Dying piece by piece: carbohydrate dynamics in aspen (Populus tremuloides) seedlings under severe carbon stress

Residual water losses mediate the trade-off between growth and drought survival across saplings of 12 tropical rainforest tree species with contrasting hydraulic strategies

Knowledge of the physiological mechanisms underlying species vulnerability to drought is critical for better understanding patterns of tree mortality. Investigating plant adaptive strategies to drought should thus help to fill this knowledge gap, especially in tropical rainforests exhibiting high functional diversity. In a semi-controlled drought experiment using 12 rainforest tree species, we investigated the diversity in hydraulic strategies and whether they determined the ability of saplings to use stored non-structural carbohydrates during an extreme imposed drought. We further explored the importance of water- and carbon-use strategies in relation to drought survival through a modelling approach. Hydraulic strategies varied considerably across species with a continuum between dehydration tolerance and avoidance. During dehydration leading to hydraulic failure and irrespective of hydraulic strategies, species showed strong declines in whole-plant starch concentrations and maintenance, or even increases in soluble sugar concentrations, potentially favouring osmotic adjustments. Residual water losses mediated the trade-off between time to hydraulic failure and growth, indicating that dehydration avoidance is an effective drought-survival strategy linked to the 'fast-slow' continuum of plant performance at the sapling stage. Further investigations on residual water losses may be key to understanding the response of tropical rainforest tree communities to climate change.

The mobilization and transport of newly fixed carbon are driven by plant water use in an experimental rainforest under drought

Non-structural carbohydrates (NSCs) are building blocks for biomass and fuel metabolic processes. However, it remains unclear how tropical forests mobilize, export, and transport NSCs to cope with extreme droughts. We combined drought manipulation and ecosystem 13CO2 pulse-labeling in an enclosed rainforest at Biosphere 2, assessed changes in NSCs, and traced newly assimilated carbohydrates in plant species with diverse hydraulic traits and canopy positions. We show that drought caused a depletion of leaf starch reserves and slowed export and transport of newly assimilated carbohydrates below ground. Drought effects were more pronounced in conservative canopy trees with limited supply of new photosynthates and relatively constant water status than in those with continual photosynthetic supply and deteriorated water status. We provide experimental evidence that local utilization, export, and transport of newly assimilated carbon are closely coupled with plant water use in canopy trees. We highlight that these processes are critical for understanding and predicting tree resistance and ecosystem fluxes in tropical forest under drought.

Tracing carbon and nitrogen reserve remobilization during spring leaf flush and growth following defoliation

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. However, we have a poor understanding of how reserves are regulated and whether distance between source and sink tissues affects remobilization. This leads to uncertainty about which reserves-and how much-are available to support plant functions like leaf growth. To better understand the source of remobilized reserves and constraints on their allocation, we created aspen saplings with organ-specific labeled reserves by using stable isotopes (13C,15N) and grafting unlabeled or labeled stems to labeled or unlabeled root stocks. We first determined which organs had imported root or stem-derived C and N reserves after spring leaf flush. We then further tested spatial and temporal variation in reserve remobilization and import by comparing 1) upper and lower canopy leaves, 2) early and late leaves, and 3) early flush and re-flush leaves after defoliation. During spring flush, remobilized root C and N reserves were preferentially allocated to sinks closer to the reserve source (i.e., lower vs upper canopy leaves). However, the reduced import of 13C in late versus early leaves indicates reliance on C reserves declined over time. Following defoliation, re-flush leaves imported the same proportion of root N as spring flush leaves, but they imported a lower proportion of root C. This lower import of reserve C suggests that, after defoliation, leaf re-flush rely more heavily on current photosynthate, which may explain the reduced leaf mass recovery of re-flush canopies (31% of initial leaf mass). The reduced reliance on reserves occurred even though roots retained significant starch concentrations (~5% dry wt), suggesting aspen prioritizes the maintenance of root reserves at the expense of fast canopy recovery.

Divergent effects of warming on nonstructural carbohydrates in woody plants: a meta-analysis

Nonstructural carbohydrates (NSC, including soluble sugars and starch) are essential for supporting growth and survival of woody plants, and play multifunctional roles in various ecophysiological processes that are being rapidly changed by climate warming. However, it still remains unclear whether there is a consistent response pattern of NSC dynamics in woody plants to climate warming across organ types and species taxa. Here, based on a compiled database of 52 woody plant species worldwide, we conducted a meta-analysis to investigate the effects of experimental warming on NSC dynamics. Our results indicated that the responses of NSC dynamics to warming were primarily driven by the fluctuations of starch, while soluble sugars did not undergo significant changes. The effects of warming on NSC shifted from negative to positive with the extension of warming duration, while the negative warming effects on NSC became more pronounced as warming magnitude increased. Overall, our study showed the divergent responses of NSC and its components in different organs of woody plants to experimental warming, suggesting a potentially changed carbon (C) balance in woody plants in future global warming. Thus, our findings highlight that predicting future changes in plant functions and terrestrial C cycle requires a mechanism understanding of how NSC is linked to a specific global change driver.

