Pre-drought Levels Research Articles

Short-term drought exposure decelerated growth and photosynthetic activities in chili pepper (Capsicum annuum L.)

Chili pepper (Capsicum annuum L.) is a preferred vegetable by local farmers at a riparian wetland in Indonesia due to high market demand and its economic value. Chili pepper can be grown immediately after rice harvesting, concurrently with the early dry season. Three chili pepper varieties, namely Genie, Romario, and Laris were evaluated for their adaptability to drought condition. Drought treatment was imposed for 7 days until soil water potential was dropped to around −30 to −40 kPa. Morphological and photosynthetic traits were measured at pre-, during, and post-drought exposures. Growth analysis was calculated based on measured morphological traits. Results indicated that the number of leaves (NoL), total leaf area (TLA), total accumulated dry matter (TDM), and relative growth rate (RGR) were negatively affected by drought. Water content of shoot (SWC), leaf (LWC), and root (RWC) were significantly decreased during drought exposure; yet, able to fully recover at 7 days after drought treatment was terminated. Decrease in shoot root ratio (SRR) and leaf weight ratio (LWR); reversely, increase the root weight ratio (RWR) under drought condition, was not associated with enhancement of root growth. Rather, it was due to less negatively effect of drought on roots than on the above-ground organs. Simultaneously, chlorophyll fluorescence, LWC, stomatal conductance, transpiration and photosynthetic rates declined during drought exposure. However, 7 days after drought treatment was terminated, all traits were able to return to pre-drought levels. Among chili pepper varieties studied, Genie exhibited better overall performance than Laris and Romario did under drought stress condition.

Drought Exacerbates Negative Consequences of High‐Intensity Cattle Grazing in a Semiarid Grassland

Grasslands managed for grazing are the largest land-use category globally, with a significant proportion of these grasslands occurring in semiarid and arid regions. In such dryland systems, the effect of grazing on native plant diversity has been equivocal, some studies suggesting that grazing reduces native plant diversity, others that grazing increases or has little impact on diversity. One impediment toward generalizing grazing effects on diversity in this region is that high levels of interannual variation in precipitation may obfuscate vegetative response patterns. By analyzing a long-term data set collected over a 20-yr period in a semiarid grassland, we explicitly evaluated the role of climate in regulating the effect of cattle grazing on plant communities, finding that climate interacted with grazing intensity to shape grassland communities. Community composition of plots that were intensively grazed varied considerably in response to climatic variation and native species richness was low relative to ungrazed and moderately grazed plots. Following a severe drought in 2002, exotic species richness rapidly increased in the high-intensity grazing plots. While this pattern was mirrored in the other treatments, exotic species richness increased to a greater extent and was slower to return to pre-drought levels in the high-intensity grazing plots. Overall, moderate grazing, even compared to grazing cessation, stabilized grassland communities through time, increased resilience to drought, and maintained the highest levels of native plant diversity and lowest levels of exotic diversity. These findings suggest that grazing, at moderate levels, may support grassland resilience to climate change in semiarid regions. However, grazing that exceeds tolerances, particularly in combination with extreme climatic events, like drought, can alter plant composition over relatively long timescales and possibly increase invasibility by nonnative species.

Lagging recovery for tropical forests

Ecology Tropical forest aboveground carbon (AGC) stocks have yet to recover from the extremely hot and dry weather associated with the 2015–2016 El Nino event. Wigneron et al. used low-frequency microwave satellite data to monitor AGC changes from 2014 to 2017. By the end of 2017, AGC stocks had not reached 2014 predrought levels and continued to decline in some areas. The slow rate of recovery could be due, in part, to enhanced forest mortality and/or unaccounted deforestation and degradation, specifically in African humid forests. Such continuity in long-term records may improve understanding of climate change on ecosystems. Sci. Adv. 10.1126/sciadv.aay4603 (2020).

Drought exacerbates negative consequences of high-intensity cattle grazing in a semiarid grassland.

