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Above-Ground Dry Matter Dynamics of the Grass Layer of a South African Tree Savanna

J. O. Grunow, H. T. Groeneveld and S. H. C. Du Toit
Journal of Ecology
Vol. 68, No. 3 (Nov., 1980), pp. 877-889
DOI: 10.2307/2259463
Stable URL: http://www.jstor.org/stable/2259463
Page Count: 13
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Above-Ground Dry Matter Dynamics of the Grass Layer of a South African Tree Savanna
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Abstract

(1) The annual cycle of above-ground biomass change in various components and species of the herb layer in broadleaved deciduous savanna was determined by harvest techniques. `Difference methods' were used to determine the mean flow rate between components such as above-ground grazeable live biomass, standing-dead biomass, and litter. (2) Above-ground net primary productivity was calculated more accurately by integration of a `growth rate' curve, derived from a curve of live biomass during the year, than by harvest techniques. The live biomass curve was fitted to field-harvest results using two opposing modified Gompertz functions. One represented `growth' and the other `death'. `Growth rate' and `death rate' were derived from these. The difference between them gave the `rate of change of biomass'. These curves also give functions for instantaneous rates, and are useful for computer simulations. (3) Above-ground net productivity of the herb layer, in the open between trees, was 76, and for grasses alone 60 g m-2 yr-1 (both figures include 10 g m-2 yr-1 eaten by insects). Averaged over 3 yr, the peak rate of biomass accumulation of the grass layer, in the open, was 5.2 g m-2 week-1 on 2 December (in the austral summer) but in one year it was as high as 9.3 g m-2 week-1. These figures corresponded to 0.1 and 0.14 g g-1 live biomass per day on a dry matter basis. (4) Peak `grazeable' live biomass above ground was reached between December and mid-February and in the grass layer amounted to 88 g m-2 in the open between trees, 54 g m-2 beneath them, and 78 g m-2 overall. Peak current-year live biomass of the tree and shrub layer was 134 g m-2. Excluding twigs, 188 g m-2 was potentially edible by large herbivores, but species selectivity and restricted browsing height result in less-than-complete use of this potential. The mean total root standing-crop for grass and tree layers was 3700 g m-3 beneath trees and 864 g m-3 between them. The overall mean root standing crop of the herb layer was 301 g m-3. (5) Production figures for Nylsvley are compared with those for grasslands in other parts of the world.

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