Populus root salicinoid phenolic glycosides are not mobilized to support metabolism and regrowth under carbon limited conditions

Remobilization of carbon storage compounds in trees is crucial for the resilience to disturbances, stress, and the requirements of their perennial lifestyle, all of which can impact photosynthetic carbon gain. Trees contain abundant non-structural carbohydrates (NSC) in the form of starch and sugars for long term carbon storage, yet questions remain about the ability of trees to remobilize non-conventional carbon compounds under stress. Aspens, like other members of the genus Populus, have abundant specialized metabolites called salicinoid phenolic glycosides, which contain a core glucose moiety. In this study, we hypothesized that the glucose-containing salicinoids could be remobilized as an additional carbon source during severe carbon limitation. We made use of genetically modified hybrid aspen (Populus tremula x P. alba) with minimal salicinoid content and compared these to control plants with high salicinoid content during resprouting (suckering) in dark (carbon limited) conditions. As salicinoids are abundant anti-herbivore compounds, identification of such a secondary function for salicinoids may provide insight to the evolutionary pressures that drive their accumulation. Our results show that salicinoid biosynthesis is maintained during carbon limitation and suggests that salicinoids are not remobilized as a carbon source for regenerating shoot tissue. However, we found that salicinoid-producing aspens had reduced resprouting capacity per available root biomass when compared to salicinoid-deficient aspens. Therefore, our work shows that the constitutive salicinoid production in aspens can reduce the capacity for resprouting and survival in carbon limited conditions.

Root water uptake depth determines the hydraulic vulnerability of temperate European tree species during the extreme 2018 drought

We took advantage of the European 2018 drought and assessed the mechanisms causing differences in drought vulnerability among mature individuals of nine co-occurring tree species at the Swiss Canopy Crane II site in Switzerland. Throughout the drought we monitored leaf water status and determined native embolism formation in the canopy of the trees as indicators of drought vulnerability. We also determined hydraulic vulnerability thresholds (Ψ₁₂ -, Ψ₅₀ - and Ψ₈₈ -values), corresponding hydraulic safety margins (HSMs) and carbohydrate reserves for all species as well as total average leaf area per tree, and used stable isotopes to assess differences in root water uptake depth among the nine species as variables predicting differences in drought vulnerability among species. Marked differences in drought vulnerability were observed among the nine tree species. Six species maintained their water potentials above hydraulic thresholds, while three species, Fagus sylvatica, Carpinus betulus and Picea abies, were pushed beyond their hydraulic thresholds and showed loss of hydraulic conductivity in their canopies at the end of the drought. Embolism resistance thresholds and associated HSMs did not explain why the co-existing species differed in their drought vulnerability, neither did their degree of isohydry, nor their regulation of carbohydrate reserves. Instead, differences in structural-morphological traits, in particular root water uptake depth, were associated with the risk of reaching hydraulic vulnerability thresholds and embolism formation among the nine species. Our study shows that structural-morphological traits, such as root water uptake depth, determine how quickly different species approach hydraulic vulnerability thresholds during a drought event and can thus explain species differences in drought vulnerability among mature field-grown trees.

Warming and elevated CO2 alter tamarack C fluxes, growth and mortality: evidence for heat stress-related C starvation in the absence of water stress

Climate warming is increasing the frequency of climate-induced tree mortality events. While drought combined with heat is considered the primary cause of this mortality, little is known about whether moderately high temperatures alone can induce mortality, or whether rising CO2 would prevent mortality at high growth temperatures. We grew tamarack (Larix laricina) under ambient (400 p.p.m.) and elevated (750 p.p.m.) CO2 concentrations combined with ambient, ambient +4 °C and ambient +8 °C growth temperatures to investigate whether high growth temperatures lead to carbon (C) limitations and mortality. Growth at +8 °C led to 40% mortality in the ambient CO2 (8TAC) treatment, but no mortality in the elevated CO2 treatment. Thermal acclimation of respiration led to similar leaf C balances across the warming treatments, despite a lack of photosynthetic acclimation. Photosynthesis was stimulated under elevated CO2, increasing seedling growth, but not leaf C concentrations. However, growth and foliar C concentrations were lowest in the +8 °C treatments, even with elevated CO2. Dying 8TAC seedlings had lower needle C concentrations and lower ratios of photosynthesis to respiration than healthy 8TAC seedlings, indicating that C limitations were likely the cause of seedling mortality under high growth temperatures.

Negative effects of long-term phosphorus additions on understory plants in a primary tropical forest

Human activities have disturbed global phosphorus (P) cycling by introducing substantial amounts of P to natural ecosystems. Although natural P gradients and fertilization studies have found that plant community traits are closely related to P availability, it remains unclear how increased P supply affects plant growth and diversity in P-deficient tropical forests. We used a decadal P-addition experiment (2007-2017) to study the effects of increased P input on plant growth and diversity in understory layer in tropical forests. We monitored the dynamics of seedling growth, survival rate, and diversity of understory plants throughout the fertilization period under control and P addition at 15 g P m^-2 yr^-1. To identify the drivers of responses, P concentration, photosynthesis rate and nonstructural carbon were analyzed. Results showed that long-term P addition significantly increased P concentrations both in soil pools and plant tissues. However, P addition did not increase the light-saturated photosynthesis rate or growth rate of the understory plants. Furthermore, P addition significantly decreased the survival rate of seedlings and reduced the species richness and density of understory plants. The negative effects of P addition may be attributed to an increased carbon cost due to the tissue maintenance of plants with higher P concentrations. These findings indicate that increased P supply alone is not necessary to benefit the growth of plants in ecosystems with low P availability, and P inputs can inhibit understory plants and may alter community composition. Therefore, we appeal to a need for caution when inputting P to tropical forests ecosystems.

