Vegetation changes without grazing, Flat Top Hill, Central Otago

Permanent monitoring sites in Flat Top Hill Conservation Area in Central Otago were resurveyed in the spring of 1997, following a four-year interval in which rabbit numbers had been very low, and stock absent. All study sites showed considerable vegetation change between 1993 and 1997. The most significant changes in the vegetation were large increases in overall vascular species richness and diversity, and particularly increases in the richness and abundance of exotic species. Exotic grasses (mainly Anthoxanthum odoratum , but also Aira caryophyllea, Dactylis glomerata and Holcus lanatus i n particular communities) have increased dramatically, but there were also significant increases in native grasses (Dichelachne crinita , three Elymus species, Festuca novae-zelandiae, Poa cita and Rytidosperma clavatum). The woody exotic species Thymus vulgaris and Rosa rubiginosa increased significantly, but Sedum acre decreased markedly in abundance from 1993 to 1997. Native species characteristic of `scabweed communities' in the driest habitats on Flat Top Hill (particularly Raoulia australis and Poa maniototo , but also Colobanthus brevisepalus, Poa lindsayi, Raoulia beauverdii, Rytidosperma buchananii and Rytidosperma thomsonii) have decreased in abundance and/or distribution over the four years. The three native ephemeral species recorded on Flat Top Hill in September 1993 (Ceratocephalus pungens, Myosotis pygmaea var. minutiflora and Myosurus minimus ssp. novae-zelandiae) all decreased in abundance from 1993 to 1997. Only Myosotis pygmaea var. minutiflora was recorded in study sites in September 1997. Myosurtis minimus ssp. novae-zelandiae was still present in the Conservation Area, but Ceratocephalus pungens was not found despite searches of previously known locations. Probably, the decrease in native ephemerals, and of the perennial native species characteristic of scabweed communities, is due to the increase of exotic grasses in the previously sparsely vegetated habitats preferred by these species. Two different management regimes in different parts of Flat Top Hill Conservation Area are recommended to maintain the key conservation values of the area. If populations of native ephemeral species are to be maintained on Flat Top Hill, it is suggested that grazing be reintroduced in two peripheral areas, the low ridge west of Butchers Creek, and the gravel fan east of Butchers Dam. It is recommended that remainder of the conservation area should presently remain securely rabbit-fenced and free of stock, and that low rabbit numbers are maintained. There is an immediate and ongoing requirement for the control of Rosa rubiginosa and Lupinus arboreus , particularly east of the main ridge. The effects of management should be reappraised following vegetation monitoring at regular intervals in the future.