Increased Frequency Of Droughts Research Articles

Poor acclimation to experimental field drought in subalpine forest tree seedlings.

The ability of tree species to acclimate and tolerate projected increases in drought frequency and intensity has fundamental implications for future forest dynamics with climate change. Inquiries to date on the drought tolerance capacities of tree species, however, have focused almost exclusively on mature trees with scant in situ work on seedlings, despite the central role that regeneration dynamics play in forest responses to changing conditions. We subjected naturally established seedlings of co-dominant subalpine conifer species (Abies lasiocarpa and Picea engelmannii) in the southern Rocky Mountains to 2 years of in situ summer precipitation exclusion, simulating summer drought conditions similar to a failure of the North American monsoon. We compared the morphological and physiological responses of seedlings growing in drought vs. ambient conditions to assess the relative changes in drought tolerance traits as a function of seedling size. Drought treatments had a marked impact on soil moisture: volumetric water content averaged ≈5–8 % in drought treatments and ≈8–12 % in ambient controls. We detected no significant shifts in morphology (e.g. root biomass, leaf:stem area ratio) in response to drought for either species, but net photosynthesis in drought treatments was 78 % lower for spruce and 37 % lower for fir. Greater stomatal control associated with increasing stem diameter conferred greater water use efficiencies in larger seedlings in both species but was not significantly different between drought and ambient conditions, suggesting an overall lack of responsivity to water stress and a prioritization of carbon gain over investment in drought mitigation traits. These results indicate a canonization of traits that, while useful for early seedling establishment, may portend substantial vulnerability of subalpine seedling populations to prolonged or recurrent droughts, especially for spruce.

Dynamic variation of meteorological drought and its relationships with agricultural drought across China

Under the background of climate warming, the global water cycle is gradually accelerating, which will lead to increased drought frequency and severity. China is one of the countries with the highest drought risk and the most serious drought impact, accompanied by abnormal and variable drought characteristics in recent decades. However, the dynamic variation of meteorological drought and its complicated relationships with agricultural drought are still unclear across China. In this study, the characteristics of meteorological drought were comprehensively identified across China during 1960–2019. The spatio-temporal variations and gridded trend characteristics of meteorological drought were revealed during the study period. Subsequently, the return period of meteorological drought was quantitatively determined based on copula models. Finally, the relationships between meteorological and agricultural drought were explicitly clarified. The results indicated that: (1) meteorological drought showed an increasing trend across China from 1960 to 2019, especially since the 1990s; (2) the most serious meteorological drought occurred in 2011, with a minimum standardized precipitation evapotranspiration index (SPEI) value of –0.81; (3) the percentage of drought area (PDA) with an increasing trend in spring, summer, autumn and winter was 85.8%, 77.4%, 97.7% and 12.9%, respectively; (4) the most appropriate copula model of meteorological drought severity and duration was Frank copula; and (5) the propagation time from meteorological to agricultural drought was longer in winter and shorter in summer, which can determine the appropriate irrigation period and predict the occurrence of agricultural drought. This study sheds new viewpoints into the identification of meteorological drought variation and its relationships with agricultural drought, which can also be applied in other areas.

A Carica papaya L. genotype with low leaf chlorophyll concentration copes successfully with soil water stress in the field

In the context of increasing drought frequencies, water sensitive plant species need to have defined irrigation thresholds. One such species is papaya (Carica papaya L.), with a current demand for the selection of drought tolerance genotypes. The aim of this study was to evaluate the growth, leaf gas exchanges, sap flow and chlorophyll (Chl) fluorescence in two papaya genotypes (pale-green leaf ‘Golden’ - G and dark-green leaf ‘Aliança’ – AL) under soil water deficit (Ψsoil) conditions. The experiments were carried out in the field, in an unconsolidated alluvial sediment soil type. The two papaya genotypes, contrasting in their leaf Chl contents, were grown under irrigated (I, Ψsoil of -12 kPa), and non-irrigated (NI, Ψsoil gradually diminished from - 23 to -311 kPa) conditions for 41 days. Leaf gas exchanges, xylem sap-flow, green intensity (SPAD index), plant height, trunk diameter and leaf area were measured five times, following Ψsoil decreases. At the lowest Ψsoil (-311 kPa), the Chl photochemistry and fruit number measurements were performed. Intrinsic water use efficiency was higher in the GNI than in the ALNI genotype. Leaf temperature and photochemical efficiency was not affected by soil water restrictions in GNI and ALNI. The water deficit effect on growth traits was linked to whole canopy assimilation, due to the higher leaf area in ALI than GI, or in ALNI than in GNI, once the leaf assimilation rate was similar between those treatments. The ψsoil of ca. -30 kPa can be considered as a threshold for papaya irrigation for the soil type studied here. Soil water stress did not affect the number of fruits in GNI, whilst this was reduced by 83% in ALNI compared to irrigated plants, indicating that pale-green leaf papaya genotypes are an alternative to cope with water stress.

