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Remote Sensing Applications: Society and

Remote Sensing Applications: Society and Environment 3 (2016) 36–44

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A remote sensing perspective of alpine grasslands on theTibetan Plateau: Better or worse under “Tibet Warming”?

Cuizhen Wang n

Department of Geography, University of South Carolina, 709 Bull Street, Columbia, SC 29208, USA

a r t i c l e i n f o

Article history:Received 5 August 2015Received in revised form28 December 2015Accepted 28 December 2015Available online 1 February 2016

Keywords:Tibetan PlateauAlpine grasslandClimate changeSatellite time seriesRemote sensing

x.doi.org/10.1016/j.rsase.2015.12.00285/& 2016 Elsevier B.V. All rights reserved.

+1 803 777 5867.ail address: cwang@mailbox.sc.edu

a b s t r a c t

The Tibetan Plateau is a unique cold and dry highland widely known as the Earth's 3rdPole. Its fragile ecosystems, especially alpine grasslands comprising 60% of the plateau, aresensitive to climate change that has been experiencing a distinct warming trend in pastdecades. Due to limited in-situ accessibility, studies of alpine grasslands have been heavilyrelying on satellite observations since 1980s. This paper gives an overview of satelliteremote sensing of alpine grasslands on the plateau, and controversy findings about theirphenological trends and climatic impacts. Implications of cryospheric and hydrologicprocesses such as snow/glacier melting, lake area change and permafrost retreat on thewarming plateau are also discussed. This study reveals that satellite-extracted spatio-temporal patterns of alpine grasslands should be interpreted with caution. Under therapidly changing climate, alpine ecosystems and their evolution paths on the TibetanPlateau need more comprehensive, integrated investigation.

& 2016 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362. Distributions of alpine grasslands on the plateau . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383. Alpine grasslands in a changing climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.1. Trends of grassland changes on the Plateau. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.2. Climatic impacts on alpine grasslands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4. Implications of cryospheric/hydrologic processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404.1. Snow and glacier melting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404.2. Permafrost retreat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404.3. Lake area change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5. Summary and research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

1. Introduction

The Tibetan Plateau in central Asia is the largest high-elevation geological structure on Earth. Covering an area of2.5 million km2 at elevation 44000 m, it is recognized as

C. Wang / Remote Sensing Applications: Society and Environment 3 (2016) 36–44 37

the “Roof of the World” and “the 3rd Pole of the Earth”(Zhang et al., 2002). The plateau is bordered with highmountains including the Himalaya along the south and theKunlun, Arjin and Qilian in the north that extend to thePamir highlands on the west end. Topographic features ofthe plateau are distinct on the Google Earth image in Fig. 1.

Climate of the interior plateau is characterized as coldand dry with annual mean temperature of 3–5 °C (Yu andXu, 2009). The average monthly mean temperaturereaches above 0 °C between April and October although itvaries dramatically across the plateau (Liu and Chen,2000). Precipitation is strongly affected by Asian andIndian summer monsoons (Wu, 2005), carrying pre-cipitation in June–September with a gradient from over1000 mm in the southeast to less than 100 mm in thenorthwest. Precipitation in winter is limited (Lu et al.,2007; Su et al., 2011). The long-term precipitation recordsat the plateau level did not have a clear trend althoughsome studies reported various local trends in differentperiods and sub-regions (Lin and Zhao, 1996; Yu and Xu,2009; Zhang et al., 2013). Recent studies also reported anoverall increase of precipitation in selected months in theeastern plateau and decrease in the west (Xu et al., 2008;Wang et al., 2015a).

The plateau is undergoing rapid warming that isbelieved higher in winter than growing seasons (Yanget al., 2010), in agreement with that of the northernhemisphere (Myneni et al., 1997; Tucker et al., 2001). Uponthe climatic records in 1955–1996, temperature increased0.16 °C/decade for the annual mean and a doubled rate(0.32 °C/decade) for the winter mean (Liu and Chen, 2000).A more recent study over 90 meteorological stations in1961–2007 found an unprecedented increase rate of0.36 °C/decade (Wang et al., 2008). Yu and Xu (2009)

Fig. 1. The Tibetan Plateau in central Asia. The true-color image in the inset is(2015b) and ACAS (2001). (For interpretation of the references to color in this fi

reported a 1.7 °C increase of annual mean temperaturesince 1970. All these records were much higher than theglobal increase in the same period (IPCC et al., 2007).Under the SRES A1B scenario (a “middle of the road”estimate of future emission, IPCC et al., 2007), a 4 °CWarming on the plateau may occur in the next 100 years(Wang et al., 2008). Here we refer this strong warmingtrend as “Tibet Warming”.

