The Effects of Morphology, Orientation and Position of Grass Diaspores on Seedling Survival

Abstract

(1) The orientation and position of 2193 germinating diaspores of eleven species of grasses were recorded as a result of observations in an open Eucalyptus forest over a period of 5 years. During three of these years further data were collected on the life histories of 1269 seedlings of nine of these species. (2) Amongst the species observed three gross structural forms of diaspore (actively-awned, passively-awned, and unawned) were recognized; each represented a distinct strategy for the recruitment of seedling populations. (3) While only sixteen seedlings of the actively-awned species Dichelachne micrantha and Themeda australis were observed in the area, all germinated from diaspores buried beneath the soil surface and only in situations where the soil was not surface crusting. These observations support the view that movements of the active awn propel the diaspore into cracks in the soil surface. (4) Seedlings of passively-awned species arose either from diaspores that were found in a standing orientation with their callused ends anchored in the soil or from diaspores that lay unanchored and almost horizontal on the soil surface. Germinating unawned diaspores were either lying on the surface or buried. (5) Anchorage of a diaspore is necessary for seedling survival. Spreading passive awns serve to rotate a falling diaspore into a position that increases its chances of landing in a standing orientation with the embedded antrosely-bristled callus providing the necessary anchorage. (6) Unawned diaspores achieve anchorages as a result of passive burial and it is postulated that dormancy delays the germination of many of these until effective burial is achieved. (7) Following a forest fire, a three-fold increase in the density of grass seedlings was recorded and the relative abundance of seedlings from unappendaged diaspores rose dramatically. (8) Since the morphology of appendaged diaspores promotes their germination on or near the soil surface, many such diaspores are destroyed by fire. In contrast, burial of unappendaged diaspores not only achieves anchorage but protects the seed from the extreme temperatures generated in a fire. Moreover, it is suggested that fire may release many of these seeds from dormancy.