Revisiting factors affecting deciduousness in tropical rainforests at a study site in coastal lowland rainforest in NE Australia

The proportion of deciduous trees has been analysed in a marginally tropical coastal lowland rainforest between the Daintree River and Cape Tribulation, NE Queensland, Australia, in relation to the climate, soil fertility and evolutionary aspects. Due to the relatively low seasonality index of 0.676 and the low inter-annual variation of rainfall (0.238) in the study area we expected a lower proportion of deciduous canopy trees in this rainforest compared to the monsoonal tropical northern part of Australia with an extensive dry season (seasonality index between 0.91 and 1.01). The extremely low proportion of 6% deciduous species is, however, in contrast to the proportions in other areas with similar seasonality and may be attributable to 1. the relatively high rainfall of 85 mm on average during each of the three driest months, 2. extremely wet conditions and consequently high runoffs (up to 65 % of the annual rainfall) which decreases the water availability in the soil during the dry season and contributes to low soil fertility due to erosion and nutrient losses, and 3. the presence of a high proportion of old lineages and primitive families of angiosperms related to evolutionary aspects of the development of deciduousness in angiosperms. Most leaf-shedding trees in this area can be classified as brevi-deciduous, shedding their leaves for about 2 to 3 weeks at the onset of the dry season, suggesting that decreasing day length and temperature may trigger leaf abscission. Species with a brevideciduous habit may have evolved here at times when evergreen forests were restricted to refugia. Climatically similar conditions are found in rainforests of humid and subhumid regions of Eastern Madagascar reflected by a low canopy height which is also observed in the study area. Reduced growth rates seem to be a more advantageous strategy than a deciduous habit.