Analysis of long-term dynamics of crowns of sessile oaks at the stand level by means of spatial statistics

We analysed crown dynamics at the stand level in three plots of an even-aged sessile oak stand (one unthinned reference plot and two plots of different thinning intensities) of a long-term experiment in Rhineland Palatinate, Germany. Crown projections and stem locations were recorded in 1977, 1994 and 2004. Spatial structures of stems and crowns were analysed using the index of Clark and Evans and Ripley's K-function. In the unthinned plot, despite crowns being highly asymmetric, it was shown that crowns did not expand in a way that makes their distribution more regular. In this dense plot with slender trees, the stability constraint was probably more important than the search for light. In the thinned plots, spatial distributions of both stems and crowns were found to be regular, in general with more regular distributions for crowns than for stems. The more regular spatial distributions of crowns were attributed to the effect of thinning and to the subsequent plastic reaction of the crowns of the remaining trees. The plastic reaction of crowns toward more regular distributions was stronger in the case of moderate thinning than in the case of intensive thinning. In the latter case, the lower plot density was probably less limiting for the access to light. Finally, the benefit of the plasticity of crowns in the thinned plots was quantified in terms of canopy cover at the stand level. Crown plasticity enabled a higher space occupation by the canopy and a reduction of overlaps between neighbouring crowns. The results of this study suggested that trees of a stand can adapt to their environment in order to find a trade-off between different functions like mechanical stability, access to light and the physiological cost associated with adaptive growth. (c) 2008 Elsevier B.V. All rights reserved.