Title: Juvenile birch in Sweden – selected stem characteristics for interior and furniture applications
Subject: Forestry industry production systems
Faculty: Faculty of technology
Date: Thursday 26 September 2019 at 3.00 pm
Place: Södrasalen (M1083), building M, Växjö
External reviewer: Professor emeritus Mats Nylinder, SLU
Chairperson: Dr Harald Säll, department of forestry and wood technology, Linnaeus University
Supervisor: Associate professor Åsa Blom, department of forestry and wood technology, Linnaeus University
Examiner: Professor Johan Bergh, department of forestry and wood technology, Linnaeus University
The demand for raw material for the furniture industry is growing. Large volumes of juvenile silver birch or downy birch stems are available in Sweden from pre-commercial thinning operations in forests. These stems are not currently utilised at a large industrial scale. The majority of research conducted which focusses on birch stems have not been performed in young trees and research often exclude the stem central part around the pith, resulting in inadequate knowledge about the juvenile part of the wood. Better knowledge about birch material properties could encourage the use of the wood for various purposes.
The initial literature review in this thesis highlighted some property requirements for the use of juvenile birch as a furniture material, and subsequently identified areas where knowledge about the physical characteristics of juvenile birch is limited. Consequently, the objectives of this thesis are to explore characteristics such as bark thickness, wood to bark bonding ratio after drying, density and growth ring-width variations, and the anatomical growth response to fertilisation.
The material studied came from mixed birch and Norway spruce stands at two sites in Southern Sweden, Asa and Toftaholm. The birch stems were naturally regenerated silver birch and downy birch, with breast height diameters between 30-83 mm. Fertilised and unfertilised silver birch trees were sampled at Toftaholm, and unfertilised stems of silver birch and downy birch were sampled at Asa. The stem characteristics from pith to bark (radial direction) and along the stem (longitudinal direction) were measured. The wood to bark bonding ratio after drying was calculated as the percent of the stem circumferences which full contact between the wood and the bark. Oven-dry density and basic wood density for silver birch was determined through the water displacement method. The impact of ring width on wood density was statistically analysed and image analysis of the wood anatomy was conducted to further understand this relationship.
Bark thickness along the stem had the highest deviation in the section closest to the stump. The wood to bark bonding ratio after drying was measured for juvenile downy birch. The ratio seems to depend more on the stem diameter than the position in the stem. These results are relevant for processors wanting to estimate wood volume under bark. The wood to bark bonding ratio was highest for diameters between 30-39 mm, and this relationship was not correlated with the sampling height along the stem.
The density variation in radial and longitudinal direction were studied for juvenile silver birch. Density seemed to be negatively correlated with growth rate, i.e. ring width. This relationship was true for stems at the same site and between sites, irrespective of management or growing conditions. As expected, the mean wood density was lower in the fertilised trees than in the unfertilised trees. Towards the bark, radial density increase was higher in slow grown trees. The longitudinal density variation in young silver birch trees was low. Quantitative wood anatomy studies confirmed that the fertilised juvenile birch had a younger cambium, thinner cell wall thickness, and fewer vessels per mm2, when compared to unfertilised trees of the same diameter class.
Overall, the knowledge acquired in this study may facilitate the industrial use of juvenile birch stems and wood in interior and furniture applications. The role of wood anatomy in determining the mechanical performance of juvenile birch stems should be further explored, as this could reveal new opportunities for the utilisation of juvenile birch.