| Abstract: | Abstract This paper demonstrates the changes in structure, floristics and forest floor light regimes across the boundary between open forest and rainforest at Kirrama in northeastern Queensland. Hemispherical photographs 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 zones were identified across the boundary: (A) open forest, (B) tall open forest, (C) tall open forest with a rainforest under-storey, (D) young rainforest, and (E) mature rainforest. During the summer (September–March) direct PPFD declined semi-exponentially across the boundary, while 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 relative 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 percentages of direct radiation relative to that above the forest (direct site factors) ranged from 2.9–38.3% at the same sites, respectively. Daily PPFD in the rainforest understorey is probably insufficient for the shade intolerant (pioneer) species which prefer the more open environments on the rainforest margin where light intensities and soil temperatures are higher. Towards the rainforest end of zone C, light conditions appear to be similar to those experienced within small treefall gaps in rainforests. Typically, such light conditions are preferred by shade tolerant (primary) tree species that are unable to grow and reproduce successfully in the adjacent shaded understorey and to compete with the fast growing pioneer tree species in the more open end of zone C. |
