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.