Mechanical stability of slender tree stems 141 The Mechanical Control of Growth

Extensive research since the 1970s has demonstrated in many herbaceous and woody species that several growth parameters are affected in response to external mechanical loading (see [116] for a recent review). This general mechanoperceptive syndrome is known as thigmomorphogenesis [114] and is likely to be found in most erect plants, although variability exists in quantitative amounts between both species and genotypes (see, e.g., [117-119]). As shown in Figure 1.4, the major responses of trees to a mechanical stimulation of their aerial parts are (i) a decrease in biomass allocation to aerial parts, thereby favoring root growth (see Section 1.4.3 for more details on specific morphological and anatomical responses of roots), (ii) a clear stimulation of the secondary diametrical growth, (iii) a decrease in primary extension growth, and (iv) changes in the density and specific mechanical properties of the wood, usually toward a lower material stiffness and a higher strain at material failure (see, e.g., [73,119,120]). The effect of branching on tree sway characteristics has been recently investigated by Sellier and Fourcaud [121], who showed that tree adaptation to wind must also be considered with regards to tree architecture.

A better insight into the quantitative understanding of the signal perceived by a plant was shown by Coutand and co-workers [122,123]. These authors demonstrated that plants tend to perceive strain, not stress, and that the control of primary growth can only be explained assuming that a systemic signal is produced integrating the overall field of strain all over the living tissues in the stem. On this basis, they were able to produce a quantitative response curve for the mechanical control of stem extension growth. Although demonstrated on the apical growth of tomato (Lycoper-sicon esculentum Mill.), this analysis has now also been carried out for radial growth of poplar saplings (Populus sp.) [Coutand et al., unpublished data].

Even if the thigmomorphogenetic responses of trees to mechanical loading are now well documented, the ecological significance in terms of acclimation to wind or buckling largely remains to be addressed. Because of the difficulty in carrying out experiments on trees growing in natural conditions, few experiments have been conducted whereby the acclimation response of trees to wind has been measured in the field. Since the seminal studies by Jacobs [124], studies of thigmomorphogenesis in natural conditions involve comparisons between staked and free-standing trees. The effects of staking or guying on stem morphology are considerable. However, when trees are staked, a nonnatural situation is provoked whereby the stimulus is negligible in the trunk and an unnatural stimulus remains in the branches. Therefore, conditions are not realistic. Because growth response curves are highly nonlinear [122,125], the perception of a tree to a mechanical signal, e.g., wind, may actually occur at very low loading levels. It would be necessary to carry out more dose-response experiments, whereby trees are subjected to different levels of wind loading to determine better their thigmomorphogenetic responses. Moreover, most studies have been conducted on isolated trees in greenhouses or growth chambers, and recent evidence indicates that the photomorphogenetic response to shade in

FIGURE 1.4 Thigmomorphogenetic responses in Wild Cherry saplings (Prunus avium L. cv. "monteil"). Three treatments were applied. Control: free standing submitted to natural wind sways; S: completely staked (trunk and branches); S+B: completely staked but with artificial bending of the trunk for 1 minute every 3 hours. (A) Typical morphologies of the stem and root system of plants subjected to treatments S and S+B. (B) Changes in dry matter partitioning between the shoot and root systems due to bending treatments. (Modified from Coutand et al., 2003.)

FIGURE 1.4 Thigmomorphogenetic responses in Wild Cherry saplings (Prunus avium L. cv. "monteil"). Three treatments were applied. Control: free standing submitted to natural wind sways; S: completely staked (trunk and branches); S+B: completely staked but with artificial bending of the trunk for 1 minute every 3 hours. (A) Typical morphologies of the stem and root system of plants subjected to treatments S and S+B. (B) Changes in dry matter partitioning between the shoot and root systems due to bending treatments. (Modified from Coutand et al., 2003.)

dense canopy may reduce thigmomorphogenetic responses (e.g., see [42,126], but see Mitchell [127]) or induce them when trees are grown in full sunlight [128]. The use of artificial fans to imitate wind loading on trees may be useful for identifying thigmomorphogenetic responses [90], but provides little information with regard to the natural significance of thigmomorphogenesis. Experiments where forest trees would be subjected to artificial loading by the use of fans, for example to simulate turbulence would require huge facilities (see, e.g., [129,130]). Recently, a new technique to demonstrate the occurrence of significant thigmomorphogenetic acclimation to wind in natural conditions has been proposed by Moulia and Combes [131]. These authors studied the variability in the difference between staked and free-standing plant canopies over several growing periods in alfalfa (Medicago sativa L.). Moulia and Combes [131] showed that the month-to-month variability in wind speed when winds were moderate was able to explain 65% of the reduction in aerial biomass and 41% of the reduction in total canopy height, thereby demonstrating highly significant thigmomophogenetic effects in dense canopies under natural conditions. However, similar studies on trees remain to be conducted and would take an extremely long time to carry out. A less cumbersome alternative is to study spatial changes in morphology associated with obvious natural gradients in wind conditions.

When applied to rain forest conditions where the evapotranspirative effects of wind can be neglected, this method has shown that thigmomorphogenetic acclimation is ongoing [132]. However, such an approach is only correlative and limited to very special conditions such as ridge crests and shelters in tropical rain forest. Of particular interest would be a study of the correlation between morphology and wind speed within a canopy using the natural spatial variability in wind speeds. A prerequisite for this would be to record wind-induced sways all over the canopy by using video recording and image correlation techniques for the kinematic tracking of wind-induced canopy movements [133]. However, long-term studies still remain to be conducted.

Assuming that thigmomorphogenetic acclimation does occur in nature, the second central question is whether these responses are adaptive or not, i.e., are the performance vs. mechanical constraints improved and are there consequences on the fitness of the individual in its environment? Because both the performance vs. the mechanical constraint and the thigmomorphogenetic "syndrome" involve several variables, qualitative inferences are uncertain, and only direct measurements of plant performance or the use of a mechanical model can help to determine the exact effect on plant performance vs. mechanical constraints. Very few analyses of this kind have been carried out. Concerning wind loading, it has been postulated that thigmomor-phogenesis might be involved in allowing trees to reach a certain shape. This shape will permit a spatially homogeneous distribution of wind-induced stresses for wind conditions. Achieving such a constant stress is adaptive and even optimal in that all parts of the trees would display the same safety factor against material failure [47,52,134].

Mattheck [135] made a significant contribution to the old hypothesis of constant stress design [47,53] by providing a dynamic biomechanical model of stress equalization through growth. Mattheck and Bethge [136] also described a wide range of shapes that could be explained qualitatively through the constant stress hypothesis. However, no direct quantitative testing of the model's prediction has ever been produced. Moreover, subsequent studies that have attempted to verify the constant stress hypothesis have used fairly detailed modeling of the wind loads involved

[137.138] and have even dismissed this hypothesis for wind loads on trees (but see

[138.139]). More indirect tests comparing the height-to-diameter ratio have also been reviewed and not found convincing [51]. Although not optimal in terms of constant stress, thigmomorphogenesis is likely to improve the overall strength of a tree's structure but to an extent that remains to be quantified, and with strategies that still have to be studied.

Thigmomorphogenesis can also increase a tree's stability against buckling under self-weight. This phenomenon has been tested experimentally by Tateno [46] on mulberry trees (Morus bombycis Koidz).

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