A tree of many faces: Why are there different crown types in Norway spruce (Picea abies [L.] Karst.)?

Norway spruce (Picea abies [L.] Karst.) has a pronounced ability to create different crown types embracing strongly hypotonic, epi-hypotonic, strongly amphitonic types as well as respective intermediate ones. Data of Holzer and Schultze (1987) were reanalyzed in order to identify major environmental components that may shape ecotypes and contribute to Gruber's (1989) hypothesis that phenotypic plasticity is different among different crown types. Environmental variables and crown types were assessed by Principal Components Analysis. The first principal component explained 74% of the variation that was mainly loaded by different temperature variables and altitude while the second principal component explained additional 23% mainly loaded by precipitation variables. Orientation had a statistically significant but small effect. Covariance analysis demonstrated that age had modified crown type in a way that more hypotonic types were phenotypically more variable. Overlaps between crown-type distributions were evaluated by Schoener's Index, which may range from 'zero' (no ecotypic overlap) to 'one' (complete ecotypic overlap). In the present paper this index resulted in pairwise values varying from 0.21 to 0.86. The ecotypic overlap matrix was symmetric, i.e. ecotypic pairs increased gradually with stepwise crown-type graduation. We discussed the adaptation strategy of Norway spruce based on our results and propose that adaptation in this species regarding crown architecture is mainly caused by adaptive differentiation in higher altitudes while in lower elevations phenotypic plasticity is the dominating factor.