Effect of repeated harvesting on biomass production and sprouting of Betula pubescens

The effect of repeated coppicing and rotation length on the biomass production and sprouting of downy birch (Betula pubescens Ehrh.) was studied in two-field experiments established in central and northern Finland. The harvesting cycles studied were 1,2,4 and 8 years in central Finland and 1,2,4, 8, 12 and 16 years in north Finland. The randomized block design with four replications was used. Biomass was harvested, weighed and subsampled for determining the moisture content. Consecutive 1- and 2-year rotations led to a decrease in the dominant height of birches. Only 13% of the original birches were able to sprout after four 1-year rotations and 22% after two 2-year rotations. Some birches were, however, able to sprout for even more than 10 years. Annual harvests led to a rapid decrease in biomass production. Already the second 1-year-rotation yielded three times less biomass than the first and subsequent production decreased further. 4-year rotations, even though yielding much less biomass than longer rotations, did not lead to a decrease of the coppicing capacity or biomass production during consecutive rotations. The mean annual dry-mass production was very low when 1- or 2-year rotations were applied. 8-year rotations in central Finland yielded 7-8 times more and in northern Finland 9-13 times more than 1- and 2-year rotations. The longer the rotation, the higher the mean annual biomass production. The highest mean annual increment (MAI) was achieved with the longest (i.e. 16-year) rotation. Compared with the 1-year rotation, the MAI for the 16-year rotation was 20 times higher. The results show that downy birch is not suitable for biomass production when using short rotations. Three annual or biannual cuttings reduce the sprouting capacity and growth of birch considerably and help to control coppicing in softwood plantations. (C) 2001 Elsevier Science Ltd. All rights reserved.