Pre-drought Levels Research Articles

Transcriptomic and physiological responses of Quercus acutissima and Quercus palustris to drought stress and rewatering.

Establishment of oak seedlings, which is an important factor in forest restoration, is affected by drought that hampers the survival, growth, and development of seedlings. Therefore, it is necessary to understand how seedlings respond to and recover from water-shortage stress. We subjected seedlings of two oak species, Quercus acutissima and Quercus palustris, to drought stress for one month and then rewatered them for six days to observe physiological and genetic expression changes. Phenotypically, the growth of Q. acutissima was reduced and severe wilting and recovery failure were observed in Q. palustris after an increase in plant temperature. The two species differed in several physiological parameters during drought stress and recovery. Although the photosynthesis-related indicators did not change in Q. acutissima, they were decreased in Q. palustris. Moreover, during drought, content of soluble sugars was significantly increased in both species, but it recovered to original levels only in Q. acutissima. Malondialdehyde content increased in both the species during drought, but it did not recover in Q. palustris after rewatering. Among the antioxidant enzymes, only superoxide dismutase activity increased in Q. acutissima during drought, whereas activities of ascorbate peroxidase, catalase, and glutathione reductase increased in Q. palustris. Abscisic acid levels were increased and then maintained in Q. acutissima, but recovered to previous levels after rewatering in Q. palustris. RNA samples from the control, drought, recovery day 1, and recovery day 6 treatment groups were compared using transcriptome analysis. Q. acutissima exhibited 832 and 1076 differentially expressed genes (DEGs) related to drought response and recovery, respectively, whereas Q. palustris exhibited 3947 and 1587 DEGs, respectively under these conditions. Gene ontology enrichment of DEGs revealed "response to water," "apoplast," and "Protein self-association" to be common to both the species. However, in the heatmap analysis of genes related to sucrose and starch synthesis, glycolysis, antioxidants, and hormones, the two species exhibited very different transcriptome responses. Nevertheless, the levels of most DEGs returned to their pre-drought levels after rewatering. These results provide a basic foundation for understanding the physiological and genetic expression responses of oak seedlings to drought stress and recovery.

Responses of stem growth and canopy greenness of temperate conifers to dry spells

Dry spells strongly influence biomass production in forest ecosystems. Their effects may last several years following a drought event, prolonging growth reduction and therefore restricting carbon sequestration. Yet, our understanding of the impact of dry spells on the vitality of trees' above-ground biomass components (e.g., stems and leaves) at a landscape level remains limited. We analyzed the responses of Pinus sylvestris and Picea abies to the four most severe drought years in topographically complex sites. To represent stem growth and canopy greenness, we used chronologies of tree-ring width and time series of the Normalized Difference Vegetation Index (NDVI). We analyzed the responses of radial tree growth and NDVI to dry spells using superposed epoch analysis and further explored this relationship using mixed-effect models. Our results show a stronger and more persistent response of radial growth to dry spells and faster recovery of canopy greenness. Canopy greenness started to recover the year after the dry spell, whereas radial tree growth remained reduced for the two subsequent years and did not recover the pre-drought level until the fourth year after the event. Stem growth and canopy greenness were influenced by climatic conditions during and after drought events, while the effect of topography was marginal. The opposite responses of stem growth and canopy greenness following drought events suggest a different impact of dry spells on trees´ sink and source compartments. These results underscore the crucial importance of understanding the complexities of tree growth as a major sink of atmospheric carbon.

Drought increases Norway spruce susceptibility to the Eurasian spruce bark beetle and its associated fungi.

Drought affects the complex interactions between Norway spruce, the bark beetle Ips typographus and associated microorganisms. We investigated the interplay of tree water status, defense and carbohydrate reserves with the incidence of bark beetle attack and infection of associated fungi in mature spruce trees. We installed roofs to induce a 2-yr moderate drought in a managed spruce stand to examine a maximum of 10 roof and 10 control trees for resin flow (RF), predawn twig water potentials, terpene, phenolic and carbohydrate bark concentrations, and bark beetle borings in field bioassays before and after inoculation with Endoconidiophora polonica and Grosmannia penicillata. Drought-stressed trees showed more attacks and significantly longer fungal lesions than controls, but maintained terpene resin defenses at predrought levels. Reduced RF and lower mono- and diterpene, but not phenolic concentrations were linked with increased host selection. Bark beetle attack and fungi stimulated chemical defenses, yet G. penicillata reduced phenolic and carbohydrate contents. Chemical defenses did not decrease under mild, prolonged drought in our simulated small-scale biotic infestations. However, during natural mass attacks, reductions in carbon fixation under drought, in combination with fungal consumption of carbohydrates, may deplete tree defenses and facilitate colonization by I. typographus.