Coastal heathland vegetation is surprisingly resistant to experimental drought across successional stages and latitude

In the last decade, several major dwarf‐shrub dieback events have occurred in northern European coastal heathlands. These dieback events occur after extended periods with sub‐zero temperatures under snow‐free conditions and clear skies, suggesting that coastal heathlands have low resistance to winter drought. As climate projections forecast increased drought frequency, intensity, and duration, coastal heathlands are likely to experience more such diebacks in the future. There are, however, few empirical studies of drought impacts and responses on plant communities in humid oceanic ecosystems. We established a drought experiment with two distinct levels of intensified drought to identify responses and thresholds of drought resistance in coastal heathland vegetation. We repeated the experiment in two regions, separated by five degrees latitude, to represent different bioclimatic conditions within the coastal heathlands' wide latitudinal range in Europe. As coastal heathlands are semi‐natural habitats managed by prescribed fire, and we repeated the experiment across three post‐fire successional phases within each region. Plant community structure, annual primary production, and primary and secondary growth of the dominant dwarf‐shrub Calluna vulgaris varied between climate regions. To our surprise, these wide‐ranging vegetation‐ and plant‐level response variables were largely unaffected by the drought treatments. Consequently, our results suggest that northern, coastal heathland vegetation is relatively resistant to substantial intensification in drought. This experiment represents the world's wettest (2200 mm year−1) and northernmost (65°8'N) drought experiment to date, thus filling important knowledge gaps on ecological drought responses in high‐precipitation and high‐latitude ecosystems across multiple phases of plant community succession.

Drought Resilience Debt Drives NPP Decline in the Amazon Forest

AbstractClimate change has substantially increased the frequency of extreme droughts in the Amazon basin, generating concern about impacts on the world's largest tropical forest, which contributes about one‐seventh of the global vegetation carbon sink. Most research to understand drought impacts has focused on the immediate influences of such events, neglecting post‐drought effects on ecosystems recovery. Since ecological processes are influenced by antecedent conditions, we analyzed whether extreme droughts affect vegetation growth (i.e., net primary productivity, NPP) recovery. Here, we evaluated the NPP in the Amazon basin from 2003 to 2020, a period in which drought frequency was almost double the decadal incidence of the last century. We show that NPP does respond to the coupled impacts of individual droughts and the post‐drought impacts during ecosystem recovery. In particular, our results reveal that the ecosystems undergoing recovery show NPP about 13% lower than reference values based on the pre‐drought state or in areas undisturbed by drought. NPP deficits have consistently increased with the extreme droughts of 2005, 2010, and 2015 due to the combined effects of disturbances magnitude and the length of recovery. If the expected increase in drought frequency and intensity does occur, reduced recovery may lead the Amazon Forest to an alternative ecosystem state with lower carbon uptake, contributing to a warming global climate.

The Genetic Control of Stomatal Development in Barley: New Solutions for Enhanced Water-Use Efficiency in Drought-Prone Environments

Increased drought frequency due to climate change is limiting the agronomic performance of cereal crops globally, where cultivars often experience negative impacts on yield. Stomata are the living interface responsible for >90% of plant water loss through transpiration. Thus, stomata are a prospective target for improving drought tolerance by enhancing water-use efficiency (WUE) in economically important cereals. Reducing stomatal density through molecular approaches has been shown to improve WUE in many plant species, including the commercial cereals barley, rice, wheat and maize. Rice with reduced stomatal density exhibit yields 27% higher than controls under drought conditions, reflecting the amenability of grasses to stomatal density modification. This review presents a comprehensive overview of stomatal development, with a specific emphasis on the genetic improvement of WUE in the grass lineage. Improved understanding of the genetic regulation of stomatal development in the grasses, provides significant promise to improve cereal adaptivity in drought-prone environments whilst maximising yield potential. Rapid advances in gene-editing and ‘omics’ technologies may allow for accelerated adaption of future commercial varieties to water restriction. This may be achieved through a combination of genomic sequencing data and CRISPR-Cas9-directed genetic modification approaches.

