Understanding the mechanisms behind nitrogen deposition impacts on montane Racomitrium heath
Low doses of N addition to sub-arctic Racomitrium lanuginosum – Carex bigelowii moss heath caused reduced cover of the dominant bryophyte, Racomitrium, and spread of graminoids, indicating that the habitat’s critical load was exceeded. N addition directly reduced growth in some bryophytes and caused physiological toxicity in Racomitrium. The sedge, Carex, responded to habitat eutrophication by increased cover and leaf turnover rate, decreasing light availability to ground layer species. Light reduction decreased growth in some bryophytes, Racomitrium again showing greatest sensitivity. Light reduction resulting from N pollution may be as important to bryophytes as direct N toxicity. Responses can be species specific, leading to competitive replacement in moss communities. Long-term high N doses led to loss of the moss mat, and slight decrease in Carex cover, suggesting presence of the mat may positively benefit Carex. Degradation was especially notable with high reduced-N addition, in which only grasses and crustose lichens remained in the bare habitat. Differential toxicity of ion type on bryophytes should be considered when setting habitat critical loads. Herbivore exclusion led to increased Racomitrium growth and greater Carex biomass, but there was no interaction with effects of N addition. It is likely that the two drivers will act synergistically, causing loss of Racomitrium and spread of N-demanding grasses within the heath. This thesis has improved mechanistic understanding of atmospheric N deposition impacts on natural and semi-natural ecosystems. It has demonstrated the importance of considering vascular plant – bryophyte and herbivore interactions when predicting community responses to drivers of change, and identified that anthropogenic N pollution is key to the loss of sensitive bryophytes and spread of graminoids, causing degradation of montane Racomitrium heath.