Invasion of coastal Acacia communities by Chrysanthemoides

Abstract

Many of the distributed coastal sand-dune communities in New South Wales are being invaded by the South Africa shrub, Chrysanthemoides monilifera ssp. Rotundata (boneseed, bitou bush). In some stands, this has resulted in displacement of the previous dominant, Acacia longifolia var. Sophorae (coastal wattle). In South Africa, closely related Acacia ssp. Are invasive, so a study of both Chrysanthemoides and Acacia provided a means of better understanding the invasion process. The main factors in the success of Chrysanthemoides in Australia are flowering mainly from autumn to spring, a yearly output of over 4000 seeds m-2 , efficient dispersal particularly by birds, and low predation. Acacia flowers only in spring and produces c. 100 seeds m-2, which are poorly dispersed and highly predated. Although remaining Acacia seeds have greater longevity in the soil than Chrysanthemoides, in established stands there are c. 60 times more viable Chryssanthemoides seeds in the soil than Acacia. By contrast in South Africa lower predation of the Australian species in up to 50 times more Acacia seeds than of the native Chrysanthemoides. In unburnt areas in Australia there were 500 times more seedlings of Chrysanthemoides than Acacia. Differences in seedling strategies were apparent in that Chrysanthemoides was able to avoid water stress to some extent by rapid root development and early closure of stomates as leaf water potential dropped. By contrast, Acacia was more typical of Australian sclerophylls and was able to tolerate lower leaf water potentials and under even severe water stress had low mortality except in mixtures where the greater transpiration of Chrysanthemoides resulted in similar mortality of both species. Chrysanthemoides outcompeted Acacia in terms of biomass when well-watered but not under water stress. Under controlled conditions, the potential invasiveness of Acacia was demonstrated in its having the higher rate of CO2 assimilation per unit leaf area but both species had similar rates in the field and Chrysanthemoides had the greater leaf area. Regenerative strategies of the two species after fire also differed. In unburnt areas there was a bank of persistent, slow growing seedlings of Chrysanthemoides but very few seedlings of Acacia or other native species. Fire killed adult acacias but anhanced germination which resulted in a 13-fold increase in subsequent seedling density. By contrast, 26% of adult Chrysanthemoides resprouted and while 30% of seeds were killed subsequent Chrysanthemoides seedlings still outnumbered those of Acacia by some 20 times. Seedlings of both species responded by greater growth rates in burnt areas but Chrysanthemoides was more precocious in that flowering and seedling occurred within 12 months of emergence. Measures to control Chrysanthemoides by a program of double-burning were successful in limiting resprouting to 5% or less of plants but a problem still remained of seedlings which emerged from deeply buried seeds. Thus other measures such as biological control for which there is ample potential need to be assessed.