Increase In Drought Severity Research Articles

Observed changes in seasonal drought characteristics and their possible potential drivers over Pakistan

AbstractLong‐term drought monitoring and its assessment are of great importance for meteorological disaster risk management. The recurrent spells of heat waves and droughts have severely affected the environmental conditions worldwide, including Pakistan. The present work sought to investigate the spatiotemporal changes in drought characteristics over Pakistan during Rabi and Kharif cropping seasons. The role of large‐scale circulation and interannual mode of climate variability is further explored to identify the physical mechanisms associated with droughts in the region. Monthly precipitation and temperature data (1983–2019) from 53 meteorological stations were used to study drought characteristics, using the standardized precipitation evapotranspiration index (SPEI). The nonparametric Mann–Kendall, Sen's Slope, and Sequential Mann–Kendall tests were applied on the drought index to determine the statistical significance and magnitude of the historical trend. The state‐of‐the‐art Bayesian Dynamic Linear model was further used to analyse large‐scale climate drivers of droughts, revealed an increase in drought severity, mostly over arid to semiarid regions for both cropping seasons. While temperature played a significant role in defining droughts over dry and hot seasons, rainfall is influential over the western disturbances influenced region. The analysis of atmospheric circulation patterns revealed that large‐scale changes in wind speed, air temperature, relative humidity, and geopotential height anomalies are the likely drivers of droughts in the region. We found that Niño4, sea surface temperature, and multivariate El Niño‐Southern Oscillation (ENSO4.0) Index are the most influential factors for seasonal droughts across Pakistan. Overall, the findings provide a better understanding of drought‐prone areas in the region, and this information is of potential use for mitigating and managing drought risks.

Vegetation resistance and resilience to a decade-long dry period in the temperate grasslands in China.

The duration of climate anomalies has been increasing across the globe, leading to ecosystem function loss. Thus, we need to understand the responses of the ecosystem to long‐term climate anomalies. It remains unclear how ecosystem resistance and resilience respond to long‐term climate anomalies, for example, continuous dry years at a regional scale. Taking the opportunity of a 13‐year dry period in the temperate grasslands in northern China, we quantified the resistance and resilience of the grassland in response to this periodic dry period. We found vegetation resistance to the dry period increased with mean annual precipitation (MAP), while resilience increased at first until at MAP of 250 mm and then decreased slightly. No trade‐off between resistance and resilience was detected when MAP < 250 mm. Our results highlight that xeric ecosystems are most vulnerable to the long‐term dry period. Given expected increases in drought severity and duration in the coming decades, our findings may be helpful to identify vulnerable ecosystems in the world for the purpose of adaptation.

Drought characteristics for the south region of Brazil from 1983 to 2012: meteorological and hydrological approaches

An effective way to classify and evaluate droughts is the adjustment of hydrological series into drought indices. With this purpose, two different drought indices were applied to series of the South Region of Brazil, a location with the majority of hydrological studies not focused in low extremes. The Standardized Precipitation Index (SPI) and the Streamflow Drought Index (SDI) were applied for meteorological and hydrological analysis. The main focus of the study was to identify trends in these indices, and to analyze possible spatial patterns over the study area. From the results, it was verified the increase in drought severity in southern part of the study area, while decreasing trends were observed in eastern and northwestern parts. In the hydrological seasonal analysis, results have evidenced that in months with usually lower flows than the yearly average the rates are becoming even lower, while in months with higher flows the rates are increasing.

Live fast, die young: Climate shifts may favor Great Basin bristlecone pine or limber pine in sub-alpine forest establishment

