Abstract: Active mechanisms of re-orientation are necessary to maintain the verticality of tree stems. They are achieved through the production of reaction wood, associated with circumferential variations of three factors related to cambial activity: maturation strain, longitudinal modulus of elasticity (MOE) and eccentric growth. These factors were measured on 17 mature trees from different botanical families and geographical locations. Various patterns of circumferential variation of these factors were identified. A biomechanical analysis based on beam theory was performed to quantify the individual impact of each factor. The main factor of re-orientation is the circumferential variation of maturation strains. However, this factor alone explains only 57% of the re-orientations. Other factors also have an effect through their interaction with maturation strains. Eccentric growth is generally associated with heterogeneity of maturation strains, and has an important complementary role, by increasing the width of wood with high maturation strain. Without this factor, the efficiency of re-orientations would be reduced by 31% for angiosperms and 26% for gymnosperms. In the case of angiosperms, MOE is often larger in tension wood than in normal wood. Without these variations, the efficiency of re-orientations would be reduced by 13%. In the case of gymnosperm trees, MOE of compression wood is lower than that of normal wood, so that re-orientation efficiency would be increased by 24% without this factor of variations.

Abstract: Functional ecology has long considered the support function as important, but its biomechanical complexity is only just being elucidated. We show here that it can be described on the basis of four biomechanical traits, two safety traits against winds and self-buckling, and two motricity traits involved in sustaining an upright position, tropic motion velocity (MV) and posture control (PC). All these traits are integrated at the tree scale, combining tree size and shape together with wood properties. The assumption of trait constancy has been used to derive allometric scaling laws, but it was more recently found that observing their variations among environments and functional groups, or during ontogeny, provides more insights into adaptive syndromes of tree shape and wood properties. However, oversimpli-fed expressions have often been used, possibly concealing key adaptive drivers. An extreme case of oversimplification is the use of wood basic density as a proxy for safety. Actually, as wood density is involved in stiffiness, loads, and construction costs, the impact of its variations on safety is non-trivial. Moreover, other wood features, especially the microfibril angle (MFA), are also involved. Furthermore, wood is not only stiff and strong, but it also acts as a motor for MV and PC. The relevant wood trait for this is maturation strain asymmetry. Maturation strains vary with cell-wall characteristics such as MFA, rather than with wood density. Finally, the need for further studies about the ecological relevance of branching patterns, motricity traits, and growth responses to mechanical loads is discussed. © The Author 2013.

Abstract: Plant tissues shrink and swell in response to changes in water pressure. These strains can be easily measured, e.g., at the surface of tree stems, to obtain indirect information about plant water status and other physiological parameters. We developed a mechanical model to clarify how water pressure is transmitted to cell walls and causes shrinkage of plant tissues, particularly in the case of thick-walled cells such as wood fibers. Our analysis shows that the stress inside the fiber cell walls is lower than the water tension. The difference is accounted for by a stress transmission factor that depends on two main effects. The first effect is the dilution of the stress through the cell wall, because water acts at the lumen border and is transmitted to the cuter border of the cell, which has a larger circumference. The second effect is the partial conversion of radial stress into tangential stress. Both effects are quantified as functions of parameters of the cell wall structure and its mechanical properties.

Abstract: The different hypotheses about buttress function and formation mainly involve mechanical theory. Forces were applied to two trees of Sloanea spp.. a tropical genus that develops typical thin buttresses. and the three-dimensional strains were measured at different parts of the trunk base. Risks of failure were greater on the buttress sides, where shear and tangential stresses are greater, not on the ridges. in spite of high longitudinal (parallel to the grain) stresses. A simple beam model, computed from the second moment of area of digitized cross sections, is consistent with longitudinal strain variations but cannot predict accurately variations with height. Patterns of longitudinal strain variation along ridges are very different in the two individuals, owing to a pronounced lateral curvature in one specimen. The constant stress hypothesis is discussed based on these results. Without chronological data during the development of the tree. it cannot be proved that buttress formation is activated by stress or strain.

Abstract: The intensification of the hydrological cycle, with an observed and modeled increase in drought incidence and severity, underscores the need to quantify drought effects on carbon cycling and the terrestrial sink. FLUXNET, a global network of eddy covariance towers, provides dense data streams of meteorological data, and through flux partitioning and gap filling algorithms, estimates of net ecosystem productivity (F-NEP), gross ecosystem productivity (P), and ecosystem respiration (R). We analyzed the functional relationship of these three carbon fluxes relative to evaporative fraction (EF), an index of drought and site water status, using monthly data records from 238 micrometeorological tower sites distributed globally across 11 biomes. The analysis was based on relative anomalies of both EF and carbon fluxes and focused on drought episodes by biome and climatic season. Globally P was approximate to 50% more sensitive to a drought event than R. Network-wide drought-induced decreases in carbon flux averaged -16.6 and -9.3 g C m-2 month-1 for P and R, i.e., drought events induced a net decline in the terrestrial sink. However, in evergreen forests and wetlands drought was coincident with an increase in P or R during parts of the growing season. The most robust relationships between carbon flux and EF occurred during climatic spring for F-NEP and in climatic summer for P and R. Upscaling flux sensitivities to a global map showed that spatial patterns for all three carbon fluxes were linked to the distribution of croplands. Agricultural areas exhibited the highest sensitivity whereas the tropical region had minimal sensitivity to drought. Combining gridded flux sensitivities with their uncertainties and the spatial grid of FLUXNET revealed that a more robust quantification of carbon flux response to drought requires additional towers in all biomes of Africa and Asia as well as in the cropland, shrubland, savannah, and wetland biomes globally.