Tree size and herbivory determine below-canopy grass quality and species composition in savannahs

Abstract

Large single-standing trees are rapidly declining in savannahs, ecosystems supporting a high diversity of large herbivorous mammals. Savannah trees are important as they support both a unique flora and fauna. The herbaceous layer in particular responds to the structural and functional properties of a tree. As shrubland expands stem thickening occurs and large trees are replaced by smaller trees. Here we examine whether small trees are as effective in providing advantages for grasses growing beneath their crowns as large trees are. The role of herbivory in this positive tree-grass interaction is also investigated. We assessed soil and grass nutrient content, structural properties, and herbaceous species composition beneath trees of three size classes and under two grazing regimes in a South African savannah. We found that grass leaf content (N and P) beneath the crowns of particularly large (ca. 3.5 m) and very large trees (ca. 9 m) was as much as 40% greater than the same grass species not growing under a tree canopy, whereas nutrient contents of grasses did not differ beneath small trees (\2.3 m). Moderate herbivory enhanced these effects slightly. Grass species composition differed beneath and beyond the tree canopy but not between tree size classes. As large trees significantly improve the grass nutrient quality for grazers in contrast to smaller trees, the decline of the former should be halted. The presence of trees further increases grass species diversity and patchiness by favouring shade-tolerant species. Both grazing wildlife and livestock will benefit from the presence of large trees because of their structural and functional importance for savannahs.

Grazing ungulates select for grasses growing beneath trees in African savannas

“…Higher nitrogen and phosphorus contents in grasses growing beneath trees compared to grasses growing outside of tree canopies were found in both dry and wet season in KNP (Treydte et al 2008). Further, several studies reported that particularly P. maximum, a preferred forage grass species, grew significantly more often beneath tree canopies (East and Felker 1993;Treydte et al 2009a), which could also be confirmed by our study. Moreover, Ben-Shahar and Coe (1992) found that zebra and blue wildebeest move to patches where the grass community contains a high proportion of nutritional species such as P. maximum.…”

“…Owen-Smith (2002) likewise termed high nutrient quality patches as "limited intake resource", providing highly nutritious food but only in limited abundance. A previous study in KNP indicated that grass leaf nitrogen contents were up to 40% higher beneath large trees compared to the same grass species growing outside of tree canopies (Treydte et al 2009a). Higher nitrogen and phosphorus contents in grasses growing beneath trees compared to grasses growing outside of tree canopies were found in both dry and wet season in KNP (Treydte et al 2008).…”

Can diverse herbivore communities increase landscape heterogeneity? Comparing wild and domestic herbivore assemblages in a South African savanna

“…In general, the presence of tall trees enhanced grass N and P contents at all study sites inside KNP, which confirmed the findings of (Treydte et al, 2009). In contrast to wildlife sites, no influence of trees on grass leaf nutrient content was found for livestock sites.…”

Impact of communal land use and conservation on woody vegetation structure in the Lowveld savannas of South Africa

“…The persistence of large single standing trees in the communal areas is highly beneficial to biodiversity and savanna function (Belsky et al, 1993;Manning et al, 2006;Treydte et al, 2009). The local abundance of rare tree species is a concern in communal areas (Shackleton et al, 1994).…”

“…These classes relate to the influence of fire and herbivory, their function in the landscape, as well as human utilization (Belsky et al, 1993;Scholes and Walker, 1993;Bond and Midgley, 2000;Shackleton and Shackleton, 2004;Neke et al, 2006;Treydte et al, 2009).…”