Mechanics of the Compression Wood Response: II. On the Location, Action, and Distribution of Compression Wood Formation

A new method for simulation of cross-sectional growth provided detailed information on the location of normal wood and compression wood increments in two tilted white pine (Pinus strobus L.) leaders. These data were combined with data on stiffness, slope, and curvature changes over a 16-week period to make the mechanical analysis. The location of compression wood changed from the under side to a flank side and then to the upper side of the leader as the geotropic stimulus decreased, owing to compression wood action. Its location shifted back to a flank side when the direction of movement of the leader reversed. A model for this action, based on elongation strains, was developed and predicted the observed curvature changes with elongation strains of 0.3 to 0.5%, or a maximal compressive stress of 60 to 300 kilograms per square centimeter. After tilting, new wood formation was distributed so as to maintain consistent strain levels along the leaders in bending under gravitational loads. The computed effective elastic moduli were about the same for the two leaders throughout the season.     

How does a gymnosperm branch (Pseudotsuga menziesii) assume the hydraulic status of a main stem when it takes over as leader?

In most gymnosperms, resistance to the flow of water per unit path length through the main stem is less than that of lateral branches. Using branches, leaders, and branches that have replaced missing leaders ('branch-leaders'), we tested the hypothesis that branch-leaders are at a hydraulic disadvantage. Reduced xylem transport efficiency in branch-leaders relative to leaders could be expected both because of an initial disparity in hydraulic capacity, and because of the relatively impermeable compression wood formed in branch-leaders during shoot reorientation. By subsampling branch-leaders, we also tested the hypothesis that opposite wood (formed directly opposite compression wood) is more permeable than normal wood, and could, therefore, compensate for the presence of compression wood at the whole shoot level. Fifteen months after leader removal, branch-leaders were intermediate between branches and leaders in their ability to supply foliage with water, suggesting a transition towards leader status that was not yet complete. Increased hydraulic capacity in branch-leaders was the result of increased xylem cross-sectional area per unit foliage, rather than an increase in permeability. Among subsampled wood types from basal branch-leader segments, opposite wood was significantly less permeable than normal wood, suggesting that it does not compensate for the presence of compression wood.     

Wood Specific Gravity Variations and Biomass of Central African Tree Species: The Simple Choice of the Outer Wood

Context

Wood specific gravity is a key element in tropical forest ecology. It integrates many aspects of tree mechanical properties and functioning and is an important predictor of tree biomass. Wood specific gravity varies widely among and within species and also within individual trees. Notably, contrasted patterns of radial variation of wood specific gravity have been demonstrated and related to regeneration guilds (light demanding vs. shade-bearing). However, although being repeatedly invoked as a potential source of error when estimating the biomass of trees, both intraspecific and radial variations remain little studied. In this study we characterized detailed pith-to-bark wood specific gravity profiles among contrasted species prominently contributing to the biomass of the forest, i.e., the dominant species, and we quantified the consequences of such variations on the biomass.

Methods

Radial profiles of wood density at 8% moisture content were compiled for 14 dominant species in the Democratic Republic of Congo, adapting a unique 3D X-ray scanning technique at very high spatial resolution on core samples. Mean wood density estimates were validated by water displacement measurements. Wood density profiles were converted to wood specific gravity and linear mixed models were used to decompose the radial variance. Potential errors in biomass estimation were assessed by comparing the biomass estimated from the wood specific gravity measured from pith-to-bark profiles, from global repositories, and from partial information (outer wood or inner wood).

Results

Wood specific gravity profiles from pith-to-bark presented positive, neutral and negative trends. Positive trends mainly characterized light-demanding species, increasing up to 1.8 g.cm^-3 per meter for Piptadeniastrum africanum, and negative trends characterized shade-bearing species, decreasing up to 1 g.cm^-3 per meter for Strombosia pustulata. The linear mixed model showed the greater part of wood specific gravity variance was explained by species only (45%) followed by a redundant part between species and regeneration guilds (36%). Despite substantial variation in wood specific gravity profiles among species and regeneration guilds, we found that values from the outer wood were strongly correlated to values from the whole profile, without any significant bias. In addition, we found that wood specific gravity from the DRYAD global repository may strongly differ depending on the species (up to 40% for Dialium pachyphyllum).

