A comparison of drought tolerance between seedlings of a native and an exotic invasive species typical of Fynbos riparian habitats

Background/Question/Methods

Exotic invasive plant species in riparian habitats in the Fynbos in the Western Cape of South Africa have reduced streamflow, compromised habitat quality for other species, and outcompeted native species.  What is not clear is why the most problematic invasive species are able to outcompete native species and spread.  Previous studies in the riparian Fynbos of the Western Cape suggest that one of the most invasive exotic species, Acacia mearnsii may be more drought tolerant than native species, and that may contribute to its invasive potential.  However, this study examined attributes of adults of native species and A. mearnsii, yet competition during recruitment ultimately determines survival.  We were interested in determining whether observed differences in drought tolerance in adults would also be observed in seedlings.  We compared drought tolerance of co-occurring juvenile individuals of A. mearnsii and a native species, Brabejum. stellatifolium, on the Holsloot River near Rawsonville in the Western Cape of South Africa   We expected to find that A. mearnsii seedlings would have lower P₅₀ values; these values indicate the xylem water potential at 50% loss of hydraulic conductivity, and similar water potentials.  Vulnerability curves were determined for 6 individuals of each species, and P₅₀ values were calculated.  In addition, P₁₀₀ values were calculated, and mid-day and pre-dawn water potentials were measured. 

Results/Conclusions

Mean values of P₅₀ for B. stellatifolium and A. mearnsii juveniles were, -1.3 MPa and -1.5MPa respectively.  Acacia mearnsii juveniles had lower pre-dawn water potentials than B. stellatifolium, -0.69MPa and -0.26Mpa respectively and slightly lower mid-day water potentials,-2.45MPa for A. mearnsii, and -2.04 MPa for B. stellatifolium.  These results suggest that seedlings of A. mearnsii are subjected to increased water stress relative to B stellatifolium, yet do not exhibit increased adaptation to drought as indicated by the P₅₀ values of the seedlings.  Mid-day water potentials of both species were more negative than P₅₀ values suggesting both species are maximizing stomatal conductance at the expense of vulnerability to catastrophic embolism.  The results of this study suggest that both A. mearnsii and B. stellatifolium may become more conservative in water use over time since previous studies suggest adult individuals never experience mid-day water potentials more negative than P₅₀ values.  Our results suggest the invasive potential of the exotic A. mearnsii is not linked to increased drought tolerance relative to native species in the seedling stage.