Wood specific gravity (WSG) is a strong predictor of tree performance across environmental gradients. Yet it remains unclear how anatomical elements linked to different wood functions contribute to ...variation in WSG in branches and roots across tropical forests. We examined WSG and wood anatomy in white sand, clay terra firme and seasonally flooded forests in French Guiana, spanning broad environmental gradients found throughout Amazonia. We measured 15 traits relating to branches and small woody roots in 113 species representing the 15 most abundant species in each habitat and representative species from seven monophyletic lineages occurring in all habitats. Fiber traits appear to be major determinants of WSG, independent of vessel traits, in branches and roots. Fiber traits and branch and root WSG increased from seasonally flooded species to clay terra firme species and lastly to white sand species. Branch and root wood traits were strongly phylogenetically constrained. Lineages differed in wood design, but exhibited similar variation in wood structure across habitats. We conclude that tropical trees can invest differently in support and transport to respond to environmental conditions. Wind disturbance and drought stress represent significant filters driving tree distribution of Amazonian forests; hence we suggest that biophysical explanations should receive more attention.
Key message
Mechanical acclimation of young poplars (
Populus tremula × Populus alba
, INRA 717-1B4) submitted to periodic stem bending is mainly driven by compressive strains. Flexure wood and ...compressive flexure wood exhibit higher mechanical resilience and lower mechanical damage.
Context
It is well known that thigmomorphogenesis modulates tree growth and the anatomical structure of wood. However, nothing is known about the mechanical behaviour of the tissues of fresh wood formed under mechanical stimulation.
Aims
We investigated the elastic and plastic properties of the fresh wood of young poplar trees (
Populus tremula × Populus alba
, INRA 717-1B4) submitted to periodic controlled stem bending that mimics the mechanical effect of wind.
Methods
For a set of trees, we applied symmetrical bending treatments, which led to the formation of “flexure wood”. For another set of trees, asymmetrical bending treatments, including compression (or tension) only, were applied and generated specific wood formation: “compressive flexure wood” and “tensile flexure wood”. We investigated the elastic and plastic properties of these woods at the stem and at the local tissue levels.
Results
The results revealed that fresh wood formed under compressive treatments is more resistant to damage (damage reduced by 44%) and a higher mechanical resilience (+ 33%), suggesting that this tissue is able to withstand higher mechanical strains than “normal wood”. This improvement could explain the higher mechanical strength of the stem to bending (+ 42%).
Conclusion
When trees experience repetitive mechanical stimulations, they adjust the plastic plastic behaviour of their wood in a way that improves the mechanical safety. This demonstrates the adaptive benefit of the mechanical acclimation of trees.
• Key message
This paper investigates the juvenility limit and structure–property relationship in secondary quality beech (
Fagus sylvatica
L.) and oak (
Quercus petraea
(Matt.) Liebl.). The juvenile ...wood occupies a very small area near the pith. The stabilization of the different parameters varies over time. Adding the microfibril angle (MFA) and the grain angle to the MOE prediction model significantly improves the quality of the model, despite little variation in both parameters.
• Context
Using secondary qualities and small logs of hardwoods such as beech and oak for engineered wood products is an increasingly important issue due to the technological challenges of processing smaller logs and denser woods. Secondary quality hardwoods are expected to have less variation in mechanical properties compared to softwoods with high juvenile wood content.
• Aims
The first objective of this study was to investigate the radial variation in wood properties of suppressed growth beech and oak trees obtained from thinning operations. The second objective was to develop a model to predict the mechanical properties of these hardwood species based on their structural parameters.
• Methods
The microfibril angle, ring wood density, and ring width from the pith to the bark were determined using an X-ray densitometer. The modulus of elasticity and modulus of rupture were evaluated on the small clear specimen using a three-point bending test. The wood density, grain angle, and microfibril angle of this small clear specimen were also measured.
