Growth is by itself a mechanical constraint. Indeed, from a mechanical point of view, a small initial bending should be amplified by growth because in any cross section of the trunk, growth increases bending loads due to self-weight. Thus, bending curvature is increased and stiffened by continuing radial growth in an amount depending on the relative rates of bending moment and cross-sectional stiffness increases [37,62-64]. This dynamic and continuous growth constraint has rarely been analyzed carefully and has never been considered in ecological studies. In some cases such constraints may be considerable, such as sudden increase of loads (e.g. leaf flushes or heavy fruit production) on slender flexible stems, which is followed by cambial growth that adjusts the curved shape . However, it is clear that without any biological control of verticality, e.g., a selection of the most vertical trees, or the action of gravitropism to restore verticality (see Section 1.4.2), any given degree of stem lean at a given height should increase significantly with growth. Studying two populations of saplings of Goupia glabra Aubl. (shade-intolerant species of the rainforest in French Guiana) in understory and full light conditions, we found that the lean never increases and even decreases in the most competitive (understory) environment (Figure 1.2). Therefore, these data provide
■ Understory o Full light
FIGURE 1.2 Variation in stem lean (%) between 0- and 2-m height in two populations of Goupia glabra Aubl. saplings from the French Guiana tropical rainforest (Fournier and Jaouen, unpublished data). Lean in seedlings grown in full light (white circles) does not increase with diameter breast height (DBH) (Spearman R is not significant); in understory seedlings (black squares), the lean was found to decrease (Spearman R = 0.40, P = 0.006).
evidence of the existence of biological reactions to the gravitational mechanical constraint at the population level.
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