Abstract
ABSTRACTHigh-latitude systems in northwestern Canada and Alaska have warmed rapidly. The aim of this study was to examine how a remotely sensed proxy of vegetation productivity varied among mountain ranges with respect to elevation and climate from 2002–2017. Our study area included high-latitude mountains in Alaska, USA, and Yukon Territory, Canada, ranging from cold arctic mountains in the tundra biome to warmer interior mountains areas within the boreal biome. We used the annual maximum Normalized Difference Vegetation Index (NDVI) data from the 250-m MODIS NDVI product as a proxy of maximum growing season photosynthetic activity. The long-term (16-year) and interannual pattern of maximum NDVI was investigated with respect to elevation, July temperature, and July precipitation classes within four climatic mountain regions. The July temperature lapse rate was consistently linear, whereas the long-term maximum NDVI lapse rate was nonlinear. At lower elevations, the high-precipitation region had the highest NDVI, whereas the interior mountains region had the highest NDVI at higher elevations. The long-term maximum NDVI was negatively correlated with July precipitation for areas with July temperature below 12°C. Above 12°C, NDVI was positively correlated with July precipitation, with the greatest rate of NDVI increase with precipitation at the warmest July temperature class. The pattern of interannual peak NDVI with respect to July temperature was not as strong as the long-term pattern; however, the only interannual negative correlation between peak NDVI and July temperature was at lower elevations within the interior mountains. We concluded that among a regional climatic gradient of mountain areas, low growing season temperature and length were likely constraining vegetation productivity, and lower growing season moisture may be an important constraint at the warmest interior mountains region.
Highlights
Alaska climate warming and tundra productivityClimate warming at high latitudes has been substantial over the past few decades (IPCC 2013), and many biotic and abiotic changes have occurred (Hinzman et al 2005; Post et al 2013; Showstack 2015)
We investigated the interannual peak summer Normalized Difference Vegetation Index (NDVI)
As an example from this study, within the interior mountains class, high NDVI broadleaf forest occurred above a July threshold temperature of 12°C, whereas spruce occurred above a July threshold of 11°C, and lower NDVI dwarf shrub rarely occurred in July temperature classes warmer than 13°C (Figure 7)
Summary
Alaska climate warming and tundra productivityClimate warming at high latitudes has been substantial over the past few decades (IPCC 2013), and many biotic and abiotic changes have occurred (Hinzman et al 2005; Post et al 2013; Showstack 2015). Over the past 50 years, the highest rate of climate warming in North America has occurred in Alaska and northwest Canada (Clegg and Hu 2010). This warming has led to record-setting changes including maximum sea-ice retreat and amplified arctic warming (Melillo, Richmond, and Yohe 2014). As sea ice has declined in arctic Alaska, autumn monthly temperatures have risen by up to 7°C over the past 3 decades (Wendler, Moore, and Galloway 2014). The summer climate regime of interior Alaska is the warmest in the past 200 years (Barber et al 2004). Alaska recently experienced the warmest winter in 90 years (Walsh et al 2017), and the length of the unfrozen period has increased by nearly 50 percent in the last century (Wendler and Suhlski 2009)
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