Most biological C resides in trees. Humans have been increasing atmospheric levels of CO2, exerting feedbacks on both humans and trees. Now humanity seeks for ways to increase C sequestration, and hence understanding tree C allocation is pivotal to all life on Earth. In 2015 we presented the first full description of tree C allocation dynamics that accounts for all C fluxes and pools in a tree. In the years that passed, we have been applying the mass balance and isotopic 13C labeling approaches to study tree C allocation in the field and greenhouse, at levels spanning from ecological to molecular. We have been advancing knowledge on tree C allocation to (I) more tree species, representing more functional groups. (II) more growing conditions, including heat, drought, and elevated CO2. (III) research focused on belowground allocation: root growth, exudation, transfer to mycorrhizal fungi and beyond. (IV) research focused on molecular mechanisms of C storage and carbohydrate management in specific tree tissues.

Among the many recent findings, we showed that (1) about half of all assimilated C is respired back to the atmosphere, but variations between tree species are large. (2) in the Mediterranean, conifers allocate more C belowground than evergreen broadleaf species. (3) under drought, while C sequestration (source) decreases significantly, C sinks remain the same, but partition less to respiration and more belowground. (4) under drought and heat, C sinks rely on decomposition of C reserves such as starch. (5) under elevated CO2, C sequestration increases in proportion to CO2 level, but not tree growth. C allocation patterns change in a species-specific manner. (6) CO2 responses continue up until other minerals run out. (7) a lot of uncertainty revolves around belowground C allocation. Root growth dynamics measured in the field are distinctive from stem or leaf growth. (8) root exudation accounts for 10% of all assimilated C but is decoupled from assimilation dynamics. (9) C transfer to mycorrhizal fungi is more rapid and diverse than previously thought. (10) part of the mycorrhizal C finds its way to neighboring trees through the mycorrhizal network. (11) at the molecular level, daily starch metabolism gene expression is different from the stress-mode starch metabolism pattern, operating unique beta amylases and starch synthases. (12) many other gene families are involved in C allocation, e.g., vacuole hexose transports, which regulate glucose levels in leaf and stem cells. (13) overexpressing one of these transporter genes produced fast-growing mutant poplar trees.

Taken together, these findings are crucial to all life on Earth, and particularly to us humans, at any level: scientists; stakeholders; and the wide public. Who cares about tree carbon allocation? Everybody!

How to cite: klein, T.: Who cares about tree carbon allocation?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2461, https://doi.org/10.5194/egusphere-egu24-2461, 2024.

Pulse labeling experiments remain an invaluable tool for tracing element allocation dynamics following environmental changes, ecological disturbance or extreme events. They are primarily used on the small scale, from single organisms to maximally the plot scale. Thus, upscaling of their outcomes is frequently challenging because of a lack of spatial representativeness and potentially missed interactions due to excluded ecosystem components. As we tackle a black box when studying belowground processes, the uncertainties of upscaling from small-scale labeling studies increase further. Therefore, we conducted a complete ecosystem pulse labeling drought stress experiment to explore the impact of extreme droughts on tropical rainforest’s belowground C allocation using the “ecosystem in a box model” of Biosphere 2 in Arizona.

The atmosphere of the tropical forest was exposed to a ¹³CO₂ pulse for several hours under ambient and drought conditions. Besides continuous monitoring (leaf, stem and soil ¹³CO₂ respiration), we performed regular post-pulse soil sampling campaigns to trace ecosystem belowground C allocations and monitor C partitioning at the soil–microbe-root interface. We aimed to identify key drought-adaptation strategies such as i) increased C allocation into subsoil layers which were expected to have higher moisture than dried-out topsoils and ii) increased relative C investment into specific rhizodeposits and mycorrhizal fungi that both may foster plant nutrition even from dry soil.

We observed a high allocation of assimilated ¹³C tracer into topsoil roots under drought, but this C allocation did not contribute to a higher root biomass. This suggests that tropical plants might modify their root composition by forming osmolytes or increasing lignin content to resist the high topsoil drought stress. The rhizodeposition (allocation of assimilated C into rhizospheres soil) increased mainly in the subsoil under drought, suggesting that trees aim to keep rhizo-microbial activity high in subsoils, where moisture was still available throughout the drought period. Whereas under ambient conditions the Gram-negative microorganisms - the most abundant rhizosphere inhabitants - profited most from the ¹³C allocated to the rhizosphere microbiome, we observed under drought conditions a high ¹³C allocation into the 18:2w6,9 biomarker representative for saprotroph and ectomycorrhizal fungi. This suggests trees invest C into their mycorrhizal partners most likely hoping for improved nutrient uptake via the small, drought-resistant fungi able to exploit not yet dried-out microhabitats in soils. Generally, we found pronounced plot- and thus plant-specific differences in belowground C allocation, suggesting species- or functional plant type specific drought response strategies belowground.