Grasslands managed for grazing are the largest land-use category globally, with a significant proportion of these grasslands occurring in semiarid and arid regions. In such dryland systems, the effect of grazing on native plant diversity has been equivocal, some studies suggesting that grazing reduces native plant diversity, others that grazing increases or has little impact on diversity. One impediment toward generalizing grazing effects on diversity in this region is that high levels of interannual variation in precipitation may obfuscate vegetative response patterns. By analyzing a long-term data set collected over a 20-yr period in a semiarid grassland, we explicitly evaluated the role of climate in regulating the effect of cattle grazing on plant communities, finding that climate interacted with grazing intensity to shape grassland communities. Community composition of plots that were intensively grazed varied considerably in response to climatic variation and native species richness was low relative to ungrazed and moderately grazed plots. Following a severe drought in 2002, exotic species richness rapidly increased in the high-intensity grazing plots. While this pattern was mirrored in the other treatments, exotic species richness increased to a greater extent and was slower to return to pre-drought levels in the high-intensity grazing plots. Overall, moderate grazing, even compared to grazing cessation, stabilized grassland communities through time, increased resilience to drought, and maintained the highest levels of native plant diversity and lowest levels of exotic diversity. These findings suggest that grazing, at moderate levels, may support grassland resilience to climate change in semiarid regions. However, grazing that exceeds tolerances, particularly in combination with extreme climatic events, like drought, can alter plant composition over relatively long timescales and possibly increase invasibility by nonnative species.

Drought Effects on Tectona grandis Water Regulation Are Mediated by Thinning, but the Effects of Thinning Are Temporary

Results from tropical planted forests have demonstrated that tree plantations can have variable growth and water use patterns in response to drought. Yet research on how specific species will perform during a drought and whether this response can be mediated through forest management is still poorly understood. We took advantage of the 2015-2016 El Nino drought in central Panama to test the effects of thinning on sap flux density, transpiration, and growth of planted Tectona grandis (teak), a non-native species introduced to Panama for timber production. Despite a reduction in growth of teak during drought for control and thinned sites, tree sap flux density of thinned sites significantly temporarily increased after the thinning. Sap flux density (Js) for teak is strongly driven by changes in vapor pressure deficit (VPD), temperature, and radiation; however, Js declines as temperature rises above 28C and VPD is above 0.5, suggesting a temperature threshold that could be problematic as droughts and temperatures increase in unison in the future. At the stand-level, all sites reduced transpiration during the drought. Although diameter growth and transpiration declined during drought, the leaf area index after the drought ended returned to pre-drought levels.

Declines and Resilience of Communities of Leaf Chewing Insects on Missouri Oaks Following Spring Frost and Summer Drought

Extreme weather events dramatically impact populations of individual insect species but the consequences of such events for entire insect communities are not well documented. We present evidence that late spring frosts and summer drought negatively affect the community of insects found on Missouri oaks (Quercus alba and Q. velutina), amounting to a 23-186 fold decrease depending on the oak species, insect size and feeding guild, and the specific weather event. Spring faunas required three years or more following spring frosts to reach pre-event levels, whereas summer faunas were more resilient, rebounding within 1-2 years. The impact of summer drought on leaf tying caterpillars, however, lasted longer than that of the spring frost; it was five years before numbers of leaf ties reached pre-drought levels. Smaller species of leaf tying Lepidoptera took longer to recover than larger species following the drought. Overall, we found no evidence for a general decline in abundance, even a modest one, during the 20 years of study of faunas on oak trees in southeastern Missouri. However, the effects of drought reported here are likely to increase with time as the climate in the Midwest U.S. is only expected to become warmer and drier during the summer months. Similarly, the risk of late season frost damage to trees is expected to increase with predicted earlier onset of spring. Understanding the impact of such weather events on insect communities influences our ability to predict how habitat and landscape management, or lack thereof, will influence future patterns of insect abundance and diversity.

