Original Articles

Patterns of seed production and shrub association in two palatable Karoo shrub species under contrasting land use intensities


Seed production and shrub association patterns of the two palatable shrubs. Tripteris sinuatum and Tetragoma froticosa were investigated on heavily grazed communal and lightly grazed commercial rangeland in the succulent karoo, Namaqualand. Seed production in both these species was substantially reduced on the communal rangeland relative to the commercial rangeland. Seed production in T. sinuatum was most severely affected, with seed production on the communal rangeland two orders of magnitude lower than on the commercial rangeland. The proportion of T. sinuatum and T. froticosa shrubs growing unassociated with other shrubs on the communal rangeland was reduced by approximately 50%. Lycium ferocissimum was most effective as a refuge plant for T. sinuatum in terms of the mean seed production per plant. Ruschia robusta was however most important in terms of the overall amount of seed produced by T. sinuatum and T. froticosa on the communal rangeland, due to the large number of times this association occurred. Refuge plants did not appear to be particularly important for T. froticosa in terms of seed production, but were important in terms of providing safe sites for recruitment. The results of this study suggest that unpalatable or thorny species such as L. ferocissimum play an important role in providing refuge sites for palatable species that might otherwise become entirely lost from overgrazed rangelands.

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