Parkland trees and Hedgerows

The Ecosystem Overviews provide a summary introduction to the main habitat types covered in APIS and the main air pollutant pressures to those habitats in the UK. In specific locations, e.g.. close to a major source, other pollutants may be a concern for a habitat and the user should use the searches by location or by habitat/pollutant in these cases.

Introduction

Parkland trees and hedgerows are expected to be at particular risk from exposure to gaseous and particulate air pollution. This is because many pollutants deposit faster on aerodynamically rough surfaces, as a result exposed parkland trees and hedgerows are effective scavengers of air pollutants. Hedgerows are under additional risk as there are often boundary features to roads or agricultural land, which may be sources of air pollutants.

BAP habitats: Ancient and/or species-rich hedgerows (priority); Lowland wood-pasture and parkland (priority).

Main Pollutant Impacts

Nitrogen deposition and Ammonia

Ammonia (NH3) emissions from intensive farmland lead to high rates of local deposition (Sutton et al. 1998). Hedgerows are often only a few metres from ammonia emitting fields, i.e. after the land spreading of manure or slurry, and maybe subject to deposition well in excess of critical loads and levels. The biological responses focus mainly on the epiphytes and ground flora, with effects of the latter expected to parallel changes in woodlands. The issue of NH3 impacts on parkland trees has been recognised, for example, where parkland is heavily grazed or fertilised, leading to concerns regarding epiphyte populations (Woodin and Farmer 1993).Although enhanced levels of NOx occur close to roads, NOx deposits much more slowly than NH3, consequently impacts on hedgerows are expected to be less from this source. Direct effects of NOx include damage to sensitive mosses, liverworts and lichens which receive their nutrients from the atmosphere. Direct damage is also possible to tree species sensitive to air pollution.

Persistent organic pollutants (POPs)

Dispersal of biocides used in agriculture is expected to affect hedgerow plant and animal species composition. The very local scale impacts may be considered as due to "spray drift". Field surveys carried out by Gove et al (2007) found that the abundance of the most sensitive species was highest in woodland margins adjacent to fields with low agrichemical inputs and lowest beside high-input farmland. These differences were seen at least 4 m into woodland margins but were not found at 10 m. They recommend the adoption of no-spray buffer zones of at least 5 m to protect the majority of woodland species from the impacts of agrichemicals applied to adjacent land.

Heavy metals

Close to sources a considerable fraction of heavy metals is contained in supermicron particles, which deposit much more rapidly than fine particles. In addition, deposition rates are largest to aerodynamically rough surfaces such as trees and bushes. Therefore urban parklands and roadside hedgerows are expected to receive a particularly large deposition of particulate pollutants, especially heavy metals, which may subsequently accumulate in the food chain. Although studies are beginning to quantify the rates of deposition, there is a need to clarify the precise scale of the ecological impacts.

Ozone

For ozone impacts see the Woodlands overview.

References: 

Sutton, M.A.; Milford, C.; Dragosits, U.; Place, C.J.; Singles, R.J.; Smith, R.I.; Pitcairn, C.E.R.; Fowler, D.; Hill, J.; ApSimon, H.M.; Ross, C.; Hill, R.; Jarvis, S.C.; Pain, B.F.; Phillips, V.C.; Harrison, R.; Moss, D.; Webb, J.; Espenhahn, S.E.; Lee, D.S.; Hornung, M.; Ullyett, J.; Bull, K.R.; Emmett, B.A.; Lowe, J.; Wyers, G.P. 1998 Dispersion, deposition and impacts of atmospheric ammonia: quantifying local budgets and spatial variability Environmental Pollutution (Nitrogen Conference Special Issue) 102 349-361