Analysis of Forest Inventory Data Shows Disparity in Tree Mortality and Resistance in Texas National Forests

Abstract In 2011, the forestlands of Texas were affected by one of the most severe droughts in the state’s history, killing an estimated 65 million trees in the East Texas region. This study uses USDA Forest Service Forest Inventory and Analysis data and a standardized precipitation-evapotranspiration index to estimate major tree species responses to the 2011 drought in Texas’ four national forests. We used an analysis of variance and a linear mixed-effect model to evaluate factors contributing to tree mortality and growth. We found that tree mortality rates varied greatly during the drought period relative to predrought levels; higher stand age, higher stand density, and higher mean summer temperature contributed to tree mortality volume. Study Implications: Drought is a recurring disturbance agent and is likely to continue to increase in frequency and severity in the future. We provide an understanding of an exceptional drought as a disturbance agent and its impacts on tree mortality and resistance. The effects of the 2011 drought on national forests in Texas have not previously been evaluated. Texas’ national forests represent over 256,975 ha (635,000 ac) in the state’s eastern region and nearly 10% of all public land in Texas. Our results are useful to decision makers in developing strategies and protecting forestlands in the face of potential future droughts.

Recovery of root hydraulic conductance and xylem vessel diameter following prolonged water deficit of maize

AbstractTo sustain crop growth following soil water deficit, it is essential to have rapid recovery of leaf gas exchange. One basis for rapid recovery would be the rapid return of root hydraulic conductance to predrought levels to support water transport to the plant shoot. In this study, transpiration and root hydraulic conductance were measured over a 9‐day recovery period following an initial water‐deficit treatment of three maize (Zea mays L.) cultivars. In addition, xylem vessel diameter was measured during the recovery period at different positions from the root tip. The initiation of recovery in the transpiration rate occurred in all three cultivars within 3 days after watering, although the root conductance and transpiration rate of one cultivar were much lower than the other two cultivars. Root conductance and vessel diameter increased more slowly than the recovery of transpiration. There was an indication that transpiration could have been limited by root hydraulic conductance only when its values were low during recovery. The relationship between hydraulic conductance and vessel diameter did not, however, indicate a Poiseuille's law relationship between the two variables. Overall, cultivar variability in recovery from water deficit of the transpiration rate and root hydraulic conductance indicates that the selection for rapid recovery of leaf gas exchange following water deficit could be an important component of genotype selection in maize breeding programs.

Assessing Terrestrial Water Storage Variations in Southern Spain Using Rainfall Estimates and GRACE Data

This paper investigates the relationship between rainfall, groundwater and Gravity Recovery and Climate Experiment (GRACE) data to generate regional-scale estimates of terrestrial water storage variations in the Andalucía region of southern Spain. These estimates can provide information on groundwater depletion (caused by periods of low rainfall or droughts) and groundwater recovery. The spatial distribution of groundwater bodies in southern Spain is complex and current in situ groundwater monitoring methods are deficient, particularly in terms of obtaining representative samples and in implementing and maintaining groundwater monitoring networks. The alternative approach proposed here is to investigate the relationship between precipitation time series and changes in the terrestrial water storage estimated from GRACE observations. The results were validated against the estimated fluctuation in regional groundwater. The maximum correlation between the mean groundwater level and the GRACE observations is 0.69 and this occurs at a lag of one month because the variation in gravity is immediate, but rainfall water requires around one month to travel across the vadose zone before it reaches the groundwater table. Using graphical methods of accumulated deviations from the mean, we show that, in general, groundwater storage follows the smooth, multi-year trends of terrestrial water storage but with less short-term trends; the same is true of rainfall, for which the local trends are more pronounced. There is hysteresis-like behaviour in the variations in terrestrial water storage and in the variations of groundwater. In practical terms, this study shows that, despite the abnormal dryness of the Iberian Peninsula during the 2004–2010 drought, the depleted groundwater storage in Andalucía recovered almost to its pre-drought level by 2016. In addition, groundwater storage and terrestrial water storage show very similar trends but with a delay in the groundwater trend.

