Allocation to carbon storage pools in Norway spruce saplings under drought and low CO2

N addition rebalances the carbon and nitrogen metabolisms of Leymus chinensis through leaf N investment

Intensifying nitrogen (N) deposition disturbs the growth of grassland plants due to an imbalance between their carbon (C) and N metabolism. However, it's unclear how plant physiological strategies restore balance. We investigated the effects of multiple N addition levels (0-25 g N m^-2 yr^-1) on the coordination of C and N metabolism in a dominant grass (Leymus chinensis) in a semiarid grassland in northern China. To do so, we evaluated photosynthetic parameters, leaf N allocation, C- and N-based metabolites, and metabolic enzymes. We found that a moderate N level (10 g N m^-2 yr^-1) promoted carboxylation and electron transport by allocating more N to the photosynthetic apparatus and increasing ribulose bisphosphate carboxylase/oxygenase activity, thereby increasing photosynthetic capacity. The highest N level (25 g N m^-2 yr^-1) promoted N investment in nonphotosynthetic pathways and increased the free amino acids in the leaves. N addition stimulated the accumulation of C and N compounds across organs by activating sucrose phosphate synthase, nitrate reductase, and glutamine synthetase. This enhancement triggered a transformation of primary metabolites (nonstructural carbohydrates, proteins, amino acids) to secondary metabolites (flavonoids, phenols, and alkaloids) for temporary storage or as defense compounds. Citric acid, as the C skeleton for enhanced N metabolism, decreased significantly, and malic acid increased by catalysis of phosphoenolpyruvate carboxylase. Our findings show the adaptability of L. chinensis to different N-addition levels by adjusting its allocations of C and N metabolic compounds and confirm the roles of C and N coordination by grassland plants in these adaptations.

Primary and secondary host selection by Ips typographus depends on Norway spruce crown characteristics and phenolic-based defenses

Climate change is expected to intensify bark beetle population outbreaks in forests globally, affecting biodiversity and trajectories of change. Aspects of individual tree resistance remain poorly quantified, particularly with regard to the role of phenolic compounds, hindering robust predictions of forest response to future conditions. In 2003, we conducted a mechanical wounding experiment in a Norway spruce forest that coincided with an outbreak of the bark beetle, Ips typographus. We collected phloem samples from 97 trees and monitored tree survival for 5 months. Using high-performance liquid chromatography, we quantified induced changes in the concentrations of phenolics. Classification and regression tools were used to evaluate relationships between phenolic production and bark beetle resistance, in the context of other survival factors. The proximity of beetle source populations was a principal determinant of survival. Proxy measures of tree vigor, such as crown defoliation, mediated tree resistance. Controlling for these factors, synthesis of catechin was found to exponentially increase tree survival probability. However, even resistant trees were susceptible in late season due to high insect population growth. Our results show that incorporating trait-mediated effects improves predictions of survival. Using an integrated analytical approach, we demonstrate that phenolics play a direct role in tree defense to herbivory.

Drought affects the fate of non-structural carbohydrates in hinoki cypress

Tree species that close stomata early in response to drought are likely to suffer from an imbalance between limited carbohydrate supply due to reduced photosynthesis and metabolic demand. Our objective was to clarify the dynamic responses of non-structural carbohydrates to drought in a water-saving species, the hinoki cypress (Chamaecyparis obtusa Sieb. et Zucc.). To this end, we pulse-labeled young trees with 13CO2 10 days after the beginning of the drought treatment. Trees were harvested 7 days later, early during drought progression, and 86 days later when they had suffered from a long and severe drought. The labeled carbon (C) was traced in phloem extract, in the organic matter and starch of all the organs, and in the soluble sugars (sucrose, glucose and fructose) of the most metabolically active organs (foliage, green branches and fine roots). No drought-related changes in labeled C partitioning between belowground and aboveground organs were observed. The C allocation between non-structural carbohydrates was altered early during drought progression: starch concentration was lower by half in the photosynthetic organs, while the concentration of almost all soluble sugars tended to increase. The preferential allocation of labeled C to glucose and fructose reflected an increased demand for soluble sugars for osmotic adjustment. After 3 months of a lethal drought, the concentrations of soluble sugars and starch were admittedly lower in drought-stressed trees than in the controls, but the pool of non-structural carbohydrates was far from completely depleted. However, the allocation to storage had been impaired by drought; photosynthesis and the sugar translocation rate had also been reduced by drought. Failure to maintain cell turgor through osmoregulation and to refill embolized xylem due to the depletion in soluble sugars in the roots could have resulted in tree mortality in hinoki cypress, though the total pool of carbohydrate was not completely depleted.

