Study

The use of natural and man-made birch Betula spp. stumps by saproxylic beetles, Bryngelsdalen (Värmlands) and Bräcke ängar (Dalsland), Sweden

  • Published source details Jonsell M., Nittérus K. & Stighäll K. (2004) Saproxylic beetles in natural and man-made deciduous high stumps retained for conservation. Biological Conservation, 118, 163-173

Summary

Intensive management of the Fennoscandian forests have resulted in a lack of dead wood and deciduous trees, contributing to biodiversity loss. In Sweden, of the 1,000 or so species of saproxylic beetle (i.e., beetles which depend on decaying wood in their life-cycle), more than 400 are considered threatened or are red-listed, and the majority of these are confined to deciduous forest. Consequently, conservation management techniques that increase the amount of dead deciduous wood have been pursued. In this study, the degree to which birch Betula spp. stump creation benefits saproxylic beetle biodiversity is investigated.

Study sites: Bryngelsdalen (Värmland) had a long history of continuous afforestation. However, most of the area has subsequently been intensively managed for timber production. The forest is mainly coniferous (dominated by Norway spruce Picea abies) but with some deciduous trees. The specific study site within this area was less intensively managed, with many old trees, a high proportion of deciduous trees, and a multi-layered stand structure. In winter 1994-1995 part of the stand was clear-felled, but a large number of deciduous trees were retained.

Bräcke ängar (Dalsland) has naturally regenerated, mostly with aspen Populus tremula and birch Betula spp. since agriculture began to diminish in the area over the last 50 years. Spruce is also abundant. In these recently regenerated stands, self-thinning has not created many dead trees. Man-made stumps were created in the dense deciduous stands, surrounded by either sparsely wooded grazed stands, or next to open ground (e.g., fields, clear-cut, lakes).

Stumps: Man-made stumps were created in March 1995 using explosives placed at about 5 m above the ground on birch trees. The explosives, when detonated, created a ragged edged stump resembling that of a natural one undergoing decay. The stumps were sampled between 22 and 24 September 1997 and again between 4 and 6 October 2000. At Bräcke ängar, several of the man-made stumps still had live cambium in 1997, which was still present in some stumps in 2000. However, only dead or partially dead stumps were sampled.

Sampling: Natural stumps were located in the same study sites. Each stump was measured (diameter at breast height) and put into a decay class:

(1) fresh wood, dead for one summer
(2) older wood, but still hard
(3) wood so decayed it can be torn apart with a knife or by hand

Due to a lack of natural stumps there were less natural (n = 39) than man-made (n = 42) stumps, and more natural than man-made stumps in advanced decay. Average diameter was 28 cm for all classes.

A 0.25 m² piece of bark, including any loose material from beneath the bark, was stripped from each stump at breast height and transported to the laboratory in textile bags. The samples were placed in Tullgren-funnels and kept for at least 24 hr under a 40 W light bulb. All adult saproxylic beetles and their larvae of two families, Elateridae and Pyrochroidae, were identified to species level.

Natural stumps had 68 species of beetle, of which 10 were red-listed. By comparison, man-made stumps had 58 species of beetle, of which five were red-listed. Decay class (combining natural and man-made stumps) had an affect on biodiversity:

Class (1) stumps (n = 7) - 21 species, of which two were red-listed;
Class (2) stumps (n = 40) - 62 species, of which six were red-listed;
Class (3) stumps (n = 32) - 58 species, of which seven were red-listed.

Consequently, man-made birch stumps hold a lot of saproxylic beetle biodiversity, but less than is present in natural stumps. Furthermore, it appears that older stumps are better for saproxylic beetle biodiversity than the newest Class (1) birch stumps.


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