Declining resistance of vegetation productivity to droughts across global biomes

Terrestrial vegetation productivity or gross primary productivity (GPP) is the main driver of the land carbon sink, and frequent and severe droughts largely affect the interannual variability of vegetation productivity and therefore land carbon sink. However, a comprehensive evaluation of spatiotemporal variation in drought response of global vegetation productivity is still lacking. Here, we used five long-term satellite-derived time series to quantify the global pattern and temporal changes of drought resistance (the ability to maintain its original levels during drought) and resilience (the ability to recover to pre-drought levels after droughts) during 1982–2015. Temporally, observed drought resistance showed a significantly declining trend across different biomes (P < 0.001), with evergreen broadleaf forests decreasing most (P < 0.001). Moreover, drought resilience showed a slight decreasing trend (P < 0.05) and has not decreased significantly among biomes. We attributed this declined resistance predominantly to temperature and soil moisture. Earth system models underestimate the drought resistance and resilience for the historical period, and project declining drought resistance and resilience to the end of 21st century. Our finding of the significant declining resistance but no increase in resilience implies the vulnerability of terrestrial carbon sinks to withstand future droughts.

An experimental assessment of carbon and nitrogen allocation in Pinus pinea populations under drought stress and rewatering treatment

Global drought effects will increase with climate change and, accordingly, plant species will be affected at different scales. Carbon (C) and nitrogen (N) are critical for plant growth and development. Drought stress disturbs the process of adjusting C and N acquisition and allocation in plants. The present study aimed to determine the N (%), C (%), C/N ratio, carbon mass (CM, g), and nitrogen mass (NM, g) in different plant organs (needle, shoot, stem, and root) in 2-years-old stone pine (Pinus pinea L.) seedlings from seven different populations of Türkiye under drought stress and rewatering treatment. The seedlings were subjected to drought until a predawn water potential between − 4.5 MPa to − 4.0 MPa. The return of the seedlings to their pre-drought level was determined by using resistance (RS), recovery (RC), and resilience (RSL) indices. The N concentration in each organ showed significant differences between the populations in response to drought while the C concentration differed between the populations only in roots. In drought, the N (%) and NM (g) values in needles increased in the populations and the increase was 50% higher in northern populations. The N (%) and NM (g) values in shoots increased almost twice in response to drought. Although C (%) and CM (g) decreased in needles and shoots in drought, they increased in stems and roots. The stem C/N ratio increased by more than 100% in some populations in drought. Root C/N ratios increased by more than 50% in drought, while they generally decreased below control values with rewatering. The C/N ratio in the stem showed great resistance and recovery, while resilience stayed negative. In conclusion, stone pine populations generally demonstrated high recovery in terms of N concentration in roots after drought. Compared to northern populations, southern populations showed more resistance to drought in terms of needle and shoot N concentration. The needle and shoot N (%) of northern populations demonstrated high resilience capacity.

Riparian forest response to extreme drought is influenced by climatic context and canopy structure

Droughts significantly impact forest ecosystems, reducing forest health and productivity, compromising ecosystem functioning, and nature-based solutions for climate change. The response and resilience of riparian forests to drought are poorly understood despite their key role in the functioning of aquatic and terrestrial ecosystems. Here we investigate riparian forest drought responses and resilience to an extreme drought event at a regional scale. We also examine how drought event characteristics, average climate conditions, topography, soil, vegetation structure, and functional diversity shape the resilience of riparian forests to drought. We used a time series of the Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) to calculate the resistance to and recovery after an extreme drought (2017–2018) in 49 sites across an Atlantic-Mediterranean climate gradient in North Portugal. We used generalized additive models and multi-model inference to understand which factors best explained drought responses. We found a trade-off between drought resistance and recovery (maximum r = −0.5) and contrasting strategies across the climatic gradient of the study area. Riparian forests in the Atlantic regions showed comparatively higher resistance, while Mediterranean forests recovered more. Canopy structure and climate context were the most relevant predictors of resistance and recovery. However, median NDVI and NDWI had not returned to pre-drought levels (RcNDWI mean = 1.21, RcNDVI mean = 1.01) three years after the event. Our study shows that riparian forests have contrasting drought response strategies and may be susceptible to extended legacy effects associated with extreme and/or recurring droughts, similarly to upland forests. This work highlights the drought vulnerability of riparian ecosystems and emphasises the need for further studies on long-term resilience to droughts.

