Introduction

In the living tree, the sapwood, in contrast with heartwood, is physiologically active, conducting water and nutrients from roots to leaves (^{Bamber, 1985}; ^{Hillis, 1987}) and storing food materials (^{Bamber, 1985}). The transformation of sapwood into heartwood is characterized by the death of parenchyma cells (^{Hillis, 1987}), development of tyloses in the vessels of many species (^{Bamber, 1976}) and the biosynthesis of nonstructural compounds, leading to an important accumulation of extractives and to the differences in physical and chemical properties between sapwood and heartwood (^{Sellin, 1994}). Heartwood and sapwood in a tree vary with a large number of factors, including species, age, climate, rate of growth, foliage area, site quality and tree vitality, and have been the subject of several reviews (^{Pinto et al., 2004}; ^{Climent et al., 2002}).

Heartwood and sapwood have different properties and their proportion within the tree will have a significant impact on the utilization of wood (^{Climent et al., 2002}). For pulping, heartwood is at a disadvantage as its extractives can affect the process and product properties. For solid wood applications the different properties of heartwood and sapwood influence drying, durability, and aesthetic values for the consumer (^{Pinto et al., 2004}; ^{Morais and Pereira, 2007}). When there is a large colour difference between sapwood and heartwood, selection of wood components by color also plays a significant role in some timber application (^{Dzifa et al., 2004}).

A study on the variation of wood properties of Kyere wood indicated that the wood density and mechanical properties decreased along longitudinal position from the bottom up the stem. Site also had significant impact on the wood properties. Wood samples collected from the site with the highest mean annual rainfall had the least density and strength properties (^{Ayarkwa, 1998}).

Albizia julibrissin or Persian silk tree is legume specie in the genus Albizia. The global distribution of this species is in the North Anatolian, northern Iran, Caucasus, Sinai, Japan, Cyprus, Yugoslavia, Bulgaria, and probably planted in Australia (^{Mozaffarian, 2003}). There are two tree species in this genus, Albizia julibrissin that grow in temperate and cool temperate northern forests of Iran, and A. lebbeck grows only in tropical regions of Iran (^{Sabeti, 1975}; ^{Mozaffarian 1996}). This species is used for making soap, hair shampoo and UV protectors and probably other compounds (^{Nehdi, 2011}; ^{Panahian and Rahnama 2010}).

The information about the effect of longitudinal position on the wood different properties is not available for Albizzia julibrissin (silk wood) in Iran. Therefore, to use this material properly and efficiently, it is a requisite to know its different properties.

Objetives

The objectives of this study were: (a) to examine the variations of wood density and mechanical strength properties (MOE and MOR) along longitudinal position, (b) to compare wood properties between heartwood and sapwood, and (c) to determine the relationship between wood density and mechanical properties in Persian silk wood (Albizzia julibrissin).

Material and methods

Results

Wood density

Average and standard deviation of wood density along longitudinal position in hartwood and sapwood are listed in Table 1. The analysis of variance (Anova) indicated that the longitudinal position and heartwood-sapwood did affect significantly wood density. The interaction effects between longitudinal position and heartwood-sapwood were not significant on wood density (Table 2). The mean of wood density along longitudinal position from the base upward decreased in heartwood and sapwood. The relationship between longitudinal position and wood density in heartwood (R²=0.459) is stronger than in sapwood (R²= 0.431). The average of wood density in heartwood is higher compared to the sapwood (439 kg/m³ vs 394 kg/m³⁾.

Table 1 The variation of wood properties in different stem height levels for silk wood. 

Uppercase and lowercase letters respectively show significant differences among longitudinal position and between heartwood-sapwood.

Table 2 F-value (in Anova results) for wood properties along longitudinal position. 

Relationship among wood properties

The dependence of static bending properties (MOE and MOR) on the oven-dry density was modeled using simple regression models (Fig. 1 and Fig. 2). These relationships in sapwood (R² density-MOR = 0.368, R² density-MOE = 0.174) are higher compared to the heartwood (R² density-MOR = 0.139, R² density-MOE = 0.138).

Figure 1 The relationship between wood density and modulus of elasticity (MOE) in heartwood and sapwood. 

Figure 2 The relationship between wood density and modulus of rupture (MOR) in heart wood and sapwood. 

Discussion

The wood density, MOE and MOR in heartwood is higher compared to sapwood. These differences are related to the chemical properties in herawood and sapwood. Significant amount of extractives are deposited in the hearwood, up to two or three times more than in sapwood (^{Panshin and de Zeeuw, 1980}). Our observation of silkwood behavior are in accodance with the studies of ^{Panshin and de Zeeuw (1980)} ("Type 4 woods: specific gravity of the wood exhibit a general decrease from pith to bark in the stem". Examples of North American hardwoods: Fagus sylvatica, Liriodendrum tulipifera, Populus spp., Prunus serotina and Quercus falcata), ^{Morais and Pereira (2007}; Eucalyptus globulus Labill.) and ^{Pinto et al., (2004}; a conifer, Pinus pinaster Ait). ^{Panshin and de Zeeuw (1980)} also point out that "among the hardwoods there is almost even division between reported increases and decreases in specific gravity from pith to bark".

Within-tree wood density and mechanical properties decreased along the stem, from the base upwards; however, wood density, MOE and MOR was the highest at 5% of total tree height. Similar patterns of wood density and mechanical properties variation in the longitudinal direction have also been reported by several researchers:

^{Panshin and de Zeeuw (1980}. "Static bending properties... decrease upward in the stem for Pinus resinosa Ait. and Shorea almon Foxw.); ^{Ayarkwa (1998}; Pterygota macrocarpa K. schum); and ^{Kord et al., (2010}; Populus euramericana). This may be due to the fact that butt log of the same tree has more mature wood than the top log which consists mainly of juvenile wood (^{Panshin and de Zeeuw, 1980}). Juvenile wood is explained by ^{Kolzlowski (1971}) and ^{Larson (1969}) as being the results of the relative abundance of growth regulators and carbohydrates in the cambial zone near the crown. Juvenile wood density and mechanical properties were lower than that of mature wood. The lower density and strength properties of the wood near the top may be due to the thin walls of the cells of the wood, the lower cellulose content and crystallinity of the wood compared with that of the matured wood in the log at the butt (^{Zobel and Sprague, 1998}).

Positive relationship was found between wood density and mechanical strength properties in heartwood and sapwood. Also, the relationship between wood density and MOE is weaker than the relationship between wood density and MOR in heartwood and sapwood. A similar trend has also been reported by several researchers for various species (^{Zhang, 1997}; ^{Zobel and Van Buijtenen, 1989}). Wood density had important role on the variation of mechanical properties.

Conclusions

In the present research, the wood density and mechanical properties of heartwood and sapwood in silkwood were determined. The following conclusions were drawn from the study: