Asymmetric tree growth at the stand level: Random crown patterns and the response to slope

Asymmetric tree growth is an adaptation to maximise photosynthesis by growing in response to gaps and neighbours, topographical site conditions or incoming solar radiation. Whereas spatial statistics have been widely used to study the distribution of trunk locations, less research has been undertaken to analyse the distribution of crown centres and asymmetric growth at the stand level. It is generally assumed that trees optimise light harvesting via more regular crown patterns. In this study, we primarily ask whether random crown patterns can be found in deciduous and coniferous forests located in continental Europe. Here, we analysed the spatial patterns of trunks, crowns and crowns of overstory trees in different deciduous and coniferous stands, using the scale-dependent g-function and Monte Carlo simulations. We also tested whether the extent of asymmetric growth, that is the crown vector length between the stem-base position and the centroid of the projected crown area, would be greater in angiosperms than in gymnosperms. Finally, we applied circular statistics to test whether trees preferentially bend in slope direction or towards incoming solar radiation. In the deciduous stands, patterns of crowns and upper crowns were random. Response to large-scale heterogeneity in light was strong, because trees bent significantly in downward direction of the slopes. The extent of asymmetric growth was significantly greater in angiosperms than in gymnosperms. The patterns of crowns and upper crowns were regular in a mixed coniferous stand but random in a dense stand with regularly planted Douglas-fir. Mechanical instability caused mutual crown support and attraction between the crowns in this dense stand. The even-aged, slender Douglas-fir clustered significantly in downward direction of the slope. In none of the four stands, trees clustered in southerly direction towards incoming solar radiation.