Global exposure of population and land‐use to meteorological droughts under different warming levels and SSPs: A CORDEX‐based study

AbstractGlobal warming is likely to cause a progressive drought increase in some regions, but how population and natural resources will be affected is still underexplored. This study focuses on global population, forests, croplands and pastures exposure to meteorological drought hazard in the 21st century, expressed as frequency and severity of drought events. As input, we use a large ensemble of climate simulations from the Coordinated Regional Climate Downscaling Experiment (CORDEX), population projections from the NASA‐SEDAC dataset and land‐use projections from the Land‐Use Harmonization 2 project for 1981–2100. The exposure to drought hazard is presented for five Shared Socioeconomic Pathways (SSP1‐SSP5) at four Global Warming Levels (GWLs: 1.5°C to 4°C). Results show that considering only Standardized Precipitation Index (SPI; based on precipitation), the SSP3 at GWL4 projects the largest fraction of the global population (14%) to experience an increase in drought frequency and severity (versus 1981–2010), with this value increasing to 60% if temperature is considered (indirectly included in the Standardized Precipitation‐Evapotranspiration Index, SPEI). With SPEI, considering the highest GWL for each SSP, 8 (for SSP2, SSP4, SSP5) and 11 (SSP3) billion people, that is, more than 90%, will be affected by at least one unprecedented drought. For SSP5 at GWL4, approximately 2 × 106 km2 of forests and croplands (respectively, 6% and 11%) and 1.5 × 106 km2 of pastures (19%) will be exposed to increased drought frequency and severity according to SPI, but for SPEI this extent will rise to 17 × 106 km2 of forests (49%), 6 × 106 km2 of pastures (78%) and 12 × 106 km2 of croplands (67%), being mid‐latitudes the most affected. The projected likely increase of drought frequency and severity significantly increases population and land‐use exposure to drought, even at low GWLs, thus extensive mitigation and adaptation efforts are needed to avoid the most severe impacts of climate change.

Changing pattern of drought in Nepal and associated atmospheric circulation

Understanding the spatial and temporal characteristics of drought is vital for the agricultural, hydrological, ecological, and socio-economic systems. This study investigates the spatial and temporal variation of drought characteristics in Nepal using the Standardized Precipitation Evapotranspiration Index (SPEI) based on observational monthly temperature and precipitation data from 1987 to 2017. The drought variation was observed in the Western, Central, and Eastern regions of Nepal at short-term (SPEI4) and long-term (SPEI12) timescales, revealing the increased drought frequency, duration, and severity in the recent decade. The frequency of short-term drought (SPEI4 ≤ -1) has elevated by 15%, 17%, and 15% in Western, Central, and Eastern regions, respectively, during 2005–2017 compared to 1987–2004. Moreover, the interdecadal increase of drought characteristics was prominent after 2004, revealed by the SPEI12 with aggravated and prolonged drought episodes. The summer drought index (SPEI4-Sep) showed an interannual variation in the Western region at a seasonal timescale, whereas decadal variation in the Central and Eastern regions. Meanwhile, summer drought events increased five times in the Central region, two times in the Eastern Region, and did not observe any change in the Western region after 2004. The large-scale atmospheric circulations revealed the negative summer precipitation anomalies due to weakening wind anomalies with anti-cyclonic circulation and moisture divergence over the Indo-Gangetic plain and Nepal, respectively, leads to enhanced drought after 2004. Furthermore, the moisture transport from the Arabian Sea and the Bay of Bengal to the study region is weaker, resulting in more drought events, especially in the Central and Eastern regions.

