Increased Drought Intensity Research Articles

Multi-year monitoring land surface phenology in relation to climatic variables using MODIS-NDVI time-series in Mediterranean forest, Northeast Tunisia

The Mediterranean region is one of the most vulnerable regions to climate change. The majority of climate models forecast a rise in temperatures and less rainfall, which have been observed in recent decades. These changes will affect several vegetation properties, especially phenological dynamics and traits, by increasing drought intensity and recurrence. In this climate change context, the present study aims to assess the evolution of vegetation state and its relation with the climate dynamics in the Mediterranean forest region of northeast Tunisia using Land Surface Phenology (LSP) metrics and the vegetation index (NDVI) analysis from 2000 to 2017. To conduct this work, we used precipitation and temperature data from the two closest weather stations and 16-day NDVI composite images from the MODIS satellite source, with 250-m spatial resolution. Three phenological metrics— start of season (SOS), end of season (EOS), and length of season (LOS) — were obtained and compared for different vegetation types. The LSP variation in response to climatic metrics was also analyzed. The results showed that the LSP in our study area changed significantly during the 2000–2017 period, which includes an average 7.8 days delay in the SOS, an average advance in the EOS by 5 days, and LOS shortened by an average 12.8 days. Autumn (Pr_9) and spring (Pr_3 and P3_4) precipitations, as well as maximum temperature (Tx9+10), represent the best climate parameters to explain the changes in LSP. Both the NDVI and SPEI showed a significant high correlation (p < 0.001) on longer time scales. LSP and NDVI proved useful tools for monitoring the vegetation state according to climate for better planning territory purposes.

GRACE-based estimates of groundwater variations over North America from 2002 to 2017

GRACE-based estimates for groundwater storage (GWS) changes in North America substantially depend upon correction of glacial isostatic adjustment (GIA) effects, which are usually removed with GIA models. In this study, GIA effects are eliminated by employing an independent separation approach with the aid of Global Navigation Satellite System (GNSS) vertical velocity data. Our goal is to provide an independent estimate for monthly GWS changes within North America in 1-degree-grids and their trends over the whole GRACE mission lifetime from April 2002 to June 2017. This estimate is derived from the release-6 version of GRACE monthly level-2 data, GNSS data, land surface models for soil moisture and snow water equivalent, and satellite altimetric lake level data. We find a GWS anomaly in form of an increasing trend in Saskatchewan, which affects the Saskatchewan Province and the states of Montana, North Dakota and Minnesota, and 4 GWS anomalies with declining trends in Nevada, California, Arizona and Texas, respectively. The monthly changes of these GWS anomalies, except for the one in Nevada, are validated by well level data. We provide results for average monthly GWS changes and the trends for the 5 anomalies but also in separate form for the 13 affected states or provinces. The increasing trends of the Saskatchewan GWS anomaly and the affected 3 states are related to increasing precipitation and can be elucidated by the decreasing drought intensity level. On the contrary, the declining trends in GWS can be explained by weakening precipitation and are mostly supported by the increasing drought intensity level in the other 4 anomalies and the affected states, which are Nevada, California, Arizona, New Mexico, Texas, Oklahoma, Kansas, and Colorado. Our estimates of monthly GWS changes and their trends can serve as alternative and beneficial input for the sustainable management of groundwater resources in North America.

Climatic Aridity Shapes Post-Fire Interactions between Ceanothus spp. and Douglas-Fir (Pseudotsuga menziesii) across the Klamath Mountains

Climate change is leading to increased drought intensity and fire frequency, creating early-successional landscapes with novel disturbance–recovery dynamics. In the Klamath Mountains of northwestern California and southwestern Oregon, early-successional interactions between nitrogen (N)-fixing shrubs (Ceanothus spp.) and long-lived conifers (Douglas-fir) are especially important determinants of forest development. We sampled post-fire vegetation and soil biogeochemistry in 57 plots along gradients of time since fire (7–28 years) and climatic water deficit (aridity). We found that Ceanothus biomass increased, and Douglas-fir biomass decreased with increasing aridity. High aridity and Ceanothus biomass interacted with lower soil C:N more than either factor alone. Ceanothus biomass was initially high after fire and declined with time, suggesting a large initial pulse of N-fixation that could enhance N availability for establishing Douglas-fir. We conclude that future increases in aridity and wildfire frequency will likely limit post-fire Douglas-fir establishment, though Ceanothus may ameliorate some of these impacts through benefits to microclimate and soils. Results from this study contribute to our understanding of the effects of climate change and wildfires on interspecific interactions and forest dynamics. Management seeking to accelerate forest recovery after high-severity fire should emphasize early-successional conifer establishment while maintaining N-fixing shrubs to enhance soil fertility.