Climate has a larger effect than stand basal area on wood density in Pinus ponderosa var. scopulorum in the southwestern USA

Stand basal area of ponderosa pine ( Pinus ponderosa var. scopulorum Engelm.) in the US Southwest has little effect on the density of the wood produced, but climatic fluctuations have a strong effect. Wood density increases during drought, particularly if the drought occurs in late winter/early spring. Future droughts, as are predicted to increase in the US Southwest, may lead to production of smaller radial increments of higher density wood in ponderosa pine. Forest restoration treatments in the US Southwest are generating large quantities of small-diameter logs. Due to negative perceptions about ponderosa pine wood quality, this material is often seen as a “waste disposal” problem rather than a high-value resource. Our objective was to understand more about variation in southwestern US ponderosa pine wood density, an important indicator of wood quality. Specifically, we investigated the effect of stand basal area on wood density, and the effect of annual and quarterly climatic variation on wood density. We collected samples from 54 trees grown at six different basal area levels from a replicated stand density experiment. Pith-to-bark strips were used in an X-ray densitometer to obtain annual density and growth measurements from 1919 to the present. Stand density had a strong effect on growth rate, but little effect on wood density. However, climatic variation did influence wood density, which increased in drought years before quickly returning to pre-drought levels. Stand basal area is not a good indicator of wood density for foresters planning to utilize material from timber harvests in the southwestern USA. Future droughts, as are predicted to increase in the region, will likely reduce wood volume production but may increase wood density in ponderosa pine.

High ecosystem stability of evergreen broadleaf forests under severe droughts.

Global increase in drought occurrences threatens the stability of terrestrial ecosystem functioning. Evergreen broadleaf forests (EBFs) keep leaves throughout the year, and therefore could experience higher drought risks than other biomes. However, the recent temporal variability of global vegetation productivity or land carbon sink is mainly driven by non-evergreen ecosystems, such as semiarid grasslands, croplands, and boreal forests. Thus, we hypothesize that EBFs have higher stability than other biomes under the increasingly extreme droughts. Here we use long-term Standardized Precipitation and Evaporation Index (SPEI) data and satellite-derived Enhanced Vegetation Index (EVI) products to quantify the temporal stability (ratio of mean annual EVI to its SD), resistance (ability to maintain its original levels during droughts), and resilience (rate of EVI recovering to pre-drought levels) at biome and global scales. We identified significantly increasing trends of annual drought severity (SPEI range: -0.08 to -1.80), area (areal fraction range: 2%-19%), and duration (month range: 7.9-9.1) in the EBF biome over 2000-2014. However, EBFs showed the highest resistance of EVI to droughts, but no significant differences in resilience of EVI to droughts were found among biomes (forests, grasslands, savannas, and shrublands). Global resistance and resilience of EVI to droughts were largely affected by temperature and solar radiation. These findings suggest that EBFs have higher stability than other biomes despite the greater drought exposure. Thus, the conservation of EBFs is critical for stabilizing global vegetation productivity and land carbon sink under more-intense climate extremes in the future.

Nitrogen Addition Increases the Sensitivity of Photosynthesis to Drought and Re-watering Differentially in C3 Versus C4 Grass Species.

Global change factors, such as variation in precipitation regimes and nitrogen (N) deposition, are likely to occur simultaneously and may have profound impacts on the relative abundance of grasses differing in functional traits, such as C3 and C4 species. We conducted an extreme drought and re-watering experiment to understand differences in the resistance and recovery abilities of C3 and C4 grasses under different N deposition scenarios. A C3 perennial grass (Leymus chinensis) and two C4 grasses (annual species Chloris virgata and perennial species Hemarthria altissima) that co-occur in Northeast China were selected as experimental plants. For both C3 and C4 grasses, N addition caused a strong increase in biomass and resulted in more severe drought stress, leading to a change in the dominant photosynthetic limitation during the drought periods. Although N addition increased antioxidant enzyme activities and protective solute concentrations, the carbon fixing capacity did not fully recover to pre-drought levels by the end of the re-watering period. N addition resulted in lower resilience under the drought conditions and lower resistance at the end of the re-watering. However, N addition led to faster recovery of photosynthesis, especially in the C3 grass, which indicate that the effect of N addition on photosynthesis during drought was asymmetric, especially in the plants with different photosynthetic nitrogen use efficiency (PNUE). These findings demonstrated that nitrogen deposition may significant alter the susceptibility of C3 and C4 grass species to drought stress and re-watering, highlighting the asymmetry between resistance and resilience and to improve our understanding about plant responses to climate change.

