Environmental and Experimental Botany 81 (2012) 18–25 Contents lists available at SciVerse ScienceDirect Environmental and Experimental Botany journa l h o me pa g e: www.elsevier.com/locate/envexpbot Vegetation recovery following extreme winter warming events in the sub-Arctic estimated using NDVI from remote sensing and handheld passive proximal sensors S. Bokhorst a,b,∗ , H. Tømmervik b , T.V. Callaghan a,c , G.K. Phoenix a , J.W. Bjerke b a Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, UK b Norwegian Institute for Nature Research NINA, FRAM – High North Research Centre on Climate and the Environment, NO-9296 Tromsø, Norway c Royal Swedish Academy of Sciences, Lilla Frescativägen 4A, SE-114 18 Stockholm, Sweden a r t i c l e i n f o Article history: Received 25 August 2011 Received in revised form 2 February 2012 Accepted 24 February 2012 Keywords: Dwarf shrubs Extreme weather Feather moss Image analysis Peltigera lichens Snow melt a b s t r a c t Extreme winter warming events in the sub-Arctic have caused considerable vegetation damage due to rapid changes in temperature and loss of snow cover. The frequency of extreme weather is expected to increase due to climate change thereby increasing the potential for recurring vegetation damage in Arctic regions. Here we present data on vegetation recovery from one such natural event and multiple experimental simulations in the sub-Arctic using remote sensing, handheld passive proximal sensors and ground surveys. Normalized difference vegetation index (NDVI) recovered fast (2 years), from the 26% decline following one natural extreme winter warming event. Recovery was associated with declines in dead Empetrum nigrum (dominant dwarf shrub) from ground surveys. However, E. nigrum healthy leaf NDVI was also reduced (16%) following this winter warming event in experimental plots (both control and treatments), suggesting that non-obvious plant damage (i.e., physiological stress) had occurred in addition to the dead E. nigrum shoots that was considered responsible for the regional 26% NDVI decline. Plot and leaf level NDVI provided useful additional information that could not be obtained from vegetation surveys and regional remote sensing (MODIS) alone. The major damage of an extreme winter warming event appears to be relatively transitory. However, potential knock-on effects on higher trophic levels (e.g., rodents, reindeer, and bear) could be unpre- dictable and large. Repeated warming events year after year, which can be expected under winter climate warming, could result in damage that may take much longer to recover. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Extreme weather events are expected to increase in frequency due to climate change (ACIA, 2005; Christensen et al., 2007). Such extreme events can surpass lethal thresholds of species leading to population crashes with knock-on effects throughout the ecosys- tem (Gaines and Denny, 1993; Jentsch et al., 2007). Increased species mortality following extreme weather is generally the result of the unseasonal weather conditions during the event or changes in the physical environment resulting in extremes of temperature and moisture availability (Barrett et al., 2008). In the Arctic for instance, sudden winter warming events that melt the snow layer have already been implicated in population crashes of soil animals, ∗ Corresponding author. Current address: Department of Forest Ecology and Man- agement, Swedish University of Agricultural Sciences, SE901-83 Umeå, Sweden. Tel.: +46 090 7868614; fax: +46 090 7868166. E-mail addresses: stef.bokhorst@slu.se, stefbokhorst@hotmail.com (S. Bokhorst). plants, and their consumers such as voles, reindeer and musk ox (Forchhammer and Boertmann, 1993; Robinson et al., 1998; Coulson et al., 2000; Aanes et al., 2002; Putkonen and Roe, 2003; Bartsch et al., 2010) due to the much lower subnivean tempera- tures or ice layer formations. Given that Arctic regions are expected to experience the greatest climatic change during winter months and with higher temperatures predicted for winter and early spring (Schwartz et al., 2006; Beniston et al., 2007; Callaghan et al., 2010), these regions will likely experience more extreme weather events with large consequences for ecosystem development. A recent occurrence of extreme weather in the sub-Arctic was the winter warming event that occurred in northern Scandinavia during December 2007 (Bokhorst et al., 2009). A 12-day period with temperatures between 2 and 10 ◦ C caused snow melt across at least 1400 km 2 exposing the vegetation to at first unseasonally warm, followed by much colder ambient temperatures which veg- etation was exposed to due to the lack of snow cover. A large decline (26%) in NDVI (normalized difference vegetation index) was observed due to this event between the summers of 2007 and 0098-8472/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2012.02.011