Poster, International Biogeography Society (IBS) 6th Biennial Meeting, Miami, USA: 09.01.2013 - 13.01.2013
Tree lines have been an important research focus in biogeography for many decades, as tree lines are, per definition, the boarder between the subalpine forest and alpine environments. Theoretically, island tree lines experience specific drivers unique to island ecosystems such as isolation-induced absence of adapted high-elevation tree species, immature soils (at least on volcanic islands), exposition to high oceanic winds and only a small Massenerhebungseffekt due to relatively small island area. Here, we test how isolation, island area, island maximum elevation and geology influence the global pattern of island tree line elevations. We collected a global dataset by applying a stratified design using the Global Island Database, GoogleEarth and literature to identify maximum tree line elevations. Islands ranged from 65°N to 55°S. Maximum tree line elevations spanned between 110 and 4061 m showing a single hump near the equator. Island area and island maximum elevation had a positive effect on tree line elevation in contrast to isolation, even when only testing for volcanic islands. When focusing on the subtropics and tropics no latitudinal effect was visible but tree lines were significantly higher on continental than on volcanic islands. The tropical peak may be attributed to diurnal temperature regimes, which do not differ strongly between islands and the continent. In contrast to continents the subtropical double hump is lost on islands likely owing to the reduced seasonality and Massenerhebungseffekt. The theory of island biogeography would suggest an influence of geology and isolation, yet it does not seem to play a role in global island tree line patterns.