Creating landscapes as natural systems

A summarised extract by Leonie Alexander

This appendix describes the natural characteristics of plants and plant communities and gives a brief insight into the basic requirements of plants, how they interact in communities and how they function as part of an ecosystem.

With this knowledge the designer can analyse the site conditions and use the predicted outcomes in terms of species and plant communities and how they develop over time to steer the design process. This will result in roadside landscapes which have the dual benefit of being visually attractive and good for wildlife. Ultimately what survives on the ground is the combination of species best suited to the conditions and, given sufficient time, the initial vegetation of any scheme will come to reflect its environment and management. The 'landscape' is not just the finished product as envisaged by the designer but it is the whole process of change from bare ground to self sustaining plant communities. The designer should harness this quality of change and control and direct it to a desired outcome.

Wildlife throughout Scotland is increasingly pushed to the edge. Roadsides offer an opportunity to make a positive contribution to local biodiversity by encompassing locally diverse ground conditions, linking up and expanding existing habitats and copying with new insight what nature achieves on its own. New roads tend to include substantial cuttings and embankments because of the constraints of road design. These are essentially dry slopes often flushed at the bottom, with different aspect and different substrate. In Scotland, approximately one third of roadsides are flat or gently sloping and two thirds are steep cuttings or embankments. A range of habitats can potentially be created including woodland edge, mixed scrub, grassland, wetland, open water and acid heath.

In summary, for the Learning from nature approach to be successful, a proper understanding of the physical environment of the site above and below ground, its context, the effects of management and the availability and suitability of species will be essential. In essence making the best use of the natural resources available demands a move away from formula planting where a standardised substrate is created and non specific species mixes planted.

1. Basic plant requirements

For plants to spread, three conditions must be satisfied. The physical conditions of soil and drainage must be suitable, the degree of competition from other plants must be tolerable and the propagule must be able to get to the site. These issues are dealt with in the three sections - soils, competition and plant dispersal and colonisation.

Plants do not just grow in groups or communities, they grow in particular places where they are influenced by the physical environment and by other organisms. In natural conditions, different species occupy different habitats reflecting their different ecological preferences. Some species are tolerant of a wide range of conditions such as common oak and sheep's fescue but most species have narrow ecological tolerances and can therefore only grow and persist in a narrow range of habitats.

There are two categories of external factors which affect plants. The first is stress and includes anything which restricts photosynthesis like shortage of light, water or mineral nutrients. The second is disturbance which results in the partial or total destruction of the plant biomass. Three basic plant strategies have evolved in response to stress and disturbance and species can be categorised as having characteristics of competitors, stress tolerators or ruderals.

Competitors exploit conditions of low stress and low disturbance. Characteristically they have high potential growth rates, tall stature and a tendency to form patches by spreading vigorously above and below ground. Typical examples would be rosebay willowherb or tall herb communities growing in rich fertile soils including valerian, angelica, marsh thistle and meadowsweet.

Stress-tolerators are associated with high stress and low disturbance where productivity is low and where resource availability is brief and unpredictable. These species frequently invest in strong anti-herbivore defences to protect the foliage and include slow growing trees such as sessile oak and beech and woodland herbaceous plants like dog violet and wood sanicle.

Ruderals are characteristic of low stress and high disturbance and these species are typified by a relatively high growth rate during the seedling phase and the early onset of reproduction. Species which can sustain frequently and severely disturbed habitats such as arable weeds and road margin species fall into this category and include groundsel and chickweed.

2. Soils

The factor which fundamentally drives the development of the landscape is the nature of the substrate. On existing Scottish motorways, the variation in soils generally appears to reflect the locality, i.e. there is little imported material of distant origin.

In relation to road construction, the key point about soils is in differentiating the 'topsoil' from the 'subsoil' during the excavation phase and storing the two separately. The top layer or A horizon contains the seed bank, humus and most of the plant roots and rhizomes of the original vegetation. The second layer or B horizon is equivalent to subsoil and in most cases this is the desired substrate for establishing new vegetation cover with none of the problems of pernicious weed seed banks or high levels of nutrients which favour vigorous growth of a few species. The actual depths of the A and B horizon differ depending on the site and require to be defined on site and not rely on standard specifications.

The application of a thin layer of top soil (150mm or less) in areas to be planted with trees results in the trees rooting into the subsoil and the shallower rooting weed species and grasses benefitting from the richer superficial layer. The depth of topsoil on Scottish roads was found to range from 75mm - 300mm with typical depth of 150 - 300mm. Planted areas at the lower end of the range therefore will have resulted in slow tree growth coupled with enhanced competition from grasses thereby requiring application of herbicide. In some circumstances, the seed bank can be incorporated into the design if the route of the road cuts through important habitats. To ensure success, careful differentiation of the A and B horizons, careful storage and minimal storage times are required.