Main Conclusion

Therefore, when estimating forest biomass in specific sites, we recommend the systematic collection of outer wood samples on dominant species. This should prevent the main errors in biomass estimations resulting from wood specific gravity and allow for the collection of new information to explore the intraspecific variation of mechanical properties of trees.     

Stem-righting mechanism in gymnosperm trees deduced from limitations in compression wood development

BACKGROUND AND AIMS: In response to inclination stimuli, gymnosperm trees undergo corrective growth during which compression wood develops on the lower side of the inclined stem. High compressive growth stress is generated in the compression wood region and is an important factor in righting the stem. The aims of the study were to elucidate how the generation of compressive growth stress in the compression wood region is involved in the righting response and thus to determine a righting mechanism for tree saplings. METHODS: Cryptomeria japonica saplings were grown at inclinations of 0 degrees (vertical) to 50 degrees. At each inclination angle, the growth stress on the lower side of the inclined stem was investigated, together with the degree of compression-wood development such as the width of the current growth layer and lignin content, and the upward bending moment. KEY RESULTS: Growth stress, the degree of compression wood development, and the upward moment grew as the stem inclination angle increased from 0 to 30 degrees, but did not rise further at inclinations > 30 degrees. CONCLUSIONS: The results suggest the following righting mechanism for gymnosperm saplings. As the stem inclination is elevated from 0 to 30 degrees, the degree of compression wood development increases to force the sapling back to its original orientation; at inclinations > 30 degrees, the maximum degree of compression wood is formed and additional time is needed for the stem to reorient itself.     

The effect of temperature on mechanical properties of standing lodgepole pine trees

To estimate strength parameters of living lodgepole pine stems over a range of temperatures (-16 to +17°C), trees were winched near or past the point of breakage, during which the applied force and deflection of the stem were measured. Trees were 43 years old, 10 m tall, and since the experiments were conducted in the late winter and early spring, when the soil was frozen and the roots were held rigid, the resistance of the stem to deflection could be isolated from the resistances of the root and soil. Static flexure theory for cantilever beams was used to estimate stress, strain, Young's modulus (E), and modulus of rupture (MOR) of the stem. Trees were stiffer and stronger in the winter when wood was frozen, with a nearly 50% increase in E and MOR compared with the spring, when wood was thawed. In winter stems failed on the tension side, while in spring stems buckled on the compression side. Compared with strength estimations reported in the literature from small samples of clear green wood at standard temperatures, modulus of elasticity (MOE) estimates of the whole stem were 35% lower in spring, and in winter MOR exceeded published values by 53%. This suggests that the sway behavior of trees is probably temperature dependent in northern forests and whole-tree strength characteristics should be considered in wind sway models used in these regions.     

Elevated atmospheric CO2 alters wood production, wood quality and wood strength of Scots pine (Pinus sylvestris L) after three years of enrichment

Scots pine ( Pinus sylvestris L.) trees were grown in open top chambers for three years under ambient and elevated CO₂ concentrations. The trees were aged 3 y at the beginning of the CO₂ exposure, and the effects of the treatment on total stem volume, stem wood biomass, wood quality and wood anatomy were examined at the end of the exposure. The elevated CO₂ treatment lead to a 49% and 38% increase in stem biomass and stem wood volume, respectively. However, no significant effects of the elevated CO₂ treatment on wood density were observed, neither when green wood density was estimated from stem biomass and stem volume, nor when oven-dry wood density was measured on small wood samples. Under elevated CO₂ significantly wider growth rings were observed. The effect of elevated CO₂ on growth ring width was primarily the result of an increase in earlywood width. Wood compression strength decreased under elevated CO₂ conditions, which could be explained by significantly larger tracheids and the increased earlywood band, that has thinner walls and larger cavities. A significant decrease of the number of resin canals in the third growth ring was observed under the elevated treatment; this might indicate that trees produced and contained less resin, which has implications for disease and pest resistance. So, although wood volume yield in Scots pine increased significantly with elevated CO₂ after three years of treatment, wood density remained unchanged, while wood strength decreased. Whilst wood volume and stem biomass production may increase in this major boreal forest tree species, wood quality and resin production might decrease under future elevated CO₂ conditions.     

SOME OBSERVATIONS ON THE MECHANISM OF ORIENTATION MOVEMENT OF WOODY STEMS

The force that induces orientation movement of inclined or bent woody stems is generated in reaction wood even in young terminal stems with a large proportion of soft tissues to secondary xylem. Compression wood formed in pine as a response to inclination expands longitudinally after the stresses are released by sawing it from the stem. The increment of length of compression wood when sawed is equal to the decrement of its length which occurs during drying. This suggests that stresses developed by compression wood in the stem are related to imbibition of water by its cell walls. Not all compression wood develops tensile forces in the stem. Neutral compression wood was observed in the lower portion of inclined stems of pine. Tension wood in poplar develops contractile forces in the stem during its aestival maturation. However, when harvested before developing contractile forces in situ, it develops such forces during drying. This suggests that in poplar the mechanism which produces forces responsible for orientation bending also involves changes in cell wall hydration.     

The relationship between stem and branch wood specific gravity and the ability of each measure to predict leaf area

A few trait axes that represent differential biomass allocation may summarize plant life-history strategies. Here we examine one of these axes described by wood specific gravity. Wood specific gravity represents the location of a species on a continuum of the rate of growth vs. the likelihood of mechanical failure, ranging from rapid volumetric growth/increased probability of mechanical failure to slow volumetric growth/decreased probability of mechanical failure. Wood specific gravity has been quantified primarily using three separate methods: a section from terminal branch, a section from the main stem or from a trunk wood core. What is unclear is how comparable these methods are and whether one or the other is a better predictor of other important plant traits such as leaf area. Here we measured stem and branch wood specific gravities from individual trees and shrubs in a tropical rain forest, quantified their relationship and determined their ability to predict leaf area. Stem and branch measures were highly correlated with each measure having a weak correlation with leaf area in trees and strong correlation with leaf area in shrubs. These results indicate that various methodologies for measuring wood specific gravity are comparable, and thus less destructive methods than are currently used are available to determine values for this important trait.     

Oscillation frequencies of tapered plant stems

Free oscillations of upright plant stems, or in technical terms, slender tapered rods with one end free, can be described by considering the equilibrium between bending moments in the form of a differential equation with appropriate boundary conditions. For stems with apical loads, where the mass of the stem is negligible, Mathematica 4.0 returns solutions for tapering modes α = 0, 0.5, and 1. For other values of α, including cases where the modulus of elasticity varies over the length of the stem, approximations leading to an upper and a lower estimate of the frequency of oscillation can be derived. For the limiting case of ω = 0, the differential equation is identical with Greenhill's equation for the stability against Euler buckling of a top-loaded slender pole. For stems without top loads, Mathematica 4.0 returns solutions only for two limiting cases, zero gravity (realized approximately for oscillations in a horizontal orientation of the stem) and for ω = 0 (Greenhill's equation). Approximations can be derived for all other cases. As an example, the oscillation of an Arundo donax plant stem is described.     

