Natural colonization as a means to upscale restoration of subarctic woodlands in Iceland

Data on birch colonization was collected from 10 different areas dominated by mountain birch across Iceland. In each area, four 100 m long and 2 m wide belt transects, were laid out at least 100 m apart perpendicular to the woodland edge into the adjacent open landscape. Along each transect, all birch seedlings with mature leaves were registered and their size measured as the length of the longest shoot from the stem base at the soil surface. The birch seedlings were divided into three size classes: I: <2 cm, II: 2- 20 cm and III: >20 cm, thus including both seedlings and older saplings. All catkin-bearing trees within 5 meters of the transects were also registered.

The microsite occupied by each seedling in size class I was recorded using 15 different types. The microsite cover along the transects was measured using the point-intercept method (Floyd & Anderson 1987), in 50 x 50 cm quadrats at 10 m intervals along the transects. We recorded microsites at 25 regularly spaced points within the quadrats. To account for rare microsite types, which were not recorded by the 25 points, all additional microsite types present within each quadrat were recorded.

Seedling densities were related to local climate data and ecological variables, including: annual precipitation, mean wind speed and dominant wind direction from September–December (the main period for birch seed dispersal), mean growing season (May – August) temperature, elevation, soil type, dominant height of the woodland canopy and total woodland area, and presence of domestic sheep.