Fallen retention aspen trees on clear-cuts can be important habitats for red-listed polypores: a case study in Finland

Published source details Junninen K., Penttilä R. & Martikainen P. (2007) Fallen retention aspen trees on clear-cuts can be important habitats for red-listed polypores: a case study in Finland. Biodiversity and Conservation, 16, 475-490.
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Summary

Green-tree retention is a forestry management practice which aims to reduce negative effects of clear-cut logging on forest biodiversity. Retention trees, which are left in otherwise clear-cut areas, may act as refugia for forest species e.g. invertebrates and fungi, and enhance structural heterogeneity. In Fennoscandian boreal forests, these often old and decaying retention trees, may provide important habitat for saproxylic species. In this study, the value of retained aspen Populous tremula trees in maintaining assemblages of wood-decaying polypore (bracket) fungi (Basidiomycota) in clear-cuts was investigated, after the retention trees had died, fallen and started to decay.

Study area: The study was undertaken in Savonranta, eastern Finland, in clear-cut and old growth forest within the southern boreal zone. The area is rich in aspen and provides an example of green-tree retention in 1980s, when such retention was not yet a common practice; since this time the trees have aged and become suitable habitats for polypore fungi.

Data collection: Data were collected at two old-growth forests (Muhamäki and Raatelamminsalo) dominated by Norway spruce Picea abies with abundant aspen, silver birch Betula pendula and Scots pine Pinus sylvestris) about 5 km apart, and adjacent clear-cut areas. The clear-cuts were of similar forest before logging; the only trees retained were aspens. The aspen trunks studied fall into five categories:

1). OG-n2: old-growth forest, naturally died trees at decay stage 2 (wood quite
hard, knife penetrating to 1–2 cm into the wood);

2). CC-n2: clear-cut, naturally died retention trees at decay stage 2;

3). OG-n3: old-growth forest, naturally died trees at decay stage 3 (wood rather
soft, knife penetrating several centimetres into the wood);

4). OG-k3: old-growth forest, trees killed by notching (with herbicides), at
decay stage 3;

5). CC-k3: clear-cut, retention trees killed by notching (with herbicides), at
decay stage 3.

Only dead fallen aspens with a diameter of at least 30 cm at breast height (1.3 m) were included, and almost all fulfilling the criteria were investigated.

Trees at decay stage 2 were investigated at Raatelamminsalo and adjacent clear-cut on 30 September and 1 October 1996. The retained aspens were living trees at the time of logging (1991), but by 1996 a large proportion had fallen and were at decay stage 2; 25 trees were investigated in the forest and on the clear-cut.

All retained aspens at Muhamäki had been killed by notching (with herbicides) 1 year before logging (1984) and the majority had reached decay stage 3 by the time of the study. These trees (OG-k3 and CC-k3, 20 of each) were inventoried during 11–13 October 1997. To compare the polypore assemblages on notched trees with trees that had died naturally, a corresponding OG-n3 inventory (20 trees) was carried out in Raatelamminsalo on 13–14 October 1997.

Altogether 110 trunks were investigated by carefully searching for polypore fruiting bodies on the trunk and branches. On an individual tree all fruiting bodies of the same species were considered to be one record. Dead fruiting bodies of perennial species were excluded.

A total of 499 polypores of 46 species were found on the fallen aspens. The average number of species per trunk was 4.5, all had at least one species and the highest number recorded on one trunk was 10. The most common species were Trametes ochracea, Rigidoporus corticola, Phellinus tremulae and Bjerkandera adusta, which together accounted for 42% of the total number recorded. Of the 46 species, 15 were found only once or twice.

The decayed trunks on the clear-cut areas hosted more species and more red-listed species than did trunks within the adjacent old growth forest. Most of the polypore species with more than two records were found in both habitats.

Conclusions: These results suggest that many aspen-associated polypores are able to live and reproduce in sun-exposed habitats, if the quality and quantity of dead wood is sufficient. This unexpected result, however, may be partly due to the exceptionally great abundance of aspen in the study area. The authors consider that in the long term, the local benefits of fallen retention aspens will be limited unless the local continuity of large aspens, both living and dead, is ensured.

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