Long-term effects of atmospheric deposition on British plant species richness.

The effects of atmospheric pollution on plant species richness (n) are of widespread concern. We carried out a modelling exercise to estimate how n in British semi-natural ecosystems responded to atmospheric deposition of nitrogen (N) and sulphur (S) between 1800 and 2010. We derived a simple four-parameter equation relating n to measured soil pH, and to net primary productivity (NPP), calculated with the N14CP ecosystem model. Parameters were estimated from a large data set (n = 1156) of species richness in four vegetation classes, unimproved grassland, dwarf shrub heath, peatland, and broadleaved woodland, obtained in 2007. The equation performed reasonably well in comparisons with independent observations of n. We used the equation, in combination with modelled estimates of NPP (from N14CP) and soil pH (from the CHUM-AM hydrochemical model), to calculate changes in average n over time at seven sites across Britain, assuming that variations in n were due only to variations in atmospheric deposition. At two of the sites, two vegetation classes were present, making a total of nine site/vegetation combinations. In four cases, n was affected about equally by pH and NPP, while in another four the effect of pH was dominant. The ninth site, a chalk grassland, was affected only by NPP, since soil pH was assumed constant. Our analysis suggests that the combination of increased NPP, due to fertilization by N, and decreased soil pH, primarily due to S, caused an average species loss of 39% (range 23-100%) between 1800 and the late 20th Century. The modelling suggests that in recent years n has begun to increase, almost entirely due to reductions in S and consequent increases in soil pH, but there are also indications of recent slight recovery from the eutrophying effects of N.