Shifting patch dynamics of floodplain woodlands in an arid savanna region: effects of soil form, flooding and large herbivores

Abstract

• Monospecific woodland groves were associated with soil forms and flooding regimes in a floodplain. • Extreme floods removed trees and created substrates for woodland regeneration. • Damage by elephants and meso-browsers affected regeneration dynamics. • Flooding and herbivory interactively generated a shifting woodland mosaic anchored by soil. Our study addressed the regeneration dynamics of the patch mosaic of monospecific woodland groves in the floodplains of the Luvuvhu and Limpopo rivers following the removal of human settlements to create the Makuleke Contractual Park falling within the northern Kruger National Park. We anticipated that subsequent increases by elephants and meso-browsers like impalas would deflect or even prevent the re-establishment of these groves through the damage that they inflicted on plants. We compared the responses exhibited in groves of fever trees ( Vachellia xanthophloea ) associated with layered clay alluvium with those of ana trees ( Faidherbia albida ) associated with deep sandy deposits. Multi-decadal changes in the woodland mosaic were identified using aerial photographs and Google Earth imagery supported by onsite inspections and related to flood events and soil features. Woodland regeneration following recent major floods was tracked using onsite inspections over five years recording sapling growth and disappearance along with the breakage, bark removal and defoliation imposed by elephants, medium-sized browsing ruminants and porcupines. Fever trees were concentrated near the confluence of the two rivers where floodwaters inundated this region. Extreme floods carried away flanking ana trees while leaving sandy deposits open for recolonization. Elephants pushed over tall fever trees but not the ana trees, while regenerating saplings of both tree species were broken or uprooted. Browsing impalas defoliated juvenile trees contributing to their demise. These impacts contributed to the contraction or even disappearance of some fever tree groves and suppressed the growth and establishment of ana tree saplings. While these herbivores contributed to the shifting patch mosaic of these monospecific woodlands, the local distribution of these woodlands was associated with the soil features that they favoured.