| Summary: | Low doses of N addition to sub-arctic <i>Racomitrium lanuginosum </i>– <i>Carex bigelowii</i> moss heath caused reduced cover of the dominant bryophyte, <i>Racomitrium</i>, 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 <i>Racomitrium. </i>The sedge, <i>Carex</i>, 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, <i>Racomitrium </i>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 <i>Carex </i>cover, suggesting presence of the mat may positively benefit <i>Carex. </i>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 <i>Racomitrium </i>growth and greater <i>Carex </i>biomass, but there was no interaction with effects of N addition. It is likely that the two drivers will act synergistically, causing loss of <i>Racomitrium </i>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 <i>Racomitrium </i>heath.
|
|---|