Resilience of seed production to a severe El Niño-induced drought across functional groups and dispersal types.

More frequent and severe El Niño Southern Oscillations (ENSO) are causing episodic periods of decreased rainfall. Although the effects of these ENSO-induced droughts on tree growth and mortality have been well studied, the impacts on other demographic rates such as reproduction are less well known. We use a four-year seed rain dataset encompassing the most severe ENSO-induced drought in more than 30years to assess the resilience (i.e., resistance and recovery) of the seed composition and abundance of three forest types in a tropical dry forest. We found that forest types showed distinct differences in the timing, duration, and intensity of drought during the ENSO event, which likely mediated seed composition shifts and resilience. Drought-deciduous species were particularly sensitive to the drought with overall poor resilience of seed production, whereby seed abundance of this functional group failed to recover to predrought levels even two years after the drought. Liana and wind-dispersed species were able to maintain seed production both during and after drought, suggesting that ENSO events promote early successional species or species with a colonization strategy. Combined, these results suggest that ENSO-induced drought mediates the establishment of functional groups and dispersal types suited for early successional conditions with more open canopies and reduced competition among plants. The effects of the ENSO-induced drought on seed composition and abundance were still evident two years after the event suggesting the recovery of seed production requires multiple years that may lead to shifts in forest composition and structure in the long term, with potential consequences for higher trophic levels like frugivores.

Non-invasive imaging shows no evidence of embolism repair after drought in tree species of two genera.

Drought stress can result in significant impairment of the plant hydraulic system via blockage of xylem conduits by gas emboli. Recovery after drought stress is an essential component of plant survival but is still a poorly understood process. In this study, we examined the capacity of woody species from two genera (Eucalyptus and Quercus) to refill embolized xylem vessels during a cycle of drought and recovery. Observations were made on intact plants of Eucalyptus calmudulensis, E. grandis, E. saligna and Quercus palustris using X-ray microtomography. We found no evidence of an effective xylem refilling mechanism in any of the plant species. Despite rehydration and recovery of plant water potential to near pre-drought levels, embolized vessels were not refilled up to 72 h after rewatering. In E. saligna, water droplets accumulated in previously air-filled vessels for a very small percentage of vessels. However, no instances of complete refilling that would restore embolized vessels to hydraulic function were observed. Our observations suggest that rapid refilling of embolized vessels after drought may not be a wide spread mechanism in woody plants and that embolism formed during drought represents long term cost to the plant hydraulic system.

Remarkable increase in tree density and fuelwood production in the croplands of northern Nigeria

This paper examines trends in woody vegetation and tree species composition in the Sudan zone of West Africa, using the Kano region of northern Nigeria as a case study. The study compares data on tree density, fuelwood production and tree species composition from fieldwork conducted in 1981 and 2016, as well as on several dates of aerial and satellite images since the 1960s. Recent satellite-based reports of greening in arid West Africa as a response to recovery from droughts in the 1970s and 1980s, are examined to explain the observed trends. Tree densities in the goods and services hinterland of Kano city have at least doubled since the drought period, and no decline, rather a slight increase was observed during the drought decades. This contradicts reports of woody vegetation trends from the more arid and less densely populated Sahel zone, which generally observed decline during the drought years and current recovery to pre-drought levels. The remarkable increase in tree numbers in Kano region is accompanied by increasing fuelwood production as suggested by greater concentration by farmers on tree species highly valued for fuel, at the expense of other traditional species. The main driver of such trends is thought to be rapid population growth in the context of a remaining dependence on wood as fuel by both urban and rural populations in Nigeria. Climate is thought to play only a minor role in explaining the trends. These observations confirm trends in fuelwood production observed in Kano region more than three decades previously, and indicate a somewhat Boserupian response to Malthusian-type pressures on available resources. Nevertheless, a return to rainfall levels of the drought decades combined with climate change predictions of increasing temperatures in dryland Africa, may have serious consequences for rural households if energy sources are not diversified.

