The effects of vegetation development upon achene production in the woody weed, groundsel bush (Baccharis halimifolia L.)

Abstract

An examination of the achene production of groundsel bush (Baccharis halimifolia L.) populations growing within two exotic softwood stands of differing ages revealed that substantial reductions in this variable occurred following canopy closure. Mean reproductive efficiency, estimated as the number of achenes per gram dry weight of understorey bushes within the older stand, was only 31% of the corresponding value for bushes growing within a stand prior to the occurrence of canopy closure. Only small differences were found in both cumulative germination percentage and germination rate of achenes collected from each population.

Female bushes, when shaded to a level of 14% open sky radiation, demonstrated a 30% reduction in the number of flowers initiated per head and a 25% reduction in percentage achene fill. Floral initiation was found to be independent of the production of oil reserves.

Whilst dramatic reductions in the achene production of understorey individuals occur with plantation development, it is suggested that the density of unmanipulated populations of B. halimifolia may prevent the invasion and development of dominance by native tree species.