Leaf phenology as an optimal strategy for carbon gain in plants1 K. Kikuzawa Abstract: Since leaves are essentially energy-gaining organs, the arrangement of leaves in time (leaf phenology) and in space (canopy architecture) in both seasonal and nonseasonal environments can be viewed as a central element in plant strategies for carbon gain. Interrelationships among leaf longevity, leaf habit, and leaf-emergence pattern together with shoot architecture affect plant productivity. Leaf longevity is shown to maximize carbon gain through three parameters: leaf photosynthetic rate, the decrease in photosynthetic rate with leaf age, and the initial construction costs of the leaf. This theoretical approach has been extended to seasonal environments and effectively simulated the geographical pattern of leaf habits. To avoid self-shading, plants adopt two alternative modes of leaf emergence. One is successive leaf emergence, in which plants expand one leaf at a time on a shoot; this unshaded leaf utilizes full sunlight and is only replaced by a second leaf when its photosynthetic ability declines. Plants with successive leaf emergence attain high production and have straight shoots with multilayered canopy architecture. The alternative is simultaneous leaf emergence on shoots inclined to minimize self-shading through a monolayered canopy architecture. By the inclination of the shoot, each leaf on the shoot can receive sufficient light. Plants with simultaneous leaf emergence utilize the entire growing period effectively. Taken together and in the context of shoot and canopy architecture these interrelationships among leaf longevity, habit, and emergence pattern provide the basis for a synthetic theory of leaf phenology. Key words: leaf phenology, leaf longevity, leaf emergence, evergreen, deciduous habit. RCsumd : Puisque les feuilles constituent essentiellement des organes d'obtention d'energie, la disposition des feuilles dans le temps (phCnologie) et dans l'espace (architecture de la canopee), que ce soit dans les environnements saisonniers ou non, peut Ctre considCree comme un ClCment crucial dans la strategic des plantes pour obtenir leur carbone. Les interactions entre la IongtvitC, le comportement et le patron d'emergence des feuilles ainsi que I'architecture des tiges affectent la productivite de la plante. On dCmontre que trois paramktres lies B la longevite des feuilles contribuent B maximiser le gain en carbone : le taux de photosynthkse foliaire, la diminution du taux photosynthCtique avec l'ige, et le coct initial de construction de la feuille. Cette approche thCorique a ete ttendue ii des environnements saisonniers et permet de simuler le patron geographique des comportements foliaires. Pour Cviter leur propre ombre, les plantes utilisent deux modes alternatifs pour I'Cmergence des feuilles. Un de ces modes consiste B produire les feuilles en succession, les feuilles prenant leur expansion une i une sur la tige; chaque feuille peut utiliser la totalit6 de la lumikre sans recevoir d'ombre et n'est remplacee par une deuxikme feuille que lorsque la capacite photosynthetique de la premikre diminue. Les plantes utilisant l'emergence successive des feuilles atteignent une forte productivitt et montrent des tiges droites avec une architecture i plusieurs Ctages. Le mode alternatif implique une emergence simultanCe des feuilles sur une tige inclinCe, minimisant la projection de I'ombre sur elle mCme, et associee avec une architecture sur un seul Ctage. L'inclinaison de la tige permet a chaque feuille de recevoir suffisamment de lumikre. Les plantes dont les feuilles emergent simultanCment utilisent la totalit6 de la pCriode de croissance. Dans leur ensemble et dans un contexte d'architecture de la tige et de la canopee, ces interrelations entre la IongCvitC des feuilles, le port et le patron d'emergence constituent la base d'une theorie synthitique de la phCnologie foliaire. Mots cl6s : phCnologie foliaire, IongCvitC foliaire, Cmergence des feuilles, feuilles persistentes, port dCcidu. [Traduit par la redaction] Received June 1, 1994. Introduction K. Kikuzawa. Hokkaido Forestry Research Institute, Phenology has been defined as the timing of biological events Koshunai, Bibai, Hokkaido, Japan 079-01. and their relationship to seasonal climatic changes (Lieth ' Adapted from a contribution to a symposium Phenology 1974). This definition effectively characterizes the timing of and seasonality of vvoody plants: the effects of clitnate seasonal events in temperate regions, where patterns of leaf change at the 15th International Botanical Congress held emergence and leaf longevity seem to be governed mainly by at Yokohama, Japan in August 1993. seasonally changing temperatures. Various types of shoot- Can. J. Bot. 73: 158- 163 (1995). Printed in Canada / ImprimC au CanadaKikuzawa elongation and leaf-emergence patterns, however, have been reported even in aseasonal tropical regions (Koriba 1948; Boojh and Ramakrishnan 1982; Lowman 1992). Thus a more general theoretical framework for phenological studies, which is applicable to nonseasonal as well as seasonal environments, is required. Leaf phenology, which can be redefined as the arrange- ments of leaves in time, could also be considered an aspect of the light-harvesting strategy of plants. Since plants harvest light by expanding leaves to gain energy and to produce organic matter, the arrangement of leaves in time and space must be a central element in any strategy of plant carbon gain. In this view, three points generally must be considered in the study of leaf phenology (Harada and Takada 1988; Kikuzawa 1989): (i) leaf longevity, (ii) leaf habit, and (iii) the timing of leaf emergence. In this paper, I will focus on leaf longevity and leaf-emergence patterns. Two types of leaf-emergence patterns are found in temper- ate forests (Kikuzawa 1983, 1989): simultaneous and succes- sive emergence. Analogous types are reported in tropical forest (Lowman 1992). Leaf longevities in temperate forests also vary. Mean leaf longevities of alder are only about 90 days (Kikuzawa 1978), while in other deciduous trees, such as maples and oaks, longevities