Plant Ecology

, Volume 170, Issue 1, pp 93–105 | Cite as

The influence of savanna trees on nutrient, water and light availability and the understorey vegetation

  • Fulco Ludwig
  • Hans de Kroon
  • Frank Berendse
  • Herbert H.T. Prins


In an East African savanna herbaceous layer productivity and species composition were studied around Acacia tortilis trees of three different age classes, as well as around dead trees and in open grassland patches. The effects of trees on nutrient, light and water availability were measured to obtain an insight into which resources determine changes in productivity and composition of the herbaceous layer. Soil nutrient availability increased with tree age and size and was lowest in open grassland and highest under dead trees. The lower N:P ratios of grasses from open grassland compared to grasses from under trees suggested that productivity in open grassland was limited by nitrogen, while under trees the limiting nutrient was probably P. N:P ratios of grasses growing under bushes and small trees were intermediate between large trees and open grassland indicating that the understorey of Acacia trees seemed to change gradually from a N-limited to a P-limited vegetation. Soil moisture contents were lower under than those outside of canopies of large Acacia trees suggesting that water competition between trees and grasses was important. Species composition of the herbaceous layer under Acacia trees was completely different from the vegetation in open grassland. Also the vegetation under bushes of Acacia tortilis was different from both open grassland and the understorey of large trees. The main factor causing differences in species composition was probably nutrient availability because species compositions were similar for stands of similar soil nutrient concentrations even when light and water availability was different. Changes in species composition did not result in differences in above-ground biomass, which was remarkably similar under different sized trees and in open grassland. The only exception was around dead trees where herbaceous plant production was 60% higher than under living trees. The results suggest that herbaceous layer productivity did not increase under trees by a higher soil nutrient availability, probably because grass production was limited by competition for water. This was consistent with the high plant production around dead trees because when trees die, water competition disappears but the high soil nutrient availability remains. Hence, in addition to tree soil nutrient enrichment, below-ground competition for water appears to be an important process regulating tree-grass interactions in semi-arid savanna.

Competition East-Africa Facilitation Soil nutrients Soil water Tree-grass interaction 