In summary, quantification of ecosystem C belowground allocation patterns at the plant-microbe-soil interface enables us to disentangle distinct belowground drought response strategies of tropical rainforests. This is essential to support those ecosystem traits that increase tropical rainforest’s resistance and resilience to climate change.

How to cite: Dippold, M. A., Bai, X., Shi, L., Acharya, P., Schmuecker, N., Ingrisch, J., Meeran, K., Daber, E., Fudyma, J., Hildebrand, G., Honeker, L. K., Kühnhammer, K., Gil-Loaiza, J., Huang, J., Zhang, X., Tfaily, M. M., Ladd, N., Meredith, L. K., and Werner, C.: Belowground C allocation of tropical rainforests under drought: an ecosystem 13CO2 labeling experiment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17755, https://doi.org/10.5194/egusphere-egu24-17755, 2024.

Boreal forests, as critical components of the global carbon (C) cycle, fix annually substantial amounts of atmospheric C. However, the timescales at which this C is cycled through the various ecosystem compartments are yet not well understood. To elucidate the temporal dynamics between photosynthesis, allocation and respiration of C, we assessed the radiocarbon (¹⁴C) to understand the fate of C in a boreal forest ecosystem. Samples from a boreal forest stand at the ICOS station Svartberget (SVB) in northern Sweden were collected, including vegetation, soil cores, atmospheric CO₂ and the ¹⁴CO₂ values from incubated topsoil were used to interpret D in different ecosystem pools. Additionally, we conducted comprehensive analyses of Δ¹⁴CO₂ released from forest floor soil respiration (FFSR) over a 24-hour cycle and calculated the Δ¹⁴C signature of the total ecosystem respiration following the Miller-Tans approach. We show that vegetation pools presented a positive D indicated by the enrichment with bomb ¹⁴C (produced mostly between 1950 and 1964), suggesting a fast-cycling rate (in the order of months to years) for living biomass and intermediate for dead biomass (years to decades). In contrast, soils showed a negative D, indicating minimal incorporation of bomb ¹⁴C. FFSR showed diurnal Δ¹⁴C variability with an average value close to the atmosphere (-2.33‰ in summer 2022 at SVB), suggesting that the output flux is dominated by autotrophic respiration of recently fixed and post-bomb labile C. Calculations for Δ¹⁴C in ecosystem respiration (166 ± 66.2‰), which is enriched in comparison to FFSR, in ecosystem respiration. Although the boreal forest stores significant amounts of C in the soil, , where it is cycled relatively fast. Only minimal amounts of recent C are incorporated and stabilised over long time scales. The potential of the boreal forest to mitigate climate change has to be further studied emphasizing the critical role of soil organic carbon persistence, where the ecosystem-atmosphere ¹⁴C disequilibrium may provide powerful insights.

How to cite: Tangarife-Escobar, A., Guggenberger, G., Feng, X., Muñoz, E., Chanca, I., Peichl, M., Smith, P., and Sierra, C.: Radiocarbon isotopic disequilibrium shows little incorporation of carbon in soils and fast cycling in a boreal forest ecosystem, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9134, https://doi.org/10.5194/egusphere-egu24-9134, 2024.

Severe drought impacts soil organic carbon (SOC) cycling. Yet, there is limited understanding of drought effects on rhizosphere C allocation and its fate in the soil, since belowground C contributes to new SOC formation while fueling soil microbial communities and SOC mineralization. Here, we quantified rhizosphere C inputs and losses in a 17-year long irrigation experiment in a dry Scots pine forest using ¹³C-enriched soil ingrowth bags with different mesh sizes. Fungal and bacterial communities inside the ingrowth bags and in adjacent soils were analyzed by Illumina MiSeq sequencing.

After two years, net new SOC accumulation was 5 times greater in root-accessible vs root-exclusion bags (1000- vs 20-μm mesh). Irrigation stimulated new SOC formation in the first year as compared to natural drought both in root-accessible (+26%) and root-exclusion bags (+47%). Losses of “old” ¹³C-enriched SOC increased under irrigation in root-accessible (+88%) and root-exclusion bags (+32%), resulting in an overall balanced effect of irrigation on SOC. After two years, irrigation showed a limited effect on net new SOC formation both in root-accessible and root-exclusion bags despite a 70% greater root ingrowth under irrigation. We attribute the lacking irrigation effect on rhizosphere-derived SOC to higher respiratory losses of new soil C, which is in line with +55% old C losses by irrigation in root-accessible bags after two years. These findings indicate a faster C cycling in the rhizosphere under irrigation with enhanced C inputs, which were however rapidly mineralized, resulting in negligible net effects. The increased belowground C allocation and increased C turnover in the rhizosphere under irrigation were paralleled by shifts in fungal and bacterial communities in ingrowth bags as well as in adjacent soils. Fungal and bacterial community structures were also shaped by the presence of roots in the bags.

