Mechanical Properties and Attachment of Dicotyledonous Climbers

All twiners, tendril climbers, and branch-angle climbers of dicotyledons show a drop of Estr from young to old stages of growth. These changes in mechanical properties probably reflect a need for twining and tendril plants to be protected against extreme forces generated by movement of the host trees. Climbers that twine from one tree to another risk experiencing extremely high forces during high winds and after failure of branches and trunks. The twining strategy therefore requires highly compliant older stems that can withstand severe stresses and strains. Being strongly attached to a support also means that once the stem is attached, the part of the stem that is below the point of attachment has little need of stem stiffness to partially support the stem or lean against surrounding vegetation to retain its position. Mature trunks of large-bodied twining species are often so compliant that they hang in tension from the point of attachment above. Such stems can have very low structural Young's moduli as reflected in some of the values presented among twiners.

Tendrils can also form an extremely firm attachment to the host supports, but the attachment is usually weaker than for stem twiners because the tendrilar organs are often derived from modified stipules, leaves, or fertile axes, which have narrow diameters. Like stem twiners, tendril climbers also have little need of stem stiffness in basal parts of the plant once they are firmly attached at many points.

Hook climbers are generally less firmly attached to the host plant than either stem twiners or tendril climbers. Hooks and curved spines can be effective means of attaching to host supports, but attachment is constrained by the internal angle and length of the hook. They can also be dislodged by movement of the host branches. Compared with twiners, hook climbers are considerably less firmly attached to the host plant, particularly in young stages. Movement and swaying of the host plant could potentially dislodge the climber from the support rather than exert extreme stresses on the main stem as in twiners. Furthermore, relatively stiff, older parts would ensure that the plant stem would maintain its position in the vegetation even though one or more hooks had become unattached. After detachment, the young apical part of the plant would remain in the vicinity of potential supports. This is quite different from the effect of removing the attachment from a mature or aging stem twiner because such stems can buckle and collapse in coils because of their own weight. A hook-climbing strategy with relatively stiff older stems is probably adopted by many different plant groups and with different developmental constraints [20]. Among the plants tested, two of the three woody plants, Bougainvillea and Rosa, retain relatively high Young's moduli in older stages of development, with even higher values found in the old stages of Rosa. Interestingly, both climb via less specialized curved spines rather than acutely angled hooks. In species of Strychnos tested from French Guiana, early growth of apical branches bear open hooks that are green and sensitive to touch (Figure 2.6e). They can engage small-diameter supports, and the limb of the hook eventually thickens and closes around the supporting host branch. Strychnos, like all the dicotyledons that produce active attachment organs, shows a large reduction in Estr to just below 2000 MNm-2 from relatively large values of over 12,000 MNm-2 found in young stages.

Plants climbing via wide-angled branches show high values of Estr in young stages, generally higher than those of twiners and consistent with the idea that young stages of growth require stem stiffness for attachment. Wide-angled branches can interlock with the branches of neighboring trees and provide very firm anchorage. It is important that young stages of growth be stiff and not deflect when the plant has established its climbing phase. The branch-angle climbers of Croton also show marked drops in Estr toward the base of the plant. Even though the attachment is not as active as in tendril and twining species, branch anchorage can be very effective and can be coupled with very low values of Estr in old stems [21].

For the three plants grouped as leaning climbers, general observations indicate that branches and leaves do not anchor the plant in the vegetation. We saw previously that the adult stems of Croton that were tested can be suspended from the host canopy via their wide-angled attachment and finally produce highly compliant wood. Unlike climbing species of Clematis, the petioles of C. recta do not twine extensively around supports. Similarly the two leaning species of Lonicera tested differ from the more typical climbing species in that they do not twine around neighboring stems. All three plants produce little change in Estr from young to old stages of development and from near the apex to the base. The values do remain relatively high, and this kind of pattern has been termed "semi-self-supporting" [10]. Such growth forms retain relatively stiff mechanical properties through their growth trajectory. They form little attachment with host vegetation apart from simply leaning.

Most dicotyledonous species that show a marked decrease in Estr during development, such as stem twiners and tendril climbers, have active attachment organs. The data also indicate that some species (e.g., Croton) that climb by wide-angled branches are also capable of producing compliant wood. The production of branches with many potential points of anchorage might finally permit a firm connection with the surrounding vegetation, but twining stems are always more firmly bound.

Interestingly, both Strychnos and Croton eventually produce older stages of growth with relatively low values of Estr. Recent studies have shown that these plants have relatively extended "stiff" phases of development, and this is possibly linked to their type of attachment as well as other potential environmental factors such as light availability [9,20]. The type of attachment shown by these plants via many potential points of attachment and the degree of security this affords to the climbing plant differs considerably from the mechanical constraints imposed by firmly twining or attaching via tendrils.

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