The ‘growing season’ of trees is often assumed to be coupled with climatology (e.g., summer vs winter) and visual canopy phenology cues (e.g., leaf emergence in spring and senescence in autumn). However, green leaves are not always photosynthetically active and actual tree radial growth via cambial cell division is ‘invisible’ since it is hard to see and occurs at micrometer resolution. Therefore, despite the presence of apparently green vegetation, trees may not be assimilating carbon or growing. Here, we study photosynthesis and tree-growth at near-instantaneous timescales using in-situ and satellite remote sensing, point dendrometers, quantitative wood anatomy, and Pulse Amplitude Modulated chlorophyll fluorescence. Tree and leaf-level measurements are being made on eight oak (Quercus spp.) trees in a temperate forest in southern New York, USA. We find that oak trees commence radial growth in the first week of April approximately one-month prior to canopy development that is not completed until the first week of May. Additionally, the development of foliar photosynthetic capacity lags leaf expansion by nearly two weeks. Further, we find that oak growth for the season is completed by late July while photosynthetic activity is maintained for three additional months until early November. Finally, we examine the growth climate sensitivity across a network of 16 oak tree-ring width chronologies distributed across the northeastern US. These relationships suggest that oak earlywood growth relies on carbon assimilated in prior year autumn while oak latewood relies on current year assimilated carbon. Therefore, photosynthesis and tree-growth in Northeastern US oaks occurs asynchronously, since trees don’t reach peak photosynthetic performance the moment leaves emerge or grow through the ‘growing season’.

How to cite: Rao, M. P., Pacheco-Solana, A., Griffin, K., Jensen, J. E., Pederson, N., Oryan, B., Nixon, T., Rodriguez Caton, M. R., Andreu Hayles, L., Hise, J., Peñuelas, J., and Magney, T.: Temporal decoupling between carbon assimilation and tree growth in temperate oaks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8562, https://doi.org/10.5194/egusphere-egu23-8562, 2023.

Assimilated non-structural carbohydrates (NSC) can be stored as reserves in plants and remobilized during periods of asynchrony between carbon acquisition and carbon demand to fuel essential metabolic functions and growth. However, the framework of NSC allocation to reserves and their remobilization remains unclear, especially for mature trees which potentially can store large quantities of reserves. Here, we explore the role of stem reserves and potential constraints in their remobilization in large Betula papyrifera trees. To explore reserve remobilizations between organs, we use different patterns of phloem girdling to induce carbon stress and to isolate crown, stem, and root NSC storage pools.  Our results suggest that NSC reserves in the stem tissues may not be easily remobilized to other, more distant, organ sinks. However, we also found that some root reserves may be allocated toward the lowermost stem/root collar position, indicating that under carbon limiting conditions the roots might not be the strongest sink for NSC reserves. We suggest this response represents an adaptive recovery strategy for a collar-sprouting species like B. papyrifera, which occupies areas prone to aboveground disturbance. We also found that storage capacity of tissues (here stem wood and crown) can far exceed the concentrations that are normally stored in these trees.  Additionally, we found stem NSC concentrations positively correlated with disease resistance and branch water content.  These relationships suggest that a maintenance of greater stem reserves may be required to support other important roles directly or indirectly, such as defense or spring leaf flush, respectively.  Overall, it appears that the remobilization of different organ reserve storage pools is regulated somewhat autonomously, which, particularly under carbon limiting conditions, could potentially limit the sharing of reserves within a large tree.

How to cite: Landhäusser, S. M., Fermaniuk, C., Fleurial, K. G., and Wiley, E.: The role of the bole: constraints in the remobilization of stem reserves under experimental carbon limitation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4386, https://doi.org/10.5194/egusphere-egu23-4386, 2023.

Woody plants rely on the remobilization of carbon (C) and nitrogen (N) reserves to support growth and survival when resource demand exceeds supply at seasonally predictable times like spring leaf flush and following unpredictable disturbances like defoliation. Despite their importance, we still have a poor understanding of how reserve remobilization is regulated and whether remobilization and the allocation of mobilized reserves is constrained by distance between source and sink tissues. This leads to uncertainty in which reserves—and how much—are actually available to support plant functions like leaf growth during spring flush or following defoliation. To better understand the source of remobilized reserves and constraints on their allocation, we used stable isotopes (¹³C,¹⁵N) to label C and N reserve pools in aspen (Populus tremuloides Michx.) saplings, and then grafted unlabeled and labelled stems to labelled and unlabeled root stocks, to create organ-specific labelled reserves.  We then tested for differences in reliance on reserves from different organs between 1) upper and lower leaves 2) early and late leaves and 3) early flush and reflush leaves produced after defoliation.  During spring flush, both C and N reserves were preferentially allocated to sinks nearer the reserve source (i.e the roots), but reliance on C reserves was reduced over time.  Additionally, N appeared to be preferentially remobilized from sources closer to the developing leaves.  However, following defoliation, we found that reflush leaves imported the same proportion of N from the roots as spring flush leaves, but that a lower proportion of C was imported from root reserves. The lower import of reserve C suggests reflush leaves must rely more on their own photosynthetic gains to fuel leaf growth, which may explain the reduced total leaf mass of reflush canopies (31% of initial mass).  The reduced import of reserves occurred even though roots retained significant starch reserves (~5% dry wt), suggesting aspen prioritizes the maintenance of root C reserves at the expense of fast canopy recovery.

