Native Plant Selection And Plant Re Colonisation Environmental Sciences
Currently, there are approximately 1,017 kilometres of motorway in the Republic of Ireland with a view to possibly extending this to 1100 kilometres by 2015. (Wikipedia, 2011). Therefore, because of this vast sprawling motorway network which will possibly be extended throughout the country, I will seek to critically evaluate the vegetation suitable for this network with a view to sustaining the features of the local landscape and its surrounding habitat.
The planting of trees and shrubs along a motorway provides important means screening for both the driver and passengers. In an urban area, a motorway can be lined with intrusive properties. Buildings or industrial estates can be unsightly while travelling along a motorway. Vegetation here has an important role to play here as they can help naturally mask these surrounding unsightly features. Trees or tall shrubs are a suitable screen for multi-storey houses or developments while small shrubs can be a suitable veil for lower single story buildings. Both of these do not only act as a screen but also function as important noise and wind barriers. (Department for Transport 1992, p.2/1)
While it is important to provide adequate screening in built up areas through means of visual mitigation, this can be difficult to maintain year round as deciduous species lose their foliage in the winter months. This can be avoided when a mix of evergreen and deciduous species are planted. (Department for Transport 2004, p.5/17).
On the other hand, overplanting on a motorway for screening can prove to be detrimental to the surrounding landscape. If the surrounding landscape is already particularly scenic, tall trees and dense shrubs can mask this. An example of this can be seen in the below image:
'The problem: M40, Oxfordshire When this planting matures an attractive view, in which the road is not a dominant element, will be lost. It might have been better either to have accepted the view or to have broken it up within intermittent planting'
(Department for Transport, 1992, p.2/2).
Here, were are advised that that varied and intermittent planting is necessary because too much screening can lead to the view of the surrounding landscape and features being masked though the dense shield of greenery once it matures. This would be especially the case of evergreen woodland species which would block the view in all seasons and grow taller than some deciduous trees or shrubs.
Therefore, we are already discovering that planting of plant species along a motorway involves specific planning whereby the natural surrounding landscape is not deprived of its features. It is also important to note also that woodland planting would normally take place on a wide cutting where trees would be clear of overhead cables and important signs or structures. The variety of species selected should reflect the local landscape while not overly shielding it. (Department for Transport, 2004, pp.5/17-5/18).
This above measure of selecting variegated species is also important for the driver and their personal experience with their surroundings along the road corridor. The National Roads Authority (2006, p.24) highlight the importance of driver fatigue in the planning of landscaping along motorway as 'the frequent use of a single or few relatively large treatments along a road corridor can become repetitive, leading to a loss of perceived naturalness, and may contribute to driver monotony and fatigue.'
They note how it is important to have a variation in the vegetation along the motorway particularly at high speeds of up to 120 km/h. It therefore should be considered to avoid a single species of high canopy trees or woodland species in an area of an attractive featured landscape.
'The evergreen holly is a native species which forms the shrub layer in some of our oldest woods. It is another visually attractive small tree very suitable for gardens as a specimen tree or as a hedge, slow growing and very dense. It is possible to take cuttings from holly. A small side shoot about 15 cm with the 'heel' where it joins the main branch should be selected and grown in a suitable sand/loam mix. September is the best time to take cuttings, which are best grown under shelter. Seedlings or cuttings should be moved once they are well established (but under 20cm tall) and when the soil is warm'
(The Tree Council of Ireland, 2008, P.26)Instead, the natural landscape should be maintained and any vegetation planted should blend with the surroundings features and not conceal them. The woodland screening mix should therefore consist of local native species that reflect the local landscape. The National Roads Authority (2008, p.165) provide examples of woodland species that can be used to reflect the local landscape; Populus tremula (Aspen) Salicaceae (family), Fraxinus excelsior (Ash) Oleaceae (family), Quercus robur (Oak) Fagaceae (family), Betula pendula (Birch) Betulaceae (family), Alnus glutinsosa (Alder) Betulaceae (family) and evergreen species such as; Ligustrum vulgare (Wild Privet) Oleaceae (family), Ilex aquifolium (Holly) Aquifoliaceae (family) and Ulex europaeus (Gorse) Fabaceae (family).
