Vegetation changes across edges of rainforest remnants

Abstract

Fragmentation of Australian rainforests has left many small isolated remnants surrounded by agricultural land. An understanding of the vegetation dynamics across the edges of such remnants is important for assessment of their conservation value. We examined change in vegetation across the edges of remnants of temperate rainforest from the Yarrawa Brush, Robertson, New South Wales, Australia. We compared the effects of remnant size, level of disturbance and topographical position on four groups of plants—core rainforest, early coloniser rainforest, weed and pasture species—across abrupt boundaries between rainforest and cleared agricultural land. We used plant species as bio-indicators to assess the width of edge zones, based on abrupt changes in abundance of weeds and coloniser seedlings, and increases in core rainforest species from the remnant edge toward the interior. The width of these edge zones varied from 4 to 16 m. The vegetation composition in the interior of remnants was distinctly different from that at the edge. Rainforest species were most abundant inside remnants. There was a peak in species richness of colonising plants at the edge of remnants, and a peak in species richness of weeds just outside the edge that resulted in an elevated total species richness at remnant edges. Increased disturbance resulted in increased edge widths, greater density and species richness of colonisers and weeds, and reduced density and species richness of core rainforest plants. Sites with minor disturbance had an abrupt increase in core rainforest species as one moved across the edge zone into the interior of the remnant, while coloniser species peaked in the edge zone and then decreased. In contrast, at sites with major disturbance the increase in core rainforest species was more gradual and both the richness and density of coloniser species maintained high values into the interior of the remnant rather than just in the edge zone. There was an increase in total species richness with increasing remnant size. The level of disturbance was a key determinant of vegetation composition, and hence of conservation value of remnants, as it has a direct influence on whether these rainforest remnants are able to expand their boundaries or continue to decrease in area of effective rainforest core. Disturbance was due to cattle and other agricultural practices. We agree with previous studies which suggest that, because of disturbance events accompanying fragmentation, these remnants are unlikely to be an accurate representation of the floristic composition of the original Yarrawa Brush.