Wood Formation Modeling - A Research Review and Future Perspectives

Wood formation has received considerable attention across various research fields as a key process to model. Historical and contemporary models of wood formation from various disciplines have encapsulated hypotheses such as the influence of external (e.g., climatic) or internal (e.g., hormonal) factors on the successive stages of wood cell differentiation. This review covers 17 wood formation models from three different disciplines, the earliest from 1968 and the latest from 2020. The described processes, as well as their external and internal drivers and their level of complexity, are discussed. This work is the first systematic cataloging, characterization, and process-focused review of wood formation models. Remaining open questions concerning wood formation processes are identified, and relate to: (1) the extent of hormonal influence on the final tree ring structure; (2) the mechanism underlying the transition from earlywood to latewood in extratropical regions; and (3) the extent to which carbon plays a role as "active" driver or "passive" substrate for growth. We conclude by arguing that wood formation models remain to be fully exploited, with the potential to contribute to studies concerning individual tree carbon sequestration-storage dynamics and regional to global carbon sequestration dynamics in terrestrial vegetation models.

Contrasting life-history traits of black spruce and jack pine influence their physiological response to drought and growth recovery in northeastern boreal Canada

An increase in frequency, intensity and duration of drought events affects forested ecosystems. Trees react to these changes by adjusting stomatal conductance to maximize the trade-off between carbon gains and water losses. A better understanding of the consequences of these drought-induced physiological adjustments for tree growth could help inferring future productivity potentials of boreal forests. Here, we used samples from a forest inventory network in Canada where a decline in growth rates of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) occurred in 1988-1992, an exceptionally dry period, to verify if this growth decline resulted from physiological adjustments of trees to drought. We measured carbon and oxygen isotope ratios in growth rings of 95 spruces and 49 pines spanning 1985-1993. We used ¹³C discrimination (Δ¹³C) and ¹⁸O enrichment (Δ¹⁸O) as proxies for intrinsic water use efficiency and stomatal conductance, respectively. We studied how inter-annual variability in isotopic signals was linked to climate moisture index, vapor pressure deficit and annual snowfall amount. We found significantly lower Δ¹³C values over 1988-1990, and significantly higher Δ¹⁸O values in 1988-1989 and 1991 compared to the 1985-1993 averages. We also observed that a low climatic water balance and a high vapor pressure deficit were linked with low Δ¹³C and high Δ¹⁸O in the two study species, in parallel with low growth rates. The latter effect persisted into the year following drought for black spruce, but not for jack pine. These findings highlight that small differences in physiological parameters between species could translate into large differences in post-drought recovery. The stronger and longer lasting impact on black spruce compared to jack pine suggests a less efficient carbon use and a lower acclimation potential to future warmer and drier climate conditions.

Plasticity of the xylem vulnerability to embolism in Populus tremula x alba relies on pit quantity properties rather than on pit structure

Knowledge on variations of drought resistance traits are needed to predict the potential of trees to acclimate to coming severe drought events. Xylem vulnerability to embolism is a key parameter related to such droughts, and its phenotypic variability relies mainly on environmental plasticity. We investigated the structural determinants controlling the plasticity of vulnerability to embolism, focusing on the key elements involved in the air bubble entry in vessels, especially the intervessel pits. Poplar saplings (Populus tremula x alba (Aiton) Sm., 1804) grown in contrasted water availability or light exposure exhibited differences in the vulnerability to embolism (P50) in a range of 0.76 MPa. We then characterized the structural changes in features related to pit quantity and pit structure, from the pit ultrastructure to the organization of xylem vessels, using different microscopy techniques (transmission electron microscopy, scanning electron microscopy, light microscopy). A multispectral combination of X-ray microtomography and light microscopy analysis allowed measuring the vulnerability of each single vessel and testing some of the relationships between structural traits and vulnerability to embolism inside the xylem. The pit ultrastructure did not change, whereas the vessel dimensions increased with the vulnerability to embolism and the grouping index and fraction of intervessel cell wall both decreased with the vulnerability to embolism. These findings hold when comparing between trees or between the vessels inside the xylem of an individual tree. These results evidenced that plasticity of vulnerability to embolism in hybrid poplar occurs through changes in the pit quantity properties such as pit area and vessel grouping rather than changes on the pit structure.

Manipulating phloem transport affects wood formation but not local nonstructural carbon reserves in an evergreen conifer

How variations in carbon supply affect wood formation remains poorly understood in particular in mature forest trees. To elucidate how carbon supply affects carbon allocation and wood formation, we attempted to manipulate carbon supply to the cambial region by phloem girdling and compression during the mid- and late-growing season and measured effects on structural development, CO₂ efflux and nonstructural carbon reserves in stems of mature white pines. Wood formation and stem CO₂ efflux varied with a location relative to treatment (i.e., above or below the restriction). We observed up to twice as many tracheids formed above versus below the treatment after the phloem transport manipulation, whereas the cell-wall area decreased only slightly below the treatments, and cell size did not change relative to the control. Nonstructural carbon reserves in the xylem, needles and roots were largely unaffected by the treatments. Our results suggest that low and high carbon supply affects wood formation, primarily through a strong effect on cell proliferation, and respiration, but local nonstructural carbon concentrations appear to be maintained homeostatically. This contrasts with reports of decoupling of source activity and wood formation at the whole-tree or ecosystem level, highlighting the need to better understand organ-specific responses, within-tree feedbacks, as well as phenological and ontogenetic effects on sink-source dynamics.

