Abstract To determine the role of leaf mechanical prop- erties in altering foliar inclination angles, and the nutri- ent and carbon costs of specific foliar angle variation patterns along the canopy, leaf structural and biomechan- ical characteristics, biomass partitioning into support, and foliar nitrogen and carbon concentrations were stud- ied in the temperate deciduous species Liriodendron tu- lipifera L., which possesses large leaves on long peti- oles. We used beam theory to model leaf lamina as a uni- form load, and estimated both the lamina and petiole flexural stiffness, which characterizes the resistance to bending of foliar elements at a common load and length. Petiole and lamina vertical inclination angles with re- spect to horizontal increased with increasing average daily integrated photon flux density (Q int ). Yet, the light effects on lamina inclination angle were primary deter- mined by the petiole inclination angle. Although the pet- ioles and laminas became longer, and the lamina loads increased with increasing Q int , the flexural stiffness of both lamina and petiole increased to compensate for this, such that the lamina vertical displacement was only weakly related to Q int . In addition, increases and decreas- es in the petiole inclination angle with respect to the hor- izontal effectively reduced the distance of lamina load from the axis of rotation, thereby reducing the bending moments and lamina inclination due to gravity. We dem- onstrate that large investments, up to 30% of total leaf biomass, in petiole and large veins are necessary to maintain the lamina at a specific position, but also that light has no direct effect on the fractional biomass in- vestment in support. However, we provide evidence that apart from light availability, structural and chemical characteristics of the foliage may also be affected by wa- ter stress, magnitude of which scales positively with Q int . Keywords Carbon partitioning · Dry mass per unit area · Flexural stiffness · Support costs · Light interception Introduction Plants may enhance whole canopy light interception by increasing the total foliar area, or by increasing the effi- ciency of unit leaf area for light interception. Changes in branch and foliar inclination angles provide an important way to modify the light interception capacity of the fo- liage. As the canopy elements become more horizontal, the interception efficiency of both direct and diffuse irra- diance increases (Heilman et al. 1996; Hikosaka and Hirose 1997; Muraoka et al. 1998; Pearcy and Valladares 1999; Utsugi 1999; Valladares and Pearcy 2000). Ac- cordingly, a canopy with horizontal leaves is particularly advantageous in understorey low-light environment. However, horizontal leaves may result in large within- canopy shading, because they do not allow light penetra- tion into deeper foliage layers. With increasing light availability, steeper inclination angles become increas- ingly profitable, because they allow more uniform distri- bution of light within the canopy, and thus, exposition of a greater photosynthesizing foliar area to light (Duncan 1971; Valladares 1999; Valladares and Pearcy 2000). Studies indicate that plants do have more horizontal leaves in low light environments, and more steeply ori- ented leaves in open environments (Knapp and Smith 1997; Muraoka et al. 1998; Valladares and Pearcy 2000; Valladares et al. 2000). Furthermore, there exists a con- tinuous vertical light gradient along the plant canopies, and leaf inclination angles become increasingly vertical with increasing irradiance along this gradient (Miller 1967; van Elsacker and Impens 1984; Hollinger 1989; Heilman et al. 1996; Utsugi 1999). Despite the frequently observed and ecologically rele- vant correlations between long-term leaf light environ- Ü. Niinemets ( ✉ ) Department of Plant Physiology, Institute of Molecular and Cell Biology, University of Tartu, Riia 23, 51010 Tartu, Estonia e-mail: ylo@zbi.ee Fax: +372-7-366021 S. Fleck Department of Plant Ecology, University of Bayreuth, 95440 Bayreuth, Germany Oecologia (2002) 132:21–33 DOI 10.1007/s00442-002-0902-z ECOPHYSIOLOGY Ülo Niinemets · Stefan Fleck Petiole mechanics, leaf inclination, morphology, and investment in support in relation to light availability in the canopy of Liriodendron tulipifera Received: 21 February 2001 / Accepted: 11 February 2002 / Published online: 30 April 2002 © Springer-Verlag 2002