Elevated CO2 causes different growth stimulation, water- and nitrogen-use efficiencies, and leaf ultrastructure responses in two conifer species under intra- and interspecific competition

The continuously increasing atmospheric carbon dioxide concentration ([CO2]) has substantial effects on plant growth, and on the composition and structure of forests. However, how plants respond to elevated [CO2] (e[CO2]) under intra- and interspecific competition has been largely overlooked. In this study, we employed Abies faxoniana Rehder & Wilson and Picea purpurea Mast. seedlings to explore the effects of e[CO2] (700 p.p.m.) and plant-plant competition on plant growth, physiological and morphological traits, and leaf ultrastructure. We found that e[CO2] stimulated plant growth, photosynthesis and nonstructural carbohydrates (NSC), affected morphological traits and leaf ultrastructure, and enhanced water- and nitrogen (N)- use efficiencies in A. faxoniana and P. purpurea. Under interspecific competition and e[CO2], P. purpurea showed a higher biomass accumulation, photosynthetic capacity and rate of ectomycorrhizal infection, and higher water- and N-use efficiencies compared with A. faxoniana. However, under intraspecific competition and e[CO2], the two conifers showed no differences in biomass accumulation, photosynthetic capacity, and water- and N-use efficiencies. In addition, under interspecific competition and e[CO2], A. faxoniana exhibited higher NSC levels in leaves as well as more frequent and greater starch granules, which may indicate carbohydrate limitation. Consequently, we concluded that under interspecific competition, P. purpurea possesses a positive growth and adjustment strategy (e.g. a higher photosynthetic capacity and rate of ectomycorrhizal infection, and higher water- and N-use efficiencies), while A. faxoniana likely suffers from carbohydrate limitation to cope with rising [CO2]. Our study highlights that plant-plant competition should be taken into consideration when assessing the impact of rising [CO2] on the plant growth and physiological performance.

Seeking the "point of no return" in the sequence of events leading to mortality of mature trees

Drought-related tree mortality is increasing globally, but the sequence of events leading to it remains poorly understood. To identify this sequence, we used a 2016 tree mortality event in a semi-arid pine forest where dendrometry and sap flow measurements were carried out in 31 trees, of which seven died. A comparative analysis revealed three stages leading to mortality. First, a decrease in tree diameter in all dying trees, but not in the surviving trees, 8 months "prior to the visual signs of mortality" (PVSM; e.g., near complete canopy browning). Second, a decay to near zero in the diurnal stem swelling/shrinkage dynamics, reflecting the loss of stem radial water flow in the dying trees, 6 months PVSM. Third, cessation of stem sap flow 3 months PVSM. Eventual mortality could therefore be detected long before visual signs were observed, and the three stages identified here demonstrated the differential effects of drought on stem growth, water storage capacity and soil water uptake. The results indicated that breakdown of stem radial water flow and phloem function is a critical element in defining the "point of no return" in the sequence of events leading to mortality of mature trees.

Declining carbohydrate content of Sitka-spruce treesdying from seawater exposure

Increasing sea levels associated with climate change threaten the survival of coastal forests, yet the mechanisms by which seawater exposure causes tree death remain poorly understood. Despite the potentially crucial role of nonstructural carbohydrate (NSC) reserves in tree survival, their dynamics in the process of death under seawater exposure are unknown. Here we monitored progressive tree mortality and associated NSC storage in Sitka-spruce (Picea sitchensis) trees dying under ecosystem-scale increases in seawater exposure in western Washington, USA. All trees exposed to seawater, because of monthly tidal intrusion, experienced declining crown foliage during the sampling period, and individuals with a lower percentage of live foliated crown (PLFC) died faster. Tree PLFC was strongly correlated with subsurface salinity and needle ion contents. Total NSC concentrations in trees declined remarkably with crown decline, and reached extremely low levels at tree death (2.4% and 1.6% in leaves and branches, respectively, and 0.4% in stems and roots). Starch in all tissues was almost completely consumed, while sugars remained at a homeostatic level in foliage. The decreasing NSC with closer proximity to death and near zero starch at death are evidences that carbon starvation occurred during Sitka-spruce mortality during seawater exposure. Our results highlight the importance of carbon storage as an indicator of tree mortality risks under seawater exposure.