'The one definitely native poplar is aspen (all other
poplars may be assumed to be introduced, although
the black poplar is still being argued about). Aspen
will grow into a full sized tree. The leaves make a
distinctive sound as they rattle gently in the wind,
and they have a sweet smell in the spring. Poplars produce seeds on catkins, but also spread vegetatively by suckers i.e. new shoots growing up from the roots. It is
easiest to propagate aspen by cutting through roots and transplanting a sucker'
(The Tree Council of Ireland, 2008, P.19)
It is also important to note how existing native hedgerows and woodland species should be retained where possible and if are necessary to be removed during construction works, post motorway landscape planting should focus on the natural native species that existed so there is no impact on local vegetation and flora. This is of crucial importance to local wildlife and protected conservation species.
An example of how post motorway construction planting is achieved is seen in the below image:
(Department for Transport, 2004, P.5/46).
Here we see how hedgerows are planted along the road corridor linking them in with existing hedgerows that exist in the surrounding fields. This provides important visual integration but also achieves benefits for the local wildlife as they have a corridor of vegetation to travel along. In the development of a natural habitat along motorways, hedgerows are of huge importance to wildlife conservation.
Richard Nairn (2012) affirms that:
'new roadside plantings can create a 'woodland edge' with tall canopy trees such as oak, ash or Scots pine set well back from the carriageway and a progressively lower fringe of small trees and understory plants closer to the road. This helps to replicate natural habitats which have been depleted in Ireland for several centuries.'
Therefore, if a motorway is developed along or nearby a woodland area, it would be necessary to replicate the planting of the natural locally occurring species for the benefit of blending it with the local surroundings.
'Good practice: M27, Hampshire A variety of species has been used. The numbers of individuals in each species group and the size of the clumps, are in keeping with the scale of the road.'
(Department for Transport, 1992, P.3/1).
In the above examples we seed how a collection similar species are selected to naturally blend with the surroundings and we note the importance of how species are selected to grow naturally as they would occur within these surroundings. This not only benefits the visual interest but wildlife protection is highlighted.
In addition to wildlife found on the wood woodland edge, the National Roads Authority have found that birds and bats 'fly higher above roads when crossing between existing woodland on either side of a road carriageway' than when the motorway has lined hedgerows with a high or low wooded canopy. Moreover, although most motorways should be predominantly straight sections, if curved trees and shrubs 'should be set back' to discourage wildlife crossing and improve visibility for motorway users.
(National Roads Authority, 2006, p.28)
Hawthorn or white thorn was planted in hedges throughout our countryside. Its sweet smelling 'May' blossom is a feature in that month, and in autumn and winter the deep red haws colour the bare twigs. They are among the berries most favoured by birds. Only untrimmed hawthorn can flower and fruit freely, but hedges have to be cut to keep them stock proof. Hawthorn hedges may be trimmed regularly, or left for several years and then laid by cutting part way through the main stems and laying these horizontally through the hedge. Even old hawthorn hedges will regenerate if trunks are cut back to base and left to sprout again. Like many other shrubs, hawthorn also grows in woodland where there is enough light - in open glades, along 'rides' through the woodland, or along the edge. A single tree may be left in a field as a 'fairy thorn', especially where there maybe an archaeological site.'
(The Tree Council of Ireland, 2008, P.42)Shrubs add an important feature to the woodland edge planting. Shrubs not only benefit the environment ecologically but also aesthetically as they can function to soften the appearance of a woodland edge and provide form. Crataegus monogyna (Hawthorn) Rosaceae (family), vastly seen planted throughout Ireland is a good example of low growing motorway shrub that acts this way.
