Tree size and herbivory determine below-canopy grass quality and species composition in savannahs |
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Authors: | A C Treydte C C Grant F Jeltsch |
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Institution: | (1) Plant Ecology & Nature Conservation, University of Potsdam, Maulbeerallee 2, 14469 Potsdam, Germany;(2) Systems Ecology, Scientific Services, Kruger National Park, Skukuza, South Africa |
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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. |
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