Evaluation of morphological, physiological, and biochemical traits for assessing drought resistance in eleven tree species

The frequency and severity of drought are expected to increase due to climate change; therefore, selection of tree species for afforestation should consider drought resistance of the species for maximum survival and conservation of natural habitats. In this study, three soil moisture regimes: control (100% precipitation), mild drought (40% reduction in precipitation), and severe drought (80% reduction in precipitation) were applied to six gymnosperm and five angiosperm species for two consecutive years. We quantified the drought resistance index based on the root collar diameter and assessed the correlation between species drought resistance and other morphological, physiological, and biochemical traits by regression analysis. The prolonged drought stress altered the morphological, physiological, and biochemical traits, but the responses were species-specific. The species with high drought resistance had high leaf mass per area (LMA), photosynthetic rate (P_n), and midday leaf water potential (Ψ_MD), and low carbon isotopic discrimination (δ¹³C), flavonoid and polyphenol content, superoxide dismutase and DPPH radical scavenging activity. The highly drought-resistant species had a relatively less decrease in leaf size, P_n, and predawn leaf water potential (Ψ_PD), and less increase in δ¹³C, abscisic acid and sucrose content, and LMA compared to the control. The interannual variation in drought resistance (∆R_d) was positively correlated with the species hydroscopic slope (isohydric and anisohydric). Korean pine was highly resistant, sawtooth oak, hinoki cypress, East Asian white birch, East Asian ash, and mono maple were highly susceptible, and Korean red pine, Japanese larch, Sargent cherry, needle fir, and black pine were moderate in drought resistance under long-term drought. These findings will help species selection for afforestation programs and establishment of sustainable forests, especially of drought-tolerant species, under increased frequency and intensity of spring and summer droughts.

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.

Drought stress improved the capacity of Rhizophagus irregularis for inducing the accumulation of oleuropein and mannitol in olive (Olea europaea) roots

Olive trees are often subjected to a prolonged dry season with low water availability, which induces oxidative stress. Arbuscular mycorrhizal (AM) symbioses can improve olive plant tolerance to water deficit. This study investigated several aspects related to drought tolerance in AM fungi olive plants. Non-AM and AM plants were grown under well-watered or drought-stressed conditions, and mycorrhizal growth response, neutral lipid fatty acid (NLFA)16:1ω5 and phospholipid fatty acid (PLFA) 16:1ω5 in roots (intraradical mycelium) and in soil (extraradical mycelium), carbohydrates (monosaccharides, disaccharides and polyols) and phenolic compounds (phenolic alcohols, flavonoids, lignans, secoiridoids and hydroxycinnamic acid derivatives) were determined. Results showed that the amounts of PLFA 16:1ω5 and NLFA 16:1ω5 were significantly influenced by drought stress conditions. The NLFA 16:1ω5/PLFA 16:1ω5 ratio showed a dramatic decrease (-62%) with the application of water deficit stress, indicating that AM fungi allocated low carbon to storage structures under stress conditions. Mannitol and verbascoside are the main compounds detected in the roots of well-watered plants, whereas oleuropein and mannitol are the main compounds differentially accumulated in the roots of water-stressed plants. The oleuropein/verbascoside ratio increased in the case of drought-stressed AM plants by 30%, while the mannitol/oleuropein ratio was decreased by 46%, when compared to the non-AM stressed plants. Mycorrhization therefore oriented the flux toward the biosynthetic pathway of oleuropein and the data suggest that sugar and phenolic compound metabolism may have been redirected to the formation of oleuropein in roots of AM stressed plants, that may underlie their enhanced tolerance to drought stress.

Rhizosphere activity in an old-growth forest reacts rapidly to changes in soil moisture and shapes whole-tree carbon allocation

Drought alters carbon (C) allocation within trees, thereby impairing tree growth. Recovery of root and leaf functioning and prioritized C supply to sink tissues after drought may compensate for drought-induced reduction of assimilation and growth. It remains unclear if C allocation to sink tissues during and following drought is controlled by altered sink metabolic activities or by the availability of new assimilates. Understanding such mechanisms is required to predict forests' resilience to a changing climate. We investigated the impact of drought and drought release on C allocation in a 100-y-old Scots pine forest. We applied ¹³CO₂ pulse labeling to naturally dry control and long-term irrigated trees and tracked the fate of the label in above- and belowground C pools and fluxes. Allocation of new assimilates belowground was ca. 53% lower under nonirrigated conditions. A short rainfall event, which led to a temporary increase in the soil water content (SWC) in the topsoil, strongly increased the amounts of C transported belowground in the nonirrigated plots to values comparable to those in the irrigated plots. This switch in allocation patterns was congruent with a tipping point at around 15% SWC in the response of the respiratory activity of soil microbes. These results indicate that the metabolic sink activity in the rhizosphere and its modulation by soil moisture can drive C allocation within adult trees and ecosystems. Even a subtle increase in soil moisture can lead to a rapid recovery of belowground functions that in turn affects the direction of C transport in trees.