Trees and shrubs can function to break the emphasis of the motorway corridor, soften hard motorways and barriers but intermittent planting also benefits biodiversity conservation rather than mass planting of non beneficial plant species. Also, the greater the variety of plant species, the greater the variety of wildlife which acts as vital nature reserve for them to travel along.
The variation of high canopy trees and low canopy trees species along a motorway is of relevance to bird species as some prefer to nest at different levels: 'Chaffinches and Greenfinches like to nest well above ground level, while Wrens, Robins and Dunnocks are happier lower down.' (Donegal County Council, 2004)
In the creation of a woodland edge, Acer campestre (Field Maple) Sapindaceae (family) provides us with autumn colour when lined with shrubs through intermittent planting. The shrubs can also provide a beautiful array of flowers in spring and summer which are not only beneficial aesthetically but influence insect pollinators. Commencing with Blackthorn in April Hawthorn which is then followed by Gorse, Cherry, Plum, Crabapple, Rowan, Elder, Guelder Rose, Honeysuckle, Wild Rose and Bramble. After flowering, most of these plants reach their fruiting period all together, just when bird populations are at their highest, and all need the fruit and berry feast to build them up for the winter. (Donegal County Council, 2004)
'Perhaps the best known and most widely
distributed of our native shrubs, gorse is also
known as whin or furze. There are two types, the
common or European gorse, and the western or
mountain gorse. The common gorse is a very
suitable shrub component along the edge of new
woodland, and also makes an excellent hedge.
Gorse is well known for flowering almost all the
year round, and its spiny 'leaves' are evergreen.
Gorse supports many insects and spiders, which
in turn provide food for small birds, which may
nest in the excellent shelter provided by these dense spiny bushes. It is often under-estimated as a wildlife resource.'
(The Tree Council of Ireland, 2008, P.40)
'A climber rather than a shrub,honeysuckle is a common component of native deciduous woodlands. The heads of pink and golden trumpet shapedflowers have a powerful sweet scent attractive to moths,which take the nectar. Theflowers mature to bright red berries that are much enjoyed by birds such as coal tits. Honeysuckle may be grown on a fence or over dead timber, or up the wall of a building with the help of wire supports. It may be cut back and trimmed hard in a hedge without ill effects. Honeysuckle will grow from berries and also from cuttings. Best of all is to 'layer' a branch i.e. peg it down into the soil while still attached to the parent plant - it will sprout roots and may then be cut off and transplanted.'
(The Tree Council of Ireland, 2008, P.43)
However, a major wildlife concern that should be noted along motorways with berry plants is that they can attract wildlife which can be dangerous at such close distances to fast moving traffic so careful planting and selection should be considered. An additional concern would be the planting of Gorse in areas that they prone to fires.
The National Roads Authority (2006, pp.10-11) have a policy setting out the selection of native species of non native as they promote conservation of biodiversity and give the landscape a unique character of Irishness while also reducing the possibility of diseases from foreign non native plant species.
Moreover, due to the small percentage of native plant species that remain in Ireland, it is important to encourage their use in landscaping of motorways and their encouraging the fauna protection via a symbiotic relationship.
Native species have been proven to be more beneficial for wildlife as they have lived in equilibrium for thousands of years whereas wildlife has found it difficult to adapt to introduced non native plant species Moreover, when selecting species of trees to plant on motorways, it is important to understand that some trees are slow growing and although Oak is a native tree and provides important feeding station for insects, the length of time it takes to grow should be considered as faster growing species such as ash may prove to be more beneficial on a larger scale planting along motorways due to their considerable growth time difference. (Boylan, C. 2012)
As we have seen, motorway planting involves trying to blend the new roads into their existing landscape. Another landscape planting that can take place to achieve this is grassland planting to match open green belt environment or more pastoral surroundings. The National Roads authority emphasise the importance here of sourcing indigenous seed and 'robust low-maintenance grassland treatments' and 'While Red Clover may be included in the grass seed mix, incorporating aggressive leguminous species such as White Clover (common in many grass seed mixes) should be avoided, as these will facilitate an increase in soil fertility, thereby increasing maintenance inputs including the need for more frequent mowing'
(National Roads Authority, 2006, pp.79-81)
On steep embankments, hydro seeding is used to spread grass and flower mix seed. However, with clay and sandstone present the soils will be lighter and the gradient cannot be too steep.Here the roots of plants form and important function in holding the structure of the embankment together. While Clover forms and important species in grassland seed mix that for form as a nitrogen fixer. (Hedgerowmobile, 2004)
Grassland also has an important role in nature conservation and natural re colonisation of grassland species also hence the choice of indigenous seed sourcing. The images contrast the grasslands choices and their results.