The importance of frictional interactions in maintaining the stability of the twining habit

The stability of twining vines under gravitational loads suggests an important role for friction. The coefficient of friction, μ, between vine stems and wood is high, often five times greater than between leather and wood, as determined by slip tests on an inclined plane. Stem trichomes function like ratchets to facilitate climbing upward (or to facilitate slipping if the stem is inverted). A mathematical model predicts large masses (kg) must be applied to the base of a twining vine to cause slipping. Vines slip as predicted when μ is low and arc length on the pole is short, and they break before slipping when μ is large or arc length is long. In contrast, twining vines are unstable in compression, collapsing when small masses (<10 g) are hung from the top of the vine. However, if the loads are applied below the uppermost gyre, the stabilizing tensional effect dominates. Therefore, in nature vines twining on a cylindrical support are stable under gravitational loads, unless these loads occur near the apex. A corollary is that a short apical coil can hold up large masses of maturing shoot.     

Physical model of water in a split-root system

Summary A physical model, based on Darcy's law and an Ohm's-law analogy, was developed to show that water can move from a wetter side of a root system to a drier side or vice versa. In the model, a wick in the form of an inverted Y was used, with the two ends of the Y in separate beakers and the third end (stem) of the Y extending into the air. The left root, right root, and stem were about 6,5, and 4 cm long, respectively. The difference in total head (potential) between the left root and the right root was varied for different potentials applied to the stem. Experiments were done either in a darkened laboratory or with a sunlamp shining on the stem. The stem was thus exposed to low-evaporation (in the dark) or high-evaporation (with the sunlamp) conditions. Total heads (sum of head due to gravity and head due to pressure-other heads were negligible) and flows of water were calculated or measured for each part of the split-root system (left root, right root, crown, stem). The results showed that the direction and quantity of water flowing in each part of the system depended upon the total head for the stem, crown, and each half of the root (the flow could be up, down, left, or right), and that the gravity component of the total head was important in moving water down the plant when light intensity was low.     

A model to simulate the gravitropic response and internal stresses in trees,considering the progressive maturation of wood

Key message

The developed model of gravitropism takes non-instantaneous maturation of wood into account which enabled to correctly simulate different gravitropic phases and realistic internal stress profiles.

Abstract

A new biomechanical model of tree movement in relation to gravity (gravitropism) is proposed in this study. The modelling of the progressive maturation of wood cells is taken into account, as well as spatio-temporal variations in maturation strains (MS) and mechanical properties. MS were identified using an inverse method that allows the model to fit the gravitropic reaction observed experimentally. For this purpose, the curvature during righting movement, the geometry and the mass distribution of a two-year-old poplar tree was measured. The identified MS are higher than expected, which shows the underestimation of MS by usual measurements. By using the same mechanical parameters and MS as an input, the model gives satisfying results in terms of shape modelling for different trees up to 32 days after tree tilting. The model is able to simulate the latency phase observed in the tree righting movement, and the internal stress profile in the trunk is realistic (low compressive value in the central part of the trunk and zero stress in newly formed cells). The next development of the model will aim to simulate the end of the gravitropic phase in relation with the regulation of MS by the tree.     

The influence of stem guying on radial growth,stem form and internal resin features in radiata pine

Key message

Stem guying to prevent wind-induced swaying of radiata pine trees resulted in significant changes in radial growth, but did not affect the frequency of compression wood or resin features.

Abstract

Mechanical stress resulting from wind forces acting on trees can cause a number of direct and indirect effects ranging from microscopic changes in cambial activity through to stem breakage and uprooting. To better understand these effects on radial stem growth and wood properties, an experiment was established in a 13-year-old radiata pine (Pinus radiata D Don) stand in which 20 trees were guyed to prevent them from swaying. Radial growth was monitored in these trees and 20 matched controls at monthly intervals for 5 years. The trees were then felled and radial growth, resin features and compression wood were assessed on cross-sectional discs taken at fixed locations up the stem. There was a significant reduction in radial growth at breast height (1.4 m above the ground) in the guyed trees, but an increase in growth immediately above the guying point. A total of 277 resin features were observed in the growth rings formed following guying. The overall frequency of such features was related to height within the stem and annual ring number. No effect of stem guying was found on the incidence of compression wood. Interestingly, the distribution of resin features also did not differ between guyed and un-guyed trees. There was no evidence of a link between stem restraint as a result of guying and the incidence of resin features, suggesting that other factors, such as soil moisture may be more influential.     

