Contrasting patterns of tree mortality in late-successional Picea abies stands in two areas in northern Fennoscandia

Question What are tree mortality rates and how and why do they vary in late-successional Picea abies-dominated forests? Do observed tree mortality patterns allow comparative assessment of models of long-term stand development? Location Northern boreal Fennoscandia. Methods We measured stand structure in 10 stands in two different areas. We determined age distributions and constructed a chronology of tree deaths by cross-dating the years of death of randomly sampled dead trees. Results The stands in the two areas had contrasting tree age distributions, despite similar live tree structure. In one area, stands were relatively even-aged and originated following a stand-replacing fire 317 years earlier. The stands in the second area had an uneven age structure and virtually no signs of past fires, suggesting a very long period since the last major disturbance. The younger stands were characterized by a high mortality rate and inter-annual variation, which we attributed to senescence of the relatively even-aged stands approaching the maximum age of P. abies. In contrast, the tree mortality rates in the older stands were low and relatively stable. Conclusions Patterns of tree mortality were, to a large extent, dependent on the time since the last stand-replacing disturbance, suggesting that northern boreal P. abies stands eventually reach a shifting mosaic state maintained through small-scale dynamics, but the time needed to reach this state appears to be lengthy; even 300 years after a forest fire stands showed changes in patterns of tree mortality that were related to the developmental stage of the stands.