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.

Towards better representations of carbon allocation in vegetation: a conceptual framework and mathematical tool

The representation of carbon allocation (CA) in ecosystem differs tremendously among models, resulting in diverse responses of carbon cycling and storage to global change. Several studies have highlighted discrepancies between empirical observations and model predictions, attributing these differences to problems of model structure. We analyzed the mathematical representation of CA in models using concepts from dynamical systems theory; we reviewed a representative sample of models of CA in vegetation and developed a model database within the Python package bgc-md. We asked whether these representations can be generalized as a linear system, or whether a more general framework is needed to accommodate nonlinearities. Some of the vegetation systems simulated with the reviewed models have a fixed partitioning of photosynthetic products, independent of environmental forcing. Vegetation is often represented as a linear system without storage compartments. Yet, other structures with nonlinearities have also been proposed, with important consequences on the temporal trajectories of ecosystem carbon compartments. The proposed mathematical framework unifies the representation of alternative CA schemes, facilitating their classification according to mathematical properties as well as their potential temporal behaviour. It can represent complex processes in a compact form, which can potentially facilitate dialog among empiricists, theoreticians, and modellers.

The partitioning of gross primary production for young Eucalyptus tereticornis trees under experimental warming and altered water availability

The allocation of carbon (C) is an important component of tree physiology that influences growth and ecosystem C storage. Allocation is challenging to measure, and its sensitivity to environmental changes such as warming and altered water availability is uncertain. We exposed young Eucalyptus tereticornis trees to +3°C warming and elimination of summer precipitation in the field using whole-tree chambers. We calculated C allocation terms using detailed measurements of growth and continuous whole-crown CO₂ and water exchange measurements. Trees grew from small saplings to nearly 9 m height during this 15-month experiment. Warming accelerated growth and leaf area development, and it increased the partitioning of gross primary production (GPP) to aboveground respiration and growth while decreasing partitioning below ground. Eliminating summer precipitation reduced C gain and growth but did not impact GPP partitioning. Trees utilized deep soil water and avoided strongly negative water potentials. Warming increased growth respiration, but maintenance respiration acclimated homeostatically. The increasing growth in the warmed treatment resulted in higher rates of respiration, even with complete acclimation of maintenance respiration. Warming-induced stimulations of tree growth likely involve increased C allocation above ground, particularly to leaf area development, whereas reduced water availability may not stimulate allocation to roots.

Homoeostatic maintenance of nonstructural carbohydrates during the 2015-2016 El Niño drought across a tropical forest precipitation gradient

Nonstructural carbohydrates (NSCs) are essential for maintenance of plant metabolism and may be sensitive to short- and long-term climatic variation. NSC variation in moist tropical forests has rarely been studied, so regulation of NSCs in these systems is poorly understood. We measured foliar and branch NSC content in 23 tree species at three sites located across a large precipitation gradient in Panama during the 2015-2016 El Niño to examine how short- and long-term climatic variation impact carbohydrate dynamics. There was no significant difference in total NSCs as the drought progressed (leaf P = 0.32, branch P = 0.30) nor across the rainfall gradient (leaf P = 0.91, branch P = 0.96). Foliar soluble sugars decreased while starch increased over the duration of the dry period, suggesting greater partitioning of NSCs to storage than metabolism or transport as drought progressed. There was a large variation across species at all sites, but total foliar NSCs were positively correlated with leaf mass per area, whereas branch sugars were positively related to leaf temperature and negatively correlated with daily photosynthesis and wood density. The NSC homoeostasis across a wide range of conditions suggests that NSCs are an allocation priority in moist tropical forests.

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.

The sweet side of global change-dynamic responses of non-structural carbohydrates to drought, elevated CO2 and nitrogen fertilization in tree species

Non-structural carbohydrates (NSC) play a central role in plant functioning as energy carriers and building blocks for primary and secondary metabolism. Many studies have investigated how environmental and anthropogenic changes, like increasingly frequent and severe drought episodes, elevated CO2 and atmospheric nitrogen (N) deposition, influence NSC concentrations in individual trees. However, this wealth of data has not been analyzed yet to identify general trends using a common statistical framework. A thorough understanding of tree responses to global change is required for making realistic predictions of vegetation dynamics. Here we compiled data from 57 experimental studies on 71 tree species and conducted a meta-analysis to evaluate general responses of stored soluble sugars, starch and total NSC (soluble sugars + starch) concentrations in different tree organs (foliage, above-ground wood and roots) to drought, elevated CO2 and N deposition. We found that drought significantly decreased total NSC in roots (-17.3%), but not in foliage and above-ground woody tissues (bole, branch, stem and/or twig). Elevated CO2 significantly increased total NSC in foliage (+26.2%) and roots (+12.8%), but not in above-ground wood. By contrast, total NSC significantly decreased in roots (-17.9%), increased in above-ground wood (+6.1%), but was unaffected in foliage from N fertilization. In addition, the response of NSC to three global change drivers was strongly affected by tree taxonomic type, leaf habit, tree age and treatment intensity. Our results pave the way for a better understanding of general tree function responses to drought, elevated CO2 and N fertilization. The existing data also reveal that more long-term studies on mature trees that allow testing interactions between these factors are urgently needed to provide a basis for forecasting tree responses to environmental change at the global scale.

