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• Hydraulic failure, but not carbon starvation, occurred towards mortality in all species under both drought treatments. • Rapid xylem repair after drought was not observed. • The recovery of photosynthesis was decoupled from water potential recovery. The impact of drought stress on dominant tree species of subtropical China, and the capacity of these species to recover from drought, is relatively unknown. Three ecologically important tree species (Syzygium rehderianum , Castanopsis chinensis and Schima superba), with contrasting drought sensitivities, were grown in pots under well-watered conditions in the glasshouse before being subjected to two drought treatments (i.e. higher intensity and shorter-duration fast drought(FD);lower intensity and longer-duration slow drought(SD)) towards mortality, and then re-watered to assess the rate of physiological recovery. Dry mass production and physiological traits related to water and carbon relations were measured over the experimental period. FD led to faster declines in hydraulic and photosynthetic function than SD in all species, while hydraulic failure occurred towards mortality in all species under both drought treatments. Although NSC responses varied among drought treatments and species, carbon starvation was not observed. S. rehderianum exhibited more rapid declines in leaf water potential and gas exchange traits, and shorter time-to-mortality, followed by C. chinensis and S. superba. Following re-watering, all three species showed rapid recovery of leaf water potential, while rapid recovery in stem hydraulic conductivity was not evident in any species. Furthermore, C. chinensis exhibited full rapid recovery of photosynthesis, but photosynthetic recovery in S. rehderianum and S. superba was slower than recovery of water potential, reflecting differential stomatal and biochemical limitations. The response of the three species to drought was strongly correlated with xylem embolism resistance, but recovery following drought was complex and partially associated with stomatal and biochemical traits. Under future longer and severe droughts, these species may diverge in drought responses and recovery, perhaps driving shifts in forest community structure in subtropical China. [ABSTRACT FROM AUTHOR]

Highlights • Northeast China is the center of maize production in the country. • Assess varying impacts of drought across different maize growth stages in Northeast China. • Probability of extreme events in irrigation water shortage will increase in the future. • Irrigation infrastructure and efficient irrigation scheme and technologies are of great importance. Abstract Northeast Farming Region of China (NFR) produces about one-third of the national maize output. Shortage of crop irrigation water is one of the main threat to the stable level of maize production in the NFR. Previous studies on the sensitivity of maize production to drought are typically based on field experiments and treat the maize growing season as a whole, with rare attention to the varying impacts of drought across different maize growth stages. Given the importance of NFR on China's food security, it is crucial to optimize the irrigation schedule to mitigate the adverse effects of drought. In this study, we employ Agro-ecological Zone (AEZ) model to investigate how climate change affects irrigation water requirement (IWR) of maize during different growth stages and under different climate change scenarios. Results indicate that the NFR would experience a substantial increase in the probability of extremely shortage of crop irrigation water under future climate change. The ensemble simulation under future climate projections indicates more frequent demands for irrigation with substantially increased amount in the mid-season stage (G3) when maize is more sensitive to water deficit compared with other stages. These findings indicate that earlier planning of irrigation infrastructure and development of more efficient irrigation scheme and technologies is of great importance to secure maize production in the region. [ABSTRACT FROM AUTHOR]

Drought and vegetation conditions within the Damqu River Basin, part of the Yangtze River Source Region (YRSR), are assessed here using the standardized precipitation index (SPI), the standardized precipitation evapotranspiration index (SPEI), the normalized difference vegetation index (NDVI), and the leaf area index (LAI). We utilized Sen’s method, least squares regression method, linear regression and Pearson’s correlation analysis to study variations in drought and vegetation indices and the drought effect on vegetation between 1988 and 2015. Results reveal that droughts occurred at a 25% frequency over this period; SPI and SPEI analyses show that 1994, 1999, 2005, and 2010 were change points and that the basin was characterized by varying drought and humidity trends. Subsequent to 2010, both SPI and SPEI decreased within the basin, while 1995, 2000, 2004, and 2010 were change points for NDVI and LAI while the watershed exhibited variable trends in vegetation reduction and increase. The NDVI-annual values of 63.36% regions and the LAI-summer values of 68.39% areas within the basin were decreased during 1988–2015 and 2000–2015, respectively. Subsequent to 2010, both NDVI and LAI decreased within the basin and significant positive correlations at inter-annual and inter-summer time scales were seen in both drought and vegetation indices; drought has exerted a lag effect on vegetation as shown by significant positive correlations between annual SPI/SPEI values and following year NDVI/LAI values. [ABSTRACT FROM AUTHOR]

Droughts are natural disasters that pose significant threats to agricultural production as well as living conditions, and a spatial-temporal difference analysis of agricultural drought risk can help determine the spatial distribution and temporal variation of the drought risk within a region. Moreover, this type of analysis can provide a theoretical basis for the identification, prevention, and mitigation of drought disasters. In this study, the overall dispersion and local aggregation of projection points were based on research by Friedman and Tukey (IEEE Trans on Computer 23:881-890, 1974). In this work, high-dimensional samples were clustered by cluster analysis. The clustering results were represented by the clustering matrix, which determined the local density in the projection index. This method avoids the problem of determining a cutoff radius. An improved projection pursuit model is proposed that combines cluster analysis and the projection pursuit model, which offer advantages for classification and assessment, respectively. The improved model was applied to analyze the agricultural drought risk of 13 cities in Heilongjiang Province over 6 years (2004, 2006, 2008, 2010, 2012, and 2014). The risk of an agricultural drought disaster was characterized by 14 indicators and the following four aspects: hazard, exposure, sensitivity, and resistance capacity. The spatial distribution and temporal variation characteristics of the agricultural drought risk in Heilongjiang Province were analyzed. The spatial distribution results indicated that Suihua, Qigihar, Daqing, Harbin, and Jiamusi are located in high-risk areas, Daxing’anling and Yichun are located in low-risk areas, and the differences among the regions were primarily caused by the aspects exposure and resistance capacity. The temporal variation results indicated that the risk of agricultural drought in most areas presented an initially increasing and then decreasing trend. A higher value for the exposure aspect increased the risk of drought, whereas a higher value for the resistance capacity aspect reduced the risk of drought. Over the long term, the exposure level of the region presented limited increases, whereas the resistance capacity presented considerable increases. Therefore, the risk of agricultural drought in Heilongjiang Province will continue to exhibit a decreasing trend. [ABSTRACT FROM AUTHOR]

