Reforestation in the far north : comparing effects of the native tree species Betula pubescens and the non-native Pinus contorta in Iceland

Abstract: The use of non-native tree species in forestry is both praised and questioned. Foresters have often promoted their use, mainly because of higher growth rates, resilience to pests and diseases and improved survival under harsh conditions. Nevertheless, non-native tree species can also have negative impact on biodiversity and ecosystem functioning, especially when introduced to former treeless vegetation. In this master thesis, I have used data from the Icelandic Forest Inventory to compare the differences in understory vegetation, berry production, game potential and esthetical value between the non-native tree species Pinus contorta and the native tree species Betula pubescens in Iceland. Species included in the analyse were: Empetrum nigrum, Carex vaginata, Geranium sylvaticum, Vaccinium uliginosum, Rubus saxatilis, Vaccinium myrtillus, Bistorta vivipara in addition to vegetation cover of bryophytes, fungi, lichen, pteridophytes and herbs. Data was divided into three age classes; young forest, middle age forest and old forest. The inventory plots covered the whole Iceland. My result showed that bryophyte cover was significantly higher in old stands of P. contorta compared to B. pubescens. Arguably this is linked to the more acidic soils and humid forest floor that P. contorta are associated with. The presence of Dryas octopetala, Vaccinium uliginosum and E. nigrum were higher in P. contorta stands than B. pubescens in middle age forest. The presence of D. octopetala was higher in older stands compared to young stands, regardless tree species, which probably is connected with sheep grazing in Iceland. D. octopetala is a highly palatable plant and the high presence of sheep in Iceland is probably reducing its range. Due to the common practise of fencing around plantations this probably favoured D. octopetala. In other words it is likely that the fencing of the plantations was the main reason for higher abundance in older forest, not the forest per se. In the middle age class there were a significantly higher presence of V. ulignosum and E. nigrum. The denser structure of P. contorta was probably favourable to V. ulignosum and E. nigrum due to less wind, more acidic soils and higher moisture content at the forest floor than B. pubescens. Most variables did not show a significant difference between P. contorta and B. pubescens. One reason for this is that the cover data is collected in categories by the Icelandic Forest Inventory, which made it harder to detect small significant differences. My results indicates that plantation of P. contorta will result in a different understory than B. pubescens. Furthermore, a large difference could be seen between the different age classes indicating the importance of following stand development over time in order to fully understand the effects of establishment of the two tree species.