Ecological factors that determine the nature of plant communities
Three main types of factors determine the initial nature of the plant and animal communities found in urban rangelands: soil and climate (both of which are environmental factors); sowing or invasion (colonization processes); and mowing (rarely grazing) or even soil upheaval and trampling (disturbance factors).
The influence of soils
The soils of urban areas can vary enormously. They encompass any of the typical soil types found in rural areas, as well as substrates derived from building rubble and post-industrial wastes that can show extreme chemical and physical properties (see Chapter 17). Not surprisingly the plants and animals that thrive in these different materials vary greatly. Some sites have substrates derived from materials that are the legacy of industrial activities. The chemical and physical nature of these materials can give rise to unusual plant and animal assemblages. The longer sites take to revegetate, the more diverse and unusual are the flora and fauna that develop (Ash et al. 1994). It is believed this reflects factors of stress and disturbance that limit aggressive competitive species, allowing a more diverse community to establish because less competitive species can survive and those adapted to high stress are also favored.
Not all rangelands have undergone disturbance, but on redevelopment sites, the soil will have undergone such acute disturbance that it affects the physical and chemical character of the substrates. Where soils have been moved and re-spread during demolition and construction work sudden boundaries may be created between different soil types. In some situations this abrupt soil physical change can contribute to the development of more diversity in the plant and animal communities — for example, perched water tables may form that could create boggy areas. However, unless it is extreme, this variation is also often masked, especially if a layer of uniform topsoil has been spread at depth, competitive grasses are sown, and uniform mowing applied.
It has been well established that it is easy for vegetation to grow on crushed building waste from demolition sites. Rubble and finer material with a high proportion of mortar are free-draining and alkaline, rich in some nutrients but low in organic matter and hence low in nitrogen. This can be rectified, if necessary, by the use of fertilizer or by sowing nitrogen-fixing species such as clovers (Bradshaw and Chadwick 1980). Such amelioration is often necessary to establish successful amenity plantings, but for naturally colonizing communities it is not needed as the species suited to the prevailing conditions will be the ones that establish and form the dominant community.
These soil materials have characteristics that are not very different to some natural substrates such as glacial tills, so there are species that show adaptions for the conditions. In these settings the growth of smaller herbs and grasses is partly determined by the percentage of fine particles, but for trees and woody plants a blocky open substrate even with extensive voids, is a perfectly acceptable rooting medium; they find rumbles easier to grow in than clay rich soils that have been heavily compacted and denatured by construction work.
Extreme soil compaction is common on demolition and construction sites, and the most fertile soils in their natural state, such as clay loams, can become the most hostile to plant growth and to some soil fauna, once denatured. They exhibit problems of drainage, poor aeration (which can happen even on dry soils), and even the physical impedance of root growth.
Depending on topography and climate these patches are either seasonally or almost permanently wet. The grasslands can develop patches of distinctive communities that include higher density of creeping buttercup (Ranunculus repens), docks (Rumex spp), and different grasses to those normally found, such as Yorkshire fog (Holcus lanatus) and rough-stalked meadow grass (Poa pratensis). Soft rush (Junicus effuses) may also establish although unless the site is permanently wet and fertile, the individuals often remain dwarfed and may not reach flowering size. In some cases extensive mats of moss dominate the sward (e.g. Brachythecium rutabellum).
Where soil movement by machinery has taken place, compacted layers that impede drainage may not be on the surface but may be at depth when soil has been redistributed on top. A second, less severe but more widespread factor affecting drainage is surface compaction from vehicles and also from people walking, especially along ‘desire lines' — the name given to the preferred routes that people use to cross open space, when the designed paths do not go where they most often want to go. The species that do well on trampled ground must have some tolerance of poor drainage and low aeration in winter but must also contend with drying out and poor water infiltration in summer, as well as show an ability to survive physical abrasion and crushing. These often include creeping bent (Agrostis stolonijera) and plantain (Plantago major).
Another typical chemical problem is contamination from heavy metals and hydrocarbons dumped or spilled onto soil or aerial deposited pollution (which is becoming less severe now in post-industrial societies) — these can make soils toxic, but unless the toxicity is extreme, the effect is rarely lethal and mostly shows as a decrease in vigor and density of cover. Hydrocarbons gradually oxidase or are digested by soil microbes; heavy metals are more persistent but the severity of heavy metal toxicity is largely dependent on pH and the presence of binding agents — organic matter and phosphate ions (Bradshaw and Chadwick 1980).