On the Factor Light in Plant Communities and its Importance for Matter Production

Abstract

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. 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. 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. 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 m2 per m2 ground area) of the observed herbosa is 4 to 7 or more. The light intensity decreases by and large exponentially through the leaf layers. It thus follows the equation where I is the light intensity in the plant community, I0 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. 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. 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. 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. 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. APPENDIX: NOTES ON THE TRANSLATION Monsi and Saeki had a very unusual way of writing. Their German is grammatically correct, but their style is (or has become since the 1950s) unusual. This makes the German text sometimes difficult to understand, even for native speakers. I have tried to keep as much of Monsi's and Saeki's writing style as possible when translating the text into English. Monsi and Saeki often use technical terms that are no longer used, or are used today with slightly different meanings. For some of their expressions, we would use a different term today. For example, they very often use the word ‘Helligkeit’ which means brightness. Most authors these days would use the term ‘light intensity’ because it sounds more quantitative than does ‘brightness’. However, Monsi and Saeki chose ‘brightness’, and I have decided to keep it that way when translating the paper. A more liberal translation into English might be easier to read, but would do less justice to the authors' own particular style. The notes below refer to a number of words that have become largely forgotten in plant ecology, ‘herbosa’, ‘lignosa’ and ‘synusia’. These go back to the Swiss botanist Rubel, and giving his definitions actually tells us something about the background Monsi and Saeki were coming from scientifically. Herbosa. Monsi and Saeki follow the ‘ecological-physiognomical’ classification of plant communities by Rubel (1930) into lignosa (woody communities), herbosa (herbaceous communities) and deserta (desert communities). Interestingly, the definitions of Rubel for these classes do not centre on the ‘woodiness’ of the communities. Lignosa are defined as ‘plant communities which consist of trees, shrubs, and herbs, which are closed on immobile ground in such a manner that the plants of the upper storeys create ecological conditions for the plants of the lower storeys significantly different from the conditions when these were dominant themselves’. Herbosa are ‘plant communities which consist of grasses, herbs, and non-woody cryptogams, which cover the immobile ground in such a manner that the incident light is used by the dominant species such that the soil surface is influenced’. Deserta are ‘communities which cannot cover the mobile and immobile ground in such a manner that they have a significant influence on a lower storey, i.e. they are open plant communities or cryptogam crusts’. The rationale behind this classification clearly influenced Monsi's and Saeki's thinking about the microenvironment as modified by plants. See Rubel E. 1930. Pflanzengesellschaften der Erde. Bern/Berlin: Verlag Hans Huber. Synusia. An ecological grouping within a plant community. It often corresponds with ‘layer’, but not always, such as in epiphyte communities. See Rubel E. 1930. Pflanzengesellschaften der Erde. Bern/Berlin: Verlag Hans Huber.