Repeated clearing as a mechanism for savanna recovery following bush encroachment

Authors: WEDEL, EMILY - Kansas State University; NIPPERT, JESSE - Kansas State University; O'Connor, Rory; NKUNA, PEACE - South African Environmental Observation Network; SWEMMER, ANTHONY - South African Environmental Observation Network

Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2024
Publication Date: 5/3/2024
Citation: Wedel, E.R., Nippert, J.B., O'Connor, R.C., Nkuna, P., Swemmer, A.M. 2024. Repeated clearing as a mechanism for savanna recovery following bush encroachment. Journal of Applied Ecology. 61(7):1520-1530. https://doi.org/10.1111/1365-2664.14666.
DOI: https://doi.org/10.1111/1365-2664.14666

Interpretive Summary: Woody plants are expanding in prevalence in grasslands and savannas around the world. In South Africa woody plant expansion has a profound effect on perennial grass biomass and water availability, as well as the types of native ungulates that are present. We repeatedly cut the dominant woody expanding species, Colophospermum mopane, and measured its growth responses as well as the surrounding grass responses to the cutting treatment. We found that repeated clearing of C. mopane caused decreased nonstructural carbohydrate reserves in its roots, and that there was an increase in soil moisture longer into the growing season which resulted in increased grass biomass. Clearing C. mopane opens up the lowveld savanna canopy for more grass growth which alters herbivore distribution at the site, and provides the desired results for restoring the semi-arid lowveld savanna.

Technical Abstract: 1. Many savannas are experiencing increased cover of trees and shrubs, resulting in reduced herbaceous productivity, shifts in savanna functional structure and potential reductions in ecotourism. Clearing woody plants has been suggested as an effective management strategy to mitigate these effects and restore these systems to an open state with higher rates of grass production and herbivory. This study investigated the effectiveness of repeated shrub clearing as a tool to mitigate bush encroachment in a semi-arid savanna in southern Africa. 2. We present data from a 7-year experiment in the Mthimkhulu Game Reserve bordering Kruger National Park, South Africa. Colophospermum mopane stems and resprouting shoots were basally cut 2–3 times per year (2015–2022) in three pairs of treatment and control plots of 60'×'60'm. We monitored changes in soil moisture, grass biomass and herbivore activity via dung counts. We assessed C. mopane physiological responses to repeated cutting using non-structural carbohydrates and stable water isotopes to infer changes to energy storage and functional rooting depth, respectively. 3. The cleared treatment had higher soil moisture and grass biomass than the control treatment. Dung counts showed impala and buffalo visited the cleared treatment more frequently than the control treatment. 4. Repeated cutting had limited effects on C. mopane survival in the first 2–3'years after initial clearing, but 80% of individuals were dead after 7'years. Repeatedly cut C. mopane had lower belowground starch concentrations and used water from shallower soil depths than C. mopane in control plots. 5. Synthesis and applications. Repeated cutting increased soil moisture availability and grass biomass, and attracted charismatic grazing herbivores. While more costly than once-off clearing methods, this practice created more employment opportunities for a neighbouring rural community. Transforming portions of the ecosystem to a grass-dominated state may increase ecotourism potential through improved game viewing in open systems.