Convex Relationships in Ecosystems Containing Mixtures of Trees and Grass |
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Authors: | RJ Scholes |
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Institution: | (1) Environment and Forest Technology, CSIR Division of Water, P.O. Box 395, Pretoria, 0001, South Africa |
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Abstract: | The relationship between grass production andthe quantity of trees in mixed tree-grassecosystems (savannas) is convex for
all or mostof its range. In other words, the grassproduction declines more steeply per unitincrease in tree quantity at low
tree coverthan at high tree cover. Since much of theeconomic value in savannas is ultimatelyderived from grass, and the main
mechanismcontrolling the tree-grass balance is dependenton the production of grassy fuel for fires,this non-linearity has
the effect of creatingtwo savanna configurations. One has a low treedensity and supports a viable grazingenterprise, while
the other has dense treecover and a frequently non-viable grazingenterprise. The non-linearity is suggested hereto have two
main sources: the geometry of thespatial interaction between tree root systemand grasses, and the effect of differingphenology
(the time course of leaf areaexposure) on the acquisition of water andnutrients. The existence of the non-linearityreduces
the resilience of thegenerally-preferred “open” configuration, andincreases the resilience of the less-desirable“closed” configuration. |
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Keywords: | competition primary productivity resilience savannas |
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