The interaction of gap age and microsite for herb layer species in a near-natural spruce forest

Question: To what degree does the regeneration of understorey forest species depend on gaps of different age and on gap-induced and non-gap-induced microsites? Do species preferences for a specific microsite change with the developmental stage of the gap? How do different species in the understorey interact over time? Location: Near-natural spruce forest on Mt. Brocken in the Harz National Park, Germany. Methods: We established 90 study plots, stratified according to different gap age classes and undisturbed forest, and including subplots with three different gap-induced types of microsites (logs, stumps and root plates) and two non-gap-induced microsites (moss-covered rocks and ordinary forest ground). Results: Significant interactions of species were encountered with gap age as well as with microsite type, light availability and competition. While shoot densities of Vaccinium myrtillus were highest at intermediate gap age, Calamagrostis villosa and Trientalis europaea showed highest densities in the oldest gaps. The species preferred different microsites but had higher densities on non-gap-induced microsites, and their preferences changed over time. Unexpectedly, species shoot densities were not always negatively affected by densities of competing species. Conclusion: The results confirmed the importance of gaps for regeneration of forest herb layer species, but pointed to a much higher importance of microsites that were not induced by gaps compared to gap-induced microsites. Niche differentiation between different herb layer species can be conceived as species-specific preferences for microsite types that change with gap age, as a result of light conditions, degree of decay of logs and root plates and presence of competitors.