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 Figure 3.2 Regular spacing in pot plants 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 root-medium 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.
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