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.

在冰岛十处由山地白桦树主导的区域收集了白桦树殖民化的数据。在每个区域，均沿林缘向外至相邻开阔景观方向，设置了四条长度为100米、宽度为2米的带状样带，样带间至少相距100米。沿每条样带，记录所有具有成熟叶片的白桦树幼苗，并测量其从土壤表面茎基到最长枝条的长度的尺寸。白桦树幼苗被分为三个尺寸等级：I级：小于2厘米，II级：2-20厘米，III级：大于20厘米，从而包括了幼苗和较老的树苗。距离样带5米范围内的所有带状果序树也被记录。对于尺寸等级I的每棵幼苗，记录了15种不同的微生境类型。沿样带的微生境覆盖率采用点截取法（Floyd & Anderson 1987）进行测量，在样带10米间隔的50 x 50厘米样方内进行。在样方内25个规则分布的点记录微生境。为了弥补25个点未能记录的稀有微生境类型，记录了每个样方内所有额外的微生境类型。幼苗密度与当地气候数据以及生态变量相关联，包括：年降水量、9月至12月（白桦树种子散布的主要时期）的平均风速和主导风向、平均生长季节（5月至8月）的温度、海拔、土壤类型、林冠主导高度、总林地面积以及家畜的存在。