Coordination between liquid and gas phase conductance enables survival during water stress in an isohydrodynamic Eucalyptus cultivar

The regulation on transpiration and water status is critical for tree survival during drought. Eucalyptus is introduced and widely planted in southern China for timber production. Eucalyptus plantations are expanding into rugged terrains with shallow soils and soil water capacitance. With the ongoing decline in precipitation and the typical monsoon climate in this region, the trees suffer regular seasonal drought, yet their response mechanisms to drought remain obscure. We examined leaf water status, stomatal and hydraulic behavior, and whole-tree transpiration of a wildly planted eucalyptus cultivar, Eucalyptus urophylla × Eucalyptus grandis, across seasons with different water availability in order to identify its response strategy to drought. During the study period, both predawn (ΨPD) and midday (ΨMD) leaf water potential varied considerably, but the water potential gradient between the roots and leaves (ΔΨS-L), measured as the difference between ΨPD and ΨMD, remained constant, indicating an isohydrodynamic regulation on water status under drought. Despite strong stomatal down regulation on transpiration in response to increasing evaporative demand, leaf water potential declined to levels approaching -2 MPa at midday in the driest months, inducing more than 60% loss of xylem hydraulic conductance. Both stomatal and whole-tree hydraulic conductance linearly declined with increasing drought intensity. Whole-tree hydraulic conductance was exponentially correlated with the sensitivity of stomatal conductance to VPD. These results demonstrated that stomatal and hydraulic regulation on transpiration of eucalyptus was coupled through tight correlation between stomatal response to evaporative demand and whole-tree hydraulic conductance. This coordination between liquid and gas phase conductance found in this study was helpful to keep a balance between allowable maximum transpiration and hydraulic safety of eucalyptus under varying soil and atmosphere moisture conditions.

Drought Trends in the Polish Carpathian Mts. in the Years 1991–2020

Mountains are highly sensitive to the effects of climate change, including extreme short- and long-term weather phenomena. Therefore, in spite of relatively high annual precipitation totals, mountains might become endangered by droughts. The paper presents drought trends in the Polish Carpathians located in Central Europe. Data from the period 1991–2020 from 12 meteorological stations located in various vertical climate zones of the mountains were used to define drought conditions using the following indices: Standardized Precipitation (SPI), Standardized Precipitation Evapotranspiration (SPEI), Relative Precipitation (RPI) and Sielianinov. Additionally, four forest drought indices were used in order to estimate the impact of drought on beech as a typical Carpathian tree species, i.e., the Ellenberg (EQ), Forestry Aridity (FAI), Mayr Tetratherm (MT) and De Martonne Aridity (AI) indices. Statistically significant but weak trends were obtained for the 6-month SPI for four stations (indicating an increase in seasonal to mid-term precipitation), for the 1-month SPEI for three stations, for the 3-month SPEI for four stations, and for MT for all stations (indicating an increase in drought intensity). The analysis of dry month frequency according to particular indices shows that at most of the stations during the last decade of the study period, the frequency of dry months was much higher than in previous decades, especially in the cold half-year. Two zones of the Polish Carpathians are the most prone to drought occurrence: the peak zone due to the shift in climatic vertical zones triggered by the air temperature increase, and the forelands and foothills, together with basins located about 200–400 m a.s.l., where the mean annual air temperature is the highest in all the vertical profile, the annual sums of precipitation are very diversified, and the conditions for beech are already unfavorable.