As a result of climatic warming, tree species ranges are generally expected to move upslope in elevation. Although this upward range migration is likely determined principally by temperature, other factors such as habitat and soil moisture availability contribute to a species’ ability to establish in new areas. Throughout the montane ecosystems of the western US, effective drying is predicted, resulting from increasing temperatures and potentially reduced precipitation. The tree species that can better establish in these future high-elevation forests will likely expand their ranges while potentially excluding other species. Using a greenhouse experiment, we compared the response of limber pine and Great Basin bristlecone pine, the two dominant Great Basin sub-alpine species, to various levels of drought at different stages during their establishment. We found that during the first year of establishment, limber pine had greater diameter growth but lower height growth than bristlecone pine, while during the second year of establishment, limber pine had greater diameter and height growth rates across all treatments. During the post-germination period and in the second year of establishment, bristlecone pine seedlings had a higher survival rate than limber pine. During the first year of establishment there was no determinable difference in survival between the species. Limber pine displayed earlier mortality and lower survival under nearly all treatments across both first-and second-year periods. In general, an increase in drought severity corresponded to an overall earlier mortality onset as well as decreased survival in both species. However, in the first year of establishment under the no water treatment, both species showed later mortality than all other treatments, even having longer survival durations than a treatment with slightly more water. Limber pine seedlings effectively grew at higher rates than bristlecone pine seedlings, while dying younger. The different response of each species to drought stress during the establishment phase suggests an asymmetric competition under two possible climate change scenarios: a warmer, wetter future may favor limber pine, while a warmer, drier future may favor GB bristlecone pine. For the Great Basin’s sub-alpine forest community, this foreshadows a more complex future change in tree demographics than simple upslope migration.

Assessment of Rangeland Degradation in New Mexico Using Time Series Segmentation and Residual Trend Analysis (TSS-RESTREND)

Rangelands provide significant socioeconomic and environmental benefits to humans. However, climate variability and anthropogenic drivers can negatively impact rangeland productivity. The main goal of this study was to investigate structural and productivity changes in rangeland ecosystems in New Mexico (NM), in the southwestern United States of America during the 1984–2015 period. This goal was achieved by applying the time series segmented residual trend analysis (TSS-RESTREND) method, using datasets of the normalized difference vegetation index (NDVI) from the Global Inventory Modeling and Mapping Studies and precipitation from Parameter elevation Regressions on Independent Slopes Model (PRISM), and developing an assessment framework. The results indicated that about 17.6% and 12.8% of NM experienced a decrease and an increase in productivity, respectively. More than half of the state (55.6%) had insignificant change productivity, 10.8% was classified as indeterminant, and 3.2% was considered as agriculture. A decrease in productivity was observed in 2.2%, 4.5%, and 1.7% of NM’s grassland, shrubland, and ever green forest land cover classes, respectively. Significant decrease in productivity was observed in the northeastern and southeastern quadrants of NM while significant increase was observed in northwestern, southwestern, and a small portion of the southeastern quadrants. The timing of detected breakpoints coincided with some of NM’s drought events as indicated by the self-calibrated Palmar Drought Severity Index as their number increased since 2000s following a similar increase in drought severity. Some breakpoints were concurrent with some fire events. The combination of these two types of disturbances can partly explain the emergence of breakpoints with degradation in productivity. Using the breakpoint assessment framework developed in this study, the observed degradation based on the TSS-RESTREND showed only 55% agreement with the Rangeland Productivity Monitoring Service (RPMS) data. There was an agreement between the TSS-RESTREND and RPMS on the occurrence of significant degradation in productivity over the grasslands and shrublands within the Arizona/NM Tablelands and in the Chihuahua Desert ecoregions, respectively. This assessment of NM’s vegetation productivity is critical to support the decision-making process for rangeland management; address challenges related to the sustainability of forage supply and livestock production; conserve the biodiversity of rangelands ecosystems; and increase their resilience. Future analysis should consider the effects of rising temperatures and drought on rangeland degradation and productivity.

Impacts of droughts on rainfall use efficiency in different climatic zones and land uses in Iran

The frequency, duration, and severity of droughts have increased substantially in recent decades, especially in arid and semi-arid regions of the world. Understanding the responses of Rainfall Use Efficiency (RUE) to drought in different ecosystems and climatic zones is not well understood. In this study, spatiotemporal patterns of RUE and Standardized Precipitation Index (SPI) were investigated in four biomes (agriculture, bare land, forest, and range) and eight climatic zones in Iran from 2000 to 2014 using the global MODIS data (MOD17A3) of a 1-km resolution and precipitation data from meteorological stations across the country. A decreasing spatial trend in RUE was observed from west to east and north-west to south-east implying the role of altitude on spatial variation of precipitation. Based on the proposed RUE classification, nearly 60% of Iran’s lands were classified in class “extremely low” and experienced mild droughts during the study period. The increase of drought severity has resulted in an increase in RUE. Temporal variation of national average RUE in all land uses showed a similar trend and the maximum RUE was observed in agricultural lands followed by forest, range, and bare lands. RUE strongly responded to SPI variations across the eight climatic zones and the four biomes. We found significant and negative slopes of linear regression equations for RUE against SPI across all climatic zones, except the arid zone, representing higher accuracy in humid zones than in arid zones. Land uses showed a significant and negative regression, except in agricultural lands.

