A potential explanation for invasion of riparian fynbos by Acacia mearnsii in the Western Cape of South Africa
In the Western Cape of South Africa, Acacia mearnsii is an invasive alien plant species that is capable of transforming riparian forests into near monocultures. Previous studies on adult individuals suggest that A. mearnsii reduces surface water availability (Dye et al, 2001) as a result of high transpiration rates, and may be more drought tolerant than native species (Crous etal, 2012.), but these studies do not explain how A. mearnsii successfully invades. Our study builds on previous work that compared hydraulic properties of seedlings of A. mearnsii and a native species, Brabejum stellatifolium (Martinez et al, 2013). Results of this study suggested that seedlings of A. mearnsii have greater xylem specific conductivity, and consequently may out-compete B. stellatifolium for water. During January of 2014, we measured diurnal rates of photosynthesis, pre-dawn and midday water potential, and xylem specific conductivity on five pairs of A. mearnsii and B. stellatifolium seedlings on the Holsloit River to expand the scope of the previous study and address the question of whether A mearnsii outcompetes B. stellatifolium by decreasing water availability or outcompetes B. stellatifolium as a result of a higher xylem specific conductivity supporting higher rates of photosynthesis.
Midday water potentials, and indicator of water stress, did not differ significantly between A. mearnsii and B. stellatifolium (-2.58 MPa and -2.88 MPa respectively), but A mearnsii had significantly lower pre-dawn water potentials (-0.58 MPa compared to -0.34 MPa; p<0.01.) Xylem specific conductivity was significantly greater facilitating higher photosynthetic rates (6.49 umol m-2 s-1 compared to 4.35 umol m-2 s-1 for A mearnsii and B. stellatifolium respectively), and A. mearnsii maintained higher photosynthetic rates over the entire course of the day. These results agree with results of the previous study (Martinez et al, 2013), but suggest that competition between A. mearnsii and B. stellatifolium has less to do with A. mearnsii lowering water availability for B. stellatifolium, and more to do with greater xylem specific conductivity supporting higher photosynthetic rates potentially increasing growth rates. Higher seedling growth rates for A mearnsii would reduce light availability for native species and may explain why A. mearnsii is able to dominate riparian fynbos communities once it has colonized following flood or fire-- common agents of disturbance in riparian fynbos. Our results suggest Working for Water, a program designed to provide jobs to remove A. mearnsiiadults in invaded streams should expand its focus to removal of seedlings following disturbance.