Temporal controls on crown nonstructural carbohydrates in southwestern US tree species

In trees, large uncertainties remain in how nonstructural carbohydrates (NSCs) respond to variation in water availability in natural, intact ecosystems. Variation in NSC pools reflects temporal fluctuations in supply and demand, as well as physiological coordination across tree organs in ways that differ across species and NSC fractions (e.g., soluble sugars vs starch). Using landscape-scale crown (leaves and twigs) NSC concentration measurements in three foundation tree species (Populus tremuloides, Pinus edulis, Juniperus osteosperma), we evaluated in situ, seasonal variation in NSC responses to moisture stress on three timescales: short-term (via predawn water potential), seasonal (via leaf δ13C) and annual (via current year's ring width index). Crown NSC responses to moisture stress appeared to depend on hydraulic strategy, where J. osteosperma appears to regulate osmotic potentials (via higher sugar concentrations), P. edulis NSC responses suggest respiratory depletion and P. tremuloides responses were consistent with direct sink limitations. We also show that overly simplistic models can mask seasonal and tissue variation in NSC responses, as well as strong interactions among moisture stress at different timescales. In general, our results suggest large seasonal variation in crown NSC concentrations reflecting the multiple cofunctions of NSCs in plant tissues, including storage, growth and osmotic regulation of hydraulically vulnerable leaves. We emphasize that crown NSC pool size cannot be viewed as a simple physiological metric of stress; in situ NSC dynamics are complex, varying temporally, across species, among NSC fractions and among tissue types.

Splitting the Difference: Heterogeneous Soil Moisture Availability Affects Aboveground and Belowground Reserve and Mass Allocation in Trembling Aspen

When exploring the impact of resource availability on perennial plants, artificial treatments often apply conditions homogeneously across space and time, even though this rarely reflects conditions in natural systems. To investigate the effects of spatially heterogeneous soil moisture on morphological and physiological responses, trembling aspen (Populus tremuloides) saplings were used in a split-pot experiment. Following the division of the root systems, saplings were established for a full year and then subjected to either heterogeneous (portion of the root system exposed to non-lethal drought) or homogeneous (whole root system exposed to non-lethal drought or well-watered) treatments. Above- and belowground growth and non-structural carbohydrate (NSC) reserves (soluble sugars and starch) were measured to determine how allocation of reserves and mass between and within organs changed in response to variation in soil moisture availability. In contrast to saplings in the homogeneous drought treatment, which experienced reduced shoot growth, leaf abscission and fine root loss, saplings exposed to the heterogeneous conditions maintained similar aboveground growth and increased root system allocation compared to well-watered saplings. Interestingly under heterogeneous soil moisture conditions, the portion of the root system that was resource limited had no root dieback and increased carbon reserve concentrations, while the portion of the root system that was not resource limited added new roots (30% increase). Overall, saplings subjected to the heterogeneous soil moisture regime over-compensated belowground, both in mass and NSC reserves. These results indicate that the differential allocation of mass or reserves between above- and belowground organs, but also within the root system can occur. While the mechanisms and processes involved in these patterns are not clear, these responses could be interpreted as adaptations and acclimations to preserve the integrity of the entire sapling and suggests that different portions of plant organs might respond autonomously to local conditions. This study provides further appreciation of the complexity of the mechanisms by which plants manage heterogeneous conditions and offers evidence that spatial and temporal variability of resource availability, particularly belowground, needs to be accounted for when extrapolating and modeling stress responses at larger temporal and spatial scales.

Woody tissue photosynthesis delays drought stress in Populus tremula trees and maintains starch reserves in branch xylem tissues

Photosynthesis in woody tissues (P_wt ) is less sensitive to water shortage than in leaves, hence, P_wt might be a crucial carbon source to alleviate drought stress. To evaluate the impact of P_wt on tree drought tolerance, woody tissues of 4-m-tall drought-stressed Populus tremula trees were subjected to a light-exclusion treatment across the entire plant to inhibit P_wt . Xylem water potential (Ψ_xylem ), sap flow ( FH2O ), leaf net photosynthesis (P_n,l ), stem diameter variations (ΔD), in vivo acoustic emissions in stems (AEs) and nonstructural carbohydrate concentrations ([NSC]) were monitored to comprehensively assess water and carbon relations at whole-tree level. Under well-watered conditions, P_wt kept Ψ_xylem at a higher level, lowered FH2O and had no effect on [NSC]. Under drought, Ψ_xylem , FH2O and P_n,l in light-excluded trees rapidly decreased in concert with reductions in branch xylem starch concentration. Moreover, sub-daily patterns of ΔD, FH2O and AEs were strongly related, suggesting that in vivo AEs may inform not only about embolism events, but also about capacitive release and replenishment of stem water pools. Results highlight the importance of P_wt in maintaining xylem hydraulic integrity under drought conditions and in sustaining NSC pools to potentially limit increases in xylem tension.

Production of constitutive and induced secondary metabolites is coordinated with growth and storage in Norway spruce saplings

A mechanistic understanding of how trees balance the trade-offs between growth, storage and defense is limited but crucial for predicting tree responses to abiotic and biotic stresses. Here we investigated how trees allocate storage of non-structural carbohydrates (NSC) to growth and constitutive and induced secondary metabolites (SM). We exposed Norway spruce (Picea abies) saplings to 5 weeks of complete darkness to induce light and/or carbon limitation and then applied methyl jasmonate (MeJA) to simulate biotic attack. We measured changes in biomass, NSC (sum of soluble sugars and starches), and constitutive and induced SM (sum of phenolic compounds and terpenoids) in current-year developing and previous-year mature needles and branches, as well as volatiles emitted from the canopy. Under darkness, NSC storage was preferentially used for constitutive biosynthesis of monoterpenes rather than biosynthesis of stilbenes and growth of developing organs, while SM stored in mature organs cannot be remobilized and recycled. Furthermore, MeJA-induced production of SM was constrained by low NSC availability in developing organs but not in mature organs grown in the dark. Emissions of volatiles were suppressed in the dark but after 1 h of re-illumination, emissions of both constitutive and induced monoterpene hydrocarbons recovered rapidly, whereas emissions of linalool and sesquiterpene produced via de novo synthesis did not recover. Our results highlight that light and/or carbon limitation may constrain constitutive and JA-induced biosynthesis of SM in coordination with growth, NSC storage and mobilization.

