Dracophyllum scrub expansion on subantarctic Campbell Island, New Zealand

Campbell Island lies c. 600 km to the south of New Zealand in the Southern Pacific Ocean and is covered mostly by Dracophyllum scrub, tussock-grassland, and upland tundra associations. European contact with the island began in the early 1800s. The earliest accounts (1840) and photographs of the island's vegetation (1888 and 1907) indicate that scrub was highly restricted, occurring mainly as shoreline fringes and extending upslope in sheltered gullies. The island's vegetation was severely modified by widespread burning possibly from the early 1800s up to 1931 and sheep grazing from 1895 to 1991. Photographs have been retaken periodically from fixed photo-points (the places photos were taken from in 1888, 1894, 1907, and 1941) since the 1960s, to document changes in the distribution of scrub. Photo-sequences show that Dracophyllum scrub cover has increased since photographs were taken in the 1800s. To estimate the changing distribution of Dracophyllum scrub since 1888, 33 of these photographic sequences were analysed. Scrub has increased since the earliest photographs, in particular since 1941. Yearly mean rate of increase in percent scrub cover, calculated from each photo-sequence, ranged from 0.02% to 1.85%. Some sequences did not feature any change in scrub cover. Increase has occurred partly as a thickening and expansion of pre-existing scrub patches. The photographic sequences were also analysed in terms of landscape elements to determine if topography was a factor influencing change in scrub. Dracophyllum scrub generally expanded and became more dense earlier on the foreshore, lower slopes and gullies. Expansion on the mid slopes down to the foreshore and on flat bogs occurred in later stages of photo-sequences. Upper slopes (>200 m a.s.l.) remained clear of scrub. Fifteen Dracophyllum stems were sampled from each of 17 plots along three transect types. Plots within any transect type had the same fertility and similar drainage conditions as each other. The age structure of the Dracophyllum population was estimated from annual rings. Graphs were drawn of Dracophyllum age structure, and related to changing land use and regional climate change to assess their relative influences. The photographic record and data on the Dracophyllum population age structure show that Dracophyllum scrub expanded dramatically from the 1930s. Phases in scrub recruitment coincided with major changes in land use. Broad climate trends of warming and drying may have encouraged recruitment, but expansion of Dracophyllum seems to be driven mainly by land use changes. The timing and magnitude of Dracophyllum expansion are determined by the presence or absence of burning and/or grazing. Differences in site fertility and differences in drainage, probably affected by climate trends, also play a role in where Dracophyllum expands. Dracophyllum established preferentially in better-drained sites and grew faster on fertile sites at lower elevations. The dramatic expansion of Dracophyllum scrub has implications for understanding human impacts and climate change, and for the management of Campbell Island for conservation values.