Snag degradation pathways of four North American boreal tree species

Abstract

Many studies have highlighted the importance of deadwood, whether standing (snags) or fallen, in boreal ecosystems dynamics. However, a dearth of literature exists regarding the persistence and degradation pathways of these structures in northeastern American boreal species.Degradation pathways were examined in four tree species of the eastern boreal mixedwood and conifer forests of Canada: Trembling aspen (Populus tremuloides Michx.), balsam fir (Abies balsamea (L.) Mill.), jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill.) BSP). Sampling was conducted in unharvested forests of northwestern Québec. Discs where collected on 363 snags and logs and dendrochronological analyses conducted to determine year of death by crossdating.Survival curves were constructed for each species and degradation pathways were documented based on the shape of the relationship, lag time if applicable (time period from death to fall onset) and half-life (time required for half the stems to fall).Most species survival curves exhibited a reverse sigmoid function. Trembling aspen, jack pine and to a lesser extent balsam fir presented significant lag times before initiation of a period with high fall rates. Black spruce experienced more snag losses during the first years following death. Jack pine was clearly the most persistent species due to snags high resistance to fall and stem breakage. Within the range of tree diameters in our study area, diameter per se did not significantly influence fall probability. Differential degradation pathways of the four species translate individualistic responses that are related to species autecology. This has important implications for all ecological functions related to deadwood and should be considered when planning retention strategies of legacy trees in an ecosystem management perspective.