Root proliferation in perennial grasses of low and high palatability

Root proliferation of desirable (Stipa clarazii and S. tenuis) and undesirable (S. ambigua) perennial grasses was studied in semiarid rangelands of Central Argentina (40 degrees 39'S, 62 degrees 54'W) in 1998. On 17 September, soil cores were removed from the edge of the plant, metal structures lined with screen mesh (hereafter called bags) were buried in the holes, and root-free soil was placed into these structures. Numbers of green tillers and circumference per plant had previously been determined. Since plants were of unequal size among species, root length and root dry weight data are reported on a per green tiller basis. Half of the plants was defoliated to 5 cm stubble height on 17 September and/or 12 October, while the other half remained undefoliated (controls). Bags were destructively harvested either 20 days after the first defoliation (first sampling) or 56 days after the second defoliation (second sampling) by digging out soil very carefully around each bag. Roots were washed from soil, root length estimated by the line intercept method, root dry weight determined after oven-drying, and root length per unit root dry weight calculated from the two measured variables. Root length and dry weight were more than 96% greater on defoliated and undefoliated plants of S. clarazii than on those of S. tenuis or S. ambigua for both sampling dates. Root length per unit root dry weight, however, was more than 43% greater (p < 0.05) in S. tenuis than in S. clarazii and S. ambigua during the second sampling. Defoliated plants had a similar root length and root dry weight than undefoliated plants in all three species, although plants of S. tenuis defoliated twice showed a greater (p < 0.05) root length than undefoliated controls. Root length and root dry weight were similar between sampling periods, except on undefoliated plants of S. tenuis which had a greater (p < 0.05) root length and root dry weight at the first than at the second sampling. Although root length per unit root dry weight may be greater in S. tenuis than in S. clarazii and S. ambigua, greater root length and dry weight increases in S. clarazii after defoliation appear determinant in contributing to explain its greater competitive ability and defoliation tolerance when compared with the other two species. Nomenclature of taxa followed Cano (1988).