Biota modelling

Biota is the animal and plant life of a particular region or habitat. Modelling the impact of air pollution on biota undertaken by WGE goes in two directions: modelling the negative impact of high concentrations of tropospheric ozone on vegetation, and modelling vegetation changes – or biodiversity changes – due to high nitrogen deposition.

Impacts of ozone on vegetation

ICP Vegetation also works in collaboration with EMEP MSC-West (European Monitoring and Evaluation Programme, Meteorological Synthesising Centre-West). The EMEP chemical transport model is used to provide ozone flux values at a large scale, for example, for Europe or globally. The DO3SE model is embedded within EMEP. The emissions data used depends on the scale of the model output required, for example, for the UK, anthropogenic emissions of NOx, NH3, SO2, primary PM2.5, primary coarse PM, CO and non-methane volatile organic compounds (NMVOCs) are derived from the National Atmospheric Emission Inventory estimate (NAEI External link, opens in new window.).

The DO3SE model (Deposition of O3 for Stomatal Exchange External link, opens in new window.), developed by researchers at the University of York, can be used to determine the Phytotoxic O3 Dose above a threshold flux of Y (PODY) which is the accumulated stomatal O3 uptake during a specified time period. The model requires inputs of data including maximum stomatal conductance (gmax) for the species/vegetation type, ozone concentration at the top of the canopy, light levels, temperature and Vapor Pressure Deficit (VPD).

Critical loads for ecosystems

The modelling of Critical Loads requires information on land cover, soil, ecosystem type and climate data. The Critical Loads calculated from this information are referred to as Simple Mass Balance Critical Loads. Critical Loads have been defined for several pollutants, e.g. sulfur, nitrogen and heavy metals. Another way is to derive ecosystem specific Critical Loads for Nitrogen from observations and experiments.

Such Critical Loads are called Empirical Critical Loads ClempN. CLempN are in almost all cases based on observed changes in the structure and functioning of ecosystems, primarily in:

• Species abundance, composition and/or diversity
• N leaching, decomposition or mineralisation rate.

Critical loads and their exceedances are the most commonly used indicators of the effects of air pollution on ecosystems. The basic idea of the critical load approach is to balance the depositions to which an ecosystem is exposed with the capacity of that ecosystem to buffer the input (e.g., acid input buffered by the availability of base cations through the rate of weathering), or to remove it from the system (e.g., nitrogen through harvesting) without causing harmful effects inside or outside the system.

Biodiversity

The resulting so-called Empirical Critical Loads are a suitable indicator to identify risks and damages to biodiversity at the ecosystem level. Such values for different habitats were first established more than 30 years ago and are continuously updated in light of the latest scientific findings. While Empirical Critical Loads show a direct link between air pollution and risks to biodiversity, Simple Mass Balance Critical Loads mentioned above show these effects only indirectly.

Biodiversity change modelling is a step further, however, it will require the development of new models and expertise. It also requires more data on species composition, which is not currently collected by ICPs under the WGE to the same extent as geochemical data.

Modelling changes in biodiversity is underpinned by geochemical models and therefore needs to be seen as an additional step, not a replacement, for the work done previously. Using models to calculate critical loads based on biodiversity changes is an additional way to assess how much air pollution ecosystems can tolerate without being damaged beyond tolerable levels. The Habitat suitability index, which was developed under the ICP Modelling & Mapping, appears to be an indicator on which this work could be built, regardless of the habitat or model used. This modeling work is still under development, and to date the modeled biodiversity changes have not been used for policy purposes.

As a result of eutrophication from nitrogen deposition, changes in nitrogen availability affect the population size of a species, leading to changes in the composition of biological communities and ecosystems. By evaluating these effects in experiments or gradient studies, it is possible to establish dose-response relationships and confirm that excessive atmospheric nitrogen deposition negatively affects species communities and thus poses a serious threat to biodiversity.