Non-structural Carbohydrate Pools Research Articles

Seasonal patterns of nonstructural carbohydrate storage and mobilization in two tree species with distinct life-history traits.

Nonstructural carbohydrates (NSC) are essential for tree growth and adaptation, yet our understanding of the seasonal storage and mobilization dynamics of whole-tree NSC is still limited, especially when tree functional types are involved. Here, Quercus acutissima Carruth. and Pinus massoniana Lamb. with distinct life-history traits (i.e., a deciduous broadleaf species vs. an evergreen coniferous species) were studied to assess the size and seasonal fluctuations of organ and whole-tree NSC pools with a focus on comparing differences in carbon resource mobilization patterns between the two species. We sampled the organs (leaf, branch, stem, and root) of the target trees repeatedly over four seasons of the year. Then, NSC concentrations in each organ were paired with biomass estimates from the allometric model to generate whole-tree NSC pools. The seasonal dynamics of the whole-tree NSC of Q. acutissima and P. massoniana reached the peak in autumn and summer, respectively. The starch pools of the two species were supplemented in the growing season while the soluble sugar pools were the largest in the dormant season. Seasonal dynamics of organ-level NSC concentrations and pools were affected by organ type and tree species, with above-ground organs generally increasing during the growing season and P. massoniana roots decreasing during the growing season. In addition, the whole-tree NSC pools of P. massoniana were larger but Q. acutissima showed larger seasonal fluctuations, indicating that larger storage was not associated with more pronounced seasonal fluctuations. We also found that the branch and root were the most dynamic organs of Q. acutissima and P. massoniana, respectively, and were the major suppliers of NSC to support tree growth activities. These results provide fundamental insights into the dynamics and mobilization patterns of NSC at the whole-tree level, and have important implications for investigating environmental adaptions of different tree functional types.

Evaluating key climatic and ecophysiological parameters of worldwide tree mortality with a process-based BGC model and machine learning algorithms

Drought-induced tree mortality has been increasing worldwide under climate change; therefore, forests will become more vulnerable as warming continues. Meanwhile, carbon starvation and hydraulic failure have been proposed as main drought-induced mortality mechanisms, mostly validated through individual tree-level experiments. However, there lack of a unified way to monitor and assess tree mortality across the different biomes and climate regions. In this sense, process-based biogeochemical (BGC) modeling may be an effective tool for simulating and understanding ecophysiological processes for tree mortality at large spatial scales. In this study, a hydraulic vulnerability curve for percentage loss of conductivity (PLC) was added to the BGC-NSCs model, the modified version of the BIOME-BGC with two additional non-structural carbohydrates (NSCs) pools. And then, we simulate the model at the sites around the world where tree mortality were reported. Using sensitivity analysis and machine learning algorithms for hydraulic stress, PLC and NSCs showed a high sensitivity and significance to tree mortality within the modeling framework. The model simulations also reveal the relationship between PLC and NSCs based on mortality stress intensity, plant functional types, and climate conditions, further validated with the results of previous experiment studies at the plot scale. This study proposes a potential to estimate eocphysiological variables at the regional scale using the BGC model, and to use high sensitivity variable, such as PLC and NSCs, as effective diagnostics for hydraulic stress across different biomes and climate regions.

