Abstract:
Shrub cover in treeless areas of the arctic has increased in the past 50 years. Spruce seedling establishment in areas near the forest boundary has also increased suggesting tree line advance. In order to describe the effects of shrubs on seedling establishment, we measured shrub cover in study plots south of the Alaska Range. In each of ten 50 m x 50 m study plots located at either elevational treeline (five plots) or above treeline (five plots), 10 sample points were systematically selected, and 10 additional sample points coincided with naturally established white spruce (Picea glauca) seedlings. At each point, the presence of all woody species was recorded at six heights: at ground level, < 0.25 m, 0.25 m to 1 m, 1 to 2 m, 2 to 4 m, and > 4 m above ground level. Results indicate almost ubiquitous cover of woody plants in the two height classes under 0.25 m at both systematic sampling points and at spruce seedlings. The frequency of each higher shrub height category at sampled points with seedlings differed significantly from the frequency at the systematic points (χ2 = 26.3, p =.0000337). Points with seedlings had less shrub cover overall than the systematic points, particularly in the tallest height categories. The frequency of tall shrubs at points with seedlings was about half that at the systematic points. Very similar patterns existed in both the tree line and above tree line study plots. These results suggest that seedlings preferentially establish in sites without tall shrubs. Shrubs may therefore limit establishment of white spruce at tree line, and continued shrub expansion may inhibit forest migration.
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Collecting data in Plot MT5Collecting data in plot MT5. Plants in the foreground include blue joint grass (Calamagrostis canadensis), and bear saxifrage (Boykinia richardsonii)Collecting data in plot MT5. Plants in the foreground include lingonberry (Vaccinium vitis-ideae), bunchberry (Cornus canadensis), and crowberry (Empetrum nigrum)Collecting data in plot MT5. Plants in the foreground include alpine blueberry (Vaccinium uliginosum) , lingonberry (Vaccinium vitis-ideae), bunchberry (Cornus canadensis), and crowberry (Empetrum nigrum)Collecting data in plot MT5. Plants in the foreground include bunchberry (Cornus canadensis), fireweed (Epilobium latifolium), and wild rose (Rosa acicularis).Collecting data in plot MT5
The above treeline transect from Plot MA5.Collecting data in plot MA5
Collecting data in plot MA4 White spruce seedlings (here marked with pink flagging) are locally abundant at treeline and above treeline. Plot MT1. Two tagged white spruce seedlings in treeline plot 1 at Monahan Flats. The low shrub cover is typical of spruce establishment microhabitat and includes alpine blueberry (Vaccinium uliginosum), shrub birch (Betula nana/glandulosa), and crowberry (Empetrum nigrum).
Collecting shrub height data at a spruce seedling in plot MA4 Spruce seedling in study plot. Collecting data in plot MA4
The above treeline transect near plot MA2. The treeline transect is in the spruce forest in the middle distance.Kite aerial photo of part of the “At Treeline” transect at Monahan Flats.Kite aerial photo of part of the “At Treeline” transect at Monahan Flats.
All 10 plots combined (at and above treeline). NDVI zenithal image from a systematic sampling site in an above treeline plot. Green denotes NDVI values greater than 0.1. From a photo taken by an infrared NDVI camera.
NDVI zenithal image from a systematic sampling site in a treeline forest plot. Green denotes NDVI values greater than 0.1. From a photo taken by an infrared NDVI camera.False color NIR image. NIR reflectance is displayed as red.Panorama of the above treeline transect at Monahan Flats. Dashed line marks the location of five 50x50m plots. The treeline transect is down the hill in the distance in the forest of spruce trees.Kite aerial photo of plot 4 above treeline at Monahan Flats. White spruce seedlings were marked with pink flagging before photography. Circles are locations of white spruce seedlings.Kite aerial photo of plot 1 in the above treeline transect at the Monahan Flats study area. White spruce seedlings (white triangles) were marked with pink flagging before photography. Spruce seedlings are more abundant in areas of low stature woody plants (e.g., crowberry, lignonberry, cassiope) and lichens which appear grayish.
