Summer Drought Research Articles

Ectomycorrhizal fungi with hydrophobic mycelia and rhizomorphs dominate in young pine trees surviving experimental drought stress

Mycorrhizal fungi can help plants to cope with drought, but research on the fungal communities that are more resistant to drought or alleviate drought stress of trees is still scarce. In this study, we investigated effects of drought on soil fungal communities and explored potential fungal traits related to drought resistance under greenhouse conditions. We manipulated water availability in pine seedlings belonging to three Spanish Pinus pinaster populations from geographical areas subjected to contrasting summer drought. A set of plant ecophysiological traits were quantified and soil fungi was quantified and profiled using ergosterol and Pacific Biosciences sequencing. Abundance of ectomycorrhizal (ECM) fungi in plants subjected to drought was lower than in well-watered plants. Most ECM taxa in plants surviving drought had long exploration types and were taxa typically forming rhizomorphs and hydrophobic mycelia. By contrast, ECM taxa in well-watered plants had wider range of distinct exploration types. No differences in fungal communities were found among P. pinaster populations. No associations between ECM fungi and plant ecophysiological traits were found, but significant interactions between drought treatments and belowground plant biomass were found for the relative abundances of ECM fungi, particularly ECM with long exploration types. Plants subjected to drought may benefit by associating to ECM taxa previously shown to transport water efficiently.

Modeling moisture redistribution from selective non-uniform application of biochar on Palouse hills

Precision agriculture is most effective in areas where significant in-field variation occurs. The Palouse region of the Pacific Northwest in the US, a vast area of undulating fertile farmland, has relatively high in-field variation in water retention and crop yield due to regional topography and uneven soil erosion. The regional agricultural systems depend on the soil at or near field capacity towards the end of a wet spring to support crops throughout the summer drought period. Dryland agricultural systems and high in-field variation and changing climate make water retention management practices throughout the region critical. A finite element vadose zone transport model was developed and used to understand the benefits of the targeted application of biochar on water retention and water redistribution in a representative hillslope. The model utilizes measured soil hydraulic properties to predict soil moisture distribution over the dry season. A Redwood Sawdust and Wheat Straw biochar was amended at 4% and 7% concentrations by mass. Biochar amended soils showed an increase in water retention and apparent reduction in unsaturated hydraulic conductivity as the soil approached saturated conditions. After two months of bare field evaporation, the model showed that biochar impacts water redistribution in the soil profile, contributing to positive and negative changes and a net increase in water retention. Model outputs with biochar showed increased retention in and around the amendment area, although the magnitude between outputs varied, with some samples showing minimal effectiveness. Despite the differences in magnitude with targeted biochar amendment, these results indicate that biochar can change water redistribution (up to 0.5%) in a soil profile. Additionally, the developed model shows promise as a field and regional level management tool to determine the best return on investment from biochar application when applied in a targeted manner.

Changes in the Physiological and Morphometric Characteristics and Biomass Distribution of Forage Grasses Growing under Conditions of Drought and Silicon Application.

Research on mitigating the effects of water scarcity by applying silicon to perennial grasses is still insufficient. This study was conducted to investigate the effect of spring and summer droughts and silicon applications on gas exchange parameters; the morphometric characteristics of root systems; and the biomass distribution of Festulolium braunii, Festuca arundinacea, and Lolium perenne cultivars. Plants were treated with a drought during the tillering phase once a year (during spring or summer regrowth) for 21 days. Foliar nutrition with silicon was applied twice under the drought conditions. Grasses in a pot experiment were cut three times during vegetation. The plants that were exposed to the drought had lower values of the gas exchange parameters than those that were well watered. The beneficial effect of silicon was related to the reduction of excessive water loss through transpiration during the spring drought. Under the drought and silicon applications, the water use efficiency, root dry mass, and length increased compared to the control. Moreover, silicon increased the proportion of both the finer and thicker roots in F. braunii and L. perenne, while the distribution of the root diameter changed least in the more resistant F. arundinacea. Silicon also reduced the carbon content in the roots and increased root carbon accumulation. Our results indicated that Si may help perennial forage grasses cope better with drought stress. This was due to the allocation of carbon to the roots to develop the fine root network, increasing the length and root biomass and improving the water use efficiency.

