Rit Mógilsár - 2019, Side 77

R i t M ó g i l s á r | 77

Growth and age of Downy Birch on Skeiðarársandur
Hulda Margrét Birkisdóttir1*, Guðrún Óskarsdóttir1, Ólafur Eggertsson2, Þóra
Ellen Þórhallsdóttir1 & Kristín Svavarsdóttir3
1University of Iceland; 2The Icelandic Forest Service;
3The Soil Conservation Service of Iceland
*hmb7@hi.is
Abstract
In recent decades, downy birch (Betula pubescens) has colonized the sparsely vege-
tated outwash plain of Skeiðarársandur. Selected aspects of the birch population have
been monitored since 2004 but an in-depth study of its dynamics was initiated in 2017.
The research reported here focuses on the population biology. It encompasses 4 study
sites but here we present preliminary results from one site (A4) in the central part of
the plain. This was probably the second area that the birch colonised. In the summer
of 2018, all birch trees were counted on belt transects (750 m²). Annual growth was
measured as length increments on dominant shoots. For tree age, tree rings were
counted in 72 randomly selected and harvested trees (>20 cm in length). Recruitment
is highly spatially variable across the four study sites with seedling and sapling density
by far the highest in the area presented here (A4). The size distribution was highly
right skewed and the age distribution left skewed (plants >20 cm). The oldest
harvested tree was 21 yrs old but >90% of the population were younger than 5 yrs. The
tallest tree in the sample (N=7074 plants) was 263 cm long but only 1% of the trees
were >20 cm long. The mean annual shoot increment was 13.1 cm in 2016, 10.0 cm in
2017 and 9.3 cm in 2018. This appears to be high compared to earlier estimates of birch
growth rates in Iceland. For all three years, the mean annual growth increment was
greater in larger trees (>100 cm long) than in plants <100 cm.
Key words: Birch, annual shoot increment, age distribution, size distribution,
Skeiðarársandur