The geometry of light capture in plantation Eucalyptus globulus labill

The nature and spatial distribution of leaf area, especially leaf size, density and orientation, strongly regulates light harvesting as well as the exchange of carbon and water between plants and atmosphere (Pearcy and Yang 1996). The occupation of three-dimensional space and the characteristics of branches on which leaves are arranged, combine to generate wide variations in shoot, branch and tree level architectures and corresponding variations in leaf angles/orientations and self-shading. How do these variations in the spatial arrangement of leaves and branches influence light interception and other physiological processes (Falster and Westoby 2003)? Unlike many deciduous northern hemisphere species, for which radiative transfer models have been generated (Chen et al. 1994; Planchais and Sinoquet 1998; Genard et al. 2000; Farque et al. 2001; Naumburg et al. 2001), the genus Eucalyptus L’Herit has a large number of species with pendulous (steeply inclined) leaves. These high leaf angles, may reduce midday canopy heat loads and susceptibility to photoinhibition (King 1997) as well as reducing light interception and photosynthesis in the middle of the day (Anderson 1981). Despite the dominance of eucalypts in Australian landscapes, their leaf architecture is poorly quantified and for only a handful of species (Falster and Westoby 2003) do we have rigorous analysis of leaf angles and other architectural features. Numerous studies highlight close relationships between intercepted radiation and leaf characteristics including leaf mass (Meir et al. 2000), specific leaf area (Han et al. 2003) and leaf [N] (Meir et al. 2000; Le Roux et al. 2001). The light-nitrogen-photosynthesis association remains a focus of current forest research (Warren and Adams 2001). Additions of nutrients to plants, particularly nitrogen, can increase productivity via increases in photosynthetic function (Linder and Rook 1984; Hirose and Werger 1994) and increases in leaf area (Smethurst et al. 2003). Nonetheless the efficiency of radiation capture remains a key driver of photosynthesis (Palmroth et al. 2002). Whilst there are some studies of irradiance, leaf morphology and shoot architecture (Niinemets and Kull 1995; Niinemets and Kull 1995; Sprugel et al. 1996; James and Bell 2000a), the role of nutrition is not well understood (Palmroth et al. 2002). Presented here is the initial analysis of branch architecture and light capture properties of Eucalyptus globulus Labill. grown in plantations in south-east Australia. Three-dimensional digitising technology was used to capture leaf display and branch architecture, at different heights and orientations within a crown, of four-year-old trees. These initial results form part of a larger study that aims to explore the interactions between branch architecture, light environments, photosynthesis and nutrient availability in plantation E. globulus.