European mistletoe (Viscum
album L.) is a hemi-parasitic shrub that grows attached on a
tree crown. Mistletoe is widely spread in southwest of Lithuania
(Navasaitis, 2004). Due to global climate change, it outspreads
also in the northern Lithuania. Several studies have shown that
mistletoe increases tree crown defoliation and the radial growth
and finally the invaded trees dry off (Dobbertin et al., 2006;
Weber et al., 2007; Kanat et al., 2010; Rigling et al., 2010).
However, F.H. Schweingruber (1996) states that the damage extent
caused by mistletoe is not clear. The influence of mistletoe to
the radial growth of trees was not investigated in Lithuania up to
Investigations on the effect of mistletoe to the radial growth of poplar (Populus nigra L.) were carried out in Kaunas Botanical Garden. For the purpose of the research, 19 poplars with a different number of mistletoes were chosen.
According to a number of
mistletoe shrubs in the crown of poplars, presence of stem decay
and stem diameter, trees were grouped into four groups: 1gr -
trees with stem decay, 2gr - trees with less than the 50cm
diameter and with up to 10 mistletoe shrubs, 3gr - trees with less
than the 50 cm diameter and with more than 15 mistletoe shrubs,
and 4gr - poplars with bigger than the 50 cm diameter and with
more than the 20 mistletoe shrubs.
The research has shown that poplars with the highest number of mistletoes were characterized by the highest radial growth (3gr and 4 gr). This points out that the number and size of mistletoes positively correlate to the radial growth rates of investigated poplars. The especially poor radial growth was typical for trees with stem decay (1 gr). Our investigations revealed also the negative influence of mistletoes. We observed that the shrub of mistletoe dies of the branch part beyond the mistletoe. For example, after four years of mistletoe growing on a branch, its diameter from 2.3 cm below the mistletoe decreased to 1.3 cm above the shrub. With the breakage of a dry branch increases the fungus infection probability. For example, a Fusarium genus fungi was isolated from a dead branch during mycological investigations.
A picture was taken in Kaunas Botanical Garden on 15th July of 2010. A lime tree with mistletoes is still alive, while all adjacent trees have dried off because of molded roots during the trench excavation.