Alpine ecosystems exist close to the biological limitsand are extremely vulnerable to climate change (Grabherret al., 1994). Alpine grasslands cover more than 60% of theTibetan Plateau (Wang et al., 2015b) and therefore, it is ofgreat importance to understand their responses to TibetWarming. Due to physical difficulties in accessing the land,historical records and in-situ observations are limitedespecially in the vast, uninhabited interior plateau. From1970s, around 100 meteorological stations have beenestablished on the plateau (Chen et al., 2006). In spite ofthe dominant coverage of alpine grasslands, only 47 sta-tions are located within this ecosystem, mostly in alpinemeadows and steppes in the eastern and central plateauwith relatively mild climate and better accessibility (Wanget al., 2015a).

Since 1980s, the spatially and temporally extensivesatellite data have provided rich information for vegeta-tion monitoring on the plateau. Both satellite observationsand bioclimatic model simulations suggest that climatechange and terrestrial responses are spatially unequivocal(Hansen et al., 1999; Beniston, 2003; Kawabata et al.,2010). This paper gives an overview of satellite-assistedmonitoring of alpine grasslands on the Tibetan Plateau,and further discusses the influencing factors of theirchanges in the past decades.

from the Google Earth. The land cover map is modified from Wang et al.gure legend, the reader is referred to the web version of this article.)

C. Wang / Remote Sensing Applications: Society and Environment 3 (2016) 36–4438

2. Distributions of alpine grasslands on the plateau

Controlled by local climates transiting from the SouthernHimalaya Subtropical (warm, moist) in the southeast to theKunlun High-cold (arid, frigid) in the northwest (Zheng etal., 1979), alpine grasses on the plateau transit from alpinemeadows, alpine steppes to alpine desert grasses along theclimatic gradient (Wang, 1988). Spatial distributions ofthese alpine grasses, however, are not clear due to insuffi-cient in-situ observations. Several global land cover pro-ducts have been developed, e.g. the 1-km AVHRR-derivedIGBP DISCover (Loveland et al., 2000), the 8-km AVHRR landcover (DeFries et al., 1998), the MODIS global land cover(Friedl et al., 2002) and the SPOT VEGETATION Global LandCover (GLC-2000) (Bartholomé and Belward, 2005).Although valuable for global observations, these productsfail to delineate alpine grasses on the Tibetan Plateau (Zhouet al., 2001; Nemani et al., 2003). With limited image scenesat coarse resolutions, cloud noises and mixed pixels mayoutperform their subtle spectral differences. In these pro-ducts, the plateau is often grouped into the herbaceous andbarren lands. In 2001, the Chinese National Vegetation Atlas(1:1,000,000) was published by the Academician of theChinese Academy of Sciences (ACAS, 2001). The VegetationAtlas on the plateau is a result of long-term efforts based onfield surveys in 1960–70s and image classification from theLandsat MSS in 1970s and TM since 1980s. Although it

Table 1Satellite-assisted studies about the trends and driving factors of alpine grasslan

Trends Detailed results Influencing fa

Positive Increased grass over, greenness; biomass Temperature a

Temperature

PrecipitationAdvanced SOS Spring temper

Winter temperTemperature

Longer growing season (at stations) Temperature (Negative Degraded alpine meadows (TRS) Warmer and d

evapotranspira

Increased snowHuman activitPermafrost retSoil thawing-f

Delayed spring SOS Winter temperSpatially varying Increased NDVI in the south and west;

decrease in northeastAnnual effectiv

Delayed SOS in the west, and advancedin the east

Localized soilPrecipitation

Temporally varying No NPP trends in 1982–1990; increase in1991–1999 (TRS)

Increased sola

Increased greenness in 1982–1991;decrease in 1992–2002

Temperature

Improved swamp meadows in 2000onward

Enforcement otion restoratio

Advanced SOS in 1980–90s, delayed SOSin 1990s–2006

Temperature (increase)

No clear trends

reaches good accuracies in the eastern plateau that is fairlyaccessible (and therefore with good ground truth informa-tion), high uncertainties remain in the vast interior plateauthat is barely inhabited.

