RATIONALE. Tree-top overpasses
Tree-top overpasses or canopy bridges have been used successfully in different parts of the world to mitigate transport infrastructure impacts on arboreal species (Das et al., 2009; Teixeira et al., 2013; Birot et al., 2019). However, in Europe the application of these measures is still limited. Some preliminary studies have shown that they are effective for species such as hazel dormouse and red squirrel in the UK (White and Hughes, 2019).
These overpasses are installed in the main as standalone bridges, although they may also be incorporated in underpasses to enhance use by arboreal species (Animex, 2021). Specific engineering options are required when installing these overpasses to ensure stability, but certain technical recommendations improve effectiveness, including extending the edges of the bridge well into the vegetation; providing multiple entrances with openings along the sides; and providing shelter options at all times (White and Hughes, 2019).
General description and targets
For climbing mammals, special types of passages may be needed. Squirrels or pine and stone martens readily cross roads and railway lines and fences are no obstacle to them, but where traffic is heavy, the risk of mortality is high. Edible and garden dormouse on the other hand rarely descend to the ground and prefer to cross roads at points where the branches of trees get close to each other.
Wildlife overpasses will be readily used by squirrels and martens, whereas they may only be suitable for dormice when there is adequate tree cover. However, passages designed or adapted to allow climbing animals to cross the infrastructure above the traffic may be a good alternative to reduce mortality risk for such particular target species. In a few countries these treetop overpasses have been constructed but currently research data on the topic in Europe is limited (see Rationale box. Tree-top overpasses). So far clear recommendations cannot yet be given. The first indications are, however, that these passages are indeed used by squirrels and dormice and in other parts of the world by monkeys or possums and other climbing species.
- Tree-top overpasses should be considered
- In wooded areas with important populations of dormice, red squirrels and pine martens where other suitable solutions, such as landscape or wildlife overpasses, cannot be constructed.
- Where mortality of target species due to collisions with traffic is concentrated.
- In large parks in towns and cities where a high road mortality of squirrels is registered and overpasses are not available.
- Taut enough for animals to pass along.
- Safe from predators.
- Places for small animals to hide.
- Good connections to trees and bushes on either side of the infrastructure.
- Safe in relation to road users.
- Tree top overpasses of flammable rope or wood should not be installed in areas where prevention of forest fire spread is a high priority. In these cases metal structures are preferred.
The design of tree-top overpasses depends on the type of road. On minor local roads the crowns of trees are often close enough together to enable climbing animals to move from tree to tree. When the distance is too big, a rope, rope ladder or other walkway can provide a connection (Figure 5.5.21A-B). On wider roads and in other situations where the distance between tree crowns is too great, the connection needs more stability (Figure 5.5.21C). Ropes and also constructions of steel cables with a small pathway in-between have been implemented. These structures have to be wide enough for animals to walk on.
- The bridges should be linked both side of the road on trees and with feeder ropes.
- Squirrels will use ropes with a diameter of 4-10 cm.
- Rope ladders with a width of 30 cm have been installed in some locations.
- Walkways of two steel cables with a net between (20-30 cm) have also been deployed.
- Planting of trees and shrubs and additional ropes and planks can facilitate animal access to the tree top overpass.
- On broad motorways, installations for traffic signs erected over the road can be adapted with a wooden walkway, shelters and hiding places.
Points for special attention
Protection from predators is an important accompanying measure. On an open rope or walkway, an additional thin rope above the passage can help prevent attacks by birds of prey.