Allelochemic control of biomass allocation in interacting shrub species.

Aqueous leachates derived from canopy phyllodes of invasiveAcacia cyclops affected growth of a range of shrub species independently of nutrient input effects. All plants showed a sublethal phytotoxic response. Root mass was generally less adversely affected than shoot mass and, while decreasing significantly in response to the 10% concentration, showed no such response to the 1% solution. Root-shoot biomass ratios increased, except inEuphorbia burmannii, which may recognize intrinsic root architecture limitations on extensive exploitation of toxin-free soil. Application of surface plant litter from underA. cyclops canopies stimulated the production of basal stems inProtasparagus capensis andEriocephalus racemosus but was insufficient to significantly reduce root-shoot ratios. Plant growth inhibition was maximized by canopy leachate compounded by surface litter effects inAnthospermum spathulatum. The net effect of leachate at high concentration on biomass allocation in certain shrub species may help explain their patterns of association and disassociation withA. cyclops.