Alpine grasses could be better identified from their growthcycles and timing of biological events along the growingseason. de Beurs and Henebry (2004) defined the land surfacephenology to measure the timing of periodic vegetationgrowth from frequent satellite observations. Alpine grasseshave typical one-season growth cycles between May andSeptember although their season lengths vary geographically.

Based on the phenological differences of alpinegrasses, Wang et al. (2015b) extracted their spatial dis-tributions that followed a distinct transition from thesoutheast to northwest of the plateau (Fig. 1). In com-parison with the Landsat-classified map (ACAS, 2001),these spatial patterns of alpine meadows, alpine steppesand alpine desert grasses showed a much better agree-ment with climatic gradients on the plateau. Additionally,the southeastern plateau controlled by the Indian mon-soon is primarily subtropical-montane evergreen/decid-uous forests and scrubs (Zheng et al., 1979). Cultivatedlands are scattered along the northeastern and southernends where also occupy the majority of population on theplateau. These non-alpine lands are not considered in thisreview paper.

d change on the Tibetan Plateau.

ctors Source

nd precipitation Zhong et al., 2010; Wang et al.,2008Xu and Liu, 2007Zhang et al., 2014Xu et al., 2008

ature Yang and Piao, 2006; Hu et al.,2011

ature Ding et al., 2011Zhang et al., 2013

2.5 days/year) Liu et al., 2006rier climate; increased potentialtion

Mao et al., 2008; Song et al.,2009Li et al., 2000

fall and spring rainfall Piao et al., 2003ies (overgrazing) Zhang et al., 2007reat Wang et al., 2011reezing Chen et al., 2011ature Yu et al., 2010e precipitation Ding et al., 2007

temperature and moisture Jin et al., 2013Wang et al., 2015a

r radiation and temperature Piao et al., 2006

Liang et al., 2007

f key national projects for vegeta-n

Song et al., 2009

1.4 days of SOS advance per 1 °C Piao et al., 2011

Shen et al., 2013; Yang et al.,2005; Hu et al., 2011

C. Wang / Remote Sensing Applications: Society and Environment 3 (2016) 36–44 39

3. Alpine grasslands in a changing climate

3.1. Trends of grassland changes on the Plateau

Biotic responses of climatic warming have been fre-quently observed in aspects of phenological variations ofterrestrial ecosystems. Intensive investigation of land sur-face phenology on the plateau has been conducted since1980s. Among the published literatures, commonly appliedsatellite image series include the 15-day, 8-km GlobalInventory Modeling and Mapping Studies (GIMMS) AVHRRnormalized difference vegetation index (NDVI) products(e.g., Zhang et al., 2007; Liang et al., 2007; Yu et al., 2010;Piao et al., 2011; Ding et al., 2007, 2011), the 10-day, 8-kmPathfinder AVHRR land (PAL) database (Yang et al., 2005;Mao et al., 2008; Hu et al., 2011; Xu and Liu, 2007), the1-km SPOT VEGETATION NDVI data sets (Zhong et al., 2010;Shen et al., 2013; Zhang et al., 2013) and the 1-km Terra/Aqua MODIS series (Wang et al., 2008, 2015a, 2015b). Insmaller study areas such as the Tri-River Source region (TRS,headwaters of Yellow River, Yangtze River, and Lancang (theupper reaches of Mekong) River), the 30-m Landsat TM/ETMþ images have been used to reveal the temporalchanges of grasslands (Song et al., 2009; Wang et al., 2011).

Changes of alpine grasslands in past decades have beencommonly recognized. Their trends across the plateau,however, are not in good agreement. Due to different datasources at varying spatial/temporal resolutions, geographiclocations, time periods and area extents of these studies onthe plateau, controversy findings have been documented.Table 1 lists a group of satellite-assisted studies showingdifferent trends of alpine grasslands and their influencingfactors. Both biophysical (e.g., greenness, grass cover, netprimary production (NPP), biomass) and phenological (e.g.,start of season (SOS), end of season (EOS), season length)changes of alpine grasslands are summarized in the table.

