Competition between plants and promoting diversity in landscape vegetation
Promoting diversity in vegetation is primarily about reducing the vigour of potential dominant species—it is simply not enough to include a larger number of species in a mix—that greater diversity of species has to be resistant to competition and elimination from aggressive species. Dominant species are those that, in the absence of constraining factors, tend to eliminate other species through competition, resulting in low diversity or mono-specific stands of vegetation. It is easy to think of plants as being essentially passive organisms, unlike animals that actively hunt and compete with each other for food resources. However, where resources are abundant, plants can be equally competitive, fighting for the same unit of water, nutrient or light, and often in an aggressive manner, moving both roots, shoots and foliage to capture those resources. In this situation, in the absence of constraining factors, the best competitor for those resources will tend also to be the winner in terms of space, eventually excluding less competitive species. This pattern holds for fertile, productive ‘high energy’ environments, but tends to fall apart when certain constraining factors are introduced to a habitat or ecosystem. It is therefore of great importance to understand what the constraining factors are that can increase the diversity of plant communities (through reducing the vigour of aggressive species), and equally to understand how to put together plant mixes with complementary competitive abilities so that no one species tends to eliminate all others. The most appropriate basis for our purposes to help understand how plants interact with themselves and with their environment in this context is Grime’s Plant Strategy Theory (CSR theory). The CSR model has proved to be a remarkably powerful tool for predicting how plants and other organisms react to changes within their environment (Dickinson and Murphy 1998). Whilst the model has been used in nature conservation management, there has been only very limited application to the functioning of non semi-natural vegetation (although, for example, see Hitchmough (1994)).
The basic starting point for CSR theory is that there are two fundamental sets of environmental threats that limit the growth and survival of aggressive, potentially dominant species: those that hinder the functioning of the plant, and thereby its growth rate and production of biomass, or those that physically damage or destroy plant tissues or biomass already present. The first set of threats is termed stress factors, involving constraints that affect the physiological processes of the plant. Such factors include extreme low or high temperatures, heavy shade, drought or low nutrient availability. The second set of threats is termed disturbance factors and include grazing, cultivation and trampling. Every habitat on the earth’s surface can be defined by the relative combinations of stress and disturbance factors that operate on it. Over the course of evolutionary time, natural selection has resulted in plants that grow in environments subject to such pressures developing adaptations that aid their survival and regeneration in those environments. What is remarkable is that unrelated species growing in geographically separated parts of the world show very similar responses to the same sorts of environmental pressures or constraints. Grime (1979) has identified three basic responses or ‘strategies’ for survival in environments that are subject to the various combinations of high and low stress or disturbance (Table 4.1)
The combination of low environmental stress and disturbance is characteristic of typical ‘productive’ conditions (i. e. where nutrients and water are not in limited supply and regular physical damage is rare) that encourage vigorous plant growth and the dominance of aggressive species that has been previously discussed. Such conditions may be found, for example, on abandoned fertile agricultural fields, old unworked allotments or gardens, or unmanaged productive grasslands: species that are well adapted to these environments tend to be tall herbaceous perennials, have spreading clonal growth and rapid summer growth rates. They are extremely effective competitors and tend to dominate vegetation, crowding out less vigorous species and resulting in low-diversity stands. Common competitors, or C-strategists, of northern Europe include rosebay willowherb, Chamerion angustifolium, and stinging nettle, Urtica dioica. In effect, the competitive strategy is to maximise the capture of resources (light, water and nutrients) and to invest these in further growth to capture still more resources.