No carbon limitation after lower crown loss in Pinus radiata

BACKGROUND AND AIMS

Biotic and abiotic stressors can cause different defoliation patterns within trees. Foliar pathogens of conifers commonly prefer older needles and infection with defoliation that progresses from the bottom crown to the top. The functional role of the lower crown of trees is a key question to address the impact of defoliation caused by foliar pathogens.

METHODS

A 2 year artificial defoliation experiment was performed using two genotypes of grafted Pinus radiata to investigate the effects of lower-crown defoliation on carbon (C) assimilation and allocation. Grafts received one of the following treatments in consecutive years: control-control, control-defoliated, defoliated-control and defoliated-defoliated.

RESULTS

No upregulation of photosynthesis either biochemically or through stomatal control was observed in response to defoliation. The root:shoot ratio and leaf mass were not affected by any treatment, suggesting prioritization of crown regrowth following defoliation. In genotype B, defoliation appeared to impose C shortage and caused reduced above-ground growth and sugar storage in roots, while in genotype A, neither growth nor storage was altered. Root C storage in genotype B decreased only transiently and recovered over the second growing season.

CONCLUSIONS

In genotype A, the contribution of the lower crown to the whole-tree C uptake appears to be negligible, presumably conferring resilience to foliar pathogens affecting the lower crown. Our results suggest that there is no C limitation after lower-crown defoliation in P. radiata grafts. Further, our findings imply genotype-specific defoliation tolerance in P. radiata.

No preferential carbon-allocation to storage over growth in clipped birch and oak saplings

Herbivory is one of the most globally distributed disturbances affecting carbon (C)-cycling in trees, yet our understanding of how it alters tree C-allocation to different functions such as storage, growth or rhizodeposition is still limited. Prioritized C-allocation to storage replenishment vs growth could explain the fast recovery of C-storage pools frequently observed in growth-reduced defoliated trees. We performed continuous 13C-labeling coupled to clipping to quantify the effects of simulated browsing on the growth, leaf morphology and relative allocation of stored vs recently assimilated C to the growth (bulk biomass) and non-structural carbohydrate (NSC) stores (soluble sugars and starch) of the different organs of two tree species: diffuse-porous (Betula pubescens Ehrh.) and ring-porous (Quercus petraea [Matt.] Liebl.). Carbon-transfers from plants to bulk and rhizosphere soil were also evaluated. Clipped birch and oak trees shifted their C-allocation patterns above-ground as a means to recover from defoliation. However, such increased allocation to current-year stems and leaves did not entail reductions in the allocation to the rhizosphere, which remained unchanged between clipped and control trees of both species. Betula pubescens and Q. petraea showed differences in their vulnerability and recovery strategies to clipping, the ring-porous species being less affected in terms of growth and architecture by clipping than the diffuse-porous. These contrasting patterns could be partly explained by differences in their C cycling after clipping. Defoliated oaks showed a faster recovery of their canopy biomass, which was supported by increased allocation of new C, but associated with large decreases in their fine root biomass. Following clipping, both species recovered NSC pools to a larger extent than growth, but the allocation of 13C-labeled photo-assimilates into storage compounds was not increased as compared with controls. Despite their different response to clipping, our results indicate no preventative allocation into storage occurred during the first year after clipping in either of the species.

Root Traits Determine Variation in Nonstructural Carbohydrates (NSCs) under Different Drought Intensities and Soil Substrates in Three Temperate Tree Species

Nonstructural carbohydrates (NSCs) are a key factor in the physiological regulation of plants and can reflect buffering capacity of plants under diverse environmental conditions. The effects of diverse environmental conditions on plant NSCs and tissue or organ scales have been thoroughly studied, but their effects on fine root (root diameter < 2 mm) NSC concentrations are still not completely understood. Our aims were to explore the synergistic fluctuations in root traits and NSC concentrations under diverse environmental conditions. This study was conducted on two-year-old temperate seedling tree species (Juglans mandshurica Maxim., Fraxinus mandshurica Rupr., and Phellodendron amurense Rupr.) with different drought intensities and soil substrates. The specific root length (SRL) and specific root surface area (SRA) were significantly affected by drought intensities and soil substrates, while the root tissue density (RTD) and average diameter (AD) were not significantly affected by water intensities and soil substrates in all three species. The root C, N, and P concentration did not change according to drought stress but were significantly affected by the soil substrates in all three species. Similarly, the soluble sugar (SS) and starch (ST) concentrations were significantly affected by both the drought stress and the soil substrates in all three species. The AD explained 6.8% of the total variations in soluble sugar, while the SRL explains 32.1% of the total variation in starch. The root tip C, N, and P concentrations were not significantly correlated with NSCs under different treatments. The total variations in root tip morphology, chemistry, and NSC concentrations are greater among species than compared to different drought intensities and soil substrates. However, the root NSC concentrations were closely related to root morphological traits (SRL and AD) rather than chemical traits. On the basis of different soil resources, the species with thinner diameters have higher SS concentrations, while those of a thicker diameter have higher ST concentrations.

