An evaluation of the uniform stress hypothesis based on stem geometry in selected North American conifers

The uniform stress hypothesis of stem formation was evaluated by comparing stem taper of Abies balsamea, Abies lasiocarpa, Picea rubens, Pinus contorta, Pinus elliottii, Pinus palustris, Pinus ponderosa, Pinus taeda, and Pseudotsuga menziesii to the taper expected if stems develop to uniformly distribute bending stress. The comparison was conducted by regressing stem diameter at height h (D_h) against bending moment at h (M_h) using the model D_h=J (M_h)^' where J and ' are fitted coefficients, and testing for '=0.333, the hypothesized value. Twelve curves were fitted with the model. Seven of the fitted values of ' were significantly different from 0.333, but eight of the values were within ᆞ% of 0.333 and eleven values were within ᆣ% of 0.333. Where the fitted value of ' was >15% of 0.333, residuals were biased with height. Fit by relative height, values of ' were within ᆞ% of 0.333 for large portions of these stems. While most of the fitted values of ' support the uniform-stress hypothesis, the values of ' for Pseudotsuga menziesii trees clearly did not. Many of the fitted values of J were inversely related to the modulus of elasticity (E) of green wood reported for these species. With the exception of Pseudotsuga menziesii, growing conditions appeared to account for extraordinary values of J. Increases in J with stem height corresponded with reported decreases in E with height. The covariance between J and E suggests some regulation of bending curvature by adjustments in cross-sectional area. These results suggest that stems taper to maintain a uniform bending curvature and that when E is relatively constant within and among stems, diameter along the stem or across stems can be predicted from bending moment using a simple power function.     

Estradiol distribution during the development and expression of reproductive structures in<Emphasis Type="Italic"> Populus tremuloides</Emphasis> Michx.

Estradiol (E₂) was measured in spring and winter tissues in several populations of dioecious Populus tremuloides Michx. and in four trees with perfect flowers. A correlation between the concentration of E₂ and sex expression was observed. In catkins, E₂ concentrations increased before anthesis, peaked during flowering, and decreased as flowers matured. The increases were concurrent with sporogenesis and development of gametophytes. Statistically significant seasonal variation in E₂ concentration was also evident. Dormant winter tissues showed significantly lower E₂ concentrations than spring tissues. Branches exposed to relatively more light had significantly higher E₂ content in both spring and winter. Seasonal variation in E₂ content was also observed in trees of the same sex, among branches of the same tree, and among different organs.     

Mechanical properties of spruce and beech wood grown in elevated CO2

Biomechanical responses of stems of 6- to 7-year-old spruce [Picea abies (L.) Karst.] and beech (Fagus sylvatica L) trees were studied after 4 years of growth in elevated atmospheric CO₂ in combination with a nitrogen treatment and on two different soil types. At the end of the treatment, stems were harvested and tested in fresh and air-dried status. Bending characteristics of juvenile wood (modulus of elasticity, termed rigidity) were determined by bending tests. Fracture characteristics (termed toughness) were determined by stroke-pendulum tests. From the base disk of each stem densitometric data were obtained. In spruce, wood produced under elevated CO₂ was tougher on both soil types; enhanced N deposition made wood less rigid only on acidic soils. In contrast, beech wood samples showed no significant reaction to CO₂ but were significantly tougher under high nitrogen depositions on acidic soil. Effects on wood density of both CO₂ and N treatments were not significant, but wood density was higher on acidic soil and so were rigidity and toughness (soil effect). Different genotypes of spruce and beech reacted significantly differently to the treatments. Some genotypes reacted strongly to CO₂ or N, whereas others did not react or showed interactions between CO₂ and N. This underlines the importance of genetic diversity in tree communities.     