Environmental stress and disturbance tend to limit the ability of competitive species to dominate. Restricted availability of resources (stress) prevents
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Table 4.1. Combinations of environmental stress and disturbance resulting in the three basic plant response strategies
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rapid growth (both in height and spread), thereby allowing species better adapted to growth under harsh conditions. Where resources are in very limited supply (i. e. in stressed environments), plants have evolved very different strategies. Rather than exhibiting rapid rates of growth, stress-tolerant species tend to be slow growing and evergreen, with specialised physiologies and often with modified protective tissues. Vegetation tends to be unproductive, relatively sparse and with low biomass. In such ‘low energy systems’ (Dickinson and Murphy 1998), plants tend to reproduce primarily through vegetative growth rather than by seed. In effect, the stress-tolerant strategy is one of thrift: to make the most of captured resources by sitting tight rather than investing in rapid growth to capture more resources. The nature of competition between plants in such environments has been the main area of controversy in the development of CSR theory. Examples of relatively stressed habitats include low-fertility acid or calcareous grasslands and the understory habitat of woodlands.
Environments where the disturbance or destruction of vegetation is a regular occurrence have given rise to plant strategies that either avoid or enable rapid recovery from that disturbance. Although naturally disturbed environments include screes and landslides, shingle beaches and sand dunes, the majority of disturbed environments are human-influenced (e. g. cultivated fields and agricultural grasslands). Plants adapted to such environments tend to show rapid growth rates and a reliance on reproduction through seed as well as vegetative expansion. For example, annuals are adapted to regular severe disturbance: their rapid growth rate enables them to take quick advantage of bare ground following a disturbance event, and copious seed production ensures their survival into future generations before another disturbance. Biennials and short-lived perennials are similarly adapted to disturbances on a longer time-cycle. In effect, the disturbance tolerant strategy or ruderal stategy (named after the roadside habitats from which the disturbance-tolerant life-history was first described) is an insurance policy: investing resources in mechanisms that ensure a rapid response to predictable patterns of disturbance (Figure 4.1).
The three main strategies listed above are extremes. In reality, most species exhibit combinations of traits from the different strategies depending upon the exact environmental conditions to which they are adapted. The crucial point is that, in terms of the maintenance of diversity in vegetation, low stress combined with low disturbance is not good, favouring the aggressive competitor species. Equally, combinations involving high intensities of stress and/or disturbance produce hostile conditions for plant growth, restricting vegetation to a limited number of highly adapted species. In general, greatest species diversity is promoted at moderate intensities of environmental stress and/or disturbance. This is easily illustrated with reference to various grassland types. The more species-rich semi-natural grassland types tend to occur on relatively low fertility, free - draining acid or calcareous soils (moderately stressed) or, in the case of traditional hay meadows, on relatively fertile sites subject to moderate disturbance (hay cutting and after-grazing). The addition of fertilisers (reducing stress) or the removal of maintenance (reducing disturbance) will result in these grasslands becoming dominated by aggressive competitive grasses, with an associated loss of diversity.
The CSR model can be readily adapted to aid understanding of how designed vegetation functions. In the majority of landscape contexts, ‘stress’ generally equates to a lack of availability of resources (water, light and nutrients) and, in particular, nutrient
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1 Plant selection. Matching species with the same ecological strategies is one aspect of
ensuring ecological compatibility with site conditions. For example, creating meadowlike herbaceous communities on fertile productive sites using stress-tolerant species from plant communities typical of low-nutrient free-draining calcareous soils (as is often recommended in the UK) will be unsuccessful without high management intervention. However, more vigorous species with a higher competitive element may be a far better option. As well as matching species to site, the CSR system also enables species matching within a planting mix so that competitive elimination with planted material is diminished and co-existence enhanced. A range of British native herbaceous species have been classified according to the CSR system (Grime et al. 1988). However, apart from some preliminary suggestions by Hitchmough (1994), there has been no attempt to date at classifying non-native species for landscape planting purposes.
2 Vegetation management. The CSR model provides an elegant framework for predicting
the effect of different management regimes on the performance and diversity of vegetation. Again, there has to date been little application of the model away from semi-natural rural vegetation, although O. Gilbert (1989) has classified a range of urban vegetation types according to their predominant vegetation strategies. We return to this matter at the end of this chapter.