Overall, our results in this long-term irrigation experiment imply that naturally dry conditions slow SOC cycling, suppressing both rhizosphere C inputs and losses. The reduced supply of belowground C leads to cascading effects on soil microbial community composition under drought.

How to cite: Guidi, C., Frey, B., Gavazov, K., Han, X., Peter, M., Mayer, M., Zhang, Y., Stierli, B., Brunner, I., and Hagedorn, F.: Drought suppresses rhizosphere carbon inputs and losses with cascading effects on soil microbial communities in a pine forest, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12463, https://doi.org/10.5194/egusphere-egu24-12463, 2024.

Plant carbon (C) allocation describes the distribution of carbon among different organs and processes and is sensitive to environmental conditions. As climate change proceeds, it will introduce additional uncertainties to the forest’s function as a crucial terrestrial carbon sink. Previous studies have explored the effect of single environmental variables on plant carbon allocation. Still, they cannot provide insights into the combined effects of changing environment in the future. To understand how European tree species will alter their C allocation after acclimating to the future environment, beech and spruce seedlings were grown in controlled environment facilities (CEFs) of different scenarios for three years. The scenarios represent the present condition of 1987 to 2016 (PC) and, in accordance with the IPCC scenarios, a mitigation scenario (RCP2.6) and a worst-case scenario (RCP8.5) of 2017 to 2100. This implies an increase in air temperature by approximately 1°C (RCP2.6) and 3°C (RCP8.5), an increase in CO2 concentration by approximately 30 ppm (RCP2.6) and 500 ppm (RCP8.5), and changes in other variables over the three years, including the irradiance, the relative humidity, and the O₃ concentration. After three years of treatment, the plants were labeled with ¹³C-enriched CO₂ for three days to understand the allocation and turnover of new photoassimilates. Both beech and spruce had greater biomass under RCP8.5 compared to RCP2.6 and PC, accompanied by enhanced allocation to belowground biomass. The concentration of non-structural carbohydrates (NSC) showed no significant difference across the scenarios, neither in leaves nor fine roots. Yet, the mean residence time of carbon (MRT) of the soil respiratory pool had shortened in the RCP scenarios in both species. Specifically for beech, a compartmental model showed an increased pool size of mobile carbon and confirmed the shortened MRT of the mobile carbon pool under RCP8.5. The unchanged NSC concentration in the sink organ with the shortened MRT has indicated a more rapid carbon turnover under both RCP scenarios accompanied by more substantial C allocation to the immediate respiration. Additionally, the fixed C was substantially invested in biomass growth, i.e., structural carbon, only under RCP8.5, which indicates that the doubled CO₂ concentration has alleviated the environmental stress. In contrast, the minor increase in CO2 concentration under RCP2.6 had no such effect. We thus recommend that the relationship between C fixation and biomass growth should be interpreted more cautiously under changing environmental conditions in the future.  

How to cite: Gu, Q.-L., Baumgarten, M., Jakli, B., Rammig, A., Grams, T., and Hesse, B.: Do beech and spruce change their carbon allocation under future environmental conditions? Lessons learned from a three-year growth chamber experiment. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5755, https://doi.org/10.5194/egusphere-egu24-5755, 2024.

Tree mortality from increasing drought events have been reported worldwide, with the potential to threaten both forest health and services such as timber production. Tree species’ resilience to drought events is dependent on a range of inter-connected mechanisms, such as gas exchange and hydraulic regulation, which further affect processes such as non-structural carbohydrate (NSC) storage, growth, and stress signaling. The effect of drought on these numerous plant processes have been well studied, however knowledge gaps remain regarding how these processes respond in concert to different drought intensities, as well as the recovery ability following stress release. In a greenhouse setting, we investigated how variable drought intensities (control, mild, severe) affected photosynthetic assimilation (A_net), NSC storage, and growth at multiple timepoints throughout a five-week drought and five-week recovery period in two-year-old Douglas fir (Pseudotsuga menziesii). Here, we show how system-wide physiological stress tracks drought intensity, with severe drought leading to an earlier cessation of A_net, greater utilization of starch to increase soluble sugars, and reduced growth as compared to the mild and control treatments. This study offers a holistic view of plant-physiological responses to drought stress intensity, and underscores how physiological damage sustained during severe drought events can have long-lasting impacts on forest health.