Variation of non-structural carbohydrates in silver birch (Betula pendula Roth) wood

Non-structural carbohydrates in silver birch (Betula pendula Roth) wood were analysed in a 7-year-old clone and in five mature stems. The analysis was conducted to obtain more detailed information on seasonal fluctuation of these components and of the tree-to-tree variation and within stem variation. The sugars were analysed by GLC-MS. The smallest total soluble sugar amounts (consisting of sucrose, fructose, glucose, raffinose and myo-inositol) in young trees were measured during mid-summer (ca. 0.3%) and the largest while in dormancy (ca. 1.6% on wood dry weight basis). Raffinose was detected in autumn as a minor component. The proportion of monosaccharides and the amount of myo-inositol were largest during growth. Compared to other studies silver birch showed more evident seasonal fluctuation in soluble sugars than evergreen tree species. The sugar amount in mature stems was approximately at the same level as in young trees that had the same felling time. Tree-to-tree variation in the non-structural carbohydrates in the mature wood was fairly large. However, the amount of total soluble sugars, sucrose and glucose showed significant variation within the stem. The amount of these sugars was largest in samples that were taken close to the cambium. Starch was also detected close to pith. According to the heartwood definition and starch measurement results in this paper, it could be stated that silver birch does not form heartwood.     

Intra-specific competition and the radial development of wood density, microfibril angle and modulus of elasticity in plantation-grown Eucalyptus nitens

In eucalypt plantations managed for solid-wood products, radial trends in wood density, microfibril angle (MFA) and stiffness (modulus of elasticity, MoE) are properties of potential commercial importance that can be affected by competition from neighbouring trees. In this study, wood properties at breast height (1.3?m) were studied on radial strips prepared from 12-mm pith-to-bark wood cores taken from 20 trees in a 22-year-old Eucalyptus nitens (Deane and Maiden) Maiden thinning trial in north-eastern Tasmania, Australia. Thinning treatments were applied at age 6?years. Trees were sampled from each of the 200, 400?stems ha^?1 and unthinned control treatments. Intra-specific competition for each sampled tree was estimated using the basal area growth of surrounding trees. SilviScan? technology was used to produce radial profiles of wood density, MFA and MoE. Results indicate a reduction in intra-specific competition through thinning of E. nitens plantations at an early age leads to a transient increase in MFA but has no significant effect on wood density or the intra-annual cycle of wood density. The correlation between the level of intra-specific competition and initial change in MFA following thinning, and a significant relationship between tree shape and mean MFA at breast height suggests the change in MFA is a post-thinning response to increased exposure and wind sway.     

Bending of apricot tree branches under the weight of axillary growth: test of a mechanical model with experimental data

Stem orientation is an important factor for fruit tree growth and branching habit since it influences fruit production as well as training practices. A mechanical model of the bending of a stem under axillary load was written and evaluated using experimental data on apricot trees (Prunus armeniaca L.). A set of 15 1-year-old stems of various shapes was observed during the early stage of the growing season when radial growth is still negligible and the loading of the stem increases considerably. The structural modulus of elasticity (MOE) of the stems was estimated through in situ bending tests assuming homogeneous material behaviour. The effect of viscoelasticity was observed through creep tests performed on similar stems during winter. Inputs of the model are initial shape, initial diameter, and final load, defined at various positions along the stem. The final shape was simulated based on different mechanical assumptions, and compared to observations. Assuming small deflections resulted in an underestimate of the mean slope variation of 48%, accounting for large displacements reduced this underestimate to 29% and accounting for viscoelasticity reduced it further to 14%. An adjustment of the structural MOE to fit the final shape led to an excellent fit of the data in most cases, the residual errors for some axes being attributed to material heterogeneity. The use of biomechanical models to predict the shape of fruit trees based on growth parameters, provided adequate assumptions are made, is discussed.     

Stem Diameter Growth of Scots Pine Trees after Increased Mechanical Load in the Crown during Dormancy and (or) Growth

