Natural colonization

One of the major factors controlling the composition of the grassland is whether it is sown or turfed or develops only though natural colonization. The general trend for sites that vegetate through natural invasion is that the early successional stages are dominated by wind dispersed mobile and usually ephemeral plants such as Matricaria chamomilla, mixed with a range of the plants that emerge from propagules contained in re-used and displaced soils, mostly grasses but also perennial forbs, like rumex and convolvulus arvensis. The resulting communities can be highly variable with factors such as the nature of the substrate and the distance from sources of colonization having a major influence on what develops.

These ephemerals usually give way to taller herbaceous clump forming species, in turn giving way to grasses where, as regeneration gaps close, the broadleaved components maintain a presence by vegetative spread more than reproduction from seed. However, there are many exceptions to these patterns; for example it is common to find that scrub species can colonize in the earliest stages alongside annual and perennial herbs. Gilbert (1989) reviews many of these communities in detail. Here are a consistent set of species that become familiar to urban naturalists and also (if not nameable) to those children who enjoy free range play on urban grasslands. Mostly these arise from wind-blown seed or from the soil seed bank, while some of the colonists are garden escapes or their descendants, such as Michaelmas daisies (Aster nova-belgii or hybrids), and goldenrods (solidago species such as S. canadensis). Evening primrose (oenothera) is another frequent colonist especially of rocky ground. Amongst early shrubs, goat willow (Salix caprea) and butterfly bush (Buddleia japonica) are the typical first colonists.

Pockets of redistributed topsoil from other sites can bring propagules of persistent perennial garden weeds that spread from root fragments such as nettles (Urtica dioica), couch grass (Elyinus repens), ground elder (Aegopodium podagraria), and horsetail (Equisetum amuse) or even Japanese knotweed (Reynoutria japonica). The likelihood of these perennials persisting in their new location depends on levels of disturbance — if they are in the path of regular mowing or strimming they will disappear sooner than if they are located on site fringes, or grow amidst piles of rubble that prevents the mowers from getting to them.

Around the edges of these sites and especially adjacent to car parks, piles of stony gravel and rubble are often dumped — these develop their own xerophytic communities. The plants found there may include species that have adaptations for surviving drought and low nutrient availability. Most typical will be legumes (such as Trifolium repens or Lotus corniculatus) as well as stress resistant species (such as sedum alba and rumex acetosa). Species of coastal origin seem particularly well adapted to these soils such as bucks horn plantain (Plantago coronopsis), small toadflax (Chaernorrhinum minus), or herb Robert (Geranium robertianum). On a more fertile imported and spread topsoil, a more typical collection of grassland forbs begins the succession — Rumex and Taraxacum come early rapidly joined by grasses.

Winged herbivorous invertebrates will be rapid colonists, especially those that feed on early colonizing plants such as the cinnabar moth (Tyria jacobae) that feeds on ragwort (Senecio jabaea). Adult stage butterflies can become abundant quite quickly, feeding on some of the flowers on site or in adjacent gardens, especially appreciating the late summer flowering plants like Aster.

The speed with which they appear depends on the wider metapopulation in the area living in gardens and other green spaces nearby.

Spiders are usually amongst the earliest colonists, including money spiders (Linyphiidae) that can blow in and also larger species such as zebra jumping spiders (Salticus scenicus) that are so widespread in gardens that they have many sources to colonize from. Spiders and beetles clearly appreciate the open but complex physical structure of rubbles and stony ground — situations that are becoming recognized in the UK as Inland Kock habitats (Tucker et al. 2005). Slugs and snails can come in rapidly as well, depending on the proximity of nearby colonization sources, probably gardens, and also pockets of garden soil incorporated in the new habitat. Pockets of buried organic matter may support high densities of decomposer organisms.

Highly unusual species assemblages have been found in these early successions, especially of invertebrates, and some of these have been regarded as being of conservation significance (Eyre et al. 2002). However, we have inadequate baseline data to evaluate the real scarcity of these species, nor do we understand their dispersal patterns well enough to judge how far they travel. The identification of ephemeral colonizers of disturbed land that appear to be rare causes a conservation conundrum. Should resources be dedicated to maintaining their survival? This could be achieved by, for example, periodic re-disturbance of early successional sites, as is sometimes proposed (Gemmell and Connell 1984). It is hard to find examples of this approach being initiated and maintained except on small scales within urban nature parks or similar places where environmental education is focused that justifies the effort (Emery 1986). Normally no local government is likely to spend money on such arcane land management.

Lichens and mosses are also rapid colonists, especially calcicole species that appreciate the calcium, magnesium, potassium and other minerals found in rubbles. Every layer rapidly develops a food web that often goes unnoticed. For example Gilbert (1976) showed how a simple concrete post supported lichens such as Lecanorion dispersae, which were grazed on by mites, Ameronothrus maculatus which in turn are predated by bugs Temnosthethus pusilltis, all of which ultimately fed a decomposer community of collembids. Soil invertebrates may colonize from patches of existing undisturbed soil, or arrive rapidly, usually as eggs carried in soil particles by birds and other mobile animals. If the site has had any plants introduced from nurseries, these will also usually carry soil and root associated biota with them.

