Strong Coupling of Shoot Assimilation and Soil Respiration during Drought and Recovery Periods in Beech As Indicated by Natural Abundance δ13C Measurements

Extreme precipitation reduces the recent photosynthetic carbon isotope signal detected in ecosystem respiration in an old-growth temperate forest

The successful utilization of stable carbon isotope approaches in investigating forest carbon dynamics has relied on the assumption that the carbon isotope compositions (δ13C) therein have detectable temporal variations. However, interpreting the δ13C signal transfer can be challenging, given the complexities involved in disentangling the effect of a single environmental factor, the isotopic dilution effect from background CO2 and the lack of high-resolution δ13C measurements. In this study, we conducted continuous in situ monitoring of atmospheric CO2 (δ13Ca) across a canopy profile in an old-growth temperate forest in northeast China during the normal year 2020 and the wet year 2021. Both years exhibited similar temperature conditions in terms of both seasonal variations and annual averages. We tracked the natural carbon isotope composition from δ13Ca to photosynthate (δ13Cp) and to ecosystem respiration (δ13CReco). We observed significant differences in δ13Ca between the two years. Contrary to in 2020, in 2021 there was a δ13Ca valley in the middle of the growing season, attributed to surges in soil CO2 efflux induced by precipitation, while in 2020 values peaked during that period. Despite substantial and similar seasonal variations in canopy photosynthetic discrimination (Δ13Ccanopy) in the two years, the variability of δ13Cp in 2021 was significantly lower than in 2020, due to corresponding differences in δ13Ca. Furthermore, unlike in 2020, we found almost no changes in δ13CReco in 2021, which we ascribed to the imprint of the δ13Cp signal on above-ground respiration and, more importantly, to the contribution of stable δ13C signals from soil heterotrophic respired CO2. Our findings suggest that extreme precipitation can impede the detectability of recent photosynthetic δ13C signals in ecosystem respiration in forests, thus complicating the interpretation of above- and below-ground carbon linkage using δ13CReco. This study provides new insights for unravelling precipitation-related variations in forest carbon dynamics using stable isotope techniques.

Unveiling intra-population functional variability patterns in a European beech (Fagus sylvatica L.) population from the southern range edge: drought resistance, post-drought recovery and phenotypic plasticity

Understanding covariation patterns of drought resistance, post-drought recovery and phenotypic plasticity, and their variability at the intra-population level are crucial for predicting forest vulnerability to increasing aridity. This knowledge is particularly urgent at the trailing range edge since, in these areas, tree species are proximal to their ecological niche boundaries. While this proximity increases their susceptibility, these populations are recognized as valuable genetic reservoirs against environmental stressors. The conservation of this genetic variability is critical for the adaptive capacity of the species in the current context of climate change. Here we examined intra-population patterns of stem basal growth, gas exchange and other leaf functional traits in response to an experimental drought in seedlings of 16 open-pollinated families within a marginal population of European beech (Fagus sylvatica L.) from its southern range edge. We found a high degree of intra-population variation in leaf functional traits, photosynthetic performance, growth patterns and phenotypic plasticity in response to water availability. Low phenotypic plasticity was associated with higher resistance to drought. Both drought resistance and post-drought recovery of photosynthetic performance varied between maternal lines. However, drought resistance and post-drought recovery exhibited independent variation. We also found intra-population variation in stomatal sensitivity to soil drying, but it was not associated with either drought resistance or post-drought recovery. We conclude that an inverse relationship between phenotypic plasticity and drought resistance is not necessarily a sign of maladaptive plasticity, but rather it may reflect stability of functional performance and hence adaptation to withstand drought. The independent variation found between drought resistance and post-drought recovery should facilitate to some extent microevolution and adaption to increasing aridity. The observed variability in stomatal sensitivity to soil drying was consistent with previous findings at other scales (e.g., inter-specific variation, inter-population variation) that challenge the iso-anisohydric concept as a reliable surrogate of drought tolerance.

