Rit Mógilsár - 2019, Page 76

76 | F a g r á ð s t e f n a 2 0 1 9

Colonization of downy birch in early succession
Guðrún Óskarsdóttir1*, Hulda Margrét Birkisdóttir1, Kristín Svavarsdóttir2 & Þóra
Ellen Þórhallsdóttir1
1University of Iceland; 2The Soil Conservation Service of Iceland
*gudrun@na.is
Abstract
Downy birch (Betula pubescens) is the dominant species in long lived and stable forests
and woodlands in Iceland but it can also colonize early-successional sites. A recent
example of the latter is the sudden establishment and rapid expansion of birch on
Skeiðarársandur outwash plain. This must be due to long distance dispersal as the
nearest seed source is >10 km away. Birch is believed to have colonized initially in one
or a few major events, when the many sequential conditions necessary for successful
dispersal and establishment must all have been favourable. This includes a large seed
crop of high quality, conditions for dispersal at the right time of year, safe sites for
germination and establishment and favourable conditions for survival and growth.
Here, we report on selected life history stages that are likely to be limiting for the birch
population on Skeiðarársandur, namely seed quality and seedling and sapling
establishment and their relationship with environmental factors. Seed quality in the
birch population on Skeiðarársandur has been extremely poor with germination rates
varying from <2% to a maximum of 22% in 2017. A survey in 2018 revealed very high
variation in seedling and sapling density in two areas where birch had already
established. Sapling density in one area was 0.05 plants m-2 but in the other it was 9.4
plants m-2, mostly plants with <4 leaves.
Key words: Downy birch, Betula pubescens, early succession, seed quality, birch density