How to cite: Alongi, F., Knüver, T., Ziegler, Y., and Ruehr, N.: Connecting gas exchange, carbohydrate storage, and growth dynamics during variable drought stress and recovery in Douglas fir, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8782, https://doi.org/10.5194/egusphere-egu24-8782, 2024.

Plants play a critical role in the surficial environment, influencing energy transfer, the global carbon (C) and nitrogen (N) cycles, and climate change. Knowledge of botanical and climatic controls on terrestrial C and N-cycling within and across ecosystems is central to understanding plant ecophysiology. In this study, we examined the effects of climate and forest composition on plant C and N, and systematically measured foliar δ¹³C and δ¹⁵N along an altitudinal gradient ranging 1900 to 5200 m, across three transects spanning west to east Himalayas. Total C and N content in plants significantly decreased with altitude, except for TOC in central and western Himalayan gymnosperms. Precipitation and temperature gradients differentiated 76% of the variation in TN and δ¹⁵N, and only 2.5% in TOC and δ¹³C stocks in the Himalayan plants. We report a complex climatic and topographic control on the C and N allocation in montane ecosystems, quantified via isotopic signature and abundance, linking plant ecophysiology with resource availability. C and N being complementary in several foliar biochemical processes, their mutual abundance was realised, examined and inferred in previously unexplored montane ecosystems and climate. In addition, the spatial distribution of foliar-isotope-abundance helped cluster plant responses, eventually leading to the construction of a spatially comprehensive map known as a dual isoscape.

How to cite: Sanyal, P. and Dasgupta, B.: What controls carbon and nitrogen allocation in montane vegetation: A case study from the Himalayas , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-500, https://doi.org/10.5194/egusphere-egu24-500, 2024.

Forest biomass resulting from tree radial growth can remain on the landscape over decadal to centennial timescales and plays a critical role in forest carbon cycling. However, visually green vegetation may not be a good proxy for carbon allocation to growth as the phenology and environmental sensitivity of photosynthesis may be different from radial growth. Here we investigate the decoupling between photosynthesis and tree radial growth across intra to interannual timescales for seven North American oak species (Quercus spp.) at four sites (Lamont Sanctuary, NY; Morton Arboretum, IL; Pace Forest, VA; & Tonzi Vaira, CA, USA). Using point dendrometers and wood anatomy, we find that oak trees generally commenced radial growth (cell division and expansion) one month prior to full canopy development and peak carbon assimilation estimated using eddy covariance, satellite and in-situ remote sensing, and leaf-level chlorophyll fluorescence. Further, radial growth was essentially completed by early summer, two to three months prior to the early autumn end of the photosynthetic activity, and before the annual peak in temperature and vapour pressure deficit (VPD) and lowest soil moisture. This suggests that high summer aridity limits carbon allocation to growth more strongly than assimilation. Tree-ring width chronologies for these species across North America further supports that results that earlywood growth depends on prior season climate and assimilated carbon while latewood growth ends by early-summer and responds primarily to current year climate variability. In summary, temporal decoupling between radial growth and photosynthesis and the stronger constraint of summer aridity on growth than photosynthesis appears to be widespread among multiple North American temperate oak species. As summers continue to warm and dry under climate change, this source-sink (or photosynthesis-growth) decoupling needs to be better resolved to constrain forest carbon cycling, as increasing aridity will likely influence the ability of trees to allocate carbon to long-term storage as woody biomass.

How to cite: Rao, M. P., Pacheco-Solana, A., Jensen, J. E., Li, R., Griffin, K. L., Pederson, N., McCormack, L. M., Verfaillie, J., Yang, X., Baldocchi, D., Andreu-Hayles, L., Oryan, B., Hise, J., Rodriguez-Catón, M., Turner, A., Eitel, J., Scanlon, T. M., Pierrat, Z., Penuelas, J., and Magney, T.: Summer aridity decouples growth from carbon assimilation in temperate oaks, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1124, https://doi.org/10.5194/egusphere-egu24-1124, 2024.

Carbon (C) allocation is the process by which photosynthate is partitioned to different functional pools, including leaves, woody tissues and fine roots. A long-standing, qualitative theory explains patterns of C allocation as maximizing growth subject to the availability of different resources. Here we outline a quantitative model based on this theory. We define net carbon profit (NCP) as the total C taken up by photosynthesis, minus the costs of constructing and maintaining leaves and the below-ground C investments required to supply them with water and nutrients. We hypothesize that leaf area index (LAI) tends to the value that maximizes gross primary production; this leads to an explicit prediction of maximum (energy-limited) LAI. We assume that the demands of leaf and root production are satisfied with highest priority, and that excess C is allocated to stems in such a way as to maximize height growth and therefore competitive success. High NCP is predicted not only in tropical and subtropical forests, but also in the Pacific Northwest of the USA and in SE Australia, where the tallest trees are found. Moreover, wood density can be related to woody biomass turnover time, τ (estimated from biomass data and net primary production) – trees with denser wood have longer lifespans. In highly productive ecosystems τ tends to be small, e.g. tropical forests; τ can be larger in temperate and boreal forests. We combine predicted τ with biomass-density relationships and mechanical constraints to predict maximum tree heights, testing our predictions using the global Forest Carbon database (ForC) and observations of maximum vegetation height.

