Individual shrubs, large scale grass cover and seasonal rainfall explain invertebrate-derived macropore density in a semi-arid Namibian savanna

Macropores created by invertebrates improve ecosystem functions such as soil properties and hydrological processes. In semi-arid savannas, where water is the main limiting resource and precipitation is scarce macropores might increase infiltration, and thereby improve water availability for plants. Macropores may therefore represent a buffering mechanism counteracting degradation in the form of shrub-encroachment. We investigated the interacting effects of vegetation structures at small scales and vegetation cover at landscape scales, and seasonality on invertebrate macropores. First, macropore density and size distribution was measured at open soil, in direct proximity to perennial grass tussocks and shrubs, at grass dominated, intermediate and shrub dominated sites. Secondly, we recorded macropores on randomly chosen plots along a shrub cover gradient at three points in time within the rainy season. Individual shrubs and the amount of large scale grass cover increased macropore densities. Interestingly, macropore numbers were highest at the beginning of the rainy season. We argue that macropore densities reflect the activity and influence of soil macrofauna on ecosystem functioning, which is greatest at highly heterogeneous vegetation. Rangeland management should aim for high grass cover with scattered shrubs for sustainable soil health, by applying appropriate stocking rates, selectively removing shrubs or reseeding perennial grasses. •We studied savanna bush-encroachment effects on invertebrate macropore distribution.•The effect of vegetation on invertebrate macropore distribution is scale dependent.•The number of macropores beneath shrubs was three times higher than next to grasses.•Landscape wide grass-cover positively affected macropore densities.•First rains after dry season induced higher macropore numbers than at later stages.