Atmos. Chem. Phys., 16, 9563–9577, 2016 www.atmos-chem-phys.net/16/9563/2016/ doi:10.5194/acp-16-9563-2016 © Author(s) 2016. CC Attribution 3.0 License. Trends in normalized difference vegetation index (NDVI) associated with urban development in northern West Siberia Igor Esau1, Victoria V. Miles1, Richard Davy1, Martin W. Miles2, and Anna Kurchatova3 1Nansen Environmental and Remote Sensing Centre/Bjerknes Centre for Climate Research, Bergen, Norway 2Uni Research Climate/Bjerknes Centre for Climate Research, Bergen, Norway, Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado, USA 3Institute of the Earth’s Cryosphere, Tyumen Oil and Gas University, Tyumen, Russia Correspondence to: Igor Esau ([email protected]) Received: 17 January 2016 – Published in Atmos. Chem. Phys. Discuss.: 18 March 2016 Revised: 21 June 2016 – Accepted: 27 June 2016 – Published: 1 August 2016 Abstract. Exploration and exploitation of oil and gas re- with the urban heat islands and succession of more produc- serves of northern West Siberia has promoted rapid indus- tive shrub and tree species growing on warmer sandy soils. trialization and urban development in the region. This devel- opment leaves significant footprints on the sensitive northern environment, which is already stressed by the global warm- ing. This study reports the region-wide changes in the veg- 1 Introduction etation cover as well as the corresponding changes in and around 28 selected urbanized areas. The study utilizes the Significant shifts in the vegetation land cover and biologi- normalized difference vegetation index (NDVI) from high- cal productivity manifest rapid climate change in the north- resolution (250 m) MODIS data acquired for summer months ern high latitudes (Hinzman et al., 2005; Groisman and Gut- (June through August) over 15 years (2000–2014). The re- man, 2013). As in other polar regions, biomes of northern sults reveal the increase of NDVI (or “greening”) over the West Siberia (Fig. 1), hereafter referred to as NWS, respond northern (tundra and tundra-forest) part of the region. Simul- to global warming with complex patterns of multidirectional taneously, the southern, forested part shows the widespread changes (Bunn and Goetz, 2006; Lloyd and Bunn, 2007; decrease of NDVI (or “browning”). These region-wide pat- MacDonald et al., 2008; Walker et al., 2009; Bhatt et al., terns are, however, highly fragmented. The statistically sig- 2013). Although the patterns are geographically fragmented nificant NDVI trends occupy only a small fraction of the re- (Elsakov and Teljatnikov, 2013; Barichivich et al., 2014; gion. Urbanization destroys the vegetation cover within the Guay et al., 2014), the northern biomes (tundra and forest- developed areas and at about 5–10 km distance around them. tundra) demonstrate widespread increase of the vegetation The studied urbanized areas have the NDVI values by 15 to productivity (“greening”). Its attribution to tall shrub and 45 % lower than the corresponding areas at 20–40 km dis- graminoids in tundra ecosystems (Frost and Epstein, 2014) tance. The largest NDVI reduction is typical for the newly and enhanced tree growth in forest-tundra ecosystems (Ur- developed areas, whereas the older areas show recovery of ban et al., 2014) suggested that we observe transitions to al- the vegetation cover. The study reveals a robust indication ternative, potentially more productive ecosystems rather than of the accelerated greening near the older urban areas. Many anomalously enhanced biological productivity in response to Siberian cities become greener even against the wider brown- global warming (Kumpula et al., 2012; Macias-Fauria et al., ing trends at their background. Literature discussion suggests 2012). These environmental shifts have been unfolding for at that the observed urban greening could be associated not only least the last 3 decades (Keeling et al., 1996; Myneni et al., with special tending of the within-city green areas but also 1997) albeit at a slower pace since 2003 (Bhatt et al., 2013). Simultaneously, the southern biomes (northern and mid- dle taiga forest to the south of about 65◦ N) demonstrate de- Published by Copernicus Publications on behalf of the European Geosciences Union. 