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.
Earth's forests face grave challenges in the Anthropocene, including hotter droughts increasingly associated with widespread forest die-off events. But despite the vital importance of forests to ...global ecosystem services, their fates in a warming world remain highly uncertain. Lacking is quantitative determination of commonality in climate anomalies associated with pulses of tree mortality-from published, field-documented mortality events-required for understanding the role of extreme climate events in overall global tree die-off patterns. Here we established a geo-referenced global database documenting climate-induced mortality events spanning all tree-supporting biomes and continents, from 154 peer-reviewed studies since 1970. Our analysis quantifies a global "hotter-drought fingerprint" from these tree-mortality sites-effectively a hotter and drier climate signal for tree mortality-across 675 locations encompassing 1,303 plots. Frequency of these observed mortality-year climate conditions strongly increases nonlinearly under projected warming. Our database also provides initial footing for further community-developed, quantitative, ground-based monitoring of global tree mortality.
Drought-induced tree mortality results from an interaction of several mechanisms. Plant water and carbon relations are interdependent and assessments of their individual contributions are difficult. ...Because drought always affects both plant hydration and carbon assimilation, it is challenging to disentangle their concomitant effects on carbon balance and carbon translocation. Here, we report results of a manipulation experiment specifically designed to separate drought effects on carbon and water relations from those on carbon translocation.
In a glasshouse experiment, we manipulated the carbon balance of Norway spruce saplings exposed to either drought or carbon starvation (CO2 withdrawal), or both treatments, and compared the dynamics of carbon exchange, allocation and storage in different tissues.
Drought killed trees much faster than did carbon starvation. Storage C pools were not depleted at death for droughted trees as they were for starved, well-watered trees. Hence drought has a significant detrimental effect on a plant's ability to utilize stored carbon.
Unless they can be transported to where they are needed, sufficient carbon reserves alone will not assure survival of a drought except under specific conditions, such as moderate drought, or in species that maintain plant water relations required for carbon re-mobilization.
Forest disturbance regimes are expected to intensify as Earth's climate changes. Quantifying forest vulnerability to disturbances and understanding the underlying mechanisms is crucial to develop ...mitigation and adaptation strategies. However, observational evidence is largely missing at regional to continental scales. Here, we quantify the vulnerability of European forests to fires, windthrows and insect outbreaks during the period 1979-2018 by integrating machine learning with disturbance data and satellite products. We show that about 33.4 billion tonnes of forest biomass could be seriously affected by these disturbances, with higher relative losses when exposed to windthrows (40%) and fires (34%) compared to insect outbreaks (26%). The spatial pattern in vulnerability is strongly controlled by the interplay between forest characteristics and background climate. Hotspot regions for vulnerability are located at the borders of the climate envelope, in both southern and northern Europe. There is a clear trend in overall forest vulnerability that is driven by a warming-induced reduction in plant defence mechanisms to insect outbreaks, especially at high latitudes.
•Tree stems are more than a conduit for phloem C transport; they are a significant C sink.•Isotopes demonstrate most phloem-transported C exchanges with NSC stored in stems.•Exchange timescales are ...days, weeks or years, reflecting processes from mixing to remobilization.
For trees to survive, they must allocate resources between sources and sinks to maintain proper function. The vertical transport pathway in tree stems is essential for carbohydrates and other solutes to move between the canopy and the root system. To date, research and models emphasize the role of tree stems as ‘express’ sugar highways. However, recent investigations using isotopic markers suggest that there is considerable storage and exchange of phloem-transported sugars with older carbon (C) reserves within the stem. Thus, we suggest that stems play an important role not only in long-distance transport, but also in the regulation of the tree's overall C balance. A quantitative partitioning of stem C inputs among storage and sinks, including tissue growth, respiration, and export to roots, is still lacking. Combining methods to better quantify the dynamics and controls of C storage and remobilization in the stem will help to resolve central questions of allocation and C balance in trees.
Tropical woody plants store ∼230 petagrams of carbon (PgC) in their aboveground living biomass. This review suggests that these stocks are currently growing in primary forests at rates that have ...decreased in recent decades. Droughts are an important mechanism in reducing forest C uptake and stocks by decreasing photosynthesis, elevating tree mortality, increasing autotrophic respiration, and promoting wildfires. Tropical forests were a C source to the atmosphere during the 2015-2016 El Niño-related drought, with some estimates suggesting that up to 2.3 PgC were released. With continued climate change, the intensity and frequency of droughts and fires will likely increase. It is unclear at what point the impacts of severe, repeated disturbances by drought and fires could exceed tropical forests' capacity to recover. Although specific threshold conditions beyond which ecosystem properties could lead to alternative stable states are largely unknown, the growing body of scientific evidence points to such threshold conditions becoming more likely as climate and land use change across the tropics.