Repeated summer drought changes the radial xylem sap flow profile in mature Norway spruce but not in European beech

Water consumption of trees is one of the most important processes connected to their survival under ongoing climate change and extreme events such as drought. Radial profiles of xylem sap flow density are an integral component to quantify the water transport for the level of an individual tree and that of ecosystems. However, knowledge of such radial profiles, in particular under stress, is very scarce. Here we show the radial profile of the xylem sap flow density in mature European beech (Fagus sylvatica L) and Norway spruce (Picea abies (L) Karst.) under repeated summer drought induced by throughfall exclusion (TE) and subsequent recovery compared to untreated control trees (CO). We measured xylem sap flow density (udaily in L dm−2 d−1) down to 8 cm sapwood depth at breast height using two different approaches, a thermal dissipation system and the heat field deformation method. In beech, repeated throughfall exclusion did not affect the radial xylem sap flow profile. However, in spruce, udaily was strongly reduced across the profile under repeated drought, changing the profile from a linear to a logarithmic regression. Even two years after drought release, the xylem sap flow profile did not fully recover in TE spruce. The reduction of udaily along the radial profile was accompanied by a reduction of the leaf area in TE spruce by c. 50%, while sapwood depth remained constant. The reduction of the xylem sap flow density along the profile reduced the calculated water consumption of TE spruce trees by more than 33% compared to CO trees, also after drought release. The impact of stressors such as repeated drought on the xylem sap flow density across the radial profile and its consequences for trees’ and stands’ water consumption needs to be addressed in more detail to minimize uncertainties in quantifying ecosystem water cycles.

Simulating the Impacts of Drought and Warming in Summer and Autumn on the Productivity of Subtropical Coniferous Forests

The impacts of drought and/or warming on forests have received great attention in recent decades. Although the extreme drought and/or warming events significantly changed the forest demography and regional carbon cycle, the seasonality quantifying the impacts of these climate extremes with different severities on the productivity of subtropical coniferous forests remains poorly understood. This study evaluated the effects of seasonal drought and/or warming on the net primary productivity (NPP) of subtropical coniferous forests (i.e., Cunninghamia lanceolata and Pinus massoniana forests) from Hengyang–Shaoyang Basin in southern China using the Ecosystem Demography model, Version 2.2 (ED-2.2) and based on the datasets from forest inventory, meteorological reanalysis, and remotely sensed products. The results showed that the goodness of fit of the DBH-height allometric equations was better than that of the default in ED-2.2 after model calibration; the ED-2.2 model qualitatively captured the seasonality of NPP in the subtropical coniferous forests; and the mismatch between simulated annual NPP and MODIS-NPP (MOD17A3HGF) became smaller over time. The effect of seasonal drought on NPP was greater than that of warming; the decline rate of NPP gradually increased and decreased with time (from July to October) under the seasonal drought and warming scenarios, respectively; NPP decreased more seriously under the combined drought-warming scenario in October, with an average decrease of 31.72%, than the drought-only and warming-only scenarios; seasonal drought had an obvious legacy impact on productivity recovery of subtropical coniferous forests, but it was not the case for warming. With the increase in drought severity, the average values of soil available water and NPP together showed a downward trend. With the increase in warming severity, the average values of canopy air space temperature increased, but NPP decreased. Seasonal drought and/or warming limit forest production through decreasing soil moisture and/or increasing canopy air space temperature, which impact on plant photosynthesis and productivity, respectively. Our results highlight the significance of taking into account the impacts of seasonal warming and drought when evaluating the productivity of subtropical coniferous forests, as well as the significance of enhancing the resistance and resilience of forests to future, more severe global climate change.

