Solar radiation regimes in rainforest understoreys, gaps and clearings, with special reference to Northeast Queensland

This thesis examines the temporal and spatial distribution of solar radiation above and beneath lowland, upland and montane rainforest in the wet tropics region of northeast Queensland. Because solar radiation or light is regarded as the major limiting resource in rainforests, an attempt is made to quantify light availability to tree seedlings and saplings growing in characteristic micro-environments, such as within the understorey beneath an intact canopy, within treefall gaps of varying sizes and configurations, and across the rainforest-open forest boundary (ecotone).\ud \ud Several hypotheses concerned with solar radiation in relation to rainforest structure and function are presented. Among these, it is suggested that prevalent cloud cover in the wet tropics region will reduce the incidence of sunflecks in the understorey, but a lower canopy density will facilitate higher levels of background diffuse radiation available to plants in the understorey. It is also hypothesised that reduction in photosynthetically active radiation by cloudiness decreases productive capacities of montane rainforests compared with lowland rainforests in the study area. Another hypothesis proposes that it is naive to consider only gap size and latitude as the main factors affecting light availability within tree fall gaps and associated vegetation dynamics.\ud \ud To test the various hypotheses, direct and indirect methods have been used to measure light availability within the characteristic rainforest micro-environments. The direct measurements have been made with radiometric and photometric sensors, and this thesis includes construction details for a purpose-built photometric sensor. The indirect measurements have been made with hemispherical (fisheye) canopy photographs, and this is considered as the first attempt to model light availability above and beneath rainforest canopies under a range of sky conditions. This was achieved by combining the well-known manual technique for determining direct and diffuse site factors beneath vegetation from fisheye photographs with the computerised Sky-Canopy-Gap-Irradiance (SCANGIR) Model presented in this thesis. The same model is also used to demonstrate the effects of latitude, topography and sky conditions on light availability within hypothetical rainforest gaps and clearings.\ud \ud The results of computer simulations have shown that latitude has a profound effect on the availability of direct light in treefall gaps and that this effect may explain the general decrease in regional rainforest diversity from tropical to temperate eastern Australia. It was shown that at 400 South, the latitude corresponding with temperate rainforest, there is theoretically no time of the day throughout the year when direct light reaches the centre of small-to-Iarge treefall gaps, except on moderately steep north-facing slopes. It is proposed that the combined effects of latitude and topography on the availability of direct light will have an enormous influence on rainforest function, particularly gap-phase dynamics.\ud \ud The results of field measurements within the understoreys of lowland, upland and montane rainforests have shown that daily solar radiation levels are 2- to 5-times higher than those reported for equatorial rainforests. Field surveys within upland and montane rainforest have shown that sunfleck activity is very limited because of the cloudy trade-wind coast climate, and this suggests that sunflecks play only a small role in the annual carbon balance of these forests. This thesis also presents the results of what is regarded as the first measurements of light availability in the understorey of a tropical rainforest before and immediately after a tropical cyclone. On the basis of 20 fisheye photographs it was shown that potential light levels increased by 60% following slightto- moderate canopy damage caused by the cyclone, and that such a change would undoubtedly have implications for the ecophysiology and dynamics of tree seedlings and saplings growing in the understorey.\ud \ud It was found that seasonal changes in light availability within small and large treefall gaps in lowland and upland rainforests in the study area are greater than those found within similar treefall gaps in equatorial rainforests. Results of modelling light availability within several treefall gaps have shown that seasonal variations in direct light in small and large elliptical gaps that are orientated east-west are greater than those found for the exact same gaps orientated north-south. It was also found that there are order-of-magnitude differences in light availability across the range of small-to-large tree fall gaps, and this undoubtedly controls the microclimate and ecophysiology of tree seedlings and saplings growing in these gaps.\ud \ud The study of light availability within a montane tropical (cloud) forest in the study area has shown that reduction in photosynthetically active radiation by cloudiness does not appear to limit the rates of photosynthesis for plants growing in gap and understorey micro-environments because median light levels are much higher than known light compensation points reported for understorey plants elsewhere. These results lend support to the Transpiration Theory as the main reason for the lower primary productivity of cloud forests.\ud \ud The study of light availability across the rainforest-open forest boundary has shown that towards the rainforest end of the transition zone (ecotone), light conditions are similar to those experienced within small treefall gaps in rainforests, while towards the open forest end of the ecotone, light conditions are similar to those experienced within large treefall gaps. It is argued that understanding the light availability continuum and associated plant responses across the ecotone are essential for conservation and management of remaining rainforest in the study area because a substantial portion remains as small, isolated patches.\ud \ud The methods and results presented in this thesis have other implications for management of remaining rainforest and also for reforestation of degraded land formerly occupied by rainforest. It is suggested that the SCANGIR Model and gap theory could be applied by forest managers to promote the eventual dominance of disturbed sites by shade-tolerant (primary) tree species. The SCANGIR Model has numerous applications in both selective and strip clear-cut timber harvesting techniques, whereby 'ideal' light conditions for regeneration of useful shade-tolerant tree species could be achieved by felling trees in a particular direction or altering the width and orientation of the clearcut strips in accordance with latitude, forest height and local topography