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Spatial and temporal patterns of greenness on the Yamal Peninsula, Russia: interactions of
ecological and social factors affecting the Arctic normalized difference vegetation index
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2009 Environ. Res. Lett. 4 045004
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IOP PUBLISHING ENVIRONMENTAL RESEARCH LETTERS
Environ. Res. Lett. 4 (2009) 045004 (16pp) doi:10.1088/1748-9326/4/4/045004
Spatial and temporal patterns of greennesson the Yamal Peninsula, Russia:interactions of ecological and social factorsaffecting the Arctic normalized differencevegetation indexD A Walker1, M O Leibman2, H E Epstein3, B C Forbes4,U S Bhatt1, M K Raynolds1, J C Comiso5, A A Gubarkov2,A V Khomutov2, G J Jia6, E Kaarlejarvi4, J O Kaplan7,T Kumpula8, P Kuss9, G Matyshak10, N G Moskalenko2,P Orekhov2, V E Romanovsky1, N G Ukraientseva2 and Q Yu3
1 University of Alaska Fairbanks, Fairbanks, AK, USA2 Earth Cryosphere Institute, Russian Academy of Science, Siberian Branch, Tyumen, Russia3 University of Virginia, Charlottesville, VA, USA4 Arctic Center, University of Lapland, Rovaniemi, Finland5 NASA Goddard Space Flight Center, MD, USA6 Chinese Academy of Sciences, Institute for Atmospheric Physics, Beijing,Peoples Republic of China7 Swiss Federal Institute for Forest Snow and Landscape Research, Birmensdorf, Switzerland8 University of Joensuu, Joensuu, Finland9 University of Berne, Berne, Switzerland10 Moscow State University, Moscow, Russia
Received 30 March 2009Accepted for publication 3 July 2009Published 15 October 2009Online at stacks.iop.org/ERL/4/045004
AbstractThe causes of a greening trend detected in the Arctic using the normalized difference vegetation index (NDVI) are stillpoorly understood. Changes in NDVI are a result of multiple ecological and social factors that affect tundra net primaryproductivity. Here we use a 25 year time series of AVHRR-derived NDVI data (AVHRR: advanced very high resolutionradiometer), climate analysis, a global geographic information database and ground-based studies to examine the spatialand temporal patterns of vegetation greenness on the Yamal Peninsula, Russia. We assess the effects of climate change,gas-field development, reindeer grazing and permafrost degradation. In contrast to the case for Arctic North America,there has not been a significant trend in summer temperature or NDVI, and much of the pattern of NDVI in this region isdue to disturbances. There has been a 37% change in early-summer coastal sea-ice concentration, a 4% increase insummer land temperatures and a 7% change in the average time-integrated NDVI over the length of the satelliteobservations. Gas-field infrastructure is not currently extensive enough to affect regional NDVI patterns. The effect ofreindeer is difficult to quantitatively assess because of the lack of control areas where reindeer are excluded. Many of thegreenest landscapes on the Yamal are associated with landslides and drainage networks that have resulted from ongoingrapid permafrost degradation. A warming climate and enhanced winter snow are likely to exacerbate positive feedbacksbetween climate and permafrost thawing. We present a diagram that summarizes the social and ecological factors thatinfluence Arctic NDVI. The NDVI should be viewed as a powerful monitoring tool that integrates the cumulative effect ofa multitude of factors affecting Arctic land-cover change.
Keywords: Nentsy, reindeer herding, gas development, Bovanenkovo, plants, disturbance, climate change, infrastructure
1748-9326/09/045004+16$30.00 2009 IOP Publishing Ltd Printed in the UK1
Environ. Res. Lett. 4 (2009) 045004 D A Walker et al
1.1. Overview: using NDVI as a tool to look at land-coverchange on the Yamal
The Yamal region in northwest Siberia is a hot spot ofArctic land-cover change due to rapid resource development,active geomorphic changes, climate change, and growth ofthe local reindeer herds, (Vilchek 1996, Forbes 1999a, 1999b,2008, Forbes et al 2009, Dobrinsky 1997, Melnikov andGrechishchev 2002, Moskalenko 2005, Walker et al 2009c).A thin layer of tundra vegetation provides key resources to thepeople and animals of the Yamal and protects the underlyingpermafrost from thaw. We are using the normalized differencevegetation index (NDVI) in conjunction with detailed ground-level surveys to better understand how the major forces ofchange affect the vegetation.