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  1. Akpo L.E. 1997. Phenological interaction between trees and understorey herbaceous vegetation of a sahelian semi-arid savanna. Plant Ecology 131: 241–248.Google Scholar
  2. Aerts R. and Chapin F.S. 2000. The mineral nutrition of wild plants revisited: a re-evaluation of processes and patterns. Advances in Ecological Research 30: 1–67.Google Scholar
  3. Amundson R.G., Ali A.R. and Belsky A.J. 1995. Stomatal responsiveness to changing light intensity increases rain-use efficiency of below-crown vegetation in tropical savannas. Journal of Arid Environments 29: 139–153.Google Scholar
  4. Anderson L.J., Brumbaugh M.S. and Jackson R.B. 2001.Water and tree-understorey interactions: a natural experiment in a savanna with oak wilt. Ecology 82: 33–49.Google Scholar
  5. Anderson M.C. 1964. Studies of the woodland light climate I. The photographic computation of light conditions. Journal of Ecology 52: 27–41.Google Scholar
  6. Belsky A.J. 1994. Influences of trees on savanna productivity: test of shade, nutrients, and tree-grass competition. Ecology 75: 922–932.Google Scholar
  7. Belsky A.J., Amundson R.G., Duxbury J.M., Riha S.J., Ali A.R. and Mwonga S.M. 1989. The effects of trees on their physical, chemical, and biological environments in a semi-arid savanna in Kenya. Journal of Applied Ecology 26: 1005–1024.Google Scholar
  8. Belsky A.J., Mwonga S.M., Amundson R.G., Duxbury J.M. and Ali A.R. 1993. Comparative effects of isoloated trees on their undercanopy environments in high-and low-rainfall savannas. Journal of Applied Ecology 30: 143–155.Google Scholar
  9. Bernhard-Reversat F. 1982. Biogeochemical cycle of nitrogen in a semi-arid savanna. Oikos 38: 321–332.Google Scholar
  10. Bray R.H. and Kurtz L.T. 1945. Determination of total, organic and available phosphorus in soils. Soil Science 59: 39–45.Google Scholar
  11. Caldwell M.M., Dawson T.E. and Richards J.H. 1998. Hydraulic lift: consequences of water efflux for the roots of plants. Oecologia 113: 151–161.Google Scholar
  12. Callaway R.M., Nadkarni N.M. and Mahall B.E. 1991. Facilitation and interference of Quercus douglasii on understorey productivity in Central California. Ecology 72: 1484–1499.Google Scholar
  13. Clayton W.D. and Renvoize S.A. 1982. Flora of Tropical East Africa. (Part 1-3). Gramineae.Google Scholar
  14. Dawson T.E. 1993. Hydraulic lift and water use by plants: implications for water balance, performance and plant-plant interactions. Oecologia 95: 565–574.Google Scholar
  15. Durr P.A. and Rangel J. 2000. The response of Panicum maximum to a simulated subcanopy environment. Tropical Grasslands 34: 110–117.Google Scholar
  16. Georgiadis N.J. 1989. Microhabitat variation in an African savanna: effects of woody cover and herbivores in Kenya. Journal of Tropical Ecology 5: 93–108.Google Scholar
  17. Joffre R. and Rambal S. 1993. How tree cover influences the water balance of Mediterranean rangelands. Ecology 74: 570–582.Google Scholar
  18. Kellman M. 1979. Soil enrichment by neotropical savanna trees. Journal of Ecology 67: 565–577.Google Scholar
  19. Koerselman W. and Meuleman A.F.M. 1996. The vegetation N:P ratio a new tool to detect the nature of nutrient limitation. Journal of Applied Ecology 33: 1441–1450.Google Scholar
  20. Ludwig F. 2001. Tree-grass interactions on an East African savanna: the effect of nutrients, shade and hydraulic lift. Tropical Resource Management Papers 39. Wageningen University.Google Scholar
  21. Ludwig F., de Kroon H., Prins H.H.T. and Berendse F. 2001. The effect nutrients and shade on tree - grass interactions on an East African savanna. Journal of Vegetation Science 12: 579–588.Google Scholar
  22. Ludwig F., Dawson T.E., de Kroon H., Berendse F. and Prins H.H.T. 2003. Hydraulic lift in Acacia tortilis trees on an East African savanna. Oecologia (in press).Google Scholar
  23. McClaren M.P. and Bartolome J.W. 1989. Effect of Quercus douglasii (Fagaceae) on herbaceous understorey along a rainfall gradient. Madroño 36: 141–153.Google Scholar
  24. Mordelet P. and Menaut J.C. 1995. Influence of trees on aboveground production dynamics of grasses in a humid savanna. Journal of Vegetation Science 6: 223–228.Google Scholar
  25. Novozamski I., Houba V.J.G., Van Eck R. and Van Vark W. 1983. A novel digestion technique for multi-element plant analysis. Communications in Soil Science and Plant Analysis 14: 239–249.Google Scholar
  26. Prins H.H.T. 1996. Ecology and Behaviour of the African Buffalo. Chapman & Hall, London.Google Scholar
  27. Prins H.H.T. and van der Jeugd H.P. 1993. Herbivore population crashes and woodland structure in East Africa. Journal of Ecology 81: 305–314.Google Scholar
  28. Scholes R.J. 1990. The influence of soil fertility on the ecology of Southern African savannas. Journal of Biogeography 17: 415–419.Google Scholar
  29. Scholes R.J. and Archer S.R. 1997. Tree-grass interactions in savannas. Annual Review of Ecology and Systematics 28: 517–544.Google Scholar
  30. SPSS 1996. SPSS for windows release 7.5. SPSS Inc., Chicago, Illinois, USA.Google Scholar
  31. Ter Steege H. 1994. Hemiphot: a programme to analyze vegetation indices, light and light quality from hemispherical photographs. Tropenbos Documents No 3. Tropenbos, Wageningen, Netherlands.Google Scholar
  32. Tiedeman A.R. and Klemmedson J.O. 1977. Effect of mequite trees on vegetation and soils in the desert grassland. Journal of Range Management 30: 361–367.Google Scholar
  33. Tiedeman A.R. and Klemmedson J.O. 1986. Long term effect of mesquite removal on soil characteristics: I. Nutrient and bulk density. Soil Science Society of America Journal 50: 472–475.Google Scholar
  34. Van de Vijver C.A.D.M. 1999. Fire and life in Tarangire; effects of burning and herbivory on an East African savanna system. PhD Dissertation, Wageningen University, Netherlands.Google Scholar
  35. Van de Vijver C.A.D.M., Foley C.A. and Olff H. 1999. Changes in the woody component of an East African savanna during 25 years. Journal of Tropical Ecology 15: 545–564.Google Scholar
  36. Verhoeven J.T.A., Koerselman W. and Meuleman A.F.M. 1996. Nitrogen-or phosphorus-limited growth in herbaceous, wet vegetation: relations with atmospheric inputs and management regimes. Trends in Ecology and Evolution 11: 494–497.Google Scholar
  37. Voeten M.M. and Prins H.H.T. 1999. Resource partitioning between sympatric wild and domestic herbivores in the Tarangire region of Tanzania. Oecologia 120: 287–294.Google Scholar
  38. Weltzin J.F. and Coughenour M.B. 1990. Savanna tree influence on understorey vegetation and soil nutrients in northwestern Kenya. Journal of Vegetation Science 1: 325–334.Google Scholar

Copyright information

© Kluwer Academic Publishers 2004

Authors and Affiliations

  • Fulco Ludwig
    • 1
  • Hans de Kroon
    • 1
  • Frank Berendse
    • 1
  • Herbert H.T. Prins
    • 1
  1. 1.Sub-department of Nature ConservationWageningen UniversityWageningenThe Netherlands

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