On the factor light in plant communities and its importance for matter production

Ecological-sociological investigations of plant communities must be based on the matter production of plants. The matter production is discussed in this study on the basis of light intensity and leaf quantity in a plant community. 1. The productive structure of a plant community is demonstrated by the stratified-clipping method. Here, we separate plant materials into photosynthetic and non-photosynthetic tissues. With diagrams of production structure, we can distinguish two main types of plant communities, the broad-leaf type and the grass-type. 2. The light intensity in forest communities is generally 2-20 % of the ambient light. The darkest community in our observations was a bamboo thicket of Phyllostachys, with a relative brightness in summer of 0.2-0.5 %. Under these conditions, some plants of the ground flora become naturally etiolated. 3. Within well-growing herbosa, it is relatively dark compared with forest shade, i.e. the relative brightness is generally only 2-3 %. The leaf area of herbosa is not less than that of lignosa. Generally, the leaf area index (in m ² per m ² ground area) of the observed herbosa is 4 to 7 or more. 4. The light intensity decreases by and large exponentially through the leaf layers. It thus follows the equation I = I ₀e ^-KF where I is the light intensity in the plant community, I ₀ is the incoming light, K is the extinction coefficient, and F is the leaf area index. K is nearly constant for the same plant communities, but varies widely between different communities: our observations yielded values between 0.3 and 2. But, generally, K for the grass-type is approx. 0.3-0.5, and for the broad-leaf type approx. 0.7-1. 5. The light extinction in a leaf community is theoretically discussed with regard to leaf position and leaf angle. We show clearly that the extinction coefficient for ideally distributed horizontal leaves is 1, while for inclined leaves it can decrease with the leaf angle down to 0.44 (for a leaf angle of 90°). A mosaic-like leaf distribution increases K. 6. The development of the production structure of a plant community, especially of the photosynthetic system, is strongly constrained by matter production. For example, the lower layer can only develop according to the remaining productivity, which corresponds to the brightness under the leaf area of the upper layer. This phenomenon is clearly demonstrated by observations of the seasonal development of Phragmites- Sanguisorba associations and of vine communities. The lower community disappears with the growth of the upper community. 7. A theoretical equation is formulated for the calculation of matter production for a leaf community. With this equation, we can calculate total production on the basis of the light-assimilation curve for a single leaf, the leaf area index and the extinction coefficient. There is an optimal leaf area for the maximum productivity under a given incoming light intensity. A smaller extinction coefficient is advantageous for matter production under stronger light, while a larger one is more advantageous under weaker light. 8. We tested the theoretical productivity equation with Boysen Jensen's experimental data for Sinapis and Avena cultures. The calculated productivity agrees quite well with the observed one.