono- and sesquiterpenes from terrestrial vegetation plays a significant role in ecological interactions and atmospheric chemistry. Previous research has suggested that global emissions of these hydrocarbons are largely driven by responses to abiotic stress and can be simulated using a fixed exponential relationship (β coefficient) between different forest ecosystems and environmental conditions. However, our meta-analysis of published emission data (89 studies/835 β coefficients) reveals that the relationship between mono- and sesquiterpene emissions and temperature is more complex than previously thought. We have found that co-occurring environmental stresses can amplify the temperature sensitivity of monoterpene emissions, which is primarily related to the specific plant functional type (PFT). In contrast, the temperature sensitivity of sesquiterpene emissions decreases over the years. On average, warmer ecosystems appear more sensitive, indicating that plants adjust their emission rates in response to thermal stress. When a PFT-dependent β coefficient for monoterpenes was implemented in a biogenic emission model and coupled with a chemistry-climate model, it was found that atmospheric processes are highly sensitive to this coefficient and subject to amplified variations under rising temperatures.

How to cite: Bourtsoukidis, E., Pozzer, A., Williams, J., Makowski, D., Peñuelas, J., Matthaios, V., Economou, T., Lazoglou, G., Yañez-Serrano, A. M., Nölscher, A., Lelieveld, J., Ciais, P., Vrekoussis, M., Daskalakis, N., and Sciare, J.: The temperature sensitivity of mono- and sesquiterpene emissions from terrestrial vegetation: Insights from a meta-analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6641, https://doi.org/10.5194/egusphere-egu23-6641, 2023.

According to climate projections, extreme summer drought conditions as those striking Central and Southern Europe in 2022 will become more frequent under climate change. Consequently, studying forests’ response to such extreme conditions may reveal important insights on how forests will cope with anticipated climate conditions. Of particular interest are questions related to forest-type specific drought sensitivities (e.g. broadleaved vs. coniferous, Mediterranean vs. temperate) and the existence of legacy effects from previous droughts (e.g. the extreme 2018 drought, see Buras et al 2020).

While many approaches exist to address these questions at local scale, satellite borne remote sensing offers the opportunity to tackle these topics at large scale. Here, the MODIS mission provides a valuable source of information due to a relatively long observational period since the year 2000 at sufficient spatial resolution (250 m x 250 m) and a high sampling frequency (daily images which are used to compute 16-day maximum value composites). In context of monitoring forests' response to environmental conditions, MODIS NDVI renders a frequently considered data source since it reflects canopy greenness and consequently mirrors – among others – early leaf coloration and senescence as direct responses of trees to extreme drought. Yet, MODIS NDVI time-series need to pass a multi-step processing chain to mask poor-quality pixels, remove remaining outliers, gap-fill, and finally apply a pixel-specific standardization to achieve relative measures of canopy greenness. The recently launched European Forest Condition (EFCM, Buras et al 2021) provides correspondingly processed data, which can be used to monitor forest canopy condition in Europe through space and time.

Here, we present first insights on the impact of the 2022 drought on European forest ecosystems based on the EFCM. Preliminary results indicate the drought 2022 to supersede previous droughts with regards to the spatial extent of severely affected pixels, thus breaking the former record from the 2018 drought (Buras et al 2020) just four years later. Our analyses suggest that legacy effects from previous years have contributed to this development. Moreover, we found different drought sensitivities of different forest types. In combination, these factors draw a complex picture of forests climate-change resilience, which we here seek to disentangle. Corresponding knowledge will likely provide valuable empirical information to improve model-based projections of tree-species performance under anticipated climate change.

Buras A, Rammig A and Zang C S 2020 Quantifying impacts of the 2018 drought on European ecosystems in comparison to 2003 Biogeosciences 17 1655–72

Buras A, Rammig A and Zang C S 2021 The European Forest Condition Monitor: Using Remotely Sensed Forest Greenness to Identify Hot Spots of Forest Decline Frontiers in Plant Science 12 2355

How to cite: Buras, A., Meyer, B., and Rammig, A.: Record reduction in European forest canopy greenness during the 2022 drought, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8927, https://doi.org/10.5194/egusphere-egu23-8927, 2023.