Suchozemští plži přírodní rezervace Plané loučky a přírodních památek Častava, Hvězda a Kurfürstovo rameno v CHKO Litovelské Pomoraví [Terrestrial gastropods of Plané loučky Nature Reserve and Častava, Hvězda, and Kurfürstovo rameno Nature Monuments in Litovelské Pomoraví PLA

In 2022, four small-scale protected areas (SPA) within Litovelské Pomoraví Protected Landscape Area were surveyed for terrestrial molluscs. All localities are floodplain forest fragments with oxbow lakes, pools and marshes in the Morava river catchment. The most species-rich SPA was Hvězda Natural Monument (NM), with 38 species recorded. The threatened species Cochlicopa nitens (Endangered), known from this area from previous surveys, was not recorded despite high sampling effort. This sensitive hygrophilous species disappeared probably due to droughts occurring in earlier years. However, other sensitive hygrophilous species such as Euconulus alderi (Nearly Threatened) and Vertigo antivertigo were recorded. The latter was also found in Plané loučky Nature Reserve (NR; 27 spp. in total) and Častava NM (19 spp. in total). Relatively common floodplain forest species Perforatella bidentata was recorded in Plané loučky NR and Hvězda NM; however, it was absent in Častava NM and Kurfürstovo rameno NM (20 spp. in total). The partial absence of this species and the complete absence of substantially declining floodplain species Pseudotrichia rubiginosa (Nearly Threatened) and other sensitive taxa indicate habitat degradation caused mainly by stream regulations and water level decrease and manipulation. The presence of invasive plant species known to decrease terrestrial mollusc species diversity and abundance are of the essence as well. Highly invasive species Arion vulgaris was the most abundant species in all of the surveyed SPAs, except for Hvězda NM, where it was present in lower numbers. Over some twenty years, this species spread from anthropogenic areas to natural habitats to become one of the most prominent features of floodplain biocoenoses in Litovelské Pomoraví and elsewhere. To improve the habitat conservation status, it is important to maintain favourable water level in streams and water bodies adjacent to SPAs, especially during the critical periods of summer droughts. Furthermore, it is advised to focus the management on neophyte extirpation.

Effects of drought and climate factors on vegetation dynamics in Central Asia from 1982 to 2020

Ecological security and ecosystem stability in Central Asia depend heavily on the local vegetation. Vegetation dynamics and the response and hysteresis relationships to climate factors and drought on multiple scales over long time series in the region still need to be further explored. Using the net primary productivity (NPP) values as the vegetation change index of interest, in this study, we analyzed vegetation dynamics in Central Asia from 1982 to 2020 and assessed the responses and time lags of vegetation to climate factors and drought. The results showed that NPP gradually decreased from north to south and from east to west. Vegetation was distributed along both sides of the mountains. The temperatures rose from northeast to southwest, while precipitation gradually increased from southwest to northeast. The proportion of dry and wet years was as follows: normal (56.41%) > slightly dry (28.2%) > slightly humid (15.39%). Precipitation and drought conditions were positively correlated with NPP during the growing season, while temperature was negatively correlated with NPP. Increased spring temperature, precipitation, and drought conditions positively affected vegetation, while sustained summer temperature resulted in suppressed vegetation growth. Autumn vegetation was positively affected by temperature and drought, and precipitation was negatively correlated with autumn vegetation. Increasing winter temperatures promoted vegetation growth. The time lag between NPP and temperature gradually increased from northeast to southwest, and the time lag between NPP and precipitation gradually increased from south to north. Spring temperatures had the greatest beneficial impact on forestlands; summer climatic factors and drought had little effect on shrublands; the autumn climate exhibited small differences in its influence of each plant type; and winter temperatures had the greatest positive effect on grasslands. No time lag effect was found between any of the four vegetation types and precipitation. A one-month lag was found between cultivated lands and temperature; a two-month lag was found between forestlands and temperature; and a one-month lag was found between forestlands and drought and between shrublands and drought. The results can provide a scientific foundation for the sustainable development and management of ecosystems.