Plant respiration: Controlled by photosynthesis or biomass?

Two simplifying hypotheses have been proposed for whole-plant respiration. One links respiration to photosynthesis; the other to biomass. Using a first-principles carbon balance model with a prescribed live woody biomass turnover, applied at a forest research site where multidecadal measurements are available for comparison, we show that if turnover is fast the accumulation of respiring biomass is low and respiration depends primarily on photosynthesis; while if turnover is slow the accumulation of respiring biomass is high and respiration depends primarily on biomass. But the first scenario is inconsistent with evidence for substantial carry-over of fixed carbon between years, while the second implies far too great an increase in respiration during stand development-leading to depleted carbohydrate reserves and an unrealistically high mortality risk. These two mutually incompatible hypotheses are thus both incorrect. Respiration is not linearly related either to photosynthesis or to biomass, but it is more strongly controlled by recent photosynthates (and reserve availability) than by total biomass.

Non-structural carbohydrate and hydraulic dynamics during drought and recovery in Fraxinus ornus and Ostrya carpinifolia saplings

The maintenance of hydraulic function during and after a drought event is crucial for tree survival, but the importance of non-structural carbohydrates (NSCs) in the recovery phase is still debated. We tested whether higher NSC availability facilitates post-drought hydraulic recovery, by applying a short-term drought (S_dr) and a long-term drought combined with shading (L_dr+sh) in Fraxinus ornus and Ostrya carpinifolia. Plants were then re-irrigated and recovery was checked 24 h later, by measuring water potential, stem percentage loss of hydraulic conductance (PLC) and NSC content. The relative magnitude of hydraulic and carbon constraints was also assessed in desiccated plants. During drought, PLC increased only in F. ornus, while it was maintained almost constant in O. carpinifolia due to tighter stomatal control of xylem pressure (i.e. more isohydric). In F. ornus, only S_dr plants maintained high NSC contents at the end of drought and, when re-irrigated, recovered PLC to control values. Whereas hydraulic failure was ubiquitous, only F. ornus depleted NSC reserves at mortality. Our results suggest that preserving higher NSC content at the end of a drought can be important for the hydraulic resilience of trees.

Dead or dying? Quantifying the point of no return from hydraulic failure in drought-induced tree mortality

Determining physiological mechanisms and thresholds for climate-driven tree die-off could help improve global predictions of future terrestrial carbon sinks. We directly tested for the lethal threshold in hydraulic failure - an inability to move water due to drought-induced xylem embolism - in a pine sapling experiment. In a glasshouse experiment, we exposed loblolly pine (Pinus taeda) saplings (n = 83) to drought-induced water stress ranging from mild to lethal. Before rewatering to relieve drought stress, we measured native hydraulic conductivity and foliar color change. We monitored all measured individuals for survival or mortality. We found a lethal threshold at 80% loss of hydraulic conductivity - a point of hydraulic failure beyond which it is more likely trees will die, than survive, and describe mortality risk across all levels of water stress. Foliar color changes lagged behind hydraulic failure - best predicting when trees had been dead for some time, rather than when they were dying. Our direct measurement of native conductivity, while monitoring the same individuals for survival or mortality, quantifies a continuous probability of mortality risk from hydraulic failure. Predicting tree die-off events and understanding the mechanism involved requires knowledge not only of when trees are dead, but when they begin dying - having passed the point of no return.

Elevated temperature differently affects growth, photosynthetic capacity, nutrient absorption and leaf ultrastructure of Abies faxoniana and Picea purpurea under intra- and interspecific competition

There is a limited understanding of the impacts of global warming on intra- and interspecific plant competition. Resolving this knowledge gap is important for predicting the potential influence of global warming on forests, particularly on high-altitude trees, which are more sensitive to warming. In the present study, effects of intra- and interspecific competition on plant growth and associated physiological, structural and chemical traits were investigated in Abies faxoniana and Picea purpurea seedlings under control (ambient temperature) and elevated temperature (ET, 2 °C above ambient temperature) conditions for 2 years. We found that A. faxoniana and P. purpurea grown under intra- and interspecific competition showed significant differences in dry matter accumulation (DMA), photosynthetic capacity, nutrient absorption, non-structural carbohydrate (NSC) contents and leaf ultrastructure under ET conditions. ET increased leaf, stem and root DMA of both conifers under both competition patterns. Moreover, under ET and interspecific competition, P. purpurea had overall superior competitive capacity characterized by higher organ (leaf, stem and root) and total DMA, height growth rate, net photosynthetic rate, specific leaf area, water use efficiency (δ13C), leaf and root N and NSC concentrations and greater plasticity for absorption of different soil N forms. Thus, the growth of P. purpurea benefitted from the presence of A. faxoniana under ET. Our results demonstrated that ET significantly affects the asymmetric competition patterns in subalpine conifer species. Potential alteration of plant competitive interactions by global warming can influence the composition, structure and functioning of subalpine coniferous forests.

