No carbon storage in growth-limited trees in a semi-arid woodland

Water, not carbon, drives drought-constraints on stem terpene defense against simulated bark beetle attack in Pinus edulis

Drought predisposes forest trees to bark beetle-induced mortality, but the physiological mechanisms remain unclear. While drought-induced water and carbon limitations have been implicated in defensive failure and tree susceptibility, evidence demonstrating how these factors interact is scarce. We withheld water from mature, potted Pinus edulis and subsequently applied a double-stem girdle to inhibit carbohydrate transport from the crown and roots. Within this isolated segment we then elicited a defense response by inoculating trees with a bark beetle-fungal symbiont (Ophiostoma sp.). We quantified local mono- and sesquiterpenes (MST), nonstructural carbohydrates (NSC), and pressure potential of the inner bark. Both drought-stressed and watered trees had similar NSC concentrations just before inoculation and depleted NSC similarly following inoculation, yet MST induction (i.e. increased concentration and altered composition) was constrained only in drought-stressed trees. Thus, NSC consumption was largely unrelated to de novo MST synthesis. Instead, stoichiometric calculations show that induction originated largely from stored resin. Watered trees experiencing higher pressure potentials consistently induced higher MST concentrations. We demonstrate the importance of preformed resin toward an induced MST response in a semi-arid conifer where drought-constraints on defense occurred through biophysical limitations (i.e. reduced turgor hindering resin transport) rather than through substrate limitation.

Stem Growth and Dehydration Responses of Mediterranean Tree Species to Atmospheric and Soil Drought

Stem growth responses to soil and atmospheric drought are critical to forecasting the tree carbon sink strength. Yet, responses of drought-prone forests remain uncertain despite global aridification trends. Stem diameter variations at an hourly resolution were monitored in five Mediterranean tree species from a mesic and a xeric site for 6 and 12 years. Stem growth and dehydration responses to soil (REW) and atmospheric (VPD) drought were explored at different timescales. Annually, growth was determined by the number of growing days and hours. Seasonally, growth was bimodal (autumn growth ≈ 8%-18% of annual growth), varying among species and sites across the hydrometeorological space, while dehydration consistently responded to REW. Sub-daily, substantial growth occurred during daytime, with nighttime-to-daytime ratios ranging between 1.2 and 3.5 (Arbutus unedo ≈ Quercus faginea < Quercus ilex < Pinus halepensis in the mesic site, and Juniperus thurifera < P. halepensis in the xeric site). Overall, time windows favourable for growth were limited by soil (rather than atmospheric) drought, modulating annual and seasonal growth in Mediterranean species, and stems maintained non-negligible growth during daytime. These patterns contrast with observations from wetter or cooler biomes, demonstrating the growth plasticity of drought-prone species to more arid climate conditions.

Drought induces opposite changes in organ carbon and soil organic carbon to increase resistance on moso bamboo

Introduction

The variety of organs carbon concentration may be important for tree survival rate, drought resistance and tree subsequent recovery. However, it remains unclear how drought affect structural carbohydrate (SC) and non-structural carbohydrate (NSC) export and transport on clonal plant, which can be correlated with sustain physiological metabolism and group drought resistance by resource sharing. To better understand the adaption ability of clone plants to drought and the linkage of organ carbon with soil organic carbon (SOC) fractions, we assessed how long-term drought affects organ carbon and its impact on SOC fractions among moso bamboo (Phyllostachys edulis) ramets.

Methods

Throughfall exclusion included two treatments with simulated drought (TE) and control sample (CK, natural rainfall) by waterproofing panels, which excluded 70-80% of the precipitation. We measured the SC and NSC of leaves, branches and roots as well as soil organic carbon components on three ramets, which emerged in 2017 (grandma, GB), ramets that appeared in 2018 (mother bamboo, MB) and ramets emerging in 2019 (current-year bamboo, CB).

Results

The results showed that there was significant difference on lignin, cellulose: lignin ratio and soluble sugar in leaves and roots (p<0.05) instead of branches (p>0.05). Effect of drought on SC and NSC varied with different organs and ramet age. Drought significantly increased soluble sugar concentration of leaves and roots by 15.5-31.0% and 10.6-24.8% for current-year bamboo. Compared with CK, drought decreased SOC by 6.7-19.1%, microbial biomass carbon (MBC) by 55.3-68.7%, readily oxidizable carbon (ROC) by 11.2-29.8%, particulate organic carbon (POC) by 25.1-47.4% but no effect on mineral-associated organic carbon (MOC). Drought changed the relationships of carbon components between plant organs and soil. In the control treatments, SC of leaves were significantly positively correlated with ROC, NSC of branches were positively correlated with ROC and MBC, NSC of roots were significantly positively correlated with SOC.