Modelling of the hygroelastic behaviour of normal and compression wood tracheids

Compression wood conifer tracheids show different swelling and stiffness properties than those of usual normal wood, which has a practical function in the living plant: when a conifer shoot is moved from its vertical position, compression wood is formed in the under part of the shoot. The growth rate of the compression wood is faster than in the upper part resulting in a renewed horizontal growth. The actuating and load-carrying function of the compression wood is addressed, on the basis of its special ultrastructure and shape of the tracheids. As a first step, a quantitative model is developed to predict the difference of moisture-induced expansion and axial stiffness between normal wood and compression wood. The model is based on a state space approach using concentric cylinders with anisotropic helical structure for each cell-wall layer, whose hygroelastic properties are in turn determined by a self-consistent concentric cylinder assemblage of the constituent wood polymers. The predicted properties compare well with experimental results found in the literature. Significant differences in both stiffness and hygroexpansion are found for normal and compression wood, primarily due to the large difference in microfibril angle and lignin content. On the basis of these numerical results, some functional arguments for the reason of high microfibril angle, high lignin content and cylindrical structure of compression wood tracheids are supported.     

Extrapolation of thermodilution curves obtained during a pause in artificial ventilation

The feasibility of three mathematical models to extrapolate the tail of thermodilution curves, when flectures are present in the descending limb, was tested in anesthetized pigs. The models were a local random walk model (LDRW), a log-normal distribution, and a two-compartment model. First, the accuracy of the extrapolation of the tail by each model was tested on two undisturbed curves by taking the truncation at five different points on the descending limb. The extrapolated curve area obtained from each model was compared with total area of the undisturbed curve. Next, dilution curves obtained during inspiratory hold maneuvers and characterized by deflection points were analyzed, taking the truncation just before deflection. The estimates of cardiac output by the models were compared with electromagnetically measured flow in the pulmonary artery. The area of the dilution curve was estimated more accurately when more information on the descending limb was available. The LDRW model and the log-normal distribution were superior to the two-compartment model regarding accuracy of cardiac output estimation and root-mean-square errors of the fit. Both models estimated curve area with an error less than 5% when truncation of the descending limb occurred below 60% of the peak value. In circumstances of mechanical ventilation, where only short periods of constant flow will be present, analyses of dilution curves based on the LDRW model or the log-normal distribution are recommended.     

Load-sharing in the lumbosacral spine in neutral standing & flexed postures – A combined finite element and inverse static study

Understanding load-sharing in the spine during in-vivo conditions is critical for better spinal implant design and testing. Previous studies of load-sharing that considered actual spinal geometry applied compressive follower load, with or without moment, to simulate muscle forces. Other studies used musculoskeletal models, which include muscle forces, but model the discs by simple beams or spherical joints and ignore the articular facet joints.This study investigated load-sharing in neutral standing and flexed postures using a detailed Finite Element (FE) model of the ligamentous lumbosacral spine, where muscle forces, gravity loads and intra-abdominal pressure, as predicted by a musculoskeletal model of the upper body, are input into the FE model. Flexion was simulated by applying vertebral rotations following spine rhythm measured in a previous in-vivo study, to the musculoskeletal model. The FE model predicted intradiscal pressure (IDP), strains in the annular fibers, contact forces in the facet joints, and forces in the ligaments. The disc forces and moments were determined using equilibrium equations, which considered the applied loads, including muscle forces and IDP, as well as forces in the ligaments and facet joints predicted by the FE model. Load-sharing was calculated as the portion of the total spinal load carried along the spine by each individual spinal structure. The results revealed that spinal loads which increased substantially from the upright to the flexed posture were mainly supported by the discs in the upright posture, whereas the ligaments’ contribution in resisting shear, compression, and moment was more significant in the flexed posture.     

