Spatial distribution and morphology of shoots in the variant crown form of Rhododendron reticulatum

Abstract

Rhododendron reticulatum D. Don ex G.Don is a common understory shrub in western Japan that exhibits highly plastic crown architecture and occurs in various light environments. I investigated how functional differentiation and spatial distribution of long and short shoots contribute to the plasticity of crown architecture of R. reticulatum. Crown form was derived from the crown depth/width ratio. Crown depth/width ratio was higher in sun-lit crowns. In crowns with higher depth/width ratio, long shoots were distributed in upper positions of the crown. Long shoots grew vertically and horizontally. In crowns with lower depth/width ratio, long shoots were arranged in the outer position of the crown and grew outward. Within neighboring shoots, long shoots had less mass than their paired short shoots. Results suggest that long shoots of R. reticulatum function to expand the crown and to reduce leaf overlap in multilayer crowns, which are found in high-light environments, while both long and short shoots function to minimize leaf overlap in monolayer crowns, which are found in shaded environments. Plasticity of crown architecture by altering shoot position and shoot morphology allows growth under various light environments in the forest understory.