The dynamic nature of plant communities
Any acceptance of an ecologically-informed approach to planting must fully embrace the concept of change. The common perception that plant communities in the wild are relatively static, with little alteration in their composition or appearance from year to year, is of course a misconception: change is fundamental to the processes that operate within semi-natural plant communities. Indeed, it could be said that every ecological principle that a designer or manager needs to be aware of is related in some way to this dynamic nature of plant communities. Change is apparent and important in all timescales, and for our purposes can be broken down into three main categories:
- changes in the way a unit of vegetation develops over a single growing season or year
(processes related to the different rates of development and performance of component species, and generally referred to as phenological change)
- changes in the abundance, performance or visual presence of component species, or the
overall biomass of the plant community between different years (generally referred to as fluctuations or cycles)
- longer-term changes in the character, composition or type of vegetation (generally
referred to as successional change).
Change also operates at all spatial scales, whether this be at the level of two plants side by side competing with each other for space or resources, or the interaction between two plant community or vegetation types (again linked to competition), or at the largest landscape scale where the manner in which different vegetation units are linked together can affect the way that plants and animals (including humans) can move around any given area. Processes operating at all these scales manifest themselves in the vertical and horizontal structure of vegetation, and in the very survival and long-term integrity and persistence of any given vegetation type.
Vegetation change is partly driven by the obvious changes within the lifecycles of individual plants and populations of plants—establishment, growth, maturity, reproduction and regeneration, senescence and death—but is equally tied up with physical environmental factors and constraints, competition and plant-plant interactions, and, crucially, with the nature of the landscape context and the surrounding vegetation types.
Ecologically-informed or ‘sustainable’ planting has been defined as designed vegetation that maintains its integrity over successive generations with minimal resource inputs (Dunnett 1995). In order to disentangle this statement, we will first consider factors that maintain the integrity of vegetation. That is, how do more than one species co-exist in any given unit of space, and continue to co-exist? The question of how biodiversity is promoted and maintained, and its importance to the functioning of ecosystems, has been one of the fundamental questions in plant ecology and is the subject of much current debate. It also has great relevance to the aesthetics and functioning of designed vegetation. We shall then consider patterns of vegetation change over different scales of time and space.
