FAIR-CT98-5038 The influence of the site factors "wind exposure" and "slope" on wood quality

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Objectives:

A detailed experiment to investigate the effect of wind exposure on Sitka spruce wood quality was established. This is an important consideration for British timber production and is the first work on this subject in this country.

Activities and Results:

A major effort was made at the beginning of the project to identify suitable sites for the work. It was quickly realised that finding forest sites on which everything but the wind exposure remained the same would be extremely difficult, if not impossible. Therefore, it was decided that a single site would be used, which had a variation in wind exposure. The most suitable possibility was a stand with a clearly defined westerly edge in which the wind exposure would be a function of the distance of trees from the edge. This removes worries about variations in temperature, rainfall, soil nutrition etc. The requirement that the trees would be large enough to pass through a conventional sawmill and provide sawn pieces on the inside and outside of the trunk restricted the search to stands planted at least 40 years ago. A rigorous search for such a site was conducted throughout Britain by contacting Forestry Commission offices and Forest Research field stations. Potential sites were visited to determine their suitability and the final site chosen was near Lochgllphead, Argyll.

Prior to beginning the tree mechanical measurements the characteristics of the trees at different distances from the forest edge were obtained. Statistical analysis was carried out to determine the number of trees required to be measured to provide a statistically robust data set and to fit in with standard British mensurational protocol. From this a detailed experimental protocol was drawn up prior to the beginning of any formal work. The experimental work in the field was extremely thorough and established the basic physical characteristics of the 15 trees in each of four rows as well as the competition from neighbouring trees. The mechanical measurements consisted of attaching strain gauges at eight heights on each tree and pulling the tree with known loads to establish the Modulus of Elasticity (MOE) as a function of height. In addition sway tests were conducted with the strain gauges attached to the stem in order to determine the resonant frequency and damping of the trees. All data were logged to portable computer and the system was specifically designed for the project. The fieldwork was very intensive and conducted over many weeks.

Following the mechanical measurements the trees were felled and 4m logs cut from the base, centre and top of the trunk. Discs were cut from the ends of each section. The stein diameter was measured at Im intervals and the branches from each metre section weighed. The sections were transported to a sawmill and battens of approximately 100 x 50-mm cut from the base and top logs, The batten,; are now being measured at BRE to determine the green timber value of MOE and then they will be dried to determine the level of distortion.

Conclusions:

By following each piece of timber from the forest through the sawmill and to final analysis it will be possible to determine the effects of wind exposure on the properties of industrially prepared timber. This link between research and the industrial sector is a key part of the work. Furthermore, every effort has been made to integrate the work with other work on timber quality in the Forestry Commission in order to establish an integrated database on the timber properties of British softwoods. This has required taking some additional measurements not required for the actual project but which enhance the value of the data obtained.

Keywords: Sitka, spruce, wind, exposure, slope, growth, mechanical properties, wood quality

Contacts

Scientific Supervisor

• Barry GARDINER / Forestry Commission Research Agency Northern Research Station / UNITED KINGDOM