Ecophysiological traits of invasive alien Acacia cyclops compared to co‐occuring native species in Strandveld vegetation of the Cape Floristic Region

Abstract

AbstractTree invasions of Mediterranean‐climate ecosystems pose a significant threat to both biodiversity and functioning, by excluding native species, altering soil nutrient status and depleting water resources. In order to attain greater relative biomass associated with successful invasion in these characteristically resource‐poor environments, invasive species must have novel traits that enable better acquisition (e.g. deep roots) or exploitation of different resources (e.g. N2 fixation) and/or more efficient use of available resources than native species. We compared the ecophysiological and morphological traits of three abundant native species to those of the invasive Australian tree species, Acacia cyclops. This species is widely invasive in the Mediterranean‐climate coastal vegetation of South Africa that includes the Strandveld vegetation type. A. cyclops had 30–50% greater foliar N concentrations (P < 0.001) in comparison with the native species and lower foliar δ15N values that may indicate N2 fixation. Additionally, A. cyclops maintained higher photosynthetic rates over the dry summer season (ca. 15 μmol m−2 s−1) than the native species. These higher photosynthetic rates may result from sustained access to water due to deeper rooting abilities as indicated by the more negative δD values (P < 0.001) of A. cyclops (−43‰) in comparison with the some native species (−29 to −37‰). Acacia cyclops did not, however, exhibit greater water use efficiencies or photosynthetic nitrogen use efficiencies (P > 0.05) compared to native species. Invasiveness of A. cyclops into this resource‐limited Mediterranean‐climate ecosystem appears to be supported by greater resource acquisition, possibly partially through N2 fixation and greater rooting depth, rather than greater resource use efficiency or conservation.