Crop plans and yield predictions for Norway spruce (Picea abies (L.) Karst.) and birch (Betula pendula Roth Betula pubescens Ehrh.) mixtures

摘要

In young Norway spruce (Picea abies (L.) Karst.) stands containing an even-aged component of self-propagated birches (Betula pendula Roth and B. pubescens Ehrh.) the forest manager has the choice of removing all broad-leaved trees or of accepting a mixed stand of Norway spruce under a birch shelter. Results of stand growth simulations show that the total stand yield will be higher if the broad-leaved trees are retained. The spatial distribution pattern of trees had little influence on the outcome of the growth simulation. The simulations also showed that spruce yields will be similar, regardless of whether the spruce is grown in pure stands or is grown under a birch shelter (either 600 birches ha-', all removed at age 25, or 1 200 birches ha-', removed in two steps at age 20 and 30). The simulations showed that the highest total yield was obtained when a shelter of 2 000 birches ha-' was left after the pre-commercial thinning, then thinned at a e 20 and finally all birch was removed at age 30. The birch shelter can produce ca. 100 mf ha-' in middle Sweden if it is composed of B. pendula. The corresponding yield for a shelter of B. pubescens will be 80 m3 ha-' or less. Although height growth of the spruce will be suppressed by the birch shelter, the growth rate will increase once the shelter has been removed. Estimates of biomass production were made for both Norway spruce and birch. Mean annual dry matter production increases from 1.4 tonne ha-' yr-P with a shelter of 600 birches, to 2.4 tonne ha-' yr-' for 3 000 birches. All yield estimates refer to age 20. The harvesting cost, based on manual felling and tractor extraction, was estimated at ca. 40 SEK/MWh (200 SEK/ton dry matter). By comparison, a commercially harvested Salix plantation costs about 24 SEK/MWh. However, the cost of establishing and maintaining a Salix plantation is ca. 35 SEK/MWh, while no such costs are incurred in the production of fuelwood under the birch shelterwood system.