Discussion

Overall, our results suggest that drought strengthened the linkage of plant organ carbon and soil carbon cycling among moso bamboo ramets in ecosystem studies, which are critical for predicting tree resistance and management in subtropical forest under drought.

Contrasting survival strategies for seedlings of two northern conifer species to extreme droughts and floods

Lowland northern white-cedar (Thuja occidentalis L.) forests are increasingly exposed to extreme droughts and floods that cause tree mortality. However, it is not clear the extent to which these events may differentially affect regeneration of cedar and its increasingly common associate, balsam fir (Abies balsamea (L.) Mill.). To test this, we measured how seedlings of cedar and fir were able to avoid, resist and recover from experimental drought and flood treatments of different lengths (8 to 66 days). Overall, we found that cedar exhibited a strategy of stress resistance and growth recovery (resilience) from moderate drought and flood stress. Fir, on the other hand, appears to be adapted to avoid drought and flood stress and exhibited overall lower growth resilience. In drought treatments, we found evidence of different stomatal behaviors. Cedar used available water quickly and therefore experienced more drought stress than fir, but cedar was able to survive at water potentials > 3 MPa below key hydraulic thresholds. On the other hand, fir employed a more conservative water-use strategy and therefore avoided extremely low water potential. In response to flood treatments, cedar survival was higher and only reached 50% if exposed to 23.1 days of flooding in contrast to only 7.4 days to reach 50% mortality for fir. In both droughts and floods, many stressed cedar were able to maintain partially brown canopies and often survived the stress, albeit with reduced growth, suggesting a strategy of resistance and resilience. In contrast, fir that experienced drought or flood stress had a threshold-type responses and they either had full live canopies with little effect on growth or they died suggesting reliance on a strategy of drought avoidance. Combined with increasingly variable precipitation regimes, seasonal flooding and complex microtopography that can provide safe sites in these forests, these results inform conservation and management of lowland cedar stands.

Local carbon reserves are insufficient for phloem terpene induction during drought in Pinus edulis in response to bark beetle-associated fungi

Stomatal closure during drought inhibits carbon uptake and may reduce a tree's defensive capacity. Limited carbon availability during drought may increase a tree's mortality risk, particularly if drought constrains trees' capacity to rapidly produce defenses during biotic attack. We parameterized a new model of conifer defense using physiological data on carbon reserves and chemical defenses before and after a simulated bark beetle attack in mature Pinus edulis under experimental drought. Attack was simulated using inoculations with a consistent bluestain fungus (Ophiostoma sp.) of Ips confusus, the main bark beetle colonizing this tree, to induce a defensive response. Trees with more carbon reserves produced more defenses but measured phloem carbon reserves only accounted for c. 23% of the induced defensive response. Our model predicted universal mortality if local reserves alone supported defense production, suggesting substantial remobilization and transport of stored resin or carbon reserves to the inoculation site. Our results show that de novo terpene synthesis represents only a fraction of the total measured phloem terpenes in P. edulis following fungal inoculation. Without direct attribution of phloem terpene concentrations to available carbon, many studies may be overestimating the scale and importance of de novo terpene synthesis in a tree's induced defense response.

A Preliminary Study on the Whole-Plant Regulations of the Shrub Campylotropis polyantha in Response to Hostile Dryland Conditions

Drylands cover more than 40% of global land surface and will continue to expand by 10% at the end of this century. Understanding the resistance mechanisms of native species is of particular importance for vegetation restoration and management in drylands. In the present study, metabolome of a dominant shrub Campylotropis polyantha in a dry-hot valley were investigated. Compared to plants grown at the wetter site, C. polyantha tended to slow down carbon (C) assimilation to prevent water loss concurrent with low foliar reactive oxygen species and sugar concentrations at the drier and hotter site. Nitrogen (N) assimilation and turn over were stimulated under stressful conditions and higher leaf N content was kept at the expense of root N pools. At the drier site, roots contained more water but less N compounds derived from the citric acid cycle. The site had little effect on metabolites partitioning between leaves and roots. Generally, roots contained more C but less N. Aromatic compounds were differently impacted by site conditions. The present study, for the first time, uncovers the apparent metabolic adaptations of C. polyantha to hostile dryland conditions. However, due to the limited number of samples, we are cautious about drawing general conclusions regarding the resistance mechanisms. Further studies with a broader spatial range and larger time scale are therefore recommended to provide more robust information for vegetation restoration and management in dryland areas under a changing climate.