Blanket bog CO2 flux driven by plant functional type during summer drought

AbstractRecent climate predictions for the United Kingdom expect a nationwide shift towards drier and warmer summers, increasing the risk of more frequent and severe drought events. Such shifts in weather patterns impede functioning of global peatlands, especially rare intact blanket bogs abundant in Scotland and representing nearly a quarter of the UK's soil carbon. In this in situ study, carbon dioxide (CO2) fluxes from dominant peatland plant functional types (PFTs) such as Sphagnum spp., graminoids, ericoids and other key cover types (i.e., pools and bare peat) were measured and compared across upland and low‐lying blanket bog margins and centres, immediately before and during a summer drought in 2018, and over the subsequent year. During that period, most sites acted as net sources of CO2 to the atmosphere. Our results showed that net ecosystem exchange (NEE) was limited by water availability during the drought, with ericoid shrubs showing the highest drought resilience, followed by graminoids (which were still limited in GPP in 2019) and Sphagnum mosses. Diverging NEE estimates were observed across centre and margin areas of the blanket bogs, with highest variability across the upland site where signs of active erosion were visible. Overall, our study suggests that estimating growing season carbon fluxes from in situ peatland PFT and cover types can help us better understand global climate change impacts on the dynamics and trajectories of peatland C cycles.

Retrieving soil moisture from grape growing areas using multi-feature and stacking-based ensemble learning modeling

Soil moisture (SM) is an essential parameter for crop growth and development, and temporal and spatial variation in SM in agricultural fields varies by crop type due to corresponding crop growing characteristics and cultivating patterns. Few studies have performed SM retrieval in grape growing areas, and SM estimation using only spectral reflectance (SR) or derived drought indices may not be wholly accurate. In this study, seven features based on evapotranspiration (ET), land surface temperature (LST), and SR were derived from Moderate Resolution Imaging Spectroradiometer (MODIS) data (8-day temporal resolution and 500 m spatial resolution) and integrated with topography feature to determine SM during the main growth stages of grape in the eastern foothills of the Helan Mountains from 2009 to 2018. A total of 25-period models covering April–October (8-day temporal resolution) were constructed. Finally, the models were achieved to retrieve SM, and the spatiotemporal distribution pattern was analyzed. The stacking ensemble algorithm can integrate multiple models for better retrieval accuracy and overcome the limitations of individual machine learning models. We also compared the SM estimation accuracy of three single machine learning models (Category boosting, random forest, and gradient boosting decision tree) with the ensemble model using the stacking algorithm. The results indicated that the stacking-based ensemble model could retrieve SM more accurately and stably than any individual machine learning algorithm, and the stacking-based ensemble model showed good applicability during the main growth stages of grape. The average coefficient of determination (R2) and root mean square error (RMSE) of the 25-period multi-feature stacking-based models were 0.7504 and 0.0245 m3/m3. Overall, the application results showed that spring and summer droughts were more severe than autumn droughts in the study area during the main growth stages of grape. Our findings indicate that using multiple features and the stacking-based ensemble model could improve SM estimation accuracy in grape growing areas.

Subsurface Lateral Flows Buffer Riparian Water Stress Against Snow Drought

AbstractIn the Sierra Nevada, CA, the Mediterranean climate exposes montane forests to water stress during the summer drought. Normally, spring snowmelt alleviates summer water stress, especially in riparian ecosystems that receive subsurface lateral inputs from groundwater. However, snow drought could potentially eliminate these beneficial effects. This research investigates how subsurface lateral redistribution mediates hillslope‐scale vegetation responses to snow drought. We apply a spatially‐distributed ecohydrologic model (Regional Hydro‐Ecologic Simulation System (RHESSys)) to a snow‐dominated watershed in the Sierra Nevada. We incorporate observed sap flow data along a hillslope to estimate relative differences in water stress for upslope and riparian sites, and constrain RHESSys drainage parameter uncertainty. Our model results show that subsurface lateral inputs buffer riparian water stress against snow drought. For all drought types, both upslope and riparian sites experience substantial losses of net primary productivity (NPP), and on average upslope sites are more adversely affected (upslope loss of NPP = 45% vs. riparian = 28%). Dry droughts from a lack of rain or snow induce substantial loss of biomass for upslope and riparian trees because of low annual precipitation, but a small snowpack retained during cold, dry years slightly alleviates these effects and delays water stress by 2 weeks in riparian areas and 3 weeks upslope. While riparian forests are often buffered against drought stress, our study also shows that riparian forests can suffer from water stress. We found 21% of drought scenarios where riparian trees experience greater water stress than upslope trees that were mostly associated with wet and warm snow droughts.

Climate-induced long-term changes in the phenology of Mediterranean fungi

Long-term responses in the phenology of Mediterranean macrofungi to climatic changes are poorly documented. Here, we address this issue by comparing the fruiting patterns of 159 fungal species in Southern France between the first half of the 19th century and the first decade of the 21st century. We used a trait-based approach to assess the influence of phenology and morphology of fungal fruit bodies and their site ecology and biogeography on the response to climate change. We show that early autumnal fruiters, epigeous species and species with affinities for cold climates now start to fruit on average 16.4, 17.3 and 17.3 d later compared to their emergence dates in the 19th century, while late fruiters, hypogeous species and Mediterranean-restricted species did not change their fruiting date. Among ecological guilds, saproxylic species and pine-associated mutualists delayed their autumnal emergence by 32.5 and 19.2 d, likely in response to a delayed rewetting of litter and woody debris after extended summer drought. Our results suggest that long-term climate warming in the Mediterranean was accompanied by contrasting changes in the emergence of fungal fruit bodies according to ecological guilds, sporocarp life-forms and forest types.

The Howling Storm: Weather, Climate, and the American Civil War by Kenneth W. Noe

Reviewed by: The Howling Storm: Weather, Climate, and the American Civil War by Kenneth W. Noe Adam H. Petty (bio) The Howling Storm: Weather, Climate, and the American Civil War. Kenneth W. Noe. Baton Rouge: Louisiana States University Press, 2020. ISBN 978-0-8071-7320-6. 680 pp., cloth, $59.95. Kenneth Noe’s latest book, The Howling Storm, investigates the role weather played in the American Civil War. In particular, it focuses on weather’s effect on military campaigns. The book is foremost a narrative history. Most of its chapters are dedicated to retelling events in Virginia or the western theater, although there are occasional excursions to the Trans-Mississippi. The weather’s impact on Northern and Southern agriculture also receives some attention in Noe’s book, as does the soldiers’ yearly struggle to make themselves comfortable in winter quarters. While The Howling Storm is not encyclopedic, it covers a remarkable amount of ground for a single volume. As for the book’s argument, Noe contends that “a full understanding of the war requires regrounding it in the physical environment” (1). Not surprisingly, he concludes that “weather shaped every campaign, often more decisively than we have fathomed” (9). Moreover, in many instances, the weather “favored the Union cause and—perhaps even more—emancipation” (493). Better equipped boys in blue could withstand the elements with greater ease then their counterparts in gray. He also observes that Northern agriculture did not suffer the [End Page 433] catastrophic damage that spring rains and summer droughts inflicted on the Confederacy, resulting in food shortages south of the Mason-Dixon line. The Howling Storm, with just shy of five hundred pages of text, is not a quick read. Nor is it a page-turner. The many descriptions of soldiers suffering through the mud, rain, heat, and cold might prove monotonous to some readers. That being said, The Howling Storm is a very useful book. Its achievement lies in gathering an enormous amount of information about Civil War weather in one volume. In the past, good military historians included information about the weather in their campaign studies, but Noe’s book provides a relatively compact resource that historians can reference to see what the weather was up to at crucial moments of the Civil War. Because of the book’s broad approach, Noe is able to see larger patterns in the weather, conclusions that would escape more focused studies. For instance, he will note when a cold snap or a torrential downpour that affected one campaign was part of a larger weather system that was also affecting operations in a different theater. One concern that I have about this book is that it could mislead those who read it to think that weather was the most important factor controlling the events of the war. I do not mean to say that Noe asserts this or intended this outcome. Rather, it is simply a danger that comes with relying on any one analytical lens to tell a story. While Noe’s work provides a nice corrective to those who might have been tempted to ignore the weather’s influence, I think it is best read in conjunction with other military histories of the war, allowing readers to weigh the merits of weather as an explanatory tool. The Howling Storm is a great resource for those wanting to add a little information about the weather to their Civil War course, especially for historians who do not specialize in military history. All in all, lay readers and historians alike owe Noe a collective “Huzzah!” for this mammoth work and important contribution to Civil War history. Adam H. Petty History Department Church of Jesus Christ of Latter-Day Saints Adam H. Petty ADAM H. PETTY is a historian and documentary editor with the Joseph Smith Papers. He is the author of The Battle of the Wilderness in Myth and Memory: Reconsidering Virginia’s Most Notorious Civil War Battlefield (2019). Copyright © 2022 The Kent State University Press