How to cite: Ding, R., Nóbrega, R., and Prentice, I. C.: Predicting biomass partitioning and maximum tree height based on optimality principles, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5515, https://doi.org/10.5194/egusphere-egu24-5515, 2024.

In recent years, we have increased dramatically our knowledge of the carbon cycle and its flow throughout the terrestrial ecosystems. During the 1990-2021 period terrestrial biosphere provided a net sink of about 21% of carbon dioxide emitted by fossil fuel burning with the major part occurring in forests. The notion of an active carbon sink of the terrestrial biosphere is driving the current debate of climate mitigation and its contribution to achieve carbon neutrality by 2050 to limit global warming within 2°C as set out in the Paris Agreement. Current knowledge is based on an outstanding wealth of systematic observations from the atmospheric gaseous exchanges, space and aircraft observation of tropospheric CO2 concentration down to flux towers and soil and leaves respiratory and photosynthesis measurements. However, achieving a solid scientific background about the real effectiveness of terrestrial carbon to support policy targets require an in depth analysis of the capacity of the biosphere to sustain long term carbon sequestration throughout the century. Main challenges affecting flows of carbon are to what extent climate extremes in both space and time domain may pulse carbon emissions to become dominant compared to mean carbon uptake by the terrestrial biosphere, the role of disturbances by biotic events that are occurring at increasing temporal frequency and the role of forest management to substantially regulate the flow of carbon through the long living wood products and their fate (nature based solutions, material substitutions and renewable energy).

The classic definitions of GPP, NPP, RE (ecosystem respiration) should be expanded to include the stochastic nature of abiotic and biotic disturbance and the human role on forest management to be able to provide a complete picture on the potential role of terrestrial ecosystems in supporting carbon neutrality targets. The presentation will address the complexity of carbon flows through the comprehensive chain, analyse research gaps and emerging monitoring technologies needed to better monitor the terrestrial biosphere.

How to cite: Valentini, R., Boukhris, I., Buonocore, L., Yates, J., and Chiriaco, M. V.: The complexity of carbon flow of the terrestrial biosphere: challenging questions and new scale of observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19348, https://doi.org/10.5194/egusphere-egu24-19348, 2024.

Tree growth is a key uncertainty in projections of forest productivity and the global carbon cycle. While global vegetation models commonly represent tree growth as a carbon assimilation (source)-driven process, accumulating evidence points toward widespread non-photosynthetic (sink) limitations. Notably, growth biophysical potential, defined as the upper-limit to tree growth imposed by temperature and turgor constraints on cell division, has been suggested to be a potent driver of observed decoupling between tree growth and photosynthesis. Understanding the interplay between biophysical potential and photosynthesis and how to accommodate it parsimoniously in models remains a challenge.

Here, we use a soil-plant-atmosphere continuum model together with a regional network of forest structure and annual, radial tree growth observations extending over three decades to simulate tree photosynthesis and biophysical potential along an aridity gradient and across five tree species in NE Spain. We then apply a linear modelling framework to quantify the relative importance of photosynthesis, biophysical potential and their interactions to predict annual tree growth along the aridity gradient.

Overall similar relative importance of photosynthesis and biophysical potential was underlain by strong variations with climate, photosynthesis being more relevant at wet sites and biophysical potential at dry sites. Observed spatial and temporal trends further suggested that tree growth is primarily limited by biophysical potential under dry conditions and that disregarding it could lead to underestimating tree growth decline with increased aridity under climate change.

Our results support the idea that biophysical potential is an important component of sink limitations to tree radial growth. Its representation in vegetation models could accommodate spatially and temporally dynamic source-sink limitations on tree growth.

How to cite: Cabon, A., Ameztegui, A., Anderegg, W. R. L., Martínez-Vilalta, J., and De Cáceres, M.: Interplay of photosynthesis and biophysical potential to model tree growth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5745, https://doi.org/10.5194/egusphere-egu24-5745, 2024.

