Arctic Summers are Getting Longer

This date was near the end of the growing season when just a few plants were still green, such as the patch of green a few feet to the right of the snoozing muskox.

A photo taken by a remote automated camera at the Mt. Noak climate monitoring station in Cape Krusenstern National Monument on 5 September 2020. This camera showed that in 2020 the winter snow cover arrived here on October 15.

The snow-free season and length of time that vegetation is green have gotten longer over the past 20 years in Alaska's Arctic national parks. The Arctic Inventory & Monitoring Network monitors changes in the timing and length of the snow season and the green season in the five northern Alaska parks using remote automated cameras and satellite images. We have one camera in each park that takes five photos per day, year-round. The camera views are very local in scope, but provide important verification of the satellite results. We use images from MODIS satellites to monitor the coming and going of the snow cover, the timing of spring green-up and fall senescence, and the maximum greenness of vegetation. These satellites have been sending back daily images since 2000.

The average date of winter snow-off and start of spring green-up in the Arctic parks, from 2000 to 2020, based on MODIS satellite images.

Satellite images show that the start and the end of both the continuous snow season and the green vegetation season vary by about a month between years. over the past 20 years in the five parks, the snow-free and green seasons have become longer by about a week in the spring and a week in the fall on average, though we are a bit uncertain of the fall dates because of cloudiness and low sun angles then. The lengthening growing season has been accompanied by a slight increase in the peak summer greenness in many locations, probably due to vegetation becoming both denser and taller. These large year-to-year variations and long-term changes in the length and timing of the growing season present challenges to wildlife, especially migratory species like caribou and birds.

Start of the Green Season and Normalized Difference Vegetation Index in Alaska’s Arctic National Parks

Abstract

Daily Normalized Difference Vegetation Index (NDVI) values from the MODIS Aqua and Terra satellites were compared with on-the-ground camera observations at five locations in northern Alaska. Over half of the spring rise in NDVI was due to the transition from the snow-covered landscape to the snow-free surface prior to the deciduous leaf-out. In the fall after the green season, NDVI fluctuated between an intermediate level representing senesced vegetation and lower values representing clouds and intermittent snow, and then dropped to constant low levels after establishment of the permanent winter snow cover. The NDVI value of snow-free surfaces after fall leaf senescence was estimated from multi-year data using a 90th percentile smoothing spline curve fit to a plot of daily NDVI values vs. ordinal date. This curve typically showed a flat region of intermediate NDVI values in the fall that represent cloud- and snow-free days with senesced vegetation. This “fall plateau” was readily identified in a large systematic sample of MODIS NDVI values across the study area, in typical tundra, shrub, and boreal forest environments. The NDVI level of the fall plateau can be extrapolated to the spring rising leg of the annual NDVI curve to approximate the true start of green season.

Swanson, D. K. 2021. Start of the green season and normalized difference vegetation index in Alaska's Arctic national parks. Remote Sensing 13(13): 2554.