Tree crowns as meeting points of diversity generating mechanisms - a test with epiphytic lichens in a temperate forest
Forest canopies are hotspots of biodiversity even in temperate forests but which and how many ecological mechanisms contribute to the high diversity remains elusive. This biodiversity is not distributed evenly throughout the complex fractal structures formed by individual tree crowns. They are non-stationary, constantly expose new surface habitat via growth, and create contrasting abiotic conditions. These features give rise to a range of vertical gradients in habitat optimality, heterogeneity, available surface area and time for succession - all known to be mechanisms shaping diversity patterns. Using a canopy crane facility and epiphytic lichens on Fraxinus excelsior and Quercus robur as model system, we aim to assess the relative importance as well as the interplay of these mechanisms in shaping biodiversity patterns within tree canopies by detecting their distinct mechanistic fingerprints. Lichen species richness exhibited a hump-shaped vertical pattern, skewed towards the top of the crown. This pattern was observable at both the level of individual plots and that of aggregate height layers and it was correlated with lichen cover. Also, a vertical gradient in species composition was found and could be related to species traits known to reflect successional niches such as dispersal mode and growth form. Habitat heterogeneity and available surface area have been found to have little effect on vertical lichen diversity patterns. We conclude that the vertical lichen diversity patterns in the tree crown are mainly shaped by the successional accumulation of species along a branch age gradient and a pronounced vertical gradient in environmental optimality from harshly exposed young branches at the top crown over suitable habitats with a balance in light and humidity towards the limiting light conditions in the dim understory. At the level of the whole canopy, successional and environmental niche dynamics jointly operate to generate lichen diversity.