Phenological shifts of the alien colonizer Bunias orientalis: Image-based analysis of temporal niche separation

Phenological development of the colonizer Bunias orientalis L. was investigated in permanent plots in herbaceous vegetation, using photography and image analysis. In all habitats but one, B. orientalis showed a two-phased rosette growth behaviour, with a first peak cover reached in spring, roughly coinciding with an early peak cover of the matrix vegetation, and a second autumn growth flush occurring in a phase of reduced matrix vegetation cover. This seasonally bimodal growth pattern of B. orientalis appears to partly compensate for its competitive inferiority in crowded herbaceous vegetation. Small or overshadowed individuals of this species particularly profit from higher above-ground performance or release from overlap in autumn. The significance of temporal niche separation for survival and growth of B. orientalis individuals varies with habitat conditions, being most apparent in occasionally disturbed habitats with a relatively low frequency of gap formation. Despite some limitations, image analysis proved to be useful for phenological investigations within herbaceous plant stands.