The NDVI is an index of vegetation greenness. Greenplants have high reflectivity in the near-infrared wavelengthsand absorb red wavelengths for photosynthesis. Thenormalized difference vegetation index (NDVI) is definedby the equation: NDVI = (NIRVIS)/(NIR + VIS), whereNIR is the reflectance of the Earths surface in the near-infrared channel (0.7251.1 m) and VIS is the reflectancein the visible portion of the spectrum or the red channel(0.50.68 m) (Tucker 1976, Tucker and Sellers 1986). Theobserved spatial and temporal changes to vegetation greennessrepresent changes to the fraction of photosynthetically activeradiation (fPAR) that is absorbed by the leaves and stems ofthe plant canopy, which is in turn a function of numerousproperties of the vegetation including its vertical and horizontalstructure, species composition, phenological stage, and healthof the plants.
The NDVI has recently gained a lot of attention becausea long-term trend of increased NDVI has been detected inparts of the Arctic (Myneni et al 1997, Jia et al 2003,2004, Stow et al 2004, Jia et al 2006, Verbyla 2008,Goetz et al 2009). For example, in northern Alaska, thetime-integrated NDVI has increased 20% during the periodof satellite observations (19822007) (Walker et al 2009a,2009b, 2009c). These changes are not, however, universal.Much smaller changes have been observed in parts of theEurasia tundra (Walker et al 2009a, 2009b, 2009c). Thecauses of these changes are not well understood but havebeen attributed to a variety of factors, including an increasein the extent and abundance of shrubs (Sturm et al 2001,Tape et al 2006, Lantz 2008), local effects of anthropogenicdisturbances and differential response of different vegetationtypes (Munger 2007), northward movement of trees fromthe sub-Arctic (Lloyd 2005, Lloyd and Bunn 2007), andlonger growing seasons and increasing land temperatures (Jiaet al 2003, Goetz et al 2005). Analysis of circumpolarpatterns of NDVI in relationship to variables in a circumpolargeographic information system have documented the influenceof land temperature, regional floras, glacial history, bedrockand soil chemistry, local nutrient availability, and disturbancepatterns on NDVI (Walker et al 2003, Raynolds et al 2006,2007, Munger et al 2008, Raynolds and Walker 2008, 2009).Recently, rapid changes in sea-ice concentrations (Comiso et al
2008) have raised the questions regarding the influence of sea-ice changes on land temperatures, permafrost, and associatedecosystem processes (Lawrence et al 2008, Bhatt et al 2007,2008, Walker et al 2009a). The Yamal offers an opportunity totake a closer look at the spatial and temporal patterns of NDVIin this region and to look at the possible underlying causes ofany trends.
A major question in our study is: how do various formsof physical disturbance affect Yamal greening patterns? Manystudies of disturbances in the Arctic have shown that plantproduction often increases on disturbed sites because of thepositive influence of increased soil temperatures and enhancednutrient regimes (Ebersole 1985, Shaver and Chapin III 1995,Walker 1996, Shaver et al 1998, Forbes and Sumina 1999,Forbes et al 2001, Walker et al 2006). For example, landscape-scale studies of vegetation response to climate and disturbancesin the Mackenzie River delta region of northwestern Canada,indicate that increased the frequency of disturbances suchas fire and landslides results in the introduction of abundantshrubs (Lantz 2008). On the Yamal, we were looking fordisturbances that occur at a large enough scale to affect theregional NDVI patterns.
1.2. Physical environment
The Yamal Peninsula (figure 1) is about 250 km wide and isbounded on the west and north by the Kara Sea and on the eastby the embayment of the Ob River. The maximum elevationsin the interior parts of the Peninsula are only about 4590 m(Tsibulsky et al 1995). The Peninsula was unglaciated duringthe last glaciation (Forman et al 2002). Most of the Peninsulais built of sandy and clayey marine, alluvial and lacustrinesediments deposited during and following the middle andlate Quaternary marine transgressions. Most of the depositsare saline within the permafrost, and some are saline in theactive layer (the layer of soil above the permafrost that meltsannually) (Trofimov 1975). Hilltops in sandy areas are oftenwindblown with sand hollows, some covering large areas.Meandering rivers and streams have cut broad valleys throughthe terrace deposits creating lowlands that are occupied bypolygonal peat lands, thaw lakes and drained thaw-lake basins.
The zonal vegetation ranges from dominantly low-