title: Does background nitrogen deposition affect the response of boreal vegetation to fertilization?
abstract: Forest floor vegetation is an important component of forest biodiversity, and numerous studies have shown that N input alters the vegetation. In some cases, however, the effects of experimental N addition have been small or absent. Two alternative hypotheses have been suggested: (a) competition from the tree layer confounds the response to N, or (b) N response in areas with high background deposition is limited by N saturation. Neither of these hypotheses has so far been explicitly tested. Here, we compile data on forest floor vegetation from N addition experiments, in which the forest had been clear-cut, along an N deposition gradient ranging from 4 to 16 kg ha(-1) year(-1) in Sweden. We analyzed the effects of N addition and its interaction with N deposition on common species and thereby tested the second hypothesis in an environment without the confounding effects of the tree layer. The results show that the effects of the experimental N addition are significantly influenced by background N deposition: the N addition effects are smaller in areas with high N deposition than in areas with low N deposition, despite the fact that the highest N deposition in this study can be considered moderate from an international perspective. The results are important when assessing the reliability of results from N addition experiments on forest floor vegetation in areas with moderate to high background N deposition. We conclude that the interacting effects of N addition and N deposition need to be included when assessing long-term N sensitivity of plant communities.
authors.affiliation: Southern Swedish Forest Research Centre, Swedish University of Agricultural Sciences, Alnarp, Sweden, per-ola.hedwall@slu.se.
authors.forename: P O
authors.initials: PO
authors.lastname: Hedwall
authors.forename: A
authors.initials: A
authors.lastname: Nordin
authors.forename: J
authors.initials: J
authors.lastname: Strengbom
authors.forename: J
authors.initials: J
authors.lastname: Brunet
authors.forename: B
authors.initials: B
authors.lastname: Olsson
mesh_terms: D044822:Biodiversity; D017753:Ecosystem; D016468:Forestry; D009584:Nitrogen / Q000378:metabolism*; D018521:Plant Physiological Phenomena; D013548:Sweden; D014197:Trees
chemical_list: D009584:Nitrogen