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
. 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.
|Figure 3.1 A limestone valley
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.
|Figure 3.2 Regular spacing in pot plants
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
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
) 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.
For a marsh willow-herb (Epilobium palustre
), its only habitat is in
slightly acidic ponds. In contrast, a species such as a blackberry (Rubus
) may be found in more than one habitat, e.g. heath land,
woodland and in hedges. The common rat (Rattus norvegicus
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).
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
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.
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.
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.