Carbohydrate dynamics of three dominant species in a Chinese savanna under precipitation exclusion

The potential impact of drought on the carbon balance in plants has gained great attention. Non-structural carbohydrate (NSC) dynamics have been suggested as an important trait reflecting carbon balance under drought conditions. However, NSC dynamics under drought and the response mechanisms of NSC to drought remain unclear, especially in water-limited savanna ecosystems. A precipitation exclusion experiment was performed to simulate different drought intensities in a savanna ecosystem in Yuanjiang valley in southwestern China. Growth, total NSC concentration and diurnal change of NSC were determined for the leaves and non-photosynthetic organs of three dominant species (Lannea coromandelica, Polyalthia cerasoides and Heteropogon contortus) throughout the growing season. Drought significantly reduced the growth of all the three species. Total NSC concentration averaged ~8.1%, varying with species, organ and sampling period, and did not significantly decrease under drought stress. By contrast, the diurnal change of NSC in these three species increased under drought stress. These results indicate that these three dominant species did not undergo carbon limitation. Thus, relative change in NSC is a more sensitive and effective indicator than carbon reserves in evaluation of plant carbon balance. These findings provide new insights for the understanding of carbon balance and the mechanisms of carbon starvation.

Tree growth and water-use in hyper-arid Acacia occurs during the hottest and driest season

Drought-induced tree mortality has been recently increasing and is expected to increase further under warming climate. Conversely, tree species that survive under arid conditions might provide vital information on successful drought resistance strategies. Although Acacia (Vachellia) species dominate many of the globe's deserts, little is known about their growth dynamics and water-use in situ. Stem diameter dynamics, leaf phenology, and sap flow were monitored during 3 consecutive years in five Acacia raddiana trees and five Acacia tortilis trees in the Arid Arava Valley, southern Israel (annual precipitation 20-70 mm, restricted to October-May). We hypothesized that stem growth and other tree activities are synchronized with, and limited to single rainfall or flashflood events. Unexpectedly, cambial growth of both Acacia species was arrested during the wet season, and occurred during most of the dry season, coinciding with maximum daily temperatures as high as 45 °C and vapor pressure deficit of up to 9 kPa. Summer growth was correlated with peak sap flow in June, with almost year-round activity and foliage cover. To the best of our knowledge, these are the harshest drought conditions ever documented permitting cambial growth. These findings point to the possibility that summer cambial growth in Acacia under hyper-arid conditions relies on concurrent leaf gas exchange, which is in turn permitted by access to deep soil water. Soil water can support low-density tree populations despite heat and drought, as long as recharge is kept above a minimum threshold.

Non-structural carbohydrate dynamics associated with drought-induced die-off in woody species of a shrubland community

BACKGROUND AND AIMS

The relationship between plant carbon economy and drought responses of co-occurring woody species can be assessed by comparing carbohydrate (C) dynamics following drought and rain periods, relating these dynamics to species' functional traits. We studied nine woody species coexisting in a continental Mediterranean shrubland that experienced severe drought effects followed by rain.

METHODS

We measured total non-structural carbohydrates (NSC) and soluble sugars (SS) in roots and stems during drought and after an autumn rain pulse in plants exhibiting leaf loss and in undefoliated ones. We explored whether their dynamics were related to foliage recovery and functional traits (height [H], specific leaf area [SLA], wood density [WD]).

KEY RESULTS

During drought, NSC concentrations were overall lower in stems and roots of plants experiencing leaf loss, while SS decreases were smaller. Roots had higher NSC concentrations than stems. After the rain, NSC concentrations continued to decrease, while SS increased. Green foliage recovered after rain, particularly in plants previously experiencing higher leaf loss, independently of NSC concentrations during drought. Species with lower WD tended to have more SS during drought and lower SS increases after rain. In low-WD species, plants with severe leaf loss had lower NSC relative to undefoliated ones. No significant relationship was found between H or SLA and C content or dynamics.

CONCLUSIONS

Our community-level study reveals that, while responses were species-specific, C stocks overall diminished in plants affected by prolonged drought and did not increase after a pulse of seasonal rain. Dynamics were faster for SS than NSC. We found limited depletion of SS, consistent with their role in basal metabolic, transport and signalling functions. In a scenario of increased drought under climate change, NSC stocks in woody plants are expected to decrease differentially in coexisting species, with potential implications for their adaptive abilities and community dynamics.