The key plant nutrients are nitrogen and phosphorus. Nitrogen is only stored in organic matter and released by slow decomposition. It is, however, highly mobile in the soil and can be taken up by the roots from a large volume of soil. In contrast, phosphate is immobile and can be taken up from only a narrow sleeve of soil around each root. Phosphorus is important in semi-natural systems as it is most often in short supply and therefore a limiting factor to growth even if nitrogen is abundant. On the whole, fast growing species require high nutrient levels and will not persist in low nutrient conditions and planting schemes can therefore take account of this by selecting species appropriate for the nutrient levels which occur. With the exception of very poor soils, low soil nutrient levels are generally associated with species richness and therefore with valued wildlife habitats.

The below ground structure of plants is of course as important as the above ground structure and this should be borne in mind when considering substrate treatment in terms of mechanical resistance to roots, aeration, fertility and moisture and the question of species selection. The preparation of the seed or planting bed is critical. Inevitably the construction phase of new roads or upgrading will result in compaction of soil which reduces root growth of trees and herbaceous plants. Substrates investigated on Scottish roads revealed a compacted layer at all sites tested. In almost all cases, ripping will be essential to aerate and prevent water logging.

Trees grow best where root penetration is good and competition is low and the addition of fertiliser does not generally improve establishment. They therefore grow perfectly well in subsoil or topsoils which are not over endowed with agricultural fertiliser. Less fertile soils have the added advantage of providing better opportunities for fungal growth particularly mycorrhizal associates of trees and shrubs and will not support weed growth with which young trees cannot compete. Another advantage of using subsoil is the lack of a seed bank of competitive species.

3. Succession

Succession is the directional change of plant communities for example from an area of bare ground to a cover of annuals followed by biennials to perennial herbs and grasses and on to pioneer scrub vegetation. Disturbance and the process of succession are part of the natural environment and the successional changes in plant communities over time should be encompassed as part of the design process. Road verges in some areas are subject to relatively frequent disturbance and this should be viewed as an opportunity to develop a mosaic of habitats in different successional stages. Succession is often simplistically thought of as abandoned grassland changing to scrub and then to woodland. It is in fact more complex. Rank grassland dominated by false oat grass for example, progresses very slowly to scrub particularly where a layer of leaf litter has developed.

4. Plant dispersal and colonisation

Evolution has built into all species a capacity by which they can be dispersed and can invade existing habitats. There are some basic rules - species which exploit scarce, scattered habitats have good powers of colonisation especially if those habitats are short lived e.g seasonal pools or bare ground. In contrast, species which have evolved in habitats that were extensive and effectively permanent have poor powers of colonisation as with woodland plants. There is little chance of a woodland flora developing on a roadside verge. 'Weeds' are characterised by specialised powers of dispersal and their whole biology is attuned to rapid invasion and multiplication. Other species invade by seeds which are adapted to allow survival and growth of the early seedling stages in the face of competition from pre-existing vegetation. As this is dependent on seeds which are heavier and less mobile, invasion is less predictable and it is true that species will not always respond to opportunities. A habitat may therefore be provided but many species will not colonise without assistance.

Successful colonisation of plant species therefore depends on a number of parameters - the acidity or alkalinity of the substrate, the soil structure and particle size, nutrient levels, toxicity, past working practices, the time since being exposed and which species colonise first. Some of these are predictable or measurable parameters, others less so.

5. Competition

Plants interact with one another in numerous and subtle ways. The most straightforward interaction is straight competition where two species make demands on the same resources and where there is insufficient to supply both. Competition below ground is well illustrated by grasses competing for water with newly planted trees. Where competition exists, the plant community is inherently unstable and will change until an equilibrium is reached.

Competitive exclusion is characteristic of sites which contain an abundance of resources with low disturbance. This could apply to roadside verges with relatively fertile soil where a few species adapted to these conditions will outcompete others and shade out gaps for regeneration resulting in overall reduced diversity. False oat grass dominated grassland is the classic roadside verge community where the grass dominates to the exclusion of all else.

Management of grassland limits the competitive ability of the faster growing grasses and more invasive species. If species rich grassland has been created then cutting is necessary to limit the growth of the first species to establish and restrict their ability to suppress the growth of those species which are still to germinate. Establishment of grassland on substrates with low fertility allows the co-existence of a large number of non competitive species as competitors are curtailed by the low nutrient levels.