Understory seedlings of Quercus mongolica survive by phenological escape

Understanding understory seedling regeneration mechanisms is important for the sustainable development of temperate primary forests in the context of increasingly intense climate warming events. The poor regeneration of dominant tree species, however, is one of the biggest challenges it faces at the moment. Especially, the regeneration of the shade-intolerant Quercus mongolica seedling is difficult in primary forests, which contrasts with the extreme abundance of understory seedlings in secondary forests. The mechanism behind the interesting phenomenon is still unknown. This study used in-situ monitoring and nursery-controlled experiment to investigate the survival rate, growth performance, as well as nonstructural carbohydrate (NSC) concentrations and pools of various organ tissues of seedlings for two consecutive years, further analyze the understory light availability and simulate the foliage carbon (C) gain in the secondary and primary forest. Results suggested that seedlings in the secondary forest had greater biomass allocation aboveground, height and specific leaf area (SLA) in summer, which allowed the seedling to survive longer in the canopy closure period. High light availability and positive C gain in early spring and late autumn are key factors affecting the growth and survival of understory seedlings in the secondary forest, whereas seedlings in the primary forest had annual negative carbon gain. Through the growing season, the total NSC concentrations of seedlings gradually decreased, whereas those of seedlings in the secondary forest increased significantly in autumn, and were mainly stored in roots for winter consumption and the following year's summer shade period, which was verified by the nursery-controlled experiment that simulated autumn enhanced light availability improved seedling survival rate and NSC pools. In conclusion, our results revealed the survival trade-off strategies of Quercus mongolica seedlings and highlighted the necessity of high light availability during the spring and autumn phenological periods for shade-intolerant tree seedling recruitment.

Nitrogen addition improves drought tolerance in Camellia japonica (Naidong) seedlings

The northernmost population of Camellia japonica globally is a unique ecotype within the species. Water deficit and atmospheric nitrogen deposition are two critical environmental factors that influence plant growth and primary productivity, particularly in the context of environmental change. However, the interactive effects of drought and nitrogen deposition on the physiology and growth of Naidong seedlings have not been investigated. To address this gap, we conducted a greenhouse experiment to investigate the individual and combined influences of different water and nitrogen (N) levels on growth, leaf morphology and physiology, and biomass and carbohydrate allocation of three-year-old Naidong seedlings. The results demonstrated that drought reduced height, basal diameter, specific leaf area, biomass accumulation, net photosynthesis, and non-structural carbohydrate (NSC) pool in the root of seedlings. However, water use efficiency (WUE) increased under slight drought conditions. Moderate (6 g N m−2 year−1) nitrogen addition enhanced seedling biomass accumulation, carbon fixation capacity, and WUE. Furthermore, a significant interaction between water and nitrogen deposition on Fv/Fm, soluble sugars concentration in the stem, and starch concentration in the root of Naidong seedlings was discovered. Drought and nitrogen deposition exhibited antagonistic effects on seedling biomass accumulation and photosynthetic capacity. The adverse impact of drought on growth and physiological responses could be partially compensated by increased nitrogen availability. This study highlights the need to include more variables when assessing a species' vulnerability to shifts in environmental conditions in the context of global change studies. Furthermore, our findings provide supportive data for the survival of C. japonica under future climate scenarios.

Modeling of non-structural carbohydrate dynamics by the spatially explicit individual-based dynamic global vegetation model SEIB-DGVM (SEIB-DGVM-NSC version 1.0)

Abstract. Forest dynamics need to be considered when estimating the global carbon budget. The alteration of forest structure and function under a changing climate and expanding human activity could lead to a reduction of forest canopy cover and a spread of lower-biomass ecosystems in warm and dry regions. A non-structural carbohydrate (NSC) acts as a storage buffer between carbon supplied by assimilation and carbon consumed by, inter alia, respiration, reproduction, and pests. Estimation of NSC concentrations in a tree is very important for accurate projection of future forest dynamics. We developed a new NSC module for incorporation into a spatially explicit individual-based dynamic global vegetation model (SEIB-DGVM) to validate the simulated NSC dynamics with observations. NSC pools were simulated in three plant organs: leaves, trunk, and roots. The seasonal dynamics of the NSCs varied among plant species, and the sizes of the NSC pools inferred from observations differed between the boreal, temperate, and tropical climates. The NSC models were therefore validated for each of the three climatic regions at both point and global scales to assess the performance of the models. The modeled NSCs showed good agreement in seasonality with the observed NSCs at four sites – Canada (boreal), Austria and Switzerland (temperate), and Panama (tropical) – and in mean values for three climate zones derived from the global NSC dataset. The SEIB-DGVM-NSC version 1.0 is expected to enable simulation of biome shifts caused by the changes in NSC dynamics worldwide. These dynamics will contribute to changes in not only the global carbon cycle but also in forest structure and demography at a global scale.

