Self-assembling plants and integration across ecological scales.

BACKGROUND AND AIMS: Although individual plants exhibit much complex behaviour in response to environmental stimuli, they appear to do so without any identifiable centres of organization. We review a special class of model with the aim of testing whether plants can effectively be self-assembling, modular-driven organisms, in the sense that whole-plant organization and behaviour emerges solely from the interactions of much smaller structural elements. We also review evidence that still higher-level behaviour, at the population and community levels of organization, can emerge from this same source. METHODS: In previous work we devised a special cellular automaton (CA) model of plant growth. This comprises a section depicting a two-dimensional plant in its above- and below-ground environments. The whole plant is represented by branching structures made up from identical 'modules'. The activity of these modules is driven by morphological, physiological and reproductive rulesets derived from comparative plant ecology, a feature which lends itself to experimentation at several ecological scales. KEY RESULTS: From real experiments using virtual plants we show that the model can reproduce a very wide range of whole-plant-, population- and community-level behaviour. All of these properties emerge successfully from a ruleset acting only at the level of the CA module. CONCLUSIONS: The CA model can, with advantage, be driven by C-S-R plant strategy theory. As this theory can ascribe a functional classification to any temperate angiosperm on the basis of a few simple tests, any community of such plants can be redescribed in terms of its 'functional signature' and the net environment that it experiences. To a valuable first approximation, therefore, a C-S-R version of the CA model can simulate the most essential properties both of natural vegetation and of its environment. We have thus achieved a position from which we can test a plethora of high-level community processes, such as diversity, vulnerability, resistance, resilience, stability, and habitat-community heterogeneity--processes which, if investigated on the scales truly required for a full understanding, would fall beyond the practical scope of even the largest real-life investigation.

Growth of Pyrola rotundifolia ssp. maritima in relation to shade.

The maritime race of the larger wintergreen [Pyrola rotundifolia L. ssp. maritima (Kenyon) E. F. Warb.] was investigated in relation to its light environment at Braunton Burrows in south-west England. Measurements made at the site of its greatest abundance indicated that, in July, Pyrola commonly experiences an irradiance which is below 10% of full daylight with some habitats providing less than 1%. The species also occurs in situations receiving up to 100% daylight; at or towards this extreme the foliage is yellowish-green instead of deep green, giving an impression of reduced luxuriance. The mean specific leaf area of plants growing in the field was found to be inversely related to mean percentage irradiance received at each site. However, the slope of this trend was slight. Detached, rooted rosettes were removed from the field and grown for 85 days in a glasshouse under treatments which provided 100, 20.2 and 6.1% daylight. The plants in all treatments appeared normal but growth was slow. