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

The effects of atmospheric pollution on plant species richness (n_sp) are of widespread concern. We carried out a modelling exercise to estimate how n_sp in British semi-natural ecosystems responded to atmospheric deposition of nitrogen (N_dep) and sulphur (S_dep) between 1800 and 2010. We derived a simple four-parameter equation relating n_sp 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_sp. 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_sp over time at seven sites across Britain, assuming that variations in n_sp 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_sp 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_dep, and decreased soil pH, primarily due to S_dep, 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_sp has begun to increase, almost entirely due to reductions in S_dep and consequent increases in soil pH, but there are also indications of recent slight recovery from the eutrophying effects of N_dep.

The impact of atmospheric acid deposition on tree growth and forest understory vegetation in the Athabasca Oil Sands Region

Atmospheric acid deposition is of major concern in the Athabasca Oil Sands Region (AOSR) in northern Alberta, Canada, which is home to the third largest oil reserve in the world. After decades of oil sands production in the AOSR, the potential impact of deposition on forest health, including tree growth and understory biodiversity, is still not clear. We evaluated the relationship of modelled/interpolate atmospheric deposition of nitrogen (N), sulphur (S), base cations (BC), and derived potential acid input (PAI) from surface oil sands mining with: (1) the radial growth (i.e. basal area increment; BAI) of jack pine (Pinus banksiana Lamb.) trees using data from two decadal time periods, prior to (1957-1966) and during (2001-2010) active oil sands development in the AOSR; and (2) forest understory vegetation (abundance, diversity, and composition), which is an important component of forest biodiversity. BAI of jack pine trees varied with N, S, and BC deposition between the two time periods, and with the direction of the site relative to main emission sources. Growth was higher in areas close to the oil sands surface mining operations prior to and after oil sands development. BAI was also positively related to atmospheric deposition in the recent period, but these relationships were weaker in the active period versus the non-active period. Understory vegetation - including vascular plant cover, richness, and diversity - increased in relation to modelled atmospheric N and S deposition. There was limited correlation between soil pH or the BC:Al ratio (indicators of soil acidification) and BAI and understory vegetation responses. No evidence was found for detrimental effects of atmospheric emissions (and subsequent deposition) from oil sands production on tree growth or forest understory vegetation. The results, if anything, suggest a fertilization effect due to enhanced atmospheric deposition of nitrogen compounds.

Critical loads of acidity and exceedances for 1138 lakes and ponds in the Canadian Arctic

Sulphur emissions associated with increased anthropogenic activity, such as resource extraction and marine shipping, may lead to the acidification of aquatic freshwater systems in the Arctic. In the current study, acid sensitivity (based on a critical load (CL(Ac)) approach) of 1138 lakes and ponds in the Canadian Arctic was quantified using the Steady-State Water Chemistry (SSWC) model. Ecosystem protection was based on an Acid Neutralising Capacity limit (ANC_limit) for Arctic Char. Acidification risk was estimated under modelled sulphur deposition for the year 2010 (two scenarios). Overall, surface water CL(Ac) values for the Canadian Arctic were low (median = 35.8 meq·m^-2·yr^-1, mean = 96.3 meq·m^-2·yr^-1), with approximately 40% (n = 455) of sites estimated to be sensitive to acidification (CL(Ac) < 20 meq·m^-2·yr^-1). Higher CL(Ac) values were found in Yukon (434.5 meq·m^-2·yr^-1, n = 40), Ellesmere Is. (262.2 meq·m^-2·yr^-1, n = 143), and Southampton Is. (251.5 meq·m^-2·yr^-1, n = 35), while lower CL(Ac) values were found in Melville Is. (5.5 meq·m^-2·yr^-1, n = 48), Banks Is. (18.4 meq·m^-2·yr^-1, n = 45), and Bylot Is. (20.4 meq·m^-2·yr^-1, n = 36). Under modelled deposition for 2010, 12-12.5% (n = 136-142) of all sites were exceeded. The highest proportion of exceeded sites were observed in Ellef Ringnes Is. (48-60%; n = 12-15), Melville Is. (38-44%; n = 18-21), and Northwest Territories (24-26%; n = 23-26).

Decline in atmospheric sulphur deposition and changes in climate are the major drivers of long-term change in grassland plant communities in Scotland

The predicted long lag time between a decrease in atmospheric deposition and a measured response in vegetation has generally excluded the investigation of vegetation recovery from the impacts of atmospheric deposition. However, policy-makers require such evidence to assess whether policy decisions to reduce emissions will have a positive impact on habitats. Here we have shown that 40 years after the peak of SO_x emissions, decreases in SO_x are related to significant changes in species richness and cover in Scottish Calcareous, Mestrophic, Nardus and Wet grasslands. Using a survey of vegetation plots across Scotland, first carried out between 1958 and 1987 and resurveyed between 2012 and 2014, we test whether temporal changes in species richness and cover of bryophytes, Cyperaceae, forbs, Poaceae, and Juncaceae can be explained by changes in sulphur and nitrogen deposition, climate and/or grazing intensity, and whether these patterns differ between six grassland habitats: Acid, Calcareous, Lolium, Nardus, Mesotrophic and Wet grasslands. The results indicate that Calcareous, Mesotrophic, Nardus and Wet grasslands in Scotland are starting to recover from the UK peak of SO_x deposition in the 1970's. A decline in the cover of grasses, an increase in cover of bryophytes and forbs and the development of a more diverse sward (a reversal of the impacts of increased SO_x) was related to decreased SO_x deposition. However there was no evidence of a recovery from SO_x deposition in the Acid or Lolium grasslands. Despite a decline in NO_x deposition between the two surveys we found no evidence of a reversal of the impacts of increased N deposition. The climate also changed significantly between the two surveys, becoming warmer and wetter. This change in climate was related to significant changes in both the cover and species richness of bryophytes, Cyperaceae, forbs, Poaceae and Juncaceae but the changes differed between habitats.

Modelling impacts of atmospheric deposition and temperature on long-term DOC trends

It is increasingly recognised that widespread and substantial increases in Dissolved organic carbon (DOC) concentrations in remote surface, and soil, waters in recent decades are linked to declining acid deposition. Effects of rising pH and declining ionic strength on DOC solubility have been proposed as potential dominant mechanisms. However, since DOC in these systems is derived mainly from recently-fixed carbon, and since organic matter decomposition rates are considered sensitive to temperature, uncertainty persists over the extent to which other drivers that could influence DOC production. Such potential drivers include fertilisation by nitrogen (N) and global warming. We therefore ran the dynamic soil chemistry model MADOC for a range of UK soils, for which time series data are available, to consider the likely relative importance of decreased deposition of sulphate and chloride, accumulation of reactive N, and higher temperatures, on soil DOC production in different soils. Modelled patterns of DOC change generally agreed favourably with measurements collated over 10-20years, but differed markedly between sites. While the acidifying effect of sulphur deposition appeared to be the predominant control on the observed soil water DOC trends in all the soils considered other than a blanket peat, the model suggested that over the long term, the effects of nitrogen deposition on N-limited soils may have been sufficient to raise the "acid recovery DOC baseline" significantly. In contrast, reductions in non-marine chloride deposition and effects of long term warming appeared to have been relatively unimportant. The suggestion that future DOC concentrations might exceed preindustrial levels as a consequence of nitrogen pollution has important implications for drinking water catchment management and the setting and pursuit of appropriate restoration targets, but findings still require validation from reliable centennial-scale proxy records, such as those being developed using palaeolimnological techniques.