6. Association

Plants are neither independent of one another nor independent of their surroundings. On the most fertile substrates with deep well drained soils and high nutrient availability, there will be a small assemblage of species with high potential growth rates which make efficient use of resources and which thrive at the expense of other less competitive species. In contrast to this in highly stressed environments eg on very shallow soils susceptible to drought, plants must be highly specialised to survive which results in a higher species diversity where no one species is able to swamp the others.

Associations of large numbers of species generally occur in areas of intermediate fertility where competitive species are kept in check by the environment and where niches are available for more narrowly adapted plants. At the design stage, species selection can therefore favour the spread of individuals of less vigorous species if aggressive species are omitted from the initial mixes.

Association of species in, for example, woodland, creates the structure of the habitat with shade tolerant species with low colonising ability as the woodland ground flora, a shade tolerant shrub layer and a light demanding canopy. These associations can be copied to create effective landscapes and rich, diverse habitats.

7. Natural characteristics

Diversity of species

The diversity of species is affected by locality, substrate and management but where climate and management are the same, there are fewer species on acid soils than on neutral and highest numbers on calcareous soils. Highly fertile substrates created artificially by agricultural practices and society are rare in nature.

Diversity of habitats

Ecotones are where one habitat merges into another with no harsh edges. These gradual transitions between habitats are better for wildlife than a sudden change. So for example, woodland merging into scrub and then long to short grassland is more valuable than a woodland edge with adjacent close mown grassland. These habitat transitions offer a greater number of niches and therefore a greater number of species are able to occupy them. Intimate mixes of vegetation structures are often very valuable. Wet grassland will have higher wildlife value if it is accompanied by a transition to drier grassland and associated with features such as permanently damp hollows and temporary pools.

Habitats which are generally not considered such as soft rock cliffs, dead wood, bare ground and river shingle are of great importance to invertebrates. Bare ground on almost any substrate will be used by a wide range of invertebrate species for basking, hunting, burrowing and nesting. Invertebrates are a forgotten group when it comes to making management decisions but their requirements can often be incorporated into design. The two most important factors for maintenance of invertebrate interest are habitat continuity and structural variation. Although diversity both between and within habitats is in many cases desirable, sites should be designed and managed for their strengths and not for diversity per se.

Rarity

It is possible to create suitable conditions for plant species which are becoming less common as a result of intensification of land management elsewhere. This can result in high nature conservation benefits as recognised in Holland where roadside verge grassland management practice is devised to encourage common and widespread plant species and butterflies which were recognised to be in decline. The majority of herbivorous invertebrates including most rare species feed on common plants and a site with large quantities of common plants with a history of sustained management and varied vegetation is a high quality habitat.

Corridors

The theory that plants and animals can colonise new sites along green corridors and that these act as general purpose conduits for a wide range of species is hard to substantiate. There are also claims that they could allow re-colonisation of sites where species for some reason have died out and that for animals they allow movement between separate areas in different stages of life and at different times of season. There is not much evidence of the use of corridors as thoroughfares but it is true to say that if the habitat is right suitable species may spread in. It is therefore the suitability of habitat that is more crucial than the fact that it is long and thin and theoretically links different areas.

Topography

Varied topography is vastly better for wildlife than averaging everything off. This applies to landscape scale topography and micro topography and the landscape designer should aim to work with what is there. Varied topography provides shelter and forces structural variation on the vegetation. It is likely to have areas of shallower soils with low nutrients and deeper pockets, south facing slopes which will be drier and wetter north facing slopes.

Biodiversity

Biodiversity describes all the living things that live in a particular area. The importance of the concept in terms of road verge management is to aim to achieve the best range of species for that particular locality. This will be done by encompassing diverse substrates both in terms of structure, aspect, slope and chemical composition. Where planting mixes are being devised biodiversity is addressed by ensuring that the species mix is representative of the best local examples in terms of species, their relative frequency and distribution. As different plant and animal species thrive at different times during the development of any ecosystem, a range of successional stages is beneficial. So if the whole roadside verge in any region is considered as 'the resource', different sections can be at different stages from bare substrate to well developed tree cover and consideration can be given to both the range of habitat types and the diversity within them.

Acknowledgements

This document has been produced by the Scottish Office Development Department
with assistance and advice from;

Steve Young, (AERC) ;

Mark Turnbull, (Turnbull Jeffrey Partnership);

Leonie Alexander, (S.W.T. Environmental Services);

John Aldhous;

Christine Bradley, (AERC);

Alan Macfarlane

& everyone who responded to the Review for Discussion.

For further information and contributions to develop the policy further, please contact:

Tim Lemon or Gareth Collins

The Scottish Office,

Victoria Quay,

Edinburgh,

EH6 6QQ

Tel. 0131 244 7211