Interestingly, some studies reported an improvedvegetation greening on the plateau while others found thegeneral trend of degradation. Some studies claimed thatthe change of alpine grasslands was fragmented and notstatistically significant. Many studies also recognized thespatially and temporally varying patterns of alpine grass-land changes across such a large geographic region. Asshown in Table 1, however, the spatial and temporaltrends do not agree in these studies.

3.2. Climatic impacts on alpine grasslands

The relationships between alpine grassland changesand climatic factors have been commonly examined toidentify the driving forces on the change (also listed inTable 1). Human settlement on the plateau remains low.Therefore most studies treat climate as the primaryinfluencing factor in this region.

Climatic impacts on the change of alpine grasslands,however, are unclear on the plateau level. Nemani et al.(2003) predicted that both temperature and precipitationwere potential climatic constraints to plant growth incentral Asia (where the Tibetan Plateau is located). Aslisted in Table 1, most studies suggested temperature asthe primary driver of alpine vegetation growth, although

some studies found a positive trend (Hu et al., 2011; Liuet al., 2006; Zhang et al., 2013), while others attributed thedegradation of alpine grasslands to a drier and warmerclimate (Mao et al., 2008; Song et al., 2009). As an inter-esting example, Yu et al. (2010) suggested that theincreased winter temperature caused the delay of springgreen-up and shorter growing season in alpine meadows/steppes, possibly explained by the later fulfillment ofwinter chilling. Chen et al. (2011) rebutted their study byrelating the delayed spring phenology to grasslanddegradation coupled with soil thawing-freezing processeson the plateau. In permafrost regions, Zhou et al. (2015)found that grass cover change responding to climatewarming was complicated by soil water released frompermafrost thawing. Yi and Zhou (2011) also suggestedthat the delayed spring phenology may be related toincreased contamination on the plateau. Shen (2011)concluded that the effect of winter warming on springphenology did not follow a simple correlation.

Some studies claimed precipitation as the primary con-trol of grassland change. In a scenario of 2 °C increase on theplateau, Li et al. (2000) simulated the potential evapo-transpiration of alpine meadows and found that it cannot bebalanced until the precipitation reached a 15% increase. Piaoet al. (2003) suggested that increased snowfall and rainfallmay shorten growing season and reduce the incoming solarradiation, and thus reduce vegetation greenness on theplateau. Ding et al. (2007) claimed that the spatial dis-tributions of annual maximum NDVI trends in 1982–1999agreed with the change of the annual effective precipitation.With finer-resolution MODIS time series in 2000–2010,Wang et al. (2015a) reported the geographically oppositetrends of alpine phenology between the western and east-ern plateau that cannot be explained by the unidirectionalclimate warming, but the pattern agreed well with thesubtle precipitation change at station level. Specifically,stations in the eastern plateau revealed an increase ofmonthly precipitation and a decrease in the west. Whilesignificant at station levels, their study also claimed that thetrends did not satisfy at all stations, and were not significantfor annual precipitation.

Under the umbrella of Tibet Warming, the geo-graphically different precipitation trends in alpine grass-lands may be explained by the change of atmosphericcirculations (Yao et al., 2012). In consistence with thedecreased precipitation in the southwestern plateau(Wang et al., 2015a), the Global Precipitation ClimatologyProject (GPCP) reported a large-scale decrease of pre-cipitation in 1979–2010 in the Himalayas Mountain, whichwas closely related to the weakening trend of the Indianmonsoon as reported in recent studies (Wu, 2005;Thompson et al., 2006). Numerical models of atmosphericgeneral circulation showed that Tibet Warming may beinterrelated with the increased summer rainfall in EastAsia (Wang et al., 2008), which was in consistence withthe improved growth and advanced spring greening in thenortheast (Wang et al., 2015a). By examining climatic datarecords at the 66 weather stations above 2000 m, Liu andYin (2001) found that the precipitation anomalies on theeastern Plateau were closely associated with the NorthAtlantic Oscillation. With the ERA-40 reanalysis in a

C. Wang / Remote Sensing Applications: Society and Environment 3 (2016) 36–4440

coupled climate model (ECHAM5/MPIOM), Bothe et al.(2010) found that summer drought and wetness events onthe Tibetan Plateau were associated with the large-scalehigh pressure anomalies of atmospheric circulations in theNorth Atlantic/European sector concurrent with increasingsea surface temperatures in the Indo-Pacific Tropics. Thesestudies suggested that macro-scale atmospheric circula-tions should be further studied to explore the determinantfactors of climatic variability across such a large orographyas the Tibetan Plateau.

