Aspen (Populus tremula L.) and goat willow (Salix caprea L.) are two important tree species in the aspect of nature conservation. Due to the elimination of forest fires, a former negative attitude towards deciduous trees in forestry and a strong herbivore pressure, these tree species have been negatively affected during the last fifty years. The aim of this study was to investigate whether the occurrence of aspen and goat willow could be related to either human impact or to site properties, such as ground moisture and vegetation type. A side study included an inventory of a number of cyanobacterial tree lichens on aspen and goat willow in order to try to relate the occurrence of these lichens species to stand history and stand properties.
The study was performed in Hamra State Forest in northern Dalarna. Nature reserves were compared with stands in managed areas. In addition, two managed stands with an extraordinary high proportion of deciduous trees were investigated. In total 171 goat willows and 29 aspens from twelve stands were included in the study. The stand history was surveyed for each stand by studying old maps and by dating tree rings. At each aspen or goat willow, basal area and stand properties was recorded, in order to see whether these tree species occurred in deviating or representative parts of the stand. In addition, the distance from each aspen or goat willow to the nearest place with signs of strong human disturbance was measured.
The result show that the density of goat willow increases from forest interiors towards edges. The density of aspen and goat willow was highest in the managed stands with a high proportion of deciduous trees, and lowest in the other managed stands. The goat willow was generally growing in places richer than the average of the stand. It was also more common at sites within the stand that had a high proportion of other deciduous trees. Most aspens and willows had a DBH between 11 and 20 cm, indicating a low establishment of young trees. In one of the studied reserves, 24 of 25 aspens and goat willows present in 1926 had died in 1998.
Cyanobacterial lichens on goat willow had the highest frequency in one of the two managed deciduous rich stands. This stand had also the highest level of ground moisture. However, the occurrence of lichens was not related, neither to ground moisture nor to vegetation type around the tree. A positive relationship was found between lichen-frequency and the DBH of the trunk. According to this study, cyanobacterial lichens are not dependent of forest stands with no, or little, human impact. It is more likely that other factors, such as air humidity or bark chemistry, are more important for the occurrence of these species.
In this study, the occurrence of aspen and goat willow could often be related to various distur-bances, including those caused by humans. The present establishment in both the old managed stands and the nature reserves was sparse. In the future, these tree species will probably have minor chances of regeneration within the reserves, if not prescribed disturbances, such as burning, are undertaken. In the managed forest in boreal Sweden, the abundance of aspen and goat willow in general is highest in the smaller diameter...