Ensemble Drought Exposure Projection for Multifactorial Interactive Effects of Climate Change and Population Dynamics: Application to the Pearl River Basin

AbstractWith a changing climate, drought has become a common natural disaster. In our study, population exposure to drought over the Pearl River Basin (PRB) under climate change is investigated. Drought frequency is evaluated through the Standardized Precipitation Evapotranspiration Index (SPEI). The data needed for SPEI calculation are obtained based on the ensemble of multiple global climate model (GCM) outputs. Population exposure to drought for the future is assessed by combining drought frequency under two emission scenarios (RCP4.5 and RCP8.5) with three downscaled population scenarios (SSP1, SSP2, and SSP3). Moreover, the main contribution (and their interactions) of GCM, RCP, and SSP to the sources of uncertainty on population exposure projections is explored through multilevel factorial analysis. Results indicate that the temperature and precipitation would continually increase for the future, and the increase in drought frequency is more substantial during the 2080s than in the 2050s. Meanwhile, population exposure to drought accounts for 21.60% of the total population in 1976–2005 over the PRB area. During the 2050s, it would decrease to 11.98–12.28% under RCP4.5 and 14.15–14.40% under RCP8.5, respectively. By the 2080s, population exposure would slightly reduce under RCP4.5 and increase to 28.86–29.44% under RCP8.5. GCM is the primary uncertainty source of drought exposure in the 2050s, with contribution rates of 72.41%, 57.47%, 51.10%, and 78.71% to the four responses. In comparison, by the 2080s, RCP is the primary contributing factor, with contribution rates of 53.91%, 44.92%, 64.20%, and 48.00%, respectively.

Non-native plant removal and high rainfall years promote post-fire recovery of Artemisia californica in southern California sage scrub.

Non-native plant invasions, changes in fire regime, and increasing drought stress all pose important threats to biodiverse mediterranean-climate shrublands. These factors can also interact, with fire and drought potentially creating opportunities for non-native species to establish dominance before native shrubs recover. We carried out post-fire demographic monitoring of the common native shrub Artemisia californica in a southern California sage scrub fragment for 7 years, including several with very low rainfall. Experimental removals of non-native plants were included for the first 4 years. We quantified A. californica post-fire crown resprouting and seedling emergence, and tested effects of precipitation, non-native plants, and their interactions on seedling and adult survival. Only 7 A. californica were confirmed as resprouts; almost all individuals established after the fire from seedlings, with 90% of emergence occurring in the second growing year after fire (spring 2015). Higher spring precipitation increased both adult and seedling survival. Non-native grasses and forbs rapidly recolonized control plots, but the removal treatment reduced non-native cover by nearly 60%. For seedlings, non-native removal reduced the probability of dropping leaves by start of summer drought and increased survival both directly and through positive interactions with rainfall. Non-native removal also reduced mortality in smaller adult plants. By 2020, mean A. californica canopy area was nearly four times greater in non-native removal plots. These findings reinforce the high vulnerability of sage scrub habitat to post-fire loss of shrub cover and potential type conversion, particularly with increasing drought frequency and in stands and species with limited crown resprouting. Yet they also illustrate the potential for targeted management of non-natives immediately after fire to promote recovery of native shrubs in this increasingly endangered community.

Effects of Throughfall Exclusion on Photosynthetic Traits in Mature Japanese Cedar (Cryptomeria japonica (L. f.) D. Don.)

As climate change progresses, it is becoming more crucial to understand how timber species respond to increased drought frequency and severity. Photosynthetic traits in a 40-year-old clonal Japanese cedar (Cryptomeria japonica) plantation were assessed under artificial drought stress using a roof to exclude rainfall and a control with no exclusion. C. japonica is a commercial tree that is native to Japan and has high growth on mesic sites. The maximum carboxylation rate (Vcmax), maximum electron transfer rate (Jmax), and dark respiration rate (Rd) in current-year shoots in the upper canopy were determined from spring to autumn over two growing seasons. In addition, the photosynthetic rate at light saturation (Pmax), stomatal conductance (gs), and intrinsic water use efficiency (WUEi) were measured in the morning and afternoon during the same period. Leaf mass per unit area (LMA) and nitrogen concentration (N) were also measured. The values of Vcmax, Jmax, Rd, N, and LMA did not differ between the two plots. By contrast, significantly lower Pmax and gs and higher WUEi were found in the drought plot, and the reduction in Pmax was accompanied by low gs values. Midday depressions in Pmax and gs were more pronounced in the drought plot relative to the control and were related to higher WUEi. Under drought conditions, mature Japanese cedar experienced little change in photosynthetic capacity, foliar N, or LMA, but they did tend to close the stomata to regulate transpiration, thus avoiding drought-induced damage to the photosynthetic machinery and improving WUEi.

The Influence of Tilia tomentosa Moench on Plant Species Diversity and Composition in Mesophilic Forests of Western Romania–A Potential Tree Species for Warming Forests in Central Europe?