Carbon stored in live ponderosa pines in the Sierra Nevada will not return to pre-drought (2012) levels during the 21st century due to bark beetle outbreaks

Outbreaks of several bark beetle species can develop rapidly in response to drought and may result in large transfers of carbon (C) stored in live trees to C stored in dead trees (10s of Tg C yr-1 in the western U.S. alone), which over time will be released back to the atmosphere. The western pine beetle (WPB) outbreak incited by the 2012–2015 mega-drought in the Sierra Nevada, California, U.S., could portend more frequent and/or severe bark beetle outbreaks as the temperature warms and drought frequency and intensity increase in the future. However, changes in the frequency and/or severity (resultant levels of host tree mortality) of beetle outbreaks are difficult to predict as outbreaks are complex with non-linear and eruptive processes primarily driven by interactions among beetle populations, the demography of hosts and other tree species, and climate and weather. Using an insect phenology and tree defense model, we projected the future likelihood of WPB outbreaks in the Sierra Nevada with climate drivers from different Earth System Models. Our goal was to understand how host (ponderosa pine, PIPO) recovery and future warming and drought affect the frequency and severity of WPB outbreaks and their C consequences. Our projections suggested that by 2100 the C stored in live PIPO (mean: 1.98 kg C m-2, 95% CI: 1.74–2.21 kg C m-2) will not return to levels that occurred before the 2012–2015 drought (2012: ∼2.30 kg C m-2) due to future WPB outbreaks. However, differences in climate models indicate a wide range of possible WPB outbreak frequencies and severities. Our results suggest that total plot basal area is the most significant factor in the mortality rate of PIPO by WPB in any given year, followed by drought severity and temperature. High levels of host basal area, higher temperature, and extreme drought all contribute to the frequency and severity of future WPB outbreaks. While PIPO basal area may decline under increased drought and warming, limiting high-stand basal area (&amp;gt;60 m2 ha-1) may reduce the severity of future WPB outbreaks in the Sierra Nevada.