• Results
The results show that the juvenile wood in oak has a wider ring and higher microfibril angle, whereas it has wider latewood and higher microfibril angle in beech. For both species, the juvenile wood occupies a very small area, less than 5 cm from the pith. The mechanical properties of oak and beech wood from suppressed growth trees are comparable to properties reported in the literature for dominant trees. The modulus of elasticity of oak was best predicted using wood density, grain angle, and microfibril angle. The modulus of rupture of oak is better predicted with wood density and grain angle, whereas it is best predicted with wood density alone for beech.
• Conclusion
Juvenile wood found in the suppressed growth trees of both hardwoods can be used in place of mature wood. It is important to take structural parameters into account when predicting the mechanical properties of hardwood species.
Key message
In French Guiana, the leaf and cambium phenologies should not be considered only as exogenous-driven processes, as the dry season, but also as endogenous-driven, as tree development ...stage.
Studies of the periodicity of wood formation provide essential data on tree age and on factors that control tree growth. The aim of this work was to investigate cambial phenology and its relation with leaf phenology and climatic seasonality in two briefly deciduous tropical rainforest species belonging to the genus
Parkia.
Wood microcores were collected every 15 days from April 2009 to February 2012 from five trees of each species. The microcores were stained with cresyl violet acetate to facilitate counting the number of cells in the cambial zone, in the radial enlargement zone and wall-thickening zone. At the same time, we observed leaf shedding pattern in the crown of the same trees. In both species, cambial activity was significantly reduced during the leafless period. In
P. nitida
, these two concomitant events were observed during the dry season whereas in
P. velutina
they can occur anytime in the year with no apparent link with seasonality. In conclusion, the period of reduced cambial activity in some tropical rainforest trees may be independent of rainfall seasonality and not necessarily follow an annual cycle. It appears that leaf phenology is a good proxy to estimate cambial activity.
Key message
This study presents a novel histologic approach to quantify the intra-annual dynamics of carbon sequestration in forming wood. This innovative approach, based on repeated measurements of ...xylem apparent density, is more direct, and more accurate than the previously published cellular-based approach. Moreover, this new approach, which was tested here on softwoods, is also applicable to hardwoods without any modification.
Context
Forest ecosystems are key players of the terrestrial carbon cycle. Indeed, wood represents the principal carbon pool of terrestrial biomass, accumulated in trees through cambial activity.
Aims
Here, we present a novel, simple, and fast approach to accurately estimate the intra-annual dynamics of aboveground woody biomass production based on image analysis of forming xylem sections.
Methods
During the 2015 growing season, we weekly collected wood samples (microcores) containing the forming xylem on seven Norway spruces (
Picea abies
(L.) Karst), grown in Hesse forest (North-East France). The microcores were prepared to allow the observation of the forming tissues with an optical microscope. Xylem apparent density and radial increment were then measured directly on images of the histological sections. In order to compare our “histologic approach” with the previously published “cellular approach,” we also counted the number of tracheids in each differentiation zones, and measured the tracheid dimensions all along the last-formed tree ring.
Results
The two approaches yielded comparable meaningful results, describing xylem size increase and aboveground woody biomass production as bell-shaped curves culminating in May and June respectively. However, the histologic approach provided a shorter time lag between xylem size increase and biomass production than the cellular one.
Conclusion
Better quantification of the shift between stem growth in size and in biomass will require addressing the knowledge gap regarding lignin deposition kinetics. Nevertheless, our novel histologic approach is simpler and more direct than the cellular one, and may open the way to a first quantification of intra-annual dynamics of woody biomass production in angiosperms, where the cellular approach is hardly applicable.
The characterization of fibers extracted from leaflet, the empty fruit bunches, leaf sheath, and spath of palm tree was performed. The fibers were extracted using three different procedures through ...chemical and / or enzymatic methods. The raw fibers studied have xylose contents between 13-22% and glucose content between 30% and 45%. The microfibrillar angle (MFA) values are in the order: bunch > spath > leaf sheath >> leaflet. Spath and leaf sheath, which naturally occur in a woven form present poor mechanical strength but could be readily used to produce cheap composites. Leaflet fibers extracted from date palm tree exhibiting a low MFA (16 degrees), a high cellulose content, and cellulose crystallinity present the highest ultimate tensile strengths (approximate to 1250 N.mm(-2)).
