Using birch as shelter with Sitka spruce – description and outcome of the Kronoberg approach

Abstract

The feasibility of Sitka spruce and birch mixtures on nutritionally impoverished sites was investigated based on experimental data and single tree growth model simulations. The Kronoberg silvicultural approach was considered as a management option, which is traditionally applied to naturally regenerating birch and Norway spruce sites with two major economic objectives: bioenergy production from the initial thinning of birch, and production of good quality sprucesawlog at clearfell. The silvicultural system was tested using field data from planted mixed species trials to initialise model simulations under three proposed scenarios a) a no thin pure Sitka spruce stand (SS-NT), b) a 50:50 birch/Sitka spruce mixture under a no thin silvicultural scenario (M-NT) and c) the Kronoberg management approach (KM) adapted for planted birch/spruce mixtures. Analysis of simulated harvest and growth data suggest that gradual removal ofbirch from the mixed stands is required to maximise timber volume production of Sitka spruce. Economic analysis shows that the SS-NT and M-NT scenarios, where stands are clearfelled at 40 years or earlier, for bioenergy or timber sale, may not be commercially viable, based on net present value (NPV) using discount rates above 2%. The KM approach yielded positive NPV returns at discount rates of 3 to 6%. The potential use of this alternative silvicultural system fornutritionally poor sites in Ireland requires further field testing to validate model simulations and economic assumptions used in the study.