Individual and interactive effects of warming and nitrogen supply on CO2 fluxes and carbon allocation in subarctic grassland

Climate warming has been suggested to impact high latitude grasslands severely, potentially causing considerable carbon (C) losses from soil. Warming can also stimulate nitrogen (N) turnover, but it is largely unclear whether and how altered N availability impacts belowground C dynamics. Even less is known about the individual and interactive effects of warming and N availability on the fate of recently photosynthesized C in soil. On a 10-year geothermal warming gradient in Iceland, we studied the effects of soil warming and N addition on CO2 fluxes and the fate of recently photosynthesized C through CO2 flux measurements and a 13 CO2 pulse-labeling experiment. Under warming, ecosystem respiration exceeded maximum gross primary productivity, causing increased net CO2 emissions. N addition treatments revealed that, surprisingly, the plants in the warmed soil were N limited, which constrained primary productivity and decreased recently assimilated C in shoots and roots. In soil, microbes were increasingly C limited under warming and increased microbial uptake of recent C. Soil respiration was increased by warming and was fueled by increased belowground inputs and turnover of recently photosynthesized C. Our findings suggest that a decade of warming seemed to have induced a N limitation in plants and a C limitation by soil microbes. This caused a decrease in net ecosystem CO2 uptake and accelerated the respiratory release of photosynthesized C, which decreased the C sequestration potential of the grassland. Our study highlights the importance of belowground C allocation and C-N interactions in the C dynamics of subarctic ecosystems in a warmer world.

Tree organ growth and carbon allocation dynamics impact the magnitude and δ13C signal of stem and soil CO2 fluxes

Incomplete knowledge of carbon (C) allocation dynamics in trees hinders accurate modeling and future predictions of tree growth. We studied C allocation dynamics in a mature Pinus sylvestris L. dominated forest with a novel analytical approach, allowing the first comparison of: (i) magnitude and δ13C of shoot, stem and soil CO2 fluxes (Ashoot, Rstem and Rsoil), (ii) concentration and δ13C of compound-specific and/or bulk non-structural carbohydrates (NSCs) in phloem and roots and (iii) growth of stem and fine roots. Results showed a significant effect of phloem NSC concentrations on tracheid growth, and both variables significantly impacted Rstem. Also, concentrations of root NSCs, especially starch, had a significant effect on fine root growth, although no effect of root NSC concentrations or root growth was detected on Rsoil. Time series analysis between δ13C of Ashoot and δ13C of Rstem or δ13C of Rsoil revealed strengthened C allocation to stem or roots under high C demands. Furthermore, we detected a significant correlation between δ13C of Rstem and δ13C of phloem sucrose and glucose, but not for starch or water-soluble carbohydrates. Our results indicate the need to include C allocation dynamics into tree growth models. We recommend using compound-specific concentration and δ13C analysis to reveal C allocation processes that may not be detected by the conventional approach that utilizes bulk organic matter.

Warming and elevated CO2 intensify drought and recovery responses of grassland carbon allocation to soil respiration

Photosynthesis and soil respiration represent the two largest fluxes of CO₂ in terrestrial ecosystems and are tightly linked through belowground carbon (C) allocation. Drought has been suggested to impact the allocation of recently assimilated C to soil respiration; however, it is largely unknown how drought effects are altered by a future warmer climate under elevated atmospheric CO₂ (eT_eCO₂ ). In a multifactor experiment on managed C3 grassland, we studied the individual and interactive effects of drought and eT_eCO₂ (drought, eT_eCO₂ , drought × eT_eCO₂ ) on ecosystem C dynamics. We performed two in situ ¹³ CO₂ pulse-labeling campaigns to trace the fate of recent C during peak drought and recovery. eT_eCO₂ increased soil respiration and the fraction of recently assimilated C in soil respiration. During drought, plant C uptake was reduced by c. 50% in both ambient and eT_eCO₂ conditions. Soil respiration and the amount and proportion of ¹³ C respired from soil were reduced (by 32%, 70% and 30%, respectively), the effect being more pronounced under eT_eCO₂ (50%, 84%, 70%). Under drought, the diel coupling of photosynthesis and SR persisted only in the eT_eCO₂ scenario, likely caused by dynamic shifts in the use of freshly assimilated C between storage and respiration. Drought did not affect the fraction of recent C remaining in plant biomass under ambient and eT_eCO₂ , but reduced the small fraction remaining in soil under eT_eCO₂ . After rewetting, C uptake and the proportion of recent C in soil respiration recovered more rapidly under eT_eCO₂ compared to ambient conditions. Overall, our findings suggest that in a warmer climate under elevated CO₂ drought effects on the fate of recent C will be amplified and the coupling of photosynthesis and soil respiration will be sustained. To predict the future dynamics of terrestrial C cycling, such interactive effects of multiple global change factors should be considered.