Other influencing factors to alpine grasslands werefound to be the increased evapotranspiration (Song et al.,2009) and solar radiation (Piao et al., 2006), permafrostretreat (Wang et al., 2011), and human activities includingovergrazing (Zhang et al., 2007) as well as the imple-mentation of national key projects to restore alpine eco-systems (Song et al., 2009). Applying a process-basedTerrestrial Ecosystem Model, Jin et al. (2013) found thatalpine phenology was more correlated to soil's localizedphysical conditions than air temperature and precipitation.Nevertheless, extrapolation based on currently limitedevidence should be treated carefully, and long-term dataset is needed to address the climatic control of alpinegrasslands.

4. Implications of cryospheric/hydrologic processes

The warming plateau accelerates its hydrological pro-cesses such as freeze-thaw cycling, deglaciation and per-mafrost retreat, which in turn affect soil thermal regimesand growth of alpine grasslands (Wang et al., 2006; Chengand Wu, 2007; Yi et al., 2013). Investigations of theseprocesses could help us better understand the complicatedprocesses and their underlying consequences on alpinegrasslands in this unique region.

4.1. Snow and glacier melting

Persistent snow cover on the plateau is primarilylocated in the southern and western edges within largemountain ridges with elevation higher than 6000 m (Puet al., 2007). The largest inter-annual variations of snowcover fractions occurs in the late fall and winter months.Different from previous suggestions (e.g. Piao et al., 2003),Pu et al. (2007) suggested that increased snowfall inwinter may not have significant impacts on alpine grass-lands. Wang et al. (2015) explored the MODIS snow coverproducts in 2001–2010 and found that snow in December–February and March–May played a significant role onalpine grasslands; however the impacts could be negativeor positive depending on different geographic areas on theplateau.

The Tibetan Plateau has the largest area of glaciers outof the Polar Regions. Similar to snow covers on the plateau,they are mostly located in the high mountains. Tibetanglaciers provide headwaters of most of Asia's great riversystems (Yao et al., 2012; Immerzeel et al., 2010). Theretreat of glaciers in the Himalayas has been observedfrom satellite imagery (Liu et al., 2010; Zhang et al., 2011;Bolch et al., 2012), although those in central Plateau were

relatively stable (Yao et al., 2012; Xu et al., 2009). Glaciermelting affects water supplies to alpine grasslands in thecatchment. However, few studies have been done to esti-mate the long-term contribution of glacier melting towater balance in the plateau, and high uncertaintiesremain because of the difficulties in quantifying thechanging glacial mass (Lei et al., 2012).

4.2. Permafrost retreat

Permafrost covers more than 1.5 million km2 on thePlateau (Zhao et al., 2004; Yang et al., 2010). Continuouspermafrost occupies about one third of permafrost area,mostly in central and northwestern Plateau. Other per-mafrost areas are predominantly discontinuous islandpermafrost and sporadic island permafrost across thePlateau (Nan et al., 2005; Cheng and Wu, 2007). Frozensoils are distributed in a range of 10–312 m in depth acrossthe plateau, and water held in the permafrost is estimatedtwice of that in glaciers (Wu et al., 2010; Zhao et al., 2010).Past studies (Ji, 1996) reported that the upper permafrostlayers had warmed 0.2–0.3 °C in average since 1970s,leading to large-scale thawing and disappearance of per-mafrost. Drilling records reveal the lower elevation limit ofpermafrost moved up to 25 m in the north and 50–80 m inthe south in the past 20–30 years (Zhao et al., 2004; Chengand Wu, 2007; Yang et al., 2010). Ni (2000) modeled thepotential changes of permafrost under the warming cli-mate, and estimated that the boundary between con-tinuous and discontinuous permafrost on the plateauwould shift toward the north by up to 1–2° in latitude.

