GROWTH-RATES OF SELECTED AUSTRALIAN TROPICAL RAIN-FOREST TREE SPECIESUNDER CONTROLLED CONDITIONS

Controlled environment treatments were applied to assess the effects o f temperature on the seedling mortality and growth rates of Toona aust ralis and Flindersia brayleyana, two tropical rainforest tree species from northeast Queensland, Australia. Past workers have assigned these two species to the same ecological niche in terms of their response t o canopy disturbance and gap-phase regeneration; however, their geogra phic ranges are very different. The hypothesis was that the species co nfined to the warm tropics (F. brayleyana) would have higher seedling mortality and a slower growth rate at lower temperatures than the spec ies that occurs over a wide latitudinal range from the warm tropics to cooler temperate environments (T. australis). Significant differences were found in the growth rates of these two species in the warm (29/2 2 degrees C) and cool (22/10 degrees C), but not the intermediate (24/ 16 degrees C), day/night temperature regimes. Their growth rates both decreased with decreasing temperature, but the decrease was significan tly less for F. brayleyana which had the faster growth rate and lower seedling mortality in the cool regime. These results led to the reject ion of the hypothesis and a test of the assignment of these two specie s to the same ecological niche. The test involved monitoring their gro wth to sapling-size in the intermediate temperature regime together wi th four other co-occurring tropical rainforest tree species belonging to different ecological niches. The growth rates and proportions of ab oveground biomass allocated to woody tissue distinguished T. australis and a fast-growing pioneer species from F. brayleyana and three prima ry forest species. The stem heights and aboveground biomass of T. aust ralis and the pioneer species exceeded the other four species by facto rs ranging from two to five. It is concluded that T. australis does no t belong to the same ecological niche as F. brayleyana, and it is reco mmended that more research be conducted on the ecotypic temperature re sponses of the taxon T. australis.