Michiel P. Veldhuis, Ruth A. Howison, Rienk W. Fokkema, Elske Tielens and Han Olff
1. Large herbivores play a key role in creating spatial heterogeneity through the formation of grazing lawns. Recent research suggests that the currently accepted nutrient-based theory on the formation of these grazing lawns cannot universally explain their formation in all ecosystems where they are found.
2. We developed and investigated an alternative hypothesis on grazing lawn formation and maintenance based on herbivore effects on the plant–soil water balance. We propose that large herbivores change the soil water balance in grazing lawns through defoliation and soil compaction, causing a shift in vegetation composition towards a drought-tolerant plant community.
3. Investigating this idea in a tropical savanna, we indeed found profound differences in grazing lawn soil properties and water balance. In particular, defoliation increased soil temperatures and potential evaporation rates while soil compaction increased bulk density and decreased water infiltration rates, especially on fine-textured soils. Soil moisture was therefore generally much lower in grazing lawns than in adjacent bunch grass areas.
4. Furthermore, we found that grazing lawn species show drought-tolerant traits, with higher leaf sodium levels, suggesting evolutionary adaptation to these herbivore-induced dry conditions. However, leaf water potentials did not differ between grazing lawn and bunch grass species.
5. Synthesis. This study shows that large herbivores might form grazing lawns through previously underestimated effects on water balance. Thus, future studies on large herbivore effects on vegetation should increasingly focus on additional pathways of soil compaction and defoliation. While nutrient-based processes driving grazing lawn formation may operate during the wet season in savannas, we suggest that water balance-based processes are additionally important during the dry season.
Figure: Overview of the influence of large herbivores on grass nutritional quality. The diagram shows five main pathways by which herbivores affect grass nutritional quality. Pathways 1,2 and 3 encompass the current accepted nutrient-based theory: 1) increased N-mineralization through defecation, 2) compensatory growth after defoliation and 3) increased litter quality through changes in plant community composition towards grazing-tolerant highly nutritious grass species. Pathways 4 and 5 are investigated in this study and operate in semi-arid systems through changes in water balance: 4) decreased vegetation cover through defoliation increases soil temperature and bare soil evaporation and 5) soil compaction decreases water infiltration. Together, pathways 4 and 5 decrease soil moisture which in turn increases grass nutritional quality, since plant adaptations to reduce evaporative water loss also reduces photosynthetic carbon fixation. This results in changes in plant carbon to nutrient ratios.