Effects of defoliatingStipa tenuisandPiptochaetium napostaenseat different phenological stages: root growth☆

Abstract During 1991–1993, root growth after a single annual defoliation at different phenological stages (vegetative or during early, late or post-internode elongation) to 5–7 cm stubble height was determined on field-grown plants of Stipa tenuis and Piptochaetium napostaense , two important forage grasses in the semi-arid, temperate rangelands of Central Argentina. Nondefoliated plants served as controls. Root growth was estimated by root length density measurements obtained through destructive harvesting, and by using a root periscope to monitor changes in the number of roots intercepting glass tubes buried in the soil. The root periscope allowed registration of the angle of each root interception, which made it possible to measure the root system dynamics (root appearance and disappearance). The effects of defoliating S. tenuis and P. napostaense at different phenological stages on root growth varied with the methodology used. Root growth of both species was not reduced by defoliation when it was estimated from root length density determinations. However, it was lower ( p