Investigation into the drought tolerance of broadleaf street trees using chlorophyll fluorescence

Water stress extensively contributes to the high tree mortality rates experienced in urban areas. This gives rise to direct costs due to replacement, but also indirect ones linked to the loss of ecosystem services delivery. Still, there is limited knowledge on how different species, commonly used in UK street plantings, rate in their tolerance to successive drought events. This study aims to compare young specimens of broadleaf tree species for their drought response and to establish how a species response varies as a result of prior exposure to drought. Results presented here are from a preliminary experiment carried out in the UK, where two-year old trees from nine species underwent two drought and recovery cycles. Responsiveness to the different periods was evaluated by changes in maximum quantum yield of photosystem II (Fv/Fm). Species varied in their Fv/Fm reduction while in drought and in the time it took Fv/Fm to recover to a pre-drought level once trees were watered. Populus tremuloides and Robinia pseudoacacia showed the greatest Fv/Fm reduction during the first cycle, they also lost many leaves and so they were excluded from the second cycle. All other species showed a greater Fv/Fm reduction in the second cycle than in the first one, particularly Betula pendula, Quercus robur and Acer platanoides. Quercus robur and A. platanoides also took the longest to recover from drought. Results suggest that selected tree species differ in their suitability to be used in water restrictive urban sites.

Remote Sensing of Ground Deformation for Monitoring Groundwater Management Practices: Application to the Santa Clara Valley During the 2012–2015 California Drought

AbstractGroundwater management typically relies on water‐level data and spatially limited deformation measurements. While interferometric synthetic aperture radar has been used to study hydrological deformation, its limited temporal sampling can lead to biases in rapidly changing systems. Here we use 2011–2017 COSMO‐SkyMed data with revisit intervals as short as 1 day to study the response of the Santa Clara Valley (SCV) aquifer in California to the unprecedented 2012–2015 drought. Cross‐correlation and independent component analyses of deformation time series enable tracking water through the aquifer system. The aquifer properties are derived prior to and during the drought to assess the success of water‐resource management practices. Subsidence due to groundwater withdrawal dominates during 2011–2017, limited to the confined aquifer and west of the Silver Creek Fault, similar to predrought summer periods. Minimum water levels and elevations were reached in mid‐2014, but thanks to intensive groundwater management efforts the basin started to rebound in late 2014, during the deepening drought. By 2017, water levels were back to their predrought levels, while elevations had not yet fully rebounded due to the delayed poroelastic response of aquitards and their large elastic compressibility. As water levels did not reach a new lowstand, the drought led to only elastic and recoverable changes in the SCV. The SCV lost 0.09 km3 during the drought while seasonal variations amount to 0.02 km3. Analysis of surface loads due to water mass changes in the aquifer system suggests that groundwater drawdowns could influence the stress on nearby faults.

Quantification of monoterpene emission sources of a conifer species in response to experimental drought.

