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.

Wood density predicts plant damage and vegetative recovery rates caused by cyclone disturbance in tropical rainforest tree species of North Queensland, Australia

“…One consequence of the opening of the canopy following cyclone damage in Australian tropical rainforests is the response of vine species, including natives such as Calamus spp., which can dominate the understorey and often cover canopy trees (Webb 1958). Postcyclone studies in complex mesophyll vine forest on the Atherton Tablelands found an increase in the frequency of vine tangles and vine towers in damaged parts of both fragments and contiguous forest (Catterall et al 2008), and T. corymbosa has proliferated on some Mabi fragment edges which received severe damage from Tropical Cyclone Larry (T. Curran pers. obs.…”

Gray-headed Lemur (Eulemur cinereiceps) Abundance and Forest Structure Dynamics at Manombo, Madagascar

“…Such processes almost certainly have continued at Manombo. The interaction of anthropogenic and cyclonic activity at various spatial and temporal scales is complex (Burslem et al 2000, Catterall et al 2008. Given the limitations of the present study, we refrain from definitive conclusions about the relative contribution of these factors.…”

How do beetle assemblages respond to cyclonic disturbance of a fragmented tropical rainforest landscape?

“…This could happen if the increased amount of edge resulting from fragmentation increases the level of wind damage among fragments with a high proportion of edge (Laurance and Curran 2008). To the best of our knowledge, this suggestion has only been tested on rainforest vegetation and structure, where no such interaction was detected (van Bloem et al 2005;Catterall et al 2008;Grimbacher et al 2008a). Such synergies may become more important in the future because of the continued clearing and fragmentation of humid tropical forests around the globe (Hansen et al 2008) and because of the predicted increase in the intensity of tropical cyclones resulting from climate change (Webster et al 2005).…”

“…The initial eVects that tropical storms have on the structure of moist tropical forests can be very dramatic and may include widespread defoliation, branch loss, stem breakage, and toppling and uprooting of whole trees (Tanner et al 1991;Everham and Brokaw 1996;Lugo 2008). Cyclones also increase the amount of woody debris and leaf litter on the forest Xoor Catterall et al 2008). Cyclone-initiated defoliation and loss of canopy cover disrupt the role that the canopy plays in shading and buVering the microclimate of the forest Xoor (Walsh 1996;Turton and Siegenthaler 2004).…”