Development of the Monsi-Saeki theory on canopy structure and function

Abstract

Background and aims: Monsi and Saeki (1953) published the first mathematical model of canopy photosynthesis that was based on the light attenuation within a canopy and a light response of leaf photosynthesis. This paper reviews the evolution and development of their theory.

Scope: Monsi and Saeki showed that under full light conditions, canopy photosynthesis is maximized at a high leaf area index (LAI, total leaf area per unit ground area) with vertically inclined leaves, while under low light conditions, it is at a low LAI with horizontal leaves. They suggested that actual plants develop a stand structure to maximize canopy photosynthesis. Combination of the Monsi-Saeki model with the cost-benefit hypothesis in resource use led to a new canopy photosynthesis model, where leaf nitrogen distribution and associated photosynthetic capacity were taken into account. The gradient of leaf nitrogen in a canopy was shown to be a direct response to the gradient of light. This response enables plants to use light and nitrogen efficiently, two resources whose supply is limited in the natural environment.

Conclusion: The canopy photosynthesis model stimulated studies to scale-up from chloroplast biochemistry to canopy carbon gain and to analyse the resource-use strategy of species and individuals growing at different light and nitrogen availabilities. Canopy photosynthesis models are useful to analyse the size structure of populations in plant communities and to predict the structure and function of future terrestrial ecosystems.

Fig. 1.

(A) Schematic arrangements of photosynthetic tissues. (a) Three layers of photosynthetic tissues placed horizontally. (b) The same area placed with inclination. (B) Light-response curve of photosynthesis of (a) a single leaf and (b) a stand of Sinapis alba, where both are presented on a leaf area basis. Stand photosynthesis is obtained by multiplying b by 3·4 (= LAI). Canopy photosynthesis calculated with eqn (4) by Monsi and Saeki (1953) is added as open circles. Redrawn with modifications from Boysen Jensen (1932). Note that 1 klux corresponds to 18 μmol photons m⁻² s⁻¹ for daylight PAR (Larcher, 1995).

Fig. 2.

(A) Daily canopy photosynthesis of a Celosia cristata stand under full daylight (100 %) calculated from eqn (4), plotted as a function of leaf area index. Different K-values are assumed. (B) Daily maximum canopy photosynthesis as a function of relative incident radiation. Calculated from eqn (8) for different K-values. Redrawn after Saeki (1960).

Fig. 3.

Photosynthesis (P) increases with increasing enzyme content (E), but saturates at higher E. Saturation starts earlier in a light-limited (A) than light-unlimited (B) habitat. The slope of the tangent lines represents the cost (C) of manufacturing one unit of enzyme. The tangent point defines the optimal enzyme investment (E_A,opt and E_B,opt) that maximizes photosynthesis per enzyme content. See text for further explanation. Modified from Mooney and Gulmon (1979).

Fig. 4.

Daily canopy photosynthesis (P_day) versus total leaf nitrogen (N_t) under the optimal and uniform nitrogen allocation program. The optimal allocation was determined by eqn (10). Note different scales on the x-axis. (A) F_t = 8·48 m² m⁻²; (B) 4·24 m² m⁻²; (C) 2·12 m² m⁻². From Hirose and Werger (1987b).

Fig. 5.

Photon absorption per unit leaf area (Φ_area, y-axis), leaf nitrogen per unit area (n_L, x-axis) and photon absorption per unit leaf nitrogen (Φ_N, contours) of 11 species in the canopy of the Thelypterido-Phragmitetum. Φ_N = Φ_area/n_L. Relative values for Φ. From Hirose and Werger (1994).

Fig. 6.

(A) Photon absorption per unit leaf area (Φ_area, x-axis), leaf area ratio (A_M, y-axis) and photon absorption per unit above-ground mass (Φ_mass, contours) of species in the canopy of the Thelypterido-Phragmitetum. Φ_mass = Φ_area × A_M. Relative values for Φ. (B) Leaf mass ratio (f_LM, x-axis), specific leaf area (σ, y-axis) and leaf area ratio (A_M, contours) of species. A_M = f_LM × σ. From Hirose and Werger (1995).

Fig. 7.

(A–C) Relative photosynthetic rate (R, daily net photosynthesis per unit above-ground mass), (D–F) daily photon absorption per unit above-ground mass (Φ_mass), and (G–I) light use efficiency (ϵ, net photosynthesis per unit photon absorption) as a function of above-ground mass of individuals in a Xantium canadense stand. Note that R = Φ_mass × ϵ. (A, D, G) 22 July; (B, E, H) 1 August; (C, F, I) 11 August. From Hikosaka et al. (1999).