Monoterpene (MT) emissions of conifer tree species, emitted from de novo synthesis and storage pools, play an important role in plant ecology and physiology. During drought stress both emission sources are affected differently and with conventional measuring techniques they are difficult to separate. We investigated 13C labelled MT emission of eight 3-year-old Scots pine seedlings in a drought stress experiment using a dynamic gas exchange chamber system (Tree DEMON). Monoterpene, water vapour and CO2 gas exchange were measured for a 2-day normal watering, a 11-day treatment and a 3-day re-watering period. In each period all trees were 13C labelled once for 5 h. Results showed the expected decrease of MT, water vapour and CO2 gas exchange with decreasing soil water content. However, during re-watering water vapour and CO2 gas exchange recovered fast to pre-drought levels, whereas MT increased to a lower level compared to the initial non-stressed phase. The 13C labelling showed highly variable %13C values for different MTs, which ranged compound-specific from 0.5 to 95 % for unstressed trees. Overall, around 36 ± 5 % of the total emission rate originated from de novo synthesized MTs during the 2-day prior to stress period. During full drought, the de novo fraction was reduced to 3 %. For the re-watering phase de novo emissions recovered only partly to 20 %, while pool emissions reached pre-drought conditions. Thus, emissions of de novo synthesized MTs of Scots pine are down-regulated by soil drought rather than MT emissions from pools.

Persistence of a Severe Drought Increases Desertification but not Woody Dieback in Semiarid Savanna

Increases in precipitation variability, coupled with higher temperatures, will lead to greater frequencies of severe, prolonged droughts for many regions with the expectation of attendant increases in woody plant die-off events. We took advantage of a 2-yr extension of a severe drought following an initial study of woody plant dieback in a woody-encroached semiarid savanna in west-central Texas, United States. This study tests for the emergence of alternative vegetation trajectories as a result of continued drought persistence: 1) whether additional woody plant dieback occurred following the initial study, leading to a grass-dominated community, or 2) whether desertification became a major feature (defined as a loss of herbaceous cover and increase in bare ground). Neither the emergence of a grass-dominated community nor the prevalence of desertification was observed during the initial study. After 2 additional yr of drought, we found that dieback of woody plants did not increase above previously observed levels, suggesting that the prolongation of drought did not cause the emergence of a grass-dominated community in this heavily encroached rangeland. However, drought severity did lead to increases in desertification, with increases in bare ground owed to declines in grass cover. While previous research at this long-term research site suggests that desertification is transient with grasses rebounding once precipitation returns to predrought levels, rangeland managers should be aware of lags in vegetation response to drought and the increased potential for a shift toward a bare-ground dominated community following extended extreme drought. In this Texas semiarid savanna, major losses in herbaceous cover lagged behind woody plant dieback, so dieback of the woody component might hold promise as an indicator for near-term potential of desertification.

The immediate and prolonged effects of climate extremes on soil respiration in a mesic grassland

AbstractThe predicted increase in the frequency and intensity of climate extremes is expected to impact terrestrial carbon fluxes to the atmosphere, potentially changing ecosystems from carbon sinks to sources, with positive feedbacks to climate change. As the second largest terrestrial carbon flux, soil CO2efflux or soil respiration (Rs) is strongly influenced by soil temperature and moisture. Thus, climate extremes such as heat waves and extreme drought should have substantial impacts onRs. We investigated the effects of such climate extremes on growing seasonRsin a mesic grassland by experimentally imposing 2 years of extreme drought combined with midsummer heat waves. After this 2 year period, we continued to measureRsduring a recovery year. Two consecutive drought years reducedRsby about 25% each growing season; however, when normal rainfall returned during the recovery year, formerly droughted plots had higher rates ofRsthan control plots (up to +17%). The heat wave treatments had no effect onRs, alone or when combined with drought, and during the growing season, soil moisture was the primary driver ofRswith little evidence forRstemperature sensitivity. When compared to aboveground net primary production, growing seasonRswas much less sensitive to drought but was more responsive postdrought. These results are consistent with the hypothesis that ecosystems become sources of CO2during drought because carbon inputs (production) are decreased relatively more than outputs (respiration). Moreover, stimulation ofRspostdrought may lengthen the time required for net carbon exchange to return to predrought levels.