'Rank, coarse grassland is typical of much highway land and stands out in marked contrast to the surrounding agricultural land, rather than blending with it'
'Good practice: M6, Cumbria Apt use of low-maintenance grassland on the right soil conditions has ensured a fit to the landscape, as well as providing nature conservation interest'
(Department for Transport, 1992, p4/1)
It is also important to note that in the development of an open grassland landscape along motorways, maintenance will be crucial. Scrub and noxious weeds an easily invade a grassland sward where wildlife have settled. It is important therefore to ensure that scrubs such as bramble, gorse, and hawthorn do not invade valuable grassland. (Department for Transport, 2004. p.5/12-13, p.5/31)
Such scrubs form important wildlife habitats but it is necessary to ensure that the do not spread into other wildlife colonies. Careful planning and maintenance can ensure that this does not occur.
As an alternative to scrub planting in this area, large canopy trees can function to break up the emphasis of mass planting and can provide form while also allowing more diversity for wildlife and provide a framed view for the driver on the road corridor. (Department for Transport, 2004. P5/31). Species of large canopy trees here could include Fraxinus excelsior (Ash), Quercus robur (Oak), Pinus sylvestris (Scots Pine) and Quercus petraea (Sessile Oak).
On the contrary, the central median of motorways is not of importance to wildlife protection and conservation. It functions more of a division between laneways but also can have aesthetic visual impact on the motorway user. Bulb planting here can be a beneficial means of achieving good visual quality while reducing the risk of this dangerous area becoming a habitat island. Considering the location, bulbs will be low maintenance and soften the opposing hard road surface. Moreover the National Roads Authority advise against the use of berry producing shrubs and plants due to their potential appealing nature to wildlife posing a risk to potential wildlife considering their closeness to fast moving vehicles and machinery. (National Roads Authority, 2006, pp.55-56)
As an alternative, examples of shrubs that could be planted along the central median include Corylus avellana (Hazel) and Salix spp (Willow) as they do not grow to great heights or width so less thinning would be required in maintaining them.
(Department for Transport, 2004, p.5/28)
'There are several varieties of willow native to Ireland. All grow in damp soil, have
catkins or 'pussy willows' that produce seeds, but are most easily grown from cuttings,
which root very readily. The most widespread willow species are the goat willow, the rusty or grey willow (both known as 'sallies'), and the eared willow. While these generally grow on damp ground, the goat willow will also colonise rough and disturbed ground in drier areas. Willow establishes easily by wind blown seed and can also be propagated by taking cuttings approx 8 inches long from stems between half an inch and one and a half inches during dormancy, which are simply pushed into the soil to a depth of 4 inches max.'
(The Tree Council of Ireland, 2008, p.31)
Therefore to conclude, we can see that vegetation on motorways provides us with an opportunity to replace important habitats for wildlife and also provides us with the task of integrating the motorway into the existing landscape where a mix of local native species provides visual interest while also benefiting wildlife biodiversity. Motorways can be a major geographic feature of the landscape if properly maintained and if the suitable vegetation is selected when created these corridors for both wildlife and road users while they continue on their journey.
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Article name: Native Plant Selection And Plant Re Colonisation Environmental Sciences essay, research paper, dissertation