Líffræðifélag Íslands - biologia.is
Líffræðiráðstefnan 2019

Erindi/veggspjald / Talk/poster V15

Growth rates and age of downy birch on Skeiðarársandur

Höfundar / Authors: Hulda Margrét Birkisdóttir(1), Guðrún Óskarsdóttir(1), Þóra Ellen Þórhallsdóttir(1), Kristín Svavarsdóttir(2), Ólafur Eggertsson(3)

Starfsvettvangur / Affiliations: 1. University of Iceland, 2. Icelandic Soil Conservation Service 3. Icelandic Forestry Service

Kynnir / Presenter: Hulda Margrét Birksdóttir

In recent decades, downy birch (Betula pubesens) has established Skeiðarársandur, a sparsely vegetated outwash plain. In 2008, surveys of plant density and size were carried out on transects in four sites (A1-A4) in the upper central part of the plain. This was repeated in the same transects (in all 4500 m2) in 2018. All birch trees were counted. Annual growth was measured on the longest shoot of randomly selected trees. This was the main trunk for single-stemmed trees but not necessarily the widest branch for multi-stemmed or prostrate individuals. For age determination, annual rings were counted in120 randomly selected trees (>20cm in length) harvested from two sites one on the western margin of the birch area (A1) and another close to the initial site of colonization (A4). Here I present preliminary results on variation in growth rates and the age distribution and possible time of colonisation.
The mean annual shoot increment was 10.4 cm in 2016, 7.1 cm in 2017 and 7.7 cm 2018. This appears to be high compared to earlier estimates of birch growth rates in Iceland. Growth was significantly greater in A4 than in the other three sites and for all three years, the mean annual growth increment was greater in larger trees (>1 m long) than in plants <1 m. The age distributions indicate three waves of recruitment at each site. In A4 it was highly left skewed with 63% of the plants ≥15 yrs old. The oldest tree in the sample was in A4, 24 yrs old and germinated from seed in 1994.