Tropical tree stand structures on a seasonally flooded elevation gradient in northern Australia

The variation in the stem size-class distributions of five dominant tropical tree species was studied in a grid established on a 7 m elevation gradient in a monsoon forest adjacent to a treeless floodplain on the north coast of the Northern Territory. Acacia auriculiformis was widespread over the gradient. The size-class distribution of stems was irregular and poorly related to elevation. These patterns are consistent with this species’ ability to colonise rainforest gaps. Nauclea orientalis had the narrowest range on the elevation gradient and was also spatially restricted to a corner of the grid. It is unknown why the size-class distribution of this species was biased toward larger stems, with few stems < 2 cm dbh. The size-class distribution of Lophostemon lactifluus was irregular and thought to reflect phasic regeneration events; stems >= 31 cm dbh were restricted to the 3-6 m elevation classes. Syzygium nervosum appears to continuously regenerate as evidenced by the reverse-J-shaped size class distributions. A cohort of stems < 10 cm dbh may reflect recent up-slope expansion into forest dominated by A. auriculiformis and L. lactifluus. Melaleuca cajuputi appears to periodically regenerate, and a cohort of stems < 31 cm dbh may have colonised the adjoining treeless floodplain. There is some evidence that M. cajuputi is excluded from sites dominated by S. nervosum and N. orientalis. The effect of a single fire on the size-class distributions of the five species was minor, affecting a small proportion of the smaller stems.