Analysis of Spatio-Temporal Evolution Characteristics of Drought and Its Driving Factors in Yangtze River Basin Based on SPEI

Using a dataset of 114 meteorological stations in the Yangtze River Basin from 1980–2019, the standardized precipitation evapotranspiration index (SPEI) was calculated based on the Penman-Monteith evapotranspiration model for multiple time scales, and the spatial and temporal evolution characteristics and driving factors of drought in the Yangtze River Basin were analyzed by combining spatial and temporal analysis methods as well as geodetector. The main results obtained are as follows: (1) The climate of the Yangtze River Basin is an overall wet trend, and the trend of summer drought is more similar to the annual scale trend. (2) Most areas in the Yangtze River Basin showed mild drought or no drought, and there is little difference in drought condition among the Yangtze River Basin regions. The areas with drought conditions are mainly distributed in the southwest and east of the Yangtze River Basin. (3) There are significant seasonal differences in drought conditions in all regions, and the drought condition is more different in autumn compared to spring, summer and winter. (4) The average annual precipitation and elevation factors are the dominant driving factors of drought in the Yangtze River Basin, and the double-factor interaction has a greater influence on the drought variation in the Yangtze River Basin than the single-factor effect, indicating that the difference of drought condition in the Yangtze River Basin is the result of the combination of multiple factors.

Summer drought exposure, stand structure, and soil properties jointly control the growth of European beech along a steep precipitation gradient in northern Germany.

Increasing exposure to climate warming-related drought and heat threatens forest vitality in many regions on earth, with the trees' vulnerability likely depending on local climatic aridity, recent climate trends, edaphic conditions, and the drought acclimatization and adaptation of populations. Studies exploring tree species' vulnerability to climate change often have a local focus or model the species' entire distribution range, which hampers the separation of climatic and edaphic drivers of drought and heat vulnerability. We compared recent radial growth trends and the sensitivity of growth to drought and heat in central populations of a widespread and naturally dominant tree species in Europe, European beech (Fagus sylvatica), at 30 forest sites across a steep precipitation gradient (500-850 mmyear-1 ) of short length to assess the species' adaptive potential. Size-standardized basal area increment remained more constant during the period of accelerated warming since the early 1980s in populations with >360 mm growing season precipitation (April-September), while growth trends were negative at sites with <360 mm. Climatic drought in June appeared as the most influential climatic factor affecting radial growth, with a stronger effect at drier sites. A decadal decrease in the climatic water balance of the summer was identified as the most important factor leading to growth decline, which is amplified by higher stem densities. Inter-annual growth variability has increased since the early 1980s, and variability is generally higher at drier and sandier sites. Similarly, within-population growth synchrony is higher at sandier sites and has increased with a decrease in the June climatic water balance. Our results caution against predicting the drought vulnerability of trees solely from climate projections, as soil properties emerged as an important modulating factor. We conclude that beech is facing recent growth decline at drier sites in the centre of its distribution range, driven by climate change-related climate aridification.

Divergent seasonal responses of carbon fluxes to extreme droughts over China

Droughts affect the interannual variability of global land carbon fluxes and are expected to exert widespread impacts on the carbon cycle given a future climate with more intense and frequent droughts. Evidence indicates that the impacts of droughts on the carbon cycle vary in different seasons, but a quantitative examination of the seasonal differences is lacking. Here we ensemble multiple data streams for greenness indices and carbon fluxes, including those from remote sensing observations, flux tower upscaling, atmospheric inversions, and dynamic global vegetation models (DGVMs), to quantify the seasonal responses of vegetation to extreme droughts in China. We find that summer droughts cause the largest negative responses of leaf area index (LAI, with median standardized anomalies of -0.40), gross primary productivity (GPP, -0.55), and net ecosystem productivity (NEP, -0.74); notably, droughts in autumn largely suppress carbon uptake. The response patterns show a high degree of heterogeneity, and we identify the factors driving these spatial variations using the extreme gradient boosting (XGBoost) machine learning approach. Climate is the dominant driver of spring and autumn GPP responses while LAI predominantly drives summer GPP loss. Looking into the biotic factors, carry-over effects (previous-season vegetation growth affecting current-season growth) contribute substantially to the alleviation of drought stress, in that previous-season greening compensate vegetation loss during droughts. Our results not only quantify the seasonal response differences in carbon fluxes and greenness, but suggest that carbon fluxes respond more sensitively to drought than greenness. Also, we show seasonal differences in the degree to which factors contribute to drought impacts, which highlight that annual-scale drought analyses may mask spring and autumn vegetation response to droughts.