To date, few attempts have been made to assess the influence of climate change on forest ecosystems and on the relationship between tree growth and climate in humid areas of low latitudes. In this paper, we studied the response of tree growth and forest ecosystem to climate change by using Fokienia hodginsii tree-ring cores from the northern Yunnan-Guizhou Plateau, southwest of China. Tree growth correlates the highest ( r = −0.64, p < 0.01) with mean temperature (July-September), but the coefficients were changing with time as revealed by a moving correlation analysis. Tree growth is significantly ( p < 0.05) and positively correlated with January-April mean temperature from AD 1961-1987, while correlations with precipitation are insignificant. In contrast, from 1988 to 2014, tree growth correlated negatively with mean temperature of previous summer and positively with precipitation of previous August-September. This indicated that the limiting factors for tree growth have changed under different climate conditions. The meteorological data suggested that from 1961 to 1987 it was cold and wet in the study area and radial growth is limited by winter and spring temperatures. This restriction is weaker if the climate is appropriate in general. However, from 1988 to 2014, the combined effects of recent warming and decreasing precipitation have led to an increasing response of tree-ring width to drought. In addition, a large proportion of mature F. hodginsii mortality occurred from 2007 to 2013, which corresponds with a drastic reduction of radial growth (narrowest in recent 100 years). The recent drought, induced by decreasing precipitation and increasing temperature, may have passed the threshold which F. hodginsii could tolerate, causing tree growth reduction, tree growth-climate relationship change, as well as catastrophic tree mortality. All these changes may lead to further responses of the local ecosystem to climate change which should be highly regarded. [ABSTRACT FROM AUTHOR]

Background: Drought stress is one of the major natural challenges in the main tea-producing regions of China. The tea plant (Camellia sinensis) is a traditional beverage plant whose growth status directly affects tea quality. Recent studies have revealed that microRNAs (miRNAs) play key functions in plant growth and development. Although some miRNAs have been identified in C. sinensis, little is known about their roles in the drought stress response of tea plants. Results: Physiological characterization of Camellia sinensis 'Tieguanyin' under drought stress showed that the malondialdehyde concentration and electrical conductivity of leaves of drought-stressed plants increased when the chlorophyll concentration decreased under severe drought stress. We sequenced four small-RNA (sRNA) libraries constructed from leaves of plants subjected to four different treatments, normal water supply (CK); mild drought stress (T1); moderate drought stress (T2) and severe drought stress (T3). A total of 299 known mature miRNA sequences and 46 novel miRNAs were identified. Gene Ontology enrichment analysis revealed that most of the differentially expressed-miRNA target genes were related to regulation of transcription. Kyoto Encyclopedia of Genes and Genomes analysis revealed that the most highly enriched pathways under drought stress were D-alanine metabolism, sulfur metabolism, and mineral absorption pathways. Real-time quantitative PCR (qPCR) was used to validate the expression patterns of 21 miRNAs (2 up-regulated and 19 down-regulated under drought stress). The observed co-regulation of the miR166 family and their targets ATHB-14-like and ATHB-15- like indicate the presence of negative feedback regulation in miRNA pathways. Conclusions: Analyses of drought-responsive miRNAs in tea plants showed that most of differentially expressedmiRNA target genes were related to regulation of transcription. The results of study revealed that the expressions of phase-specific miRNAs vary with morphological, physiological, and biochemical changes. These findings will be useful for research on drought resistance and provide insights into the mechanisms of drought adaptation and resistance in C. sinensis. [ABSTRACT FROM AUTHOR]

Understanding the bulk organic carbon isotopic composition ( δ 13 C org ) of the surface soil in various climates is crucial for past climate reconstruction. Although the relationship between climatic factors (such as precipitation and temperature) and plant δ 13 C org are well studied at the global and regional scales, studies relating the surface soil organic matter δ 13 C org values and climatic factors remain relatively rare. In this study, 52 surface soil samples were collected from the Shiyang River drainage basin, the northeastern margin of the Tibetan Plateau, where changes in vegetation are sensitive to the climate. The δ 13 C org of the surface soils was analyzed to examine their environmental implications. Our results show that soil δ 13 C org values change from −26.58‰ to −21.73‰ with an average of −25.20‰ (±1.14‰). Binary regression analysis and linear regression analysis show that the change in the soil δ 13 C org values is significantly correlated with altitude; temperature and precipitation are responsible for the altitudinal impact on the soil δ 13 C org while air CO 2 concentrations have little effect. The sensitivity of the δ 13 C org values to the mean annual temperature is +0.066‰/°C and to the mean annual precipitation is −0.3‰/100 mm. We find that extremely low temperatures (above 2960 m a.s.l. in the study area) may result in physiological drought in plants even when precipitation is favorable, which consequently leads to higher soil δ 13 C org values. This observation is inconsistent with its direct positive role in the δ 13 C org values below 2960 m a.s.l., and the threshold is −1.2 °C in the Shiyang River drainage basin. In addition, the slope of the regression line for the soil δ 13 C org values versus altitude with favorable precipitation in our study is +1.2‰/km, which supports that a relationship of +1.1–1.3‰/km for the soil δ 13 C org values versus altitude is available on the Tibetan Plateau and the surrounding areas and further indicates that this coefficient in this region maybe potentially be useful for paleoelevation reconstruction. [ABSTRACT FROM AUTHOR]

