Earth Interactions d Volume 23 (2019) d Paper No. 4 d Page 1 Ó 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Changes in Vegetation Cover of the Arctic National Wildlife Refuge Estimated from MODIS Greenness Trends, 2000–18 Christopher Pottera NASA Ames Research Center, Moffett Field, California Received 5 October 2018; in final form 18 December 2018 ABSTRACT: Trends and transitions in the growing-season normalized dif- ference vegetation index (NDVI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite sensor at 250-m resolution were analyzed for the period from 2000 to 2018 to understand recent patterns of vegetation change in ecosystems of the Arctic National Wildlife Refuge (ANWR) in Alaska. Statistical analysis of changes in the NDVI time series was conducted using the breaks for additive seasonal and trend method (BFAST). This struc- tural change analysis indicated that NDVI breakpoints and negative 18-yr trends in vegetation greenness over the years since 2000 could be explained in large part by the impacts of severe wildfires. At least one NDVI breakpoint was detected in around 20% of the MODIS pixels within both the Porcupine River and Coleen River basins of the study area. The vast majority of vegetation cover in the ANWR Brooks Range and coastal plain ecoregions was detected with no (positive or negative) growing-season NDVI trends since the year 2000. Results suggested that most negative NDVI anomalies in the 18-yr MODIS record have a Corresponding author: Christopher Potter, [email protected] DOI: 10.1175/EI-D-18-0018.1 Unauthenticated | Downloaded 09/29/21 01:06 AM UTC Earth Interactions d Volume 23 (2019) d Paper No. 4 d Page 2 been associated with early spring thawing and elevated levels of surface moisture in low-elevation drainages of the northern ANWR ecoregions. KEYWORDS: Climate records; Remote sensing; Ecosystem effects; Forest fires; Regional effects 1. Introduction The Arctic National Wildlife Refuge (ANWR) was established by the Alaska National Interest Lands Conservation Act of 1980 and covers 19 million acres (77 000 km2) in northeast Alaska. Proponents of development in the ANWR view its 1.6 million acre (6475 km2) coastal plain as a promising onshore oil reserve (Comay et al. 2018). Nonetheless, wildlife habitats in the ANWR are vulnerable to long-lasting effects from any disturbance, in part because short growing seasons in the Arctic provide limited time for species to recover. Caribou (Rangifer tarandus) are the most numerous large animals in the ANWR. The Porcupine herd (named after the Porcupine River) numbers approximately 150 000 animals. The herd’s Arctic Village–South Brooks Range spring migration route within Alaska and the ANWR crosses the East Fork of the Chandalar River, the Sheenjek River, and upper Coleen River, and follows the Firth River into Canada where it joins the Old Crow migration route. Caribou herds will use areas in the northern foothills of the Brooks Range during summer at elevations of 300– 600 m, with vegetation communities consisting of graminoid meadows, dwarf shrub, and alpine tundra communities (Nicholson et al. 2016). Caribou consume a variety of vegetation including fungi, lichens, woody browse, graminoids, and forbs (Thompson et al. 2015; Denryter et al. 2017). Barboza et al. (2018) reported that female caribou and their calves in northern Alaska select a mixture of graminoids, browse, and forbs to achieve adequate dietary concentrations of digestible energy and nitrogen requirements. Graminoids were the most abundant forage, accounting for 77% of the digestible nitrogen and 74% of the digestible energy in forage biomass. Localized patterns of vegetation growth and decline in the ANWR may be affecting the survival and reproduction of the caribou herds, which are a crucial subsistence resource for Alaska native communities in the region. Because of changing growing-season lengths, caribou migration may be increasingly out of sync with the timing of key food resources and forage availability. Parturition (birthing of calves) precedes or coincides with the onset of the Arctic growing season and is timed to maximize the period of peak nutrient availability to mother– offspring pairs (Loe et al. 2005). The trophic mismatch hypothesis for large herbivores postulates that earlier green-up will shift peak nutrient availability away from peak nutritional demand leading to lower productivity. Recent work by Gustine et al. (2017) showed that climate-related effects on forage in the summer and autumn ANWR migration ranges corresponded more closely with the demands of female caribou and their offspring to gain mass for the next reproductive cycle. Therefore, these researchers implied that the window of time to examine the mismatch hypothesis in Arctic ungulates is not at parturition, but rather in late summer–autumn, at which time the effects of small changes in forage quality are amplified by forage