A certain number of measurable characteristics of tree leaves (morphological characteristics, absorption of light radiation, intensity of respiration and photosynthesis) are clearly linked with the presence of physiologically active pigments in the leaves. Leaf characteristics are highly and inequally influenced by changing conditions of light environment, especially those related to light intensity, light quality and duration of the daily illumination period. These modifications do not only apply to light radiation as created under laboratory conditions, but also to light conditions ensuing from the place in the crown of a single tree, the social position of the tree in a forest stand and the site factors in general. There are also changes taking place due to the progression of the vegetation period, at the end of which all species are less tolerant or more light demanding. The reaction of the leaves towards light radiation out of different regions of the spectrum is also different. The so-called blue light radiation (λmax = 440 nm) seems to be of the greatest importance in this relation, as species react quite different to its action. The biggest variation in leaf characteristics due to changing light environment was measured for oak and beech, which both react quickly and are qualified as 'photolabile species'. No important variations occur in leaves of ash and maple, which therefore are qualified as 'photostable species'. As a consequence of variable reactions to changing light conditions, the relationships between the species are continually modified, even in such a way that their potential for dominance is not constant. The classical division into tolerant and intolerant species or classification of the species based upon the degree of light demand, is highly inaccurate and it seems preferable to speak of relative light demands and relative tolerance. All these observations and conclusions bring about a clear confirmation of the necessity to recognize the individuality of the single tree, the special character of each growth condition, the own structure of each forest stand, the specific reaction to one sided modifications of environmental factors. This is especially important for an intensive sylvicultural practice. They also prove the necessity for more physiological and biochemical research to arrive at a better understanding of growth and its mechanism. Sylviculture in fact must try to regulate, on an expanded scale, the phenomens of growth, which is the exchange, absorption and transformation of energy. A practical interpretation and regulation of fundamental laws of physiology and growth will be possible as soon as a clinical form of sylviculture is created and the adequate instrumentarium developed.
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