Drought dynamics and impacts on vegetation in China from 1982 to 2011

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Ecological Engineering 75 (2015) 303307Short communicationDrought dynamics and impacts on vegetation in China from 1982 to2011Honglin Wang a, Aifang Chen a, Qianfeng Wang b, Bin He a,*aAcademy of College of Global Change and Earth System, Beijing Normal University, Beijing 100875, ChinabAcademy of Disaster Reduction and Emergency Management, Ministry of Civil Affairs and Ministry of Education, Beijing Normal University, Beijing 100875,ChinaA R T I C L E I N F OArticle history:Received 3 July 2014Received in revised form 20 November 2014Accepted 29 November 2014Available online xxxKeywords:Climate changeDroughtSPIVegetationNDVIGrowing seasonChinaA B S T R A C TWe investigated the drought dynamics and their impacts on vegetation change in China from 1982 to2011 using the standard precipitation index (SPI) and the normalized difference vegetation index (NDVI)anomaly, which were calculated from meteorological and satellite-derived NDVI data, respectively. Thetrends in the change of SPI and vegetation were explored based on the non-parametric MannKendall(MK) test and Sens slope test, and the relationship between these trends was examined. The results wereas follows: (1) For China as a whole, although the long term trend of drought-impacted areas changedlittle (0.045% /10 a) over the past 30 years, Dry trends were identified in northeastern and southwesternChina. (2) The annual vegetation growth at the national scale showed an increasing trend, with a rate of0.008%/10 a from 1982 to 2011; cropland vegetation presented the largest increase in NDVI (p < 0.05). (3)Droughts that occurred during the growing season and pre-growing season both had large negativeimpacts on vegetation growth, and significant influences were found in northern China, especially in thenorthwestern area. Compared to the northern areas, the NDVI in southern China appeared to benefit fromwarming temperatures. 2014 Elsevier B.V. All rights reserved.Contents lists available at ScienceDirectEcological Engineeringjournal homepage: www.else vie r .com/ locat e/e coleng1. IntroductionMany parts of the world have suffered from serious droughts inrecent years, which have greatly impacted socioeconomicsystems and the environment (IPCC, 2001). Major drought eventshave raised the question of whether droughts are becoming morefrequent and more severe, as projected by climate change studies(Zhao and Running, 2010). Another concern is whether drought,which is one of the major disturbances to vegetation growth, mayweaken the ability of vegetation to function as a carbon sink (Xuet al., 2011). For the improved understanding of the vulnerability ofthe terrestrial carbon cycle and the sustainable use of naturalvegetation resources (Xu et al., 2012), it is of particular importanceto understand and assess the consequences of drought onvegetation at different scales.The impacts of drought on vegetation have been extensivelystudied on regional and global scales. For instance, a recent studyindicated that droughts are counteracting the increase in global netprimary productivity (NPP) caused by global warming (Zhao and* Corresponding author. Tel.: +86 15 810 327 627.E-mail address: hebin@bnu.edu.cn (B. He).http://dx.doi.org/10.1016/j.ecoleng.2014.11.0630925-8574/ 2014 Elsevier B.V. All rights reserved.Running, 2010). On a regional scale, the severe 2003 drought inEurope caused great impacts on a variety of land-cover types, andmost ecosystems recovered to a normal state by early 2004(Gobron et al., 2005). Rainforests in the Amazon region play animportant role in the global carbon cycle, and extreme droughts inthis region in 2010 seemed to cause a significant increase in treemortality and carbon losses (Xu et al., 2011). Thus, serious droughtsare causing adverse effects on terrestrial ecosystems.Vegetation growth has significantly increased over the lastthree decades in China (He et al., 2007). This can be mainlyattributed to national warming and ecological conservationprojects, such as the Three-North Shelter Forest Program (TNSFP)(Duan et al., 2011), the BeijingTianjin Sand Source ControlProgram (Wu et al., 2013), the Grain for Green Project (Zhang et al.,2012a), and small-scale regional ecological engineering (Huanget al., 2012). During the 20th century, China experienced a series ofdrought events (Xiao et al., 2009), and the increasing droughtstress associated with warming and reduced rainfall was found tocontribute to the decrease in the growing seasons normalizeddifference vegetation index (NDVI) in northern China after the1990s (Peng et al., 2011). In 2010, southwest China suffered asevere and sustained spring drought, which reduced the regionalannual gross primary productivity (GPP) and NPP by 65 and 45 Tghttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.ecoleng.2014.11.063&domain=pdfmailto:hebin@bnu.edu.cnhttp://dx.doi.org/10.1016/j.ecoleng.2014.11.063http://dx.doi.org/10.1016/j.ecoleng.2014.11.063http://www.sciencedirect.com/science/journal/09258574www.elsevier.com/locate/ecoleng304 H. Wang et al. / Ecological Engineering 75 (2015) 303307C/a, respectively, and both the annual GPP and NPP in 2010 werethe lowest during the period of 20002010 (Zhang et al., 2012b).Obviously, drought has become one of the most importantdisturbances to vegetation growth in China. Furthermore, droughtslower the water levels of rivers, reservoirs, and lakes, which limitswater availability for ecosystem and agricultural production andleads to ecological degradation and reduced food productivity(Piao et al., 2010; Zhang et al., 2012b; Yu et al., 2013).Although previous studies have provided much informationon the relationship between drought and vegetation growth,many efforts mainly focused on the impacts of single droughtevents were concentrated on a regional scale. In addition, theresponses of vegetation to drought during the growing andnon-growing seasons are still not clear. Our main goal is to explorethe drought dynamics in China and their impacts on vegetationfrom 1982 to 2011. Firstly, the spatial-temporal characteristicsof drought are analyzed based on drought area and the trend ofdrought index. Secondly, the relationships between droughtvariations and vegetation growth are used to explore howa drought will impact vegetation growth. Our results willprovide scientific information for drought prediction and forestmanagement in China.2. Datasets and methods2.1. DatasetsThe monthly precipitation (mm) data from 19822011 at752 meteorological stations in China were collected from the ChinaMeteorological Data Sharing Service. To avoid the possible effectsof artificial shifts in the data caused by the relocationsof measurement sites and equipment observation error, themeteorological data were checked for homogeneity with referenceto a previously used method (Ren et al., 2008). Stations with lessthan 30 years of data were also rejected (Yu et al., 2013). A total of603 stations were selected for this study according to the dataavailability criteria in China.The NDVI is usually used for measuring vegetation growth. Inthis study, we used the National Oceanic and AtmosphericAdministration/Advanced Very High Resolution Radiometer(NOAA/AVHRR) NDVI dataset, which was produced by GlobalInventory Modeling and Mapping Studies (GIMMS) (Tucker et al.,2005). The dataset spans the period from 19822011 and has a0.083 spatial resolution and a 15-day interval. The GIMMS NDVIhas been corrected to remove non-vegetation effects, includingsensor degradation, inter-satellite differences, and volcanic aerosoleffects (Zhou et al., 2001). The monthly NDVI was derived from twoimages from each month using the maximum value composite(MVC) method (Holben 1986). Our analysis was mainly confined tothe growing season, which was defined as AprilOctober and wasconstantly applied across the whole country (Zhou et al., 2001).Based on vegetation maps with a scale of 1:1,000,000 from theAtlas of China's Vegetation, the vegetation was classified intofour types: forest, shrub, grassland, and cropland (Zhao et al.,2011).2.2. MethodsThe standardized precipitation index (SPI) (McKee et al., 1993)was selected to assess drought variations in China. The SPI is awidely used drought index, which is derived from precipitationdata alone to determine a water deficit and surplus and can becalculated for short or long time scales (Paulo et al., 2003). Themulti-time scale SPI allowed us to analyze the relationshipbetween drought and vegetation at various time scales. Thecriteria for drought classification was referenced in McKee's study(McKee et al., 1993).The non-parametric MannKendall (MK) test was appliedfor the SPI trend tests. The MK test is a rank-based procedure,which is suitable for detecting non-linear trends (Kendall, 1975).It is frequently used for detecting trends in hydrologicaland meteorological time series (Hamed, 2008).Confidence probabilities of 95% (p < 0.05) was considered to besignificant.The Sen slope estimator was applied to obtain the trend invegetation growth from 1982 to 2011 (Sen, 1968). This estimationdid not require the data to be distributed normally and has beenwidely applied in vegetation growth studies (Fernandes andLeblanc, 2005).The bilinear interpolation method was used to extract the NDVIvalues for the station data based on grid vegetation data with aspatial resolution of 0.083 (Vu et al., 2012). The monthly NDVIsequence data were generated at station in China and spans theperiod from 1982 to 2011. Finally, the Pearson correlation analysiswas used to explore the relationship between the NDVI anomalyand drought (SPI less than 1.0) (Madden and Williams, 1978;Trenberth and Shea, 2005). The data processing and calculationswere conducted with the Interactive Data Language (IDL) programand the ArcGIS9.3 software.3. Results3.1. Temporal and spatial characteristics of droughtThe