Prolonged duration and increased severity of agricultural droughts during 1978 to 2016 detected by ESA CCI SM in the humid Yunnan Province, Southwest China

Droughts in humid regions may have more severe consequences owing to a lack of drought-resistant measures and awareness. The Yunnan Province, in the humid southwestern part of China, experienced an extreme drought from 2009 to 2010. However, few studies have systematically explored agricultural droughts in this region. Based on revised global multi-source microwave-based SM products, the European Space Agency's Climate Change Initiative Soil Moisture (ESA CCI SM) data, the 10-day-scale soil moisture (SM) series was constructed and applied to detect droughts and analyze changes in drought characteristics, including duration, area, frequency, intensity, and severity. The results suggest that the intensity of agricultural drought in the Yunnan Province decreased, while the drought frequency increased. Drought duration increased by 20 days, collectively leading to a significant increase in drought severity in the last decade. Qujing in the east was most sensitive to agricultural drought, followed by Kunming and Chuxiong in the central area. The southwest region was prone to long drought and maintained an upward trend in terms of drought area. Regional SM with complicated temporal-spatial change patterns cannot be attributed to a single trend of “dry to dryer, wet to wetter”. This study may provide insights into drought analysis in other humid regions.

Higher susceptibility of beech to drought in comparison to oak

The expected increase in drought severity and frequency as a result of anthropogenic climate change leads to concerns about the ability of native tree species to cope with these changes. To determine the susceptibility of Fagus sylvatica (European beech) and Quercus robur (pedunculate oak) – the two dominant deciduous tree species in Central Europe – to drought, we quantified the climate sensitivity and drought-response of radial growth for both species using an array of dendroecological methods. Tree-ring data were collected from a site east of Coburg, Bavaria which had shown pronounced stress-symptoms (early leaf coloration) during the record drought of 2018. Climate-growth relationships were used to establish the sensitivity of radial growth to multiple climatic variables. The impact of specific drought events on tree growth was quantified using tolerance indices. In addition, we employed a Principal Component Gradient Analysis (PCGA) and remote sensing data (MODIS Normalized Difference Vegetation Index (NDVI)) to delineate the species specific drought responses. Using these methods we were able to show a clear difference in drought susceptibility between beech and oak. Beech displayed a higher sensitivity to temperature and the standardized precipitation evapotranspiration index (SPEI) and showed lower resistance and resilience to drought events than oak. In particular, beech was unable to fully recover from the 2003 drought, after which it expressed a stark growth decline, i.e. drought legacies, which was not observed for oak. The PCGA revealed a clear differentiation in the grouping of drought responses between beech and oak, supporting the findings of the climate-growth analysis and the tolerance indices. Correlations of NDVI and ring-width indices (RWI) indicated that under normal climatic conditions NDVI variability is linked to the start of the growing season. This is in contrast to drought years, such as 2003, where summer NDVI mirrored the drought response of beech and oak. These results reveal beech to have both a higher sensitivity to summer temperature and SPEI and a higher susceptibility to drought events. Although, in the past high plasticity and adaptability to drought have been attributed to both beech and oak, our study assigns beech a higher risk than oak to suffer from anticipated increases in drought frequency and intensity as a consequence of climate change.

Drought projections for Australia: Updated results and analysis of model simulations

To meet increasing demand for information on future drought hazard to help Australia build resilience and preparedness under a changing climate, we developed new information on drought projections for Australia and four sub-regions based on the natural resources management (NRM) zones. The information reported here includes: two drought indices (the Standardised Precipitation Index, SPI, and the Standardised Soil Moisture Index, SSMI); four drought metrics (percent time spent in droughts, mean drought duration, mean drought frequency, and mean drought intensity); and two drought categories (drought and extreme drought). The projections are developed from CMIP5 global climate model simulations of rainfall and soil moisture for the historical (1900–2005) and future (2006–2100) climates.The multi-model results project significant increases in all the drought hazard metrics, except frequency, with larger changes in the SSMI compared to SPI. The more severe drought hazard under climate change is apparent over a larger area than previously indicated, particularly in southern and eastern Australia. Although the majority of modelling results indicate more severe drought conditions, the range in the results is large, mainly because of the uncertainty in the global climate model rainfall projections. A projected decrease in rainfall results in a projected increase in drought severity (which is further enhanced by the increase in potential evapotranspiration), and a projected increase in rainfall results in a projected decrease in drought severity (moderated by the increase in potential evapotranspiration). The assessment of the ability of models to reproduce historical observations does not show clusters of models that best simulate all the different drought metrics. Unlike previously assumed, the results show that the models that best reproduce the observed rainfall are not necessarily best in simulating the drought metrics. For this reason, all the models are used here to estimate the multi-model median and range of results. The large uncertainty in the projections can be confusing to end users and present challenges in adapting to climate change. The presentation and communication of projections here will also go some way towards overcoming this challenge.

