Internal fragmentation in the rainforest: edge effects of highways, powerlines and watercourses on tropical rainforest understorey microclimate, vegetation structure and composition, physical disturbance and seedling regeneration

摘要: Human activities have led to the loss and fragmentation of rainforest around world’s tropics sub-tropics, whilst internal by linear clearings has subdivided many remaining areas rainforest, exacerbating effects initial disturbance. Linear for human infrastructure, including roads powerlines, are widespread within rainforests world Wet Tropics World Heritage Area north-eastern Queensland in particular. Edge these clearings have potential substantially increase disturbed area existing tracts of rainforest affect a wide range biota. I examined edge two artificial on rainforest understorey microclimate, physical disturbance regimes, vegetation structure and composition tree seedling survival growth. adjacent powerline clearings highways were compared with those near perennial creeks, only type of widespread natural canopy opening available complex mesophyll vine forest Palmerston Region Heritage Area northeastern Queensland (average annual precipitation 3000 – 4000 mm). Understorey microclimate highways, creeks was significantly altered from that forest interior. Light intensity elevated edges powerline clearings, with the effect being strongest creek edges. Air temperature vapour pressure deficit were clearing dry season highway in both wet seasons but not either season. In contrast, soil moisture reduced remained unaltered near powerline speed related to distance when assessed instantaneous measurements although slight increases air detected diurnal patterns using data loggers. The predominant throughout the study sites, however, at or close 0 m s-1. Canopy elevated near lowered no edge gradients These different gradients may be largely result differences fluxes latent and sensible heat each type indicating nature is least as important width in determining severity extent microclimatic effects. Variation understorey light environment red:far red ratio, which been shown other studies correlate very closely percent canopy transmittance well commonly used measures cover and light availability. Average red:far red values and reduced although proportion high light measurements (≥ 0.70) 12 all types. These indicate that anthropogenic are more ‘sealed’ (ie greater foliage density the edge) than This open was accompanied decline (due changes texture) suggesting that periodic flooding one causative factors maintaining lower foliage density thus comparatively structure. Measures provided mixed results. There greater numbers fallen trees no particular size appeared disproportionately risk tree-fall mortality. As there also number living edge, it uncertain whether mortality rate truly edge. small-scale seedlings through falling leaf litter canopy debris consistently interior. Although disturbance, measured seedlings, was elevated edges, there consistent reduction stature taller (≥ 50 cm) might expected amongst suffering greater rates trauma. Whilst damage artificial seedlings stress, perhaps a result temperatures year-round in understorey deficit lack any signal in seedling suggests this influence seedling community. Vegetation composition forest, relative abundances vines, small saplings were elevated Pioneer species abundant edge, particularly creeks. Light-demanding herbs occurred frequently and late-successional shrubs less Grasses weeds found almost exclusively also recorded large gaps low abundance within Changes proportions early-, mid- latesuccessional tree between adults juveniles suggested light conditions longer suitable recruitment early-successional near anthropogenic become more sealed since created. In contrast, availability near creek sufficient light-demanding pioneer species. Proximity did growth for eight included herbivore-exclusion experiment. pioneer species survived powerline clearing edges). Light determinant of the late-successional species. weak evidence decrease herbivory evidence an effect distance majority not experience experiment probability seedlings experiencing positive increased increasing The major observed study rather than seedling growth, suggesting ability persist understorey may an factor influencing seedling community. These indicate associated abiotic edge effects driving force behind plant community the forest powerlines was brighter, warmer drier interior slightly windier. openings cause moisture stress experience Analysis a combination degree ‘sealing’ among types caused by varying regimes properties of themselves (particularly level evaporative cooling) responsible biotic natural and openings.