Long-term effects of defoliation: incomplete recovery of a New Zealand alpine tussock grass, Chionochloa pallens, after 20 years

1. Chionochloa pallens, a New Zealand low alpine tussock grass, is an impor tant food plant of the takahe Porphyrio hochstetteri, a rare endemic flight less rail. Introduced red deer Cervus elaphus compete with the takahe for t his resource. 2. 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 carri ed out. Growth was compared between treatments (control vs. defoliated) and with a previous sampling in 1986. 3. Even after two decades, recovery was incomplete. Tiller dry weights and size (length and basal diameter) and tussock dry weight per unit area remai ned significantly depressed in the clipped plots (by about 30%) compared wi th the control plots. 4. Other features, such as biomass allocation to plant parts and mineral co ncentrations in tissues, no longer showed many significant differences betw een the treated and control plots, indicating continued convergence towards the controls. 5. However, at the present rate of recovery it is estimated that the effect s of a single severe defoliation on biomass (per tiller and per unit area) will persist for nearly three decades. 6. As C. pallens is known to be a relatively fast-growing species of Chiono chloa, recovery of other native snow tussock grasses in these alpine habita ts damaged by deer grazing is likely to take even longer.