气候和密度对刺槐径向生长和干旱脆弱性的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 气候和密度对刺槐径向生长和干旱脆弱性的影响 DOI: 10.5846/stxb202112133525 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点研发计划项目(2020YFA0608101) Effects of climate and density on the radial growth and drought vulnerability of Robinia pseudoacacia Author: Affiliation: Fund Project: National Key Research and Development Program of China (Project No. 2020YFA0608101) 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:为评估气候和竞争对刺槐径向生长、抗性和弹性的影响,使用年轮气候学方法建立河南省民权和济源不同密度刺槐的生长年表,确定不同密度刺槐径向生长与气象因子的关联,利用胸高断面积增量变化获得干旱事件前后不同密度刺槐的干旱脆弱性,旨在确定气候和密度对刺槐径向生长和干旱脆弱性影响。结果表明:在生长前期,不同密度刺槐的径向生长无显著差异,随着树木的生长,高密度刺槐的年轮宽度和胸高断面积增量(BAI)开始显著低于低密度(P<0.05)。Pearson相关分析结果显示,生长季的标准化植被蒸散指数、降水、相对湿度、温度和饱和水汽压亏缺是影响刺槐生长的重要因素。路径分析结果显示降水和温度是年尺度上影响刺槐生长关键因素。受干旱事件的影响,刺槐的年轮宽度和BAI均下降,低密度刺槐恢复力、弹性、相对弹性均显著高于高密度(P<0.05),在第1次干旱事件发生后,不同密度刺槐均恢复生长,但无法恢复到干旱前的生长水平。在多次干旱事件后,高密度刺槐相对弹性趋于或小于0,表明受多次干旱影响,其生长不能恢复到干旱前水平。随着时间的推移,济源刺槐在经历3次干旱后仍保留一定的弹性,但民权高密度刺槐在第2次干旱事件后相对弹性出现丧失,在未来干旱中其死亡风险增加。研究结果为研究气候对暖温带刺槐生长的影响提供了重要见解,并为密度影响下的刺槐恢复力和弹性研究提供了新视角。 Abstract:The aim of this study was to evaluate the effects of climate and competition on the radial growth and drought vulnerability of Robinia pseudoacacia (locust). Dendroclimatology was used to characterize the recovery, resilience, and chronology of locust in two sites varying in tree density (Minquan and Jiyuan, Henan Province). The correlations between the radial growth of locust and meteorological factors were determined, and the drought vulnerability of locust during drought periods was estimated by measuring the breast area increase (BAI). There was no significant difference in the radial growth of locust at sites varying in locust density in the early growth stage. The ring width and BAI of locust became increasingly lower at the high-density site than at the low-density site with growth (P<0.05). Pearson correlation analysis revealed that the standardized precipitation evapotranspiration index, precipitation, relative humidity, temperature, and the saturated vapor pressure deficit were important factors affecting the growth of locust during the growing season. Path analysis revealed that precipitation and temperature were the key factors affecting the annual growth of locust. The ring width and BAI of locust decreased under drought, and the recovery, resilience, and relative resilience of locust were significantly higher at the low-density site than at the high-density site (P<0.05). The growth of locust at both sites recovered after the first drought event, but growth did not return to its pre-drought level. After several drought events, the relative resilience of locust at the high-density site was equal to or less than 0, indicating that its growth did not return to the pre-drought level because of the multiple drought events. Locust at the low-density site still exhibited high resilience after three droughts; however, the relative resilience of locust at the high-density site decreased substantially, and the risk of death under future droughts increased after the second drought event. Overall, our findings provide the important insights into the effect of climate and density on the growth of locust in warm temperate areas as well as a new perspective on the study of locust recovery and resilience. 参考文献 相似文献 引证文献

East Texas forests show strong resilience to exceptional drought

Abstract Drought-induced tree mortality has increased in many parts of the world because of climate change, and in some regions, has altered forest structure, species composition and ecosystem functions. These developments have the potential to reduce forest resilience and push forests over a tipping point to an alternate state (i.e. a different forest type or a non-forest ecosystem). Texas experienced an exceptional drought in 2011, which killed an estimated 65.5 million trees in the heavily forested areas of the East Texas region. The objective of this study was to understand the effects of the 2011 drought on mortality, biomass and growth in this region and explore forest stability at species, forest and ecoregion levels. We categorized 19 years’ time-series data of 1455 Forest Inventory and Analysis plots into four periods, base (2001–2003), pre-drought (2004–2010), drought (2011–2016) and post-drought (2017–2019), and used repeated-measures analysis of variance to analyse annual mortality, biomass loss, growth rates and the Forest Stability Index to understand forest stability. Our results show that tree mortality and biomass loss increased significantly during the drought period and annual growth rates declined. During the post-drought period, mortality and biomass loss rates returned to the pre-drought level and growth partially recovered. During the drought period, FSI dropped at species, forest type and ecoregion levels but recovered during the post-drought period, except for the Oak Woods and Prairies ecoregion and for forests dominated by southern red oak. Our results indicate that East Texas forests are undergoing a reorganization and recovery stage (in terms of species composition and stand structure) but have yet to reach a tipping point. Given the increased frequency and severity of climate change-induced weather events, East Texas forests could approach a tipping point in the future if there is insufficient time between events for reorganization and recovery. Further analyses are needed to understand the fate of tree species and forests in East Texas under global climate change.

