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The impact of development

Research commissioned by the Woodland Trust has highlighted the potential impacts on the ecology of nearby ancient woodland.

Ancient woodland

Ancient woodland dates back hundreds of years and supports more threatened species than any other habitat in the UK. However, less than 300,000ha remains. It is a functionally irreplaceable resource for biodiversity that is also an important part of our cultural heritage.

Ancient woods have been fragmented for hundreds of years and by 1900 only 5% of the UK was covered by woodland. While the twentieth century saw further attrition of ancient woodland, it was the intensity of land use between woods that saw unparalleled change. Of the woods recorded on today’s ancient woodland inventories, 48% are smaller than five hectares.

Therefore, many are very vulnerable to edge effects from surrounding land use. The importance of ancient woodland is recognised in recent national planning policies across the UK and planning authorities and inspectors increasingly act to prevent its direct destruction. However, the damage and impacts posed to ancient woods by nearby development are not so widely appreciated. This research highlights how these possible damaging impacts effects ancient woodland in the UK.

What are these development impacts?

The importance of ancient woodland is recognised in national planning policy guidance across the UK and planning authorities and inspectors increasingly act to prevent its direct destruction. However, a wide range of development types affect nearby ancient woodland, including: housing, roads and motorways, commercial and industrial development; intensive livestock units; quarrying and mineral extraction; waste disposal facilities; leisure and sport and cumulative development. These cause five main impacts: chemical effects; disturbance; fragmentation; invasion by non-native plants; and cumulative effects.

All development types (with the exception of permitted development) are associated with substantial effects of at least one impact type (chemical effects, disturbance, fragmentation). In addition, some kinds of development are likely to be associated with the effects of multiple impact types. Transport and cumulative development (urbanisation and cumulative fragmentation) are likely to have the greatest impact, arising from multiple effects.

Chemical effects of development

Roads and Motorways, commercial and industrial development, intensive livestock units, energy generation and supply, quarrying and mineral extraction, waste disposal facilities, and cumulative development all have the potential to create substantial chemical impacts on nearby ancient woodland.

Chemicals, such as herbicides, pesticides, heavy metals, toxic or nutrient-rich leachates, and sulphur and nitrogen oxides, may reach ancient woodland from nearby development through a range of mechanisms. These include: aerosol or spray drift; the application of road-salt; contaminated surface and ground water flows; deposition of dust, particulate and gaseous pollution; localised acid-rain events; deliberate dumping of rubbish or garden waste into woodland; and accidental release or spillage of hazardous substances.

Chemical effects on nearby ancient woodland include: population-level responses to lethal and sub-lethal doses of toxic chemicals, or nutrient enrichment, that can significantly alter the composition of the ground flora and lichens, mosses and liverworts growing on trees or rocks; reduced tree health by inhibiting root development and retarding growth, increased drought and frost susceptibility, defoliation, or leaf discoloration, poor crown condition, and the promotion of insect damage; poisoning of animals, leading to mortality, reduced feeding rates, or species avoidance; and loss of soil micro–organisms, including tree mycorrhizae, thereby affecting decomposition and nutrient cycling.

The solution

Chemical effects arising from development should be avoided wherever possible, by maintaining minimum distances between new development and existing woodland. The construction-related chemical effects of nearby development should be managed through agreement and implementation of Environmental Management Plans. Ongoing impacts should be mitigated through the creation of woodland buffer zones of an appropriate width.

Disturbance

Evidence indicates that housing, transport, commercial and industrial development, quarrying and mineral extraction, leisure and sport, military activity, water management and cumulative development all have substantial potential to disturb nearby ancient woodland.

Development in the vicinity of ancient woods may cause direct disturbance effects as a result of: modified local hydrological regimes; vibration; noise and light pollution; vehicular collisions with wildlife; external activity visible from within the wood; an increase in wind-blown litter accumulation; and tree surgery or felling along the woodland edge for safety reasons or subsidence prevention.

Development near to ancient woodland increases the likelihood of unmanaged public access, leading to: trampling of vegetation and soil compaction; removal of dead wood or plants; acts of vandalism, and the dumping of rubbish or garden waste. Further indirect effects include predation of woodland fauna by pets or large-bodied birds that may be attracted to the area.

Disturbance may result in more frequent biologically-costly flushing events and increased mortality of animal species. Noise and light pollution interfere with interactions between species, affecting foraging and predation, reducing breeding success and thereby affecting ongoing population viability. Disturbance may, therefore, lead to species being eliminated from woods.

Engineering works or vegetation clearance near to ancient woodland may affect woodland hydrology, increasing the likelihood of water-logging or drought and leading to loss of trees and changes in species composition. Soil compaction adjacent to woodland increases water run-off and soil erosion. It can cause severe damage to tree roots, leading to tree defoliation, crown dieback, and death.

The solution

Developments likely to cause disturbance should be located away from ancient woodland, particularly those likely to modify local hydrological function. Where development is located near to ancient woodland, buffer zones should be retained to reduce the distance that disturbance penetrates. If possible, access to the woodland should be limited or managed.

Fragmentation

Ancient woodland is a highly fragmented habitat. New development may be associated with the destruction or alteration of semi-natural habitats in the vicinity of ancient woods and the creation of large areas of terrain inhospitable for woodland species. Therefore, development may increase the distances between favourable habitats that woodland species must cross to disperse, forage, or breed.

In addition, developments that create chemical or disturbance effects that penetrate nearby ancient woodland may effectively reduce woods to smaller functional habitat islands. As a result, new development may significantly fragment ancient woodland habitats, creating substantial barriers to species movement, interrupting natural flows between habitat patches, sub-dividing populations, and altering the population dynamics of associated species and communities.

Transport and urbanisation, in particular, may create major landscape-scale barriers to movement of woodland species. However, evidence suggests that all development types may lead to further isolation of ancient woods. Nevertheless, the net impact of a development on fragmentation depends on the existing land cover and land use.

For example, some new developments are associated with the creation of extensive rough ground or planted areas, which may have potential to increase connectivity if sited on previously intensive-arable farmland.

The solution

Landscape-scale connectivity of ancient woodland should be considered in all local and regional development plans. Development mitigation should seek to enhance the ability of woodland species to move between ancient woods.

Non-native plant species

The likelihood of ancient woodland being invaded by non-native plant species is increased by a range of factors associated with construction, including soil excavation and movement, altered environmental conditions and modified hydrological processes. Nutrient enrichment from developments, such as transport corridors, intensive livestock units and residential gardens, also increases the risk of non-native plant species invading woodland on an ongoing basis.

Research indicates that the proportion of non-native plant species in an area rises with increasing density of human population. As a result, evidence suggests that cumulative development is likely to promote non-native plant species invasion into ancient woodland, particularly in relation to housing, transport and energy infrastructure.

The solution

Developments that may provide ongoing sources of invasive non-native plants should not be placed close to ancient woodland. Where development does occur near ancient woodland, potential construction-related invasion pathways should be managed pro-actively using a structured approach to reduce risk.

Cumulative effects

Ancient woodland is the recipient of the sum of a wide variety of effects generated by multiple developments and development types. There are frequently long time lags before the combined impacts of chemical effects, disturbance, fragmentation and invasion by non-native plants become apparent.