Larger animals will be present already as a metapopulation on a patchwork of sites across the area; colonization therefore depends on their mode of dispersal, the presence or absence of intervening barriers, and the existence of appropriate food and shelter on the receptor site. Once vegetation cover is established, the grasslands may rapidly lose the open niches that support these early invertebrate colonists, but they do then become home for burrowing invertebrates including sometimes large populations of those that feed on plant roots. In a garden context this latter group may be regarded as lawn pests and controlled by insecticides, but in the less manicured settings of urban rangelands they are frequently unnoticed. The best known in that latter category are 'leatherjackets’ or crane fly larvae (Tipula paludosa) that can overwinter in large numbers underground. When hatched they provide a bounty for birds and bats and a focus of fascination for urban children. As the site ages and higher levels of organic matter begin to accumulate, more resources are available for decomposer organisms such as earthworms (Luinbriais terrestris).

Another burrowing invertebrate group is the mining bees such as the Tawny Mining Bee (Andrena full'd) which are found in short turf, especially where the ground is warm and dry. Each of these species can provide food for others — disease organisms, parasites and predators such as the common bee fly (Bombylius major). These invertebrates in turn provide food for the birds that use the grasslands.

The colonization and distribution of fungi in urban grasslands of this type is poorly understood. Mycorrhizal species that are associated with transplanted trees are often found fruiting in the planting beds and adjacent grasslands. It is highly likely that transplanted trees and shrubs carry the associations with them when they are introduced, and mycorrhizal colonization, perhaps aided by animals, seems to be pretty rapid. Where there are scattered young trees, the exposed nature of the landscape often means that fallen leaves are dispersed and do not accumulate (except in shop doorways), but where the trees are grouped closely with shrubs an organic leaf litter can develop that becomes home to diverse decomposer species.

Even away from woody plants fungal fruiting bodies are still found, such as inkcaps (Coprinus spp) and field mushrooms (Agaricus campestris) that sometimes appear in open grassland; presumably these only exist where they find pockets of enough organic matter to feed on. They are usually localized and may come from mycelium deposited in redistributed soils rather than colonizing from spores.

The extent to which wild herbaceous plants in urban locations are dependent on obligate symbiosis with mycorrhizal fungi is not well determined. However, there are some species for which this is known to be essential — especially terrestrial orchids such as Dactylorhiza spp. and the occasional huge populations of these plants that can develop on post-industrial land (Ash et al. 1994) hints at the possibility that there are unusually rich colonies in such soils of fungal species.

Sown or turfed grasslands

There is a limited range of turf grasses usually sown for amenity and sports use. Lolium perenne tends to dominate because it is competitive and many landscape managers appreciate its reputation of tolerating wear, but it can have a higher maintenance demand and is less tolerant of stress conditions than Agrostis or Festuca In situations where soil conditions are not favorable it can fail to maintain its dominance and be overtaken by fescues and bent grasses. More sophisticated assessment of conditions of redeveloped areas usually encourages a choice of seed mixes dominated by fescues. In situations where low nitrogen levels are anticipated then white clover (Trifolium repens) is included in the mix to improve the density and appearance of the sward.

Planted trees and shrubs

New tree and shrub plantings around and in these grasslands reflect the prevailing fashion in landscape design. An early to mid-twentieth-century preference for exotic diversity and flowering and fruiting shrubs gradually moved to favoring a dominance of monocultures of evergreen species such as Aucuba japonica or Primus laurocerasus, which were believed to have a low maintenance need compared to the long season of‘interest’, with the dense and year round shade theoretically helping with weed control. These have in turn fallen out of favor, being recognized as having in fact a long season of no interest and that, weeds aside, they become dumping sites for an understory of trash. Current fashion favors deciduous species with a much greater focus on native plants, such as alders (Alnus glutinosd) and birches (Betula pendula), both of which tolerate very poor quality soils, together with those trees that have a long history of success in urban conditions like sycamore (Acer pseudoplatanus).

Young trees and shrubs support a less diverse range of organisms than a mature, or especially over-mature, specimen of the same species would. Unsurprisingly the missing groups are decomposers and species that find shelters and niches within the increasingly complex architecture of an aging tree. However, those species that can exploit young trees can be present in high numbers and contribute to a food web that may be depauperate compared to older habitats but still important. A successful group are leaf and young stem feeding invertebrates such as the sycamore aphid (Drepanosiphumplatanoidis) (Dixon 1973). These in turn feed predatory species — birds and other insects such as ladybirds (Adalia bipunctata), lacewings (Chrysapa spp.), and hoverflies (Syrphus spp.) as well as parasites such as the wasp (Aphelinus Jlavus). Ants feed on the honeydew residue and sooty molds develop and the web of life grows.

< Prev   CONTENTS   Source   Next >