Southern Swedish Forest Research Centre, Swedish University of Agricultural Sciences, Alnarp, Sweden,
Oecologia. 2013 Oct;173(2):615-24. doi: 10.1007/s00442-013-2638-3. Epub 2013 Mar 16.
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.
森林地表植被是森林生物多样性的重要组成部分，许多研究表明，氮素输入会改变植被。然而，在某些情况下，实验性氮添加的影响很小或不存在。已经提出了两种替代假设：(a) 来自树木层的竞争使对氮的响应变得复杂，或 (b) 背景沉积高的地区的氮响应受到氮饱和的限制。到目前为止，这两种假设都没有得到明确的验证。在这里，我们汇集了来自瑞典的氮添加实验中森林地表植被的数据，这些实验中的森林已经被砍伐，氮沉积梯度范围从 4 到 16 公斤/公顷/年。我们分析了氮添加及其与氮沉积的相互作用对常见物种的影响，从而在没有树木层混杂影响的环境中检验了第二个假设。结果表明，实验性氮添加的影响受到背景氮沉积的显著影响：尽管本研究中的最高氮沉积可以从国际角度被认为是中等水平，但在氮沉积高的地区，氮添加的影响小于氮沉积低的地区。这些结果在评估中高背景氮沉积地区森林地表植被氮添加实验结果的可靠性时非常重要。我们得出结论，在评估植物群落对氮的长期敏感性时，需要考虑氮添加和氮沉积的相互作用效应。
