Boreal forests, those consisting mostly of pine and spruce trees, are often significantly affected by human imposed methods of forest restoration. Many organisms in these ecosystems rely on nutrients from fallen trees, and the removal of these dead trees as well as attempts to prevent forest fires (and other natural processes such as windstorms) threaten the survival of such organisms. This is particularly true of organisms that do not live inside of protected forest areas in which these natural processes continue without interference. While many saproxylic species (those dependent on dead wood) can live in any dead wood, some species are adapted to live off of sun-exposed dead wood in many habitats. Previous studies have shown that saproxylic beetles can reproduce in dead wood that has been produced by retaining dead wood and by cutting stumps high, allowing further exposure to the sun. In particular, these studies have shown that the method of cutting exposed stumps benefits those species that are adapted to live in sun exposed areas.
In a recent study by Djupstrom et al., researchers attempted to determine the extent to which methods of dead tree retention and high-cut stumps contribute to population growth of threatened beetle species in boreal forests. The researchers focused on the Peltis grossa population, a beetle species that is red-listed in Sweden and endangered in Norway. They chose this particular species to study the effects of producing high-cut stumps because this species lives and reproduces in standing dead wood. The study took place in the boreal forests of southern Sweden, made up mostly by Scots pine and Norway spruce trees. The study began in 1994 when 500 trees stumps were produced; 425 of which actually showed evidence of P. grossa emergence holes. These holes were first observed in 2003, while the last holes were observed in 2010. This led them the team to conclude that emergence occurs about ten years after a stump is cut and will last for another ten years.
The study evaluates the difference in P. grossa recruitment in clear-cut forests from the early 1990s and the recruitment in the experimental high-cut stumps in clear-cut forests produced in 1994. The data shows that the percentage of dead wood objects that a particular tree consists of has a correlation to the percentage of dead wood with observable P. grossa emergence holes. 3.1% of experimental clear-cut birch trees, which made up 17.2% of the dead wood objects in the area, had emergence holes, compared to the 0.9% of birch with emergence holes in ordinary clear-cuts. Even though dead birch wood made up 39.3% of the dead wood in the studied area, coarse, high-cut stumps still recruited higher rates of P. grossa. Norway spruce made up 15.6% of dead wood objects in the experimental clear cut, with 1.4% of stumps producing emergence holes, compared to the ordinary clear-cut in which Norway spruce made up 21.6% of dead wood with 3.1% of stumps exhibiting emergence holes. However, 15.6% of high-cut stumps of Norway spruce (that made up 54% of dead wood) had emergence holes, compared to the 7.7% of high-cut spruce in the ordinary clear-cut. This suggests that high-cut stumps are more habitable, and also that the density of a particular type of tree also affects the extent of P. grossa recruitment. However, the study shows that even if there is a high density of dead wood in a forest, many species such as P. grossa cannot habitate and reproduce unless the dead wood is exposed to sunlight.
The study conducted by Djupstrom et al. shows that restoring dead wood in sun-exposed forests by cultivating stumps through clear-cuts will dramatically improve the chance of habitation and reproduction of many saproxylic species adapted to live on sun-exposed dead wood. While their hypothesis was validated, they still maintain that it is important for forest preservation to both cultivate these clear cuts and to preserve dead wood in areas without high sun-exposure, because it seems these are distinct conditions that facilitate life for species adapted to the particular habitats.
Citation: Djupstrom, Line B., et al. (2012). Restoration of habitats for a threatened saproxylic beetle species in a boreal landscape by retaining dead wood on clear-cuts. Biological Conservation 155: 44-49.