Differential patterns of nitrogen nutrition and growth cost of the indigenous Vachellia sieberiana and the introduced Chromolaena odorata in the savannah environment

Vachellia sieberiana is a Fynbos legume that is highly adapted to nutrient-poor savannah ecosystems and can withstand competition from invasive shrubs like Chromolaena odorata by utilizing both atmospheric and soil nitrogen sources. The shift in the use of N resources is driven by soil-borne symbionts to the plant in the Family Rhizobiaceae, which enhance below-ground allocation to nodules and make them more efficient at fixing biological N. In this way, V. sieberiana seedlings subjected to competition have C growth costs when integrated over the whole plant growth cycle compared with V. sieberiana seedlings growing with no competition, and allocate more biomass to the below-ground structures that let them survive in impoverished soils.