Identifying the relevant carbohydrate storage pools available for remobilization in aspen roots

Nonstructural carbohydrate (NSC) remobilization remains poorly understood in trees. In particular, it remains unclear (i) which tissues (e.g., living bark or xylem) and compounds (sugars or starch) in woody plants are the main sources of remobilized carbon, (ii) to what extent these NSC pools can be depleted and (iii) whether initial NSC mass or concentration is a better predictor of regrowth potential following disturbance. To address these questions, we collected root segments from a large mature trembling aspen stand; we then allowed them to resprout (sucker) in the dark and remobilize NSC until all sprouts had died. We found that initial starch mass, not concentration, was the best predictor of subsequent sprout mass. In total, more NSC mass (~4×) was remobilized from the living inner bark than the xylem of the roots. After resprouting, root starch was generally depleted to <0.6% w/w in both tissues. In contrast, a large portion of sugars appear unavailable for remobilization: sugar concentrations were only reduced to 12% w/w in the bark and 2% in the xylem. These findings suggest that in order to test whether plant processes like resprouting are limited by storage we need to (i) measure storage in the living bark, not just the xylem, (ii) consider storage pool size-not just concentration-and (iii) carefully determine which compounds are actually components of the storage pool.

Eyes on the future - evidence for trade-offs between growth, storage and defense in Norway spruce

Carbon (C) allocation plays a central role in tree responses to environmental changes. Yet, fundamental questions remain about how trees allocate C to different sinks, for example, growth vs storage and defense. In order to elucidate allocation priorities, we manipulated the whole-tree C balance by modifying atmospheric CO₂ concentrations [CO₂ ] to create two distinct gradients of declining C availability, and compared how C was allocated among fluxes (respiration and volatile monoterpenes) and biomass C pools (total biomass, nonstructural carbohydrates (NSC) and secondary metabolites (SM)) in well-watered Norway spruce (Picea abies) saplings. Continuous isotope labelling was used to trace the fate of newly-assimilated C. Reducing [CO₂ ] to 120 ppm caused an aboveground C compensation point (i.e. net C balance was zero) and resulted in decreases in growth and respiration. By contrast, soluble sugars and SM remained relatively constant in aboveground young organs and were partially maintained with a constant allocation of newly-assimilated C, even at expense of root death from C exhaustion. We conclude that spruce trees have a conservative allocation strategy under source limitation: growth and respiration can be downregulated to maintain 'operational' concentrations of NSC while investing newly-assimilated C into future survival by producing SM.

Seasonal changes of sucrose transporter expression and sugar partitioning in common European tree species

In temperate woody species, carbon transport from source to sink tissues is a striking physiological process, particularly considering seasonal changes. The functions of different tissues can also alternate across the seasons. In this regard, phloem loading and sugar distribution are important aspects of carbon partitioning, and sucrose uptake transporters (SUTs) play a key role in these processes. Therefore, the influence of seasons and different light-dark conditions on the expression of SUTs from 3-year-old Fagus sylvatica L., Quercus robur L. and Picea abies (L.) Karst. trees were analyzed. In addition, tissue-specific sugar and starch contents under these different environmental conditions were determined. Putative SUTs were identified in the gymnosperms (Picea abies, Ginkgo biloba L.), here for the first time, and also in the angiosperms (Q. robur, F. sylvatica). The identified SUT sequences of the different tree species cluster into three types, similar to other SUTs from herbaceous and tree species. Furthermore, the sequences from angiosperm and those from gymnosperm species form distinct clusters within the three types of SUTs. In F. sylvatica, Q. robur and P. abies, the expression levels of the different SUTs during seasons showed marked variations. Because of the high expression levels of type I SUTs in bark, wood and leaves during active growing phases in spring and summer, it can be assumed that they are involved in phloem loading, sucrose retrieval and possibly in further physiological processes. The expression patterns also indicate a flexible expression in all tissues depending on physiological requirements and environmental conditions. Compared with type I SUTs, the seasonal variations of type II SUT expression were less pronounced, whereas the seasonal variations of the type III SUT expression patterns were partly reverse. In addition to the seasonal regulation, the expressions of the different SUTs were also regulated by light in a diurnal manner.

Modelling carbon sources and sinks in terrestrial vegetation

Contents Summary 652 I. Introduction 652 II. Discrepancy in predicting the effects of rising [CO₂ ] on the terrestrial C sink 655 III. Carbon and nutrient storage in plants and its modelling 656 IV. Modelling the source and the sink: a plant perspective 657 V. Plant-scale water and Carbon flux models 660 VI. Challenges for the future 662 Acknowledgements 663 Authors contributions 663 References 663 SUMMARY: The increase in atmospheric CO₂ in the future is one of the most certain projections in environmental sciences. Understanding whether vegetation carbon assimilation, growth, and changes in vegetation carbon stocks are affected by higher atmospheric CO₂ and translating this understanding in mechanistic vegetation models is of utmost importance. This is highlighted by inconsistencies between global-scale studies that attribute terrestrial carbon sinks to CO₂ stimulation of gross and net primary production on the one hand, and forest inventories, tree-scale studies, and plant physiological evidence showing a much less pronounced CO₂ fertilization effect on the other hand. Here, we review how plant carbon sources and sinks are currently described in terrestrial biosphere models. We highlight an uneven representation of complexity between the modelling of photosynthesis and other processes, such as plant respiration, direct carbon sinks, and carbon allocation, largely driven by available observations. Despite a general lack of data on carbon sink dynamics to drive model improvements, ways forward toward a mechanistic representation of plant carbon sinks are discussed, leveraging on results obtained from plant-scale models and on observations geared toward model developments.

