Fræðaþing landbúnaðarins - 13.02.2009, Page 259

MÁLSTOFA G - FRÁ SANDI TIL SKÓGAR | 257
Colonization of mountain birch (Betulapubescens) on
Skeiðarársandur
Magdalena Milli HiedlÞóra Ellen Þórhallsdóttir', Kristín Svavarsdóttir2
1LíJJrœðistofnun Háskólans Askja, Sturlugötu 7, 101 Reykjavík,
2Landgrœðsla ríkisins Keldnaholt, 112 Reykjavík
Skeiðarársandur is a huge floodplain (1,000 km2) in the South of Iceland at the foot of
Skeiðarárjökull. It is characterized by its sandy soil and mostly very sparse vegetation
(<5% cover) (Svavarsdóttir & Þórhallsdóttir 2006). In recent years, mountain birch
(Betula pubescens) has colonized several sites on Skeiðarársandur. Birch forests are
generally regarded as the ”climax vegetation“ of the Icelandic lowlands and during
the Norse settlement they may have covered around 30% of the island. Its
establishment on Skeiðarársandur demonstrates that birch can also play a role in early
stages of primary succession.
In 2004, Bryndís Marteinsdóttir and co-workers (Marteinsdóttir et al. 2007) surveyed
birch populations at four sites on Skeiðarársandur, comparing densities, size and age
distributions, and recording maximum age and fecundity.
The aim of the study reported here was twofold: 1) obtaining a more complete picture
and a better understanding of the colonization and establishment of B. pubescens on
Skeiðarársandur, 2) recording the population and life history stage changes after a
period of four years.
Three of the four previous sites were resampled and one new added. At each site, all
birch plants were sampled within a series of 5-11 belt transects (50 m long * 3 m
wide) or 50*50 m plots. Basal diameter, maximum elevation above ground, length of
the longest shoot and the annual growth increment were measured, and signs of
grazing by sheep noted. If a plant was reproductive, we counted female catkins and
collected ten for further analyses. Every lOth plant was harvested for age estimation.
The total sample size was 527 plants, greatly varying in size and density amongst sites
(Table 1). The tallest plant was 121 cm, but average height was much lower (Table 1).
Forty-seven plants, 8.9% were reproductive. Maximum number of catkins on one
plant was 643 at the site with the highest birch density.
Table 1. Summary of measured variables at four study sites on Skeiðarársandur. Sites
1, 2 and 3 were sampled in 2004 (Marteinsdóttir et al. 2007: 1=V; 2=MN; 3=MS).
Site 4 is a new site and is located approx. half-way between sites 1 and 2 (west-east
direction on the sandur). Saplings refer here to plants < 2 mm in diameter.
Site
i 2 3 4
N 53 274 181 19
Plant density plants m‘2 0.02 0.11 0.04 0.01
Mean elevation cm 19.5 25.5 28.1 11.5
Plants with catkins % 5 13 19 4
Saplings % 5.3 27.1 12.7 11.3