Tropical forests play an essential role in the carbon cycle. However, climate change threatens their ability to store carbon. Specifically, understanding the perturbation of climatic regimes on carbon uptake mechanisms is crucial. However, our knowledge concerning the spatial and temporal carbon distribution over trees and forests is limited, especially in the context of tropical forests of Central Africa. The TREE4FLUX project aims to fill these gaps for the first time in the forests of Congo Basin forests, by focusing research at different scales around the CongoFlux tower in the Yangambi Biosphere Reserve (DRC). On the forest ecosystem scale, carbon uptake can be monitored by measurements of CO2 exchanges between the atmosphere and the vegetation using the Eddy Covariance approach. Carbon assessments are also possible through tree-growth measurements within a network of permanent inventory plots. However, refining the carbon cycle at the tree scale requires a detailed study of the numerous inextricable metabolic processes that underlie tree growth, e.g. photosynthesis, wood formation, or respiration. Because they are largely controlled by various climatic drivers, climate-growth relationships over time remain hard to establish. The chronology of carbon uptake and attribution to the different mechanisms remain elusive preventing a grasp of the intra-annual variations of these periodic processes and their articulation over time. This is the case of xylogenesis or wood formation in which each phase is differently involved in the carbon cycle and sensitive to various climatic drivers. To understand the sensitivity of tree growth to climate, we need to untangle the cambium’s role in wood formation. For that purpose, monitoring cambial phenology helps characterize the distribution, allocation, and short- and long-term carbon storage in woody material. While tree growth uptakes carbon, respiration releases carbon into the atmosphere at various levels. Heterotrophic and autotrophic respirations have a decisive role in the carbon cycle at the forest scale but face significant misunderstandings in this regard. To upscale our understanding from individual tree to forest scale, we imperatively need respiration monitoring in both living and decayed trees. This requires unravelling the metabolic processes driving both autotrophic and heterotrophic respiration, i.e. the tree growth and decayed process, respectively. Characterization of carbon fluxes according to an integrative approach over climatic variations is required to understand how environmental changes affect ecosystem dynamics and their ability to provide ecosystem services

How to cite: Hicter, P., Hubau, W., and Beeckman, H.: Unravelling the carbon cycle at the tree and forest scale: a TREE4FLUX initiative in Central African Tropical Forests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15315, https://doi.org/10.5194/egusphere-egu24-15315, 2024.

The rate at which forests take up atmospheric CO₂ is critical because of their potential to mitigate climate change and their value for wood production. The allocation of carbon fixed through photosynthesis into biomass can be quantified through the tree carbon (C) use efficiency (CUE), which is determined by gross primary production (GPP) and plant respiration (Ra) via the relation CUE=(GPP-Ra)/GPP. The effect of future climate on CUE is unclear due to the highly uncertain response of plant respiration to the expected increases in temperature and possible changes in tissue nitrogen (N) concentrations that also affect GPP and Ra.

We aim to develop novel data-driven estimates of plant respiration, net primary production (NPP=GPP-Ra) and tree CUE covering the northern hemisphere boreal and temperate forests. These will be based on recent satellite-driven maps of tree living biomass, databases of N concentration measurements in tree compartments (leaves, branches, stem sapwood, roots) and the relationships between respiration rates and tissue N concentrations and temperature. Such estimates will enable the detection of spatial relationships between CUE and environmental conditions and facilitate the parameterization of dynamic global vegetation models to predict the change in Ra, NPP and CUE in response to future climate and forest management.

Here we compile an unprecedented database of N concentration measurements in tree stems, branches and roots covering all common boreal and temperate tree genera together with data available mainly for leaves from databases like TRY. We apply this database to test different hypotheses on the controls of tree tissue N concentration and allocation. We find that the variation in tree tissue N concentrations of boreal and temperate trees is controlled by their leaf type (broadleaf deciduous, needleleaf deciduous, needleleaf evergreen), growth rate (fast- vs. slow-growing), tree age/size and climate conditions. These relationships have important implications on the coupling of the C and N cycles in the vegetation, since tissue N concentrations determine photosynthesis, growth and plant respiration. Thus, by altering tissue N concentrations, changes in the distribution of tree species, in tree age/size or in climate, induced by climate change, forest management or disturbances, can affect the C sequestration potential of boreal and temperate forests.

Subsequently, we use machine learning approaches to explain the variation in tree tissue N concentrations. We combine the derived tree-level relationships between tissue N concentrations and the above-mentioned underlying drivers, tree species distribution maps, and tree tissue biomass products based on satellite remote sensing. In this way, we derive novel estimates of the spatial distribution of tissue N concentrations and contents in northern boreal and temperate forests. These will be the basis for spatial estimates of Ra, NPP and CUE in these ecosystems. Finally, we aim to identify their climate change mitigation potential by determining which tree species allocate the highest share of N to their leaves and which species exhibit the highest CUE under different climatic conditions.

