Retention of stumps on wet ground at stump-harvest and its effects on saproxylic insects

Abstract: Low stumps represent on their own up to 80% of the dead wood remaining on clear cuts and therefore supply suitable habitat for saproxylic insects i.e. insects depending on dead wood for their survival. Recent stump harvesting activities threaten this substrate of ecological importance and increase the anthropogenic negative impacts on these species. Because of technical and environmental reasons (nutrient leakage, erosion) guidelines for stump harvesting recommend to retain stumps standing in wet parts of clear cuts. However, stumps in wet positions might not be a satisfactory substrate for saproxylic insects and therefore might not be as much used as stumps in dry positions. To test this hypothesis, a total of 100 stumps (50 spruces and 50 birches) were collected on four clear cuts between 5 and 7 years old near Uppsala, Sweden. Stump samples were paired to get a balanced dataset, each pair containing two stumps of the same tree species, diameter and sun-exposure, one dry and one wet. Each sample was placed in a rearing box for 2 months. All emerging insects were sorted out down to order, family, genus or species level according to their importance in the project. Species richness, abundance and composition in each type of stumps were analyzed. Proportion of stumps inhabited, density and Shannon’s diversity and evenness were calculated for each insect order. A canonical correspondence analysis was performed to investigate the possible connections existing between insect species and the tree species and dryness of the stumps. In total, 17065 insects were collected representing 114 species out of which 96 were considered as saproxylic. An overall of 76 and 55 saproxylic species were collected on birch and spruce stumps respectively. Species richness was higher in birch stumps with 11 species in average per sample and only 4 in spruce ones .The results show that the tree species was the only factor significantly affecting both the species richness and the species abundance of a stump. The variable “Dryness” had significant effect on the samples’ species diversity with dry samples harboring a higher number of species. The proportion of stump types used by different orders clearly shows that the tree species was again the main factor influencing the species abundance of a stump. More insects were found in birch stumps, regardless of their dryness, than spruce. Wet spruce and dry birch stumps were respectively the least (7.9%) and the most used (34.1%) substrates. Coleoptera beetles were more numerous in birch stumps but did not show any preference concerning the moisture level of the stumps. The other orders showed a similar pattern with Hymenoptera, Lepidoptera insects however favoring dry birch stumps and Dipterans wet ones. Arhopalus rusticus and Curtimorda maculosa were the only species to show a significant correspondence to a substrate and were associated to spruce stumps. The results show that both birch and spruce support the life cycle of many different insect species and not only saproxylic. As wet spruce stumps presented the lowest biodiversity, it could be thus advised, in a context of biodiversity conservation, to set aside in priority birch and more generally broadleaves stumps.