Abstract
The radial permeability (gas and liquid) of the hybrid pine (Pinus elliottii var. elliottii [PEE] × Pinus caribaea var. hondurensis [PCH]) was investigated for wood samples collected from 30 trees that were 19 years of age and represented various genotypes and stocking rates. The PEE × PCH hybrid is now a very important resource for the Australian forestry industry, producing logs used to manufacture a diverse array of wood products. The permeability of wood influences many important wood properties and industrial processes. For all data combined from all radial sampling positions, there was no significant effect of genotype and stocking rate on radial permeability. Both gas and liquid permeability increased from pith to bark positions within the tree. Conversely, resin content decreased from pith to bark positions. Gas and liquid permeability were significantly positively correlated, and a highly significant negative relationship was also found between permeability (gas and liquid) and resin content.
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Radial Permeability of the Hybrid Pine (Pinus elliottii × Pinus caribaea) in Australia
William Leggate,a,* Adam Redman,b Jeff Wood,c Henri Bailleres,b and David J. Lee d
The radial permeability (gas and liquid) of the hybrid pine (Pinus elliottii var. elliottii [PEE] × Pinus caribaea var. hondurensis [PCH]) was investigated for wood samples collected from 30 trees that were 19 years of age and represented various genotypes and stocking rates. The PEE × PCH hybrid is now a very important resource for the Australian forestry industry, producing logs used to manufacture a diverse array of wood products. The permeability of wood influences many important wood properties and industrial processes. For all data combined from all radial sampling positions, there was no significant effect of genotype and stocking rate on radial permeability. Both gas and liquid permeability increased from pith to bark positions within the tree. Conversely, resin content decreased from pith to bark positions. Gas and liquid permeability were significantly positively correlated, and a highly significant negative relationship was also found between permeability (gas and liquid) and resin content.
Keywords: Wood permeability; Resin content; Hybrid pine; Pinus elliottii var. elliottii; Pinus caribaea var. hondurensis, Southern pine
Contact information: a: Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia; b: Queensland Department of Agriculture and Fisheries, Horticulture and Forestry Science, Salisbury Research Facility, 50 Evans Road, Salisbury, Queensland, 4107 Australia; c: Fenner School of Environment and Society, The Australian National University, Canberra, ACT 0200, Australia; d: Forest Industries Research Centre, University of the Sunshine Coast, Maroochydore, Queensland, 4558, Australia; *Corresponding author: william.leggate@anu.edu.au
INTRODUCTION
Australian subtropical plantation exotic pine resources include slash pine (Pinus elliotii var. elliotii) [PEE], Caribbean pine (Pinus caribaea var. hondurensis) [PCH], and locally developed hybrids known as PEE × PCH (Hybrid pine F1 and F2), as well as smaller areas of loblolly pine (Pinus taeda) (Lee 2015). The hybrid pine PEE × PCH now dominates exotic softwood plantations in Queensland with around 94,100 hectares planted and an estimated annual log production volume soon to exceed 1 million m3/year (Personal Communication, HQ Plantations 2018). The subtropical plantation exotic pine resources comprise approximately 15% of the Australian softwood plantation estate (approximately 156,600 hectares), supporting a diverse processing sector that includes sawn timber, engineered wood products, panels, landscaping, and lower grade end uses (Lee 2015).
Permeability is the property of a material that indicates how freely fluids and gases flow through them in response to a pressure gradient (Milota et al. 1994). In the case of wood, liquid and gas permeability is determined by measuring the rate of flow of fluid and gas through a wood specimen of known length and cross-sectional area while a known pressure difference is applied across it (Booker 1977). Gas and liquid permeability are usually closely correlated (Comstock and Côté 1968; Taghiyari 2012), but due to higher viscosity, molecular size, and liquid-wood interactions, liquid permeability is usually much lower. For example, water permeability in wood has been shown to be around 50 times less compared to air permeability at the same temperature (Siau 1984).
The permeability of wood to liquids and gases influences many important processes, including treatment with preservatives, wood modification systems, drying, chemical pulping, gluing, and finishing (Fogg 1968; Tesoro 1973; Hansmann et al. 2002; Zimmer et al. 2014). Generally, less permeable wood types are more difficult to treat and dry (Milota et al. 1994). Wood permeability can also influence its durability (Nicholas et al. 2005; Sandberg and Salin 2012).
Fluid migration in wood uses the vascular system developed by trees for physiological requirements (Redman 2017). Therefore, wood has a number of specific features concerning permeability (Redman et al. 2012). There are two features that are most important. The first is that wood has dramatic anisotropy ratios: permeability can be 1000 times greater in the longitudinal direction than in the transverse direction for softwoods, and this factor can be more than 106 for hardwoods. The second is that heartwood is usually much less permeable than sapwood; in softwoods pit aspiration (full or partial closure); extractives (including resins) accumulations and tylosoids are mainly responsible for this difference. Heartwood formation also entails a shift of the mean pore size towards smaller radii (Schneider and Wagner 1974).
Even though wood is much more permeable longitudinally compared with the radial and tangential directions, in wood preservation there is usually a much greater reliance on radial and tangential liquid flow through wood rather than longitudinal movement. This is because most of the surface area presented to the penetrating fluid is lateral rather than transverse, and therefore it is lateral (radial and tangential) rather than longitudinal permeability which in most cases limits the response of a species to wood treatment (Banks 1972). Many studies have shown that radial permeability of wood is usually greater than tangential permeability (Yokota 1967; Choong and Fogg 1968; Comstock 1970; Isaacs et al. 1971; Petty 1975; Palin and Petty 1983; Milota et al. 1994; Cai et al. 1997), and for some southern pines the difference has been reported to be a factor of 150 for dry wood (Erickson 1938).
