Associations between shade tolerance and wood specific gravity for conifers in contrast to angiosperm trees: Foundations of the conifer fitness-enhancing shade tolerance hypothesis

For decades, researchers have held that wood specific gravity was an indicator or surrogate for both shade tolerance and successional status. However, recent research in dry tropical forests has shown very different associations regarding wood specific gravity. Past analyses of the tolerance and wood properties of tree species have focused on pooled coniferous and angiosperm species in temperate regions; fewer analyses have been conducted separately for conifers and angiosperm species. A database was compiled for the wood properties and/or tolerance scores of 542 temperate Northern Hemisphere conifer and angiosperm trees. Plant strategy was defined by shade tolerance (T shade), drought tolerance (T drought), and polytolerance (T poly = T shade + T drought) and fundamental wood properties were represented by basic specific gravity (SGbasic), relative stiffness (MOE/SGbasic), and relative strength (MOR/SGbasic). Simple linear regressions tested the significance (p < .05) of correlations between plant strategy and wood properties. Conifers, unlike angiosperm trees, showed a negative correlation between T shade and SGbasic and a positive correlation between T shade and both MOE/SGbasic and MOR/SGbasic. Only angiosperm trees had a significant correlation between T poly and both SGbasic and MOE/SGbasic, but both conifers and angiosperm trees had a significant correlation between T drought and both SGbasic and MOE/SGbasic. Shade tolerance, as a plant strategy, has functional implications for wood properties in temperate Northern Hemisphere conifers but not in associated angiosperms. The implied functional link between wood properties (SGbasic) and shade tolerance hypothetically extends to other fitness-enhancing traits impacted by SGbasic, such as growth rates and species maximum height.

Trade-offs between xylem hydraulic efficiency and mechanical strength in Chinese evergreen and deciduous savanna species

Evergreen and deciduous species coexist in tropical dry forests and savannas, but differ in physiological mechanisms and life-history strategies. Hydraulic conductivity and mechanical support are two major functions of the xylems of woody plant species related to plant growth and survival. In this study, we measured sapwood-specific hydraulic conductivity (Ks), leaf-specific hydraulic conductivity (KL), modulus of rupture (MOR) and elasticity (MOE), xylem anatomical traits and fiber contents in the xylems of 20 woody species with contrasting leaf phenology (evergreen vs deciduous) in a Chinese savanna. Our results showed that deciduous species had significantly higher Ks and KL but lower MOR and MOE than evergreen species. Evergreen species experienced more negative seasonal minimum water potential (Pmin) than deciduous species during the dry season. Furthermore, we found trade-offs between xylem hydraulic efficiency and mechanical strength across species and within the evergreen and deciduous groups, and these trade-offs were modulated by structural and chemical traits. Both Ks and KL were significantly related to hydraulic weighted vessel diameter (Dh) across all species and within the deciduous group. Both MOR and MOE were significantly related to wood density, neutral detergent fiber and acid detergent fiber across species and within evergreen and deciduous groups. Our findings demonstrated that Chinese evergreen and deciduous savanna species diverged in xylem hydraulic and mechanical functions, reflecting conservative and acquisitive life-history strategies for evergreen and deciduous species, respectively. This study provides new information with which to understand the hydraulic and biomechanical properties and ecological strategies of savanna species in long-term dry-hot environments.

Divergence of stem biomechanics and hydraulics between Bauhinia lianas and trees

Liana abundance and biomass are increasing in neotropical and Asian tropical seasonal forests over the past decades. Stem mechanical properties and hydraulic traits influence the growth and survival of plants, yet the differences in stem mechanical and hydraulic performance between congeneric lianas and trees remain poorly understood. Here, we measured 11 stem mechanical and hydraulic traits for 10 liana species and 10 tree species from Bauhinia grown in a tropical common garden. Our results showed that Bauhinia lianas possessed lower stem mechanical strength as indicated by both modulus of elasticity and modulus of rupture, and higher stem potential hydraulic conductivity than congeneric trees. Such divergence was mainly attributed to the differentiation in liana and tree life forms. Whether the phylogenetic effect was considered or not, mechanical strength was positively correlated with wood density, vessel conduit wall reinforcement and sapwood content across species. Results of principle component analysis showed that traits related to mechanical safety and hydraulic efficiency were loaded in the opposite direction, suggesting a trade-off between biomechanics and hydraulics. Our results provide evidence for obvious differentiation in mechanical demand and hydraulic efficiency between congeneric lianas and trees.

Effect of Graphene Oxide Nanoparticles on the Physical and Mechanical Properties of Medium Density Fiberboard

In this research, the special effects of graphene oxide nanoparticle charging (0, 2, 4, 6, wt.%) on the properties of medium-density fiberboard were examined. Physical and mechanical properties of the panels were determined conferring the method of European Norm standards. The consequences exhibited substantial enhancement in mechanical properties, explicitly in modulus of rupture, modulus of elasticity and internal bonding for 2–6% nanoparticle addition in a urea–formaldehyde resin. The mechanical properties, i.e., internal bond, modulus of elasticity and modulus of rupture were improved by 28.5%, 19.22% and 38.8%, respectively. Results also show a clear enhancement in thickness swelling and water absorption. The physical properties of thickness swelling, water absorption and thermal conductivity were improved up to 50%, 19.5% and 39.79%, respectively. The addition of graphene oxide nanoparticles strongly affected the curing time of the urea–formaldehyde resin and improved its thermal stability.

