Variations of saproxylic beetle assemblages within the same white spruce logs across early and advanced decay classes

Abstract

Saproxylic species play a multitude of essential ecological roles within the forest ecosystem by undergoing a distinct succession as deadwood decays in the early to late successional stages. As of 2024, knowledge regarding saproxylic beetle community drivers, in terms of biotic interactions and larval niches, is still minimal. Striving to understand the structure, function, drivers of community assembly, and spatiotemporal dynamics of saproxylic fauna in forest ecosystems is necessary to ensure the conservation of saproxylic biodiversity. This study investigates whether there are patterns of diversity, abundance, and composition of saproxylic beetles within sections of the same white spruce log and whether there are differences or similarities between the early decay class (DC2) and advanced decay class (DC5) of white spruce logs. The study was carried out in a 10-ha non-harvested white spruce [(Picea glauca (Moench) Voss)] stand (56°79′N, 118°36′W, 758 m a.s.l.) located at the Ecosystem Management Emulating Natural Disturbances (EMEND) research site in northwestern Alberta. Both DC2 and DC5 white spruce logs were cut into five bolts, 60 cm long, with 60 cm intervals between each bolt. A total of 30 white spruce bolts were transported to Berlese funnels where saproxylic beetles were collected, and later identified to species and feeding guild. The data collected was analyzed using Excel and RStudio, using Generalized Linear Model (GLM) to compare species richness and abundance and Non-metric Multidimensional Scaling (NMS) ordination to understand community structure. Results from this study indicated that mean species richness did not differ significantly within and between decay classes. However, saproxylic beetle abundance was significantly highest in bolt two, second from the stump, and gradually declined in abundance towards bolts situated higher in the tree. Even though species richness did not differ significantly within and between decay classes, DC2 showed less similarity in species composition across the log replicates than DC5. These results indicate that saproxylic beetle assemblages are spatially aggregated within the same decay class of log. This study revealed that log sections closest to the stump are recommended to be left post-harvest to aid saproxylic beetle population persistence rather than leaving treetops. Most importantly, understanding the niche partitioning of saproxylic species at different decay stages of deadwood, in terms of competition and co-existence, can contribute to developing forest management strategies that have the least impact on saproxylic beetle populations.