Seasonal Direct Light Availability Affects Mean Leaf Orientation in a Herbaceous Multi-Species Canopy

Abstract: Relationship between leaf orientation in natural or semi-natural herbaceous communities and the local abundance of direct light has rarely been investigated. We present evidence from a permanent plot experiment that seasonal direct light availability in 40 × 40 cm plots significantly affects mean leaf orientation in a semi-natural wooded meadow. In sunny years and in well-illuminated conditions, the leaf surface is exposed predominantly to the north, and vice versa . Two groups of species are responsible for such a response: of the variation in leaf orientation, 41 % is explainable through direct light availability for light-demanding species, and 26 % for graminoid species. In shade-tolerant species and in forbs there is no sign of a regression between leaf orientation and direct light abundance. Ability to plastically react to variation in seasonal and local direct light availability appears to be a trait under considerable selective pressure only in light-demanding plants, and more characteristic of species with graminoid growth form.