Optimizing the management of Norway spruce and Scots pine mixtures on a site infected by Heterobasidion coll.

Management of a Norway spruce [Picea abies (L.) Karst.] stand established on a Heterobasidion coll.-infected site is problematic because the fungus spreads vegetatively from one generation to the next. This study determined the optimal management schedules for Norway spruce and Scots pine (Pinus sylvestris L.) mixtures growing in southern Finland with Heterobasidion coll.-colonized stumps of Norway spruce from the previous generation. The thinnings were always conducted in winter. In healthy stands, 25% pine mixture with two thinnings produced the highest soil expectation value (SEV) at 2% discounting rate, whereas in stands with logging injuries or old stumps colonized mainly by H. parviporum, 25-50% pine mixture with one delayed thinning was optimal. The proportion of decayed Norway spruce trees at the end of rotation decreased when there was a pine mixture. Increasing the number of old colonized stumps tended to increase the proportion of decayed Norway spruce trees, increase the optimal share of Scots pine and decrease SEV. With increasing discounting rate the optimal thinning time was earlier, the proportion of Scots pine at the end of rotation was higher and rotations were shorter. When the relative proportion of H. parviporum in the colonized old stumps was decreased from 90% to 75% or 50%, Scots pine mixture no longer decreased the butt rot of Norway spruce in the final felling.