Cross-sectional compression wood distribution and its relation to eccentric radial growth in Picea abies [L.] Karst

The formations of compression wood and eccentric radial increment are two phenomena that are so closely associated that they cannot be readily discussed separately. However, various studies suggest that their formation is stimulated by different mechanisms and that there is no functional relationship. While the formation of compression wood is a reaction of coniferous trees to negative gravitroptic scenarios which enables the trees to recover from displacement, one-sided radial growth promotion is stimulated by unilateral mechanical strain on the cambium for lowering stress peaks. This study offers an approach to quantify the relation of both phenomena in stem cross-sections and single growth rings.Samples of 56 Norway spruces (Picea abies [L.] Karst.) from sites differing in slope and exposure to the prevailing wind direction were examined for their cross-sectional compression wood distribution and eccentricity in radial increment. The assessment of both phenomena on stem cross-sections was based on their spectral properties in reflected light. Hyperspectral image analysis allowed detection of compression wood distribution as well as measurement of annual radial increment along eight radii, oriented by compass direction.Since the distributions of both phenomena comprise both a directional component as well as amplitude in annual reference, these data were handled as vectors. Circular statistics were applied to describe and quantify their relationship and to test both phenomena for the inducing environmental factor that stressed the tree.The direction of compression wood formation and eccentric growth is significantly positively correlated. In the majority of observations a unimodal distribution of both phenomena is given. Mostly strong one-sided compression wood formation is associated with a coincident direction of eccentric radial increment. However, stem cross-sections with a distinct direction of compression wood were observed without a dominating direction of eccentric growth, and vice versa.Bivariate statistics that consider vector direction as well as vector length have proved to be very helpful in retrospectively identifying possible initiating environmental factors for both phenomena at a given site. The location of standard and confidence ellipses in relation to the assumed direction of action is a strong indication for or against an assumed environmental factor.