Carbono em Braun-Blanquet (1932)

Physiologic-ecologic Effect of Light.—The essential importance of light for carbon assimilation is well known. Radiant energy is thus converted into chemical energy, and under its influence the decomposition of carbon dioxide takes place in the chloroplast. But in order to fulfill this function, the light rays must be absorbed by the plant. That is, in photochemical activity there is a certain correspondence between the action of hght and the amount absorbed. The greatest photosynthetic efficiency is attained in the red waves (wave length 0.66 to 0.68μ). As Ursprung (1917) has shown, there is a limited amount of photosynthesis in the invisible infra-red and ultra-violet. (Braun-Blanquet 1932:97)

Assimilation Curves. —Studies of the relation of assimilation (photosynthesis) to intensity of illumination have proved that for all species the assimilation curve at first rises sharply; that is, with gradual increase of a very feeble light there is at first a rapid increase in assimilative activity. Later the curve becomes more and more horizontal. For every species or, better, for every ecotype there comes a time sooner or later when a further increase of light is useless and no further increase of carbon dioxide assimilation takes place. Typical shade leaves show an early and rapid flattening of the curve ; the curve for sun leaves flattens more gradually (Fig. 52). (Braun-Blanquet 1932:97)

Under natural conditions plants obtain carbon dioxide from both the soil and the atmosphere. As carbon is one of the chief constituents of all organic substances, its absorption from the air plays a notable role in the gas exchange between plants and the atmosphere. The assimilation of carbon dioxide by the forest is very considerable and is subject to decided variation. The ordinary amount of atmospheric carbon dioxide averages 0.03 per cent, but the cover of trees the lower layers of the air may reach a concentration of 0.08 per [111] cent. This high percentage of carbon dioxide compensates, to a certain extent, for the low light intensity of the forest floor. (Braun-Blanquet 1932:110-1)

Gut (1929) found in general a diminution of the carbon dioxide content of the forest atmosphere during the early hours of the day, a slight increase in the afternoon, which generally continues into the evening, and a restoration to the normal by the “respiration” of the soil during the night. In the deciduous forest there is sometimes a decided but transient increase in the carbon dioxide content toward evening, which might be explained by the accumulation of the products of metabolism in the leaves to the extent that it hinders the rate of further carbon assimilation. In the forest the lowest content of atmospheric carbon dioxide, as well as its greatest variations, occurred in the spring; in the autumn its highest content was reached. (Braun-Blanquet 1932:111)

It is impossible at present to estimate the ecological importance of carbon dioxide and its variations. It seems certain that the normal atmospheric content of carbon dioxide represents a minimum rather than an optimum amount for plants. The determinations of its variations are still too fragmentary to have great ecological value. It must suffice to call the attention of phytosociologists to the possible importance of this factor. (Braun-Blanquet 1932:111)

The determinations of Lundegårdh in the Swedish oak forests, confirmed by Gut, show that the carbon dioxide content of the air is subject to greater fluctuations in the beech than in the pine forests. It therefore appears that in the deciduous forest the gas exchanges are more active but there is a slower rate of growth. (Braun-Blanquet 1932:111)

The sources of atmospheric carbon dioxide are combustion; the respiration of man, animals, and plants; volcanic emissions; and the gases arising from the sea and from the soil. (Braun-Blanquet 1932:111)

Determination of Carbon Dioxide Content of the Air.—Of the numerous methods for the determination of the carbon dioxide content of the atmosphere the only one that appears to be practicable for phytosociologists is the volumetric method of Gut. Even this is complicated and difficult. For details the reader is referred to the description by Gut (1929, pp. 18-29). (Braun-Blanquet 1932:111)

All of these “wind communities” are sharply circumscribed in area and well differentiated from the surrounding turf communities (Fig. 91). They offer a fruitful field for the study of comparative ecology. In addition to direct wind effects, exposed ridges often show indirect influences upon the habitat, by changing the composition of the soil, the soil fauna, the circulation of carbon dioxide, etc. These conditions, however, have not been investigated. The soil on exposed ridges is often less acid than adjacent soils protected from wind. Dense crowding in a community increases the wind resistance of the individual members, be they trees, shrubs, or herbs. (Braun-Blanquet 1932:97)

BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.