Spatiotemporal dynamics of intermittent stream fish metacommunities in response to prolonged drought and reconnectivity

Hydrological regimes are primary drivers of community structure and dynamics in streams with strong seasonal or annual flood and drought cycles. In the current study, we investigated the dynamics of fish metacommunities in two intermittent streams (Hickory Creek and Clear Creek) in north Texas, USA, by examining changes in diversity, abundance, assemblage structure and temporal stability associated with prolonged seasonal drought and reconnectivity. Diversity (α and γ), abundance and stability increased with initial isolation during summer drought but dramatically declined as drought or drying persisted through the winter (November–December). During post-drought reconnectivity in Hickory Creek, diversity and abundance increased and approached pre-drought levels. Abundance and body size varied greatly among species and indicated species-specific responses (i.e. mortality, recruitment, dispersal) to hydrologic fragmentation and connectivity. Ultimately, assemblage structures were significantly altered by drought in Hickory and Clear creeks, and despite a trend towards recovery in Hickory Creek, assemblages did not fully recover during the present study. Intermittent-stream fishes may be generally adapted to natural drought dynamics; however, climate change and human-mediated habitat alterations may result in prolonged and intensified drought conditions that exceed many species mechanisms of resistance or resilience having potentially large impacts on biodiversity across spatial and temporal scales.

Dominant role of greenhouse-gas forcing in the recovery of Sahel rainfall

Higher atmospheric greenhouse gases are shown to have driven the recovery of Sahelian rainfall since the 1980s. This study discounts the role of sea surface temperature changes that had previously been jointly credited as drivers of the recovery. Sahelian summer rainfall, controlled by the West African monsoon, exhibited large-amplitude multidecadal variability during the twentieth century. Particularly important was the severe drought of the 1970s and 1980s, which had widespread impacts1,2,3,4,5,6. Research into the causes of this drought has identified anthropogenic aerosol forcing3,4,7 and changes in sea surface temperatures (SSTs; refs 1, 2, 6, 8, 9, 10, 11) as the most important drivers. Since the 1980s, there has been some recovery of Sahel rainfall amounts2,3,4,5,6,11,12,13,14, although not to the pre-drought levels of the 1940s and 1950s. Here we report on experiments with the atmospheric component of a state-of-the-art global climate model to identify the causes of this recovery. Our results suggest that the direct influence of higher levels of greenhouse gases in the atmosphere was the main cause, with an additional role for changes in anthropogenic aerosol precursor emissions. We find that recent changes in SSTs, although substantial, did not have a significant impact on the recovery. The simulated response to anthropogenic greenhouse-gas and aerosol forcing is consistent with a multivariate fingerprint of the observed recovery, raising confidence in our findings. Although robust predictions are not yet possible, our results suggest that the recent recovery in Sahel rainfall amounts is most likely to be sustained or amplified in the near term.

Exposure of two Eutrema salsugineum (Thellungiella salsuginea) accessions to water deficits reveals different coping strategies in response to drought.

Eutrema salsugineum is an extremophile related to Arabidopsis. Accessions from Yukon, Canada and Shandong, China, were evaluated for their tolerance to water deficits. Plants were exposed to two periods of water deficit separated by an interval of re-watering and recovery. All plants took the same time to wilt during the first drought exposure but Yukon plants took 1 day longer than Shandong plants following the second drought treatment. Following re-watering and turgor recovery, solute potentials of Shandong leaves returned to predrought values while those of Yukon leaves were lower than predrought levels consistent with having undergone osmotic adjustment. Polar metabolites profiled in re-watered plants showed that different metabolites are accumulated by Yukon and Shandong plants recovering from a water deficit with glucose more abundant in Yukon and fructose in Shandong leaves. The drought-responsive expression of dehydrin genes RAB18, ERD1, RD29A and RD22 showed greater changes in transcript abundance in Yukon relative to Shandong leaves during both water deficits and recovery with the greatest difference in expression appearing during the second drought. We propose that the initial exposure of Yukon plants to drought renders them more resilient to water loss during a subsequent water deficit leading to delayed wilting. Yukon plants also established a high leaf water content and increased specific leaf area during the second deficit. Shandong plants undergoing the same treatment regime do not show the same beneficial drought tolerance responses and likely use drought avoidance to cope with water deficits.