Nitrate and ammonium differ in their impact on δ13C of plant metabolites and respired CO2 from tobacco leaves

The carbon isotopic composition (δ13C) of foliage is often used as proxy for plant performance. However, the effect of N O 3 - vs. N H 4 + supply on δ13C of leaf metabolites and respired CO2 is largely unknown. We supplied tobacco plants with a gradient of N O 3 - to N H 4 + concentration ratios and determined gas exchange variables, concentrations and δ13C of tricarboxylic acid (TCA) cycle intermediates, δ13C of dark-respired CO2, and activities of key enzymes nitrate reductase, malic enzyme and phosphoenolpyruvate carboxylase. Net assimilation rate, dry biomass and concentrations of organic acids and starch decreased along the gradient. In contrast, respiration rates, concentrations of intercellular CO2, soluble sugars and amino acids increased. As N O 3 - decreased, activities of all measured enzymes decreased. δ13C of CO2 and organic acids closely co-varied and were more positive under N O 3 - supply, suggesting organic acids as potential substrates for respiration. Together with estimates of intra-molecular 13C enrichment in malate, we conclude that a change in the anaplerotic reaction of the TCA cycle possibly contributes to 13C enrichment in organic acids and respired CO2 under N O 3 - supply. Thus, the effect of N O 3 - vs. N H 4 + on δ13C is highly relevant, particularly if δ13C of leaf metabolites or respiration is used as proxy for plant performance.

Drought and recovery effects on belowground respiration dynamics and the partitioning of recent carbon in managed and abandoned grassland

The supply of soil respiration with recent photoassimilates is an important and fast pathway for respiratory loss of carbon (C). To date it is unknown how drought and land-use change interactively influence the dynamics of recent C in soil-respired CO₂ . In an in situ common-garden experiment, we exposed soil-vegetation monoliths from a managed and a nearby abandoned mountain grassland to an experimental drought. Based on two ¹³ CO₂ pulse-labelling campaigns, we traced recently assimilated C in soil respiration during drought, rewetting and early recovery. Independent of grassland management, drought reduced the absolute allocation of recent C to soil respiration. Rewetting triggered a respiration pulse, which was strongly fuelled by C assimilated during drought. In comparison to the managed grassland, the abandoned grassland partitioned more recent C to belowground respiration than to root C storage under ample water supply. Interestingly, this pattern was reversed under drought. We suggest that these different response patterns reflect strategies of the managed and the abandoned grassland to enhance their respective resilience to drought, by fostering their resistance and recovery respectively. We conclude that while severe drought can override the effects of abandonment of grassland management on the respiratory dynamics of recent C, abandonment alters strategies of belowground assimilate investment, with consequences for soil-CO₂ fluxes during drought and drought-recovery.

Shoot δ13 C values as an indicator of cumulative stress: The role of re-watering during drought and salinity stress

RATIONALE

The carbon stable isotope composition (δ¹³ C value) of a plant can reflect prolonged drought and salinity, as different isotopic signals resulting from drought and salinity can be retained in plant tissue. Commonly, drought and salinity are interrupted by intermittent precipitation or irrigation. It remains unclear whether the δ¹³ C values reflect the cumulative duration of intermittent drought or salinity stress.

METHODS

Drought (5% and 10% polyethylene glycol) and salinity (35 mM and 85 mM NaCl) were imposed on annual ryegrass consistently or cyclically; throughout the treatments, the stress duration for cyclic drought/salinity was half that of the corresponding prolonged stress treatment. The shoot δ¹³ C values were measured using isotope ratio mass spectrometry.

RESULTS

Prolonged drought restrained growth and increased shoot δ¹³ C values relative to the control group. However, the shoot biomass was even lower under cyclic drought than under prolonged drought. Furthermore, the shoot δ¹³ C value under cyclic drought was close to that of the control group. The low NaCl concentration treatment actually enhanced shoot growth. The shoot δ¹³ C value varied with both duration and intensity of salinity across all groups.

CONCLUSIONS

The shoot δ¹³ C value in annual ryegrass did indicate cumulative stress from cyclic low salinity, but not that from cyclic drought, in a manner that was mediated by the effect of re-watering on the mass and allocation of the photosynthates produced during stress.

