Plant communities

Plant communities Plant communities can be viewed from the natural wild habitat to the more ordered situation in horticulture. Neighbouring plants can have a significant effect on each other, since there is competition for factors such as root space, nutrient supply and light. In the natural wild habitat, competition is usually between different species. In subsistence horticulture in the tropics, different crops are often inter-planted (see also companion planting). In Western Europe, crops are usually planted as single species communities.

Single species communities
When a plant community is made up of one species it is referred to as a monoculture. Most fields of vegetables such as carrots have a single species in them. On a football field there may be only ryegrass (Lolium) with all plants a few millimetres apart. Each plant species, whether growing in the wild or in the garden, may be considered in terms of its own characteristic spacing distance (or plant density). For example, in a decorative border, the bedding plant Alyssum will be planted at 15 cm intervals while the larger Pelargonium will require 45 cm between plants. For decorative effect, larger plants are normally placed towards the back of the border and at a wider spacing.

In a field of potatoes, the plant spacing will be closer within the row (40 cm) than between the rows (70 cm) so that suitable soil ridges can be produced to encourage tuber production, and machinery can pass unhindered along the row. In nursery stock production, small trees are often planted in a square formation with a spacing ideal for the plant species, e.g. the conifer Chamaecyparis at 1.5 metres. The recent trend in producing commercial top fruit, e.g. apples, is towards small trees (using dwarf rootstocks) in order to produce manageable plants with easily harvested fruit. This has resulted in spacing reduced from 6 to 4 metres.

Too much competition for soil space by the roots of adjacent plants, or for light by their leaves, would quickly lead to reduced growth. Three ways of overcoming this problem may be seen in the horticulturist's activities of transplanting seedlings from trays into pots, increasing the spacing of pot plants in greenhouses, and hoeing out a proportion of young vegetable seedlings from a densely sown row. An interesting horticultural practice, which reduces root competition, is the deep-bed system, in which a one metre depth of well-structured and fertilized soil enables deep root penetration. However, growers often deliberately grow plants closer to restrict growth in order to produce the correct size and the desired uniformity, as in the growing of carrots for the processing companies.

Whilst spacing is a vital aspect of plant growth, it should be realized that the grower might need to adjust the physical environment in one of many other specific ways in order to favour a chosen plant species. This may involve the selection of the correct light intensity; a rose, for example, whether in the garden, greenhouse or conservatory, will respond best to high light levels, whereas a fern will grow better in low light.

Another factor may be the artificial alteration of day length, as in the use of 'black-outs' and cyclic lighting in the commercial production of chrysanthemums to induce flowering. Correct soil acidity (pH) is a vital aspect of good growing; heathers prefer high acidity, whilst saxifrages grow more actively in non-acid (alkaline) soils. Soil texture, e.g. on golf greens, may need to be adjusted to a loamy sand type at the time of green preparation in order to reduce compaction and maintain drainage.

Each species of plant has particular requirements, and it requires the skill of the horticulturist to bring all these together. In greenhouse production, sophisticated control equipment may monitor air and rootmedium conditions every few minutes, in order to provide the ideal day and night requirements.

This aspect of single species communities emphasizes the great contrast between production horticulture and the mixed plantings in ornamental horticulture. This inter-species competition is even more marked in the natural habitat of a broad-leaved temperate woodland habitat and reaches its greatest diversity in tropical lowland forests where as many as 200 tree species may be found in one hectare.

Plant species as plant communities
The subject of ecology deals with the inter-relationship of plant (and animal) species and their environment. Below are described some of the ecological concepts which most commonly apply to the natural environment, where human interference is minimal. It will be seen, however, that such concepts also have relevance to horticulture in spite of its more controlled environment.