Carbon starvation following a decade of experimental drought consumes old reserves in Pinus edulis.

Shifts in the age or turnover time of non-structural carbohydrates (NSC) may underlie changes in tree growth under long-term increases in drought stress associated with climate change. But NSC responses to drought are challenging to quantify, due in part to large NSC stores in trees and subsequently long response times of NSC to climate variation. We measured NSC age (Δ14 C) along with a suite of ecophysiological metrics in Pinus edulis trees experiencing either extreme short-term drought (-90% ambient precipitation plot, 2020-2021) or a decade of severe drought (-45% plot, 2010-2021). We tested the hypothesis that carbon starvation - consumption exceeding synthesis and storage - increases the age of sapwood NSC. One year of extreme drought had no impact on NSC pool size or age, despite significant reductions in predawn water potential, photosynthetic rates/capacity, and twig and needle growth. By contrast, long-term drought halved the age of the sapwood NSC pool, coupled with reductions in sapwood starch concentrations (-75%), basal area increment (-39%), and bole respiration rates (-28%). Our results suggest carbon starvation takes time, as tree carbon reserves appear resilient to extreme disturbance in the short term. However, after a decade of drought, trees apparently consumed old stored NSC to support metabolism.

Non-structural carbohydrate concentrations in contrasting dry and wet years in early- and late-successional boreal forest trees

Key messagePatterns of non-structural carbohydrate allocation in dry and wet differ between birch and larch in southern boreal forest.An increasing area of boreal forests is currently switching from temperature to drought limitation. It is not sufficiently known how the trees’ non-structural carbohydrate (NSC) pools are affected by alternating dry and wet phases in drought-prone boreal forests and how NSC concentrations are related to growth responses. In the southern boreal forests of Mongolia, NSC concentrations (including soluble sugars and starch) were determined enzymatically in the stemwood of two tree species during a drought year (2017) that was preceded by another drought in 2015 and two subsequent wet years (2018/19). Betula platyphylla as a broadleaved pioneer tree showed reduced radial stem increment in the drought year and rapidly increasing growth in the following wet years. It had low concentrations of NSC and of soluble sugars in particular, suggesting that most assimilate were invested into growth in this early successional tree. Larix sibirica as a late-successional conifer also showed reduced growth in the drought year, but lagged reductions in NSC concentrations and less pronounced growth releases than birch in 2018/19, suggesting a longer aftereffect of the drought than in birch. Larch maintained much higher NSC concentrations throughout the three studied growing seasons, including the drought episode in 2017. The NSC pool in larch was primarily formed by soluble sugars in sapwood and heartwood. In contrast to birch, larch showed a high allocation priority for assimilates in soluble sugars before investment in biomass, which explains the aftereffect of the drought on both growth and NSC. We conclude that the high soluble sugar concentrations in larch are a key determinant of the extreme drought and cold tolerance of L. sibirica and, therefore, growth has lower allocation priority for carbon compared to birch.

Moisture stress limits radial mixing of non-structural carbohydrates in sapwood of trembling aspen.

Dynamics in non-structural carbohydrate (NSC) pools may underlie observed drought legacies in tree growth. We assessed how aridity influences the dynamics of different-aged NSC pools in tree sapwood at two sites with differing climate conditions ('wet' vs. 'dry') that also experienced widespread regional drought five years earlier. We used an incubation method to measure radiocarbon (Δ14C) in CO2 respired from Populus tremuloides (aspen) tree rings to evaluate NSC storage and mixing patterns, coupled with measurements of NSC (soluble sugars, starch) concentrations and respired δ13C-CO2. At a wet site, CO2 respired from rings formed during 1962-1967 was only ~ 11years old, suggesting deep sapwood mixing of NSCs as starch. At a dry site, total NSC was about one-third of wet site totals, maximum ages in deep rings were lower, and ages more rapidly increased in shallow rings then plateaued. These results suggest historically shallower mixing and/or relatively higher consumption of NSCs under dry conditions. Both sites, however, had similar aged NSC (<1yr) in the most recent six rings, indicative of deep radial mixing following relatively wet conditions during the sampling year. We suggest significant differences in NSC mixing among sites are driven by moisture stress, where aridity reduces NSC reserves and restricts the depth of radial mixing. However, dynamic climate conditions in the southwestern US resulted in more complex radial patterns of sapwood NSC age than previously described. We suggest a novel conceptual framework to understand how moisture variability might influence the dynamics of NSC mixing in the sapwood.