How to cite: Wiley, E., Hart, A., and Landhäusser, S.: Tracing the origin of imported carbon and nitrogen reserves: remobilization during spring leaf expansion and recovery following defoliation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7551, https://doi.org/10.5194/egusphere-egu23-7551, 2023.

Nonstructural carbohydrates (NSCs) play a crucial role in plant functioning and survival. Nonetheless, substantial knowledge gaps remain regarding NSC mobilization and transport in forests experiencing more frequent extreme droughts. We combined drought manipulation and 13CO2 pulse-labeling in an enclosed rainforest, and assessed changes in tissue NSC content and allocation of recent photosynthates in eight species that represent ecosystem biomass and cover different positions and hydraulic strategies. Drought reduced starch in leaves but not in stem phloem and roots across species. However, soluble sugars remained constant or increased in understory plants and anisohydric trees, and decreased only in leaves of isohydric trees with relatively constant leaf water potential and sap flow. Drought slowed export and transport of recent photosynthates, not only for anisohydric species with a strong decrease in leaf water potential and sap flow but also for isohydric species with a strong decrease in photosynthetic supply and carbohydrate levels.  We provide evidence that tropical plants under drought mobilize starch to buffer carbon deficiency, while regulating local utilization, export and transport of soluble sugars depending on position and isohydricity. We highlight the importance of plant functional types for understanding NSC dynamics and their role in determining forest carbon balance under drought. 

How to cite: Huang, J., Ladd, N., Meredith, L., Werner, C., and Lehmann, M. and the the coauthors: Plant water use strategies drive the fate of newly fixed carbon in an experimental rainforest under drought, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13892, https://doi.org/10.5194/egusphere-egu23-13892, 2023.

Plants allocate carbon to roots, shoots, respiration and rhizodeposition. The quantitative partitioning depends on genetic and environmental factors, as well as on the plant’s phenology. The first two or three pools represent most of the carbon input to the soil, depending on the ecosystem considered. Quantifying this partitioning is of major importance as roots, shoots, and rhizodeposition are constituted of several organic compounds differing in residence time. Moreover, besides being carbon inputs, these compounds modify in different ways their surroundings and might slow down or accelerate the cycling of carbon and nutrients.

One frequently used method to quantify this partitioning is the 13C labelling of atmospheric CO2, which allows to trace organic carbon in the soil-plant system. 13C labelling of CO2 can be either continuous, or as single or multiple pulses. It can be combined with gas measurements that estimate the priming effect induced by the plant inputs. This poster aims at synthesizing the literature about experimental quantification of carbon allocation to different plant pools, and comparing the methods used. It allows to identify the plant traits that predict this allocation. Moreover, we show the preliminary results of a continuous 13C labelling experiment that compares plant traits with carbon content variations in soil columns.

Whereas shoots and roots are relatively simple to quantify, rhizodeposition is not. It therefore represents a major incertitude when considering the carbon inputs to soil. Moreover, labile exudates represent an important part of rhizodeposition that induce an increase of older soil carbon mineralisation. Several experiments show that the loss of carbon induced by rhizodeposition priming effect is bigger than the input. These results are often associated to plants with high photosynthetic activity.

A trade-off is to be found when considering 13C labelling. Continuous labelling allows a quantification of net rhizodeposition which is independent of the phenology and that integrates the whole plant growth. On the other side, pulse labelling is easier and less expensive. Regarding priming effect quantification, gas flux measurements are precise but do not integrate the whole plant growth. Mass quantification through carbon content change do, but it requires very accurate estimations of these changes.

How to cite: Hulin, B. and Abiven, S.: Implication of carbon allocation in plants for soil organic matter cycling., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5105, https://doi.org/10.5194/egusphere-egu23-5105, 2023.

