Improved quality of beneath-canopy grass in South African savannas: Local and seasonal variation

Authors


  • Nomenclature: Trees: Palgrave (1983); Beentje et al. (1994) Grasses: Van Oudtshoorn (1999).

  • Ackowledgements. This study was funded by the German Research Foundation (DFG). SANPARKS and Scientific Services in Kruger Park gave permission to conduct field work, C.C. Grant, A. Delsink, S. Ronaldson, Y. Pretorius and C. van der Waal helped with logistics and stimulated fruitful discussions. We thank C.C. Grant and two anonymous reviewers for their comments on an earlier draft of this paper. Field technicians and lab assistants in South Africa and The Netherlands were of great support.

Corresponding author; Fax + 49 3119771948; E-mail treydte@uni-potsdam.de; http://www.bio.uni-potsdam.de/professuren/vegetationsoekologie-naturschutz/

Abstract

Abstract

Questions:

Do large trees improve the nutrient content and the structure of the grass layer in savannas? Does the magnitude of this improvement differ with locality (soil nutrients) and season (water availability)? Are grass structure and species composition beneath tree canopies influenced by soil fertility and season?

Location:

South Africa.

Methods:

We compared grass leaf nutrient contents and grass sward structure beneath and outside tree canopy areas in three savannas of different soil fertility during the dry and the wet seasons.

Results:

Grass nitrogen contents were twice as high during the wet season as compared to the dry season, being more strongly elevated underneath tree canopies during the wet season. Grasses had significantly less stem material and provided less dead material underneath trees on the high soil fertility site. Grass species composition differed significantly beneath and outside tree canopies, with more nutritious grass species found sub-canopy. Grass species richness was significantly lower beneath than outside of trees at the site of high soil fertility.

Conclusions:

Trees improve overall quality of savanna grasses by enhancing grass growth and nutrient uptake during the wet season, and by delaying grass wilting in the dry season. The positive effect of trees on the grass layer might attract grazing herbivores in otherwise nutrient-poor savannas. Hence, single standing large trees should be maintained to sustain high grass quality and, consequently, grazer populations in savanna habitats.

Ancillary