Modeling the effect of moisture stress on the shift in optimal and maximum temperatures for germination of Malva parviflora L. seeds: Introducing a new hydrothermal time model

Introduction Seed germination is largely controlled by the temperature and moisture content of the seedbed. Therefore, hydrothermal time models have been widely used to describe seed germination patterns in response to temperature and water potential (Ψ) of the seedbed. The majority of these models assume a Normal distribution for base water potential (Ψb(g)) to describe the variation in time to germination. In some of these models, it is assumed that the thermoinhibition of germination induced by the shift in Ψb(g) to more positive values occur only at temperatures above the optimum (To) and that the To is independent of drought stress levels. In this study, the Weibull hydrothermal time was used to quantify the Ψb(g) changes in response to temperature and to model the effect of drought stress on the shift in the optimal (To(g)) and maximum (Tm(g)) temperatures for different germination fractions of malva parviflora seeds. Materials and methods The experiment was conducted at the Seed Technology Laboratory of Agricultural Sciences and Natural Resources University of Khuzestan in 2016. Germination test was performed at eight constant temperatures of 8, 12, 16, 20, 24, 28, 32 and 36 (± 0.2) °C in light/dark conditions (12 h/12 h). In each of the above temperature regimes, seed germination response to different levels of drought stress, i.e. osmotic solutions with concentrations of 0, -0.2, -0.4, -0.6, -0.8 and -0.1 MPa was evaluated. Germination test was performed with four replications (each Petri dish as one replicate). In each replicate, 50 seeds were placed on a layer of Whatman No 1 filter paper in a 9 cm glass Petri dish, and then moistened with 7 ml distilled water or other osmotic solutions. The number of germinated seeds was counted twice every day until germination stopped at each temperature regime (when no germination occurred for 5 consecutive days). All models, having been formulated into the hydrotime and then hydrothermal models, were fitted to data using PROC NLMIXED procedure of SAS software version 9.4. Results and discussion While Ψb(g) showed a linear increase in the temperature range between Tb (base temperature) and Tm(g), the hydrotime constant (θH) decreased nonlinearly in response to increasing temperature. Based on the relationship between Ψb(g) and θH, the shape of the germination rate (GR(g)) response to temperature in the hydrothermal time model was curvilinear. The model estimated the values of θHT (hydrothermal time constant), Tb, Ψbase (base water potential at Tb), and KT (slope of the Ψb(g) response to temperature) as 1800.04 MPa °C h, 4.20 °C, -2.46 MPa, and 0.064 MPa °C-1, respectively. Both To(g) and Tm(g) decreased proportionally with increasing drought intensity and became cooler for higher germination percentiles. For example, the estimated To(50) (optimal temperature for the median) for M. parviflora seeds germinated under no water stress (Ψ=0 MPa) was 23.38 °C but dropped to 15.59 °C as water availability became minimum (Ψ=-1.0 MPa). Similarly, it was estimated that 50% of seeds would be able to germination at (or below) 42.55 °C form zero osmotic potential (Tm(50) at Ψ=0 MPa) but to attain the same germination level at -1.0 MPa, temperature should never exceed 26.99 °C (Tm(50) at Ψ=-1.0 MPa). Conclusion The hydrothermal time model not only gave good fits to germination data but also showed some adaptive properties of M. parviflora seeds to different temperature and moisture environments. With the increasing severity drought, the To(g) and Tm(g) shifted to cooler values, which mean that the seeds were able to germinate at a narrower temperature range under drought conditions.

Meteorological and hydrological droughts in Mekong River Basin and surrounding areas under climate change

Study Region Mekong River Basin and surrounding areas. Study FocusThis study investigated the impacts of climate change on future meteorological and hydrological droughts in the Mekong River Basin and its surrounding areas. Our work is based on the output of five global climate models (GCMs) and simulations using the geomorphology-based hydrological model (GBHM) for the historical (1975–2004), near future (2010–2039), middle future (2040–2069), and far future (2070–2099) periods. The meteorological droughts in the study area were measured using SPI and SPEI, while the hydrological droughts were measured using SSI. New Hydrological Insights for the RegionThe results suggest that droughts will generally reduce in the future over most of the study area, but will be more unevenly distributed with an eastward migration as compared to the historical period. Both meteorological and hydrological droughts will intensify in the near future, but will then reduce in intensity. Meteorological droughts will increase in the northeastern areas in the near future, followed by migration towards the south. Hydrological droughts showed similar aggravation followed by reduction, with upstream areas showing greater variability. In the general context of drought alleviation, southwestern China and the Mekong River estuary may suffer from a continuously increasing drought intensity in the future. This finding is based on 100-year extreme drought events.