Droughts have reduced forest carbon uptake and stocks by elevating tree mortality, increasing autotrophic respiration, and promoting wildfires.
Threshold conditions beyond which tropical forests are pushed into alternative stable states are becoming more likely as effects of droughts intensify.
Theoretically, progressive drought can force trees into negative carbon (C) balance by reducing stomatal conductance to prevent water loss, which also decreases C assimilation. At higher ...temperatures, negative C balance should be initiated at higher soil moisture because of increased respiratory demand and earlier stomatal closure. Few data are available on how these theoretical relationships integrate over the whole plant.
We exposed Thuja occidentalis to progressive drought under three temperature conditions (15, 25, and 35°C), and measured C and water fluxes using a whole-tree chamber design.
High transpiration rates at higher temperatures led to a rapid decline in soil moisture. During the progressive drought, soil moisture-driven changes in photosynthesis had a greater impact on the whole-plant C balance than respiration. The soil moisture content at which whole-plant C balance became negative increased with temperature, mainly as a result of higher respiration rates and an earlier onset of stomatal closure under a warmer condition.
Our results suggest that the effect of drought on whole-plant C balance is highly temperature-dependent. High temperature causes a negative C balance even under mild drought and may increase the risk of C starvation.
Accumulating evidence highlights increased mortality risks for trees during severe drought, particularly under warmer temperatures and increasing vapour pressure deficit (VPD). Resulting forest ...die-off events have severe consequences for ecosystem services, biophysical and biogeochemical land–atmosphere processes. Despite advances in monitoring, modelling and experimental studies of the causes and consequences of tree death from individual tree to ecosystem and global scale, a general mechanistic understanding and realistic predictions of drought mortality under future climate conditions are still lacking. We update a global tree mortality map and present a roadmap to a more holistic understanding of forest mortality across scales. We highlight priority research frontiers that promote: (1) new avenues for research on key tree ecophysiological responses to drought; (2) scaling from the tree/plot level to the ecosystem and region; (3) improvements of mortality risk predictions based on both empirical and mechanistic insights; and (4) a global monitoring network of forest mortality. In light of recent and anticipated large forest die-off events such a research agenda is timely and needed to achieve scientific understanding for realistic predictions of drought-induced tree mortality. The implementation of a sustainable network will require support by stakeholders and political authorities at the international level.
• 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 carbon isotope ratio (δ13C) in tree rings is commonly used to derive estimates of the assimilation‐to‐stomatal conductance rate of trees, that is, intrinsic water‐use efficiency (iWUE). Recent ...studies have observed increased iWUE in response to rising atmospheric CO2 concentrations (Ca), in many different species, genera and biomes. However, increasing rates of iWUE vary widely from one study to another, likely because numerous covarying factors are involved. Here, we quantified changes in iWUE of two widely distributed boreal conifers using tree samples from a forest inventory network that were collected across a wide range of growing conditions (assessed using the site index, SI), developmental stages and stand histories. Using tree‐ring isotopes analysis, we assessed the magnitude of increase in iWUE after accounting for the effects of tree size, stand age, nitrogen deposition, climate and SI. We also estimated how growth conditions have modulated tree physiological responses to rising Ca. We found that increases in tree size and stand age greatly influenced iWUE. The effect of Ca on iWUE was strongly reduced after accounting for these two variables. iWUE increased in response to Ca, mostly in trees growing on fertile stands, whereas iWUE remained almost unchanged on poor sites. Our results suggest that past studies could have overestimated the CO2 effect on iWUE, potentially leading to biased inferences about the future net carbon balance of the boreal forest. We also observed that this CO2 effect is weakening, which could affect the future capacity of trees to resist and recover from drought episodes.
In this study, we show that the increase in iWUE observed for two widely distributed boreal tree species is largely the result of changes in tree biomass, growing conditions, climate change, and differences in site fertility. After accounting for these variables, the CO2 effect is strongly lessened. This finding suggests that past studies could have overestimated the capacity of trees to adjust their physiological processes in response to global warming, potentially leading to too optimistic inferences about the capacity of boreal forest to acclimate and adapt to future warmer climate, especially for black spruce growing on the least fertile areas.
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