In a context of global change, it is crucial to understand the factors and processes by which plants respond to drought and by which crops may limit the development of their aboveground biomass.

Experimental studies have showed that soil water status, soil structure and soil texture impact carbon allocation within plant and in particular the root:shoot ratio. We used a conceptual soil-plant hydraulic model to analyze the results of a meta-analysis gathering literature data of root:shoot ratio measured in controlled conditions. For each paper, information on soil water status, soil and plant traits and abiotic factors were collected. Soil hydraulic conductivity was estimated based on pedotransfer functions, when unavailable.

The results feature that the root:shoot ratio is an adaptation strategy that depends on the soil conductance in order to balance the water availability with the transpiration demand. The partitioning response varies between plant types. This study gives an explanation to current observations and shows the necessity to collect accurate soil measurements and information for further experiments.  

How to cite: Cecere, A. and Javaux, M.: Experimental investigation of the relationship between root:shoot ratio and soil-plant hydraulics., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5112, https://doi.org/10.5194/egusphere-egu23-5112, 2023.

Vegetation carbon uptake is a major sink for anthropogenic greenhouse gas emissions, yet inferring longer-term behaviour of ecosystems as carbon sinks or sources is still difficult. Here, using a time series decomposition technique and eddy covariance data, we show that while at subseasonal scales net ecosystem CO₂ uptake (NEP) is closely related to photosynthetic uptake, there is an increased importance of ecosystem respiration for determining NEP on longer time scales. 

The interannual evolution of net ecosystem CO₂ uptake (NEP) is insufficiently understood and often not well captured in state-of-the-art vegetation models and data products. This lack of understanding may in part be due to different drivers between interannual and seasonal scales affecting the two terms balancing NEP - photosynthetic uptake of CO₂ (GPP) and ecosystem respiration (R_eco).

Here, we extract timescale specific carbon flux dynamics at 20 long-running FLUXNET eddy covariance sites (>13 years) using time series decomposition to relate variability in GPP and Reco to NEP at subseasonal to interannual timescales. The results indicate that relations between NEP and GPP or R_eco, respectively, are not constant across different timescales, but that GPP and R_eco exert differential control on NEP between sub-seasonal, seasonal, and longer timescales.

• Overall, the fraction of variance in NEP explained by GPP variance is decreased at longer timescales, while the fraction of NEP variance explained by R_eco variance and by the covariance of GPP and R_eco generally increases at longer timescales.

• Regarding GPP, we find that the slopes between NEP and GPP, which could be interpreted as scale specific apparent carbon use efficiencies (NEP/GPP) are highest and most consistent at subseasonal scales, while generally smaller in magnitude and less constrained at interannual scales. This indicates that GPP and NEP are generally more strongly and directly linked at the subseasonalscale.

• Regarding R_eco, we find a positive relationship between NEP and R_eco at the seasonal scale. This is counterintuitive given NEP = GPP – R_eco, but similar to spatial relations in other studies and likely related to GPP seasonality as a common driver. In contrast, the subseasonal and interannual NEP-R_eco relations are mostly negative, as would be expected since higher respiratory loss would generally indicate lower ecosystem carbon retention, i.e. lower NEP. 

The timescale specific relations extracted here based on direct ecosystem CO₂ exchange measurements suggest an increased importance of ecosystem respiration for long-term carbon source or sink behavior for some ecosystems. These results give insight into ecosystem functioning, as well as demonstrate the utility of time series decomposition as a diagnostic of ecosystem dynamics at different timescales. Such information may eventually serve as a basis to infer turnover times of ecosystem carbon pools and better characterize interannual ecosystem carbon dynamics.

How to cite: Linscheid, N., Mahecha, M. D., Rammig, A., Bastos, A., Nelson, J. A., and Reichstein, M.: Increasing importance of ecosystem respiration for ecosystem carbon exchange dynamics from weekly to interannual timescales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8387, https://doi.org/10.5194/egusphere-egu23-8387, 2023.

Soil respiration is a highly complex process including a wide range of soil biota (autotrophic and heterotrophic functioning) and different pathways of carbon cycling (decomposition, arbon allocation), all being under the control of environmental and biotic drivers. The most important biotic driver is the photosynthetic activity of the vegetation providing supply mainly for the autotrophic component of soil respiration: plant roots and their symbiotic partners - such as arbuscular mycorrhizal fungi (AMF). By acting as a source of CO₂ and a pathway of carbon to the SOM, the role of AMF in carbon balance is unquestionable, not to mention that mycorrhizal C allocation could determine the long-term C storage potential of an ecosystem.

