Why do evergreen trees dominate the Australian seasonal tropics?

Abstract

The northern Australian woody vegetation is predominantly evergreen despite an intensely seasonal climate and a diversity of deciduous species in the regional flora. From a global climatic perspective the dominance of evergreen rather than deciduous trees in the Australian savannas is apparently anomalous when compared with other savannas of the world. However, this pattern is not unexpected in light of existing theory that emphasises photosynthetic return relative to cost of investment between deciduous and evergreen species. (a) Climatically, monsoonal Australia is more extreme in terms of rainfall seasonality and variability and high air temperatures than most other parts of the seasonally dry tropics. Existing theory predicts that extreme variability and high temperatures favour evergreen trees that can maximise the period during which leaves assimilate CO2. (b) Soil infertility is known to favour evergreens, given the physiological cost of leaf construction, and the northern Australian vegetation grows mainly on deeply weathered and infertile Tertiary regoliths. (c) These regoliths also provide stores of ground water that evergreens are able to exploit during seasonal drought, thereby maintaining near constant transpiration throughout the year. (d) Fire disturbance appears to be an important secondary factor in explaining the dominance of evergreens in the monsoon tropics, based on the fact that most deciduous tree species of the region are restricted to small fire-protected sites. (e) Evolutionary history cannot explain the predominance of evergreens, given the existence of a wide range of deciduous species, including deciduous eucalypts, in the regional tree flora.