How to cite: Thurner, M., Yu, K., Manzoni, S., Prokushkin, A., Thurner, M. A., Wang, Z., and Hickler, T.: Nitrogen concentrations and contents in boreal and temperate tree tissues – Controls, spatial distribution and impact on plant respiration, net primary production and carbon use efficiency, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3710, https://doi.org/10.5194/egusphere-egu24-3710, 2024.

As an essential nutrient, Nitrogen (N) availability is fundamental in monitoring forest productivity, and as such, understanding the effects of changing atmospheric N inputs in forest ecosystems is of high significance. While most field experiments have been employing ground fertilization to simulate Nitrogen deposition, two experimental forest sites in Italy have adopted the more advanced canopy N application approach.
Here we present findings from a case study of wood core analyses of predominantly pure, even aged, Sessile Oak (Quercus petraea L.) and European Beech (Fagus sylvatica L.) forest stands, under two treatments of below and above canopy Nitrogen application, comparing between the two methods. The potential effect of elevated N availability on total ring width, mean ring density, and their corresponding earlywood and latewood fractions are examined.
Our results indicate inconclusive effects of the treatments on the ring widths of either Q. petraea or F. sylvatica, although basal area increment patterns appeared to be affected divergently between the species and treatments. Mean and earlywood, but not latewood, densities exhibited a decrease in certain years of treatment in Q. petraea following the above canopy N application only, whereas F. sylvatica wood density showed no clear response to any of the treatments.
Thus, we describe distinct responses of these broadleaved species to the different treatment approaches, discussing potential growth patterns under increased N availability, and emphasizing the importance of considering wood density in tree biomass accumulation and Carbon storage capacity assessments.

How to cite: Minikaev, D.: Experimental Canopy Nitrogen Deposition Effects in Temperate Forests: The case of Quercus petraea L. and Fagus sylvatica L. Ring Width and Wood Density, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21519, https://doi.org/10.5194/egusphere-egu24-21519, 2024.

Nonstructural carbohydrates (NSC) management in response to drought stress is vital to understanding drought acclimation in trees. However, the molecular mechanisms underlying such processes remain unclear for most forest trees. Poplars are considered a model species for studying woody plants' molecular mechanisms. They are known to deploy a passive, symplastic sugar loading strategy, relying on sugars' concentration gradient through the plasmodesmata rather than on active sugar transport. Under drought conditions, tight regulation is needed to sustain long periods of stress and maintain water content. This study subjected young poplar trees (P. alba) to drought stress, following a combined analysis of sugar content and gene expression profile (RNAseq). We analyzed the expression of 29 gene families related to NSC signaling, translocation, and metabolism along three tree compartments (leaves, shoots, and roots). Starch depletion was evident in all organs, while soluble sugars accumulated only in the leaves, with an overall whole-tree decrease of ~30% in total sugar content. Our findings highlight relevant genes that specialize in triggering drought metabolism, starch depletion, sugar immobility, and osmotic adjustment. Broadly up-regulated genes are highlighted, as well as tissue-specific ones. This research provides a broad overview of the molecular process underlining NSC dynamics under drought in poplar trees.

How to cite: Fox, H., Ben-Dor, S., Doron-Faigenboim, A., Goldsmith, M., Klein, T., and David-Schwartz, R.: Carbohydrate dynamics in Populus trees under drought: an expression atlas of genes Carbohydrate dynamics in Populus trees under drought: an expression atlas of differentially expressed genes across leaf, stem, and root, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15757, https://doi.org/10.5194/egusphere-egu24-15757, 2024.

Respiration by trees stems constitutes a substantial proportion of autotrophic respiration in forested ecosystems and has been estimated to contribute 12 – 25 % of total ecosystem respiration, yet little is known about its associated drivers at different spatial scales. Stems are the largest contributor to forest biomass and so the respiratory consumption of stems has the potential to considerably affect carbon budgets in forest communities. As logged and degraded forests are fast becoming the most dominant land-use type throughout the tropics, it is also important to contextualise stem respiration over land use gradients. In this study we quantified stem respiration at individual tree and plot scales in nine 1-ha plots over a gradient of heavily logged to old-growth forest in Malaysian Borneo. We investigated how logging intensity, forest structure, plant functional traits, and soil chemistry influence stem respiration in logged and old-growth forest plots at both scales. We found that, at individual tree level, stem respiration rate per unit stem area was significantly higher in logged than old-growth plots, and this was consistent within most diameter classes. At the 1-ha plot scale, however, total stem respiration did not differ between forest types: the higher stem respiration rate in logged plots was offset by the higher stem area in old-growth plots. At plot level, stem respiration was driven by forest structure and soil chemistry. We found that basal area was a strong predictor of stem respiration within both forest types at plot scale; for a similar basal area, logged plots exhibited a higher stem respiration rate. Partitioning stem respiration into its growth and maintenance components at plot scale highlighted how logged plots prioritise growth in response to intense light competition, as logged plots had significantly higher allocation to growth respiration, whereas old-growth plots prioritised maintenance and cell structure. Our analysis at individual tree scale reinforced these differing priorities, as stem respiration in logged plots was driven by plant traits associated with growth and wood anatomy, as opposed to within old-growth plots where stem respiration was driven by traits associated cell structure and maintenance. These results reflect the different strategies of resource allocation for trees growing in logged and old-growth plots and adds to the growing body of research on autotrophic respiration, the least studied component of forest carbon dynamics within a very understudied yet expanding land use.