The frequency and intensity of extreme climate events is increasing globally. In 2022, Europe experienced extreme heatwaves and prolonged droughts followed by clear indications of a devastating effect across many forest sites. Instrumental measurements show that 2022 contained some of the most extreme heat and dryness conditions ever recorded across Europe, particularly during the forest growing season. Such extreme conditions that follow a number of consecutive extreme years (e.g., 2019, 2018, 2015) with short intervals in between are becoming the new norm and the response of forests in terms of canopy condition, reduced productivity, and potential feedback to the climate is not clear. It is yet not clear how the negative and positive impacts of the most recent extreme temperature and dryness conditions differ from the impact of previous extreme conditions. This study aims to: 1) compare the severity of extreme conditions in 2022 (in terms of heat and dryness) with the preceding extreme years (i.e., 2003, 2015, 2018, 2019) and 2) to quantify forest canopy response (in terms of vegetation browning) and variables responsible for the feedback from forests to the climate (transpiration and solar induced fluorescence as a proxy for photosynthetic activity) across the main classes of forest types in Europe.

For this assessment we use spatially explicit daily modelled top soil water content (0-7 cm), air temperature, and potential transpiration using the ERA5-Land spatial dataset (0.1° × 0.1°) between 1970 to 2022. Reference period for anomaly assessment is set to the 1970-2000 period and MODIS land cover type product is used to aggregate anomalies across forest types. SIF anomalies are extracted using an OCO-2 satellite based SIF dataset, and ground-based ecophysiological measurements collected across a few sites during heatwave and drought events in 2022 are compiled and used to link leaf-level processes to observed canopy response.

Our initial assessment shows that conditions in July 2018 had the largest negative impact on transpiration of European forests, but in June 2022 we observed a far larger spatial extent of positive temperature anomalies across Europe. We compare the impact of 2022 extreme conditions with that of previous extreme years, and discuss our results in the light of underlying ecophysiological mechanisms that control the response and the feedback to climate across different forest types and climate regions.

How to cite: Gharun, M., Shekhar, A., and Buchmann, N.: Assessment of the impact of 2022 extreme climate conditions on European forests, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11381, https://doi.org/10.5194/egusphere-egu23-11381, 2023.

The unprecedented heatwave that hit the Pacific northwest of North America in late June-early July 2021 impacted ecosystems and communities, yet evidence and analysis of this impact are still missing. Here we bring a unique dataset quantifying the impact on conifer trees, which are keystone species of many northwest ecosystems. Moreover, we take advantage of this exceptional event as a broad, extreme, “field experiment” to test a fundamental theory in plant physiology, and prepare our forests to a harsher future. Overall, the data collected confirm the role of hydraulic vulnerability in drought-induced injury to trees. Among the recorded species, we obtained P50 data for 27 species, represented by 64 cultivars. Plotting needle browning extent by P50 revealed important thresholds of drought sensitivity: (1) species with P50 <-6 MPa were unaffected by drought. (2) species with -6 MPa< P50 <-5 MPa had mild extent of needle browning, up to 25% of the canopy. (3) species with P50 > -5 MPa had browning of up to 95%. The sharp divergence among resistant and vulnerable conifer species according to their xylem vulnerability, all of which simultaneously exposed trees to the same extreme event at the same site, is evidence to the key role of P50, in agreement with previous assessments of drought effects on angiosperms. Among local, NW conifer species, some cultivars proved hardier than others. The aftermath of the 2021 NW heatwave should take advantage of this broad, extreme, “field experiment” to prepare our forests to a harsher future.

How to cite: Klein, T., Torres-Ruiz, J., and Albers, J.: Conifer desiccation in the 2021 NW heatwave confirms the role of hydraulic damage, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1802, https://doi.org/10.5194/egusphere-egu23-1802, 2023.

In recent decades, forests are increasingly suffering from so-called hotter droughts. This is because rising temperatures increase the atmospheric water demand under drought, thereby amplifying plant-water consumption. As a consequence, soil-water potentials are reaching more extreme values which eventually may result in xylem cavitation and possibly tree die-back. For instance, the extreme 2018 drought resulted in extraordinary die-back frequencies in numerous tree species across Central Europe such as European beech, Norway spruce, and Scots pine. Since forests render an important agent in terms of climate change mitigation, increasing forests' climate-change resilience via forest conversion is essential to preserve their integrity and consequently ecosystem services related to carbon sequestration.

