Unlike forests managed under more natural conditions (e. g., natural forests and woodlands selective cutting and the retention of seed trees for regeneration), plantation forests are typically re-established through the planting of seedlings. The important connotations of this are that spacing is controlled and that trees are typically planted in a geometric pattern which promotes efficient harvesting.

Early thinning is a term often used in conjunction with pre-commercial or sub­economic thinning and this well justified term is continually verified in research. It refers to an operation in which re-spacing is required to be carried out for the benefit of stand development, but the economic results of doing so do not necessarily justify the operations in themselves. Ahtikoski et al. (2008) use a complex calculation showing that energy wood thinnings could be financially viable if the extracted volume at least 42 m3 ha_1 for an average stem volume larger than 0.015 m3 and the unit price delivered exceeded US$12.00 MW h_1. Plantation managers can partly avoid this cost by expanding initial planting espacement and accepting the consequences of later canopy closure, but the debilitating relationship between productivity and tree size cannot be totally avoided.

However, in cases where a market for smaller roundwood has fallen away (e. g., loss of contract or closure of plant) the demand for biomass could promote early thinnings. Also, the development of a bioenergy conversion facility should stimulate denser establishment (higher number of stems per unit area) on areas that have been managed more extensively (refer Chap. 5). To promote efficiency through mechanized operations, variable row spacing can be used (e. g., closer spaced double rows which will be removed in the first thinning and more widely spaced rows that will not be thinned).

Small trees can be felled in a number of ways, largely determined by the extraction system to be used. Motor-manual felling with a chainsaw is often the most cost effective way of felling trees when they do not need to be processed in whole-tree harvesting (i. e., tree parts and crown intact). This is especially true if the trees will be chipped as they lie in the stand or if they will be extracted with a cable system (e. g., high lead or monocable). If pre-bunching or processing is required, then mechanized felling is preferable. However, opening up of strip roads can be costly if the bunches have to be laid perpendicularly into the stand, as placing them in parallel requires wider striproads, and if driven on they can be contaminated with mineral soil and stones, resulting in increased ash content of the eventual product.

An agricultural tractor fitted with a boom and a multi-tree (or accumulating) felling head can provide a low investment alternative to a feller-buncher or harvester, but versatility is dependent on terrain conditions (Russell and Mortimer 2005). In a study in Scandinavian forests, productivity levels of 3-6 m3 h_1 were obtained for felling and loading small trees onto a trailer for extraction (Belbo 2010). Small tree size thinning harvesters fitted with accumulating felling or harvesting heads are popular because of their greater stability and terrain going capability. However, all harvesting systems are highly sensitive to tree volume (Fig. 6.3).

Volume per tree, m

Fig. 6.3 Influence of direct loading and the number of trees per crane cycle on the combined felling and loading time using a Nisula 280 felling head in small trees, by increasing tree size (Belbo 2010)

The extraction of small FT or energy roundwood to roadside can be done using forestry equipped agricultural tractor/trailer units, a grapple skidder or a forwarder (Fig. 6.4). FT have a low bulk density and the load can potentially be compacted with the crane and grab. If the trees are to be extracted individually (e. g., using a monocable or a chute) or if they are to be chipped with a terrain going chipper machine they can simply be left as they fall for transpiration drying and subsequent extraction.