Biomechanical Functional Traits and Processes Involved in Height Growth Strategy

The studies mentioned previously have shown that sets of variables derived from mechanical models incorporating tree size, shape, and wood properties (available volume of wood, stem vs. crown biomass, shoot vs. root biomass, stem slenderness, taper and lean, and root and shoot architecture) are usually involved in the assessment of failure risk of trees. As often pointed out for any kind of tree functional trait [3], to describe a strategy, we must analyze those traits we estimated both at the individual and population levels. Thus, we must investigate how individual traits are influenced by both the environment (plasticity) and ontogeny. To define tree functional types, we assign greater importance to the trait variation or trajectory during ontogeny than to average values. For example, the decrease of the buckling safety factor during the early growth stages of saplings, and the maximal values reached in the most competitive environments, are more pertinent when comparing species' strategies than the mean value of risk per species. Certain size effects are physically obvious, and it is helpful to use modeling to define size-independent traits at the first order, for example buckling safety factors rather than critical height or structural mean modulus of elasticity rather than cross-sectional flexural stiffness (the product of the mean modulus of elasticity and the second moment of area of the cross section [113]). Because traits are potentially numerous, the minimal set able to describe a strategy for a given mechanical constraint in a given situation is a complex question. As far as we know, such a question is rarely considered, and traits are often chosen implicitly.

Lastly, height strategy involves not only selected morphological and anatomical features that are directly linked to tree failure, but two growth processes also exist, which allow a certain mechanical control over these features. One such process is thigmomorphogenesis [114], the phenomenon by which external mechanical loading can change (i) the biomass allocation between roots and shoots and also between their length and thickness, (ii) shoot and root architecture, (iii) organ cross-sectional shape, and (iv) internal plant structure. The second process is gravitropism [12], i.e., the phenomenon by which the verticality of a displaced stem or branch can be restored. A "hard" functional trait (see [115] for a discussion of the distinction between "hard" and "soft" traits) defining a species' strategy should be the species' sensitivity to these processes, i.e., its capacity to adapt functional growth. Such traits can also be measured experimentally by, for example, measuring the reorientation of artificially tilted stems or studying the growth response to applied mechanical loading (see Section 1.4).

1.4 the growth processes that control the

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