Stem photosynthesis contributes to non-structural carbohydrate pool and modulates xylem vulnerability to embolism in Fraxinus ornus L.

Stem photosynthesis can significantly contribute to the carbon budget of woody plants, providing an extra carbon gain that might be crucial under drought stress causing leaf photosynthesis impairment and/or a reduced phloem transport.Stems of Fraxinus ornus L. saplings were covered with aluminum foil to test the impact of inhibition of stem photosynthesis on plant vulnerability to drought. Plants were water-stressed to target xylem water potential of −3.5 MPa and were then re-irrigated to field capacity to quantify their recovery capacity. Vulnerability to xylem embolism was assessed in light-exposed and stem-shaded saplings with both the hydraulic method and in vivo with X-ray phase contrast micro-computed tomography. We also measured non-structural carbohydrate (NSC) concentration and osmotic potential in bark and wood, separately.Stem shading increased xylem vulnerability to embolism formation under drought but did not influence the recovery phase. This difference was coupled with modification of the NSC pool and impaired osmoregulation, in particular in the wood of stem-shaded saplings compared to control ones.Our results indicate stem photosynthesis as an important source of local NSCs, directly or indirectly involved in osmoregulation processes, which could be crucial to enhance the hydraulic resistance to embolism formation and to endure drought.

Root exudation of carbon and nitrogen compounds varies over the day-night cycle in pea: The role of diurnal changes in internal pools.

Rhizodeposition is the export of organic compounds from plant roots to the soil. Carbon allocation towards rhizodeposition has to be balanced with allocation for other physiological functions, which depend on both newly assimilated and stored nonstructural carbohydrate (NSC). To test whether the exudation of primary metabolites scales with plant NSC status, we studied diurnal dynamics of NSC and amino acid (AA) pools and fluxes within the plant and the rhizosphere. These diurnal dynamics were measured in the field and under hydroponic-controlled conditions. Further, C-limiting treatments offered further insight into the regulation of rhizodeposition. The exudation of primary metabolites fluctuated diurnally. The diurnal dynamics of soluble sugars (SS) and AA concentrations in tissues coincided with exudate pool fluctuations in the rhizosphere. SS and AA pools in the rhizosphere increased with NSC and AA pools in the roots. C starvation treatments offset the balance of exudates: AA exudate content in the rhizosphere significantly decreased while SS exudate content remained stable. Our results suggest that rhizodeposition is to some extent controlled by plant C:N status. We propose that SS exudation is less controlled than AA exudation because N assimilation depends on controlled C supply while SS exudation relies to a greater extent on passive diffusion mechanisms.

Data to the non-structural carbohydrates of the fructan accumulating Sphagnum species from the tropical Réunion Island in relation to their altitude zone and distribution

The prevalence of fructan-producing plants is primarily restricted to the temperate climate, while they are virtually absent from the tropical regions. Although the Sphagnum genus has a worldwide distribution and considerable ecological importance, it is relatively rare for the whole of tropical Africa. The island of Réunion is located in the tropical climate and has a rich Sphagnum flora containing 14 species. Sphagnum species - as an exception amongst mosses - synthesise fructan and have a considerable amount of sucrose as well. No data on fructan content of Sphagnum occuring in tropical habitats have been reported so far. This short paper provided data first, based on experimental results, on the non-structural carbohydrate pool, especially fructans in Sphagnum species from a tropical region, from Réunion Island. Fructans accounted for 2-4% of the total soluble carbohydrates in the examined Sphagnum species. We compared the fructan content of eight Sphagnum species collected from Réunion in relation to their altitude zone and their distribution. The species generally increased their fructan accumulation as the altitude increased. Our preliminary experimental results show that fructans as an alternative storage carbohydrate to starch may play a role in the ecological niche preference of the Sphagnum species according to altitudinal zones.