abundance, peak forage intake, and resultant mass gains in mother–offspring pairs. Unauthenticated | Downloaded 09/29/21 01:06 AM UTC Earth Interactions d Volume 23 (2019) d Paper No. 4 d Page 3 In a study of land-cover change using high-resolution aerial imagery for the ANWR, Jorgenson et al. (2018) estimated that 18% of the area had been altered over the past 50 years, mainly by wildfire and postfire succession, shrub and tree increase in the absence of fire, river erosion and deposition, and ice-wedge deg- radation. Changes in tundra communities tended to be related to landscape wetting, mainly from increased wet troughs caused by ice-wedge degradation. Boreal forest cover showed changes associated with landscape drying and decreases in lake area. Prior to this, Pattison and Welker (2014) found that decreasing snow cover was detrimental to tundra plants in northern Alaska, particularly for various species of shrubs and grasses, including the diamond-leaf willow (Salix pulchra) and tussock cottongrass (Eriophorum vaginatum). As snow depth declined, so too did general productivity of these species throughout the growing season. To evaluate satellite imagery for vegetation greening trends in northern Alaska, Pattison et al. (2015) reported on changes in species composition for coastal plain field plots in the ANWR from 1984 to 2009. They linked these changes to the trends in the normalized greenness vegetation index (NDVI) at both fine and coarse scales. Field plot data implied few changes in plant community compo- sition and no detectable increases in total vegetative cover. Deciduous shrub cover did not show large increases. Field-measured NDVI was positively correlated to deciduous and evergreen shrub composition, suggesting that these functional groups had a strong influence on NDVI values, but the field data showed that NDVI did not change over time on these tundra types. Although Landsat (30 m) NDVI was consistent with field-measured NDVI, NDVI at 1-km resolution from the National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (AVHRR) satellite record was not. AVHRR NDVI values showed increases that were in neither the field-measured nor Landsat NDVI records. This result suggested that AVHRR may be demonstrating increasing trends in NDVI that are not occurring on the ground in some Arctic tundra eco- systems. To date, most of the large-scale studies of vegetation greening or browning in Alaska have not included comprehensive structural breaks analysis, designed to simultaneously detect all major disturbances that can alter greening trend statistics and the conclusions about gradual change in vegetation cover density and tundra or shrubland health. Gradual change analysis applied to a time series is designed to test for changes in the coefficients of a regression model, and generally assumes that there is just a single change under the alternative or that the timing and the type of change are known (Zeileis et al. 2002). A structural break can occur when a time series abruptly changes at a point in time. Detection of multiple breaks or disturbances in a time series of NDVI can occur in wilderness areas as a result of periodic wildfires, insect outbreaks, and/or from repeated cycles of extreme weather events. The objective of this study was to detect both abrupt and gradual changes in vegetation cover throughout the ANWR since the year 2000 using the 250-m- resolution regional MODIS NDVI record for structural change analysis. The principal question posed in this analysis of the highest-spatial-resolution MODIS NDVI available, and the longest time series yet tested, was, Has the green vege- tation cover changed over large areas in the ANWR since the year 2000 and particularly along major caribou migration routes? Statistical analysis of changes Unauthenticated | Downloaded 09/29/21 01:06 AM UTC Earth Interactions d Volume 23 (2019) d Paper No. 4 d Page 4 Figure 1. Study area of the ANWR in northeastern Alaska showing Landsat NLCD vegetation classes (Selkowitz and Stehman 2011). in the MODIS NDVI time series was conducted using the breaks for additive seasonal and trend method (BFAST; Verbesselt et al. 2010a,b; de Jong et al. 2012). 2. Study area The ANWR covers nearly 80 000 km2 from the Beaufort Sea coast in the north, across the Brooks Range to the boreal forest and tributaries of the Yukon River in the south (Figure 1). The mean annual temperature is below freezing, and all parts of the ANWR are underlain by continuous permafrost, except for larger river valleys in the far southern portion (Jorgenson et al. 2018). Tundra vegetation predominates from the Brooks Range to the coastal plain, composed mainly of dwarf shrubs, sedges, and mosses (USFWS 2015). Boreal forest vegetation is found south of the Brooks Range. White and black spruce forests predominate in the northern portion of the subarctic lowlands, whereas the uplands support moist sedge–shrub tundra.