The influence of mowing and other disturbance

Not mown at all

There are pockets of land in the urban fabric where for various reasons no mowing is done. If the grassland community is established before woody plants invade, and the soil is fertile a dense grass mat will develop that can be resistant to further succession. These grasslands can stand for years before any significant woody plant colonization occurs.

The most effective eventual colonizers of these mats are frequently shrubs with heavy animal dispersed seed such as elders (Sambucus nigra) and especially those that can also spread vegetatively, such as brambles (Rubus fruticosus). As these grow and shade out the herbaceous mat they can create colonization opportunities for other woody species that have shade tolerant seedlings, such as oak. Other competitive species can become significant components of the sward that develops, including those that spread by wind-blown seed and that form strong aggressive clumps when established such as Aster spp., Urtica, or Aegopodium.

Where the substrate is made up of chemically or physically extreme materials more unusual plant communities can arise. For example substrates that are made up of large particles of acidic (or at least neutral) stone can become colonized by dense stands of foxglove (Digitalis purpurea) and evening primrose (Oenothera biennis). These short-lived plants gradually disappear as organic matter builds up and grasses establish enough to close regeneration niches.

Influence of periodic mowing

Grasslands that have periodic rather than regular mowing will support different species communities. The periods without cutting can provide opportunities for flowering and reproduction. The herbs that form large populations on such sites include mouse-ear hawkweed (Pillosella officinarum), Primula spp., or Lady’s Smock (Cardamine pratensis). Sometimes more adventurous landscape managers will experiment with relaxing mowing at different times of year to favor peak flowering of other grassland species such as knapweed (Centaurea nigra) and yarrow (Achillea millefolium).

Influence of regular mo wing

Experimentation is rare, and most urban grasslands overseen by adequately funded land managers are mown on a cycle of 10—14 days and cut to a height of 10—15 cm, The swards can look devoid of broadleaved species at first glance but they will have diverged away from a monoculture of the grasses that were sown and there will be many prostrate and rosette forming herbs that tolerate close cropping (Thompson et al. 2004). The species typically found include white clover (Trofilium repens), daisy (Bellisperennis), creeping speedwell (Veronica filiformis), selfheal (Prunella vulgaris), dove’s foot cranesbill (Geranium molle), creeping buttercup

(Ranunculus repens'), and creeping thistle (Cirsium arvense). In some cases populations of herbs can persist that are surprising in their tolerance of close cropping such as Primula vulgaris and even the common spotted orchid (Dactylorhiza fuchsia) .These eventually coalesce to something that superficially has the visual uniformity of a sown grassland, but is likely to remain much more diverse.

Influence of herbicides and fertilizers

The common assumption that the green expanses of urban rangelands are sterile spaces arose partly because they were dosed regularly in herbicides by the parks department. Biocides, herbicides, and pesticides are different to other forms of disturbance that destroy biomass. They may have selective or broad spectrum impact meaning that the changes in the receiving community can be complex with many interactions. Some herbicides only destroy aboveground green tissue of plants — they are approximately akin to chemical mowing — and others kill the root as well and have an impact very different to mechanical disturbance. Although this may have been a common practice in the mid-twentieth century, today this is less usual. Adverse public reaction to spraying is one reason, but probably the most significant cause of a change in practice has been budget pressures — removing weeds from low quality grassland is of little interest to local governments compared to saving money. Most greenspace managers have little reason to create species uniformity in extensive grasslands as long as the visual appearance and functional characteristics are adequate.

Herbicides are often still used around the bases of street furniture and sometimes trees, in locations that mowers cannot access easily. The use of well-chosen herbicides at tree bases is less likely to harm the tree than the use of strimniers, as strimmer damage to tree bark can be lethal.

Sward diversity may also be reduced by high soil fertility because it favors aggressive and competitive plants. Again, however, it is increasingly rare to find managers fertilizing urban grasslands. The extensive urban rangelands that this chapter addresses rarely see any such inputs. However, high fertility can arise from topsoiling with nutrient rich soil. The disturbance caused by soil lifting and re-spreading can release a nutrient flush that increases the likelihood of a dense competitive sward being established. In patches, fertility will also be increased by excreta from dogs or other animals.

Sometimes where soil nitrogen is low, but other resources, especially phosphorus, are adequate, a population of nitrogen fixing herbs gets established that can form a major component of the sward. This is much more likely where seed of these herbs has been sown, but even when establishment is patchy because of natural invasion, individual plants of species such as Trifolium repens can expand to form a colony several meters across.

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