Quantification of compression wood severity in tracheids of Pinus radiata D. Don using confocal fluorescence imaging and spectral deconvolution

Confocal fluorescence microscopy was used to examine the spectral characteristics of lignin autofluorescence in secondary cell walls of normal and compression wood from Pinus radiata. Using UV excitation, fluorescence spectra of normal and compression wood sections showed significant differences, especially in the outer secondary cell wall of tracheids, with a shift in maxima from violet to blue wavelengths between normal and compression wood. A comparison of normal wood, mild and severe compression wood, showed that the wavelength shift was intermediate in the mild compression wood compared to the severe compression wood, thus offering the possibility of quantifying the severity by measuring ratios of fluorescence at violet and blue wavelengths. Fluorescence induced by blue light, rather than UV, was less well differentiated amongst wood types. Spectral deconvolution indicated the presence of a minimum of five discrete lignin fluorophores in the cell walls of both normal and compression wood tracheids. Comparison with lignin model compounds suggest that the wavelength shift may correspond in part to increased levels of p-hydroxy type lignin in the compression wood samples. The combination of confocal fluorescence imaging and related spectral deconvolution therefore offers a novel technique for characterising cell wall lignin in situ.     

Formation and function of compression wood in gymnosperms

Westing, Arthur H. (Middlebury Coll., Vt.). 1965.Formation and function of compression wood in gymnosperms. Bot. Rev. 31: 381–480 A review with ca. 575 references. The world literature pertaining to the biology of compression wood (Rotholz; reaction wood) is evaluated critically. Compression wood is a geotropic reaction to an inertial force and is peculiar to the Coniferales, Ginkgoales, and Taxales. It is formed by the cambium (or cambial derivatives) of the lower side of inclined stems and branches, where it expandsin situ thereby tending to right the former and maintain (or restore) the inherent angle of the latter. Compression wood is stimulated to form by applications of indoleacetic acid, but under natural conditions is interpreted to result from an increased sensitization of cells on the lower side to an insignificantly changed level of endogenous auxin. A theoretical model of the perception (susception) mechanism is advanced. The mechanics of righting is discussed and the forces involved are estimated. Frequent reference is made to other geotropic phenomena of the higher plants, particularly to tension wood, the analogue of compression wood in the arborescent Dicotyledoneae. Much pertaining to the perception, formation, and function of compression wood remains to be elucidated     

Mechanical Properties of Black Locust (Robinia pseudoacacia) Wood: Correlations among Elastic and Rupture Moduli,Proportional Limit,and Tissue Density and Specific Gravity

The purpose of this paper is to determine the extent to which the physical and mechanical properties of dry and green wood samples are correlated. Samples of green (fresh) sap- and heartwood differing in density (ρ) were removed from the trunk of a black locust (Robinia pseudoacaciaL.) tree 30 years old and measuring 15 m in height. These samples were mechanically tested to determine their Young's elastic modulus (E), proportional (elastic) limit (σ_p), and modulus of rupture (σ_R). The Young's elastic modulus of green wood samples increased in magnitude to a limit with increasing cross-sectional area of the sample tested. The values of all three mechanical parameters measured for sapwood samples were consistently lower than those measured for heartwood samples with equivalent cross-sectional areas.Ewas linearly and positively correlated with the σ_pand σ_Rof heartwood tissue samples. All mechanical properties were highly correlated with the density of green heartwood. Likewise, these properties were highly correlated with the specific gravity of wood samples. Based on these results, it is concluded that either the density of fresh wood or the specific gravity of air-dried wood can be used to estimate the mechanical properties of black locust wood based on simple regression curves in the absence of extensive mechanical tests.