Recent trends in atmospheric evaporative demand in Southwest Iran: implications for change in drought severity

This study evaluates trends in climatic variables, atmospheric evaporative demand (AED) and meteorological drought between 1988 and 2018 over the southwest of Iran, where the Karkheh Basin is located. For this purpose, we used monthly time series of meteorological variables (precipitation, air temperature, wind speed and sunshine duration). The AED was estimated by the FAO-56 Penman-Monteith equation and drought severity by the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). There have been significant changes in the climate over the study domain, including a large increase in air temperature and wind speed and a decrease in relative humidity, which have driven a positive trend in the AED. The SPEI for the region has fallen, with the decrease being larger than the magnitude of change observed with the SPI. This suggests an increase in drought severity mostly associated with the rise in AED. The implications for water availability and climate change processes in the region are discussed.

Assessment of impacts of potential climate change on meteorological drought characteristics at regional scales

AbstractDrought patterns are better understood by analysing the joint dependence among long‐term drought characteristics: severity, duration, and frequency. Changes in these drought characteristics are also likely due to potential climate change, for instance, increased drought severity and duration, increased frequency of drought events, and increased spatial extent of drought impacts. In this study, we performed a regional‐scale analysis of potential changes in meteorological drought characteristics in a tropical region in India under a warming future climate scenario. The meteorological drought characteristics were computed using the Standardized Precipitation Index. The Coordinated Regional Downscaling Experiment South Asia Regional Climate Model future precipitation simulation under the RCP 4.5 scenario was bias‐corrected and utilized for future drought frequency analysis. Regionalization of meteorological drought characteristics was performed using the simple k‐means clustering technique to identify the homogeneous regions for baseline and future periods. Bivariate copula was utilized to model the dependence among the drought characteristics in order to develop the S‐D‐F curves. The changes in regionalization and regional S‐D‐F curves under future potential climate change were then investigated. The results of the study suggest an increase in drought severity by 2–3% for long‐term droughts (duration more than 5 months) in the future as indicated by upward shift and changes in the slope of the regional S‐D‐F curves. There is a projected increase in the occurrence of long‐term droughts in the future when compared to short‐term (duration of 1 to 3 months) droughts. We also observed a considerable spatial shift of drought hotspots, the severe drought‐prone regions lie in the eastern part of the study region in the future. Future projections of regional S‐D‐F curves and Spatio‐temporal drought characteristics obtained from the proposed framework may be utilized for devising drought mitigation strategies.

Elucidating Diverse Drought Characteristics from Two Meteorological Drought Indices (SPI and SPEI) in China

AbstractThis study elucidates drought characteristics in China during 1980–2015 using two commonly used meteorological drought indices: standardized precipitation index (SPI) and standardized precipitation–evapotranspiration index (SPEI). The results show that SPEI characterizes an overall increase in drought severity, area, and frequency during 1998–2015 compared with those during 1980–97, mainly due to the increasing potential evapotranspiration. By contrast, SPI does not reveal this phenomenon since precipitation does not exhibit a significant change overall. We further identify individual drought events using the three-dimensional (i.e., longitude, latitude, and time) clustering algorithm and apply the severity–area–duration (SAD) method to examine the drought spatiotemporal dynamics. Compared to SPI, SPEI identifies a lower drought frequency but with larger total drought areas overall. Additionally, SPEI identifies a greater number of severe drought events but a smaller number of slight drought events than the SPI. Approximately 30% of SPI-detected drought grids are not identified as drought by SPEI, and 40% of SPEI-detected drought grids are not recognized as drought by SPI. Both indices can roughly capture the major drought events, but SPEI-detected drought events are overall more severe than SPI. From the SAD analysis, SPI tends to identify drought as more severe over small areas within 1 million km2 and short durations less than 2 months, whereas SPEI tends to delineate drought as more severe across expansive areas larger than 3 million km2 and periods longer than 3 months. Given the fact that potential evapotranspiration increases in a warming climate, this study suggests SPEI may be more suitable than SPI in monitoring droughts under climate change.