SurEau: a mechanistic model of plant water relations under extreme drought

Key messageA new process-based model,SurEau, is described. It predicts the risk of xylem hydraulic failure under drought.ContextThe increase in drought intensity due to climate change will accentuate the risk of tree mortality. But very few process-based models are currently able to predict this mortality risk.AimsWe describe the operating principle of a new mechanistic model SurEau that computes the water balance, water relations, and hydraulics of a plant under extreme drought.MethodsSurEau is based on the formalization of key physiological processes of plant response to water stress. The hydraulic and hydric functioning of the plant is at the core of this model, which focuses on both water flows (i.e., hydraulic) and water pools (i.e., hydric) using variable hydraulic conductances. The model considers the elementary flow of water from the soil to the atmosphere through different plant organs that are described by their symplasmic and apoplasmic compartments. For each organ, the symplasm is described by a pressure-volume curve and the apoplasm by its vulnerability curve to cavitation. The model is evaluated on mature oak trees exposed to water stress.ResultsOn the tested oak trees, the model captures well the observed soil water balance, water relations, and level of embolism. A sensitivity analysis reveals that the level of embolism is strongly determined by air VPD and key physiological traits such as cuticular transpiration, resistance to cavitation, and leaf area.ConclusionThe process-based SurEau model offers new opportunities to evaluate how different species or genotypes will respond to future climatic conditions.

Regional differences in rapid evolution during severe drought.

Climate change is increasing drought intensity, threatening biodiversity. Rapid evolution of drought adaptations might be required for population persistence, particularly in rear‐edge populations that may already be closer to physiological limits. Resurrection studies are a useful tool to assess adaptation to climate change, yet these studies rarely encompass the geographic range of a species. Here, we sampled 11 populations of scarlet monkeyflower (Mimulus cardinalis), collecting seeds across the plants’ northern, central, and southern range to track trait evolution from the lowest to the greatest moisture anomaly over a 7‐year period. We grew families generated from these populations across well‐watered and terminal drought treatments in a greenhouse and quantified five traits associated with dehydration escape and avoidance. When considering pre‐drought to peak‐drought phenotypes, we find that later date of flowering evolved across the range of M. cardinalis, suggesting a shift away from dehydration escape. Instead, traits consistent with dehydration avoidance evolved, with smaller and/or thicker leaves evolving in central and southern regions. The southern region also saw a loss of plasticity in these leaf traits by the peak of the drought, whereas flowering time remained plastic across all regions. This observed shift in traits from escape to avoidance occurred only in certain regions, revealing the importance of geographic context when examining adaptations to climate change.

Temporal Trend Possibilities of Annual Rainfall and Standardized Precipitation Index in the Central Highlands, Vietnam

This paper employed the Şen’s innovative trend analysis (ITA) method to identify overall and partial trends in annual rainfall records over the Central Highlands of Vietnam. Additionally, the lowest values of Standardized Precipitation Index (SPI) for 3 and 12 months were subjected to the analysis to reveal potential changes in the greatest intensity of meteorological drought. The outcomes from a preliminary assessment by applying the normal (Gaussian) probability distribution functions (PDFs) indicate significant increases in standard deviation values of the second half subseries (current climate), thereby implying greater level of rainfall variability. The results from temporal trend analysis show that there is high domination of neutral and decreasing trends in annual rainfall records, with the estimated ITA slopes varying mainly from − 10 to − 14 mm/year. Furthermore, the low value sub-groups are mainly characterized by significant decreasing trends, which is an evident indication of possible increases in drought intensity. These results were further demonstrated by evaluating the ITA plots for the lowest values of SPI in both 3-month and 12-month timescales. The lowest values of SPI are mainly lower than − 1.5 and − 2.0 corresponding to severe and extreme drought categories, and most of the scatter points in low SPI cluster are accumulated in the lower triangular area of 1:1 line. Generally, it is expected that these findings will contribute various well-founded foundations to manage water–use activities and natural hazards associated with rainfall shortages.

