LAUSR

Abstract Invasion by invasive alien trees is a major threat to the biodiversity of the fynbos biome in South Africa. The environmental factors influencing the intensity of invasion are, however, largely unknown. In particular, the effects of soil nutrients on the invasion of alien trees in the fynbos biome have not been investigated. The Catabolic Theory predicts that trees will be more competitive than shrubs and herbaceous plants where anabolic nutrients (e.g. Mg, Mn and B) are comparatively more abundant than catabolic nutrients (e.g. P, Cu and Zn). We tested this theory at seven study sites by comparing soil nutrient concentrations in non-invaded and adjacent invaded sub-sites. Soil samples from 275 plots across 17 sub-sites were analysed for pH, Na, Mg, K, Ca, P, S, C, B, Mn, Cu and Zn. The results corroborated the Catabolic Theory. An index of anabolic to catabolic nutrients was greater in all ten sub-sites invaded by invasive alien trees than in sub-sites without invasion. Nutrients strongly related to tree invasion included B and P. As per the Catabolic Theory, poverty of B at certain sites is likely to constrain anabolism and consequently tree establishment. Some non-invaded sites had B concentrations of 0.02 mg kg−1, which is an order of magnitude less than critical deficiency thresholds in an agronomic context. By contrast, richness of P (as a result of prior fertilization) in fallow agricultural fields would, as per the Catabolic Theory, promote catabolism and the competitive strength of shrubs and/or herbaceous plants relative to trees. Practical implications of these findings include: 1) the mapping of soil properties such as B and P to reveal the inherent propensity of landscapes in the Western Cape to be invaded by trees; and 2) the application of B-binding substances such as illite and vermiculite to constrain invasion of trees in fynbos.