Shoot fluctuating asymmetry:: a new and objective stress index in Norway spruce (Picea abies)

We showed that difference in length of two opposite second-order lateral shoots on first-order branches of Norway spruce (Picea abies (L.) Karst.) can be classified as fluctuating asymmetry (FA). The FA was higher (i) in trees from a heavily polluted site than in trees from an unpolluted site, (ii) in shaded trees from the understorey of a dense stand than in trees from an open site, and (iii) after needle clipping (simulated herbivory). This is the first study to demonstrate that FA of woody plants can be assessed by measuring skeletal elements (branches) rather than photosynthetic organs (leaves or needles). Our finding significantly improves the possibilities to study plant developmental stability, in particular by allowing fast and accurate assessment of the past stress levels by using information stored in branch architecture. We concluded that FA based on shoot measurements is a practicable objective stress index in Norway spruce.