Abstract
Invasion of ecosystems by woody alien plant species is a widespread phenomenon. Interspecific competition has often been suggested as a mechanism for replacement of one species by another, but this is rarely tested. We investigated the potential of an invasive alien tree to transform vegetation by quantifying the relative abilities of the alien tree Schinus molle and dominant native trees Acacia tortilis and Rhus lancea to compete for light when growing in association within a South African semi-arid savanna. Due to dispersal of its fruits by birds, seedlings of S. molle establish under tree canopies. Using canopy symmetry as an index of ability to compete for light, we found that the alien S. molle consistently out-competes the dominant native tree species. The results also show that pod production of A. tortilis was higher when it grew alone compared to when it grew with S. molle or R. lancea. The percentage of dead branches was higher on A. tortilis trees growing in association with the S. molle. The outcome is that the alien tree will gradually increase in abundance, changing woodland structure and ecosystem processes. Our findings provide evidence for the role of competition in the process of alien plant invasions. We suggest that S. molle, previously considered a benign naturalized species in South Africa, should be declared a noxious weed in some parts of that country.
Highlights
Invasive alien plants have caused substantial changes to the structure and functioning of ecosystems in many parts of the world
The results showed that the mean coefficient of tree symmetry (TS) of S. molle was significantly higher than for both A. tortilis (F1,244 1⁄4 17.999; po0.0001; Fig. 3a) and R. lancea (F1,94 1⁄4 7.284; po0.005; Fig. 3b), indicating that S. molle is causing asymmetrical canopy growth in both those species
No difference was found between A. tortilis growing alone and when it grew with R. lancea (F1,149; p 1⁄4 0.533; Fig. 3e)
Summary
Invasive alien plants (sensu Richardson et al, 2000) have caused substantial changes to the structure and functioning of ecosystems in many parts of the world. By replacing native species and altering vegetation structure, plant invasions potentially drive changes in community pattern, soil nutrient status, species interactions, ecosystems services and disturbance regimes (Hobbs and Mooney, 1986; Richardson and van Wilgen, 2004; Traveset and Richardson, 2006). Spatial patterns in plant communities have been attributed to a range of processes, including interactions between individuals of different plant species, localized dispersal abilities, and disturbance regimes. The effects of the invaders on disturbance regimes or other ecosystem processes are generally much more dramatic than any effect attributable to competitive interactions among individuals Competitive interactions are crucial in some cases and need to be better understood to facilitate effective management strategies
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