Spacing and Transpiration of Various Sized Clumps of a Desert Grass, Hilaria Rigida

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Abstract
Various methods were used to examine within-species competition between clumps of the grass H. rigida growing in the Sonoran desert. Clumps of increasing size tended to be further from a random point as well as separated from their nearest neighboring H. rigida by greater distances. Larger clumps had more ground area for their roots. This was graphically demonstrated using polygons circumscribing individual clumps and directly measured by excavation of the roots. The larger space occupied by the larger clumps presumably reflects their pre-emption of ground water. Leaves on small clumps (< 10 culms) had a 3-fold higher rate of transpiration per unit leaf area than leaves on large clumps (> 200 culms) but the larger clumps had .apprx. 3 times as much leaf area per unit ground area. Consequently, the depth of water transpired daily did not depend on clump size. As the amount of CO2 fixed per unit ground area also did not differ with clump size, the pattern of small and large clumps may be quite persistent in time.