Satellite data and machine learning reveal the incidence of late frost defoliations on Iberian beech forests.

Climate warming is driving an advance of leaf unfolding date in temperate deciduous forests, promoting longer growing seasons and higher carbon gains. However, an earlier leaf phenology also increases the risk of late frost defoliation (LFD) events. Compiling the spatiotemporal patterns of defoliations caused by spring frost events is critical to unveil whether the balance between the current advance in leaf unfolding dates and the frequency of LFD occurrence is changing and represents a threaten for the future viability and persistence of deciduous forests. We combined satellite imagery with machine learning techniques to reconstruct the spatiotemporal patterns of LFD events for the 2003-2018 period in the Iberian range of European beech (Fagus sylvatica), at the drier distribution edge of the species. We used MODIS Vegetation Index Products to generate a Normalized Difference Vegetation Index (NDVI) time series for each 250 × 250m pixel in a total area of 1,013 km2 (16,218 pixels). A semi-supervised approach was used to train a machine learning model, in which a binary classifier called Support Vector Machine with Global Alignment Kernel was used to differentiate between late frost and non-late frost pixels. We verified the obtained estimates with photointerpretation and existing beech tree-ring chronologies to iteratively improve the model. Then, we used the model output to identify topographical and climatic factors that determined the spatial incidence of LFD. During the study period, LFD was a low recurrence phenomenon that occurred every 15.2yr on average and showed high spatiotemporal heterogeneity. Most LFD events were condensed in 5 yr and clustered in western forests (86.5% in one-fifth of the pixels) located at high elevation with lower than average precipitation. Elevation and longitude were the major LFD risk factors, followed by annual precipitation. The synergistic effects of increasing drought intensity and rising temperature combined with more frequent late frost events may determine the future performance and distribution of beech forests. This interaction might be critical at the beech drier range edge, where the concentration of LFD at high elevations could constrain beech altitudinal shifts and/or favor species with higher resistance to late frosts.

Root carbon and nutrient homeostasis determines downy oak sapling survival and recovery from drought.

The role of carbon (C) and nutrient uptake, allocation, storage and especially their interactions in survival and recovery of trees under increased frequencies and intensities of drought events is not well understood. A full factorial experiment with four soil water content regimes ranging from extreme drought to well-watered conditions and two fertilization levels was carried out. We aimed to investigate whether nutrient addition mitigates drought effects on downy oak (Quercus pubescens Willd.) and whether storage pools of non-structural carbohydrates (NSC) are modified to enhance survival after 2.5 years of drought and recovery after drought relief. Physiological traits, such as photosynthesis, predawn leaf water potential as well as tissue biomass together with pools and dynamics of NSC and nutrients at the whole-tree level were investigated. Our results showed that fertilization played a minor role in saplings’ physiological processes to cope with drought and drought relief, but reduced sapling mortality during extreme drought. Irrespective of nutrient supply, Q. pubescens showed increased soluble sugar concentration in all tissues with increasing drought intensity, mostly because of starch degradation. After 28 days of drought relief, tissue sugar concentrations decreased, reaching comparable values to those of well-watered plants. Only during the recovery process from extreme drought, root NSC concentration strongly declined, leading to an almost complete NSC depletion after 28 days of rewetting, simultaneously with new leaves flushing. These findings suggest that extreme drought can lead to root C exhaustion. After drought relief, the repair and regrowth of organs can even exacerbate the root C depletion. We concluded that under future climate conditions with repeated drought events, the insufficient and lagged C replenishment in roots might eventually lead to C starvation and further mortality.

