Adding Diversity to Grassland and Other Habitats
Clearly the more diverse and varied your site is the greater the diversity it can support. Sites with wet ditches and dry banks, with open areas in full sun and shaded margins, will have a greater overall potential and can be sown and managed in different ways. If you have an opportunity to make provision for this by planting trees or creating or enhancing features like ponds or wetland areas this will be of benefit overall. As described in the section on soils the productivity and fertility of your site will have an over-riding influence on the diversity your site can support. The discussion in this section however will assume that the soil character, potential and limitations are ‘given’ and will focus on how to get the best from each site.
Good management that follows ecological principles to work with nature is the best route to obtaining good and sustainable results. Elsewhere we describe how to best match your sowings and expectations to your site conditions, and practical ways to achieve this. The following note discusses how and why these methods are important to maintaining plant diversity.
The act of introducing species to a site by sowing seeds is a way of assisting the dispersal and regeneration of diversity. Without this assistance many species may never arrive, or arrive only by a chance event and probably too late to have much chance to establish as ground cover is already complete. Natural regeneration by naturally dispersed seeds or from the soil seed bank is often suggested but for many sites this approach has limitations.
Natural Seed Dispersal
With the exception of wind dispersed species like dandelions, willowherbs and orchids the dispersal rates of most species is very slow: only a few metres per year. Additionally, after a century of agricultural intensification and urban development, many wild plant populations are now very isolated or remote from new sites and seed is prevented from moving even slowly by distance or sterile barriers of crops and tarmac.
Soil Seed banks
Many common weed species are adapted to exploit regularly occurring gaps in disturbed or cultivated habitats. These species overcome the limitations of dispersal over distance by having a persistent bank of seeds in the soil available whenever a suitable gap opens. These species may need no assistance and will naturally regenerate unless they have been absent from a location for many generations.
Species of more stable communities like grassland and woodland in which disturbance opportunities are infrequent tend not to invest resources into viable seed banks and rely more on recent seed fall from plants close by to colonise gaps. Where a plant species is abundant in a locality and sets good seed it may colonise naturally, but for many locally scarce species some assistance is required.
Introducing seed or moving it from one part of a site to another is a good way of overcoming natural barriers to colonisation (or at the very least speeding the process up). As the establishment of new plants from seed, particularly amongst established vegetation, can be rather hit and miss it pays to consider adding seed or plants over a period of years to build the success rate and diversity rather than rely on all species being established from a single application. This is perhaps even more important with smaller sites where it is easier to lose a species altogether from the site by the loss of a few individuals.
Create Gaps and Establishment Opportunities
Once seeds have found their way on to a site the young plants that result will need the space and time necessary to establish. It is important to provide this both when re-seeding a bare site and periodically over time as not all species are long-lived and will need to re-establish replacement plants from time to time.
For new sowings it is important to have a clean seed bed at the point of sowing to give all sown species the best start. When sowing a mixture it is also important to apply the seed at a low enough rate so that each seed has both space and time to get established without too much interference and competition from its neighbours. In practice this is quite tricky to arrange as some species will establish very quickly and others slowly: our recommendations for optimising this can be found as part of our mixture specifications.
For established sowings a managed degree of disturbance and gap creation is important to maintain diversity and continuity by allowing re-establishment of existing species as well as colonisation by new species. Each time a gap is produced the particular combination of gap size, season and weather conditions will be different, and as a result the plant species which are able to exploit each opportunity will be different. This diversity of opportunity helps develop and maintain plant species diversity over time.
Annual wild flowers require large gaps that result from extensive cultivation or disturbance. Many will not even germinate unless they have open ground.
Grassland species can establish in more modest gaps as created by hooves of grazing animals, by rabbits and moles, by deliberate or accidental manmade damage such as harrowing or vehicle damage and from die back of plants (yellow rattle in particular, being an annual, is useful in this regard).
In woodland establishment opportunities arise on a large scale from gaps when a tree falls or woodland is coppiced and on a smaller scale from disturbance or leaf litter accumulation.
Practical ways of maximising these opportunities are suggested within the notes on preparation and on management for each plant community type.
Restrict Species Dominance by Management
In theory if two or more plant species grow in close proximity in competition for the same basic resources of light, water and mineral nutrients in a stable environment, without disturbance or interference, eventually one species will emerge as the best adapted to the conditions. Species diversity will fall as a consequence leading ultimately to a community dominated by one species.
In the natural world there are no completely stable uniform environments and other influences such as animals, disease and humans periodically interrupt or prevent any one species attaining dominance. The tendency however exists to some extent in most situations and needs to be managed. The ability of plants to move in response to competition is limited so competition between plants is typically exerted in two main ways.
Firstly by expansive growth, increasing in size and spread, out growing and crowding out weaker neighbours, and secondly by deposition and accumulation of persistent leaf litter which suppresses establishment and growth of potential newcomers.
In practical terms to maximise diversity managed interventions are used which interrupt the successional processes of potentially dominant plants. The type and timing of these interventions will be aimed at having the maximum impact on the vigour of competitors and the most benefit to diversity. In grassland for example mowing or grazing to remove the expansive growth of competitive tussocky grasses or invading trees and scrub, and removing or preventing the accumulation of litter at the base of the sward, both help prevent loss of diversity and interest. These principles underpin the management guidance we have outlined for each habitat type.
Diversity of approaches
Diversity can also be encouraged through varied management approaches within each zone.
In grassland for example cutting date and frequency influences the balance and composition of the sward. So, rather than cut all of a meadow in one operation mow in sections at different times through the season, vary the cutting from year to year, and leave some areas uncut in some years.
It is always worth keeping in mind that the precise conditions and timings for optimal management in a particular site can be very hard to pin down and be prescriptive for. The ‘best’ approach will vary from site to site and will change with the weather and seasons. A varied approach and experimentation with management of your own site, together with observation and taking notes, is a very rewarding way of finding out and reviewing what works for your circumstances.