Controlled environment treatments were applied to assess the effects of temperature on the seedling mortality and growth rates of Toona australis 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 to canopy disturbance and gap-phase regeneration; however, their geographic ranges are very different. The hypothesis was that the species confined to the warm tropics (F. brayleyana) would have higher seedling mortality and a slower growth rate at lower temperatures than the species 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 (29/22° C) and cool (22/10°C), but not the intermediate (24/16° C), day/night temperature regimes. Their growth rates both decreased with decreasing temperature, but the decrease was significantly less for F. brayleyana which had the faster growth rate and lower seedling mortality in the cool regime. These results led to the rejection of the hypothesis and a test of the assignment of these two species to the same ecological niche. The test involved monitoring their growth to sapling-size in the intermediate temperature regime together with four other co-occurring tropical rainforest tree species belonging to different ecological niches. The growth rates and proportions of above-ground biomass allocated to woody tissue distinguished T. australis and a fast-growing pioneer species from F. brayleyana and three primary forest species. The stem heights and aboveground biomass of T. australis and the pioneer species exceeded the other four species by factors ranging from two to five. It is concluded that T. australis does not belong to the same ecological niche as F. brayleyana, and it is recommended that more research be conducted on the ecotypic temperature responses of the taxon T. australis.
Keywords: Ecological niche; Growth rate; Seedling mortality; Temperature; Tropical rainforest.