Effects of brackish water inflow on methane-cycling microbial communities in a freshwater rewetted coastal fen

Abstract. Rewetted peatlands can be a significant source of methane (CH4), but in coastal ecosystems, input of sulfate-rich seawater could potentially mitigate these emissions. The presence of sulfate as an electron acceptor during organic matter decomposition is known to suppress methanogenesis by favoring the growth of sulfate reducers, which outcompete methanogens for substrate. We investigated the effects of a brackish water inflow on the microbial communities relative to CH4 production–consumption dynamics in a freshwater rewetted fen at the southern Baltic Sea coast after a storm surge in January 2019 and analyzed our data in context with the previous freshwater rewetted state (2014 serves as our baseline) and the conditions after a severe drought in 2018 (Fig. 1). We took peat cores at four previously sampled locations along a brackishness gradient to compare soil and pore water geochemistry as well as the microbial methane- and sulfate-cycling communities with the previous conditions. We used high-throughput sequencing and quantitative polymerase chain reaction (qPCR) to characterize pools of DNA and RNA targeting total and putatively active bacteria and archaea. Furthermore, we measured CH4 fluxes along the gradient and determined the concentrations and isotopic signatures of trace gases in the peat. We found that both the inflow effect of brackish water and the preceding drought increased the sulfate availability in the surface and pore water. Nevertheless, peat soil CH4 concentrations and the 13C compositions of CH4 and total dissolved inorganic carbon (DIC) indicated ongoing methanogenesis and little methane oxidation. Accordingly, we did not observe a decrease in absolute methanogenic archaea abundance or a substantial change in methanogenic community composition following the inflow but found that the methanogenic community had mainly changed during the preceding drought. In contrast, absolute abundances of aerobic methanotrophic bacteria decreased back to their pre-drought level after the inflow, while they had increased during the drought year. In line with the higher sulfate concentrations, the absolute abundances of sulfate-reducing bacteria (SRB) increased – as expected – by almost 3 orders of magnitude compared to the freshwater state and also exceeded abundances recorded during the drought by over 2 orders of magnitude. Against our expectations, methanotrophic archaea (ANME), capable of sulfate-mediated anaerobic methane oxidation, did not increase in abundance after the brackish water inflow. Altogether, we could find no microbial evidence for hampered methane production or increased methane consumption in the peat soil after the brackish water inflow. Because Koebsch et al. (2020) reported a new minimum in CH4 fluxes at this site since rewetting of the site in 2009, methane oxidation may, however, take place in the water column above the peat soil or in the loose organic litter on the ground. This highlights the importance of considering all compartments across the peat–water–atmosphere continuum to develop an in-depth understanding of inflow events in rewetted peatlands. We propose that the changes in microbial communities and greenhouse gas (GHG) fluxes relative to the previous freshwater rewetting state cannot be explained with the brackish water inflow alone but were potentially reinforced by a biogeochemical legacy effect of the preceding drought.

Climatic and biotic factors influencing regional declines and recovery of tropical forest biomass from the 2015/16 El Niño

The 2015/16 El Niño brought severe drought and record-breaking temperatures in the tropics. Here, using satellite-based L-band microwave vegetation optical depth, we mapped changes of above-ground biomass (AGB) during the drought and in subsequent years up to 2019. Over more than 60% of drought-affected intact forests, AGB reduced during the drought, except in the wettest part of the central Amazon, where it declined 1 y later. By the end of 2019, only 40% of AGB reduced intact forests had fully recovered to the predrought level. Using random-forest models, we found that the magnitude of AGB losses during the drought was mainly associated with regionally distinct patterns of soil water deficits and soil clay content. For the AGB recovery, we found strong influences of AGB losses during the drought and of [Formula: see text]. [Formula: see text] is a parameter related to canopy structure and is defined as the ratio of two relative height (RH) metrics of Geoscience Laser Altimeter System (GLAS) waveform data-RH25 (25% energy return height) and RH100 (100% energy return height; i.e., top canopy height). A high [Formula: see text] may reflect forests with a tall understory, thick and closed canopy, and/or without degradation. Such forests with a high [Formula: see text] ([Formula: see text] ≥ 0.3) appear to have a stronger capacity to recover than low-[Formula: see text] ones. Our results highlight the importance of forest structure when predicting the consequences of future drought stress in the tropics.

