On the relationship of phenological strategy to ecological success: the case of broomsedge in Hawaii

Abstract

Andropogon virginicus L. (broomsedge) is a perennial bunchgrass of temperate origin which was introduced to the Hawaiian Islands early in the 20th Century. Since its introduction broomsedge has become the dominant species of savanna vegetation on the windward sides of Oahu and Hawaii (the Big Island). Despite its ecological success, the phenology of broomsedge-especially the timing of its periods of active growth in the summer and dormancy (or semi-rest) in the winter-is not synchronized with the summer-dry, winter-wet moisture seasonality of its range in Hawaii. Rather, this species responds to a shortening-daylength cue by flowering in late autumn, and the onset of dormancy and the progression of subsequent phenophases seems to be mostly under endogenous control following the flowering response. The lack of synchronization between the onset of active growth by broomsedge and the onset of the rainy season does not conform to the specific assumption that the ecological dominance of the perennial bunchgrass life-form in savanna vegetation results from a coupling of phenological strategy to moisture seasonality. It also brings into question the more general assumption that a perfect coupling of phenology to the prevailing climatic conditions is a prerequisite for ecological success.