In this context, better knowledge of species-specific drought responses is highly valuable since it allows for determining critical drought thresholds beyond which the functional integrity of trees is at threat. Such knowledge may serve to parameterize dynamic vegetation models, which then can be deployed to project tree-species-specific performance under various climate change scenarios. Eventually, this may guide forest managers to select more climate-resilient tree species portfolios in terms of forest conversion.

Yet, species-specific drought responses are typically derived from plot-based physiological monitoring networks. While this approach provides valuable and highly precise data on tree physiology and it suffers from a relatively low replication. To overcome this low-spatial replication, large-scale assessments using satellite-based remote sensing appear a promising research avenue. For instance, the recently released European forest condition monitor (EFCM) provides information on forest canopy conditions in near real-time, which was shown to successfully quantify drought impacts under previous droughts (Buras et al., 2020, 2021). However, the EFCM currently does not allow for species-specific assessments given a lack of corresponding data, in particular tree-species classifications.

To overcome this research gap and to deepen our knowledge of species-specific drought responses, we here present a machine-learning-based tree-species classification using Moderate Resolution Imaging Spectroradiometer (MODIS) of Germany which is subsequently used to stratify EFCM data to quantify the species-specific response of most abundant tree-species (in particular beech, oak, spruce, and pine) to the extreme 2022 drought. Preliminary results indicated a successful calibration-validation of the tree-species classification, with the corresponding F1-scores in the order of 0.55 – 0.7 and true-skill statistics in the order of 0.6 – 0.78, indicating average to good performance. Once applied to stratify the EFCM data we provide a well-replicated large-scale assessment of tree-species-specific drought response, which will improve our understanding of species-specific climate-change resilience. Corresponding information can then be used further to parameterize dynamic vegetation models, which eventually can be deployed to obtain projections of tree performance under various climate-change scenarios.

Buras A, Rammig A and Zang C S 2020 Quantifying impacts of the 2018 drought on European ecosystems in comparison to 2003 Biogeosciences 17 1655–72

Buras A, Rammig A and Zang C S 2021 The European Forest Condition Monitor: Using Remotely Sensed Forest Greenness to Identify Hot Spots of Forest Decline Frontiers in Plant Science 12 2355

How to cite: Wang, Y., Wang, Y., Zhu, X., Rammig, A., and Buras, A.: Quantifying Tree-species Specific Responses to the Extreme 2022 Drought in Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6144, https://doi.org/10.5194/egusphere-egu23-6144, 2023.

Abstract: Climate warming poses major threats to temperate forests, but the response of plant root metabolism has remained unclear. Understanding and predicting the impact of climate warming on the root metabolome represents a grand challenge and a major opportunity to predict the belowground functioning of forests in a warmer climate. We here studied the impact of long-term soil warming (>14 years, ambient versus +4 °C) on the fine root metabolome across three seasons (spring, summer, and autumn) for two years in a spruce-dominated mountain forest in the Austrian Limestone Alps. Root primary metabolites were analyzed with a liquid chromatography-mass spectrometry metabolomics platform (LC-Orbitrap MS). A total of 44 primary metabolites were identified in roots (19 amino acids, 12 organic acids, and 13 sugars). Warming and season had significant effects on total primary metabolite concentration, but no interaction effect. Warming increased the amino acid and sugar concentrations but did not affect organic acids. This may be explained by increased activity of the protein amino acid (such as arginine, glycine, and lysine) biosynthesis and metabolism and/or of root carbohydrate metabolism and transport under warming. The non-metric multidimensional scaling (NMDS) showed that soil warming was not significantly affecting the primary metabolite profiles, but year and season had significant effects. Season impacted primary metabolite profiles through changing soil temperature and years by effects on the soil environment (soil temperature and soil moisture) and root morphology (root length, specific root area, specific root length, and root diameter). In addition, we found that the root metabolism activity in warmed plots was lower than in control plots at the same soil temperature. Our data indicated that root metabolism in long-term warmed soil undergoes thermal acclimation, which may help match root metabolism with the required nutrient uptake and assimilation.

Keywords: Soil warming, root metabolism, primary metabolites, temperate forest.

How to cite: Liu, X., Heinzle, J., Tian, Y., Salas, E., Kwatcho-Kengdo, S., Borken, W., Schindlbacher, A., and Wanek, W.: Long-term soil warming changes the quantity but not the composition of primary metabolites of tree fine roots, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11752, https://doi.org/10.5194/egusphere-egu23-11752, 2023.