Threshold Response to Extreme Drought Shifts Inter-Tree Growth Dominance in Pinus sylvestris

Many studies quantify short-term drought impact on tree growth relative to pre-drought growth averages. However, fewer studies examine the extent to which droughts of differing severity differentially impact tree growth or shape stand dynamics. Focusing on three droughts in high and low density stands of Pinus sylvestris in Scotland, we calculated pre-drought growth averages using climatically standardized antecedent growth years to assess tree level drought and post-drought growth performance as percentage growth change (PGC). We then used mixed-effects models to understand how droughts of differing severity impact tree growth and calculated indices of growth dominance (Gd), size inequality (Si), and size asymmetry (Sa) to detect changes in stand structure. Mixed-effects model results indicate that the magnitude and duration of the growth reduction during and following the more extreme drought was significantly larger compared to less severe droughts, for which we found limited evidence of drought impact. While no changes in Si or Sa were noted following any drought, we found evidence of a difference in Gd after the most extreme drought in both stand densities indicative of a threshold response, with smaller trees contributing proportionally more to stand growth relative to their size. Under less severe droughts, inter-tree variability may have partially buffered against stand-level growth change, however, a small increase in drought severity was associated with a significant reduction in average tree growth, an increase in the number of trees growing at &amp;gt;2SD below pre-drought levels and a shift in Gd toward smaller trees, indicating that a drought severity threshold in P. sylvestris may have been exceeded.

Drought reduces water uptake in beech from the drying topsoil, but no compensatory uptake occurs from deeper soil layers.

Summary The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial.We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2‐h resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use.Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre‐drought levels within 3 wk after rewetting.These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

Gas exchange and hydraulic function in seedlings of three basal angiosperm tree-species during water-withholding and re-watering

Basal angiosperms have relatively primitive hydraulic systems and may be vulnerable to drought. The coordination in hydraulic function between leaf, stem and root of a plant during and after a drought is not well understood. Here, we examined the interaction between gas exchange and hydraulic function in the leaves, stems, and intact root systems of potted-seedlings of three evergreen basal angiosperm tree-species exposed to a cycle of severe water stress and subsequent rewatering. In all three species, stomatal closure occurred at water potentials before 50% loss of stem hydraulic conductivity, because their leaves and roots were more vulnerable to drought than the stems. Manglietia insignis that has scalariform perforatiom plates was more vulnerable to drought than the other two species that have simple porforation plates. During rewatering, water potentials of D. chinensis and C. camphora were recovered after 10 days, their gas exchange were gradually recovered and reached to pre-drought levels after 3 weeks, whereas, the recovery of hydraulic function in their leaves, stems and roots lagged behind but also reached pre-drought levels after 3 weeks. Manglietia insignis displayed poor recovery in both gas exchange and hydraulic function and a high mortality. Our results reveal that drought-induced severe embolism can be recovered even in seedlings of basal angiosperms depending on species if a drought does not persist long, and earlier recovery in xylem water potential and gas exchange than hydraulic function.

Watersheds may not recover from drought.

The Millennium Drought (southeastern Australia) provided a natural experiment to challenge the assumption that watershed streamflow always recovers from drought. Seven years after the drought, the runoff (as a fraction of precipitation) had not recovered in 37% of watersheds, and the number of recovered watersheds was not increasing. When recovery did occur, it was not explained by watershed wetness. For those watersheds not recovered, ~80% showed no evidence of recovering soon, suggesting persistence within a low-runoff state. The post-drought precipitation not going to runoff was found to be likely going to increased evapotranspiration per unit of precipitation. These findings show that watersheds can have a finite resilience to disturbances and suggest that hydrological droughts can persist indefinitely after meteorological droughts.

Carbon Dioxide and Methane Flux Response and Recovery From Drought in a Hemiboreal Ombrotrophic Fen