The objective of this study was to describe the time-lagged relationship between gross primary production (GPP) and the mycorrhizal soil respiration component, so to determine the amount of carbon derived from GPP appearing as mycorrhizal mycelial respiration. Measurements of CO₂ efflux were conducted in three different treatments – i) undisturbed, root and AMF-included (R_s), ii) root-excluded (R_{basal + myc}) and iii) root- and AMF-excluded (R_basal) plots – for three consecutive years in a Central-Hungarian dry sandy grassland between July 2011 and May 2014. GPP data were derived from eddy-covariance (EC) measurements, while an automated soil respiration system (SRS) consisting of ten chambers was used for continuous and long-term measurement of soil CO₂ efflux. We analysed the relationship between mycorrhizal mycelial respiration and GPP by using cross-correlation and GAMs (generalized additive models). Besides, we used sine wave models to describe the diel pattern of basal and mycorrhizal fungi respiration in the soil together with the diel patterns of soil temperature and GPP.

Considering the whole dataset correlation between GPP and mycorrhizal fungi respiration was highest at 13.5 hours time lag, while the average difference between peak timing of mycorrhizal fungi respiration and peak timing of GPP was 15 hours. However, the time lag and the peak timing difference varied from 10-24 hours. According to the results, carbon allocation to mycorrhizal fungi is a fast process in dry grasslands and the main driver of this respiration component is the GPP.

How to cite: De Luca, G., Papp, M., Fóti, S., Pintér, K., Mészáros, Á., Nagy, Z., and Balogh, J.: Dynamics of mycorrhizal respiration in relation to GPP in a Central-Hungarian dry grassland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13662, https://doi.org/10.5194/egusphere-egu23-13662, 2023.

Soil respiration of grasslands is highly variable both in time and space reflecting the topographic characteristics, the changing environmental constrains and biological activity of the vegetation. The aim of this study was to describe the effect of gross primary productivity (GPP) and soil organic carbon content (SOC) on soil respiration under varying environmental conditions in a dry grassland site in Hungary. We made spatially explicit measurements of variables including soil respiration, aboveground biomass, green vegetation index, soil water content, and soil temperature during an 8-year study in the vegetation periods. Sampling was conducted 23 times in 80 x 60 m grids of 10 m resolution with 78 sampling points. Altitude, slope, and soil organic carbon were used as background factors at each sampling position. Site-level GPP data were derived from eddy-covariance measurements and used for the estimation of GPP at every sampling position as a weighted metric on the basis of the biomass and green vegetation index of the positions. Data were analyzed using generalized additive models (GAM). Spatially, soil respiration had negative correlation with soil temperature, altitude and slope, while it was positively correlated with soil water content, aboveground biomass, green vegetation index and SOC. Soil respiration was significantly different between SOC groups (low-medium-high carbon content), mean soil respiration increased with soil carbon content. According to the GAM analysis, the shape of the GPP response was almost linear in each SOC groups and GPP had a strong influence on soil respiration in all of the groups besides temperature and soil water content. The results suggest that GPP and the resulting belowground carbon allocation affecting mainly the autotrophic components of soil respiration have similar influence on soil respiration as the main environmental variables.

How to cite: Balogh, J., Fóti, S., de Luca, G., Mészáros, Á., Pintér, K., and Nagy, Z.: Influence of gross primary productivity and soil carbon content on soil CO2 efflux in dry grasslands, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12082, https://doi.org/10.5194/egusphere-egu23-12082, 2023.

Herbaceous dominated ecosystems are found throughout the world representing a wide variety of locations that differ in their mean annual precipitation, annual temperature ranges, light availability, and soil nutrient concentrations. This is reflected in a wide range of annual above- and belowground biomass production, plant diversity and canopy density between these ecosystems worldwide. Our study aims to understand how nutrient availability and disturbance impact species relative abundance, diversity, and annual aboveground production. Here, we present background data on herbaceous dominated ecosystems found in coastal South Carolina, USA that are part of a global network (DragNet) of herbaceous dominated ecosystem study sites distributed across the world. Dominated by both grass species and forbs, we found that within a site, there is variation in the relationship between species diversity and biomass production. Aboveground biomass and accumulated litter are not correlated suggesting that the drivers for carbon assimilation and decomposition are different. This research will serve as the foundation for future research on how nutrients and soil disturbance interact with the amount of carbon assimilation and if some species show trait variation in response to these treatments in different regions of the world.

How to cite: O'Halloran, L.: Aboveground biomass production and litter accumulation in coastal grassland ecosystems: the basis of a global study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10322, https://doi.org/10.5194/egusphere-egu23-10322, 2023.