Plant species richness in the UK appears to be benefitting from reductions in acid deposition. This finding was suggested by the authors of the 2016 paper Evidence for increases in vegetation species richness across the UK Environmental Change Network sites linked to changes in air pollution and weather patterns (Rose et al.).Here we explain their work, which involved analysing ECN's long-term vegetation monitoring data.

This paper, published in a Special Issue of the journal Ecological Indicators to mark 20 years of data collection at ECN terrestrial sites, uses detailed ECN vegetation data. The data suggests that species richness increased at the sites during the period 1993-2012.

Reference

Rose, R., Monteith, DT., Henrys, P., Smart, S., Wood, C., Morecroft, M., Andrews, C., Beaumont, D., Benham, S., Bowmaker, V., Corbett, S., Dick, J., Dodd, B., Dodd, N., Flexen, M., McKenna, C., McMillan, S., Pallett, D., Rennie, S., Schafer, S., Scott, T., Sherrin, L., Turner, A. and Watson, H. (2016). Evidence for increases in vegetation species richness across the UK Environmental Change Network sites linked to changes in air pollution and weather patterns. Ecological Indicators 68, 52-62. DOI: 10.1016/j.ecolind.2016.01.005

Why this research matters

This paper is relevant to the following issues:

• Recovery from acidification
• Air pollution
• Plant diversity
• Strategies/policies to increase biodiversity

In brief

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In more detail

The ECN vegetation monitoring programme is nationally unique with respect to the high frequency of measurement (annual to tri-annual surveys). It thus provides a particularly sensitive record of recent long-term botanical change for various habitats across the UK.

The authors analysed these monitoring records for the period 1993-2012 and revealed a network-wide increase in plant species richness (i.e. the number of species occurring within a defined area). They also found that a commonly used metric that summarises the soil pH preference of plant communities increased in open upland, open lowland and woodland habitats.

The increase in this metric, called 'Ellenberg R', suggests that plant communities were gradually shifting in composition towards species characteristic of less acid soils.

This was further supported by evidence that the majority of plant species found to be increasing in frequency at multiple sites favoured less acidic soils (i.e. with a higher pH).

These changes are therefore consistent with observations that soil water at several ECN sites and elsewhere has become gradually less acidic since 1993, in response to reductions in acid deposition.

These results therefore provide what is believed to be the first definitive evidence of widespread improvements in vegetation diversity as a consequence of partial environmental recovery from acidification. The authors note, however, that increased species richness at some lowland sites could also be linked to a series of wetter summers in recent years, and reduced occurrence of drought.

The observation that UK plant species richness appears to be benefitting from reductions in acid deposition provides a new perspective on the relationship between levels of air pollution and plant diversity. Negative spatial relationships between levels of nitrogen deposition and species richness have often been interpreted primarily as evidence for the impacts of nitrogen's nutrient enriching effects (eutrophication). However, in the UK, places that experience high levels of sulphur deposition tend also to receive high levels nitrogen deposition. This study therefore emphasises the difficulty in differentiating between the acidifying and eutrophying effects of air pollution.

The highly detailed nature of the ECN vegetation datasets should now assist scientists in their attempts to disentangle the relative importance of these effects. This in turn should provide a clearer guide to policymakers regarding acceptable levels of pollution and the relative vulnerability of different habitats to negative impacts.

The study highlights a clear policy need for a more thorough evaluation of the relative impacts of atmospheric eutrophication and acidification on botanical biodiversity, and demonstrates the value of ECN data as a particularly important component of the evidence base in this field.

This is the published abstract of the research paper

We analysed trends in vegetation monitored at regular intervals over the past two decades (1993–2012) at the twelve terrestrial Environmental Change Network (ECN) sites. We sought to determine the extent to which flora had changed and link any such changes to potential environmental drivers. We observed significant increases in species richness, both at a whole network level, and when data were analysed within Broad Habitat groupings representing the open uplands, open lowlands and woodlands. We also found comparable increases in an indicator of vegetation response to soil pH, Ellenberg R. Species characteristic of less acid soils tended to show more consistent increases in frequency across sites relative to species with a known tolerance for strongly acidic soils. These changes are, therefore, broadly consistent with a response to increases in soil solution pH observed for the majority of ECN sites that, in turn, are likely to be driven by large reductions in acid deposition in recent decades. Increases in species richness in certain habitat groupings could also be linked to increased soil moisture availability in drier lowland sites that are likely to have been influenced by a trend towards wetter summers in recent years, and possibly also to a reduction in soil nitrogen availability in some upland locations. Changes in site management are also likely to have influenced trends at certain sites, particularly with respect to agricultural practices. Our results are therefore indicative of widescale responses to major regional-scale changes in air pollution and recent weather patterns, modified by local management effects. The relative consistency of management of ECN sites over time is atypical of much of the wider countryside and it is therefore not appropriate to scale up these observations to infer national scale trends. Nevertheless the results provide an important insight into processes that may be operating nationally. It will now be necessary to test for the ubiquity of these changes using appropriate broader spatial scale survey data.