Characteristics of Spring–Summer Drought in Dry and Wet Periods in the South of European Russia

The distribution pattern of the characteristics of spring–summer droughts during periods of increased and decreased annual wetting in 12 administrative entities of the south of European Russia in 1901–2018 are studied. During this time, four periods of different humidification conditions (alternating wetter, dry, wet and again dry periods) of different durations are well traced according to anomalies in the aridity index in seven regions: Kursk, Belgorod, Voronezh, Saratov, Volgograd, and Rostov oblasts and the Republic of Kalmykia. A desiccation period was noted in Samara and Astrakhan oblasts from the beginning to the middle of the 20th century. The desiccation period then gave way to a wetter one. The most frequent change of periods with different moisture conditions was observed in Stavropol krai, where five alternating periods were identified. It has been established that the frequency of spring–summer droughts is consistent with the long-term humidification conditions. According to the standardized precipitation index, it increases on average by 1.5 times in the dry period with a slight increase in drought intensity. The conditions of increased humidification observed in all of the examined administrative subjects in the second half of the 20th century were replaced by drier conditions at the beginning of the 21st century. The spread of more arid conditions began in the eastern oblasts. The development of the dry period in the 21st century is confirmed by the distribution of the NDVI vegetation index in the negative trend. In the absence of statistically significant aridization trends for more than a century, all of the considered oblasts of the south of European Russia undergo long, intracentury periods of aridization strengthening and weakening, which are characterized by significant differences in the frequency and intensity of spring–summer droughts.

Differential effects of drought on nonstructural carbohydrate storage in seedlings and mature trees of four species in a subtropical forest

Nonstructural carbohydrates (NSC) play important roles in forest vulnerability to climate change, especially under increasing drought intensity and frequency. Understanding NSC dynamics is essential for accurately predicting the resistance and resilience of forests in response to drought. However, our knowledge of NSC responses to drought is still limited due to the lack of research in trees of different life stages. In this study, we conducted a throughfall exclusion experiment (TFE) with four subtropical evergreen tree species to examine drought effects on NSC in mature trees and understory seedlings. Our results showed the differential effects of drought on NSC dynamics of understory seedlings and mature trees. In the TFE experiment, mature trees of all four species were relatively homoeostatic with the insignificantly changed NSC pools, photosynthesis, and growth under the drought treatment compared to the control. In contrast, understory seedlings displayed significant decreases in total NSC and soluble sugars (−14.70 ± 3.66% and −16.93 ± 3.85%, respectively) with the exception of Lithocarpus glaber (L. glaber). The seedlings of L. glaber with the highest hydraulic resistance maintained or slightly increased NSC and its components in response to drought. Our study highlights the importance of life stage in assessing drought effects of trees on NSC storage and then forest C cycling, which could be incorporated into the dynamic global vegetation models (DGVMs) to better understand drought effects on forest C balance in the future.

Quantifying the Individual Contributions of Climate Change, Dam Construction, and Land Use/Land Cover Change to Hydrological Drought in a Marshy River

Hydrological drought for marshy rivers is poorly characterized and understood. Our inability to quantify hydrological drought in marshy river environments stems from the lack of understanding how wetland loss in a river basin could potentially change watershed structure, attenuation, storage, and flow characteristics. In this study, hydrological drought in a marshy river in far Northeast China at a higher latitude was assessed with a streamflow drought index (SDI). A deterministic, lumped, and conceptual Rainfall–Runoff model, the NAM (Nedbor Afstromnings Model), was used to quantify the individual contributions of climate change, land use/land cover (LULC) change, and river engineering to hydrological drought. We found that in the last five decades, the frequency of hydrological droughts has been 55% without considering LULC change and reservoir construction in this wetland-abundant area. The frequency of hydrological drought increased by 8% due to land use change and by 19% when considering both the impacts of LULC change and a reservoir construction (the Longtouqiao Reservoir). In addition to the more frequent occurrence of hydrological droughts, human activities have also increased drought intensity. These findings suggest that LULC and precipitation changes play a key role in hydrological drought, and that the effect can be significantly modified by a river dam construction.

