Seasonal variations in nutrients and secondary metabolites in semi-arid savannas depend on year and species

Abstract

Semi-arid savannas are characterised by alternating wet and dry seasons and large inter-annual rainfall fluctuations that affect plant growth. Carbon-based secondary metabolites (CBSMs) vary inversely with growth and nutrients because of the physiological trade-off between cellular growth and differentiation. We predicted that (1) nutrient concentrations decrease during the wet season, (2) CBSM concentrations increase during the wet season and (3) nutrient concentrations are lowest and CBSM concentrations are highest in the dry season. We measured nitrogen, phosphorus, total polyphenols and condensed tannins in six woody species (including one evergreen) seasonally at the Nkuhlu exclosure, Kruger National Park, South Africa, for three consecutive years, including one ‘wet’ year (above-average rainfall) and two ‘dry’ years (below-average rainfall). Neither N nor P consistently decreased during wet seasons, while CBSMs did not consistently increase. Neither N nor P in the evergreen species was consistently lowest in dry seasons, while CBSMs were not consistently highest in dry seasons. We discuss the inconsistent responses in the context of species-specificity and high inter-annual rainfall variation. We conclude that seasonal variations in N, P and CBSMs in semi-arid savannas cannot be easily generalised because they depend on species and year