Drought impacts on tree phloem: from cell-level responses to ecological significance

On-going climate change is increasing the risk of drought stress across large areas worldwide. Such drought events decrease ecosystem productivity and have been increasingly linked to tree mortality. Understanding how trees respond to water shortage is key to predicting the future of ecosystem functions. Phloem is at the core of the tree functions, moving resources such as non-structural carbohydrates, nutrients, and defence and information molecules across the whole plant. Phloem function and ability to transport resources is tightly controlled by the balance of carbon and water fluxes within the tree. As such, drought is expected to impact phloem function by decreasing the amount of available water and new photoassimilates. Yet, the effect of drought on the phloem has received surprisingly little attention in the last decades. Here we review existing knowledge on drought impacts on phloem transport from loading and unloading processes at cellular level to possible effects on long-distance transport and consequences to ecosystems via ecophysiological feedbacks. We also point to new research frontiers that need to be explored to improve our understanding of phloem function under drought. In particular, we show how phloem transport is affected differently by increasing drought intensity, from no response to a slowdown, and explore how severe drought might actually disrupt the phloem transport enough to threaten tree survival. Because transport of resources affects other organisms interacting with the tree, we also review the ecological consequences of phloem response to drought and especially predatory, mutualistic and competitive relations. Finally, as phloem is the main path for carbon from sources to sink, we show how drought can affect biogeochemical cycles through changes in phloem transport. Overall, existing knowledge is consistent with the hypotheses that phloem response to drought matters for understanding tree and ecosystem function. However, future research on a large range of species and ecosystems is urgently needed to gain a comprehensive understanding of the question.

Stored root carbohydrates can maintain root respiration for extended periods

Tight coupling between below-ground autotrophic respiration and the availability of recently assimilated carbon (C) has become a paradigm in the ecophysiological literature. Here, we show that stored carbohydrates can decouple respiration from assimilation for prolonged periods by mobilizing reserves from transport roots to absorptive roots. We permanently disrupted the below-ground transfer of recently assimilated C using stem girdling and root trenching and measured soil CO₂ efflux for over 1 yr in longleaf pine (Pinus palustris), a species that has large reserves of stored carbohydrates in roots. Soil CO₂ efflux was not influenced by girdling or trenching through the 14-month observation period. Stored carbohydrate concentrations in absorptive roots were not affected by the disrupted supply of current photosynthate for over 1 yr; however, carbohydrate concentrations in transport roots decreased. Our results indicate that root respiration can be decoupled from recent canopy assimilation and that stored carbohydrates can be mobilized from transport roots to absorptive roots to maintain respiration for over 1 yr. This refines the current paradigm that canopy assimilation and below-ground respiration are tightly coupled and provides evidence of the mechanism and dynamics responsible for decoupling the above- and below-ground processes.

Resilient Leaf Physiological Response of European Beech (Fagus sylvatica L.) to Summer Drought and Drought Release

Drought is a major environmental constraint to trees, causing severe stress and thus adversely affecting their functional integrity. European beech (Fagus sylvatica L.) is a key species in mesic forests that is commonly expected to suffer in a future climate with more intense and frequent droughts. Here, we assessed the seasonal response of leaf physiological characteristics of beech saplings to drought and drought release to investigate their potential to recover from the imposed stress and overcome previous limitations. Saplings were transplanted to model ecosystems and exposed to a simulated summer drought. Pre-dawn water potentials (ψpd), stomatal conductance (gS), intercellular CO2 concentration (ci), net-photosynthesis (AN), PSII chlorophyll fluorescence (PItot), non-structural carbohydrate concentrations (NSC; soluble sugars, starch) and carbon isotope signatures were measured in leaves throughout the growing season. Pre-dawn water potentials (ψpd), gS, ci, AN, and PItot decreased as drought progressed, and the concentration of soluble sugars increased at the expense of starch. Carbon isotopes in soluble sugars (δ13CS) showed a distinct increase under drought, suggesting, together with decreased ci, stomatal limitation of AN. Drought effects on ψpd, ci, and NSC disappeared shortly after re-watering, while full recovery of gS, AN, and PItot was delayed by 1 week. The fast recovery of NSC was reflected by a rapid decay of the drought signal in δ13C values, indicating a rapid turnover of assimilates and a reactivation of carbon metabolism. After recovery, the previously drought-exposed saplings showed a stimulation of AN and a trend toward elevated starch concentrations, which counteracted the previous drought limitations. Overall, our results suggest that the internal water relations of beech saplings and the physiological activity of leaves are restored rapidly after drought release. In the case of AN, stimulation after drought may partially compensate for limitations on photosynthetic activity during drought. Our observations suggest high resilience of beech to drought, contradicting the general belief that beech is particularly sensitive to environmental stressors.