Refuges and ecological traps: Extreme drought threatens persistence of an endangered fish in intermittent streams.

Recent droughts raise global concern over potential biodiversity loss and mitigating impacts to vulnerable species has become a management priority. However, drought impacts on populations are difficult to predict, in part, because habitat refuges can buffer organisms from harsh environmental conditions. In a global change context, more extreme droughts may turn previously suitable habitats into ecological traps, where vulnerable species can no longer persist. Here, we explore the impacts of California's recent record‐breaking drought on endangered juvenile Coho salmon. We estimated the variability of cumulative salmon survival using mark–recapture of nearly 20,000 tagged fish in intermittent stream pools during a 7‐year period encompassing drought and non‐drought conditions. We then determined the relative importance of physical habitat, streamflow, precipitation, landscape, and biological characteristics that may limit survival during drought. Our most striking result was an increase in the number of pools with reduced or zero survival during drought years and a coincident increase in spatial variability in survival among study reaches. In nearly half of the stream pools, salmon survival during drought was similar to mean survival of pools assessed during non‐drought years, indicating some pools had remarkable resistance (ability to withstand disturbance) to extreme drought. Lower survival was most attributable to longer duration of disconnection between upstream and downstream habitats, a consequence of increasing drought severity. Our results not only suggest that many pools sustain juvenile salmon in non‐drought years transition into ecological traps during drought but also highlight that some pools serve as refuges even under extreme drought conditions. Projected increases in drought severity that lead to longer droughts and greater habitat fragmentation could transform an increasing proportion of suitable habitats into ecological traps. Predicting future impacts of drought on Coho salmon and other sensitive species will require identification and protection of drought refuges and management strategies that prevent further habitat fragmentation.

Identifying Drought Status in Duhok Governorate (Iraqi Kurdistan Region) from 1998 through 2012 using Landsat Time Series Dataset

Drought is a natural hazard that has a significant impact on the various aspects (i.e., economic, agricultural, environmental, and social). This study was carried out to evaluate drought severity and frequency during the growing season (April month) in Duhok Governorate (DUG), the Iraqi Kurdistan Region (IKR), for the period from 1998 through 2012 based on Landsat-based spectral indices. In this study, 15 mosaics assembled for 15 years consist of two scenes of Landsat time series, in a total of 30 TM and ETM+ images (WRS2: 170/34 &amp; 170/35) acquired in 1998 to 2012. Annual precipitation data were collected from 18 meteorological stations distributed in the (DUG) for the study period. Drought status was investigated using the Normalized Difference Vegetation Index (NDVI), Modified Soil Adjusted Vegetation Index (MSAVI2), and Normalized Difference Water Index (NDWI). The study results showed an increase in drought severity and frequency in the (DUG) during the fifty years, particularly in 2000 and 2008. Whereas, the NDVI-based vegetation cover area has been reduced by 21.5% and 50.2% in 2000 and 2008, respectively. Additionally, the lowest values of the MSAVI2 (0.012 and 0.266) occurred in 2000 and 2008. As a result, the percentage of the vegetation cover reduction was 14.0% and 23.9%, respectively. Moreover, drop-in precipitation averages have occurred in those two drought years 2000 and 2008, as well as a significant reduction in the vegetation cover. On the other side, the most significant shrinkage in Duhok Dam (DUD) was by 1.13, 1.44, and 1.36 km2 in 2007, 2008, and 2009. It can be concluded that there are increasing drought episodes in the last two decades, declining in the water body surface area, and decreasing the precipitation averages in DUG from 1998 through 2012.

