Dead wood and polypore diversity in natural post-fire succession forests and managed stands – Lessons for biodiversity management in boreal forests

Abstract

We tested the extent to which forestry mimics natural stand-replacing fire dynamics, and what factors best explain species richness and assemblages of polyporous fungi in natural northern boreal Picea abies-dominated forests and managed forests which were either clear-cut and planted with pine, or selectively logged. The amount of coarse woody debris (CWD) and diversity of polypores in old-growth forests and in forests at three natural post-fire successional stages (young, middle-aged, old) were compared with managed forests at comparable successional stages. Fire created a high amount of CWD, providing a supply of dead wood for saproxylic species for at least 50–70y. Dead wood volumes remained high throughout the natural forest succession, exceeding at all stages 18m3ha−1, the threshold value established in the study for the occurrence of red-listed species. The richness of polypore species was highest on young post-fire sites, and their species composition, including red-listed species, was comparable to that of old-growth forests (>300y) used as references. The amount of CWD in natural post-fire forests was over two to twenty times higher than that on clear-cut sites, and species diversity on clear-cut sites was considerably lower than on sites undergoing natural succession, with no occurrences of red-listed species. A relatively diverse polypore flora was found in selectively logged forests, likely due to rather a high volume and continuity of spruce CWD. The results indicate a strong but non-linear relationship between total volume of CWD and species richness of polypores, but successional stage and naturalness also explained part of the variation. The species assemblages were influenced most by the decay stage of CWD, tree species composition and naturalness of the site. The results demonstrate the importance of old-growth and young post-fire forests as landscape-level habitats and species pools, especially for red-listed species. We recommend that old-growth spruce forests be preserved whenever possible and that high CWD volumes be generated locally for red-listed species, for example, by prescribed burning. Regeneration forests for spruce, extended rotation periods and the use of logging techniques other than clear-cutting would help in maintaining species diversity in spruce-associated species in boreal forests.