Branching pattern in Cupressaceae

In Cupressaceae, new lateral shoots are not present at each node (“continuous branching”), nor are they obviously grouped in regularly spaced tiers (“rhythmic branching”). Thus, branching pattern in Cupressaceae is referred to as "diffuse", although this does not imply that the spatial arrangement of sibling shoots along an axis is completely unpredictable or random. For example, some taxa exhibit regularly alternate branched and unbranched nodes in some axis categories (such as trunk, branch, twig, twiglet), with sibling shoots arranged in a single plane. However in most cases, branching patterns seem more complex or disturbed due to background noise. In order to characterize their variability within the family, we studied different axis categories for several species and genera (Cupressus spp., Austrocedrus chilensis, Pilgerodendron uviferum and Fitzroya cupressoides). Axes were described as a set of sequences, in which index parameters were the node ranks and attached variables were the number of twigs (for a given phyllotaxis). In opposite-decussate phyllotaxy for instance, 0, 1 or a maximum of 2 sibling shoots can develop per node and their arrangement along an axis can be resumed as a succession of 0, 1 or 2 (ex : 10101101020…). Using statistical models such as variable-order Markov chains and hidden semi-Markov chains, we highlighted hidden regularities and quantified morphological observations. Along axes, we identified homogeneous branching zones defined by contrasted proportions of the number of sibling shoots per node. We also observed that each zone comprised remarkable patterns in the succession of number of sibling shoots (for instance, a mixture of 101001 and 2002). In addition, for a given species, different branching patterns were identified for different axis categories and such patterns could be ordered and related to morphogenetic gradients