A field experiment with a 2 x2 factorial block design (W_xS_x)was conducted in northern Sweden where the mechanical loadsin the crowns of sixteen 2.5m high Scots pine (Pinus sylvestrisL.)trees were increased during one winter (W₁, dormant period)and (or) summer (S₁, growth period). Trees treated were loadedwith five 2kg bags hung over mid-crown branches close to thestem, i.e. 10kg per tree. After treatment, all trees were leftto grow untreated for one additional year. Trees were then cutat ground level and annual ring widths for the last 5 yearswere measured on stem discs taken at 1, 5, 10, 15, 20, 30 and50% of tree height. Differences between treatments were analysedwith two-way factorial ANOVA. Accumulated treatment responsewas positive for winter loading (W₁S_x) at all levels, and statisticallysignificant at 1, 15 and 20% of tree height. Summer loading(W_xS₁) had positive effects at the lowest and middle parts ofthe stem, and negative in between. No statistically significanttwo-way interaction (W xS) was observed. Results support thehypothesis that Scots pine trees can retain information aboutmechanical forces acting on their stems during winter, and respondto this during the following growth period. The results alsosuggest that stem form of trees in boreal forests may be stronglyaffected by winter conditions. Stem form; mechanical perturbation; Scots pine; Pinus sylvestris; dendrometer; diameter; growth; dormancy; thigmomorphogenesis; wind; sway     

Enhanced growth and ethylene increases spiral grain formation in Picea abies and Abies balsamea trees

Spiral grain angle in Norway spruce (Picea abies) trees and balsam fir (Abies balsamea) seedlings was investigated in relation to growth rate, endogenous and applied ethylene. Trees from stands of Norway spruce, which were irrigated and fertilised in order to enhance growth, and trees having different growth rates in non-treated stands were studied. Stem growth rate at the stand level (m³ ha^-1 year^-1) was measured annually, or by means of microscopy on stem sections as the number and size of tracheids produced. Enhanced growth increased ethylene evolution and maintained a high level of left-handed spiral grain angle in comparison to slower-growing trees. An increased number of earlywood tracheids in fast growing trees was correlated to a more left-handed spiral grain angle. Ethrel, applied to stems of balsam fir seedlings, increased the internal ethylene levels in parallel with increased left-handed spiral grain angle. The results indicate that ethylene regulates the extent of spiral grain angle.     

Laurel forests in Tenerife, Canary Islands

The efficiency of the conductive system in about 40-year-old Laurus azorica trees growing in a laurel forest was evaluated by comparing main stems and leaves (petioles) on the basis of theoretical sap flow values (1) calculated from vessel anatomy (taking vessels as ideal capillaries), (2) derived from measured dye velocity and (3) data taken from direct sap flow measurements. It was found that actual sap flow rate per wood area increases in stems from the pith towards the cambium. The outermost part of the stem is the most important part of the tree for conducting water. Maximum actually measured transpiration (sap flow rate) for the stand was practically identical to the theoretical rate calculated based on petiole anatomy, but it was about 45 times lower than that calculated based on stem anatomy. This illustrates the safety features of stem wood, which due to its high vessel density, is capable of transporting all the water required even when only a small area of its vessels is working. In the petioles, xylem is more efficiently used, but almost all vessels must work in order to supply water to leaves and any disturbance may cause leaf loss.     

Root deformation in plantations of container-grown Scots pine trees: effects on root growth,tree stability and stem straightness

Root system deformation was studied in 23 Scots pine (Pinus sylvestris L.) stands in central Sweden. The study comprised both plantations created with container-grown plants (Paperpot) and natural stands including young (7–9 year old) and older (19–24 year old) trees. Trees were measured with regards to distribution of roots, root deformation, stability, stem straightness and wood properties in stumps. Root distribution was most uniform for naturally regenerated trees. Older trees generally showed a better root distribution than young trees. The young planted trees displayed a high frequency of severely spiralled root systems, while only a few of the older trees had spiralled root systems. No severe root deformations were observed on naturally regenerated trees. Naturally regenerated trees were more stable than those which had been planted. Differences in bending moment, when trees were pulled to an angle of 10°, were considerable between young planted and naturally regenerated trees, but less pronounced for the older trees. Young planted trees had the highest frequency of severely crooked stem bases, while naturally regenerated trees had the straightest mode of growth. Tensile strength in peripheral wood samples of the stumps was substantially lower for planted than for naturally regenerated trees. Strain values to breakage of wood samples, taken from the root collar and the central- and peripheral part of the stump were lower for planted trees. The conclusions from this study are that root distribution, tree stability and stem straightness of planted Paperpot-grown trees will improve after a certain time and approach the state of naturally regenerated trees. As trees grow older, early established crooked stem bases will be compensated by radial growth and the tree will appear straighter. Inside the stem, however, problems may still remain with abnormal fibre direction and compression wood together with inferior root strength due to fibre disturbances as a result of spiralled roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

