DEMOGRAPHIC-CHANGES AND THE DEVELOPMENT OF COMPETITION IN A NATIVE-AUSTRALIAN EUCALYPT FOREST INVADED BY EXOTIC PINES

Data from the second half of a 14-year study of a eucalypt dry sclerop hyll forest invaded by Pinus radiata D. Don shows further progress in the development of a mixed eucalypt-pine stand. Earlier (Chilvers and Burdon 1983) it was clear that pines invading from an adjacent commerc ial plantation were starting to contribute their own progeny to the mi xed stand. Initially the recruitment of established pines (> 1 m high) from these seedlings was very slow. However, since 1984, increasing n umbers of seedlings have survived beyond their first year, suggesting the possibility of another wave of successful establishment in the nea r future. The established fraction of the pine population continued to grow rapidly and incurred no mortality. Many of these plants have now emerged through the top of the eucalypt canopy. In contrast, both juv enile and adult eucalypt numbers continued to decline. Despite these d emographic and visual changes, comparative analysis of the growth of e stablished pines over two consecutive 7-year periods (1974-1981; 1981- 1988) showed that competition between neighbouring plants, especially pines, is beginning to affect individual growth rates. Growth rates of pines having other pines as nearest neighbours declined 40% between 1 974-1981 and 1981-1988, while that of pines with eucalypts as nearest neighbours fell by 28%. In pine-pine nearest neighbour (NN) pairs, gro wth rates were significantly affected by the size (diameter) of the ne arest neighbour. In pine-eucalypt NN pairs growth rates of the pines w ere not affected by the size of the eucalypt, but these were significa ntly negatively correlated with the inverse of the distance to the euc alypt. More broad-based assessment of 43 pines showed a significant re lationship between their growth rates and the summed competitive effec t of other pines within a 5-m radius. No such relationship was found b etween growth rate and the summed competitive effect of eucalypts in t he 5-m zone.