Do cyclones and forest fragmentation have synergistic effects? A before-after study of rainforest vegetation structure at multiple sites

Abstract

Ecological degradation within areas of remnant forest may be amplified if the effects of fragmentation interact with the effects of other environmental disturbances such as wind storms. We used before-after comparisons to assess the effects of Tropical Cyclone Larry on remnant and continuous rainforest in the Wet Tropics uplands of north-eastern Australia. Vegetation structure was measured 3 years before the cyclone and 6 months afterwards, at eight continuous forest sites and eight remnants (6-37 ha), within 20 km of the cyclone's track. The cyclone caused extensive defoliation, felling and breakage of stems and branches (greatest among the trees >100 cm diameter which had around 50% stem loss), and increased litter and woody debris. Cyclone effects were strongly influenced by a site's spatial position (P = 0.005, 0.001 in multivariate analyses of overall damage). Maximum damage occurred 10-15 km south of the cyclone track, perhaps because of the additive effects of the west-moving air at the southern eyewall combined with the cyclone's own rapid westward movement. Most fragments were south of the cyclone track, as a consequence of spatially selective deforestation practices, and therefore, showed greatest damage. However, once the effects of spatial position were considered, the independent differences in cyclone effects between fragments and continuous forest were lost (P = 0.23, 0.41 when north-south distance was included as a covariate in analyses). The expected protection afforded by a continuous forest canopy seems to have disappeared in the face of extremely strong cyclonic winds and down-draughts. Nevertheless, an interaction between fragmentation and disturbance may yet occur, during the period of post-cyclone recovery, owing to the effects of landscape context on plant recruitment. For example, there was a higher diversity of exotic seedling germination in fragments, independent of the extent of cyclone damage. © 2008 The Authors.