The fate of recently fixed carbon after drought release: towards unravelling C storage regulation in Tilia platyphyllos and Pinus sylvestris

Carbon reserves are important for maintaining tree function during and after stress. Increasing tree mortality driven by drought globally has renewed the interest in how plants regulate allocation of recently fixed C to reserve formation. Three-year-old seedlings of two species (Tilia platyphyllos and Pinus sylvestris) were exposed to two intensities of experimental drought during ~10 weeks, and ¹³ C pulse labelling was subsequently applied with rewetting. Tracking the ¹³ C label across different organs and C compounds (soluble sugars, starch, myo-inositol, lipids and cellulose), together with the monitoring of gas exchange and C mass balances over time, allowed for the identification of variations in C allocation priorities and tree C balances that are associated with drought effects and subsequent drought release. The results demonstrate that soluble sugars accumulated in P. sylvestris under drought conditions independently of growth trends; thus, non-structural carbohydrates (NSC) formation cannot be simply considered a passive overflow process in this species. Once drought ceased, C allocation to storage was still prioritized at the expense of growth, which suggested the presence of 'drought memory effects', possibly to ensure future growth and survival. On the contrary, NSC and growth dynamics in T. platyphyllos were consistent with a passive (overflow) view of NSC formation.

Release of resource constraints allows greater carbon allocation to secondary metabolites and storage in winter wheat

The atmospheric CO₂ concentration ([CO₂ ]) is rapidly increasing, and this may have substantial impact on how plants allocate metabolic resources. A thorough understanding of allocation priorities can be achieved by modifying [CO₂ ] over a large gradient, including low [CO₂ ], thereby altering plant carbon (C) availability. Such information is of critical importance for understanding plant responses to global environmental change. We quantified the percentage of daytime whole-plant net assimilation (A) allocated to night-time respiration (R), structural growth (SG), nonstructural carbohydrates (NSC) and secondary metabolites (SMs) during 8 weeks of vegetative growth in winter wheat (Triticum aestivum) growing at low, ambient and elevated [CO₂ ] (170, 390 and 680 ppm). R/A remained relatively constant over a large gradient of [CO₂ ]. However, with increasing C availability, the fraction of assimilation allocated to biomass (SG + NSC + SMs), in particular NSC and SMs, increased. At low [CO₂ ], biomass and NSC increased in leaves but decreased in stems and roots, which may help plants achieve a functional equilibrium, that is, overcome the most severe resource limitation. These results reveal that increasing C availability from rising [CO₂ ] releases allocation constraints, thereby allowing greater investment into long-term survival in the form of NSC and SMs.

Effects of prolonged drought stress on Scots pine seedling carbon allocation

As the number of drought occurrences has been predicted to increase with increasing temperatures, it is believed that boreal forests will become particularly vulnerable to decreased growth and increased tree mortality caused by the hydraulic failure, carbon starvation and vulnerability to pests following these. Although drought-affected trees are known to have stunted growth, as well as increased allocation of carbon to roots, still not enough is known about the ways in which trees can acclimate to drought. We studied how drought stress affects belowground and aboveground carbon dynamics, as well as nitrogen uptake, in Scots pine (Pinus sylvestris L.) seedlings exposed to prolonged drought. Overall 40 Scots pine seedlings were divided into control and drought treatments over two growing seasons. Seedlings were pulse-labelled with 13CO2 and litter bags containing 15N-labelled root biomass, and these were used to follow nutrient uptake of trees. We determined photosynthesis, biomass distribution, root and rhizosphere respiration, water potential, leaf osmolalities and carbon and nitrogen assimilation patterns in both treatments. The photosynthetic rate of the drought-induced seedlings did not decrease compared to the control group, the maximum leaf specific photosynthetic rate being 0.058 and 0.045 µmol g-1 s-1 for the drought and control treatments, respectively. The effects of drought were, however, observed as lower water potentials, increased osmolalities as well as decreased growth and greater fine root-to-shoot ratio in the drought-treated seedlings. We also observed improved uptake of labelled nitrogen from soil to needles in the drought-treated seedlings. The results indicate acclimation of seedlings to long-term drought by aiming to retain sufficient water uptake with adequate allocation to roots and root-associated mycorrhizal fungi. The plants seem to control water potential with osmolysis, for which sufficient photosynthetic capability is needed.