Firstly, the structure, physiology and lifecycle properties of a plant species should be seen as closely related to its position within a habitat, giving it a competitive advantage. Small short-lived ephemerals such as groundsel (Senecio) with its rapid seed production and low dormancy are able to achieve a speedy colonization of bare ground. The spreading perennial, bramble (Rubus) has thorns that ease its climbing habit over other plant species and a tolerance of low light conditions that assumes greater importance as tree species grow above it. Woody species such as oak (Quercus) quickly create a well-developed root system that supplies the water and minerals for their dominance of the habitat. Aquatic species such as pondweed (Potamogeton spp) often have air spaces in their roots to aid oxygen and carbon dioxide diffusion.
Ecology terms
For a marsh willow-herb (Epilobium palustre), its only habitat is in slightly acidic ponds. In contrast, a species such as a blackberry (Rubus fruticosus) may be found in more than one habitat, e.g. heath land, woodland and in hedges. The common rat (Rattus norvegicus), often associated with humans, is also seen in various habitats (e.g. farms, sewers, hedgerows and food stores).

Within the term 'habitat' , distinction can be drawn between closed plant communities and open plant communities.

These two terms can only be used in a relative way because the radiation from the sun, the gases in the atmosphere, and migrant species prevent a true closed system being established within the natural environment.

A couple of general points may be added about the vegetation of the British Isles. The British Isles at present has about 1700 plant species. Fossil and pollen evidence suggests that before the Ice Age there was a much larger number of plant species, possibly comparable to the 4000 species now seen in Italy (a country with a similar land area to that of Britain). In Neolithic times, when humans began occupying this area, most of Britain was covered by mixed oak forest. Since that time progressive clearing of most of the land has occurred, especially below the altitude limits for cattle (450 m) and for crops (250 m).

Plant associations
In natural habitats, it is seen that a number of plant species (and associated animals) are grouped together, and that away from this habitat they are not commonly found. Two habitat examples can be given. In south-east Britain, in a low rainfall, chalk grassland habitat there will often be greater knapweed ( Centaurea scabiosa ), salad burnet ( Poterium sanguisorba ) and bee orchid ( Ophrys apifera ). In the very different high rainfall, acid bogs of northern Britain, cotton grass ( Eriophorum vaginatum ), bog myrtle ( Myrtus gale ) and sundew ( Drosera anglica ) are commonly found together. Other habitat species such as bluebell (in dense broadleaved woodland), bilberry (in dry acid moor), mossy saxifrage (in wet northfacing cliffs), broom (in dry acid soils) and water violet (in wet calcareous soils) can be mentioned. It should be noted that successful weeds such as chickweed are not habitat-restricted in this way.

Niche
The role of a species within its habitat.
For a Sphagnum moss, its niche would be as a dominant species within an acid bog. The term 'niche' carries with it an idea of the specialization that a species may exhibit within a community of other plants and animals. A niche involves, for plants, such factors as temperature, light intensity, humidity, pH, nutrient levels, etc. For animals such as pests and their predators, there are also factors such as preferred food and chosen time of activity determining the niche. The niche of an aphid is as a remover of phloem sugars from its host plant.

The term is sometimes hard to apply in an exact way, since each species shows a certain tolerance of the factors mentioned above, but it is useful in emphasizing specialization within a habitat. The biologist, Gause, showed that no two species can exist together if they occupy the same niche. One species will, sooner or later, start to dominate.

For the horticulturalist, the important concept here is that for each species planted in the ground, there is an ideal combination of factors to be considered if the plant is to grow well. Although this concept is an important one, it cannot be taken to an extreme. Most plants tolerate a range of conditions, but the closer the grower gets to the ideal, the more likely they are to establish a healthy plant.

Biome
A major regional or global community of organisms, such as a grassland or desert, characterized by the dominant forms of plant life and the prevailing climate.

This term refers to a wider grouping of organisms than that of a habitat. As with the term habitat, the term 'biome' is biological in emphasis, concentrating on the species present. This is in contrast to the broader ecosystem concept described below. Commonly recognized biomes would be 'temperate woodland' , 'tropical rainforest' , 'desert' , 'alpine' and 'steppe'. About 35 types of biome are recognized worldwide, the classification being based largely on climate, on whether they are landor water-based, on geology and soil, and on altitude above sea level. Each example of a biome will have within it many habitats. Different biomes may be characterized by markedly different potential for annual growth. For example, a square metre of temperate forest biome may produce ten times the growth of an alpine biome.