Trees improve grass quality for herbivores in African savannas

Abstract

The tree–grass interactions of African savannas are mainly determined by varying rainfall patterns and soil fertility. Large savanna trees are known to modify soil nutrient conditions, but whether this has an impact on the quality of herbaceous vegetation is unclear. However, if this were the case, then the removal of trees might also affect the structure and quality of the grass layer. We studied the impact of large nitrogen- and non-nitrogen fixing trees on the sub-canopy (SC) grass layer in low- and high-rainfall areas of differing soil fertility in eastern and southern Africa. We compared the structure and nutrient levels of SC grasses with those outside the canopy. Grass leaf nitrogen and phosphorus contents beneath tree canopies were elevated at all study sites and were up to 25% higher than those outside the canopy in the site of lowest rainfall and soil fertility. Grass leaf fibre and organic matter (OM) contents were slightly enhanced beneath tree canopies. At the site of highest rainfall and soil fertility, grasses beneath the canopy had significantly lower ratios of stem:leaf biomass and dead:living leaf material. Grass species composition differed significantly, with the highly nutritious Panicum spp. being most abundant underneath tree crowns. In the two drier study sites, soil nitrogen and OM contents were enhanced by 30% beneath trees. N-fixation capacity of trees did not contribute to the improved quality of grass under the canopy. We conclude that trees improve grass quality, especially in dry savannas. In otherwise nutrient-poor savanna grasslands, the greater abundance of high-quality grass species with higher contents of N and P and favourable grass structure beneath trees could attract grazing ungulates. As these benefits may be lost with tree clearance, trees should be protected in low fertility savannas and their benefits for grazing wildlife recognised in conservation strategies.