Leaf litter production and litter nutrient dynamics of invasive Acacia mearnsii and native tree species in riparian forests of the Fynbos biome, South Africa

Abstract

Alien invasive Australian tree species are most prevalent in the Cape Floristic Region (CFR), a global biodiversity hotspot. The plants pose a serious threat as they have replaced indigenous riparian forest vegetation, including many native tree species such as Brabejum stellatifolium and Metrosideros angustifolia along many watercourses. Black wattle (Acacia mearnsii) is a leguminous nitrogen fixer which makes significant contributions to available nitrogen in its new habitats in the fynbos, which is generally low in nutrients. However, the magnitude and the timing of litter associated nutrient input into riparian forests and woodlands and adjacent aquatic environments are not known. In this study, we assessed patterns of leaf litterfall, nutrient return to soil and resorption (N and P) of A. mearnsii and two co-occurring native species in two perennial streams in the Mediterranean-type Fynbos biome of the CFR. Leaf litterfall of A. mearnsii in the riparian forests was significantly greater compared to native vegetation, with two peaks in litterfall, one in mid-autumn, and the other in mid-summer at both streams. In contrast, the native vegetation only dropped leaves in the summer month of December. Acacia mearnsii also returned substantially more nutrients (N and P) during the peak leaf litterfall period (December) than co-occurring native species. However, the co-occurring native species were more efficient in the resorption of nutrients (N and P), with average resorption higher than the global averages of 52% for N and 50% for P. In contrast, A. mearnsii was relatively inefficient in resorbing N (30% mean N resorption) but was more efficient in resorbing P (76% mean P resorption). In addition to fixing N, it is possible that the A. mearnsii stands rely on the return of nutrient-rich leaf litter to soils and through specialized root systems and root symbionts assisting reuse of nutrients. The results highlight the differences in the nutritional economy between native riparian forest species and invasive A. mearnsii. Our findings provide further insight into the success of invasive leguminous woody tree species in nutrient-poor Mediterranean riparian forests.