Effect of Slope Grain on Mechanical Properties of Different Wood Species

The aim of the presented study is to determine the relationship between mechanical parameters of selected wood species (Carya sp., Fagus sylvatica L., Acer platanoides L., Fraxinus excelsior L., Ulmus minor Mill.) used for the production of hand tools and drumsticks and the grain deviation angle from the rectilinear pattern. Modulus of rupture (MOR), modulus of elasticity (MOE), elastic strain and work to maximum load (WML) in the three-point bending test were determined. The results obtained show that the values of all the mechanical parameters measured for hickory wood are higher than those obtained for domestic species. As the grain deviation angle from parallelism increases, the mechanical properties of all analyzed wood species decrease. The greatest influence of grain deviation angle on mechanical parameters was recorded for the work to maximum load values.

Genetic Improvement of Sawn-Board Stiffness and Strength in Scots Pine (Pinus sylvestris L.)

Given an overall aim of improving Scots pine structural wood quality by selective tree breeding, we investigated the potential of non-destructive acoustic sensing tools to accurately predict wood stiffness (modulus of elasticity, MOE) and strength (modulus of rupture, MOR) of sawn boards. Non-destructive measurements of wood density (DEN), acoustic velocity (VEL) and MOE were carried out at different stages of wood processing chain (standing trees, felled logs and sawn boards), whilst destructively measured stiffness and strength served as benchmark traits. All acoustic based MOE and VEL estimates proved to be good proxies (r_A > 0.65) for sawn-board stiffness while MOE_TREE, VEL_HIT and resistograph wood density (DEN_RES) measured on standing trees and MOE_LOG and VEL_FAK measured on felled logs well reflected board strength. Individual-tree narrow-sense heritability ( h i 2 ) for VEL, MOE and MOR were weak (0.05-0.26) but were substantially stronger for wood density (0.34-0.40). Moreover, additive genetic coefficients of variation for MOE and MOR were in the range from 5.4% to 9.1%, offering potential targets for exploitation by selective breeding. Consequently, selective breeding based on MOE_TREE, DEN_RES or stem straightness (STR) could improve several structural wood traits simultaneously.

Do tall tree species have higher relative stiffness than shorter species?

PREMISE OF THE STUDY

In 1757 Leonhard Euler demonstrated that to avoid bending tall columns needed to be stiffer but not stronger than shorter columns of equal diameter and material density. Many researchers have concluded that trees have a fixed stiffness to basic density ratio, and therefore, trees adjust for increasing height by adding mass to adjust stem form. But the wood science literature points to considerable variance in stiffness with respect to green wood density.

METHODS

Using the vast global repository of green wood mechanical properties, we compared relative stiffness and relative strength between taller and shorter species. For North American trees, we examined stem moisture distribution.

KEY RESULTS

For all regions of the world, taller species on average possessed greater stiffness, but not strength, than shorter species of equal basic specific gravity. We looked for a possible universal mechanism that might allow taller tree species to adjust stiffness without affecting xylem specific gravity and concluded that the evidence points to a decrease in cellulose microfibril angle in structural cell walls combined with possible increases in holocellulose percentage. The evidence is strongest for conifers. We also showed that tall conifers have the ability to adjust the distribution of xylem moisture to maximize conduction while minimizing column load.

CONCLUSIONS

Our research reveals that taller trees have developed internal stem adjustments to minimize diameter increase while attaining ever-greater heights, thus enabling these taller species to reduce energy expended on biomass accumulation while gaining greater access to solar radiation.

Microwave Non-Destructive Inspection and Prediction of Modulus of Rupture and Modulus of Elasticity of Engineered Cementitious Composites (ECCs) Using Dual-Frequency Correlation

This research article presents dual-frequency correlation models for predicting the growth of elasticity and flexural strength of engineered cementitious composites (ECCs) using microwave nondestructive inspection technique. Parallel measurements of microwave properties and mechanical properties of ECC specimens were firstly undertaken in the sense of cross-disciplinary experiments. Regression models were developed via means of nonlinear regression to the measured data. The purpose of the study is: (i) to monitor the flexural strength and elasticity growth; and (ii) to predict their mature values under the influence of different initial water contents, via microwave effective conductance at early ages. It has been demonstrated that both the modulus of rupture (MOR) and modulus of elasticity (MOE) can be accurately modeled and correlated by microwave conductance using exponential functions. The moduli developed as a function of conductance whereas the regression coefficient exhibited a linear relation with water-to-binder ratio. These findings have highlighted the effectiveness of the microwave non-destructive technique in inspecting the variation of liquid phase morphology of ECCs. The dual-frequency correlation can be used for structural health monitoring, which is not only for prediction but also provides a means of verification.

Structural determinants of increased susceptibility to dehydration-induced cavitation in post-fire resprouting chaparral shrubs

It is well established that transpiration and photosynthetic rates generally increase in resprouting shoots after fire in chaparral shrublands. By contrast, little is known about how plant hydraulic function varies during this same recovery period. We hypothesized that vascular traits, both functional and structural, would also shift in order to support this heightened level of gas exchange and growth. We examined stem xylem-specific hydraulic conductivity (K_s ) and resistance to cavitation (P₅₀ ) for eight chaparral shrub species as well as several potential xylem structural determinants of hydraulic function and compared established unburned plants and co-occurring post-fire resprouting plants. Unburned plants were generally more resistant to cavitation than resprouting plants, but the two groups did not differ in K_s . Resprouting plants had altered vessel structure compared with unburned plants, with resprouting plants having both wider diameter vessels and higher inter-vessel pit density. For biomechanics, unburned plants had both stronger and denser stem xylem tissue than resprouting plants. Shifts in hydraulic structure and function resulted in resprouting plants being more vulnerable to dehydration. The interaction between time since disturbance (i.e. resprouting versus established stands) and drought may complicate attempts to predict mortality risk of resprouting plants.