Globally peatlands store 500 Gt carbon (C), with northern blanket bogs accumulating 23 g C m−2 y−1 due to cool wet conditions. As a sink of carbon dioxide (CO2) peat bogs slow anthropogenic climate change, but warming climate increases the likelihood of drought which may reduce net ecosystem exchange (NEE) and increase soil respiration, tipping C sinks to sources. High water tables make bogs a globally important source of methane (CH4), another greenhouse gas (GHG) with a global warming potential (GWP) 34 times that of CO2. Warming may increase CH4 emissions, but drying may cause a reduction. Predicted species composition changes may also influence GHG balance, due to different traits such as erenchyma, e.g., Eriophorum vaginatum (eriophorum) and non-aerenchymatous species, e.g., Calluna vulgaris (heather). To understand how these ecosystems will respond to climate change, it is vital to measure GHG responses to drought at the species level. An automated chamber system, SkyLine2D, measured NEE and CH4 fluxes near-continuously from an ombrotrophic fen from August 2017 to September 2019. Four ecotypes were identified: sphagnum (Sphagnum spp), eriophorum, heather and water, hypothesizing that fluxes would significantly differ between ecotypes. The 2018 drought allowed comparison of fluxes between drought and non-drought years (May to September), and their recovery the following year. Methane emissions differed between ecotypes (p &amp;lt; 0.02), ordered high to low: eriophorum &amp;gt; sphagnum &amp;gt; water &amp;gt; heather, ranging from 23 to 8 mg CH4-C m−2 d−1. Daily NEE was similar between ecotypes (p &amp;gt; 0.7), but under 2018 drought conditions all ecotypes were greater sources of CO2 compared to 2019, losing 1.14 g and 0.24 g CO2-C m−2 d−1 respectively (p &amp;lt; 0.001). CH4 emissions were ca. 40% higher during 2018 than 2019, 17 mg compared to 12 mg CH4-C m−2 d−1 (p &amp;lt; 0.0001), and fluxes exhibited hysteresis with water table depth. A lag of 84–88 days was observed between rising water table and increased CH4 emissions. A significant interaction between ecotype and year showed fluxes from open water did not return to pre-drought levels. Our findings suggest that short-term drought may lead to a net increase in C emissions from northern wetlands.

2001-2018年中国-老挝交通走廊核心区植被稳定性对极端干旱的响应

中国-老挝交通走廊核心区干旱事件的频繁发生威胁着该区域生态系统的稳定性。基于Palmer干旱指数（PDSI）和增强型植被指数（EVI）数据量化了中老交通走廊核心区不同植被类型的稳定性（年均EVI与其标准差之比）及其对干旱的抵抗力（干旱期间植被结构和功能保持其原始水平的能力）和恢复力（植被恢复到干旱前水平的速度）。结果表明：（1）2001-2018年间，中老交通走廊核心区共发生5次极端干旱事件，出现极端干旱的年份有2005年、2010年、2015年、2016年和2017年，以上年份极端干旱面积占总面积的比例分别为13.37%、47.46%、10.41%、12.00%和3.05%；（2）不同植被类型对极端干旱的响应不同，中老交通走廊核心区内常绿阔叶林的稳定性显著高于其他植被类型，和其他植被类型相比，常绿阔叶林虽然暴露在干旱环境中的时间更长，但其具有更高的稳定性，在维持中老交通走廊核心区的生态系统稳定性上发挥着重要作用；（3）常绿阔叶林和木本稀树草原对极端干旱的抵抗力和恢复力显著高于混交林、草地和农田，研究区内各植被类型对极端干旱的抵抗力与温度和降水呈显著正相关关系。;The core area of China-Laos transportation corridors have been frequently affected by drought events, which threatened the stability of terrestrial ecosystem functioning. Based on the Palmer Drought Severity Index (PDSI) data and enhanced vegetation index (EVI) products, we quantified the temporal stability (ratio of mean annual EVI to its SD), resistance (ability to maintain its original levels during droughts), and resilience (rate of EVI recovering to pre-drought levels) in the core area of China-Laos transportation corridors. The results showed that:(1) there were 5 extreme drought events in the core area of China-Laos transportation corridors from 2001 to 2018, appearing in 2005, 2010, 2015, 2016 and 2017. The area of extreme drought accounted for 13.37%, 47.46%, 10.41%, 12.00% and 3.05%, respectively. (2) Different vegetation types had different responses to droughts. The temporal stability of evergreen broadleaf forests was significantly higher than that of other vegetation types. Evergreen broadleaf forests had higher stability than other vegetation types despite the greater drought exposure, which played an important role in maintaining the ecosystem stability of the corridor core area. (3) The evergreen broadleaf forests and savannas illustrated significantly higher resistance and resilience of EVI than other vegetation types. The resistance of various vegetation types to extreme drought in the study area was positively associated with temperature and precipitation.