ResearchOnline@JCU This file is part of the following reference: Turton, Stephen Michael (1991) Solar radiation regimes in rainforest understoreys, gaps and clearings, with special reference to Northeast Queensland. PhD thesis, James Cook University. Access to this file is available from: http://eprints.jcu.edu.au/28066/ If you believe that this work constitutes a copyright infringement, please contact [email protected] and quote http://eprints.jcu.edu.au/28066/ SOLAR RADIATION REGIMES IN RAINFOREST UNDERSTOREYS, GAPS AND CLEARINGS, WITH SPECIAL REFERENCE TO NORTHEAST QUEENSLAND VOLUME 1 Thesis submitted by Stephen Michael TURTON MSc (Cant) in January 1991 for the degree of Doctor of Philosophy in the Department of Geography at James Cook University of North Queensland FRONTISPIECE The bright spot that illumines the floor from 9.30 to 10 most mornings is some sapling's hope of becoming a tree. From above you see only the glorious Upper Circle, not the slums beneath. Even parasitic festoons seem gay, those tons of creeper riding on strangled boughs. But this is like judging a country by its brochure. Rainforest is envy visible and justified, in its dog-shade-dog world. A blue quandong, emerging, shows no sign of the struggle that brought it up past the death of a thousand siblings through a blanket of shade the height of two dozen men on each other's shoulders. from 'The Sun Hunters', by Mark O'Connor, (The Great Forest, 1989). I, the undersigned, the author of this thesis, understand that James Cook University of North Queensland will make it available for use within the University Library and, by microfilm or other photographic means, allow access to users in other approved libraries. All users consulting this thesis will have to sign the following statement: "In consulting this thesis I agree not to copy or closely paraphrase it in whole or in part without the written consent of the author; and to make proper written acknowledgement for any assistance which I have obtained from it." Beyond this, I do not wish to place any restriction on access to this thesis. ?O-/{-12 •••••••••••••••••••••••• ~ff.-••••••••••••••• (signature) (date) DECLARATION I declare that this thesis is my own work and has not been submitted in any form for another degree or diploma at any university or other institution of tertiary education. Information derived from the published or unpublished work of others has been acknowledged in the text and a list of references is given. Stephen M. Turton January 1991 ACKNOWLEDGMENTS Over the past six years or so a large number of people have assisted me in different ways with this research project and I regret that I am unable to list all of them. I begin by thanking my supervisor, Dr Mike Bonell (Reader in Geography / Director of the Institute of Tropical Rainforest Studies) for his interest in my work and constructive review of the manuscript. I thank my other colleagues (past and present) in the Department of Geography at James Cook University for their encouragement over the years. Thanks are extended to the following technical and cartographic staff at JCU: Clive Grant (Geography Dept.) for assisting with the construction and preparation of equipment and some of the field-work; Eddie Rowe (Geography Dept.) prepared Figs. 2.6, 2.7, 5.1, 5.3, 5.4, 5.5,5.6 and 5.7; John Ngai (Geography Dept.) prepared Figs. 6.1, and 6.2; Tom Jeston (Chemistry Dept.) for making the light sensor housing; and Reg Mercer (Electronics Section) for designing and constructing the m V integrators. I am indebted to the research, technical, library and administrative staff at CSIRO's Tropical Forest Research Centre in Atherton for providing access to their facilities. In particular I thank Dr Geoff Stocker (now retired) for his direction and ideas at the earlier stages of this research, and Greg Unwin for many helpful discussions as the research progressed. I would like to thank Professor Peter Schwertdfeger (Flinders University) for the kind loan of the linear pyranometers; Dr Gordon Duff (University of the Northern Territory) for giving consent to use his fisheye photographs taken across the open forest-rainforest boundary at Kirrama; the Photography Section of JCU; Geoff Power and Kerryn Harrison (Botany Department, JCU) for identifying some of the species on the summit of Mt Bellenden Ker (refer to Section 5.5.2.3); Dr Keith Bristow (CSIRO, Townsville) for technical advice on calibration of linear and standard pyranometers; Graham Bennett of Telecom Australia for permitting access to the summit of Mt Bellenden Ker via the cable-car; and finally Dr Dawn May (History & Politics Dept., JCU, Cairns Campus) for making time to read the manuscript. My thanks are also extended to the following voluntary observers, organised by Mr Fred Strochnetter of the Bureau of Meteorology (Brisbane), who read solar radiation integrators on a daily basis for one year: Mrs Jan Anderson, Linda and Bill Venn, Mr W.L. Burton, and Mrs Pat Perkins. I thank the following people for assisting with data prepartion and typing of the references: Lin Wallace, Karin Bryant, Chris Kiro and Marie Gilbert. Mrs Sue Skelding (Cairns Campus Secretary) typed the page numbers at quite short notice and I thank her for her help. Virtually all of this thesis was prepared by the author on a Macintosh SE microcomputer. Specifically, word-processing was completed using Word 4.0; statistical analyses and some graphs were completed using StatView 512K +; graphs and various diagrams not listed in the acknowledgements above were drawn using Cricket Graph and SuperPaint software packages; and BASIC computer programs were compiled using QuickBASIC. I lastly extend my sincere gratitude to my wife Wendy and son David for their constant support and encouragement. Wendy has contributed a considerable amount of time and effort to helping me with field-work, data preparation and proof-reading. I therefore dedicate this thesis to them. ABSTRACT 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). 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. 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. 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. 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 slight­ to-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. 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.