Foliar salicylic acid spraying effect’ on growth, seed oil content, and physiology of drought-stressed Silybum marianum L. plant

Water deficit is one of the most sever worldwide problems of agricultural systems, which influences both yield and quality. Applying compositions like salicylic acid (SA) can improve plant resistance against environmental stresses. The present study was conducted to evaluate the impact of irrigation intervals and SA application on plant growth, osmotic parameters, oil yield, and oil composition of milk thistle. A completely randomized design was applied with a factorial arrangement of two factors: irrigation intervals as the first factor, including 5 days (control), 10 days (D1), 15 days (D2), 20 days (D3) and SA application as the second factor, including 0, 0.5 and 1 mM. From the obtained results, it was clear that water deficit damaged plant growth, including relative water content (RWC), seed yield, and oil content. Results show that spraying 0.5 mM SA partially increased plant growth and seed yield parameters under control condition. Under drought stress conditions, spraying 1 mM of SA had more significant effects on higher seed yield and oil content, compared to 0.5 Mm. Drought stress increased proline content, total soluble carbohydrates (TSC), sucrose, total phenolic compounds (TPC), and DPPH free radical scavenging, while applying 1 mM of SA increased these parameters in both well-watered and water deficit conditions. In addition, the production of linolenic acid, palmitic acid, and stearic acid enhanced under drought stress, while linoleic acid and total soluble proteins (TSP) dropped by increasing drought intensity. Applying 1 mM of SA increased all these parameters more effectively than other SA treatments. According to our results, we suggest that SA can play an essential role as a regulator for improving osmotic adjustment, RWC, and yield of milk thistle plants under drought stress conditions. Therefore, the application of a proper amount of SA can be considered as a practical and straightforward strategy for increasing seed oil quality and seed yield, for both industrial and edible purposes, in arid and semi-arid areas.

Tree Growth Under Climate Change: Evidence From Xylogenesis Timings and Kinetics.

Tree growth is one of the most studied aspects of tree biology, particularly secondary growth. In the Mediterranean region, cambial activity is mostly determined by water availability. Climatic projections for the Mediterranean region predict more frequent and intense droughts, and longer periods without precipitation. To investigate tree growth under the predicted scenarios of climate change, a water manipulation experiment was conducted in a maritime pine stand (Pinus pinaster Aiton). In 2017, fifteen trees were divided into three groups: control, rain exclusion, and irrigation. Drought conditions were simulated by installing a continuous plastic sheet on the forest floor from March to September. Trees under irrigation treatment were watered twice a week in September. Cambial activity and xylem formation was monitored every 10 days from February 2017 until March 2018. Cell production was maximal around the spring equinox in all treatments. Trees under rain exclusion decreased cell production rates, xylogenesis duration, and latewood cell wall thickness. The extra irrigation in September did not produce noticeable differences in xylogenesis compared to trees in the control treatment. The synchronization of maximum cambial division rates around the vernal equinox (spring) could allow Mediterranean trees to mitigate the impact of summer drought. With the predicted increase in drought intensity and frequency, lower tree productivity, carbon sequestration, and wood biomass are expected.

Assessing Spatiotemporal Drought Dynamics and Its Related Environmental Issues in the Mekong River Delta

Drought is a major natural disaster that creates a negative impact on socio-economic development and environment. Drought indices are typically applied to characterize drought events in a meaningful way. This study aims at examining variations in agricultural drought severity based on the relationship between standardized ratio of actual and potential evapotranspiration (ET and PET), enhanced vegetation index (EVI), and land surface temperature (LST) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) platform. A new drought index, called the enhanced drought severity index (EDSI), was developed by applying spatiotemporal regression methods and time-series biophysical data derived from remote sensing. In addition, time-series trend analysis in the 2001–2018 period, along with the Mann–Kendal (MK) significance test and the Theil Sen (TS) slope, were used to examine the spatiotemporal dynamics of environmental parameters (i.e., LST, EVI, ET, and PET), and geographically weighted regression (GWR) was subsequently applied in order to analyze the local correlations among them. Results showed that a significant correlation was discovered among LST, EVI, ET, and PET, as well as their standardized ratios (|r| &gt; 0.8, p &lt; 0.01). Additionally, a high performance of the new developed drought index, showing a strong correlation between EDSI and meteorological drought indices (i.e., standardized precipitation index (SPI) or the reconnaissance drought index (RDI)), measured at meteorological stations, giving r &gt; 0.7 and a statistical significance p &lt; 0.01. Besides, it was found that the temporal tendency of this phenomenon was the increase in intensity of drought, and that coastal areas in the study area were more vulnerable to this phenomenon. This study demonstrates the effectiveness of EDSI and the potential application of integrating spatial regression and time-series data for assessing regional drought conditions.