Using a Simple Representation of Non‐Structural Carbohydrates Allocation to Predict Wood Growth in Temperate Forests

AbstractWood growth is an important process of carbon sequestration and plant physiology in forest ecosystems. Hence, understanding and predicting wood growth is central to quantifying forest carbon cycling. Current dynamic global vegetation models (DGVMs) are mainly driven by carbon inputs (e.g., Gross Primary Production, GPP). However, observations indicate that wood growth is often independent of carbon flux at the annual scale. Here, we revised a DGVM, FORCCHN2 model, to use the active non‐structural carbohydrates (NSC) pool and slow NSC pool (i.e., represented temporary and long‐term NSC storage, respectively), and to integrate the stored NSC with annual wood growth. For the diameter increments (ΔDBH) and aboveground wood growth of 506 trees in 32 plots of Harvard Forest, we tested the NSC allocation coefficients from 5% to 95%. The predictions reproduced 31.2%–55.1% of individual‐tree ΔDBH and 36.8%–43.0% of the aboveground wood increment. Trees of shade‐intolerant species invested more of their available carbon resources (i.e., NSC storage) into wood growth than shade‐tolerant species. Specifically, the shade‐tolerant trees consumed approximately 32% NSC storage by wood growth at the annual scale, while the shade‐intolerant trees consumed about 90%. Our study provides a simple framework that constructs a direct link between annual wood growth and NSC dynamics. The findings highlight the need for research into NSC storage and allocation in trees, particularly in considering NSC allocation strategies of different tree species.

Non-structural carbohydrate dynamics and growth in tomato plants grown at fluctuating light and temperature.

Fluctuations in light intensity and temperature lead to periods of asynchrony between carbon (C) supply by photosynthesis and C demand by the plant organs. Storage and remobilization of non-structural carbohydrates (NSC) are important processes that allow plants to buffer these fluctuations. We aimed to test the hypothesis that C storage and remobilization can buffer the effects of temperature and light fluctuations on growth of tomato plants. Tomato plants were grown at temperature amplitudes of 3 or 10°C (deviation around the mean of 22°C) combined with integration periods (IP) of 2 or 10 days. Temperature and light were applied in Phase (high temperature simultaneously with high light intensity, (400 μmol m–2 s–1), low temperature simultaneously with low light intensity (200 μmol m–2 s–1) or in Antiphase (high temperature with low light intensity, low temperature with high light intensity). A control treatment with constant temperature (22°C) and a constant light intensity (300 μmol m–2 s–1) was also applied. After 20 days all treatments had received the same temperature and light integral. Differences in final structural dry weight were relatively small, while NSC concentrations were highly dynamic and followed changes of light and temperature (a positive correlation with decreasing temperature and increasing light intensity). High temperature and low light intensity lead to depletion of the NSC pool, but NSC level never dropped below 8% of the plant weight and this fraction was not mobilizable. Our results suggest that growing plants under fluctuating conditions do not necessarily have detrimental effects on plant growth and may improve biomass production in plants. These findings highlight the importance in the NSC pool dynamics to buffer fluctuations of light and temperature on plant structural growth.

Spatial variations and pools of non-structural carbohydrates in young Catalpa bungei undergoing different fertilization regimes.

