Commercial Pine Invasion of South African Fynbos: A Belowground Perspective

Abstract

The Fynbos biome of South Africa is an exceptionally biodiverse, nutrient-poor environment. Commercial pine plantations have permeated South Africa’s Western Cape for over a century and many Pinus species are classified as invasive in the region. Their invasion has caused notable harm to Fynbos flora on local and regional scales, in addition to affecting ecosystem-level processes such as promoting hotter and more frequent fires and exacerbating water scarcity. Previous research surrounding this invasion has primarily focused on aboveground explanations for the competitive exclusion of native Fybnos species by pines, but this study aims to illuminate the belowground perspective. Here, the effects of Pinus radiata on various belowground competitive strategies of two native Fynbos species, Ficinia nigrescens and Phylica spicata are examined. Samples of these two native species were collected both in the presence and absence of P. radiata to compare soil mineralization rates, plant root architecture, identity and diversity of microbial communities, and status of mycorrhizal associations. The invasive presence of Pinus radiata in the Fynbos biome appears to alter conditions of soil nitrogen mineralization and competition due to its obligate association with ectomycorrhizal fungi. Competitive root strategies of Ficinia nigrescens and Phylica spicata did not change in the presence of P. radiata, suggesting a limitation of local plasticity in root traits. Rates of arbuscular mycorrhizal colonization differed in the presence of P. radiata, likely attributable to the degraded mutualism hypothesis. Nutrient acquisition strategies of Fynbos flora that were previously adaptive to resist local invasion may no longer be advantageous in the presence of pines.