Response of southern African vegetation to climate change at 1.5 and 2.0° global warming above the pre-industrial level

Climate change has severely impacted southern African vegetation over the last decades. Recent studies have shown that limiting the global warming level to well below 2 °C above pre-industrial level may reduce the catastrophic effects of anthropogenic climate change. However, there is limited knowledge on the response of vegetation to climate change at different global warming levels. This study examines how southern African vegetation may respond to climate change at 1.5 °C and 2 °C warming. An ensemble of climate projections from the CESM model were analyzed to evaluate the potential changes in timing and magnitude of vegetation index and parameters across southern African biomes for a global warming of 1.5 °C and 2 °C under the RCP8.5 scenario. The results show that the spatial pattern of the Normalized Difference Vegetation Index (NDVI) are well captured by the ensemble mean. However, the model overestimates NDVI over the east coast of South Africa while it underestimates it over Angola. The strong relationship between the NDVI and other vegetation parameters (e.g. net primary production, leaf area cover) over southern Africa, suggests that vegetation productivity is projected to decline over Mozambique, Namibia, Botswana, Zimbabwe and Zambia at 1.5 °C and 2 °C warming. However, vegetation productivity is projected to likely increase over some parts of Madagascar and South Africa. The results further show that there is a similarity in the response of vegetation parameters at 1.5 °C and 2 °C warming. The difference in projected changes of the above ground net primary production, soil carbon and surface run-off is negligible between 1.5 °C and 2 °C warming. While the model ensemble members show a range in the magnitude of changes, they all project an increase in below ground net primary production, soil carbon and photosynthesis over the southern African biomes while projecting a decline in canopy transpiration over all the biomes. Furthermore, the CESM projects an increase in drought intensity as temperature increases over the southern African biomes. The projected changes in the drought intensity appears more pronounced with the Standardized Precipitation Evapotranspiration Index (SPEI) than with the Standardized Precipitation Index (SPI). The results of the study is important in informing policy makers for climate change mitigation and adaptation efforts in southern Africa. For effective mitigation and adaptation strategy, there is a need to translate the results to make them more accessible to inform policy through the use of appropriate engagement platforms such as multi-stakeholder partnerships and workshops.

Drought sensitivity of an N2 -fixing tree may slow temperate deciduous forest recovery from disturbance.

Increased drought intensity and frequency due to climate change may reduce the abundance and activity of nitrogen (N2 )-fixing plants, which supply new N to terrestrial ecosystems. As a result, drought may indirectly reduce ecosystem productivity through its effect on the N cycle. Here, we manipulated growing season net rainfall across a series of plots in an early successional mesic deciduous forest to understand how drought affects the aboveground productivity of the N2 -fixing tree Robinia pseudoacacia and three co-occurring nonfixing tree species. We found that lower soil moisture was associated with reduced productivity of R.pseudoacacia but not of nonfixing trees. As a result, the relative biomass and density of R.pseudoacacia declined in drier soils over time. Greater aboveground biomass of R.pseudoacacia was also associated with greater total soil N, extractable inorganic N, N mineralization rates, and productivity of nonfixing trees. These soil N effects may reflect current N2 fixation by R.pseudoacacia saplings, or the legacy effect of former trees in the same location. Our results suggest that R.pseudoacacia promotes the growth of nonfixing trees in early succession through its effect on the N cycle. However, the sensitivity of R.pseudoacacia to dry soils may reduce N2 fixation under scenarios of increasing drought intensity and frequency, demonstrating a mechanism by which drought may indirectly diminish potential forest productivity and recovery rate from disturbance.