Is the soil seed bank a reliable source for passive restoration of bush-cleared semi-arid rangelands of South Africa?

Mthunzi Mndela,Casper Ignacio Madakadze,Sikhalazo Dube,Florence Nherera-Chokuda

doi:10.1186/s13717-019-0204-6

Mthunzi Mndela, Casper Ignacio Madakadze + Show 2 more

Open Access

https://doi.org/10.1186/s13717-019-0204-6

Copy DOI

Abstract

BackgroundBush clearing of encroached rangelands is crucial for restoration of herbaceous cover and diversity. Regeneration after bush clearing depends largely on the soil seed bank (SSB) size and composition. To assess the potential of the SSB to facilitate post-clearing herbaceous restoration, we examined the SSB density, composition and diversity and similarity between SSB and aboveground vegetation (AGV).MethodsThe study was conducted in semi-arid rangelands of Maseding and Kgomokgomo in North-West Province of South Africa. In each rangeland, all woody plants in three 1250 m2 plots were cut using saws and loppers, with the stumps treated with picloram. Paired observations were conducted in cleared and uncleared microsites. Soil samples were collected at three depths (0–10, 10–20 and 20–30 cm) in each microsite in April (pre-treatment), August, October and December 2016 and in April and August 2017. SSB was assessed using seedling emergence method, whereas pre- and post-clearing surveys of AGV were conducted in February 2016 and 2017, respectively.ResultsCleared microsites had significantly (p < 0.01) higher seed bank densities (1872 and 693 seeds m-2 at Maseding and Kgomokgomo, respectively) relative to uncleared microsites in August 2017, with grasses accounting for higher densities than other plant functional groups in the upper 0–10 cm. Pioneer grasses (Aristida congesta subsp. barbicollis, Brachiaria eruciformis and Tragus berteronianus) and invasive forbs (Bidens pilosa and Schkuhria pinnata) dominated the SSB in cleared microsites at Maseding, whereas succulents (Portulaca spp.) and pioneer grasses (T. berteronianus and Urochloa mosambicensis) were abundant at Kgomokgomo in August 2017. SSB and AGV were dissimilar until December 2016; thereafter, similarity increased significantly (p < 0.05) in cleared microsites (Sørensen’s coefficient = 0.60 to 0.66 at Maseding and 0.43 to 0.52 at Kgomokgomo) compared to uncleared microsites. Species diversity was highest in August 2016 and April 2017 at Maseding, but it did not differ between two microsites (p > 0.05). At Kgomokgomo, SSB was more diverse in cleared than uncleared microsites in April and August 2017.ConclusionHigh seed bank densities in the upper soil layer in cleared microsites would promote passive restoration, but regeneration is likely to be initiated by early successional species. However, close monitoring and eradication of invasive forbs and succulents is necessary following bush clearing. The tendency of SSB resembling AGV over time in cleared microsites signifies that seed production from AGV is important for passive restoration following bush clearing.

Highlights

Bush clearing is a commonly applied management tool to reduce woody plant canopy and densities in heavily encroached rangelands for restoration of grass cover, diversity and ground water recharge (Archer and Predick 2014)
The effects of bush clearing on passive restoration are determined by the method of bush clearing, as different mechanical methods disturb the soil to varying degrees (Faist et al 2015)
Knowledge of the soil seed bank dynamics is important for understanding historical land use management and further provides insights on management strategies to be applied for future restoration (Cox and Allen 2008; Jonason et al 2014)

Summary

Introduction

Bush clearing is a commonly applied management tool to reduce woody plant canopy and densities in heavily encroached rangelands for restoration of grass cover, diversity and ground water recharge (Archer and Predick 2014). In South Africa, post-clearing monitoring of passive recovery of vegetation is rarely conducted on the assumption that plants will regenerate effectively from soil stored propagules (Fourie 2007). This highlights a need to assess the size, composition and regeneration capacity of the soil seed bank. Soil seed bank indicates the past vegetation composition and potential future communities and acts as a reservoir for conservation and restoration of plant species diversity (Bakker et al 1996). To assess the potential of the SSB to facilitate post-clearing herbaceous restoration, we examined the SSB density, composition and diversity and similarity between SSB and aboveground vegetation (AGV)

Objectives

Methods

Findings

Discussion

Conclusion