Role of topography in facilitating coexistence of trees and grasses within savannas

Yeonjoo Kim, Elfatih A.B. Eltahir

Research output: Contribution to journalArticle

19 Citations (Scopus)

Abstract

[1] The factors and processes that may explain the observed coexistence of trees and grasses in savannas are not well understood. Here we propose a new hypothesis that addresses this issue. We hypothesize that "variations in elevation at relatively short horizontal scales of ∼1 km force similar variations in soil moisture and thus create significantly different hydrologic niches within any large area. Under water-limited conditions the relatively wet valleys favor trees, while the relatively dry hills favor grasses. This coexistence of trees and grasses is only possible for a window of climatic conditions that are characteristic of savannas." To test this hypothesis, numerical simulations are performed for the region of West Africa using a model that simulates vegetation dynamics, the Integrated Biosphere Simulator (IBIS), and a distributed hydrologic model, Systeme Hydrologique Europeen (SHE). IBIS is modified to include the groundwater table (GWT) as a lower boundary. The spatial distribution of GWT is simulated by SHE. At 9°N the model simulates trees even when the GWT is assumed to be infinitely deep; at 13°N the model simulates grasses even when the capillary fringe of the GWT reaches the surface. However, for the transitional climate, at 11°N, trees are simulated when the GWT is at ∼2.5 m from the surface, but grasses are simulated when the GWT is deeper than 2.5 m. These results suggest that the variability of soil moisture forced by topography can be a determinant factor of vegetation distribution within savannas. Furthermore, they confirm that this role of topography can be significant only in a certain climatic window characteristic of savannas.

Original languageEnglish
Pages (from-to)W075051-W0750513
JournalWater Resources Research
Volume40
Issue number7
DOIs
Publication statusPublished - 2004 Jul

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savanna
coexistence
grass
topography
groundwater
biosphere
simulator
soil moisture
capillary fringe
vegetation dynamics
niche
spatial distribution
valley
vegetation
climate
simulation
water

All Science Journal Classification (ASJC) codes

  • Water Science and Technology

Cite this

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title = "Role of topography in facilitating coexistence of trees and grasses within savannas",
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Role of topography in facilitating coexistence of trees and grasses within savannas. / Kim, Yeonjoo; Eltahir, Elfatih A.B.

In: Water Resources Research, Vol. 40, No. 7, 07.2004, p. W075051-W0750513.

Research output: Contribution to journalArticle

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AB - [1] The factors and processes that may explain the observed coexistence of trees and grasses in savannas are not well understood. Here we propose a new hypothesis that addresses this issue. We hypothesize that "variations in elevation at relatively short horizontal scales of ∼1 km force similar variations in soil moisture and thus create significantly different hydrologic niches within any large area. Under water-limited conditions the relatively wet valleys favor trees, while the relatively dry hills favor grasses. This coexistence of trees and grasses is only possible for a window of climatic conditions that are characteristic of savannas." To test this hypothesis, numerical simulations are performed for the region of West Africa using a model that simulates vegetation dynamics, the Integrated Biosphere Simulator (IBIS), and a distributed hydrologic model, Systeme Hydrologique Europeen (SHE). IBIS is modified to include the groundwater table (GWT) as a lower boundary. The spatial distribution of GWT is simulated by SHE. At 9°N the model simulates trees even when the GWT is assumed to be infinitely deep; at 13°N the model simulates grasses even when the capillary fringe of the GWT reaches the surface. However, for the transitional climate, at 11°N, trees are simulated when the GWT is at ∼2.5 m from the surface, but grasses are simulated when the GWT is deeper than 2.5 m. These results suggest that the variability of soil moisture forced by topography can be a determinant factor of vegetation distribution within savannas. Furthermore, they confirm that this role of topography can be significant only in a certain climatic window characteristic of savannas.

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