It is well-known that drought has considerable effects on plant traits from leaf to ecosystem scales; however, little is known about the relative contributions of various traits within or between tree species in determining the plant's sensitivity or the tolerance to drought under field conditions. We conducted a field throughfall exclusion experiment to simulate short-term drought (~67% throughfall exclusion during the dry season from October to March) and prolonged drought (~67% throughfall exclusion prolonging the dry season from October to May) and to understand the effects of drought on two dominant tree species (Michelia macclurei and Schima superba) in subtropical forests of southern China. The morphological, physiological, and nutritional responses of the two species to the two types of drought were determined. There were significantly different morphological (leaf max length, max width, leaf mass per area), physiological (leaf proline) and nutritional (P, S, N, K, Ca, Mg) responses by M. macclurei and S. superba to prolonged drought. Comparison between the drought treatments for each species indicated that the trees responded species--specifically to the short-term and prolonged drought, with S. superba exhibiting larger plasticity and higher adaption than M. macclurei. M. macclurei responded more sensitively to prolonged drought in terms of morphology, proline content, and nutritional traits and to short-term drought with regard to soluble sugars content. The differential species-specific responses to drought will allow us to estimate the changes in dominant trees in subtropical forests of China that have experienced a decade's worth of annual seasonal drought. [ABSTRACT FROM AUTHOR]

Drought is a typical disaster in the main soybean production area of northeast China. The spatiotemporal variations of drought related to soybean production based on a crop water deficit index (CWDI) and sensitivity to meteorological variables were investigated in northeast China using daily meteorological data from 87 weather stations from 1981 to 2010. Statistical analysis revealed that precipitation could not meet the water demands of soybeans during the seedling-branching, filling, and maturing stages, and excessive drought occurred more often in northeast China. The Mann-Kendall test indicated that the soybean CWDI significantly increased during the filling stage. Kriging spatial analysis showed that the most drought-prone area was located in the west of northeast China. Explanations for the spatiotemporal variations of the drought for soybean production were explored in terms of meteorological variables. Statistical analysis showed that the crop evapotranspiration, air temperature, wind speed, and number of sunshine hours were significantly higher and the precipitation and relative humidity were significantly lower in the drought-prone area than in the dry area less prone to droughts. An explored method of sensitive analysis quantitatively revealed that precipitation and humidity negatively affected the CWDI, whereas temperature, wind speed, and number of sunshine hours positively affected the CWDI. The CWDI was most sensitive to precipitation. These results not only provide valuable information for soybean planning and management but also produce important background and physical evidence for the influence of climate on the drought related to soybean production in northeast China. [ABSTRACT FROM AUTHOR]

Over the past several decades, rocky desertification has led to severe ecological problems in karst areas in South China. After a rocky desertification treatment project was completed, the vegetation coverage changed greatly and, consequently, increased the ecology water consumption (approximately equal to the actual evapotranspiration) of the regional vegetation. Thus, it intensified the regional water stresses. This study explored the changes in the actual evapotranspiration (ETa) response to the vegetation coverage changes in the rocky desertification areas in South China based on the precipitation (P), potential evapotranspiration (ETp) and NDVI (the normalized difference vegetation index) datasets. The revised Bagrov model was used to simulate the actual evapotranspiration changes with the supposed increasing NDVI. The results indicated that the average NDVI value was lower when the rocky desertification was more severe. The ETa, evapotranspiration efficiency (ETa/ETp) and potential humidity (P/ETp) generally increased with the increasing NDVI. The sensitivity of the ETa response to vegetation coverage changes varied due to different precipitation conditions and different rocky desertification severities. The ETa was more sensitive under drought conditions. When a drought occurred, the ETa exhibited an average increase of 40~60 mm with the NDVI increasing of 0.1 in the rocky desertification areas. Among the 5 different severity categories of rocky desertification, the ETa values' responses to NDVI changes were less sensitive in the severe rocky desertification areas but more sensitive in the extremely and potential rocky desertification areas. For example, with the NDVI increasing of 0.025, 0.05, 0.075, and 0.1, the corresponding ETa changes increased by an average of 2.64 mm, 10.62 mm, 19.19 mm, and 27.58 mm, respectively, in severe rocky desertification areas but by 4.94 mm, 14.99 mm, 26.80, and 37.13 mm, respectively, in extremely severe rocky desertification areas. Understanding the vegetation ecological water consumption response to the vegetation coverage changes is essential for the vegetation restoration and water stresses mitigation in rocky desertification areas. [ABSTRACT FROM AUTHOR]

Background and Aims: In arid environments, a high nitrogen content per leaf area (Narea) induced by drought can enhance water use efficiency (WUE) of photosynthesis, but may also lead to high leaf construction cost (CC). Our aim was to investigate how maximizing Narea could balance WUE and CC in an arid-adapted, widespread species along a rainfall gradient, and how such a process may be related to the drought threshold of the desert-steppe ecotone in northern China. Methods: Along rainfall gradients with a moisture index (MI) of 0·17-0·41 in northern China and the northern Tibetan Plateau, we measured leaf traits and stand variables including specific leaf area (SLA), nitrogen content relative to leaf mass and area (Nmass, Narea) and construction cost (CCmass, CCarea), δ13C (indicator of WUE), leaf area index (LAI) and foliage N-pool across populations of Artemisia ordosica. Key Results: In samples from northern China, a continuous increase of Narea with decreasing MI was achieved by a higher Nmass and constant SLA (reduced LAI and constant N-pool) in high-rainfall areas (MI > 0·29), but by a lower SLA and Nmass (reduced LAI and N-pool) in low-rainfall areas (MI ≤ 0·29). While δ13C, CCmass and CCarea continuously increased with decreasing MI, the low-rainfall group had higher Narea and δ13C at a given CCarea, compared with the high-rainfall group. Similar patterns were also found in additional data for the same species in the northern Tibetan Plateau. The observed drought threshold where MI = 0·29 corresponded well to the zonal boundary between typical and desert steppes in northern China. Conclusions: Our data indicated that below a climatic drought threshold, drought-resistant plants tend to maximize their intrinsic WUE through increased Narea at a given CCarea, which suggests a linkage between leaf functional traits and arid vegetation zonation. [ABSTRACT FROM AUTHOR]

