Competition between the indigenous legume Vachellia sieberiana and the introduced Chromolaena odorata in the savannah environment

The Leguminosae is the third largest plant family, ranking behind only the Asteraceae and Orchidaceae, and is by far the most widespread group of plants, not present only in Antarctica and the Falkland Islands. Legumes are ecologically important because of their ability to fix atmospheric N, thus regulating the input of this nutrient into ecosystems. Vachellia sieberiana is a Fynbos legume that is highly adapted to nutrient-poor savannah ecosystems. However, the presence of other species with similar nutrition requirements can promote changes in the legume’s preferences for N, including shifts from biological nitrogen fixation to the use of soil N. The noxious weed Chromolaena odorata has invaded natural ecosystems and agricultural land across South Africa due to it fast growth rates and enhanced nutrient acquisition rates. Therefore, this species is likely to affect the establishment and growth of native legumes such as V. sieberiana, particularly in phosphorus deficient soils. Yet no research has been conducted so far to understand how the presence of C. odorata affects indigenous legumes in the nutrient impoverished soils of the savanna ecosystems of southern Africa.

Root systems of V. sieberiana tree seedlings showing prominent root nodues. Image credit: A. Magadlela

In a recent study published in AoBP, Ndzwanana et al. address this gap in knowledge by investigating competition between C. odorata and V. sieberiana in nutrient-poor savannah soils and how the presence of the former modifies the patterns of growth of the latter.  It was found that V. sieberiana can withstand competition from invasive shrubs like C. odorata by utilizing both atmospheric and soil nitrogen sources. This shift in N acquisition strategy is driven by soil-borne symbionts to the plant from the Family Rhizobiaceae, which enhance below-ground allocation to nodules and make them more efficient at fixing atmospheric N.  In this way, V. sieberiana seedlings subjected to competition have greater C growth costs when integrated over the whole plant growth cycle compared with V. sieberiana seedlings growing with no competition. The allocation of biomass to the below-ground structures in these seedlings allows them to survive in impoverished soils and to out-compete C. odorata.

Researcher highlight

Anathi Magadlela grew up in a small village town called Idutywa in the Eastern Cape Province of South Africa. In 2011, he moved to Western Cape Province South Africa to conduct an MSc and PhD in plant physiology in the Botany and Zoology Dept. at Stellenbosch University, Cape Town. Anathi currently holds a lecturer position in the School of Life Sciences at the University of KwaZulu- Natal, Pietermaritzburg.

Anathi is a plant biologist, focusing on functional and evolutionary aspects of plant-soil-microbe interactions during nutrient stress. His current research focuses on the functional flexibility of legumes in natural ecosystems and sustainable agricultural systems, during nutrient stress. This research seeks to assess phenotypic and molecular adaptations of nutrient-stressed legumes, to elucidate the unique genes and proteins involved in nutrient stress adaptations of legumes and nitrogen-fixing bacteria.

Further reading

Ndzwanana, Z., Tsvuura, Z., Valentine, A. J., Pérez-Fernández, M. A., & Magadlela, A. (2019). Differential patterns of nitrogen nutrition and growth cost of the indigenous Vachellia sieberiana and the introduced Chromolaena odorata in the savannah environment. AoB PLANTS. https://doi.org/10.1093/aobpla/plz008