Increasing carbon availability stimulates growth and secondary metabolites via modulation of phytohormones in winter wheat

Phytohormones play important roles in plant acclimation to changes in environmental conditions. However, their role in whole-plant regulation of growth and secondary metabolite production under increasing atmospheric CO2 concentrations ([CO2]) is uncertain but crucially important for understanding plant responses to abiotic stresses. We grew winter wheat (Triticum aestivum) under three [CO2] (170, 390, and 680 ppm) over 10 weeks, and measured gas exchange, relative growth rate (RGR), soluble sugars, secondary metabolites, and phytohormones including abscisic acid (ABA), auxin (IAA), jasmonic acid (JA), and salicylic acid (SA) at the whole-plant level. Our results show that, at the whole-plant level, RGR positively correlated with IAA but not ABA, and secondary metabolites positively correlated with JA and JA-Ile but not SA. Moreover, soluble sugars positively correlated with IAA and JA but not ABA and SA. We conclude that increasing carbon availability stimulates growth and production of secondary metabolites via up-regulation of auxin and jasmonate levels, probably in response to sugar-mediated signalling. Future low [CO2] studies should address the role of reactive oxygen species (ROS) in leaf ABA and SA biosynthesis, and at the transcriptional level should focus on biosynthetic and, in particular, on responsive genes involved in [CO2]-induced hormonal signalling pathways.

Key knowledge and data gaps in modelling the influence of CO2 concentration on the terrestrial carbon sink

Primary productivity of terrestrial vegetation is expected to increase under the influence of increasing atmospheric carbon dioxide concentrations ([CO₂]). Depending on the fate of such additionally fixed carbon, this could lead to an increase in terrestrial carbon storage, and thus a net terrestrial sink of atmospheric carbon. Such a mechanism is generally believed to be the primary global driver behind the observed large net uptake of anthropogenic CO₂ emissions by the biosphere. Mechanisms driving CO₂ uptake in the Terrestrial Biosphere Models (TBMs) used to attribute and project terrestrial carbon sinks, including that from increased [CO₂], remain in large parts unchanged since those models were conceived two decades ago. However, there exists a large body of new data and understanding providing an opportunity to update these models, and directing towards important topics for further research. In this review we highlight recent developments in understanding of the effects of elevated [CO₂] on photosynthesis, and in particular on the fate of additionally fixed carbon within the plant with its implications for carbon turnover rates, on the regulation of photosynthesis in response to environmental limitations on in-plant carbon sinks, and on emergent ecosystem responses. We recommend possible avenues for model improvement and identify requirements for better data on core processes relevant to the understanding and modelling of the effect of increasing [CO₂] on the global terrestrial carbon sink.

Impact of interspecific competition and drought on the allocation of new assimilates in trees

In trees, the interplay between reduced carbon assimilation and the inability to transport carbohydrates to the sites of demand under drought might be one of the mechanisms leading to carbon starvation. However, we largely lack knowledge on how drought effects on new assimilate allocation differ between species with different drought sensitivities and how these effects are modified by interspecific competition. We assessed the fate of (13) C labelled assimilates in above- and belowground plant organs and in root/rhizosphere respired CO2 in saplings of drought-tolerant Norway maple (Acer platanoides) and drought-sensitive European beech (Fagus sylvatica) exposed to moderate drought, either in mono- or mixed culture. While drought reduced stomatal conductance and photosynthesis rates in both species, both maintained assimilate transport belowground. Beech even allocated more new assimilate to the roots under moderate drought compared to non-limited water supply conditions, and this pattern was even more pronounced under interspecific competition. Even though maple was a superior competitor compared to beech under non-limited soil water conditions, as indicated by the changes in above- and belowground biomass of both species in the interspecific competition treatments, we can state that beech was still able to efficiently allocate new assimilate belowground under combined drought and interspecific competition. This might be seen as a strategy to maintain root osmotic potential and to prioritise root functioning. Our results thus show that beech tolerates moderate drought stress plus competition without losing its ability to supply belowground tissues. It remains to be explored in future work if this strategy is also valid during long-term drought exposure.

Coordination between growth, phenology and carbon storage in three coexisting deciduous tree species in a temperate forest

In deciduous trees growing in temperate forests, bud break and growth in spring must rely on intrinsic carbon (C) reserves. Yet it is unclear whether growth and C storage occur simultaneously, and whether starch C in branches is sufficient for refoliation. To test in situ the relationships between growth, phenology and C utilization, we monitored stem growth, leaf phenology and stem and branch nonstructural carbohydrate (NSC) dynamics in three deciduous species: Carpinus betulus L., Fagus sylvatica L. and Quercus petraea (Matt.) Liebl. To quantify the role of NSC in C investment into growth, a C balance approach was applied. Across the three species, >95% of branchlet starch was consumed during bud break, confirming the importance of C reserves for refoliation in spring. The C balance calculation showed that 90% of the C investment in foliage (7.0-10.5 kg tree(-1) and 5-17 times the C needed for annual stem growth) was explained by simultaneous branchlet starch degradation. Carbon reserves were recovered sooner than expected, after leaf expansion, in parallel with stem growth. Carpinus had earlier leaf phenology (by ∼25 days) but delayed cambial growth (by ∼15 days) than Fagus and Quercus, the result of a competitive strategy to flush early, while having lower NSC levels.