The East Asian monsoon has a tremendous impact on agricultural production in China. An assessment of the risk of drought disaster in maize-producing regions is therefore important in ensuring a reduction in such disasters and an increase in food security. A risk assessment model, EPIC (Environmental Policy Integrated Climate) model, for maize drought disasters based on the Erosion Productivity Impact Calculator crop model is proposed for areas with the topographic characteristics of the mountainous karst region in southwest China. This region has one of the highest levels of environmental degradation in China. The results showed that the hazard risk level for the maize zone of southwest China is generally high. Most hazard index values were between 0.4 and 0.5, accounting for 47.32% of total study area. However, the risk level for drought loss was low. Most of the loss rate was <0.1, accounting for 96.24% of the total study area. The three high-risk areas were mainly distributed in the parallel ridge-valley areas in the east of Sichuan Province, the West Mountain area of Guizhou Province, and the south of Yunnan Province. These results provide a scientific basis and support for the reduction of agricultural drought disasters and an increase in food security in the southwest China maize zone. [ABSTRACT FROM AUTHOR]

Background: Cynanchum komarovii Al Iljinski is a xerophytic plant species widely distributing in the severely adverse environment of the deserts in northwest China. At present, the detailed transcriptomic and genomic data for C. komarovii are still insufficient in public databases. Results: To investigate changes of drought-responsive genes and explore the mechanisms of drought tolerance in C. komarovii, approximately 27.5 GB sequencing data were obtained using Illumina sequencing technology. After de novo assembly 148,715 unigenes were generated with an average length of 604 bp. Among these unigenes, 85,106 were annotated with gene descriptions, conserved domains, gene ontology terms, and metabolic pathways. The results showed that a great number of unigenes were significantly affected by drought stress. We identified 3134 unigenes as reliable differentially expressed genes (DEGs). During drought stress, the regulatory genes were involved in signaling transduction pathways and in controlling the expression of functional genes. Moreover, C. komarovii activated many functional genes that directly protected against stress and improved tolerance to adapt drought condition. Importantly, the DEGs were involved in biosynthesis, export, and regulation of plant cuticle, suggesting that plant cuticle may play a vital role in response to drought stress and the accumulation of cuticle may allow C. komarovii to improve the tolerance to drought stress. Conclusion: This is the first large-scale reference sequence data of C. komarovii, which enlarge the genomic resources of this species. Our comprehensive transcriptome analysis will provide a valuable resource for further investigation into the molecular adaptation of desert plants under drought condition and facilitate the exploration of drought-tolerant candidate genes. [ABSTRACT FROM AUTHOR]

Abstract: Based on pollen analysis and other evidence in a sediment profile from Huangqihai Lake, located at the limit of Asian monsoon influence, the local vegetation development and climatic change were reconstructed for the past 8600years. From 8600 to 7200cal. yr BP, the local vegetation was dominated by steppe and tree cover was low. Then pine increased gradually, and the local vegetation was pine and oak mixed forest with variable ratios of the two dominant tree taxa from 7200 to 3600cal. yr BP. From 3600 to 3000cal. yr BP, the forest was replaced by shrubland, which was attributed to a period of intense drought. From 3000 to 600cal. yr BP, the tree cover declined to its lowest levels, probably due to long-term further drought. After 600cal. yr BP, steppe has become dominant again. We found that the changes of vegetation type indicated by pollen percentages did not match existing evidence for monsoon dynamics. Changes in tree pollen concentration, however, well captured the changes in the Asian monsoon, implying different sensitivity of vegetation cover and vegetation type (e.g. replacement of steppe by forest) to the monsoon development. In addition, a shrubland-dominated stage which has not been evident in previous studies was also clear in our pollen sequences. Our study implied that the responses of marginal forest to drought were variable, depending on the drought regime. The different responses of forest to climate drying in this study provide insights into the reliable prediction of forest dynamics in the semi-arid regions. [Copyright &y& Elsevier]

Tree mortality in response to climate change induced drought has emerged as a global concern. Small changes of tree mortality rates can profoundly affect forest structure, composition, dynamics and ecosystem services such as carbon sequestration. Our analyses of longitudinal data from natural stands (82 plots) in Beijing showed that tree mortality rates have increased significantly over the two decades from 1986 to 2006. In contrast, recruitment rates decreased significantly over this period. The increase in overall mortality rates resulted from an increase in tree deaths dominantly attributed to changes in temperature and precipitation resulting in drier conditions across latitudes, elevations, tree species, and tree sizes. In addition, the results showed that mortality rates of Chinese pine ( Pinus tabuliformis) ( β = 0.0874) as a result of climate change induce drought were much smaller than oak ( Quercus) ( β = 0.1583). [ABSTRACT FROM AUTHOR]

