LIGHT ENVIRONMENTS AND FLORISTIC COMPOSITION ACROSS AN OPEN FOREST-RAIN-FOREST BOUNDARY IN NORTHEASTERN QUEENSLAND

This paper demonstrates the changes in structure, floristics and fores t floor light regimes across the boundary between open forest and rain forest at Kirrama in northeastern Queensland. Hemispherical photograph s of the canopy were used to estimate spatial and temporal variations in potential (clear-sky) direct and diffuse photosynthetic photon flux density (PPFD = 400-700 nm) across the boundary. Five vegetation zone s were identified across the boundary: (A) open forest, (B) tall open forest, (C) tall open forest with a rainforest understorey, (D) young rainforest, and (E) mature rainforest. During the summer (September-Ma rch) direct PPFD declined semi-exponentially across the boundary, whil e in the winter (April-August), the decline was more linear. However, the decline in diffuse PPFD across the boundary was linear throughout the year. Mean per cent grass cover was positively correlated and mean per cent shrub cover negatively correlated with annual average total PPFD across the boundary. Mean percentages of diffuse radiation relati ve to that above the forest (diffuse site factors) ranged from 9.8% in the mature rainforest to 66.4% in the open forest, while mean percent ages of direct radiation relative to that above the forest (direct sit e factors) ranged from 2.9-38.3% at the same sites, respectively. Dail y PPFD in the rainforest understorey is probably insufficient for the shade intolerant (pioneer) species which prefer the more open environm ents on the rainforest margin where light intensities and soil tempera tures are higher. Towards the rainforest end of zone C, light conditio ns appear to be similar to those experienced within small treefall gap s in rainforests. Typically, such light conditions are preferred by sh ade tolerant (primary) tree species that are unable to grow and reprod uce successfully in the adjacent shaded understorey and to compete wit h the fast growing pioneer tree species in the more open end of zone C .