A biomechanical perspective on the role of large stem volume and high water content in baobab trees (Adansonia spp.; Bombacaceae)

The stems of large trees serve in transport, storage, and support; however, the degree to which these roles are reflected in their morphology is not always apparent. The large, water-filled stems of baobab trees (Adansonia spp.) are generally assumed to serve a water storage function, yet recent studies indicate limited use of stored water. Through an analysis of wood structure and composition, we examined whether baobab morphology reflects biomechanical constraints rather than water storage capacity in the six Madagascar baobab species. Baobab wood has a high water content (up to 79%), low wood density (0.09-0.17 g · cm(-3)), high parenchyma content (69-88%), and living cells beyond 35 cm into the xylem from the cambium. Volumetric construction cost of the wood is several times lower than in more typical trees, and the elastic modulus approaches that of parenchyma tissue. Safety factors calculated from estimated elastic buckling heights were low, indicating that baobabs are not more overbuilt than other temperate and tropical trees, yet the energy investment in stem material is comparable to that in temperate deciduous trees. Furthermore, the elastic modulus of the wood decreases with water content, such that excessive water withdrawal from the stem could affect mechanical stability.     

Photosynthetic, hydraulic and biomechanical responses of Juglans californica shoots to wildfire

Leaf gas exchange and stem xylem hydraulic and mechanical properties were studied for unburned adults and resprouting burned Juglans californica (southern California black walnut) trees 1 year after a fire to explore possible trade-offs between mechanical and hydraulic properties of plants. The CO₂ uptake rates and stomatal conductance were 2–3 times greater for resprouting trees than for unburned adults. Both predawn and midday water potentials were more negative for unburned adult trees, indicating that the stems were experiencing greater water stress than the stems of resprouting trees. In addition, the xylem specific conductivity was similar in the two growth forms, even though the stems of resprouting trees were less vulnerable to water-stress-induced embolism than similar diameter, but older, stems of adult trees. The reduced vulnerability may have been due to less cavitation fatigue in stems of resprouts. The modulus of elasticity, modulus of rupture and xylem density were all greater for resprouts, indicating that resprouts have greater mechanical strength than do adult trees. The data suggest that there is no trade-off between stem mechanical strength and shoot hydraulic and photosynthetic efficiency in resprouts, which may have implications for the success of this species in the fire-prone plant communities of southern California.     

Water content, hydraulic conductivity, and ice formation in winter stems of Pinus contorta: a TDR case study

Stem water content, ice fraction, and losses in xylem conductivity were monitored from November 1996 to October 1997 in an even-aged stand of Pinus contorta (lodgepole pine) near Potlatch, Idaho, USA. A time domain reflectometry (TDR) probe was used to continuously monitor stem water contents and ice fractions. Stem sapwood water contents measured with TDR were not different from water contents measured gravimetrically. The liquid water content of stems ranged from 0.70 m³ m^-3 to 0.20 m³ m^-3 associated with freezing and thawing of the wood tissue. Ice fraction of the stem varied from 0-75% during the winter suggesting liquid water was always present even at ambient temperatures below -20°C. Shoot xylem tensions decreased through the winter to a minimum of ca. -1.4 MPa in February then increased to -0.4 MPa in May. Shoot xylem tensions decreased during the growing season reaching -1.7 MPa by September. Annually, low shoot water potentials were not correlated to decreases in stem hydraulic conductivity. Xylem conductivity decreased due to cavitation through the winter and was 70% of summer values by March. Decreases in xylem conductivity were correlated to low shoot water potentials and cumulative freezing and thawing events within the xylem. Xylem conductivity increased to pre-winter values by May and no reductions in xylem conductivity were observed during the growing season.     

Changes in the localization and levels of starch and lipids in cambium and phloem during cambial reactivation by artificial heating of main stems of Cryptomeria japonica trees

Background and Aims

Cambial reactivation in trees occurs from late winter to early spring when photosynthesis is minimal or almost non-existent. Reserve materials might be important for wood formation in trees. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules in cambium and phloem were examined from cambial dormancy to the start of xylem differentiation in locally heated stems of Cryptomeria japonica trees in winter.