inter-annual variation in the percentage of drought-impacted areas (SPI of less than 1.0) in China from 19822011 are shown in Fig. 1(a). Relatively large dry areas occurred inthe middle of the 1980s, the late 1990s, the early 2000s, and themost recent 12 years. The largest drought-impacted areapercentage (27.69%) occurred in 1986 and represented approxi-mately one-quarter of China's total territory. This value wasfollowed by 26.07% in 2001, 25.1% in 1997, and 22.51% in 2011.Despite the large inter-annual variability, the long-term trends inthe drought-impacted areas in China as a whole decreased slightly(by 0.045%/10 a) over the past 30 years.The annual trends of SPI for each station in China were alsoinvestigated based on the MK test. The positive and negativeresults indicated trends towards wetter and drier conditions,respectively (Fig. 1(b)). The SPI trends showed great spatialvariability. Dry trends were identified in northeastern andsouthwestern China, but significant wet trends were detected inwestern China.3.2. Trends in vegetation growth over the past 3 decadesInter-annual variations in the NDVI for different vegetationtypes are shown in Fig. 2(a). The NDVI increased remarkably for allvegetation types (p < 0.05). The largest increases appeared incroplands and shrubs, with values of 0.015/10 a and 0.01/10 arespectively, followed by 0.007/10 a for grasslands and 0.006/10 afor forests. At the national scale, the average NVDI for all annualvegetation in China demonstrated a significant increasing trend,with a rate of 0.007/10 a. The annual NDVI trend patterns weredramatically spatially heterogeneous (Fig. 2(b)). Significantincreasing trends in vegetation growth occurred in central, eastern,and southern China, and significant decreasing trends occurred inthe northwestern and northeastern areas and some parts of thesouthwestern area.Fig. 1. Percentage of drought-impacted areas (a) and annual SPI trends from 1982 to 2011 (b).H. Wang et al. / Ecological Engineering 75 (2015) 303307 3053.3. Relationship between drought and vegetationTo assess the impact of droughts occurring in the growingseason (AprilOctober) and pre-growing season (November of theprevious year to March) on vegetation, we calculated therelationships between the growing season SPI and NDVI anomaly,and between the pre-growing season SPI and growing season NDVIanomaly respectively, as shown in Fig. 3. For both conditions,significant positive relationships mainly occurred in the north-western and northern areas of China, which are arid and semi-aridregions. This indicated that droughts occurring during the growingseason or pre-growing season should have large adverse impactson vegetation growth. In addition, more stations had significantpositive relationships between growing season SPI and NDVIcompared with the pre-growing season (60 vs. 48). This indicatedthat droughts occurring in the growing season might have moreserious influences on vegetation growth than those occurring inthe pre-growing season. It is worthy to note that the vegetation innortheastern China is very sensitive to pre-growing seasondrought, but it is less sensitive to growing season drought.4. DiscussionThe trends in the drought-impacted areas showed slightchanges and were essentially consistent with the results calculatedusing Palmer Drought Severity Index (PDSI), which presented anFig. 2. Inter-annual variations of annual NDVI (a) and the spaincreasing trend of 0.50%/10 a from 1951 to 2003 (Zou, 2005).However, a significant upward trend with a rate of 3.7%/10 a from1951 to 2010 (Yu et al., 2013) was revealed based on another widelyused drought index, the Standardized Precipitation Evapotranspi-ration Index (SPEI). Through investigating the trend in the SPI, wefound that dry trends occurred in the northeast and southwestregions of China, which were similar to the areas that experiencedprecipitation decreases (Yu et al., 2013). The SPI values werecalculated solely based on precipitation data; thus, the environ-mental factors that impact the distribution of precipitation,including latitude, longitude, and topography, may also have aninfluence on the spatial patterns of the SPI (Zhai et al., 2005; Zhangand Feng, 2010).A significant increasing trend in the annual vegetation growthin China was found in our study. This result was similar to that ofPeng's study, in which the growing season (AprilOctober) NDVIwas found to significantly increase by 0.0007 year1 from 1982 to2010 on a national scale (Peng et al., 2011). The spatial patterns ofvegetation growth changes were generally consistent with otherfindings on vegetation changes in China (He et al., 2007; Xu et al.,2012). In northern China, the vegetation growth displayed adecreasing trend, which was attributed to warming temperaturesand the related increasing evapotranspiration rates and soil waterdeficits (Duan et al., 2011). The relationship between the growingseason NDVI and temperature anomalies is shown in Fig. 4. Thehigh