Droughts and the ecological future of tropical savanna vegetation

Abstract Climate change is expected to lead to more frequent, intense and longer droughts in the future, with major implications for ecosystem processes and human livelihoods. The impacts of such droughts are already evident, with vegetation dieback reported from a range of ecosystems, including savannas, in recent years. Most of our insights into the mechanisms governing vegetation drought responses have come from forests and temperate grasslands, while responses of savannas have received less attention. Because the two life forms that dominate savannas—C3 trees and C4 grasses—respond differently to the same environmental controls, savanna responses to droughts can differ from those of forests and grasslands. Drought‐driven mortality of savanna vegetation is not readily predicted by just plant drought‐tolerance traits alone, but is the net outcome of multiple factors, including drought‐avoidance strategies, landscape and neighborhood context, and impacts of past and current stressors including fire, herbivory and inter‐life form competition. Many savannas currently appear to have the capacity to recover from moderate to severe short‐term droughts, although recovery times can be substantial. Factors facilitating recovery include the resprouting ability of vegetation, enhanced flowering and seeding and post‐drought amelioration of herbivory and fire. Future increases in drought severity, length and frequency can interrupt recovery trajectories and lead to compositional shifts, and thus pose substantial threats, particularly to arid and semi‐arid savannas. Synthesis. Our understanding of, and ability to predict, savanna drought responses is currently limited by availability of relevant data, and there is an urgent need for campaigns quantifying drought‐survival traits across diverse savannas. Importantly, these campaigns must move beyond reliance on a limited set of plant functional traits to identifying suites of physiological, morphological, anatomical and structural traits or “syndromes” that encapsulate both avoidance and tolerance strategies. There is also a critical need for a global network of long‐term savanna monitoring sites as these can provide key insights into factors influencing both resistance and resilience of different savannas to droughts. Such efforts, coupled with site‐specific rainfall manipulation experiments that characterize plant trait–drought response relationships, and modelling efforts, will enable a more comprehensive understanding of savanna drought responses.

A 50-year analysis of hydrological trends and processes in a Mediterranean catchment

Abstract. The Réal Collobrier hydrological observatory in south-eastern France, managed by Irstea since 1966, constitutes a benchmark site for regional hydro-climatology. Because of the dense network of stream gauges and rain gauges available, this site provides a unique opportunity to evaluate long-term hydro-meteorological Mediterranean trends. The main catchment (70 km2) and its sub-catchments are located in the Massif des Maures of south-eastern France, close to the Mediterranean coast. The vegetation is composed of forest mainly calcified on crystalline soils (maquis of heath, cork-oak, maritime pine and chestnut). Direct human influence has been negligible over the past 50 years. The land use and land cover has remained almost unchanged, with the notable exception of a wildfire in 1990 that impacted a small sub-catchment. Therefore changes in the hydrological response of the catchments are caused by changes in climate and/or physical conditions. This study investigates changes in observational data using up to 50-year daily series of precipitation and streamflow. The analysis used several climate indices describing distinct modes of variability, at inter-annual and seasonal timescales. Trends were assessed by the Mann–Kendall method. The analysis also used hydrological indices describing drought events based on daily data for a description of low flows, in particular in terms of timing and severity. The analysis shows that there is a marked tendency towards a decrease in the water resources of the Réal Collobrier catchment in response to climate trends, with a consistent increase in drought severity and duration. But the changes are variable among the sub-catchments.

Nitrogen supply and other controls of carbon uptake of understory vegetation in a boreal Picea abies forest

In boreal forests, carbon (C) uptake by understory may be too large to be ignored and too variable in space to be assumed a constant fraction of the ecosystem gross primary production. To improve estimates of understory production in these ecosystems, we need to better account for its main controls. In this study, we estimated C uptake of field-layer vegetation, dominated by Vaccinium myrtillus, V. vitis-idaea, and Deschampsia flexuosa, in a boreal Picea abies stand in northern Sweden. Nitrogen (N) availability in the stand has been manipulated through annual N additions since 1996 at the rates of 0, 12.5, and 50 kg N ha−1 yr−1. To assess the relative importance of N supply, and interannual fluctuations in leaf biomass and weather, in controlling field-layer photosynthetic production, we calculated C uptake over eight growing seasons using a canopy photosynthesis model. Without N additions, tree leaf area index (L) was already high (8.5) and field-layer C uptake was small, 27 g C m−2 (or ∼3% of stand C uptake). An increase in tree L with N additions further reduced light availability for the understory, yet the concurrent increase in the relative abundance of the more physiologically active D. flexuosa sustained the contribution of the field-layer to stand photosynthetic production. Based on a literature survey, in which site quality or stand age generated a wide range in L, understory contribution to ecosystem C uptake increases linearly with the fraction of available light reaching the forest floor across high latitude forests. Understory contributes only ∼5% to ecosystem C uptake where trees intercept ∼80% of incoming light, increasing to 100% after clearcut tree harvest. While the availability of solar energy, both spatially and temporally, is the primary driver of understory production, our analyses suggest that the predicted increases in drought severity and frequency at high latitudes may affect understory communities more than trees. Future empirical and modeling studies should focus on functional and ecological responses to drought of not only trees but also understory species, which contribute to biodiversity and convert their photosynthates to important non-timber products.

