Tree mortality after prescribed burning in an old-growth Scots pine forest in northern Sweden

Tree mortality and input of dead trees were studied after a prescribed burning in a forest reserve in northern Sweden. The stand was a multi-layered old-growth forest. The overstorey was dominated by Scots pine (Pinus sylvestris L.) and the understorey consisted of mixed Scots pine and Norway spruce (Picea abies L. Karst.). Ground vegetation was dominated by ericaceous dwarf-shrubs and feathermosses. The stand has been affected by six forest fires during the last 500 years. The prescribed burning was a low intensity surface fire that scorched almost 90 % of the ground. Tree mortality for smaller pines and spruces (DBH < 10 cm) was over 80 % in the burned parts of the reserve. For larger pines, 10‐50 cm DBH, mortality showed a decreasing trend with increasing diameter, from 14 % in class 10‐20 cm DBH to 1.4 % in class 40‐50 cm DBH. However, pines with DBH ≥ 50 cm had a significantly higher mortality, 20 %, since a high proportion of them had open fire scars containing cavities, caused by fungi and insects, which enabled the fire to burn inside the trunks and hollow them out. The fire-induced mortality resulted in a 21 m 3 ha‐1 input of dead trees, of which 12 m 3 ha‐1 consisted of trees with DBH ≥ 30 cm. An increased mortality among larger trees after low-intensity fires has not previously been described in Fennoscandian boreal forests, probably owing to a lack of recent fires in old-growth stands. However, since large pines with open fire scars were once a common feature in the natural boreal forest, we suggest that this type of tree mortality should be mimicked in forestry practices aiming to maintain and restore natural forest biodiversity.