Enhancement of Apple Stress Resistance via Proline Elevation by Sugar Substitutes

Plants encounter numerous adversities during growth, necessitating the identification of common stress activators to bolster their resistance. However, the current understanding of these activators' mechanisms remains limited. This study identified three anti-stress activators applicable to apple trees, all of which elevate plant proline content to enhance resistance against various adversities. The results showed that the application of these sugar substitutes increased apple proline content by two to three times compared to the untreated group. Even at a lower concentration, these activators triggered plant stress resistance without compromising apple fruit quality. Therefore, these three sugar substitutes can be exogenously sprayed on apple trees to augment proline content and fortify stress resistance. Given their effectiveness and low production cost, these activators possess significant application value. Since they have been widely used in the food industry, they hold potential for broader application in plants, fostering apple industry development.

Beyond source and sink control - toward an integrated approach to understand the carbon balance in plants

A conceptual understanding on how the vegetation's carbon (C) balance is determined by source activity and sink demand is important to predict its C uptake and sequestration potential now and in the future. We have gathered trajectories of photosynthesis and growth as a function of environmental conditions described in the literature and compared them with current concepts of source and sink control. There is no clear evidence for pure source or sink control of the C balance, which contradicts recent hypotheses. Using model scenarios, we show how legacy effects via structural and functional traits and antecedent environmental conditions can alter the plant's carbon balance. We, thus, combined the concept of short-term source-sink coordination with long-term environmentally driven legacy effects that dynamically acclimate structural and functional traits over time. These acclimated traits feedback on the sensitivity of source and sink activity and thus change the plant physiological responses to environmental conditions. We postulate a whole plant C-coordination system that is primarily driven by stomatal optimization of growth to avoid a C source-sink mismatch. Therefore, we anticipate that C sequestration of forest ecosystems under future climate conditions will largely follow optimality principles that balance water and carbon resources to maximize growth in the long term.

A neutral theory of plant carbon allocation

How plants use the carbon they gain from photosynthesis remains a key area of study among plant ecologists. Although numerous theories have been presented throughout the years, the field lacks a clear null model. To fill this gap, I have developed the first null model, or neutral theory, of plant carbon allocation using probability theory, plant biochemistry and graph theory at the level of a leaf. Neutral theories have been used to establish a null hypothesis in molecular evolution and community assembly to describe how much of an ecological phenomenon can be described by chance alone. Here, the aim of a neutral theory of plant carbon allocation is to ask: how is carbon partitioned between sinks if one assumes plants do not prioritize certain sinks over others? Using the biochemical network of plant carbon metabolism, I show that, if allocation was strictly random, carbon is more likely to be allocated to storage, defense, respiration and finally growth. This 'neutral hierarchy' suggests that a sink's biochemical distance from photosynthesis plays an important role in carbon allocation patterns, highlighting the potentially adaptive role of this biochemical network for plant survival in variable environments. A brief simulation underscores that our ability to measure the carbon allocation from photosynthesis to a given sink is unreliable due to simple probabilistic rules. While neutral theory may not explain all patterns of carbon allocation, its utility is in the minimal assumptions and role as a null model against which future data should be tested.

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

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

Estimating canopy stomatal conductance and photosynthesis in apple trees by upscaling parameters from the leaf scale to the canopy scale in Jinzhong Basin on Loess Plateau

The estimations of stomatal conductance and photosynthesis performed by upscaling the parameters from the leaf scale to the canopy scale are key points in the fields of forest ecohydrology and physiology. The foundation for solving this scientific problem is determining the optimal models for calculating the leaf stomatal conductance (gl) and photosynthetic rate (Pl). In this study, we used the Jarvis model combined with modification factors, including leaf-air temperature (ΔT) and CO2 concentration inside and outside the stomata (ΔC), to estimate gl and the new Ye light-response model to estimate the Pl of apple trees in Jinzhong Basin on Loess Plateau. The results show that the modified Jarvis (JarvisΔT-ΔC) model and the new Ye light-response model could estimate gl and Pl, respectively, with very high accuracy, with R2 values of 0.926 and 0.959 for the former, and 0.987 and 0.983 for the latter in 2019 and 2021, respectively. Then, we estimated the canopy stomatal conductance (gc) and photosynthetic rate (Pc) by first dividing the apple tree canopy into sunlit and shaded leaves and then summing the contribution of sunlit and shaded gl, Pl and leaf area index. Our efforts will be a valid reference for estimating the gc and Pc of other tree or crop species in the future.