Impact of White Mistletoe (Viscum album ssp. abietis) Infection on Needles and Crown Morphology of Silver Fir (Abies alba Mill.)

Abstract

White mistletoe (Viscum album ssp. abietis (Wiesb.) Abromeit), (Santalales, Viscaceae) is a hemiparasite species and, in Romania, occurs only on silver fir (Abies alba Mill.). The species affects silver fir stands located mainly at lower elevations on the Eastern border of the silver fir natural distribution area. Along with drought and pollution, white mistletoe infection plays an important role in silver fir decline. The present study tried to quantify the mistletoe impact on crown degradation of silver fir trees, by measuring several parameters such as needle length and needle loss. The data were collected from a pure silver fir stand where more than 70% of trees were affected by mistletoe. The results indicate that both mistletoe infection and branch position into the crown influence the needles length. The reduction in needle length was more evident in the upper and middle part of the crown and smaller in the lower part of the crown, as well as in the “compensation crown” (formed by epicormic branches). The difference between needle lengths increases with the infection degree. In moderately and heavily infected trees, needles are falling in high proportion compared with low infected trees, demonstrating that mistletoe is producing crown degradation significantly reducing the photosynthetic tissue.