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Keywords:

  • Ground vegetation;
  • Multiple perturbations;
  • Nitrogen enrichment;
  • α-diversity;
  • β-diversity

Abstract

Questions

Nitrogen (N) enrichment often appears in combination with land-use related disturbances. Are there interactive effects between N addition and physical disturbances where N addition increases plant community sensitivity to disturbance?

Location

Boreal coniferous forest, Bispgården (63 °00′N, 16 °40′E), central Sweden.

Methods

We tested potential interactive effects between N addition and disturbance by evaluating effects of N fertilization on community composition of forest floor vegetation in clear-cut (disturbed) and mature (undisturbed) forests. We analysed species composition in 63 forests stands (16 clear-cut and N-fertilized, 14 clear-cut and unfertilized, 17 mature and N fertilized, and 16 mature and unfertilized). Species composition was scored by the point-intercept method at 200 random points along a 45-m transect in each stand.

Results

The N effect on plant community composition was strongly dependent on disturbance caused by clear-cutting. In undisturbed forests, there were small or no effects on community composition from N addition. In contrast, effects were large in forests first exposed to N addition and subsequently disturbed by clear-cutting. Effects of N addition differed among plant functional groups. Abundance of graminoids increased (+232%) and abundance of dwarf shrubs decreased (−44%) following disturbance in N-fertilized stands. For vascular plants, the two perturbations had contrasting effects on α- (within stand) and β- (among stands) diversity: in disturbed stands, N addition reduced α-diversity, while β-diversity increased. For bryophytes, effects of disturbance on α-diversity were smaller under N addition than ambient N, while neither N addition nor disturbance had any effect on β-diversity.

Conclusions

Effects of N addition on plant communities may be small, short-lived or even absent until exposure to a disturbance implements full effects of N, highlighting the importance of considering interactive effects with disturbance when evaluating effects of N enrichment on plant community composition and biodiversity.