Soil seed banks along a woody plant removal gradient in a semi-arid savanna of South Africa: Implications for restoration

Woody plant encroachment threatens ecosystem services and functions, thereby reducing herbaceous plant population persistence and community stability. Consequently, woody plant control projects are implemented in South Africa to restore herbaceous vegetation. Because persistent seeds drive passive restoration and vegetation trajectories, management following woody plant control entails examining soil seed bank (SSB) size and composition. However, a knowledge gap exists regarding how SSB characteristics respond along a woody plant removal gradient. This study was conducted at Roodeplaat in Gauteng Province of South Africa to assess the impact of woody density reduction [hereafter woody plant removal intensity (WPRI)] on SSB density, composition, diversity and richness. Woody plant vegetation composed of a mixture of species, mainly Vachellia robusta, Ziziphus mucronata, Euclea species and Pappea capensis. Selective tree removal was applied to downscale woody density (4065 ± 109 plants ha⁻¹) of the control (0% WPRI) to 10, 20, 50, 75 and 100% WPRIs in four blocks, resulting in four replicates per WPRI. A total of 120 soil samples (n = 20 per WPRI) were collected at 5 cm depth after 3 years of tree removal. Germination method was used to assess the SSB. Fifty-one species, mainly forbs (n = 26) and grasses (n = 16) were recorded from 32 237 seeds. The SSB densities of grasses increased from 649 to 6000 seeds m⁻² from 0 to 100% WPRI whilst sedges and forbs exhibited differential declining trends along WPRI gradient. Cyperus rotundus accounted for more than one-third of the SSB densities at 0 and 10% WPRIs, whereas Panicum maximum contributed nearly half (46%) to the SSB density at 100% WPRI. Woody plant removal increased SSB diversity (H′) and richness, but diversity peaked at 20% WPRI (H' = 1.78); thereafter it declined. Overall, our results signified that complete woody plant clearing has a potential for restoration of woody-encroached rangelands through increasing SSB size and species richness.