Is Erica tetralix abundance on wet heathlands controlled by nitrogen deposition or soil acidification?

Erica tetralix is the key species on NW European wet heathlands, where it is often found to be the dominating plant species. Consequently, it is of considerable concern that the species has decreased significantly in cover from 28% to 18% over a six-year period. In order to understand the underlying causes, a structural equation modeling (SEM) approach was applied on ecological data from 1130 wet heathland plots. Both atmospheric N deposition and soil acidification were included in the SEM. The most important causal effect revealed by the SEM was a significant negative effect of N deposition on the cover of E. tetralix, whereas soil acidity tended to have a negative effect of relatively less importance. There was no significant effect of N deposition on soil pH, which indicates that there are no major indirect effects of N deposition on the cover of E. tetralix mediated by soil acidification.