Individual study: The use of natural and man-made aspen Populus tremula stumps by saproxylic beetles, Bryngelsdalen (Värmlands) and Bräcke ängar (Dalsland), Sweden
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
Intensive management of the Fennoscandian forests have resulted in a lack of dead wood and of deciduous trees, which in turn has contributed to biodiversity loss. In Sweden, of the 1,000 or so recorded species of saproxylic beetle (i.e., those 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 aspen Populus tremula 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 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 aspen trees. When detonated, this produced ragged-edged stumps similar in appearance to those that had undergone natural 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 site. Each stump was measured (diameter) 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 = 30) than man-made (n = 57) 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 h under a 40 W light bulb. All adult saproxylic beetles and their larvae in two families, Elateridae and Pyrochroidae, were identified to species level.
Natural stumps had 62 species of beetle, of which eight were red-listed. By comparison, man-made stumps had 72 species of beetle, of which 10 were red-listed. Decay class (combining natural and man-made stumps) had an affect on biodiversity:
Class (1) stumps (n = 13), held 31 species of which two were red-listed;
Class (2) stumps (n = 53), had 71 species of which nine were red-listed;
Class (3) stumps (n = 19), with 50 species of which six were red-listed.
Consequently, man-made aspen stumps hold a lot of saproxylic beetle biodiversity, and it appears that, unsurprisingly, older stumps are better for saproxylic beetle biodiversity than the newest Class (1) stumps.
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