Theme 1: Rangeland/Grassland Ecology--Oral Sessions

Description

Savannas are characterized by the coexistence of trees and grasses, and their interactions are modified by water availability and herbivore activity. Many savannas are experiencing bush encroachment, resulting in reduced herbaceous productivity and shifts in savanna structure. This study aims to understand the effects of tree density and tree cutting on herbaceous productivity, water use, and herbivore abundance in a mopane-dominated lowveld savanna. We present data from a 4-year mopane-cutting experiment in the Mthimkhulu Game Reserve bordering Kruger National Park (South Africa). We established three 60x60 m plots for experimental manipulation where mopane stems and re-sprouting shoots were cut 2-3 times per year (2015-2019). We established transects within the plots to measure grass productivity and herbivore activity (counts of tracks and dung). Additionally, we measured root non-structural carbohydrates within cut and uncut mopane to assess the impact of cutting on energy storage. We used stable isotopes of xylem and soil water at multiple depths to infer changes in functional water use of coexisting mopane and grass species. Cutting had limited effects on mopane survival during this 5-year period, but re-sprouting stems had reduced height and starch concentrations than uncut trees. Cutting mopane resulted in shallower-soil water use in 2017 and tended to increase variability in root water uptake across multiple soil depths. The cut treatment tended to have higher grass cover and productivity than the control treatment by the 3rd growing season. Visitation of grazers increased in the cut plots relative to uncut plots by 2017, suggesting increased grass cover promotes grazer visitation. These results emphasize the importance of top-down drivers on savanna tree cover and the impacts of bush encroachment on grass biomass and herbivore presence. We suggest repeated cutting or browsing pressure is needed to suppress woody cover and increase grass production in lowveld savannas.

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Lowveld Savanna Bush Cutting Alters Tree-Grass Interactions

Savannas are characterized by the coexistence of trees and grasses, and their interactions are modified by water availability and herbivore activity. Many savannas are experiencing bush encroachment, resulting in reduced herbaceous productivity and shifts in savanna structure. This study aims to understand the effects of tree density and tree cutting on herbaceous productivity, water use, and herbivore abundance in a mopane-dominated lowveld savanna. We present data from a 4-year mopane-cutting experiment in the Mthimkhulu Game Reserve bordering Kruger National Park (South Africa). We established three 60x60 m plots for experimental manipulation where mopane stems and re-sprouting shoots were cut 2-3 times per year (2015-2019). We established transects within the plots to measure grass productivity and herbivore activity (counts of tracks and dung). Additionally, we measured root non-structural carbohydrates within cut and uncut mopane to assess the impact of cutting on energy storage. We used stable isotopes of xylem and soil water at multiple depths to infer changes in functional water use of coexisting mopane and grass species. Cutting had limited effects on mopane survival during this 5-year period, but re-sprouting stems had reduced height and starch concentrations than uncut trees. Cutting mopane resulted in shallower-soil water use in 2017 and tended to increase variability in root water uptake across multiple soil depths. The cut treatment tended to have higher grass cover and productivity than the control treatment by the 3rd growing season. Visitation of grazers increased in the cut plots relative to uncut plots by 2017, suggesting increased grass cover promotes grazer visitation. These results emphasize the importance of top-down drivers on savanna tree cover and the impacts of bush encroachment on grass biomass and herbivore presence. We suggest repeated cutting or browsing pressure is needed to suppress woody cover and increase grass production in lowveld savannas.