Colonisation of clearfelled coupes by rainforest tree species from mature mixed forest edges, Tasmania, Australia

Abstract

Mixed forest, containing a eucalypt overstorey and an understorey of rainforest tree species, accounts for approximately 20% (195,000 ha) of Tasmania's wet eucalypt forest. In wood production areas it is typically clearfelled, burnt and then sown with eucalypt seed. This management removes virtually all standing seed sources within the coupe, so that recolonisation of coupes by rainforest tree species depends largely on seed sources located at the coupe edges. We quantified the influence of mature mixed forest edges on rainforest regeneration following clearfelling by modelling the change in the density of the regeneration of the four dominant rainforest tree species ( Nothofagus cunninghamii, Atherosperma moschatum, Phyllocladus aspleniifolius and Eucryphia lucida) with increasing distance from forest edge. We also assessed the influence on rainforest tree regeneration of prevailing wind direction, age of regeneration, characteristics of the mature edge vegetation and of the competing regenerating vegetation within the coupe. Distance from edge and age of coupe were highly significant ( p < 0.01) effects in each of the species models. We found that the abundance of regeneration declined with increasing distance from edge for all four rainforest tree species, and with the exception of A. moschatum regeneration, increased with coupe age up to the age of 15 years. The abundance of N. cunninghamii and E. lucida, which are species with restricted seed dispersal, declined most steeply with increasing distance from the edge. A. moschatum, which is a species with the potential for long distance seed dispersal by wind, was more abundant than N. cunninghamii and E. lucida at distances greater than 20 m from coupe edges. More than 500 seedlings ha −1 were present at all distances from coupe edge for P. aspleniifolius, reflecting its capacity to germinate after disturbance from soil-stored and bird-dispersed seed. There were no significant differences in seedling density upwind or downwind of coupe edges, although the potential for dense regeneration of N. cunninghamii and E. lucida and for long distance dispersal of A. moschatum appeared to be greatest downwind of edges. Other variables that significantly affected the abundance of regeneration were the height of rainforest tree species in the edge vegetation ( N. cunninghamii model), the cover of rainforest tree species in the edge vegetation ( A. moschatum model) and the cover of competing eucalypt regeneration within the coupe ( P. aspleniifolius model). The proportion of rainforest tree species that regenerated vegetatively was small (3.1%). We concluded that management which maintains mature mixed forest edges, or patches of mature forest within coupes, is likely to result in greater levels of rainforest regeneration and a more rapid shift towards pre-harvest composition following logging. We use our results to demonstrate that variable retention harvesting systems, such as aggregated retention or stripfelling, which reduce the distance to rainforest seed source, would result in a greater abundance of rainforest regeneration over a larger proportion of the coupe than current clearfell, burn and sow silviculture.