Climate change challenges important native timber species in Central Europe. The introduction of non-native tree species originating from warmer climates is one option to make Central European forests compatible to global warming. This, however, requires an assessment of the species’ growth requirements, and of its impact on biodiversity in its native ranges. Silver lime (Tilia tomentosa), a moderately drought-tolerant, thermophilous tree species of South-eastern Europe is considered suitable for the future. Along three elevational transects in western Romania, we assessed the impact of changing climate and local site conditions on the abundance of this tree species and contrasted plant species diversity and composition of lime-dominated forests with mesophytic oak and beech forests. Local site conditions and disturbance histories shaped the distribution pattern of silver lime. When dominant, it reduced plant species diversity within stands due to its dense canopy. For shade-tolerant, mesophytic species, though, lime forests provided an additional habitat and extended their range into warmer environments. Thus, silver lime may have the potential as an admixed tree species forming a transitory meso-thermophilous habitat in the future. At the same time, silver lime may be limited under increasing drought frequency.

Analysis of Characteristics of Hydrological and Meteorological Drought Evolution in Southwest China

Based on the data of 82 meteorological stations and six representative hydrological stations in four provinces in Southwest China (Guizhou, Sichuan, Yunnan, Chongqing), this paper uses standardized precipitation evapotranspiration index (SPEI) and standardized runoff index (SRI) to analyze the spatial and temporal evolution characteristics of drought in the study area from 1968 to 2018. Combined with the Southwest monsoon index and historical drought data, the correlation of drought and the applicability of different drought indices were verified. The results show that: (1) SPEI-12 in Southwest China shows a downward trend from 1968 to 2018, with a linear trend rate of −0.074/10a, and SPEI-3 has a downward trend in four seasons, the maximum linear trend rate being −0.106/10a in autumn;(2) The change in SRI-12 and SRI-24 value directly reflected the decrease in SRI value, indicating that drought events are increasing in recent times, especially in the 21st century (3). Severe drought occurred in the south of Southwest China, as indicated by the increase of drought frequency in this area. The main reason for the variations in the frequency distribution of drought in Southwest China is the combined effect of the change of precipitation and evapotranspiration. (4) The correlation between hydrological drought index and disaster areas is stronger than the correlation between meteorological drought and disaster areas.

Future Seasonal Drought Conditions over the CORDEX-MENA/Arab Domain

Seasonal drought is often overlooked because its impacts are less devasting than meteorological or hydrological drought. Nevertheless, short-term drought can have significant impacts on soil moisture content, agricultural crop yield, and sand and dust storms. Using data obtained from bias-corrected regional climate modelling (RCM) outputs, future seasonal drought is investigated over the water-scarce Arab domain using SPI-3. The climate modelling outputs include three downscaled mainframe GCMs downscaled using a single RCM for two climate scenarios: RCP4.5 and RCP8.5. Results across the region exhibit spatial and temporal variability. For example, Rift Valley, in the eastern sub-Sahara, projects less frequent and less severe drought, particularly during the winter (DJF) months. Conversely, the Morocco Highlands and adjacent Mediterranean coast signals a dramatic increase in drought by end-century during winter (DJF) and spring (MAM). Moderate increase in drought indicated in the greater Mashreq in spring (MAM) can be linked to sand and dust storm risk. Thirdly, autumn drought (SON) is linked to increased forest fire risk in the Levant. Projected increases in drought frequency and severity call for adaptation measures to reduce impacts.

Climate change impacts on spatial distribution, tree-ring growth, and water use of stone pine (Pinus pinea L.) forests in the Mediterranean region and silvicultural practices to limit those impacts

Stone pine (Pinus pinea L.) has been cultivated since centuries in Mediterranean areas for its products and economic benefits, including edible pine nuts, timber, mushrooms, firewood, and grazing. However, current management objectives of stone pine stands also include recreational use, biodiversity conservation, protection from soil erosion, and CO2 fixation. Stone pine stands are considered to be among the ecosystems most vulnerable to climate change, and the current increase in drought frequency in the Mediterranean Basin has been shown to negatively impact their long-term establishment. Understanding the effects of climate change on the distribution, tree-ring growth and water use of stone pine forests can help assessing the adaptive capacity of the species, and developing management programs aimed at its conservation. This paper reviews the impacts of climate change on stone pine in the Mediterranean region. The high sensitivity of stone pine to climate change has been widely demonstrated in that: (i) climatic models predict the loss of suitable habitats and the shift of its geographical distribution in the next future; (ii) tree-ring analysis showed that winter and spring rainfalls have positive effects on growth, whereas high spring temperature has a negative effect; (iii) the strategy of stone pine to cope with water deficit affects the processes regulating its growth, including wood formation, leading to peculiar tree-ring anatomical features such as intra-annual density fluctuations. The silvicultural interventions and the most effective management strategies for stone pine forests are reviewed and discussed in the context of current climate change in the Mediterranean Basin.

