Complex Biotic Interactions Drive Long-Term Vegetation Change in a Nitrogen Enriched Boreal Forest
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The effects of experimental nitrogen (N) additions (0, 12.5, and 50 kg N ha−1 y−1) on long-term (12 years) understorey vegetation dynamics were examined in a boreal forest. The results showed that two types of natural enemies of the dominant dwarf-shrub Vaccinium myrtillus (pathogenic fungus of the species Valdensia heterodoxa and herbivorous larvae of the genus Operophtera) influenced the vegetation dynamics. The pathogenic fungus, causing premature leaf-shed of V. myrtillus, showed a strong positive N response during the initial 5-year period. For the larvae, a relatively modest N response was overshadowed by an almost 40-fold population increase during an outbreak event that followed the initial 5-year period. This outbreak occurred irrespective of N addition, resulting in V. myrtillus decline and depriving the pathogenic fungus of its substrate. Hence our study demonstrates that vegetation dynamics in this relatively species poor and seemingly simple ecosystem are driven by complex biotic interactions. Further, we show that an important component of these interactions is the temporal alternation of the two natural enemies and, resultant regulation of the dominant plant’s abundance. Finally, we emphasize that long-term data are essential to capture the complexity of this type of biotic interactions. In our case, a short-term study may have resulted in markedly different conclusions regarding effects of N enrichment and the role of biotic interactions for forest vegetation dynamics.
Keywordsbiotic control nitrogen deposition trophic interactions vegetation change
We are grateful to Ann Sehlstedt for help in monitoring the vegetation, and the staff at the Svartberget field station for assistance during fertilizer applications. The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), the Swedish Clean Air Program (SCARP) (supported by the Swedish Environmental Protection Board), and the Umeå Centre for Environmental Research (CMF) financially supported the project (all funding received by A. Nordin and L. Ericson).
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