Physiological recovery of tree water relations upon drought release-response of mature beech and spruce after five years of recurrent summer drought.

As climate change progresses, the frequency and duration of drought stress events are increasing. While the mechanisms of drought acclimation of trees has received considerable attention in recent years, the recovery processes remain critically understudied. We used a unique throughfall exclusion experiment in a mature temperate mixed forest consisting of the more isohydric Norway spruce and more anisohydric European beech, to study the recovery and resilience after drought release. We hypothesized that pre-dawn water potential (ΨPD) of both species will increase within 1 day after watering, while the recovery of stomatal conductance (gs) and the reversal of osmoregulation will be significantly delayed in the more isohydric spruce. Furthermore, we hypothesized that the xylem sap flow density (udaily) will not fully recover within the growing season due to the strong drought impact. After 5 years of summer drought, trees showed significantly reduced ΨPD, udaily and increased osmoregulation in leaves, but only isohydric spruce displayed increased leaf abscisic acid concentrations. In line with our hypothesis, ΨPD and gs recovered within 1 day in beech. Conversely, isohydric spruce showed delayed increases in ΨPD and gs. The delay in recovery of spruce was partially related to the replenishment of the stem water reservoir, as indicated by the missing response of udaily at the crown base compared with DBH level upon watering. However, udaily fully recovered only in the next growing season for beech and was still reduced in spruce. Nevertheless, in both species, osmotic acclimations of leaves were reversed within several weeks. While both species displayed full resilience to drought stress in water-related physiology, the recovery time was in several cases, e.g., udaily, ΨPD and gs, shorter for beech than for spruce. With future increases in the frequency of drought events under ongoing climate change, tree species that recover more quickly will be favored.

Delaying drought-driven leaf cell damage may be the key trait of invasive trees ensuring their success in the Mediterranean basin.

Invasive alien species (IAS) threaten the biodiversity richness of the Mediterranean basin, a drought-prone region. However, our knowledge on the adaptive strategies of IAS for facing Mediterranean drought summers is still incomplete. The aim of the present study is to compare the water relations and the critical relative water content (RWC) values leading to loss of cell rehydration capacity of two Mediterranean basin IAS (i.e., Ailanthus altissima (Mill.) Swingle and Robinia pseudoacacia L.) versus two co-occurring native species (i.e., Fraxinus ornus L.and Quercus pubescensWilld.). Study IAS showed higher values of water potential at turgor loss point and osmotic potential at full turgor, lower values of modulus of elasticity and leaf mass area but higher photosynthesis rate, even during the summer, with respect to the Mediterranean native species. These findings supported the hypothesis that IAS are characterized by a resource acquisitive strategy coupled with a safety-efficiency trade-off, compared with Mediterranean native species. However, similar leaf RWC thresholds leading to loss of cell rehydration capacity were recorded in the two groups of species. Moreover, IAS showed higher saturated water content and capacitance values compared with the co-occurring species. Overall, our results suggest that the success of Mediterranean IAS is driven by their ability to delay dehydration damage of mesophyll cells during Mediterranean summer drought, thereby supporting their distinctive high carbon assimilation rate.

