Model simulation of spatial distribution of photosynthesis in structurally differing plant communities in the Central Caucasus

The aim of the present study was to investigate the significance of structural properties for whole canopy photosynthesis (Acan). Effects of both classical parameters of canopy structure (vertical distribution of leaf area, plant area and leaf inclination) and of leaf dispersion on Acan were analysed by means of a relatively simple canopy photosynthesis model. This model was designed for photosynthetic input parameters based on measured field parameters. Eight structurally different species-rich seminatural and natural plant communities (used and abandoned pastures, hay meadows, tall herbs and dwarf shrubs) in the subalpine belt of the Central Caucasus were investigated. The validation by means of a micrometeorological approach showed that the estimate of Acan in the model corresponds well with the measurements in very differently structured, species-rich plant communities. Simulations showed that the significance of canopy properties for canopy photosynthesis essentially depends on the vertical distribution of the leaf area. Therefore this parameter was used for classifying the canopies investigated. Three types of canopies could be distinguished: type 1 with the leaf area amassed near the soil surface and the structural shape of a pyramid with a broad base; type 2 with a gradual increase in leaf area from the canopy surface to the ground (pyramid with a narrow base) and type 3 with a concentration of the leaf area in the upper canopy (inverted pyramid). The significance of the structural properties for Acan increases from type 1 to type 3 and is higher when the stand is dominated by a single species. In stands of type 1 canopy structure did not limit Acan via light climate, but rather by temperature effects. In stands of type 2 and type 3, mutual shading is an important limiting factor for canopy photosynthesis, but the structural properties of type 3 stands are better optimized for Acan. Simulations with assumed equal leaf area index and photosynthetic characteristics showed that in canopies of type 3 the structural properties (leaf inclination, leaf dispersion) are such that the major part of the leaf area in the canopy is supplied with intermediate photosynthetic photon flux densities, thus increasing daily total Acan. Sensitivity analysis of Acan to non-random leaf dispersions showed effects especially in stands of type 2 and type 3. Clumped leaf dispersions resulted in better light utilization and higher Acan. When compared with the leaf dispersions measured in the canopies, the simulation results show that the canopies optimize leaf dispersion with respect to Acan.