Models of Tree and Stand Dynamics by Annikki Mäkelä & Harry T. Valentine

Author:Annikki Mäkelä & Harry T. Valentine
Language: eng
Format: epub
ISBN: 9783030357610
Publisher: Springer International Publishing

7.3 Crown Structure

7.3.1 The Evolutionary Significance of Crown Architecture for Carbon Allocation

Crowns are plastic. Shoots appear to grow into light. If an opening is created, branches of surrounding trees will soon fill it. Also, the main direction of light appears to have an impact on crown shape. Trees in low-latitude savannas, where light comes directly from above, have wide horizontal crowns, whereas boreal conifers, where the direction of sun rays is more from the side, tend to have very narrow and tall column-like crowns. A classic text describing the plastic adaptation of trees to different light environments is the book by Horn (1971).

These observations have led to the proposition that crown structure should form so as to allow for maximum light capture under any prevailing conditions. In a pioneering study to formally test this hypothesis, Oker-Blom and Kellomäki (1982) utilised a light interception model of the type described in Sect. 4.​6.​1 to analyse the effect of crown shape on the amount of light absorbed by open-grown trees during the growing season at different latitudes. Crown shape was defined as the ratio of crown length (L c) to crown radius (R) in conical crowns in which leaves were evenly distributed. However, contrary to expectation, no optimal crown shape could be derived from these premises. The efficiency of light absorption increased both towards longer columnar crowns and towards flatter disc-like crowns at all latitudes, with minimum light absorption found at length to radius ratios between L c∕R = 1 and L c∕R = 4. Some indication of possible north-south differences in optimum crown form was nevertheless suggested by the result that the L c∕R ratio corresponding to the minimum light absorption increased with decreasing latitude, such that tall columnar crowns were relatively more beneficial for light absorption in higher than in lower latitudes.

The result by Oker-Blom and Kellomäki (1982) suggests that crown shape is not solely determined on the basis of maximisation of light absorption. Other factors, such as mechanical strength under wind force and snow loads, also have been discussed as possible traits playing a role in natural selection (Bruning 1976). From the point of view of carbon economy and allocation, the benefits from light absorption have to be weighed against the related investment in the construction of the supporting branching network (see Fig. 6.​1).

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