N-fertilization and disturbance impacts and their interaction in forest-tundra vegetation

摘要

The interaction of environmental perturbations is an important, although a seldom studied feature, when evaluating factors influencing plant community structure and potential changes in the vegetation. Since environmental perturbations commonly occur in concert, there is a need for experimental investigations in which single, combined and interactive effects of environmental factors are studied. We studied interactive effects of N-fertilization (40 kg N ha(-1) year(-1)) and disturbance, i.e. removing the vegetation and soil organic layers, in the forest-tundra ecotone in northern Finland during 2002-2005. Plant abundances were measured in a coniferous forest, mountain birch forest and tundra heath. Both N-fertilization and disturbance reinforced the proportion of deciduous (Vaccinium myrtillus) and the graminoids (i.e. Deschampsia flexuosa and Carex sp.) at the expense of evergreens in vegetation. N-fertilization also enhanced the post-disturbance recovery of graminoids. Vegetation recovery was slow in the tundra heath, where N-fertilization decreased the abundance of the evergreen Empetrum nigrum ssp. hermaphroditum. Although the changes in vegetation due to the N-fertilization and disturbance could be detected, they did not change the initial dominance of plant functional types. Taken together, our results suggest that increasing N affects the rate rather than direction of recovery after disturbance. Moreover, plant communities differ in terms of their capabilities to respond to multiple perturbations which should be taken into account when evaluating future vegetation responses under changing environment in at high-latitude and high-altitude ecosystems.