Data from: Crown asymmetry in high latitude forests: disentangling the directional effects of tree competition and solar radiation

Light foraging by trees is a fundamental process shaping forest communities. In heterogeneous light environments this behavior is expressed as plasticity of tree growth and the development of structural asymmetries. We studied the relative influence of neighborhood structure and directional solar radiation on horizontal asymmetry of tree crowns in late-successional high latitude (67-68°N) forests in northern Fennoscandia. We described crown asymmetries as crown vectors (i.e. horizontal vectors from stem center to crown center), which we obtained from canopy maps based on crown perimeter measurements in the field. To disentangle the influence of the two main determinants, inter-tree competition and directionality of above-canopy solar radiation at high latitudes, we applied circular statistical models, utilizing cylindrical distributions, to these data consisting of orientations and intensities of crown asymmetry. At the individual tree level, our model predicted crown asymmetry vectors from the current stand structure, and the predictions became better when the intensity of asymmetry (i.e. crown vector length) was higher. Competition was the main determinant of crown asymmetry for 2/3 of trees, and the model predictions improved when we incorporated the directionality of solar radiation. At the stand-level, these asymmetries had resulted in a small increment of the projected canopy area and an increased regularity of spatial structure. Our circular statistical modelling approach provided a quantitative evaluation of the relative importance of directionality of solar radiation and neighborhood stand structure, showing how both of these factors play a role in formation of crown asymmetries in high latitude forests. This approach further demonstrated the applicability of circular statistical modeling in ecological studies where the response variable has both orientation and intensity.

林木光觅行为是塑造森林群落的基础生态学过程。在异质光照环境中，该行为体现为林木生长的可塑性与结构不对称性的形成。我们以芬诺斯坎底亚北部的高纬度（北纬67°~68°）演替后期森林为研究对象，探究邻体结构与定向太阳辐射对林木冠幅水平不对称性的相对影响。我们将冠幅不对称性表征为冠幅向量（crown vectors，即从树干中心到冠幅中心的水平向量），该向量通过野外冠幅周长测量得到的冠层地图提取获得。为厘清两大核心驱动因子——林木邻体竞争与高纬度冠层上方定向太阳辐射——的影响，我们针对涵盖冠幅不对称性方向与强度特征的数据集，采用基于圆柱分布（cylindrical distributions）的圆形统计模型（circular statistical models）进行分析。在单木尺度上，我们的模型可基于当前林分结构预测冠幅不对称向量，且当不对称性强度（即冠幅向量长度）越高时，模型预测精度越高。三分之二的林木，其冠幅不对称性的核心驱动因子为邻体竞争；当纳入太阳辐射方向因子后，模型预测精度进一步提升。在林分尺度上，此类冠幅不对称性导致冠层投影面积小幅增加，同时提升了林分空间结构的规整性。我们所采用的圆形统计建模方法，实现了对太阳辐射方向性与林分邻体结构相对重要性的定量评估，阐明了二者在高纬度森林冠幅不对称性形成过程中的作用机制。该方法进一步验证了圆形统计建模在响应变量同时包含方向与强度特征的生态学研究中的适用性。