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New tree volume models to reflect the more slender than before stem forms

To operate effectively, forestry needs information on how much wood is available for different purposes, including forest conservation and carbon sink maintenance, how the tree stock is developing and what timber species are available. All calculations related to the volume and development of the tree stock require tree volume models. Examples of such calculations include forest plans drawn up by various actors, the national forest inventory and all calculations relating to logging potential and carbon sequestration. Most commonly, a model is needed to calculate the volume of a tree trunk using its diameter and length.

New models for calculating the volume of stems based on laser scanning

Since 1982, the models published by Jouko Laasanenaho in his doctoral thesis have been used in Finland. Over the years, forest management practices and tree breeding, among other things, have influenced the shape of tree stems. Therefore, the need has arisen to update the models with new data. The old models are based on stem measurements taken by climbing at the turn of the 1960s and 1970s. In the 1980s and 1990s, data was also collected from felled trees to update the models. Measuring using climbing or fallen stems is too slow and expensive, and a new method was needed. 

In addition to the old data, the new models are based on measurement data collected by ground scanning from National Forest Inventory plots. The ground scanning provides a point cloud from which the tree taper curve of vertical trees can be reconstructed without climbing. Even this method is not without problems, as obstructions to visibility caused by other trees and branches, and for example strong winds, can cause inaccuracies in the measurement result.

Mature pine forest. Photo: Erkki Oksanen

Regional and soil differences are reflected in stem form

In the new models, the volume of trees is modelled relative to a cylinder with a diameter equal to the diameter of the stump and a height equal to the length of the tree. This ratio (i.e. the tree's form factor) in turn varies with trunk size, soil (swamp or fabric) and the amount of heat in the area, as well as between the datasets used. With the new volume models, the form factor has been constrained to give a logical volume estimate for even small trees, whereas with the old models, very short trees gave much too large volume estimates. The effect of the thermal sum on the stem form is greater the further north you go.

More slender stems than before

The stem form of trees had changed over time in all tree species, becoming more slender. This change is particularly noticeable in the base of the trees, which are now less conical than before. The reduction in volume is reflected in particular in a reduction in the proportion of logs, but from the point of view of sawing, the more nodular shape is a good thing. 

The relevance of the new models was assessed by calculating the volumes of the 11 national forest inventories using the models of all three data sets. The total volume calculated with the model based on ground scanning was 1.4% lower than with the models based on the Laasasenaho data. The clear difference in volumes indicates that new models should be introduced in all forestry calculation systems.