With permafrost retreat and thinning, groundwatertable drops, root-zone soil moisture decreases and soiltemperature increases. In transitional permafrost zone,however, water released from frozen soil during perma-frost thawing actually favors vegetation growth (Zhouet al., 2015). Therefore permafrost plays an important rolein regulating the distributions of alpine vegetation, espe-cially the succession of alpine meadows into steppes anddesertification (Wang, 1998; Wang et al., 2006). Alpinemeadows are found more sensitive to permafrost changesthan alpine steppes (Wang et al., 2011). For example, thesource region of the Yangtze River is primarily coveredwith permafrost. Yang et al. (2005) found that alpinegrasslands in this area were extremely sensitive to soiltemperature and moisture in the 0–40 cm depth.

Reciprocally, grassland degradation affects soil surfacetemperature (Yi et al., 2013) and thus accelerates theimpact of thermal regime in the active layer of soil (Wanget al., 2010). Soil with high grass cover had later thawing-freezing and lower annual variability than that with lowgrass cover. While some studies (e.g. Yu et al., 2010)attributed the delayed spring phenology to temperatureincrease, the thawing-freezing processes in the active layerof permafrost may also affect the seasonal growth of alpinegrasslands (Yang et al., 2003). Hu et al. (2009) reportedthat grassland degradation on the plateau resulted inadvanced and shorter processes of soil freezing andthawing, which in turn shortens the growing season ofgrasslands. Therefore, permafrost retreat and grasslanddegradation may interact with each other and deserve

C. Wang / Remote Sensing Applications: Society and Environment 3 (2016) 36–44 41

further investigation in different permafrost areas on theplateau.

4.3. Lake area change

The Tibetan Plateau has more than 1000 lakes, most ofwhich belong to inland drainage systems (Li et al., 2014).Lake area change certainly reflects the change of wateravailability in alpine grasslands of the catchment. Climatechange directly affects the extent of Tibetan lakes. Somestudies found that most lakes in the plateau wereexpanding (Zhang et al., 2011). Others reported that theinland lake water had widespread decline in the sourceregion of the Yellow River (Lin et al., 2011), and expansionin the western and northern source region of the YangtzeRiver (Huang et al., 2011). Along with Tibet Warming,evaporation rate on the plateau is about 2–3 times higherthan precipitation (Li et al., 2007). Therefore lake areachange of this region is influenced by water recharge fromother sources than precipitation (Zhang et al., 2011).

Some studies claimed that lake area expansion insouthern plateau was a direct result of glacier melting (Wuand Zhu, 2008; Liu et al., 2010). Ma et al. (2010) alsoclaimed the 60 newly appeared lakes in the plateau to theaccelerated glacier melting. Similar results were reportedin the source regions of the Yellow and Yangtze Rivers bycomparing the TM images in 1986 and 2000 (Wang et al.,2004). Oppositely, a recent study debated that, instead ofglacier retreat, water supply in the Tibetan lakes wasactually primarily attributed by permafrost degradationalong with Tibet Warming (Li et al., 2014). With theLandsat/ICESat satellite time series in 1970–2010, thestudy found a southwest-northeast transition of lake areasfrom shrinking, stable, to rapidly expanding, in consistencewith the pattern of permafrost degradation that wasretreating from south and southeast (isolated permafrost)to northwest (continuous permafrost). Shrinking inlandlakes were also found in other studies (e.g. Lin et al., 2011;Huang et al., 2011) as a common feature in the dis-continuous permafrost regions.

Associated with snow/glacier melting, permafrost retreatand lake extent change, the dynamics of water availability insoil inevitably affect alpine grasslands in both short-termchange and long-term succession. Understanding the inter-action among these processes is critical for better predictionof future water availability in alpine grasslands on the pla-teau, as well as water sources and hydropower supplies formore than 1.5 billion people in South and East Asia.