KEY MESSAGE : Thermogravimetric analysis, performed on small samples of earlywood (EW) or latewood (LW), indicated that earlywood is more susceptible to thermal degradation than latewood. These ...results suggest a direct relationship between wood density (which depends on the EW/LW ratio and indirectly on silviculture) and the response of wood during thermo-modification processes. CONTEXT : One of the main difficulties in developing thermo-modified wood products at an industrial scale lies in the difficulty of obtaining consistent products with a stable quality (durability, dimensional stability, color). This may be due either to the thermal treatment process itself or to inter- or intra-specific heterogeneity of wood properties. AIMS : We investigated the effect of the natural variability of oak wood, particularly in density, on the degree of thermo-degradation during thermal treatments. METHODS : X-ray computed tomography was used to assess the effect of initial wood density of oak boards on their thermo-degradation. Intra-ring wood density was estimated using thermogravimetric analysis and micro-densitometry. RESULTS : X-ray CT did not allow establishment of a clear correlation between initial wood density and mass loss due to thermo-degradation, while thermogravimetric analysis, performed separately on earlywood and latewood samples, revealed a larger susceptibility to thermal degradation of the less dense earlywood samples compared to more dense latewood samples CONCLUSION : Initial wood density, which is directly controlled by the earlywood/latewood ratio modulated by silvicultural practices, directly influences thermo-degradation during thermal treatment. Initial wood density therefore appears to be a potential parameter influencing industrial thermal treatment processes.
Tension wood of Laetia procera (Poepp.) Eichl. (Flacourtiaceae), a neo-tropical forest species, shows a peculiar secondary wall structure, with an alternance of thick and thin layers, while opposite ...wood of this species has a typical secondary wall structure (S1 + S2 + S3). Samples for the study of microstructural properties were collected upon the estimation of growth stresses in the living tree, in order to analyze the correlation of the former with the latter. Investigation using optical microscopy, scanning electron microscopy and UV microspectrophotometry allowed the description of the anatomy, ultra-structure and chemistry of this peculiar polylaminate secondary wall. In the thick layers, cellulose microfibril angle is very low (i.e., microfibril orientation is close to fibre axis) and cellulose microfibrils are well organized and parallel to each other. In the thin layers, microfibrils (only observable in the inner layer) are less organized and are oriented with a large angle relative to the axis of the cell. Thick layers are lightly lignified although thin layers show a higher content of lignin, close to that of opposite wood secondary wall. The more the wood was under tensile stress, the less the secondary wall was lignified, and lower the syringyl on guaiacyl lignin units' ratio was. The innermost layer of the secondary wall looks like a typical S3 layer with large microfibril angle and lignin occurrence. The interest of this kind of structure for the understanding of stress generation is discussed.
The mechanism responsible for unusual hygro-mechanical properties of tension wood containing the gelatinous layer (G-layer) was investigated. Tension and normal wood specimens were sampled from the ...leaning stems of a 75- and a 40-year-old Kunugi oak (
Quercus
acutissima
) tree, and the moisture dependencies of the longitudinal Young’s modulus and longitudinal dimensions were measured. The results, which were analyzed in relation to the anatomical properties of the specimens, revealed that the ratio of increase in the longitudinal Young’s modulus with drying was higher in the G-layer than in the lignified layer (L-layer); the longitudinal drying shrinkage displayed a similar pattern. It was found that the lattice distance of the 200 plane in the cellulose crystallite increased with drying, moreover, the half-width of the 200 diffraction peak increased with drying, which was remarkable in the tension wood. Those results suggest that in the green state, the polysaccharide matrix in the G-layer behaves like a water-swollen gel; however, it is transformed into a condensed and hard-packed structure by strong surface tension during moisture desorption, which is a form of xero-gelation. However, in the L-layer, condensation and subsequent xero-gelation of the polysaccharide matrix was prevented by the hydrophobic lignin that mechanically reinforces the matrix.
PDF
