Preface: Structure and Function of Plant Canopies

Abstract

This section comprises a set of papers taken from those presented at a symposium held to commemorate the 50th anniversary of the Monsi–Saeki theory (1953), together with invited papers. The papers describe recent advances in the study of structure and function of plant canopies and are written by former students (and their collaborators) of Professors Monsi and Saeki. The topics cover construction and maintenance of efficient photosynthetic systems at leaf, individual plant and stand level. Canopy structure and function are analysed with respect to optimization and an evolutionarily stable strategy. A new translation of the original paper by Monsi and Saeki (1953) into English has been commissioned and is included in this section.

Professors Masami Monsi (left) and Toshiro Saeki (right) in front of the Biology Building, University of Tokyo, 1971. Photograph by S. D. Song.

M. Monsi and T. Saeki published the first model of canopy photosynthesis to be based on light attenuation within a canopy and a light-response curve of leaf photosynthesis (Monsi and Saeki, 1953). They showed that plant canopies are structured to maximize canopy photosynthesis under a given irradiance regime. It was a seminal paper on modelling and optimization of canopy structure and function, and has continued to inspire studies that test their predictions and develop more sophisticated models. A symposium was held in 2003 at Tsukuba, Japan, to commemorate the 50th anniversary of this most notable publication. This section comprises a collection of review papers presented at the symposium and papers invited later. At the symposium, Graham Farquhar presented an Annals of Botany lecture on the importance of diffuse and direct light components in canopy modelling, which will appear in a later issue of Annals of Botany (in Prep.). The article of Monsi and Saeki, which was originally written in German and published in a local journal, has now been translated into English and is published here to ensure a wider distribution among the international scientific community (Monsi and Saeki, 2005). A historic letter from P. Boysen Jensen to M. Monsi has also been translated and is added to the end of the translation. We appreciate the efforts of Marcus Schortemeyer and Graham Farquhar in translating the German into English.

The Monsi–Saeki theory evolved from the study of P. Boysen Jensen (1932) on dry matter production in plants, as described by Hirose (2005). The theory predicts an optimal leaf area index (LAI) and that canopy photosynthesis is maximized at a high LAI with vertically inclined leaves under a high irradiance, whilst under low irradiance a low LAI is predicted with horizontal leaves. The model assumed that all leaves in the canopy had the same photosynthetic capacity. However, this assumption does not hold true in real systems and Hirose (2005) describes how the Monsi–Saeki theory has been modified with leaf nitrogen distribution in a canopy being taken into account. The gradient of leaf nitrogen observed in the canopy is shown to respond directly to the light gradient. This response enables plants to use efficiently both light and nitrogen, whose supply is limited in the natural environment. The Monsi–Saeki canopy photosynthesis model stimulated studies that scaled-up from chloroplast biochemistry to canopy carbon gain and analysed the resource-use strategy of species in a vegetation stand with different light and nitrogen availabilities. The canopy photosynthesis model has also been applied to study growth of individuals in a stand and has been used to analyse development of size structure.

Earlier models assumed that the optimum for one individual was independent of the characteristics of its neighbours. This seems unlikely in vegetation stands where neighbouring plants strongly influence each other's light climate. Anten (2005) shows that optimal vegetation stands with maximum whole-stand photosynthesis are not evolutionarily stable. They can be invaded by mutants that are taller, project their leaves more horizontally or that produce greater-than-optimal leaf areas, although these mutants reduce total canopy photosynthesis. He further suggests that canopy models can contribute to our understanding of species coexistence through simultaneous analysis of the various traits that determine light capture and photosynthesis and the trade-offs between them. These trade-offs are associated with specialization for different positions in the niche space defined by the temporal and spatial heterogeneity of resources.

Within a leaf, a light gradient develops from the upper to lower surface. Terashima et al. (2005) discuss if the same logic for canopy photosynthesis can be applied to the photosynthesis of a single leaf. Leaf photosynthesis would be maximized by allocating nitrogen among chloroplasts in proportion to the light intercepted by each chloroplast. CO₂ diffusion to chloroplasts can be a limiting step in leaf photosynthesis. The authors discuss the importance of CO₂ diffusion in relation to leaf thickness, to construction and maintenance mechanisms of efficient photosynthetic systems at a leaf level, together with the roles of sugar-sensing, redox control and cytokinin in making those systems. They also suggest a new direction to study photosynthetic systems in trees with the aid of the pipe model theory.

In a plant canopy, leaves are continuously produced, senesce and fall. These processes determine the amount of leaf area in the canopy. Hikosaka (2005) reviews studies on leaf lifespan and its modelling and analyses dynamics of leaves in the canopy. Applying the canopy photosynthesis model with leaf nitrogen distribution, he develops a new model of leaf lifespan. This predicts that when leaf turnover is at a steady state, the ratio of biomass production to nitrogen uptake is equal to the ratio of litter fall to nitrogen loss, which is an inverse of the nitrogen concentration in dead leaves. Hence, it is shown that nitrogen concentration in dead leaves and nitrogen availability in the soil determine canopy photosynthesis, and that the dynamics of leaves are regulated so as to maximize the carbon gain and resource-use efficiency of the plant.

Whilst Monsi and Saeki (1953) developed their theory of canopy structure and function with herbaceous systems, the theory can be applied to forest systems as well. Among others, Kitajima et al. (2005) have studied foliage distribution and light extinction through crowns of canopy trees, using direct canopy access with a tower-crane established in a neotropical forest. They show that species with orthotropic (vertically-orientated) terminal shoots exhibited a lower extinction coefficient than those with plagiotropic (more-or-less horizontally-orientated) shoots. Within each type, later successional species were found to exhibit greater maximum LAI and total light extinction. In late-successional species, leaf position within individual shoots did not predict the light availability at the individual leaf surface, which explains the slow decline of their photosynthetic capacity with leaf age and the weak differentiation of sun and shade leaves.

We regret that Professor Toshiro Saeki died on 15 April 2004. He was suffering from lung cancer and pneumonia. Professor Masami Monsi died earlier on 21 December 1997 (Obituary, Hirose 1999). However, the work of Professors Monsi and Saeki continues to shape the minds of plant scientists and to inspire new and exciting ideas in canopy research. We would like to dedicate this collection of papers to the memory of these pioneers of mathematical modelling of canopy photosynthesis.