Methods

Electric heating tape was wrapped on one side of the stem of Cryptomeria japonica trees at breast height in winter. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules were determined by image analysis of optical digital images obtained by confocal laser scanning microscopy.

Key Results

Localized heating induced earlier cambial reactivation and xylem differentiation in stems of Cryptomeria japonica, as compared with non-heated stems. There were clear changes in the respective localizations and levels of starch and lipids (as droplets) determined in terms of relative areas on images, from cambial dormancy to the start of xylem differentiation in heated stems. In heated stems, the levels and number of starch granules fell from cambial reactivation to the start of xylem differentiation. There was a significant decrease in the relative area occupied by lipid droplets in the cambium from cambial reactivation to the start of xylem differentiation in heated stems.

Conclusions

The results showed clearly that the levels and number of storage starch granules in cambium and phloem cells and levels of lipids (as droplets) in the cambium decreased from cambial reactivation to the start of xylem differentiation in heated stems during the winter. The observations suggest that starch and lipid droplets might be needed as sources of energy for the initiation of cambial cell division and the differentiation of xylem in Cryptomeria japonica.     

The contribution of stored water to transpiration in Scots pine

Abstract. The amount of water available diurnally and annually from the storage tissues was measured in plots of Scots pine trees with four different population densities (608–3281 trees per ha) in a 40-year-old plantation in north eastern Scotland. The water storage capacity of stems, branches, and foliage was estimated from equations derived from harvested trees and measurements of relative water content. On average 64% of the water considered to be available for transpiration was in the stem sapwood and less than 5% in the phloem, cambium and foliage. Trees on the plot with the highest population density had a water storage capacity of 212 m3 ha^?1 (21.2 mm), whereas those on the plot with the lowest population density had a water storage capacity of 124 m3 ha^?1 (12.4 mm). The utilization of stored water in transpiration was estimated from seasonal and diurnal measurements of the relative water content of foliage and stem sapwood. The largest change in sapwood relative water content over a 2-week period was a reduction of 27% corresponding to extraction from the sapwood of 2.5 and 5.1 mm of water on the plots with the lowest and highest population densities, respectively. In rapidly changing weather conditions 1–1.5 mm day^?1 could be removed from the stem sapwood alone. Since transpiration rarely exceeded 3 mm day^?1, 30–50% of the transpired water was extracted from water stored in the stem sapwood over short periods. Trees on the plot with the lowest population density occasionally had slightly higher relative water contents and exhibited larger diurnal fluctuations than those on the plot with the highest population density, possibly because of differences in wood density. Sapwood water content was generally lower at times of high transpiration rate and in winter during freezing conditions. Resaturation took several months to complete during the winter.     

Variability in the rigidity of <Emphasis Type="Italic">Coffea canephora</Emphasis> Pierre stems determined by acoustic analysis

The physical characteristics of wood are not usually taken into account in the genetic improvement of perennial species cultivated for their fruits or seeds. Yet in coffee trees, stem breakage during harvesting, or lodging during their development are major defects in some cultivars. Such defects are linked to certain mechanical properties of the wood, such as stiffness, which can be characterized by the modulus of elasticity (Eb), a parameter used in the theory of mechanics of solids. The longitudinal Eb of several clones of the species Coffea canephora Pierre was assessed. The Eb was measured using a vibration analysis system developed at CIRAD, the "Bing" system. It is based on an analysis of vibrations caused by a blow to the tip of a piece of wood of known geometry. The Eb obtained in this way revealed substantial variability within the C. canephora study sample. Clone classification according to stiffness made it possible to identify the clones most susceptible to lodging. This technological characteristic of wood is highly heritable (0.64), meaning that effective improvement of this trait is feasible. This parameter could also be used as a predictor of other traits of agronomic interest, such as resistance to borer insects.     

