remote sensing Article Remote Sensing of 2000–2016 Alpine Spring Snowline Elevation in Dall Sheep Mountain Ranges of Alaska and Western Canada David Verbyla 1,* ID , Troy Hegel 2, Anne W. Nolin 3, Madelon van de Kerk 4, Thomas A. Kurkowski 5 and Laura R. Prugh 4 ID 1 School of Natural Resources and Extension, University of Alaska Fairbanks, Fairbanks, AK 99775, USA 2 Yukon Department of Environment, Whitehorse, YT Y1A 4Y9, Canada; [email protected] 3 College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA; [email protected] 4 School of Environmental and Forestry Sciences, University of Washington, Seattle, WA 98195, USA; [email protected] (M.v.d.K.); [email protected] (L.R.P.) 5 Scenarios Network for Alaska and Arctic Planning, University of Alaska Fairbanks, Fairbanks, AK 99775, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-907-474-5553 Received: 18 August 2017; Accepted: 7 November 2017; Published: 11 November 2017 Abstract: The lowest elevation of spring snow (“snowline”) is an important factor influencing recruitment and survival of wildlife in alpine areas. In this study, we assessed the spatial and temporal variability of alpine spring snowline across major Dall sheep mountain areas in Alaska and northwestern Canada. We used a daily MODIS snow fraction product to estimate the last day of 2000–2016 spring snow for each 500-m pixel within 28 mountain areas. We then developed annual (2000–2016) regression models predicting the elevation of alpine snowline during mid-May for each mountain area. MODIS-based regression estimates were compared with estimates derived using a Normalized Difference Snow Index from Landsat-8 Operational Land Imager (OLI) surface reflectance data. We also used 2000–2009 decadal climate grids to estimate total winter precipitation and mean May temperature for each of the 28 mountain areas. Based on our MODIS regression models, the 2000–2016 mean 15 May snowline elevation ranged from 339 m in the cold arctic class to 1145 m in the interior mountain class. Spring snowline estimates from MODIS and Landsat OLI were similar, with a mean absolute error of 106 m. Spring snowline elevation was significantly related to mean May temperature and total winter precipitation. The late spring of 2013 may have impacted some sheep populations, especially in the cold arctic mountain areas which were snow-covered in mid-May, while some interior mountain areas had mid-May snowlines exceeding 1000 m elevation. We found this regional (>500,000 km2) remote sensing application useful for determining the inter-annual and regional variability of spring alpine snowline among 28 mountain areas. Keywords: spring snow; alpine; Dall sheep; MODIS; MODCAG; snowline elevation; snow mapping 1. Introduction Climate warming is altering the duration, amount and timing of snowfall and these changes are especially substantial in arctic and boreal regions [1]. Changing spring snow conditions likely impact many alpine mammals. For example, earlier spring plant phenology initiated earlier emergence from hibernation, earlier weaning, increased body mass and increased survival in yellow-bellied marmots (Marmota flaviventris)[2]. Reduced snow depth during calving allowed parturient caribou (Rangifer tarandus) females to disperse up in elevation away from predator populations, thus increasing recruitment rates [3]. During later springs, upward movement may be limited by snow, resulting in Remote Sens. 2017, 9, 1157; doi:10.3390/rs9111157 www.mdpi.com/journal/remotesensing Remote Sens. 2017, 9, 1157 2 of 18 Remote Sens. 2017, 9, 1157 2 of 18 higher predation ratesrates on neonates [[3].3]. In addition, late late-winter-winter snowfall has been shown to negatively affect caribou calf growthgrowth ratesrates inin AlaskaAlaska [[4].4]. Dall sheep (Ovis dalli dalli) areare anan iconiciconic alpinealpine speciesspecies distributeddistributed acrossacross northwesternnorthwestern NorthNorth America, includingincluding major major mountain mountain ranges ranges in Alaska, in Alaska, Yukon, Yukon, the Northwest the Northwest Territories Territories and northwest and Britishnorthwest Columbia. British Columbia. The species The is aspecies rare example is a rare of example a North of American a North Americ large mammalan large thatmammal occupies that mostoccupies of its most native of rangeits native , with range genetic , with isolation genetic among isolation mountain among rangemountain populations range populations for thousands for ofthousands years [5]. of Because years there[5]. Because is limited there dispersal is limited among dispersal mountain among populations, mountain some populations, populations some may bepopulations vulnerable may as “islandbe vulnerable populations”. as “island