Changes in composition of bottom and field layer vegetation as an effect of fertilisation in coniferous forest

Most studies dealing with the effects on the ecosystem of nitrogen fertilisation or deposition show changes in the field and bottom layer vegetation of forests. According to some studies even low levels of added nitrogen result in changes in the field-layer vegetation. Effects of added nitrogen on vegetation can be divided into immediate and long-term effects. The immediate effects are caused by the toxicity of nitrogen on plants, especially bryophytes and lichens. The long-term effects are likely an effect of an accumulation of nitrogen in the system, through increased competition in the ground vegetation and decreased insolation due to a closing canopy. Ecosystem changes are dependent on several preconditions, such as: initial composition of the vegetation, initial nutrient status and light conditions. In general tall and fast growing species are favoured by addition of nitrogen, while species less responsive to increased levels of nitrogen in terms of increased growth are disfavoured. Species which can utilise nitrate through enhanced nitrate-reductase activity are promoted. In some vegetation types the interference of pathogens has been shown to mediate the change in vegetation. The species richness of a site is to large extent determined by other factors than availability of nutrients, like soil acidity. Many bryophytes and lichens decrease in abundance as an effect of increased levels of nitrogen. A plausible explanation is their low growth potential. A few studies on the activity of cyanobacteria associated with bryophytes and lichens have shown a significant decrease in activity when nitrogen was added to the site. Many studies indicate that competition between strata (e.g. from the tree canopy) is essential for the response in ground vegetation when a forest is fertilised