Trees use more non-structural carbohydrate reserves during epicormic than basal resprouting

Non-structural carbohydrates (NSCs) are crucial to support tree resprouting after disturbances that damage the crown or stem. Epicormic resprouting (from stem) could demand more from NSC reserves than basal resprouting (following top-kill), since epicormically resprouting trees need to maintain a greater living biomass. Yet, little is known about NSC use during epicormic resprouting, particularly the relative importance of stem and below-ground NSC reserves. We compared the distribution and magnitude of NSC decreases during epicormic and basal resprouting by experimentally removing crowns or stems of 14-year-old Eucalyptus obliqua L'Hér. trees in native forest, then harvesting these trees over a 10-month period (start, sprouts emerged, sprouts expanded) to measure changes in NSC concentration and mass by organ (stem, lignotuber, roots). We hypothesized that (i) NSC depletion during resprouting is primarily due to decreases in starch rather than soluble sugars concentrations; (ii) during epicormic resprouting, stem NSC concentrations are decreased irrespective of any decreases in roots; and (iii) absolute decreases in NSC mass are greater for epicormic than basal resprouting during the leafless period due to the carbon demands associated with maintaining greater living biomass. Results confirmed our hypotheses; starch was the primary storage carbohydrate, stems were an important source of starch during epicormic resprouting and carbon demands of maintenance functions were greater for epicormic resprouting, leading to greater decreases in NSC reserves. Roots were a more important starch storage organ than the lignotuber for both epicormic and basal resprouting. The proportional decrease in starch was severe for both modes of resprouting due to a long leafless period, after which trees resprouting epicormically relied on starch reserves for longer than those resprouting basally. It remains to be seen how the timing of disturbance affects the timing and vigour of resprouting, and how long-term NSC recovery differs for epicormic and basal resprouting.

Living on next to nothing: tree seedlings can survive weeks with very low carbohydrate concentrations

The usage of nonstructural carbohydrates (NSCs) to indicate carbon (C) limitation in trees requires knowledge of the minimum tissue NSC concentrations at lethal C starvation, and the NSC dynamics during and after severe C limitation. We completely darkened and subsequently released seedlings of two deciduous and two evergreen temperate tree species for varying periods. NSCs were measured in all major organs, allowing assessment of whole-seedling NSC balances. NSCs decreased fast in darkness, but seedlings survived species-specific whole-seedling starch concentrations as low as 0.4-0.8% per dry matter (DM), and sugar (sucrose, glucose and fructose) concentrations as low as 0.5-2.0% DM. After re-illumination, the refilling of NSC pools began within 3 wk, while the resumption of growth was delayed or restricted. All seedlings had died after 12 wk of darkness, and starch and sugar concentrations in most tissues were lower than 1% DM. We conclude that under the applied conditions, tree seedlings can survive several weeks with very low NSC reserves probably also using alternative C sources like lipids, proteins or hemicelluloses; lethal C starvation cannot be assumed, if NSC concentrations are higher than the minimum concentrations found in surviving seedlings; and NSC reformation after re-illumination occurs preferentially over growth.

Phloem Girdling of Norway Spruce Alters Quantity and Quality of Wood Formation in Roots Particularly Under Drought

Carbon (C) availability plays an essential role in tree growth and wood formation. We evaluated the hypothesis that a decrease in C availability (i) triggers mobilization of C reserves in the coarse roots of Picea abies to maintain growth and (ii) causes modification of wood structure notably under drought. The 6-year-old saplings were subjected to two levels of soil moisture (watered versus drought conditions) and root C status was manipulated by physically blocking phloem transport in the stem at three girdling dates (GDs). Stem girdling was done before the onset of bud break [day of the year (doy) 77], during vigorous aboveground shoot and radial stem growth (GD doy 138), and after cessation of shoot growth (GD doy 190). The effect of blockage of C transport on root growth, root phenology, and wood anatomical traits [cell lumen diameter (CLD) and cell wall thickness (CWT)] in earlywood (EW) and latewood (LW) was determined. To evaluate changes in belowground C status caused by girdling, non-structural carbohydrates (soluble sugars and starch) in coarse roots were determined at the time of girdling and after the growing season. Although fine root mass significantly decreased in response to blockage of phloem C transport, the phenology of root elongation growth was not affected. Surprisingly, radial root growth and CLD of EW tracheids in coarse roots were strikingly increased in drought-stressed trees, when girdling occurred before bud break or during aboveground stem growth. In watered trees, the growth response to girdling was less distinct, but the CWT of EW significantly increased. Starch reserves in the roots of girdled trees significantly decreased in both soil moisture treatments and at all GDs. We conclude that (i) radial growth and wood development in coarse roots of P. abies saplings are not only dependent on current photosynthates, and (ii) blockage of phloem transport induces physiological changes that outweigh drought effects imposed on root cambial activity and cell differentiation.