How to cite: Mills, M. B., Both, S., Jotan, P., Huaraca Huasco, W., Cruz, R., Pillco, M. M., Burslem, D. F. R. P., Maycock, C., Malhi, Y., Ewers, R. M., Berrio, J. C., Kaduk, J., Page, S., Robert, R., Teh, Y. A., and Riutta, T.: From tree to plot: investigating stem respiration and its drivers along a logging gradient in Sabah, Malaysian Borneo, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11207, https://doi.org/10.5194/egusphere-egu24-11207, 2024.

The understanding of forest growth is crucial for the preservation of forests in the future. Factors such as tree species, age, forest management and environmental conditions influence this growth.

Tree height and age data can be combined to describe forest growth and infer known environmental effects. In this study, we constructed height-age growth curves for 14 monospecific and mixed-species stands using ground measurements and satellite data in northern France. A random forest height model was constructed using tree species and age, area of disturbance, and 125 environmental parameters (climate, altitude, soil composition, geology, stand ownership, and proximity to road and urban areas). Through feature elimination and SHAP analysis, six key features were identified that explain forest growth and their effect on height was investigated.

The agreement between satellite and ground data justifies their simultaneous exploitation. Age and tree species emerged as the primary predictors of tree height, accounting for 49% and 10% of the variation, respectively. Post-disturbance growth is influenced by the disturbed patch area, which reveals the regeneration method, accounting for 19% of the variation. Soil pH, altitude, and summer climatic water budget have varying effects on tree height depending on age and tree species.

The agreement between satellite and ground data justifies their simultaneous exploitation. Age and tree species are the primary predictors of tree height, accounting for 49% and 10% of the variation, respectively. Post-disturbance growth is influenced by the disturbed patch area, which reveals the regeneration method, accounting for 19% of the variation. Soil pH, altitude, and summer climatic water budget have varying effects on tree height depending on age and tree species.

The integration of satellite and field data shows potential for analyzing future forest evolution.

How to cite: Pellissier-Tanon, A., Ciais, P., Schwartz, M., Fayad, I., Xu, Y., Ritter, F., de Truchis, A., Véga, C., and Leban, J.-M.: Combining satellite images with national forest inventory measurements for monitoring post-disturbance forest height growth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15187, https://doi.org/10.5194/egusphere-egu24-15187, 2024.

How to cite: Bauters, M., Van de Velde, V., W. Drake, T., Boeckx, P., Ewango, C., Doetterl, S., and Makelele, I.: Cations in Crisis: Limitation and Vulnerability of Secondary Forest Regrowth in dynamic Tropical Landscapes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12065, https://doi.org/10.5194/egusphere-egu24-12065, 2024.

Restoring tropical forest can provide a large additional carbon sink, yet knowledge of the optimal locations for reforestation programs remains uncertain. By evaluating multiple pantropical forest biomass carbon and height datasets, we find that tropical plantations and regrowth forests (TPFs) situated at elevations ≥ 300 m accumulate 1.5-fold more carbon per year in biomass than their lowland counterparts (elevations < 300 m) prior to reaching maturity. Notably, the biomass carbon accumulation rates increase significantly (P<0.001) between 300 m and 1000 m, subsequently declining at higher elevations. The main cause is the greater sensitivity of ecosystem production than respiration to elevational gradients in air temperature and vapor pressure deficit. Our analysis also shows that TPFs at mid-elevation (300 to 1000 m) grow most rapidly before ~20 to 25 years of age, while for those in the lowlands (< 300 m), maximum growth rates are attained at up to 30 years old or more. Our findings underscore the importance of accounting for elevation when executing reforestation in the tropics.

How to cite: Chen, X., Su, Y., and Li, X.: Optimal elevation for biomass carbon accumulation in tropical planted and secondary forests , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1368, https://doi.org/10.5194/egusphere-egu24-1368, 2024.