AIR1-CT92-0608 Production of Quality Wood from Broadleaves

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AIR Cluster IX - Forestry : Plant Genetics : Wood (Lignocellulose)

	Proposal No:	AIR1-CT92-0608
	Date Prepared:	November 1999
	Source:	Final Report 1996

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Final Report 1996

Introduction

The objective of this project was to take an integrated approach towards production in the forest, and subsequent industrial transformation of quality wood from broad-leaved trees on a European scale. The purpose was to contribute to knowledge and hence assist foresters in decision making. This involved producing guidelines which might help the integration of ecological, silvicultural genetic and technological knowledge. Most, though not all the investigations, have been on the two northern European oaks Quercus petraea and Q. robur.

Objectives

The aim was to provide a means of characterising woodlands, especially for identifying features which will allow the correct matching of species to sites. Young stands represent a phase in the growth of a forest when competition is fierce: stem numbers drop from several thousands to a few hundreds within a short time compared to the whole life of the stand. It is a crucial phase in stand management. One of the objectives of young stand management is to recruit desirable stems, in terms of size, quality, species, etc into the dominant layer. Hence, knowledge of processes that determine social hierarchy is a cornerstone of young stand management. A further objective was to determine the conditions that have most influence on growth, and the sites where the trees are likely to do best.

Activities and Results

Development of young stands The work was carried out in Belgium, France, Germany and the United Kingdom and covered the following main areas. In the studies, the natural development of stands was investigated as well as several aspects of competition, in order to relate site conditions to some measurable characteristics of trees and stands. They included the management of competition between trees, and between trees and ground vegetation. Among the results were:

• Species: of the species investigated, cherry grows faster in diameter and height on the sandy loam soils where the trials were carried out. Ash grows best on lower slopes, while cherry grows badly on them.
• Weeding: elimination of competing vegetation results in a considerable increase in diameter growth of all young trees.
• Spacing: trees grow larger in diameter at wider spacing, though height growth is relatively unaffected. There is a significant interaction between species and spacing for diameter growth.

An additional finding of environmental interest was the strong correlation between the age of the trees and the amount of nitrogen in water draining from the site during the period January to May 1995. The younger the plantation, the higher nitrogen content. The maximum value was 10 mg l^-1 in 5 year old stands, and the minimum 0.3 mg l^-1 in 24 year old stands.

Growth Site index curves (i.e. the height the trees reach at a predetermined age, normally 50 years) were developed for various broad-leaved trees (ash, maple, cherry and oak) in relation to site and climatic factors. For ash, topography, soil depth, water availability and underlying geology were identified as being important for predicting site index. Using these variables, a predictive model for ash growth was developed. For sycamore, site index ranges from 15.8 to 30.6 m at age 50. The average is 22.7 m and the coefficient of variation is 12 %. The variables that most influence site index are mean annual temperature, water , topography, and soil aeration. Sycamore grows best on deep moist soils, often on shaded lower slopes where most of the ground vegetation indicates reasonable fertility. For cherry site indices ranged from 15.8 to 28.8 m at age 50. The mean is 22.0 m and coefficient of variation 11.6 %. The most important variables influencing growth are total annual rainfall and the contrast between the cold season (September-March) and the warm season (April-August).

Tree and stand growth models A number of permanent sample plots were established that will provide a basis for much valuable future work for predicting rates of growth for different species. For basal area growth, a general set of equations designed for beech in a past study were used, the results of this showed:

• the development of a relationship between basal area growth and height growth
• the existence of periodic effects, probably caused by climatic fluctuations, on basal area growth
• an effect of silviculture in crown radius development through distance dependent tree models.

A general set of equations was developed for beech, based on height and basal area growth, volume and site index. Attempts to fit the site index growth models to the permanent plot data showed that the trend of annual production is a decrease from 0.7 to 0.3 m³ ha^-1 year^-1 from year 40 to year 200, with about 40% fluctuation around mean values. Most of fluctuations are periodic.

The type of wood laid down in an oak tree can have major influences on its subsequent value for various purposes. For example colour and the width of annual growth rings can influence its value for veneer and furniture making, and the amounts of tannins in the wood are important in the maturation and taste imparted by oak barrels to alcoholic drinks such as whisky and wine. It is valuable to wood users and tree breeders to know the extent to which variability between trees is caused by the environment in which they are grown, or whether the differences are heritable. The extent of variability between the characteristics of trees growing within a stand can also be of significance.

Several physical (e.g. colour, density, annual ring width) and chemical (e.g. tannins, oak lactones) properties of the wood were examined, mainly using clonal and family material of oaks from various genetic trials in Germany. The results indicated that of the chemical characteristics studied, there was a high level of heritability and, unexpectedly, major differences between the two oak species. The density of the annual rings was also highly heritable, and different between the two species. Ring width, reflecting the vigour of growth, is highly heritable, while colour characteristics had only medium to low heritabilities. There is always considerable variability between individual trees within a stand. Most wood properties influencing quality are therefore under strong genetic control, indicating that quality could be incorporated into any oak breeding or selection programme. Furthermore, present users of oak may be able to select more effectively from present sources of wood to meet specific requirements in wood quality,

The high between-tree variability was re-emphasised in further studies into the correlation between basic properties of oak veneer and the wood of the trees from which it was made. These showed a strong correlation between the tendency of veneer to vary in thickness and the roughness of the wood (which itself is negatively correlated with the density of early wood). The frequency of checks in veneer is correlated with the density of the late wood.

Transforming low-grade broad-leaved timber An important aim of this part of the project was to develop the technologies for optimising cutting and temperature criteria for converting rather low quality broad-leaved trees, especially oaks, of which there are plenty in Europe, into usable laminated veneer lumber (LVL). This is a timber composite made by gluing together thick peeled veneers for the manufacture of furniture and many other purposes. In common with other similar work, wood heterogeneity was found to reduce veneer quality. Glues tend to reduce the mass diffuser of LVL compared to solid wood. This may cause increased dimensional stability in the inner part of the material, but high moisture gradients near the surface which could lead to mechanical degradation. A small industrial trial, including the processes from harvesting to the production of glued boards, resulted in the production of 10 m³ of LVL. It was found that coppice was much more suitable for peeling than standard trees. In processing, 2-3 mm thick oak veneers are very brittle; scarifying is not recommended, and joinery proved problematic.

The trial demonstrated that:

• mechanical properties of LVL such as Young's modulus compare very favourably with those of high quality oak timber;
• variability within boards, beams and panels is far lower than in solid wood.

A practical consequence of these findings is that LVL could possibly replace imported tropical hardwoods for some purposes, such as the construction of window frames.