Plastic bimodal growth in a Mediterranean mixed-forest of Quercus ilex and Pinus halepensis

Mediterranean tree species have evolved to face seasonal water shortages, but may fail to cope with future increases in drought frequency and intensity. We investigated stem radial increment dynamics in two typical Mediterranean tree species, Aleppo pine (Pinus halepensis), a drought-avoiding species, and holm oak (Quercus ilex), a drought-tolerant species, in a mixed forest and on contrasting slope aspects (south- and north-facing). Intra- and inter-annual growth patterns were modelled using the VS-Lite2 model for each tree species and slope-aspect. Both species showed a bimodal growth pattern, with peaks coinciding with favourable conditions in spring and autumn. A bimodal growth pattern is always observed in P. halepensis, while in Q. ilex is facultative, which suggests different strategies adopted by these species to cope with summer drought. More specifically, trees on south-facing slope showed a more evident bimodal pattern and more intra-annual density fluctuations. In recent decades, the intensity of both growth peaks has diminished and drifted away due to the increased summer drought. The VS-Lite2 model reveals a niche partitioning between both species. Differences in growing season’s length and timings of growth peaks in both species are relevant for their coexistence and should be considered for estimating mixed-forest responses under climate change scenarios.

Sensitivity and threshold dynamics of Pinus strobus and Quercus spp. in response to experimental and naturally occurring severe droughts.

Increased drought frequency and severity are a pervasive global threat, yet the capacity of mesic temperate forests to maintain resilience in response to drought remains poorly understood. We deployed a throughfall removal experiment to simulate a once in a century drought in New Hampshire, USA, which coupled with the region-wide 2016 drought, intensified moisture stress beyond that experienced in the lifetimes of our study trees. To assess the sensitivity and threshold dynamics of two dominant northeastern tree genera (Quercus and Pinus), we monitored sap flux density (Js), leaf water potential and gas exchange, growth and intrinsic water-use efficiency (iWUE) for one pretreatment year (2015) and two treatment years (2016-17). Results showed that Js in pine (Pinus strobus L.) declined abruptly at a soil moisture threshold of 0.15m3m-3, whereas oak's (Quercus rubra L. and Quercus velutina Lam.) threshold was 0.11 m3m-3-a finding consistent with pine's more isohydric strategy. Nevertheless, once oaks' moisture threshold was surpassed, Js declined abruptly, suggesting that while oaks are well adapted to moderate drought, they are highly susceptible to extreme drought. The radial growth reduction in response to the 2016 drought was more than twice as great for pine as for oaks (50 vs 18%, respectively). Despite relatively high precipitation in 2017, the oaks' growth continued to decline (low recovery), whereas pine showed neutral (treatment) or improved (control) growth. The iWUE increased in 2016 for both treatment and control pines, but only in treatment oaks. Notably, pines exhibited a significant linear relationship between iWUE and precipitation across years, whereas the oaks only showed a response during the driest conditions, further underscoring the different sensitivity thresholds for these species. Our results provide new insights into how interactions between temperate forest tree species' contrasting physiologies and soil moisture thresholds influence their responses and resilience to extreme drought.