Effects of experimental stem burial on radial growth and wood anatomy of pedunculate oak

In dendrogeomorphology, abrupt changes in wood anatomy are frequently used to date the exact year of burial and exposure events. However, few studies have addressed the precision and underlying mechanisms of these changes. In a field experiment, performed in a drift-sand area in the Netherlands, we buried the stems of mature pedunculate oak trees (Quercus robur L.) up to a height of 50 cm and analysed the responses in ring width and earlywood-vessel characteristics, while monitoring the course of temperature above and below the soil surface.After 3 years of stem burial, we found no significant differences in ring width and earlywood-vessel characteristics between control and buried trees both above and below the burial level. Burial however strongly reduced temperature amplitude and the occurrence of sub-zero temperatures around the buried stems. All buried trees formed epitropic roots that grew upward into the new sediment layer, but no adventitious roots were formed on the buried stems. Irrespective of the burial treatments, we found that the mean ring width was largest at the original stem base and lowest at breast height. In contrast, vessel sizes were significantly larger at breast height compared with the stem base. Differences in vessel density barely differed between years and heights.In our field experiment on mature pedunculate oak trees, the burial of stems by 50 cm of drift sand did not induce any local growth suppression or detectable changes in wood anatomy. As wood-anatomical changes in response to burial have previously been reported for trees that had formed adventitious roots, we stress the role of adventitious-root formation as a possible trigger behind the local changes in wood anatomy, reflecting a functional change of a buried stem towards a root. Based on our field experiment, it seems unlikely that years of shallow or moderate burial events (≤50 cm) can be reconstructed using the wood structure of buried stems. As epitropic roots develop quickly after burial, dating such roots may potentially yield better estimates of burial events. Further research on the relation between adventitious root and changes in stem anatomy is needed to ascertain the precision of dating sand-burial events using tree rings.     

Laurel forests in Tenerife, Canary Islands

The general wood structure, vessel size and distribution along the stem xylem radius and in petioles were studied in Laurus azorica trees living in a Tenerife laurel forest. The fractions of volume occupied by dry matter, water and air in percentage of wood fresh volume were also studied. The wood showed a diffuse-porous structure, with solitary vessels or vessels somewhat clustered in small radially oriented groups. Vessels had a diameter ranging from 20 to 130 µm. This diameter was minimal close to the pith, increased more than 2-fold with age, and reached its maximum width close to the cambium. Vessel density decreased from 36 vessels mm^-2 near the pith to about 13 vessels mm^-2 near the cambium. Accordingly, the lumen area was small in young xylem close to the pith (0.0015 mm²), reaching a value 5 times larger (0.007 mm²) near the cambium than in the centre of the stem. Lumen area of vessels in petioles was about 1.5% of petiole cross-sectional area and thus much lower than in stems. Mean hydraulic diameter of these vessels was about 20 µm, and mean vessel density about 136 per petiole. There were only small differences in proportions of dry matter, water and air along stem radius. The relevance of each one of these fractions in the wood is discussed as evidence of the possible existence of a number of embolized vessels dispersed in the total functional cross-sectional area of the xylem.     

Extraction of tracheal sap from Citrus and analysis of its nitrogenous compounds

Tracheal sap was extracted from sections of stems (0.5 to 1.5 cm in diameter and 7.5 to 15.0 cm in length) of orange trees (Citrus sinensis (L.) Osbeck cv. Washington Navel) by using a combination of the vacuum and liquid displacement methods. The volume of sap obtained and its concentration of nitrogenous compounds were dependent on the volume of displacing liquid used for the extraction. Four ml of water-saturated 1-butanol extracted essentially all of the xylem fluid present in the stem sections without apparent production of artifacts. The time of sampling affected the nitrogen concentration of the tracheal sap, but not the content of xylem nitrogen per volume of stem material. The orientation of the stems in the tree and the diameter of the stems had an effect on their contents of xylem nitrogen, with southeastern orientation and thinner stems showing higher concentrations. We could not detect the presence of ammonium, nitrites or proteins in the tracheal sap of orange trees. Most of the nitrogen was present as amino acids and about 2% of the total in the form of nitrates. The qualitative composition of amino acids, as determined by TLC, was the same both in winter and spring tracheal sap. The main components of the sap were proline and arginine in winter, and these amino acids together with asparagine and aspartic acid in spring.