Seasonal Drought Pattern Changes Due to Climate Variability: Case Study in Afghanistan

We assessed the changes in meteorological drought severity and drought return periods during cropping seasons in Afghanistan for the period of 1901 to 2010. The droughts in the country were analyzed using the standardized precipitation evapotranspiration index (SPEI). Global Precipitation Climatology Center rainfall and Climate Research Unit temperature data both at 0.5° resolutions were used for this purpose. Seasonal drought return periods were estimated using the values of the SPEI fitted with the best distribution function. Trends in climatic variables and SPEI were assessed using modified Mann–Kendal trend test, which has the ability to remove the influence of long-term persistence on trend significance. The study revealed increases in drought severity and frequency in Afghanistan over the study period. Temperature, which increased up to 0.14 °C/decade, was the major factor influencing the decreasing trend in the SPEI values in the northwest and southwest of the country during rice- and corn-growing seasons, whereas increasing temperature and decreasing rainfall were the cause of a decrease in SPEI during wheat-growing season. We concluded that temperature plays a more significant role in decreasing the SPEI values and, therefore, more severe droughts in the future are expected due to global warming.

Disentangling the functional trait correlates of spatial aggregation in tropical forest trees.

Environmental filtering and dispersal limitation can both maintain diversity in plant communities by aggregating conspecifics, but parsing the contribution of each process has proven difficult empirically. Here, we assess the contribution of filtering and dispersal limitation to the spatial aggregation patterns of 456 tree species in a hyperdiverse Amazonian forest and find distinct functional trait correlates of interspecific variation in these processes. Spatial point process model analysis revealed that both mechanisms are important drivers of intraspecific aggregation for the majority of species. Leaf drought tolerance was correlated with species topographic distributions in this aseasonal rainforest, showing that future increases in drought severity could significantly impact community structure. In addition, seed mass was associated with the spatial scale and density of dispersal-related aggregation. Taken together, these results suggest environmental filtering and dispersal limitation act in concert to influence the spatial and functional structure of diverse forest communities.

Crop yield sensitivity of global major agricultural countries to droughts and the projected changes in the future

Understanding the potential drought impacts on agricultural production is critical for ensuring global food security. Instead of providing a deterministic estimate, this study investigates the likelihood of yield loss of wheat, maize, rice and soybeans in response to droughts of various intensities in the 10 largest producing countries. We use crop-country specific standardized precipitation index (SPI) and census yield data for 1961–2016 to build a probabilistic modeling framework for estimating yield loss risk under a moderate (−1.2 < SPI < −0.8), severe (−1.5 < SPI < −1.3), extreme (−1.9 < SPI < −1.6) and exceptional (SPI < −2.0) drought. Results show that there is >80% probability that wheat production will fall below its long-term average when experiencing an exceptional drought, especially in USA and Canada. As for maize, India shows the highest risk of yield reduction under droughts, while rice is the crop that is most vulnerable to droughts in Vietnam and Thailand. Risk of drought-driven soybean yield loss is the highest in USA, Russian and India. Yield loss risk tends to grow faster when experiencing a shift in drought severity from moderate to severe than that from extreme to the exceptional category, demonstrating the non-linear response of yield to the increase in drought severity. Sensitivity analysis shows that temperature plays an important role in determining drought impacts, through reducing or amplifying drought-driven yield loss risk. Compared to present conditions, an ensemble of 11 crop models simulated an increase in yield loss risk by 9%–12%, 5.6%–6.3%, 18.1%–19.4% and 15.1%–16.1 for wheat, maize, rice and soybeans by the end of 21st century, respectively, without considering the benefits of CO2 fertilization and adaptations. This study highlights the non-linear response of yield loss risk to the increase in drought severity. This implies that adaptations should be more targeted, considering not only the crop type and region but also the specific drought severity of interest.