Climate sensitivity and drought seasonality determine post-drought growth recovery of Quercus petraea and Quercus robur in Europe

Recent studies have identified strong relationships between delayed recovery of tree growth after drought and tree mortality caused by subsequent droughts. These observations raise concerns about forest ecosystem services and post-drought growth recovery given the projected increase in drought frequency and extremes. For quantifying the impact of extreme droughts on tree radial growth, we used a network of tree-ring width data of 1689 trees from 100 sites representing most of the distribution of two drought tolerant, deciduous oak species (Quercus petraea and Quercus robur). We first examined which climatic factors and seasons control growth of the two species and if there is any latitudinal, longitudinal or elevational trend. We then quantified the relative departure from pre-drought growth during droughts, and how fast trees were able to recover the pre-drought growth level. Our results showed that growth was more related to precipitation and climatic water balance (precipitation minus potential evapotranspiration) than to temperature. However, we did not detect any clear latitudinal, longitudinal or elevational trends except a decreasing influence of summer water balance on growth of Q. petraea with latitude. Neither species was able to maintain the pre-drought growth level during droughts. However, both species showed rapid recovery or even growth compensation after summer droughts but displayed slow recovery in response to spring droughts where none of the two species was able to fully recover the pre-drought growth-level over the three post-drought years. Collectively, our results indicate that oaks which are considered resilient to extreme droughts have also shown vulnerability when droughts occurred in spring especially at sites where long-term growth is not significantly correlated with climatic factors. This improved understanding of the role of drought seasonality and climate sensitivity of sites is key to better predict trajectories of post-drought growth recovery in response to the drier climate projected for Europe.

Forecasting standardized precipitation index using data intelligence models: regional investigation of Bangladesh

A noticeable increase in drought frequency and severity has been observed across the globe due to climate change, which attracted scientists in development of drought prediction models for mitigation of impacts. Droughts are usually monitored using drought indices (DIs), most of which are probabilistic and therefore, highly stochastic and non-linear. The current research investigated the capability of different versions of relatively well-explored machine learning (ML) models including random forest (RF), minimum probability machine regression (MPMR), M5 Tree (M5tree), extreme learning machine (ELM) and online sequential-ELM (OSELM) in predicting the most widely used DI known as standardized precipitation index (SPI) at multiple month horizons (i.e., 1, 3, 6 and 12). Models were developed using monthly rainfall data for the period of 1949–2013 at four meteorological stations namely, Barisal, Bogra, Faridpur and Mymensingh, each representing a geographical region of Bangladesh which frequently experiences droughts. The model inputs were decided based on correlation statistics and the prediction capability was evaluated using several statistical metrics including mean square error (MSE), root mean square error (RMSE), mean absolute error (MAE), correlation coefficient (R), Willmott’s Index of agreement (WI), Nash Sutcliffe efficiency (NSE), and Legates and McCabe Index (LM). The results revealed that the proposed models are reliable and robust in predicting droughts in the region. Comparison of the models revealed ELM as the best model in forecasting droughts with minimal RMSE in the range of 0.07–0.85, 0.08–0.76, 0.062–0.80 and 0.042–0.605 for Barisal, Bogra, Faridpur and Mymensingh, respectively for all the SPI scales except one-month SPI for which the RF showed the best performance with minimal RMSE of 0.57, 0.45, 0.59 and 0.42, respectively.

Drought and soil nutrients effects on symbiotic nitrogen fixation in seedlings from eight Neotropical legume species

AbstractSymbiotic nitrogen fixation is a dominant source of nitrogen to many terrestrial ecosystems and thus may influence their responses to global change. High legume species diversity and abundance are thought to lead to high rates of symbiotic nitrogen fixation in Neotropical forests. However, how changes in water and nutrient availability will affect symbiotic nitrogen fixation has only recently been explored, even as droughts begin to increase in severity and frequency in the Neotropics. To explore these effects, we grew eight species of Neotropical woody legume seedlings in a shadehouse for four months while manipulating soil water, phosphorus, and molybdenum availability. Overall, drought reduced nodule biomass, nitrogenase activity (acetylene reduction g‐1 nodule), and acetylene reduction per seedling by 33%, 27%, and 41%, respectively, but reduced seedling biomass by only 18%. Species varied in the manifestation of drought effects. For example, drought reduced the probability that nodules formed in some species, but in others reduced the nitrogenase activity or acetylene reduction per seedling. In contrast, the effects of phosphorus and molybdenum availability were more species‐specific. However, fertilization did affect (both positively and negatively) one or more symbiotic nitrogen fixation response variables in two of the eight species. Our results indicate drought reduces symbiotic nitrogen fixation in Neotropical legume seedlings, but the mechanism of response may be species‐specific. Therefore, predicting the response of symbiotic nitrogen fixation in the Neotropics to a changing climate, with expected increases in drought frequency and severity, may require grappling with the diversity of responses among nitrogen‐fixing legumes.Abstract in Portuguese is available with online material