Non-native trees alter biomass and nutrient availability in mountain streams of the Cape Floristic Region, a global biodiversity hotspot

Abstract

The invasion by alien trees of riparian zones may lead to significant alterations in the amount and timing of leaf litter inputs to freshwater ecosystems, consequently affecting stream ecological functioning. Invasive Australian Acacia tree species are among the most harmful of invading species in the Cape Floristic Region (CFR), a global biodiversity hotspot [1]. Blackwattle (Acacia mearnsii) are leguminous nitrogen fixers, making significant contributions to available biomass and nitrogen in their new habitats [2]. To-date the amount, nor the timing of additions of biomass and nutrient are known. This study assessed patterns of leaf litterfall, Biological Nitrogen Fixation and nutrient resorption (N and P) capacities of A. mearnsii and two native co-occurring species in two perennial streams in the CFR. Instream leaf litter decomposition and macroinvertebrate herbivory experiments were conducted. The results indicate significant riparian zone effects in terms of biomass and nutrient input. The annual leaf litterfall of A. mearnsii in riparian zones was nearly nine times greater compared to native vegetation, and A. mearnsii leaves were dropped twice a year, in mid-summer and mid-autumn, whereas natives dropped their leaves only once. Acacia mearnsii also retained significantly higher N concentrations in leaf litter than the native species throughout the year. The co-occurring native species were efficient at resorption of nutrients, on par with global averages. In contrast, A. mearnsii N resorption was low. In stream, invasive species leaf litter decomposed at a significantly faster rate than that of fynbos species, which was explained by differences in litter quality between species. This suggests that the structurally simpler and higher nutrient containing A. mearnsii leaf litter was preferred by macroinvertebrates. This study highlights the potential of a single alien plant species to disrupt multiple ecosystems, and provides insight to the ability of invaders to disrupt vulnerable Mediterranean-type ecosystems by creating their own environment.

[1] Richardson, D.M., Holmes, P.M., Esler, K.J., Galatowitsch, S.M., Stromberg, J.C., Kirkman, S.P., Pysˇek, P. and Hobbs, R.J. (2007) Riparian vegetation: degradation, alien plant invasions and restoration prospects. Diversity and Distributions, (13), 126-139.

[2] Morris, T. L., Esler, K. J., Barger, N. N., Jacobs, S. M. and Cramer, M. D. (2011) Ecophysiological traits associated with the competitive ability of invasive Australian acacias. Diversity and Distributions, 17(5), 898-910.