Despite the importance of non-structural carbohydrates (NSC) for growth and survival in woody plants, we know little about whole-tree NSC storage. Here, Catalpa bungei trees fertilized using different schedules, including water and fertilizer integration, hole application, and no fertilization, were used to measure the spatial variations of sugar, starch, and NSC concentrations in the leaf, branch, stem, bark, and root. By calculating the volume of whole-tree NSC pools and the contribution of distinct organs, we were also able to compare the storage under various fertilization regimes. We found that the spatial distribution patterns of each organ undergoing different fertilization regimes were remarkably similar. Height-related increases in the sugar and NSC concentrations of the leaf and bark were observed. The concentrations of sugar and NSC in the branch did not appear to vary longitudinally or horizontally. The sugar and NSC concentrations in the stem fluctuated with height, first falling and then rising. The coarse root contained larger amounts of NSC components in comparison to fine root. Contrary to no fertilization, fertilization enhanced the distribution ratio of the leaf, branch, and stem NSC pools while decreasing the distribution ratio of the root NSC pool. Particularly, the addition of fertilizer and water significantly increased the biomass of the organs, enhancing the carbon sink of each organ and whole-tree in comparison to other fertilization regimes. Our main goal was to strengthen the empirical groundwork for comprehending the functional significance of NSC allocation and stock variations at the organ-level of C. bungei trees.

FORCCHN V2.0: an individual-based model for predicting multiscale forest carbon dynamics

Abstract. Process-based ecological models are essential tools to quantify and predict forest growth and carbon cycles under the background of climate change. The accurate description of phenology and tree growth processes enables an improved understanding and predictive modeling of forest dynamics. An individual tree-based carbon model, FORCCHN2 (Forest Ecosystem Carbon Budget Model for China version 2.0), used non-structural carbohydrate (NSC) pools to couple tree growth and phenology. This model performed well in reducing uncertainty when predicting forest carbon fluxes. Here, we describe the framework in detail and provide the source code of FORCCHN2. We also present a dynamic-link library (DLL) package containing the latest version of FORCCHN2. This package has the advantage of using Fortran as an interface to make the model run fast on a daily step, and the package also allows users to call it with their preferred computer tools (e.g., MATLAB, R, Python). FORCCHN2 can be used directly to predict spring and autumn phenological dates, daily carbon fluxes (including photosynthesis, aboveground and belowground autotrophic respiration, and soil heterotrophic respiration), and biomass on plot, regional, and hemispheric scales. As case studies, we provide an example of FORCCHN2 running model validations in 78 forest sites and an example model application for the carbon dynamics of Northern Hemisphere forests. We demonstrate that FORCCHN2 can produce a reasonable agreement with flux observations. Given the potential importance of the application of this ecological model in many studies, there is substantial scope for using FORCCHN2 in fields as diverse as forest ecology, climate change, and carbon estimations.

What drives forest carbon storage? The ramifications of source-sink decoupling.

This article is a Commentary on Martínez‐Sancho et al. (2022), 236: 58–70.

Mutually inclusive mechanisms of drought-induced tree mortality.

Unprecedented tree dieback across Central Europe caused by recent global change-type drought events highlights the need for a better mechanistic understanding of drought-induced tree mortality. Although numerous physiological risk factors have been identified, the importance of two principal mechanisms, hydraulic failure and carbon starvation, is still debated. It further remains largely unresolved how the local neighborhood composition affects individual mortality risk. We studied 9435 young trees of 12 temperate species planted in a diversity experiment in 2013 to assess how hydraulic traits, carbon dynamics, pest infestation, tree height and neighborhood competition influence individual mortality risk. Following the most extreme global change-type drought since record in 2018, one third of these trees died. Across species, hydraulic safety margins (HSMs) were negatively and a shift towards a higher sugar fraction in the non-structural carbohydrate (NSC) pool positively associated with mortality risk. Moreover, trees infested by bark beetles had a higher mortality risk, and taller trees a lower mortality risk. Most neighborhood interactions were beneficial, although neighborhood effects were highly species-specific. Species that suffered more from drought, especially Larix spp. and Betula spp., tended to increase the survival probability of their neighbors and vice versa. While severe tissue dehydration marks the final stage of drought-induced tree mortality, we show that hydraulic failure is interrelated with a series of other, mutually inclusive processes. These include shifts in NSC pools driven by osmotic adjustment and/or starch depletion as well as pest infestation and are modulated by the size and species identity of a tree and its neighbors. A more holistic view that accounts for multiple causes of drought-induced tree mortality is required to improve predictions of trends in global forest dynamics and to identify mutually beneficial species combinations.