Understanding the roles of nonstructural carbohydrates in forest trees - from what we can measure to what we want to know

Contents 386 I. 386 II. 388 III. 392 IV. 392 V. 396 VI. 399 399 References 399 SUMMARY: Carbohydrates provide the building blocks for plant structures as well as versatile resources for metabolic processes. The nonstructural carbohydrates (NSC), mainly sugars and starch, fulfil distinct functional roles, including transport, energy metabolism and osmoregulation, and provide substrates for the synthesis of defence compounds or exchange with symbionts involved in nutrient acquisition or defence. At the whole-plant level, NSC storage buffers the asynchrony of supply and demand on diel, seasonal or decadal temporal scales and across plant organs. Despite its central role in plant function and in stand-level carbon cycling, our understanding of storage dynamics, its controls and response to environmental stresses is very limited, even after a century of research. This reflects the fact that often storage is defined by what we can measure, that is, NSC concentrations, and the interpretation of these as a proxy for a single function, storage, rather than the outcome of a range of NSC source and sink functions. New isotopic tools allow direct quantification of timescales involved in NSC dynamics, and show that NSC-C fixed years to decades previously is used to support tree functions. Here we review recent advances, with emphasis on the context of the interactions between NSC, drought and tree mortality.

Experimental drought and heat can delay phenological development and reduce foliar and shoot growth in semiarid trees

Higher temperatures associated with climate change are anticipated to trigger an earlier start to the growing season, which could increase the terrestrial C sink strength. Greater variability in the amount and timing of precipitation is also expected with higher temperatures, bringing increased drought stress to many ecosystems. We experimentally assessed the effects of higher temperature and drought on the foliar phenology and shoot growth of mature trees of two semiarid conifer species. We exposed field-grown trees to a ~45% reduction in precipitation with a rain-out structure ('drought'), a ~4.8 °C temperature increase with open-top chambers ('heat'), and a combination of both simultaneously ('drought + heat'). Over the 2013 growing season, drought, heat, and drought + heat treatments reduced shoot and needle growth in piñon pine (Pinus edulis) by ≥39%, while juniper (Juniperus monosperma) had low growth and little response to these treatments. Needle emergence on primary axis branches of piñon pine was delayed in heat, drought, and drought + heat treatments by 19-57 days, while secondary axis branches were less likely to produce needles in the heat treatment, and produced no needles at all in the drought + heat treatment. Growth of shoots and needles, and the timing of needle emergence correlated inversely with xylem water tension and positively with nonstructural carbohydrate concentrations. Our findings demonstrate the potential for delayed phenological development and reduced growth with higher temperatures and drought in tree species that are vulnerable to drought and reveal potential mechanistic links to physiological stress responses. Climate change projections of an earlier and longer growing season with higher temperatures, and consequent increases in terrestrial C sink strength, may be incorrect for regions where plants will face increased drought stress with climate change.

Non-structural carbon dynamics and allocation relate to growth rate and leaf habit in California oaks

Trees contain non-structural carbon (NSC), but it is unclear for how long these reserves are stored and to what degree they are used to support plant activity. We used radiocarbon ((14)C) to show that the carbon (C) in stemwood NSC can achieve ages of several decades in California oaks. We separated NSC into two fractions: soluble (∼50% sugars) and insoluble (mostly starch) NSC. Soluble NSC contained more C than insoluble NSC, but we found no consistent trend in the amount of either pool with depth in the stem. There was no systematic difference in C age between the two fractions, although ages increased with stem depth. The C in both NSC fractions was consistently younger than the structural C from which they were extracted. Together, these results indicate considerable inward mixing of NSC within the stem and rapid exchange between soluble and insoluble pools, compared with the timescale of inward mixing. We observed similar patterns in sympatric evergreen and deciduous oaks and the largest differences among tree stems with different growth rates. The (14)C signature of carbon dioxide (CO2) emitted from tree stems was higher than expected from very recent photoassimilates, indicating that the mean age of C in respiration substrates included a contribution from C fixed years previously. A simple model that tracks NSC produced each year, followed by loss (through conversion to CO2) in subsequent years, matches our observations of inward mixing of NSC in the stem and higher (14)C signature of stem CO2 efflux. Together, these data support the idea of continuous accumulation of NSC in stemwood and that 'vigor' (growth rate) and leaf habit (deciduous vs evergreen) control NSC pool size and allocation.

Paradigm shift in plant growth control

For plants to grow they need resources and appropriate conditions that these resources are converted into biomass. While acknowledging the importance of co-drivers, the classical view is still that carbon, that is, photosynthetic CO2 uptake, ranks above any other drivers of plant growth. Hence, theory and modelling of growth traditionally is carbon centric. Here, I suggest that this view is not reflecting reality, but emerged from the availability of methods and process understanding at leaf level. In most cases, poorly understood processes of tissue formation and cell growth are governing carbon demand, and thus, CO2 uptake. Carbon can only be converted into biomass to the extent chemical elements other than carbon, temperature or cell turgor permit.