The dynamic nature of plant. communities—pattern and process in. designed plant communities

Nigel Dunnett

All planting design, if it is to be successful, must to some extent be a compromise between what is desirable (artistic or creative vision) and what is possible (scientific reality). Of course, technology can be employed to push the boundaries of what is possible on any given site, but this is often at a considerable environmental cost. The great advantage of an ecologically-informed basis to planting is that it has the potential to achieve full creative vision with relatively little site modification. Having said that, even the terms ‘ecological’ or ‘naturalistic’ planting encompass a broad spectrum of approaches, ranging from pure restoration ecology (which aims to reproduce as closely as possible a target or reference of semi-natural plant community) through to ornamental plantings that may be highly naturalistic but bear no resemblance to any naturally occurring plant communities. But most points on this nature ^ art continuum (described fully in Chapter 3) can be characterised by having some degree of creativity associated with them: achieving a ‘natural’ quality is of great importance and they are therefore driven at least partly by visual principles. Even habitat creation approaches involve some form of species selection and arrangement to distil the essence of a plant community.

The scientific underpinning of the different approaches to planting that are described as ecological can also vary widely. At the most basic level, for most ecologically – informed schemes, scientific thinking will come in at the plant-selection level: making plant choices based upon the ‘right plant, right place’ philosophy. This concept is fundamental: plants are the great interpreters of site conditions and accurately reflect and mirror what might be minute changes in soil type, topography, climate and management. Choosing plants according to fitness to site reduces the need for drastic and resource­intensive site manipulation. Plants from habitats that share similar environmental constraints tend to share common traits or characteristics, and this is a tendency that can be fully exploited in planting design (Dunnett 1995). At one extreme, this may involve putting together cosmopolitan mixes of plants that are adapted to certain site conditions, but with no regard to their geographical origin. At the other extreme, plant selection may have a strong geographical element to it and may aim to reproduce the character of a plant community (rather than trying to copy it completely) that is suited to particular site conditions. This ‘biogeographic’ approach may use very attractive reference communities from widely separated countries (for example, the contemporary ‘prairie’ and ‘steppe’ perennial planting styles in Western Europe), or be much more tied into local or regional reference plant communities.

But the value of scientific understanding goes much further than simply helping to put an appropriate plant list together. Applying scientific principles can actually guide the way that plants are arranged to achieve a fully naturalistic effect, but one that also actually works as a functioning plant community into the indefinite future. Plant communities tend to show identifiable patterns in the way that different species are arranged, both horizontally and vertically—these are related not only to environmental variation but also to the characteristics of the plants themselves and how they interact. As well as patterns in space, ‘natural’ plant communities show patterns in time: they are dynamic and change over a range of timescales, as a result of ecological processes. These changes in space and over time are directly related to each other, and manifest themselves in the way that naturalistic vegetation appears and functions. In this light, it is no coincidence that one of the first ecological publications that opened people’s eyes to the dynamic nature of plant communities (and one of the most influential ecological publications of the twentieth century) was titled Pattern and Process in the Plant Community (Watt 1947). The aim of this chapter is to identify principles that enable us to understand patterns and processes in designed plant communities. The aim is not to repeat standard ecological texts but instead to provide insights into how a designed ecological landscape might function over time and space. Where ecological concepts are introduced they are clearly linked to their implications in terms of how vegetation is designed, established and managed. It should also be stressed that it is assumed that readers will be familiar with basic scientific concepts relating to the requirements for successful plant growth and these will therefore not be considered here.