Heavy droughts have frequently struck Southwestern China in recent years. A major consequence of these droughts is the loss of terrestrial water storage (TWS) which affects agriculture, civil life, and industry production and results in serious social and economic losses. The satellite gravimetry has been proved an effective way to estimate hydrological variations over the globe, and we use the monthly gravity solutions from the Gravity Recovery and Climate Experiment (GRACE) satellite mission and extract the hydrological variations from the regional gravity signal. Here we show that the recent heavy droughts in Southwestern China have caused observable nonseasonal gravity changes in the monthly GRACE solutions. Two heavy droughts in September 2009 to May 2010 and August 2011 to January 2012 have resulted in significant TWS deficit up to hundreds of gigatons in Southwestern China. It is found that the latter drought in 2011-2012 is the most severe one in Southwestern China over the decade, hitting large areas, and causing heavy TWS deficit. Affected by the recent droughts, the GRACE estimate shows a gradual decrease in the regional TWS during the recent 4 years in Southwestern China with a nominal rate of −13.3 ± 7.5 mm/yr, which indicates the severity of the TWS deficit in this area. [ABSTRACT FROM AUTHOR]

Abstract: Ecological responses to climate change are strongly regulated by long-term processes, such as changes in species composition and carbon dynamics in soil; therefore, understanding and predicting these processes require long time scale studies. Based on high-resolution lake sediment records of the Angili Nuur Lake from the forest-steppe ecotone at the southeastern margin of the Asian Gobi, we found three pronounced drought events during the mid-Holocene (∼7.0–6.3ka BP, BP means “before present”, where present means 1950 and ka=1000 years) and a subsequent graduate drying trend. During the extreme drought events, lake aquatic system showed strong desiccation and instant salty marsh expansion; whereas regional forest showed strong resilience. During the following long-term drying at millennial scale, broadleaf forest, coniferous forest and steppe successively dominated in this region, in 6.3–4.4ka BP, 4.0–2.7ka BP and since 2.7ka BP, respectively; but the changing dominance between forest and steppe did not match the climate change rate as reconstructed in previous studies, suggesting a lagged and non-proportional change of vegetation type to the range of climate change. The resilience of forest implies a strong buffering effect of biotic and abiotic factors other than climate, probably explained by diverse micro-environments of this mountainous region that allowed forest to survive during the drought spells, and also by gradual soil coarsening that retarded soil field capacity change and nutrient loss. However, under accumulative drying trend, forest was in general replaced by steppe around 2.0ka BP. [Copyright &y& Elsevier]

Poplar plantations are widely used for timber production and ecological restoration in northern China, a region that experiences frequent droughts and water scarcity. An open-path eddy-covariance (EC) system was used to continuously measure the carbon, water, and energy fluxes in a poplar plantation during the growing season (i.e., April–October) over the period 2006–2008 in the Daxing District of Beijing, China. We examined the seasonal and inter-annual variability of gross ecosystem productivity (GEP), net ecosystem exchange (NEE), and ecosystem respiration (ER). Although annual total precipitation was the lowest in 2006, natural rainfall was amended by flood irrigation. In contrast, no supplementary water was provided during a severe drought in spring (i.e., April–June), 2007, resulting in a significant reduction in net ecosystem production (NEP=−NEE). This resulted from the combined effects of larger decrease in GEP than that in ER. Despite the drought – induced reduction in NEP, the plantation forest was a strong carbon sink accumulating 591±62, 641±71, and 929±75gCm−2 year−1 for 2006, 2007, and 2008, respectively. The timing of the drought significantly affected the annual GEP. Severe drought during canopy development induced a lasting reduction in carbon exchange throughout the growing season, while the severe drought at the end of growing season did not significantly reduce carbon uptake. Additionally, irrigation reduced negative drought impacts on carbon sequestration. Overall, this fast growing poplar plantation is a strong carbon sink and is sensitive to the changes in environmental conditions. [ABSTRACT FROM AUTHOR]

Both flooding and drought are important in determining plant distribution in wetlands. However, the roles of plant's physiological response to flooding and drought in accounting for plant distribution are far from clear. To this end, three typical wetland plants with different distribution patterns (high-elevation species Miscanthus sacchariflorus, low-elevation species Carex brevicuspis and Polygonum hydropiper) in Dongting Lake were treated with three water levels (flooding 25 cm, control 0 cm, drought −25 cm), and relative growth rate (RGR), malondialdehyde (MDA) content, electrolyte leakage and proline content were investigated. The RGR of the three species decreased significantly in both flooding and drought treatments. Compared to the control, the RGR of M. sacchariflorus decreased more in the flooding treatment but less in the drought treatment compared to the other two species. The contents of MDA in the three species increased in both flooding and drought treatments, except for P. hydropiper in the flooding treatment. MDA contents increased more in M. sacchariflorus in the flooding treatment but less in the drought treatment compared to the other two species. Only M. sacchariflorus had a higher electrolyte leakage in the flooding treatment, and drought led to a higher electrolyte leakage in P. hydropiper and C. brevicuspis. Proline content increased 69.2, 66.7 and 39.6 % in P. hydropiper, C. brevicuspis and M. sacchariflorus in the flooding treatment, and increased 44.2, 13.0 and 45.3 % in the drought treatment, respectively. These results suggest that M. sacchariflorus has a higher tolerance to drought but a lower tolerance to flooding than do the other two species, which might be the intrinsic mechanisms accounting for their different distribution patterns. [ABSTRACT FROM AUTHOR]