Preferential allocation of carbohydrate reserves belowground supports disturbance-based management of American chestnut (Castanea dentata)

Non-structural carbohydrates (NSCs) are critical for the survival of trees, but the ability to accurately predict NSC levels for a given forest tree species is lacking. We evaluated NSC dynamics in American chestnut (Castanea dentata), a species of high restoration interest, to test the conventional model of seasonal and inter-organ dynamics and to facilitate informed conservation efforts. Chestnut trees were sampled over the course of one year at different phenological stages. Then, organ-specific NSC concentration data were paired with biomass estimates from a custom-built allometric model to generate NSC pool sizes. Organ-level NSC concentrations and pools generally peaked at leaf fall (October) and were lowest during shoot expansion (June), although interactions between organ type and collection period drove variation in pool sizes. Whole-tree NSC pools increased from 2.47 ± 0.42 kg at shoot expansion to 4.29 ± 0.65 kg at leaf fall. Coarse roots were the most important NSC reservoir, accounting for 46.5% − 56.6% of the total whole-tree NSC reserves, depending on the time of year. Coarse root NSC pools were larger and more dynamic than in previous studies with other temperate deciduous trees, and they were the primary supplier of NSCs to support spring leaf-out. Preferential belowground NSC allocation and the ability to mobilize root NSCs to fuel growth and metabolism suggest that chestnut could thrive under disturbance-based management.

Modeling temporal variations of non-structural carbohydrate (NSC) storages across biomes

Recent findings on non-structural carbohydrates (NSCs) highlight their importance for plant growth and survival. Process-based biogeochemical models (BGC) focusing on primary production hardly capture the lagged NSC-related effects on tree mortality and growth. This study modified BIOME-BGC by adding sugar and starch NSC pools to incorporate NSCs as a temporary carbon reservoir between production and growth. The modified BGC-NSCs were evaluated with field NSC and flux sites across different biomes. As a result, the BGC-NSCs simulated the daily variations in gross primary production (GPP), the monthly and interannual variations of starch with a mean R2 of 0.60, 0.55 and 0.62, respectively. The simulation demonstrates that the starch pool size regulates the coupling strength between production and growth, implicating the involvement of NSCs in tree growth and mortality lagging. Consequently, we proposed future modeling guidance for mapping and simulating NSC dynamics to analyze growth and die-back process under climate change.

Divergent allocations of nonstructural carbohydrates shape growth response to rainfall reduction in two subtropical plantations

Nonstructural carbohydrates (NSC) are indicators of tree carbon balance and play an important role in regulating plant growth and survival. However, our understanding of the mechanism underlying drought-induced response of NSC reserves remains limited. Here, we conducted a long-term throughfall exclusion (TFE) experiment to investigate the seasonal responses of NSC reserves to manipulative drought in two contrasting tree species (a broadleaved tree Castanopsis hystrix Miq. and a coniferous tree Pinus massoniana Lamb.) of the subtropical China. We found that in the dry season, the two tree species differed in their responses of NSC reserves to TFE at either the whole-tree level or by organs, with significantly depleted total NSC reserves in roots in both species. Under the TFE treatment, there were significant increases in the NSC pools of leaves and branches in C. hystrix, which were accompanied by significant decreases in fine root biomass and radial growth without significant changes in canopy photosynthesis; while P. massoniana exhibited significant increase in fine root biomass without significant changes in radial growth. Our results suggested that under prolonged water limitation, NSC usage for growth in C . hystrix is somewhat impaired, such that the TFE treatment resulted in NSC accumulation in aboveground organs (leaf and branch); whereas P . massoniana is capable of efficiently utilizing NSC reserves to maintain its growth under drought conditions. Our findings revealed divergent NSC allocations under experimental drought between the two contrasting tree species, which are important for better understanding the differential impacts of climate change on varying forest trees and plantation types in subtropical China.