Invasiveness Of Two Exotic Tree Species: Acacia Mearnsii De Wilde And Acacia Dealbata Link

Regimes of regeneration and dispersal, mating system and population structure of Acacia mearnsii De Wild. and Acacia dealbata Link. and their impacts on local vegetation were studied to assess their invasiveness. The regimes of regeneration and dispersal were studied through investigating soil seed bank, seedling/sprout spread and seed production of individual trees. The results showed that A. mearnsii and A. dealbata were both prolific in seed production. The former mainly reproduced sexually, while the latter sprouted heavily with high survival rate. Soil seed bank density of the two species was 28,882 seedsï¹’m^-2 and 1,475 seedsï¹’m^-2, respectively, indicating great potential for regeneration. However, surviving seedlings/sprouts were much fewer. At the edge of A. mearnsii and A. dealbata plantations, on everage there were 0.8 seedlingsï¹’m^-2 and 1.0 sproutsï¹’m^-2, spreading 1.67 mï¹’year^-1 and 1.04 mï¹’year^-1, respectively. And around the mother trees, on average there were 1.3 seedlingsï¹’m-2 and 0.6 spoutsï¹’m^-2, spreading 1.51 mï¹’year^-1 and 0.85 mï¹’year^-1, respectively. The above data indicated that the actual spread of the two species was not significant. The distance of seed dispersal in soil seed bank was approximately twice as the height of the mother trees and the distance of seedlings was about 1.5 times of the tree-height. Mating system and population structure of 3 A. mearnsii populations differing in origin and management were analyzed using isozyme markers. The results showed that there were high genetic variation within populations and no differentiation among populations. Multi-locus estimator of the outcrossing rate varied from 0.992 to 0.905, indicating viable adaptation to the environment. The species diversity, richness and evenness of 6 types of A. mearnsii and A. dealbata plantation communities were investigated to evaluate how and to what extent they affect local biodiversity. Shannon-Wiener index varied from 1.0939 to 2.7578, which was higher than local Pinus yunnanensis plantations except for pure plantation of A. mearnsii forest and A. mearnsii-Camellia oleifera, indicating that A. mearnsii and A. dealbata mixed with local tree species have not resulted in reduction of under-story species diversity. Assessment of invasion potential of the two species found their invasiveness was moderate, indicating that attention should be given to site selection and plantations. In short, A. mearnsii and A. dealbata were two popular multipurpose tree species in the subtropics of China, but caution should be taken in introduction and planting of these two species to avoid possible risk of biological invasion.