The Hexi Corridor is the most important area for desert oasis farming in northwestern China. Due to persistent drought and water shortage, sewage irrigation is widely used in this area. Heavy metal pollutants contained in the sewage could remain in the surface layer of agricultural soil and accumulate in plants. Our research used pot experiments to evaluate carrot crop (Daucus carota L.) production, heavy metal uptake, and bioavailability under single cadmium (Cd) or nickel (Ni) contamination and compound (Cd-Ni) contaminations in irrigated desert oasis soil. The results show that Cd existed in the Fe-Mn oxide bound fraction and Ni presented in the residual fraction mainly in original (control) soils. Low concentrations of Cd could promote the growth of carrots, while high concentrations of Cd significantly restrain the growth of the crops. However, Ni had a poisonous effect on the carrots even at the lowest concentrations. There was an antagonistic effect between Cd and Ni in the compound contaminated oasis soils. The bio-concentration factors (BCF) of Cd in carrots were higher than those of Ni, and the BCF of Cd and Ni in single-contaminated soils were higher than those in compound-contaminated soils. Cd and Ni contents in different parts of the carrots were correlated with the exchangeable fraction in contaminated oasis soils, which would cause potential risk to human health through the food chain. [ABSTRACT FROM AUTHOR]

Abstract: Populus euphratica is native to semi-arid regions of the Xinjiang Uyghur Autonomous Region of China, and studying its drought responses will greatly increase the understanding of how trees acclimate to drought. Water was withheld for seven weeks in four different drought stress treatments, with regime 1 being the least drought stressed and regime 4 being the most, and the poplar''s transcriptional profiles examined with Affymetrix Poplar GeneChip microarrays. The number of significantly up or down transcriptional changes increased with the severity of drought stress, with regime 1, 2, 3 and 4 showing 952, 1354, 2138 and 2360 altered transcripts, respectively. Only 277 of these were found in common across all four regimes, while 1938 transcripts were found to be unique to the individual treatments. Genes with altered transcript abundance included members of the transcription factor families AP2/EREPB, bZIP, NAC, NF-Y, WRKY, MYB and Homeobox, as well as genes for the small HSP, HSP70 and HSP90 heat shock protein families. Analysis of the transcript data from these experiments indicated that P. euphratica activates specific regulatory pathways according to the degree of drought stress it receives. These results provide important insights into the molecular mechanisms underpinning the drought stress responses of poplar, as well as providing candidates for future experimentation. [Copyright &y& Elsevier]

Drought significantly constrains higher yield of alfalfa ( Medicago sativa L.) in arid and semiarid areas all over the world. This study evaluated the responses of leaf cuticular wax constituents to drought treatment and their relations to gas-exchange indexes across six alfalfa cultivars widely grown in China. Water deficit was imposed by withholding water for 12 d during branching stage. Cuticular waxes on alfalfa leaves were dominated by primary alcohols (41.7-54.2%), alkanes (13.2-26.9%) and terpenes (17.5-28.9%), with small amount of aldehydes (1.4-3.4%) and unknown constituents (4.5-18.4%). Compared to total wax contents, the wax constituents were more sensitive to drought treatment. Drought decreased the contents of primary alcohol and increased alkanes in all cultivars. Alkane homologs, C25, C27, and C29, were all negatively correlated with photosynthetic rate, transpiration rate, stomatal conductance, and leaf water potential. Under drought conditions, both stomatal and nonstomatal factors were involved in controlling water loss from alfalfa leaves. No direct relationship was observed between wax contents and drought resistance among alfalfa cultivars. An increase in alkane content might be more important in improving drought tolerance of alfalfa under water deficit, which might be used as an index for selecting and breeding drought resistant cultivars of alfalfa. [ABSTRACT FROM AUTHOR]

Several provinces of China experienced an intense drought episode during 2009 and 2010. The drought was particularly severe in southwestern and northern China, where the accumulated precipitation from May 2009 to April 2010 was about 25% less than normal. The decline of accumulated precipitation over northern China was mostly noticeable during the summer season of 2009 and it was comparable to recent dry episodes. The southwestern China drought resulted from a sequence of dry months from summer 2009 to winter 2010, corresponding to the driest event since at least 1951. The suppression of rainfall in summer over both regions was in agreement with a weakened broad-scale South Asian summer monsoon, possibly influenced by an El Niño developing phase, whereas the extremely negative phases of the Arctic Oscillation during the winter of 2010 may have contributed to the persistence of the drought in southwestern China. The assessment of the associated impacts indicates that water reservoirs were severely affected with a ~20% reduction in the nationwide hydroelectrical production during the drought event. Furthermore, an analysis of the normalized difference vegetation index data reveals that large cropland sectors of northern and eastern China experienced up to 8 months of persistently stressed vegetation between May 2009 and July 2010, while southwestern China was relatively less affected. Such different regional vegetative responses are interpreted in terms of the land-cover type, agriculture management, and their dependence on seasonal precipitation and water availability for irrigation. [ABSTRACT FROM AUTHOR]

Abstract: Jatropha curcas L. is chosen as an ideal biodiesel crop in China because its seed kernel has high oil content (43–61%) and it does not compete with food. Its oil is non-edible, and the trees can resist drought and grow on barren and marginal lands without using arable land. This article reviews the history of Jatropha, current development status and problems in its seeds, propagation, plantation management, oil extraction, biodiesel processing and other value-added products production techniques in China. The commercial production of seed, oil and biodiesel as well as research advancement in China is also introduced and discussed. Examples about our new bred mutant and selected high-oil-yield Jatropha varieties, high-qualified produced biodiesel, and biodiesel pilot plant are presented. Finally, future prospects of Jatropha biodiesel industry in China are discussed. [Copyright &y& Elsevier]

Abstract: A simulated experiment with 5 soil water gradients was conducted to study the responses of peroxidase (POD) and the catalase (CAT) activities in Phragmites australis from Yellow River Delta on different water stresses. The results showed that: 1) the value of CAT was much higher than that of POD under all conditions; 2) a similar fluctuation change was present in both POD and CAT among the inundation levels generally, and the values of POD CAT were much higher under drought condition than those under other inundation levels, and decreased gradually with the increasing inundation levels, except CAT value has a little rise at the inundation level of 15cm, which indicated that POD and CAT were positive to drought stress and played major role in resistance to drought in oxidative defensive systems of Phragmites australis. [Copyright &y& Elsevier]

