Effects of light on branch growth and death vary at different organization levels of branching units in Sakhalin spruce

This study proves the existence of correlative inhibition, and demonstrates that role of light intensity in the growth and death varies among different levels of branching units. Within a tree crown, local light conditions vary within a branch and among different branches. Although the role of light intensity in the growth and survival of branches has been studied extensively, the effects of the spatial heterogeneity of light intensity on different levels of branching units are poorly understood. We investigated the effects of light intensity on the growth and death of primary branches (those branching off from the main stem) and secondary branches (those branching off from the primary branches) in the whole crown of Sakhalin spruce (Picea glehnii). The growth of shoot extension on a primary branch or secondary branch may be inhibited on sunlit trees compared to shaded trees when branches under relatively low local light intensity (rPPFD) levels, but it was increased with increasing rPPFD more rapidly on sunlit trees than on shaded trees. A relative importance analysis showed that, in the primary branches, branch growth was mainly influenced by rPPFD, and less influenced by its vertical position within the crown. However, the probability of death of a primary branch was equally influenced by its position within the crown and rPPFD. In contrast, rPPFD played a dominant role in both the growth and death of secondary branches. The results of our study suggest that local light intensity alone cannot fully explain the growth and survival of primary branches simultaneously, and the effects of light intensity vary among different levels of branching units.