Variation in essential oil composition within and among Tunisian Thymus algeriensis Boiss et Reut. (Lamiaceae) populations: Effect of ecological factors and incidence on antiacetylcholinesterase and antioxidant activities

The variation of the essential oil composition of fourteen Tunisian Thymus algeriensis populations and their antioxidant and antiacetylcholinesterase activities were assessed. At the species level, α-pinene, 1,8-cineole, camphor, caryophyllene oxide and carvacrol were the main compounds. Significant variation in the essential oil composition was observed between populations according to ecological factors. Six chemotypes were revealed (1,8-cineole-β-panasinsene/1,8-cineole/caryophyllene oxide/1,8-cineole-camphor-caryophyllene oxide/1,8-cineole-camphor-caryophyllene oxide-terpenyl acetate/carvacrol) with the distinction of upper arid populations (Ta 13 and Ta 14) with the dominance of carvacrol chemotype. The occurrence of the carvacrol was positively correlated to the evapotranspiration, summer drought stress and the temperature of the warmest months of the year.High chemical intrapopulation variability was also observed, revealing the presence of seven chemotypes (carvacrol/terpenyl acetate/caryophyllene oxide/linalool-linalyl acetate/1,8-cineole-β-panasinsene/1,8-cineole/1,8-cineole-camphor). The level of antioxidant and anti-acetylcholinesterase activity was linked to their chemical composition difference. The best activities were revealed for populations Ta 13 and Ta 14 from the most arid zones, characterized by their carvacrol richness.

Thinning improves growth and resilience after severe droughts in Quercus subpyrenaica coppice forests in the Spanish Pre-Pyrenees

During the past years, growth and productivity of different oak species have been constrained by water shortage in seasonally dry regions such as the Mediterranean Basin. Thinning could improve oak radial growth in these drought-prone regions through the reduction of tree competition for soil water in summer. However, we still lack adequate, long-term assessments on how lasting are thinning treatments effects and to what extent they contribute to oak growth recovery after drought. Here we aim: (i) to study the radial growth sensitivity to drought of Quercus subpyrenaica in the Spanish Pre-Pyrenees, and (ii) to verify if thinning represents a suitable option to enhance growth resistance to drought and post-drought growth recovery. We analysed basal area increment (BAI) trends in the period 1960–2020 of formerly coppiced oak stands thinned in 1984 and compared them with unthinned plots and also with coexisting Scots pine (Pinus sylvestris) growing in thinned plots. We used the Standardized Precipitation Evapotranspiration Index (SPEI) to estimate the severity of droughts and we also assessed climate-growth relationships. Oaks in thinned plots showed higher BAI (369 mm2) than those in unthinned plots (221 mm2). Growth rates remained higher in thinned than in unthinned plots also under severe drought stress. A severe summer drought in 1986 caused abrupt BAI reductions in both oaks (- 40.5%) and pines (- 40.1%). The positive effect of thinning on growth lasted for over 20 years and slightly declined as canopies closed. In the thinned plots, trees with smaller diameter showed the greatest growth release. Oaks in unthinned plots and Scots pine were more sensitive to short-term droughts in terms of growth reduction than oaks in thinned plots, while long term droughts have similar effects on oaks from both thinned and unthinned plots. Oaks were resilient to drought, showing recovery periods lasting from 1 to 2 years in both thinned and unthinned plots. However, intense and prolonged droughts could strongly reverse the expected growth enhancement of thinned plots, and a greater frequency of droughts would limit coppice growth and productivity thus lengthening the rotation periods.

A ~700 years perspective on the 21st century drying in the eastern part of Europe based on δ18O in tree ring cellulose

Numerical simulations indicate that extreme climate events (e.g., droughts, floods, heat waves) will increase in a warming world, putting enormous pressure on society and political decision-makers. To provide a long-term perspective on the variability of these extreme events, here we use a ~700 years tree-ring oxygen isotope chronology from Eastern Europe, in combination with paleo-reanalysis data, to show that the summer drying over Eastern Europe observed over the last ~150 years is to the best of our knowledge unprecedented over the last 700 years. This drying is driven by a change in the pressure patterns over Europe, characterized by a shift from zonal to a wavier flow around 1850CE, leading to extreme summer droughts and aridification. To our knowledge, this is the first and longest reconstruction of drought variability, based on stable oxygen isotopes in the tree-ring cellulose, for Eastern Europe, helping to fill a gap in the spatial coverage of paleoclimate reconstructions.