In order to better understand the relationship between vegetation vigour and moisture availability, a correlation analysis based on different vegetation types was conducted between time series of monthly Normalized Difference Vegetation Index (NDVI) and Palmer Drought Severity Index (PDSI) during the growing season from April to October within the Laohahe catchment. It was found that NDVI had good correlation with PDSI, especially for shrub and grass. The correlation between NDVI and PDSI varies significantly from one month to another. The highest value of correlation coefficients appears in June when the vegetation is growing; lower correlations are noted at the end of growing season for all vegetation types. The influence of meteorological drought on vegetation vigour is stronger in the first half of the growing season, before the vegetation reaches the peak greenness. In order to take the seasonal effect into consideration, a regression model with seasonal dummy variables was used to simulate the relationship between NDVl and PDSI. The results showed that the NDVI-PDSI relationship is significant (α = 0.05) within the growing season, and that NDVI is an effective indicator to monitor and detect droughts if seasonal timing is taken into account. [ABSTRACT FROM AUTHOR]

Abstract: Moisture is a primary limiting factor that affects plant growth. To determine the ideal amount of irrigation water for the growth of individual Populus bolleana Lauche, the major protective species around Qira oasis in southern edge of Taklimakan Desert in Xinjiang, China, we studied the moisture of soil layers with depth between 0-80cm and the response of P. bolleana''s water status such as moisture content, stomatal characteristics, leaf water potential, stem sapflow and stem diameter to different irrigation based on the field monitoring data. The results showed that the moisture content of soil layers with depth of 0-80cm gradually increased with irrigation increasing, especially when irrigation was more than 30 liters. When irrigation was 30 and 40 liters, the daily leaf water potential presented an obvious unimodal trend: the predawn and midday leaf water potential increased significantly. Along with the irrigation increased from 10 to 40 liters, sapflow and stomatal conductance followed a trend of first decreases and then increases. When irrigation was 30 liters, the daily mean stem diameter and contraction of P. bolleana reached their peaks, indicating that irrigating more than 30 liters of water could improve the water status of P. bolleana and prevent it from drought stress. Thus, we concluded that the ideal irrigation amount for the normal growth of individual P. bolleana should be 30 liters per year. [Copyright &y& Elsevier]

Abstract: Cores of thick leaf spruce from the western Qilian Mountains were subjected to densitometric analysis to obtain data for seven tree-ring parameters (tree-ring width, earlywood width, latewood width, maximum density, minimum density, mean earlywood density and mean latewood density). The chronologies were analyzed individually and then compared with each other. Most of these variables show moderately high values of common variance and mean interseries correlation except latewood and maximum densities. The relationships between the different tree-ring parameters and climate data are also presented. The growth/climate response analyses reveal that the radial growth of thick leaf spruce is the mainly controlled by total annual precipitation (July–June). Herein the residual ring width chronology was used to reconstruct annual (July–June) precipitation of Jiuquan for the period 1768–2009 A.D., and it accounted for 45.0% of the precipitation variance. Multi-taper spectral analysis reveals the existence of significant 11.1-year, 4.9-year, and 2.0- to 3.4-year periods of variability. Spatial analysis shows that the precipitation of Jiuquan has strong common signals for the northern margin of the Tibetan Plateau and Hexi Corridor. Drought events in our reconstruction are compared to the historical archives and other moisture-sensitive tree ring width series in the Hexi Corridor. The results reveal common climatic extremes over much of the Hexi Corridor. Many of these events have had profound impacts on the peoples of the Hexi Corridor over the past several centuries. [Copyright &y& Elsevier]

Abstract: Knowledge of the spatial pattern and temporal relationships between tree-growth and climatic factors are important not only for the projection of forest growth under varying climate but for dendroclimatology in general. Here, we systematically investigated tree-growth climate relationships of Picea crassifolia at upper treeline in the Qilian Mts., northwestern China. 297 trees from eleven sites, covering a large part of the natural range of this species, show increasing and partly divergent correlations with temperature in the most recent decades. The dominant signal at all sites was a strengthening of negative correlations of annual radial growth with summer temperature. In a subset of trees at six sites, a strengthening positive correlation with summer temperatures existed as well. Wetter and high altitude sites tended to show a higher percentage of trees that are positively correlated with warming temperatures, indicating that some individuals there may take advantage of warmer conditions. Divergent responses between the two sub-populations clustered by their response to climate factor were significantly stronger in the last 30 years compared to earlier time slices. In the same time frame, hydrothermal conditions of the investigation area changed to a drier and warmer combination. Drought conditions, most likely affecting the radial growth of most P. crassifolia, have been intensifying over time and expanding spatially from the middle Qilian Mts. to most of our study area during the last half century. While explanations such as methodological effects due to trend removal or human disturbance at the sampling sites might be able to explain the result at single sites, the spatial and temporal co-occurrence of large scale changes in climate and tree growth suggests a causal link between them. [Copyright &y& Elsevier]

Abstract: Droughts have become widespread in the Northern Hemisphere, including in China, where they have affected farmland resources on the Loess Plateau. Given this background, we proposed a new index, the Normalized Day–Night Surface Temperature Index (NTDI), to estimate moisture availability (m a), defined as the ratio of actual to reference evapotranspiration. The NTDI is defined as the ratio of the difference between the maximum daytime surface temperature and the minimum nighttime surface temperature, to the difference between the maximum and minimum surface temperatures estimated from meteorological data by applying energy balance equations. To calculate the index, we used data of 20 clear-sky meteorological observations made during the 2005 growing season at a natural grassland station in the Liudaogou River basin on the Loess Plateau. The NTDI showed a significant inverse exponential correlation with m a (R2  = 0.97, p < 0.001), whereas the numerator of the index (the maximum daytime surface temperature minus the minimum nighttime surface temperature) was only weakly correlated with m a (R2  = 0.24, p = 0.03). This result indicates that normalization relative to the index denominator (maximum surface temperature − minimum surface temperature) dramatically improved the accuracy of the estimate. [ABSTRACT FROM AUTHOR]