5. Summary and research needs

Due to limited physical access to the Tibetan Plateau,large-coverage frequent satellite imagery is becoming thepredominant data source to monitor this unique environ-ment. This paper gives a comprehensive review of satellite-assisted studies in aspects of alpine grassland mapping andphenological changes, their climatic consequences and theimplication of snow/glacier/permafrost/lake changes alongwith Tibet Warming. Although spatially explicit, plateau-level information could be easily extracted from satellite

time series, the controversy findings amongst past studiesindicate that the satellite-derived patterns should be inter-preted with caution. Time series of vegetation indexextracted from satellite imagery is often contaminated bytemporal variation of meteorological conditions and soilbackground, which may outperform the spectral signaturesin alpine environments. As reviewed in this paper, incon-sistencies of the plateau-level trends and driving forceswere reported. For satellite remote sensing of alpinegrasslands on the plateau, the following aspects deservefurther investigation:

(1) Spatial/temporal scales of satellite dataMany studies on the plateau relied on coarse-resolution satellite imagery such as the 8-km, 15-dayAVHRR GIMM database that became available sincethe 1980s. Due to the harsh climates, alpine grasslandsare sparse with low biomass and short growingseason, especially in alpine steppes and alpine desertsin western plateau. With reduced spatial and temporalresolutions, these data fail to reveal detailed growthcurves of alpine grasses and their inter-annual trends.Patterns of vegetation growth extracted from thesedata turn to be highly fragmented across the plateau,which could easily be mis-interpreted in geospatialanalysis. The km-scale, 8-day MODIS composite data,available since 2000, reduced the uncertainties of thespatial patterns in alpine grassland mapping. The shortperiod of MODIS data acquisition, however, limits itscapability of extracting phenological trends and exam-ining climate dependencies. Trends in such a shortperiod could be falsely significant with a small degreeof freedom in statistical tests. Further research may beconducted to integrate multi-source satellite observa-tions (spatially and temporally) to take advantage themerit of continuous satellite observations for long-termassessment of alpine grasslands in a changing climate.

(2) The accelerated anthropogenic impactsPopulation on the Tibetan Plateau is low and clusteredin the eastern and southern plateau. Constrained bythe harsh climate, human impacts on this fragileenvironment should not be ignored. Taking theQinghai-Xizang Highway/Railway systems as an exam-ple, negative effects such as the imbalanced thermalregime in the active layer of permafrost along the roadinfrastructure are inconvertible. Comprising about onethird of pasturelands in China, alpine grasslands onthe plateau are under accelerated grazing in recentdecades. Overgrazing and the associated infestation ofplateau pika (Ochotona curzoniae), the rapidly estab-lished highway/railway systems and the intensifiedtouring activities, all cast strong impacts on this fragileecosystem. Started in 1998, a series of key nationalprojects have been initiated on the plateau to combatgrassland degradation. Recovery of alpine grasslands,however, is much slower than those in temperateclimates. Long-term implementation and evaluationof these programs are needed.

(3) New satellite data on Tibetan PlateauSoil moisture controls water availability in alpinegrasslands, but has not been well documented on the

C. Wang / Remote Sensing Applications: Society and Environment 3 (2016) 36–4442

plateau. While more satellite sensors have beenavailable in past decades, their soil moisture productsare at coarse resolutions (40–100 km), and uncer-tainties remain high in examining the contribution ofsoil moisture to alpine grasses. For example, Su et al.(2011) compared the soil moisture products of theNASA AMSR-E (Njoku et al., 2003; Owe et al., 2008)and European ASCAT-L2 (Wagner et al., 2003; Bartaliset al., 2007) in three networks composing the TibetanPlateau Observatory. The study found that, at theplateau level, both products overestimated soil moist-ure by 0.2–0.3 m3 m�3 in monsoon season, and werebasically not reliable in winter. Better products such asthe new SMAP mission (launched in February 2015)reach up to 10-km grid size (http://smap.jpl.nasa.gov),although the product may be affected by the failure ofthe L-band radar in July 2015. Such data layers couldimprove our investigation of the plateau's land surfaceconditions in alpine ecosystems.

In short, by far we are still in lack of sufficient under-standing of current socio-ecological systems on theTibetan Plateau. Additional work is necessary to integratethe physiological, ecological and hydrological data fromsatellite observations to better understand the compli-cated processes on the plateau, and the evolution of alpinegrasslands under the persistent Tibet Warming.

Acknowledgments

This work was initiated from the author's collaborativeresearch with the Institute of Remote Sensing and DigitalEarth (RADI), Chinese Academy of Sciences, China. Theauthor highly appreciates Dr. Huadong Guo and colleaguesat RADI for their supportive efforts in remote sensing ofalpine grasslands on the Tibetan Plateau.

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