Nodulation Alleviates Water Stress in Vachellia Sieberiana

Abstract Species in the genus Vachellia (Fabaceae) have a global tropical and sub-tropical distribution. Numerous Vachellia species are currently observed to be expanding their indigenous ranges and increasing in dominance globally, suggesting an overarching driver. Most Vachellia species enhance nitrogen uptake mutualistically via specialized root nodule structures. Nodules contain N2-fixing rhizobia that consume host supplied carbon to catalyse atmospheric N2 into a plant useable form, a key element in plant growth. The rhizobial mutualism of some Vachellia species may be vital to understanding changing patterns of ecological success observed across the savanna precipitation gradient. Here, we investigated how the seedling root development and physiology of two dominant savanna woody species, the arid-adapted Vachellia erioloba and the mesic-adapted Vachellia sieberiana, responded to simulated drought events. Seedlings of both species were grown at 4%, 8% and 16% soil moisture content (SMC) for four months. Seedling growth and allometry of arid-adapted V. erioloba was unresponsive to water stress treatments, and no nodulation was observed, reflecting a fixed higher relative investment in belowground biomass. In contrast, V. sieberiana roots were nodulated, but developed the highest nodule biomass and growth rate when grown at the lowest soil moisture (4% SMC). These patterns suggest that effective life history strategies for the arid-30 adapted species precludes the need for rhizobial mutualism, possibly due to more “open” N cycling and lower competitive interactions in arid systems, while the more “closed” N cycling in mesic savannas, and higher competitive stress, may favour nodulation, especially under low water supply that limits root access to soil nitrogen, and signals a more competitive environment and an advantage from N2-fixing.