A large amount of carbon is stored in global forests. However, the fraction of carbon stored as plant biomass vs. soil organic carbon (SOC) varies among forest types. The extent to which biomass and SOC pools may change over the 21st century is uncertain, yet important to carbon cycle interactions with climate change. Here, we used data derived from inventories and remote sensing and CMIP6 models to examine the current and 21st century dynamics in the proportion of biomass and SOC across global forests. Our results show contrasting ecosystem carbon pools of forests in colder vs. moist warmer climates. Boreal forests currently store only 14±7% of their ecosystem carbon as plant biomass compared to 50 ±18% for moist tropical forests. We found that annual precipitation, topography, soil, and wildfire were the primary controls of these differences in forest carbon pool fractions. Under the Shared Socioeconomic Pathway (SSP5-8.5) climate scenario, CMIP6 models project that the ratio of biomass to ecosystem carbon in global forests will increase across the 21st century, with the largest increases in boreal forests compared to moist tropical forests. Changes in forest ecosystem carbon pools resulting in greater biomass fraction will affect the surface energy balance, disturbance regime, wildfire fuel loads, and ecosystem carbon balances, all of which interact with the climate system.

How to cite: Mekonnen, Z. and Riley, W.: Impacts of climate warming on biomass proportion of global forest carbon stocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16874, https://doi.org/10.5194/egusphere-egu23-16874, 2023.

Forest biomass is used as a representative factor for forest size, forest maturity, and forest productivity, so quantitative evaluation is very important not only for management and harvest but also for the evaluation of ecosystem functions and services including CO₂ absorption. The allometric equation is a method of estimating the value of each part through the relative growth rate of plants and is a methodology widely used from the past to the present. Recently, studies have shown that the relative growth system of trees is changing due to the increase in CO₂ concentration in the atmosphere and the resulting climate change, raising the need to review the previously developed relative growth and coefficients. In this study, the height-DBH relative growth relationships of four major tree species in Korea [(Pinus densiflora (PD), Larix kaempferi (LK), Quercus variabilis (QV), and Quercus mongolica (QM)] were analyzed using the 5―7th NFI data. Furthermore, these results were compared with the present yield table from the national institute for forest science. As a result of the analysis, it was found that the expected height value for the same DBH increased as the NFI progresses. For example, as a result of model analysis, the expected height for PD, LK, QV, and QM for DBH 25cm were 12.48, 19.17, 14.47, and 13.19m in the 5th NFI data, respectively. From the 7th NFI data, they were estimated as 13.61 (+9.1%), 21.58 (+12.7%), 15.76 (+8.9%), and 13.93 m (+5.6%), respectively. These results indicate that the current growth of major tree species in South Korean forests is more active in height growth than in diameter growth under climate change when compared with the height-DBH development trends by tree species identified through past survey data.

How to cite: Kim, M., Park, T., Ko, Y., Choi, G.-M., and Lee, W.-K.: Changes in Tree height–diameter allometry for temperate forests: A large periodical observational study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10187, https://doi.org/10.5194/egusphere-egu23-10187, 2023.

The observed increase in forest productivity over the industrial period is largely attributed to a stimulation of photosynthesis by increasing atmospheric CO₂ concentrations. The extent to which this fertilisation effect will persist in the future, however, is uncertain as competing limitations such as the availability of nutrients and soil moisture may prevent plants from making use of the carbon they assimilate, which may dominate the future photosynthesis response.

Many early studies of the CO₂ fertilisation effect use simulations that make use of the classical ‘big-leaf’ approach for scaling leaf photosynthesis to the canopy described by Sellers et al., (1992), which has been shown to produce unrealistically low sensitivities to light. Light limitation has the potential to significantly weaken the CO2 fertilisation effect relative to these early predictions. More realistic multi-layer canopy schemes have since been developed that produce more accurate light responses, however, the impact that this has on predicted CO₂ fertilisation seems to have gone unstudied.

In this presentation we present a modified version of the big-leaf canopy scheme. This new version produces more realistic sensitivities to light while remaining more computationally efficient than modern multi-layer canopy schemes. We examine the effect that the new big-leaf scheme has on predictions of future CO2 fertilisation and demonstrate the importance of correctly simulating the dominant limitations of photosynthesis.

How to cite: Jones, S. and Cox, P.: More realistic CO2 fertilisation in a revised big-leaf model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13959, https://doi.org/10.5194/egusphere-egu23-13959, 2023.