Snow tussock (Chionochloa) population responses to removal of sheep and European hares, Canterbury, New Zealand

Abstract

Snow tussock (Chionochloa macra and C. flavescens ssp. brevis) population structures and seedling regeneration were investigated in 10 montane-subalpine stands on formerly forested sites in the Harper-Avoca catchment, Canterbury, subjected to different histories of sheep grazing and browsing by European hares. Population structures were assessed by assigning individuals to four “age-states” based on basal diameter, number of tillers, height, and percent crown death: “seedlings”, “juvenile tussocks”, “mature tussocks”, and “senescent tussocks”. On areas subjected to c. 80 years of grazing by sheep, most snow tussocks had been destroyed. Remaining tussocks were predominantly senescent and seedlings were infrequent, suggesting a continuing decline in abundance. In contrast, stands retired from sheep-grazing for 34 or 21 years were characterised by low proportions of senescent tussocks and high proportions (>60%) of seedlings and juveniles, suggesting the onset of increases in tussock abundance. Population structures inside and outside a 10 year old exclosure showed that browsing by European hares alone was capable of inhibiting C. macra recovery. In retired stands, snow tussock seedlings were most frequent within 70 cm of mature tussocks, on microsites protected from frost-heave by short ground-tier vegetation (e.g., bryophyte mats). A lack of seedling regeneration on heavily grazed/browsed sites reflected the low basal area and poor vigour of the tussocks remaining as seed sources. Balanced management of tussock grasslands for conservation and production urgently requires further knowledge of successional processes and how these are altered by different levels of grazing and burning under different environmental conditions. Age-state analysis appears useful for assessing tussock population responses to management. Comparison of sites subjected to historically different management offers an approach that complements experimental or monitoring studies. © 1992 Taylor & Francis Group, LLC.