Long-term effects of defoliation: incomplete recovery of a New Zealand alpine tussock grass, Chionochloa pallens, after 20 years.
Chionochloa pallens, a New Zealand low alpine tussock grass, is an important food plant of the takahé (Porphyrio hochstetteri), a rare endemic flightless rail. Introduced red deer (Cervus elaphus) compete with the takahé for this resource. The experiment reported here measured the long-term rate of recovery of the grass after simulated severe deer grazing by means of a single clipping in 1977. Biomass, tillering and levels of six mineral nutrients (N, P, K, Ca, Mg, Na) were measured in tussocks 20 years after the clipping was carried out. Growth was compared between treatments (control vs. clipped) and with a previous sampling in 1986. Even after two decades, recovery was incomplete. Tiller dry weights and size (length and basal diameter) and tussock dry weight per unit area remained significantly depressed in the clipped plots (by about 30%) compared with the control plots. Other features, such as biomass allocation to plant parts and mineral concentrations in tissues, no longer showed many significant differences between the treated and control plots, indicating continued convergence towards the controls. However, at the present rate of recovery it is estimated that the effects of a single severe defoliation on biomass (per tiller and per unit area) will persist for nearly three decades. As C. pallens is known to be a relatively fast-growing species of Chionochloa, recovery of other native snow tussock grasses in these alpine habitats damaged by deer grazing is likely to take even longer.