Introduction

Whereas the mechanical performance of plant organs has often been discussed in evolutionary biology [1,2], tree biomechanics has rarely been considered in the context of functional ecology. Functional ecology aims at understanding the functions of organisms that result in fluxes of biomass or energy within an ecosystem, e.g., a forest. This discipline studies the processes controlling these fluxes, at either the scale of an individual, community, or ecosystem, with their response to natural or anthropic environmental variations.

Ecological differences among vascular land plant species arise from different ways of acquiring the same major resources of light, water, CO2, and nutrients. An ecological strategy is the manner in which species secure carbon profit, i.e., both light and CO2 absorption, during vegetative growth, and this also ensures gene transmission in the future [3]. At the present time, the relationship between biodiversity and ecosystem functioning is one of the most debated questions in ecology, and it is of great importance to identify variations in ecological strategies between species [3-6].

In this context, the field of tree biomechanics is concerned with the manner in which trees develop support structures to explore space and acquire resources, and, by feedback, to allocate biomass to the support function. The purpose of this chapter is to discuss how an understanding of the solid mechanics of materials and structures has contributed to functional ecology with examples taken from current studies in tree biomechanics.

Mechanics gives physical limits to size, form, and structure because living organisms must follow physical laws [7]. This discipline also allows several relationships between function and size, form, or structure to be explored. Solid mechanics provides the relationships between supported loads (inputs) to outputs such as displacements, strains, stresses (local distribution of loads), and safety factors against buckling or failure through given parameters [8]. These parameters may be structural geometry (shape) and material properties, e.g., critical stresses or strains leading to failure, or the relationship between stresses and strains given, e.g., in the simplest case by the modulus of elasticity (or Young's modulus) [8]. Biomechanics is much more ambitious than solid mechanics. Biomechanics aims at analyzing the behavior of an organism that performs many not explicitly specified functions using geometry and material properties fabricated by processes shaped by the complexity of both evolution and physiology. Thus, to use the framework of solid mechanics to solve biological problems concerning form and function, biomechanics involves different steps. Initially, a representation of the plant and of the supported loads using a mechanical model is necessary. This step means that initial choices must be made because models are nearly always simplifications. For example, can wind be considered as a static or a dynamic force for the problem considered? Are stems paraboloids or cylinders? Is root anchorage perfectly rigid or not? These initial choices can have huge consequences on the subsequently discussed outputs, in particular concerning the functional significance or the adaptive value of mechanical outputs, e.g., safety factors [9] or gravitropic movements. The subsequent discussions tend to be biological in nature and therefore out of the scope of engineering science.

Before dealing with several ecological questions, we first present some biome-chanical characteristics of trees and develop questions concerning height growth strategies. We then discuss successively the underlying mechanical problems and associated models, i.e., the representation of supported loads, plant shape, and material along with the biological problems, data, and hypotheses, especially those tackling the biological control of size, shape, and material properties. The practical application of biomechanics in eco-engineering [10] is then discussed.

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