Recent spread of Dracophyllum scrub on subantarctic Campbell Island, New Zealand: climatic or anthropogenic origins?
The vegetation of subantarctic Campbell Island consists mainly of lowland Dracophyllum scrub and upland tussock grassland and tundra. Soon after the island was discovered in 1810, occupation by sealers and whalers led to localized burning and cutting of scrub. Further burning and cutting took place as a result of sheep farming between 1894 and 1931. Since the earliest photographs and vegetation descriptions of the island in the late nineteenth century, scrub cover has expanded markedly. Also, since the 1960s, the island has become warmer and drier. The aim of this paper is to quantify scrub changes as depicted in photo-sequences from the island, and to establish if the spread of scrub is a result of the cessation of anthropogenic activities or a response to global warming. Location
Campbell Island, an isolated island 600 km south of New Zealand mainland (52°33.7′S, 169°09′E). Methods
Changes in scrub cover in the island were established through analysis of 33 photographic sequences in the island, the earliest photos dating back to 1888. Visual estimates were made of percentage woody cover in matching areas of repeat photographs, and of changes in scrub distribution with altitude and landscape type. Results
Dracophyllum scrub was restricted at the time of the first photographs (1888) relative to its present extent, and earlier written observations suggest that there had been little change from the 1840s. Scrub cover has subsequently increased in most areas. Most of the spread occurred after the cessation of farming. Change has mostly consisted of a thickening and expansion of pre-existing scrub patches. There is no indication that the upper elevational limit of scrub has increased. Main conclusions
Initial reduction of scrub was probably due to anthropogenic fire in the early nineteenth century, although it is possible that less favourable climates had also restricted its distribution. Failure of the scrub to regenerate significantly between 1840 and c. 1895 may also have been a consequence of generally cooler, wetter climates at this time. Rapid scrub expansion began between the end of farming and burning in 1931 and 1988 when the remaining feral sheep were removed. Sheep grazing in the absence of fire reduced competition, and encouraged regeneration and growth of woody scrub. Spread was probably assisted by a pronounced shift to warmer, drier climates in the second half of the twentieth century. Upper elevational limits of scrub have not increased, suggesting that factors other than summer temperature are controlling scrub line in this superoceanic environment.
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