negative correlations mainly in the north indicated thattial distribution of annual NDVI trends (b) for 19822011.Fig. 3. The relationship between growing season (AprilOctober) SPI and NDVI (a) and between the pre-growing season SPI (November of the previous year to March) andgrowing season NDVI anomaly (b).306 H. Wang et al. / Ecological Engineering 75 (2015) 303307increasing temperatures could lead to adverse effects on vegeta-tion growth. However, increasing trends in NDVI were also found insmall parts of North China, such as the western part of InnerMongolia. The possible reasons for this observation are thefollowing: (1) the negative effects of increasing temperature onvegetation growth were offset by increasing precipitation; (2) theencroachment of trees and shrubs into grasslands that are affectedby climate, soil formations, and other factors promoted vegetationgrowth, and thus affected the aboveground net primary produc-tivity (ANPP) and ecosystem carbon (C) and nitrogen (N) pools(Huang et al., 2012); (3) ecological engineering benefited vegeta-tion growth in this region (Peng et al., 2011). Increasing trends invegetation were observed in southern China. A widely acceptedviewpoint is that vegetation in this region is mainly controlled bytemperature. Warming temperatures have prolonged the growingseason, improved photosynthetic efficiency, and increased vege-tation productivity (Duan et al., 2011). As shown in Fig. 4, higherpositive correlations were mainly found in the south.Significant positive correlations between the SPI and NDVIanomaly were identified for both the growing and pre-growingseasons in north China (Wang et al., 2010), implying thatvegetation growth was affected not only by the growing seasonprecipitation deficit, but also by the pre-growing season droughtconditions. These results are not difficult to understand, as manyprevious studies assert that precipitation is the main factorFig. 4. Correlation between growing season NDVI and temperature anomalies.limiting vegetation growth (Zhao et al., 2011). Although thevegetation could be restored to a normal status after the end of adrought (Zhang et al., 2012b), but long duration and frequentdroughts will inevitably lead to the vegetation degradation.Significant negative correlations between the SPI and NDVIanomaly occurred in the southern region, indicating that theprecipitation deficits favored vegetation growth. This was mainlydue to the resilience and restorability of vegetation to droughtstress. Previous studies have confirmed that resilience of vegeta-tion to drought stress is stronger in humid regions than in otherregions (Vicente-Serrano et al., 2013). In addition, the cloudcoverage in southern China is markedly larger than that innorthern China (less cloud coverage for more solar radiation) (Heet al., 2007). During drought periods the increased radiation due todecreasing cloud coverage may promote vegetation growth(Saleska et al., 2007).Overall, drought is an important disturbance to vegetationgrowth in China, especially for North China. Other factors,including the influence of climate change, ecological engineering,the resilience and restorability of vegetation, the encroachment ofother biome types, etc., contribute to the complex interactionprocesses of vegetation growth and drought.5. ConclusionWe conclude that the variation trends in drought-impactedareas were slight (0.045%/10 a) during 19822011; droughtsoccurring during the growing season and pre-growing season bothhad great impacts on the NDVI; and a significant influence ofdrought on vegetation was found in northern China, especially inthe northwest. Our study on drought dynamics in China can guideagricultural production for the continued development of adapta-tion strategies to protect vulnerable ecosystems and to ensureagricultural security. The findings on the relationship betweendrought and vegetation growth can also provide a vital scientificbasis for China's drought predication and forest management.AcknowledgmentsThis work was financially supported by the National BasicResearch Development Program of China (grant no.2012CB95570001 and 2011CB952001) and the NationalNatural Science Foundation of China (grant no. 41301076) andthe State Key Program of National Natural Science of China(grant no. 41330527).H. 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ttp://refhub.elsevier.com/S0925-8574(14)00666-1/sbref0160http://refhub.elsevier.com/S0925-8574(14)00666-1/sbref0165http://refhub.elsevier.com/S0925-8574(14)00666-1/sbref0165http://refhub.elsevier.com/S0925-8574(14)00666-1/sbref0165Drought dynamics and impacts on vegetation in China from 1982to 20111 Introduction2 Datasets and methods2.1 Datasets2.2 Methods3 Results3.1 Temporal and spatial characteristics of drought3.2 Trends in vegetation growth over the past 3decades3.3 Relationship between drought and vegetation4 Discussion5 ConclusionAcknowledgmentsReferences

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