Grasses continued to trump trees at soil carbon sequestration following herbivore exclusion in a semiarid African savanna

Authors: WIGLEY, B - Nelson Mandela University; Augustine, David; COETSEE, C - Nelson Mandela University; RATNAM, J - National Centre For Biological Sciences; SANKARAN, M - National Centre For Biological Sciences

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2019
Publication Date: 5/1/2020
Citation: Wigley, B.J., Augustine, D.J., Coetsee, C., Ratnam, J., Sankaran, M. 2020. Grasses continued to trump trees at soil carbon sequestration following herbivore exclusion in a semi-arid African savanna. Ecology. 101(5). Article e03008. https://doi.org/10.1002/ecy.3008.
DOI: https://doi.org/10.1002/ecy.3008

Interpretive Summary: Both domestic and wild herbivores alter vegetation communities through grazing and browsing on plants. Changes in the vegetation can also affect the amount of organic matter, and hence the amount of carbon, that enters the soil from plant roots and dead leaves. We used three sets of long-term, large herbivore exclosures with paired controls, to study how almost two decades of herbivore removal from a savanna in Kenya affected the amount of carbon in the plant biomass aboveground (woody and grass) and in the soil. We used analyses of types of carbon in the soil to determine whether the grasses or the woody vegetation were the primary source of the carbon. The exclusion of large herbivores from this savanna, which included a diverse community of grazers, browsers, and mixed-feeding ungulates, resulted in significant increases in grass cover (~22%) and woody canopy cover (31%), translating to a large increase in aboveground carbon over two decades. Herbivore exclusion also led to a 54% increase in total soil carbon, with 71% of this derived from grasses despite substantial increases in woody cover. We attribute this surprisingly high contribution of grasses to soil C sequestration to the fact that open sparse canopies of trees (e.g. Acacia spp.) are largely beneficial for the growth of understory grasses, as well as to the increased productivity of grasses when they were no longer grazed.

Technical Abstract: While studies have shown that mammalian herbivores limit aboveground carbon storage in savannas, their effects on belowground soil carbon storage remains unclear. Using three sets of long-term, large herbivore exclosures with paired controls, we asked how almost two decades of herbivore removal from a semi-arid savanna in Laikipia, Kenya affected aboveground (woody and grass) and belowground soil carbon sequestration, and determined the major source (C3 vs. C4) of belowground carbon sequestered in the soils with and without herbivores. Large herbivore exclusion, which included a diverse community of grazers, browsers, and mixed-feeding ungulates, resulted in significant increases in grass cover (~22%), woody basal area (~8 m2 ha-1) and woody canopy cover (31%), translating to a ~8.5 t ha-1 increase in aboveground carbon over two decades. Herbivore exclusion also led to a 54% increase (20.5 t ha-1, 0-30 cm depth) in total soil carbon, with 71% of this derived from C4 grasses despite substantial increases in woody cover. We attribute this surprisingly high contribution of C4 grasses to soil C sequestration to the facilitative influence of open sparse woody canopies (e.g. Acacia spp.) on grass cover and productivity, together with reduced offtake of grass biomass with herbivore exclusion in this semi-arid system.