There populations”. are many There survival are many challenges survival in thesechallenges high latitudein these mountainshigh latitude including mountains extreme including weather extreme events weather such as events heavy snowfall,such as heavy winter snowfall, icing events winter and icing late springevents snowand late delaying spring springsnow delaying forage production spring forage [6,7] production (Figure1). [6,7] (Figure 1). (a) (b) Figure 1. (a) Typical Dall sheep habitat from Wrangell Mountains, Alaska; (b) Dall sheep ewe and Figure 1. (a) Typical Dall sheep habitat from Wrangell Mountains, Alaska; (b) Dall sheep ewe and lamb lamb (both photographs were from late August by D. Verbyla). (both photographs were from late August by D. Verbyla). Dall sheep populations have declined in recent decades in some areas. Range-wide, Dall sheep populationsDall sheep have populations declined by have 21% declined since 1990 in recent[8] but decades the causes in some remain areas. unknown Range-wide, [9]. Harsh Dall spring sheep populationsweather was haveshown declined to reduce by horn 21% sincegrowth 1990 rates [8 ]in but the the Yukon causes [10] remain and pregnant unknown Dall [9 ].sheep Harsh in springDenali weatherNational was Park shown were tofound reduce to hornavoid growth snow-covered rates in theareas Yukon [11]. [ 10However,] and pregnant no studies Dall sheephave examined in Denali Nationalimpacts of Park spring were snow found conditions to avoid on snow-covered Dall sheep populations areas [11]. across However, their norange, studies in part have due examined to a lack impactsof available of spring regional snow snow conditions products on that Dall capture sheep populations important acrossfeatures their of range,snow-covered in part due landscapes, to a lack ofsuch available as the elevation regional snow of spring products snowline. that capture important features of snow-covered landscapes, such as theThe elevation elevation of springof spring snowline. snowline is important to all alpine herbivores in this region since above this elevationThe elevation forage of is spring restricted snowline to low is importantquality fora toge all and alpine is covered herbivores with in snow. this region Below since snowline, above thissnowmelt elevation has forageoccurred is restrictedand new shoot to low growth quality provides forage and high is quality covered forage with snow.that is Belowavailable snowline, for the snowmeltfirst time after has occurreda long winter. and new Theshoot snowline growth elevation provides may high thus quality affect foragesheep by that altering is available the quantity for the firstand quality time after of forage. a long winter.Snowline The elevation snowline may elevation also affect may the thus food affect-safety sheep tradeoff by altering for sheep, thequantity because andseveral quality of their of forage. primary Snowline predators elevation occur mainly may also at lower affect elevations the food-safety [12]. tradeoff for sheep, because severalA spring of their warming primary trend predators has occurred occur mainly in recent at lower decades elevations with earlier [12]. spring snowmelt and river and Alake spring ice-out warming in northern trend hasAlaska occurred and Canada in recent [13 decades–15]. However, with earlier despite spring this snowmelt trend andin earlier river andspring lake snowmelt, ice-out inan northernoccasional Alaska late spring and could Canada impact [13– 15sheep]. However, populations despite in some this mountain trend in ranges. earlier springSpring snowmelt,2013 was unusually an occasional late lateat lower spring elevations, could impact with sheep river populationsice-out in Canada in some and mountain interior Alaska ranges. Springoccurring 2013 11 was–20 days unusually later than late atthe lower 22-year elevations, average with(Figure river 2). ice-outSnow conditions in Canada and and impact interior on Alaska lamb occurringsurvival in 11–20 the spring days later of 2013 than likely the 22-year varied average among (Figuremountain2). Snowareas. conditionsIncreased andvariability impact in on lambinter- survivalannual alpine in the springsnow may of 2013 impact likely populations varied among to mountaina greater extent areas. Increasedthan shifting variability mean conditions in inter-annual and alpineadditional snow tools may and impact products populations are therefore to a greater needed extent to than characterize shifting mean this conditionsvariability and across additional broad toolsregions and [16]. products are therefore needed to characterize this variability across broad regions [16]. Remote Sens. 2017, 9, 1157 3 of 18 Remote Sens. 2017, 9, 1157 3 of 18 Figure 2. Spring river ice-out