The biomass fractions of a tree are the stump (including roots), stem, branches and foliage (needles and leaves). Broadleaved trees and conifers have different fractions of these aboveground components (Johansson 1999a, b). For birches, the mean aboveground fractions are: stem, 75 %; branches, 18 %; and leaves, 7 %. For conifers, the mean values are 63 %, 23 % and 14 % respectively (Johansson, 1999b, c). The percentage represented by needles is higher in young than old conifers, Fig. 12.

The effect of repeated harvesting on biomass production and sprouting of downy birches growing in central and northern Finland has been studied by Hytonen and Issakainen

(2001) . Different harvesting cycles of 1, 2, 4, 8, 12 and 16 years were examined. The main results were that downy birch is not suitable for biomass production using short rotations. Most of the stumps, 87 %, did not sprout in the one year rotations, but 8-year rotations produced the same number of sprouting stumps as the longer rotations.

Reim (1929) reported that European aspen growing along the borders of farmland may produce large numbers of suckers when cultivation ceases. In a study of repeated short rotations of aspen, the number of suckers per hectare decreased with every additional rotation (Perala, 1979). The study included rotations of four or eight years and, in both cases, the number of suckers decreased over the three rotations studied.

3. Conclusions

The management methods presented here rely on the land owner having extensive and detailed knowledge of biological processes. The changes in growth of individual species and mixed stands must be known. Some of the methods are based on optimal rotation periods and adequate management of the stand, including cleaning and thinning at the correct time. Severe competition could drastically decrease tree growth. Besides the need for the site to be suitable for tree cultivation, the skill of the owners is important. The most important factor, however, is the enthusiasm and curiosity of the owner; without this, most of the methods will not produce the yields suggested in the present study.

Table 3 lists possible future management models for trees established on farmland and forest land. When operating on a small-scale, there are many alternatives and the owner can be more flexible than is possible in large-scale operations. As the possible rotation periods range from 5 to 40 years it is important to have stands of different ages to ensure a continuous supply. Efficient management of such small areas would make it possible to produce a certain amount of biomass for personal use or to sell to neighbors or local heating plants..

Figures for potential energy supply from different stand types and management options allow us to make comparisons and select appropriate ways to use available land.

Most of the methods are cheap, need a short time to establish and involve relatively straightforward management. The raw materials produced can be used to generate energy for the landowner or can be sold.