Patterns

We have seen how diversity can be maintained, through the promotion of the co­existence of species within a given area of space, but how does this actually work out on the ground? What is the visual and physical manifestation of diversity? Whilst the distribution of plants within more diverse

4.2

Examples of plants that exhibit different ‘strategies’:

(a) rosebay willowherb, Chamerion angustifolium, an aggressive, vigorous ‘competitor’;

(b) Juniper, Juniperus communiis, a slow growing, evergreen ‘stress tolerator’, growing in thin, free-
draining soil and exposed conditions on a limestone pavement in North Yorkshire;

(c) vegetation adapted to hot arid conditions on Tenerife—a very different climate but the vegetation is also evergreen and slow growing; and

(d) Poppies, a typical ‘ruderal’ species, flowering on an abandoned cultivated field

4.3

Aggregated plant distribution: hypothetical distribution of a species across an area of space showing patches of higher density imposed on a general distribution of lower density (modified from Greig-Smith (1964))

communities may at first glance, appear to be random, ecological studies indicate that this is rarely the case. The distribution of a particular species may respond to (often small-scale) spatial changes in environmental factors such as soil moisture, concentration of particular nutrients, pH and so on, and the growth form of the plant (whether it spreads vegetatively or clonally by stolons or rhizomes, or how its seed is distributed), but it is also dependent upon competitive interactions with its neighbours. For example, a walk through a semi-natural woodland or forest reveals that many species occur in clumps or groups rather than as scattered individuals. This can be a result of many factors: some species form suckering clumps, others may have established at the same time as a result of some disturbance, such as a mature tree falling down to open up a glade. The main point here is that the plants are usually distributed in patterns and these patterns can be used as a basis for the design of diverse naturalistic plantings.

The detection of patterns of plant distribution has been an area of scientific study as well as for design inspiration. At the most basic level, distribution patterns within mixtures of plants can be described according to how aggregated and segregated the component species are (Pielou 1961). The degree of aggregation of a species is an indication of the amount of association of individuals or groups of individuals of that species. In effect, it is a measure of the non-randomness of the distribution of the species. In general, most species show some form of aggregation or clumping (see Figure 4.3). This may vary from a very loose association to a dense massing.

There has been a tendency to invest these naturalistic patterns with an almost mystical quality, presenting them as a set of rules that, if plants are

Table 4.2. Possible causes of plant distribution patterns
Distribution pattern	Possible cause
Singly or small clusters	Exacting requirements for regeneration from seed rarely met in the habitat. Spread by rhizomes is strictly limited. May indicate sensitivity to intense herbivory. Surrounding species may suppress expansion. Possible allelopathic effects
Larger cluster and groups	Species exhibit limited rhizomatous growth from initial colonisation or establishment centres. Indication of competitive balance within a habitat. Possible artefact of early successional stages, reflecting distribution patterns when habitat was originally more open to invasion
Patches	Potential reflection of patchiness of the environment, for example fluctuations in soil characteristics, or previous disturbance patterns. Possible early successional stage, indicating phase of expansion of competitors
Extensive stands	Species generally have rhizomatous growth habit, stoloniferous spread, competitively excluding other species. Possible artefact of low disturbance and environmental stress. Very common in competitors and stress-tolerant competitors

laid out in these patterns, ecologically-functioning vegetation will automatically result. This is by no means the case because these patterns are, in the first place, largely a reflection. Planting according to these patterns will result in a naturalistic appearance in the short to medium term, but a long-term maintenance of these patterns will be dependent on an understanding of the underlying causes of the patterns. Table 4.2 indicates potential causes for observed plant distribution patterns. The table shows a gradient, moving from top to bottom, from individuals and small clusters through to extensive monocultural stands. A distinction is made between cluster-based distributions, whereby it is still possible to identify individual plants or clones through to more extensive patches and stands. The latter are generally associated with higher productivity systems. The distinction also reflects clonal versus non-clonal growth morphology (again this is linked to productivity). Secondly there is an indication that some patterns may be ephemeral points within successional development.

Many designers, who may not have a great deal of ecological understanding, view patterns as a means by which ‘stability’ can be achieved, independent of previously mentioned factors, such as fitness to site—i. e. if you have the pattern of a seminatural stereotype then somehow stability will emerge inherently. As indicated in Table 4.2, this is a somewhat naive view, given that observed patterns may be ephemeral points in a longer-term developmental sequence, and that patterns are an outcome of competitive interactions and environmental pressures rather than a factor that dictates how vegetation performs into the future.

The characteristic patterns that different degrees of aggregation lead to lies at the basis of the socalled German Plant Sociability school of planting design, whereby different species are assigned a sociability score according to the degree of massing that the species may typically display in the wild. Hansen and Stahl (1993) list five sociability scores (Figure 4.4):

i singly or in small clusters

ii small groups of 3-10 plants

iii larger groups of 10-20 plants

iv extensive planting in patches

v extensive planting over large areas.

The degree of segregation of two or more species gives an indication of how ‘mingled together’ or intimately associated they are. The lack of distinct boundaries between plant groups is another key characteristic of a naturalistic approach to planting. Figure 4.5 indicates a range of possible patterns for two-species mixtures.

In essence, the ecologically-informed detailed planting plans described in Chapter 9 are based upon interactions between the segregation and aggregation of the component species. The German Plant Sociability school of planting design was itself based upon detailed studies of the way plants are distributed in the wild, such as those carried out by Willy Lange (see Chapter 2). Vegetation mapping of this sort enables us to detect just how a diverse mix of species is able to co-exist. It not only provides a sense of how designers can arrange plants to achieve a naturalistic effect, but it can also indicate how mixtures can be put together to produce an extended season of display, using information both about their competitiveness and vigour, but also their growth form (degree of aggregation, as discussed above) and growth pattern through a season.