Abstract
Microfibril angle (MFA) is a property of wood cell walls that has a strong influence on end-product quality, particularly for solid timber. Forest managers, tree breeders and wood processors require more quantitative information on the inter- and intra-stem variation in MFA in order to understand the impacts of their decisions on wood quality. The aim of this study was to develop parametric models that can be used to predict the intra- and inter-stem variation in MFA in radiata pine (Pinus radiata D. Don) trees growing in New Zealand Empirical models were developed using a dataset that contained records from 347 trees in which radial profiles of MFA have been measured at different heights up the stem. Radial variation in MFA was modelled as a function of cambial age using both a modified logistic function and a modified Michaelis-Menten equation. Additional terms were added to these models to account for differences in MFA with relative height up the stem. Values of MFA ranged from more than 40° near the pith to approximately 10-15° in the outerwood. Values greater than 30° were largely confined to the inner rings of the butt logs. A variance components analysis showed that most of the variation in MFA occurred within stems, with less than 15% of the variation due to differences between sites. The final models were able to account for 57-63% of the variation in MFA and inclusion of a relative height term significantly improved the model fit. Radiata pine has a region of high microfibril angle in the first 10-15 growth rings from the pith, particularly at the base of the tree. Growth rate had a small positive influence on average MFA (wider rings resulting in higher MFA values). Site differences were small, indicating that regional variation in wood stiffness is due more to the known trends in wood density. The models developed here can be coupled to growth models to examine how the combination of site productivity and silvicultural regime affect the size of the central zone containing high MFA wood.
Highlights
Microfibril angle (MFA) is a property of wood cell walls that has a strong influence on end-product quality, for solid timber
The variance components analysis performed on the logarithm of MFA, showed that 68% of the total variation in MFA was due to radial variation within trees, 15% was due to vertical variation within stems, 3% was due to differences among trees within a stand and 14% was due to differences between sites
The modified logistic function (Eq 1) was able to explain approximately 46% of the variation in MFA based on the fixed effects alone (66% when including the random effects of site, tree and disc) (Table 2)
Summary
Microfibril angle (MFA) is a property of wood cell walls that has a strong influence on end-product quality, for solid timber. If any, empirical models exist that are able to quantify the patterns of variation that are observed in radiata pine MFA This is partly due to difficulties in collecting large datasets on MFA variation using many of the early measurement techniques, which were based on microscopy and were relatively slow (Batchelor et al 1997; Donaldson 2008; Verbelen and Stickens 1995). Other methods such as near-infrared spectroscopy (Schimleck and Evans 2002) and X-ray diffraction (Cave 1966, 1997; Evans 1999) enable data on MFA to be more readily collected on large numbers of samples. The latter method is widely used due to automation and the possibility of increased resolution of measurements of withingrowth ring variation (Evans et al 2001)
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