Evidence for increases in vegetation species richness across the UK Environmental Change Network sites linked to changes in air pollution and weather patterns
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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.