Tree stem assessments


Since stem size and shape are key determinants of tree stability, timber quality and timber volume recovery, refining and developing techniques for various tree stem assessments is a key aspect of our research.

Research objectives

The shape of trees is very important in determining timber volume recovery and hence value. The shape of trees is also an important component of their stability in wind. Branch architecture is indicative of the knot content of that timber. Knots are generally detrimental to sawn wood forest products and affect usability and value. Therefore, measuring tree shape and branch architecture has application in stand evaluation, risk assessment and timber value. Many of the current techniques are subjective (i.e. the results depend on the individual assessor) and new digital methods offer the potential to improve on this and increase measurement efficiency. We aim to investigate how these new technologies can help in tree stem assessments.

Results so far

Stem straightness

Stem straightness is the major determinant of sawlog value. Straighter stems provide more processing options and higher sawn wood recovery. Effective assessment techniques enable better quality stands or trees to be identified during timber cruises and marketed to specific processor requirements, optimising the use of forests and adding value for all. Assessing straightness during inventory measurements would enable more detailed production forecasting. For tree breeding and inventory purposes we use an established straightness scoring system based on visual assessment of straight log lengths in the lower 6 m of standing trees. This can be used to determine a stand straightness grade that projects a likely log assortment. While this system works in principle, it is strongly dependant on the person who does the assessments.

Tree stem assessments 

We are continually developing photographic and terrestrial laser scanning techniques to measure straightness.

Stem Taper

Put simply stem taper describes the relationship of tree height to diameter. However trees are not conical and the relationship between height and diameter is not constant along the tree stem. We build and develop mathematical functions that describe the shape of tree stems for a range of species. Models are used to predict stem diameter at different heights and are an integral part of modern forest inventories and harvesting technologies. These taper functions are central to our ForestGALES wind-risk and Timber Quality decision support systems for forest managers. Traditionally we measure taper using callipers along the length of felled trees. However we are investigating the application of photogrammetry, terrestrial laser scanning and harvesting data to generate these functions on a regional level.

Tree stem form


Taper measurement









Branching habits are directly related to knot volume and geometry. We make detailed studies on the size, frequency and shape of branches on our most important tree species. Our most detailed studies typically involve the painstaking physical measurements of individual branches, including aspects such as size, age, insertion angle and living or dead. We then make models of branches for different species and under different forest management scenarios. We are also interested in how the depth of the living crown affects timber production. We are pursuing terrestrial laser scanning (LiDAR) techniques as a way to non-destructively assess branches and we are interested in how we can link remote sensing, e.g. airborne LiDAR to branching habits and timber properties.

Branch measurements







Current, ongoing

Related Resources

Information Note:
Protocol for Stem Straightness Assessment in Sitka spruce

Information Note:
Timber Quality, a Pilot Study for Assessing Stem Straightness

Research Note:
Developing Methods for Assessing Scots Pine Timber Quality

Taper functions for Scots pine and Sitka spruce in Northern Britain


Paul McLean

Funders and partners

The project is co-funded by FC Scotland, FC England and Natural Resources Wales, via the Forest Products Research Programme (Wood and Timber Properties).

Forestry Commission policy






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