The growing season in 2006 existed a quite less precipitation than previous years in the eastern of Tibetan Plateau, China. In order to explore the response strategies of the grass to drought year, the variations of growth and potential reproduction of Poa crymophila between rich (2005) and poor rainfall year (2006) were studied. Reproductive shoot biomass, above- and below-ground biomass pronounced decreased in the poor rainfall year, but the ratio of above-/below-ground biomass and vegetative shoot biomass increased. Carbon content in inflorescence increased significantly in 2006, but decreased significantly in root. Nitrogen contents and the C/N ratios in plant components were not significantly different. Sexual reproduction in 2005 and 2006 was not different. Season precipitation shortage could result in the higher mortality of vernal budding shoots or tillers. The results suggested that P. crymophila response to poor rainfall year (drought) by a smaller plant body, allocating more proportional biomass to above-ground, altering more carbon to inflorescence, and altering its reproductive way to sexual. [ABSTRACT FROM AUTHOR]

Climate extremes, particularly the droughts sustaining over a prolonged period and affecting extended area (defined as "exceptional drought events"), can have longlasting effects on economic and social activities. Here we use the Chinese drought/flood proxy data of the past five hundred years to identify the cases of exceptional drought events over eastern China (east of 105°E), and to study their spatial patterns and temporal evolutions. The associated circulations for the contemporary case are analyzed using available meteorological data. Possible linkage of these cases to climatic forcing and natural climate events is also explored. After considering the intensity, duration, and spatial coverage, we identified three exceptional drought events, which occurred in 1586-1589, 1638-1641, and 1965-1966 in chronological order. They were the most severe droughts of last five centuries in eastern China, with more than 40% of affected area and the drought center encountered a significant summer rainfall reduction (about 50% or more). These three droughts all developed first in North China (34-40°N), and then either expanded southward or moved to the Yangtze River Valley (27-34°N) and the northern part of the southeastern coastal area (22-27°N). For the 1965-1966 case, the significant reduction of summer precipitation was caused by a weakening of summer monsoon and an anomalous westward and northward displacement of the western Pacific subtropical high. Our analyses also suggest that these three exceptional drought events might be triggered by large volcanic eruptions and amplified by both volcanic eruptions and El Niño events. [ABSTRACT FROM AUTHOR]

The article presents a reconstruction of drought in north-central China utilizing tree rings. A drought reconstruction for north-central China is undertaken using tree ring-width chronology developed from two sites of the Chinese pine (Pinus tabulaeformis) in the northern Helan Mountains. The reconstruction was developed by calibrating tree-ring data with the Palmer drought severity index (PDSI), an index of regional moisture conditions. The reconstruction reveals that regional drought variability was stable during the nineteenth century, but became more persistent and variable during the twentieth century. It also discloses a drying trend occurring in north-central China since mid-1930s.

[Objective] The research aimed to analyze precipitation change and agricultural drought and flood degrees during crop growth season in Binzhou. [Method] Based on monthly rainfall and average temperature data at Binzhou meteorological observatory during March - November of 1981 -2010, by using linear regression, climatic tendency rate and dry-wet coefficient, precipitation change and agricultural drought and flood degrees during crop growth season of the past 30 years in Binzhou were analyzed from natural precipitation tendency change and satisfaction degree of agricultural water demand during crop growth season. [Result] In the past 30 years, precipitation during growth season in Binzhou presented increasing tendency. Spring, summer and autumn precipitation all increased somewhat, especially summer precipitation. Monthly average rainfall distribution was very uneven, and rainfall in July and August was more. In the past 30 years, average dry-wet coefficient K value during crop growth season in Binzhou was 0.60, it overall belonged to moderate drought climate type, and occurrence frequency of drought was 97%. It belonged to serious drought climate type in spring and autumn and light drought climate type in summer. Dry-wet coefficient presented rising tendency, illustrating that climate was developing toward wet direction. Seen from mean over the years, except humid in July, it was over light drought in other months. [Conclusion] Climate was overall arid during crop growth season in Binzhou, but precipitation somewhat increased in the past 30 years. Therefore, we suggested that artificial rainfall work should be enhanced. [ABSTRACT FROM AUTHOR]

Estimating crop evapotranspiration under drought stress provides the theoretical foundation for optimizing irrigation schedule and reducing agricultural drought risk. Winter wheat water deficit plot experiments were conducted based on large-scale weighing lysimeters in the Huaibei Plain. The response of evapotranspiration to water deficit during several growth stages was analyzed; four crop coefficients were optimized and then evapotranspiration under drought stress was estimated with the dual crop coefficient method and the genetic algorithm. Drought stress not only reduced evapotranspiration during the current stage, but this influence was also transferred to the following periods. Evapotranspiration could basically return to the normal status after rewatering if the plant did not previously suffer from serious drought stress. Therefore, severe water deficit during continuous wheat growth periods should be avoided, and the deficit intensity during each stage should be controlled. The optimized Kcbini, Kcbmid, Kcbend and Kcmax were 0.32, 1.38, 0.33 and 1.39, and the root mean square error, mean absolute error, and mean relative error of the corresponding estimated evapotranspiration under drought stress were 22.83%, 25.36%, and 11.78% less than those using recommended coefficients, respectively. These optimized crop coefficients provide references for the practical application in the Huaibei Plain, this method for estimating crop evapotranspiration under drought stress can be applied to field circumstances and other regions. [ABSTRACT FROM AUTHOR]