What is it about?
Grazing lawns are very important food sources for grazing herbivores as they provide easy to digest grasses with high concentrations of essential nutrients. Several processes have been described through which herbivores actually increase these concentrations of nutrients in these grazing lawns. They thus stimulate the production of their own food, resulting in a positive feedback. We have now shown that grazing lawns also contain highly compacted and dry soils and plants responds to these dry conditions by accumulating nutrients in their leaves. Therefore, large herbivores can create grazing lawns through trampling (soil compaction) that in turn results in higher nutrient concentrations, representing an alternative way through which herbivores positively feedback on their food.
Why is it important?
Grazing lawns are an important food source for both wild and domestic herbivores. Understanding how they are created and maintained helps managing these systems. Now that we found that soil compaction and soil moisture are important factors, this will have large consequences for the possibilities to create grazing lawns on different textures (clay vs sand) and under specific rainfall conditions. We expect that grazing lawns are more easily created under clayey and low rainfall conditions. Furthermore, it provides new insights in how to restore overgrazed areas, that often consist of highly compacted dry soils.
The key finding in this paper is that feedbacks of large herbivores on vegetation is not only about nutrients, but that herbivores can also change the availability of water to plants. So far, the consumption of nutrients from vegetation and return of nutrients via dung and urine have been the main focus in plant-herbivore interactions and also the main explanation on the formation of grazing lawns. We have now shown that feedbacks on water availability through soil compaction can be very important as well.
Figure: Heterogeneous grasslands during the dry season in Hluhluwe-iMfolozi park with short grazing lawns on the bottom-right and tall bunch grasses at the top-left.