Forest tree health may be assessed using crown transparency, an estimate of foliage density regarded as fast and easy indicator suitable for long-term observation. Crown transparency mirrors tree stress that can be caused by different factors like: sudden clearing, nutrient deficiency, parasite infestation, root rot, fructification, extreme meteorological conditions and air pollution. Most often, several factors act simultaneously. Trees with a crown transparency of more than 25 percent are regarded as stressed.
Variable crown transparency in spruce trees
Over the past decades periods of increased crown transparency were observed for both beech and spruce. They can mainly be explained by meteorological extremes and, in beech, fructification and ozone. For the three oak species, temporal changes in crown transparency are uniform with slightly higher crown defoliation for downy oak compared to pedunculate and sessile oak. A tree with a transparent crown has fewer leaves to assimilate carbon dioxide and produce carbohydrates; therefore, its photosynthetic capacity and growth are reduced.
Percentage trees with a crown transparency higher than 25% based on a constant number of monitored trees.
Left: Beech, spruce and oak since 1984.
Right: Beech, spruce and the three oak species since 2009.