9564 I. Esau et al.: Trends in normalized difference vegetation index Figure 1. The 15-year mean absolute NDVImax (a) and the statistically significant (at α < 0.05) NDVImax trends for 2000–2014 (b). Atmos. Chem. Phys., 16, 9563–9577, 2016 www.atmos-chem-phys.net/16/9563/2016/ I. Esau et al.: Trends in normalized difference vegetation index 9565 crease of productivity (“browning”). This browning has been likely to persist. This point of view received strong support robustly associated with decreasing mass of the green leaves, from the plot-scale International Arctic Tundra Experiment, which has been linked to Siberian forest acclimation with- which simulated warmer climate conditions in high latitudes out the transition to an alternative ecosystem (Lapenis et al., (Walker et al., 2006; Elmendorf et al., 2012). The question 2005; Lloyd and Bunn, 2007). Although the grown trees are remains whether the plot-scale experiments and field stud- stressed by increasing temperatures, other types of vegetation ies at a few carefully selected locations, e.g., Vaskiny Dachi cover (grass- and shrub-lands, mires) and disturbed patches in the central Yamal peninsula (Leibman et al., 2015), could with young tree seedlings exhibit greening within the same be extrapolated for assessment of the larger scale transition bioclimatic zone. to alternative ecosystems. To approach this question, we use The persistent greening trends of the disturbed vegetation an opportunity provided by the above-mentioned intensive cover are the focus of this study. Over the last 30–50 years, urbanization in NWS. The urbanization created a large num- exploration of oil and gas reserves promoted rapid industri- ber of relatively large-scale (tens of kilometers across) ar- alization and urban development of NWS. It makes this part tificial disturbances across all major regional biomes. This of the circumpolar region particularly interesting for stud- study looks at systematic differences in the apparent vegeta- ies of the emerging alternative ecosystems. The development tion productivity trends between the undisturbed background has left significant footprints on the vegetation cover. These and the areas disturbed by urbanization and industrial devel- footprints were found not only at the development sites but opment. also across large distances (Walker et al., 2009; Kumpula As in several previously published studies, this study uti- et al., 2012). More specifically, Kumpula et al. (2011) re- lizes a normalized difference vegetation index (NDVI) de- ported extensive transformation from shrub- to more produc- rived from satellite data products from the Moderate Reso- tive (greener) grass- and sedge-dominated tundra in NWS lution Imaging Spectroradiometer (MODIS) onboard of the that reclaim artificial terrain disturbances at Bovanenkovskiy Earth Observing System-Terra platform satellite. Studies by gas field, Yamal peninsula (70◦ N). Similar environmental Frey and Smith (2007), Epstein et al. (2012), Macias-Fauria shifts towards more productive plant successions on dis- et al. (2012) and others demonstrated that MODIS NDVI turbed patches were found in studies of post-mined sand and could be successfully used in vegetation productivity assess- sandy loam quarries around Noyabrsk at 63◦ N (Koronatova ment for Siberian biomes. This study details the previously and Milyaeva, 2011). This transformation across open wood- reported NDVI maps of the region and expands them over the lands and forest-tundra was found to be more ambiguous. A last 15 years (2000–2014) period. The vegetation productiv- 26 % (from 2316 to 1715 g m−2/ decrease of the total above- ity of emerging alternative ecosystems is calculated at and ground biomass has been attributed to enhanced bog for- around of 28 urbanized areas, hereafter referred to as cities. mation on the background of widespread greening of better These 28 cities are located in four northern biomes ranging drained sandy patches (Moskalenko, 2013). A detailed study from tundra (Bovanenkovo) to middle taiga forest (Uray). by Sorokina (2003) revealed the increased soil temperature This paper has the following structure: Data and meth- by 1 to 4 K on the disturbed patches within forest, mire and ods of the study are described in Sect. 2. Section 3 presents tundra ecosystems. The reclaiming vegetation cover is char- the analysis and results for both the background changes in acterized by increasing share of shrubs (from 18.5 to 27.8 % NDVI and NDVI in and around the selected cities. Section 4 in the forest ecosystems) and grasses (from less than 0.1 to discusses the results in the context on previous research and 8.8 %) in its above-ground biomass. emphasize the role of the cities. Section 5 outlines conclu- Although disturbed patches occupy a relatively small frac- sions. tion in NWS, their contribution to the observed vegetation productivity changes can be disproportionally significant. For example, for the Yamal peninsula, Elsakov and Tel- 2 Data and methods jatnikov (2013) concluded that its widely referred green- This study is based on analysis of the maximum summer ing (Macias-Fauria et al., 2012) could be traced to statisti- NDVI, which we denote as NDVImax. NDVImax was ob- cally significant changes over just 4.8 % of the total area. tained