Wood permeability has been shown to vary with many factors, such as species, genetics, position in the tree, grain direction, drying conditions, moisture content, and method of storage before seasoning (Banks 1972). The range of this permeability variation is very large (Banks 1972). Permeability is closely linked to treatability, which in turn can be influenced by species, geographic origin, and growing conditions (Larnøy et al. 2008; Lande et al. 2010; Zimmer et al. 2014).
Anatomical features of wood directly influence the permeability of wood, and therefore any factors linked to wood anatomy can affect the wood permeability. Key anatomical features in softwood that influence permeability include annual ring width, latewood/earlywood proportions, the shape, size, and frequency of pits in tracheids; and whether the pits are unaspirated (open) or aspirated (closed). Other anatomical features that influence permeability include the size and frequency of radial and axial resin canals, parenchyma rays, and the amount and constitution of heartwood (Zimmer et al. 2014). Interstitial spaces created by certain drying conditions can also increase wood permeability (Bamber 1972; Booker 1990; Ahmed and Moren 2012).
Wood chemistry can also have a very important impact on wood permeability through the effect of wood extractives (including resins) that can block liquid flow (Ellwood and Ecklund 1961; Fogg 1968; Olsson et al. 2001; Baraúna et al. 2014).
The formation of resin canals and resinous blemishes in pine stems and wood has been shown to be influenced by genetic constitution and by the growing conditions, including silvicultural procedures such as stocking rate and thinning (Cown et al. 2011). This suggests that the resin content may vary in the PEE × PCH hybrid pine due to genotype and stocking intensity.
With the exception of a previous study undertaken on tangential permeability (Leggate et al. 2017), the wood permeability of the PEE × PCH hybrid pine in Australia has not been studied. Given the importance of this resource to the Australian forest industry and the significant role that permeability plays in wood processing and product performance, a study was undertaken to investigate the variation in radial wood permeability of the PEE × PCH hybrid pine resource in Australia in relation to genotype, tree stocking rate (stems per hectare), position in the tree on a horizontal axis (pith to bark), and resin content.
EXPERIMENTAL
Materials and Methods
Tree sampling
A set of 30 trees was selected from Experiment 622NC, an F1 (first filial hybrid of PEE × PCH) taxon (clones and F1 family) spacing trial planted during March 1997 within compartment 202 Donnybrook LA (26o 58’ 30” S; 152o 59’ 00” E), SF 611 Beerburrum, north of Brisbane, Queensland, Australia. The trees were 19 years old at the time of harvest in October 2016 and represented a broad range of stocking rates, from low stocking rates (i.e. wide spacing) at 200 stems per hectare (spha) and 333 spha, to medium stocking rates of 500 spha and 666 spha, to higher stocking rates of 1000 spha. The trees selected also represented three different F1 genotypes: F1 seedling (a routine plantation seedlot (a single cross family)), Clone 887 (selected for good growth and flat fine branches), and Clone 625 (selected for heavy branches as a contrast to clone 887). As per Table 1, two trees from each stocking rate and genotype combination were harvested in order to provide samples for wood property analysis.
Table 1. Tree Sampling Criteria (number of trees sampled for each genotype and stocking rate combination)
The diameter at breast height over bark (DBHOB) was recorded for each selected tree. One transverse disc (35 cm thickness) was taken from each tree at 2.34 m height.
Laboratory assessments
Samples for permeability and resin content determination were cut from three radial locations on each disc (close to the pith, mid-radius (between pith and bark), and close to the bark (Fig. 1)). The target dimensions of the permeability samples were 22 mm in diameter and 8 mm in length (flow direction). The target dimensions of the samples for resin content determination were 28 mm x 15 mm x 9 mm. The permeability samples were cut from pith-to-bark diametral strips with growth rings aligned perpendicular to the transverse surface, so that gas and liquid permeability could be measured only in the radial grain direction (Fig. 1).
Fig. 1. Permeability and resin content samples cut from disc at three radial locations from pith to bark
The cambial age (ring number from the pith), chronological age (number of years since planting), and distance from the pith were recorded for each sample. Each permeability sample was coated with an epoxy resin on its lateral surface (periphery of the disc). This was done in order to direct gas and liquid movement only in the radial direction in order to measure radial permeability.
Prior to analysis, the permeability and resin content samples were conditioned to 8% moisture content (MC) in a constant environment chamber controlled at 23 °C and 44% relative humidity. Gas (atmospheric air) and liquid (non-distilled water) radial permeability measurements were undertaken using a Porolux 1000 Porometer (IB-FT GmbH, Berlin, Germany). This is a laboratory device that can measure the pore size and permeability of materials. Gas permeability was performed on samples prior to liquid permeability. For gas permeability, samples were subjected to pressurized, atmospheric air until pressure reached the target pressure of 4200 millibars. For liquid permeability, samples were subjected to pressurized water (non-distilled) with a constant pressure of 4200 millibars for 5 min. Permeability calculations take into account the viscosity values for water (non-distilled) and for atmospheric air as influenced by temperature. The permeability test undertaken using the Porolux 1000 equipment is shown in Fig. 2. Permeability for each sample was recorded in millidarcy units (mD).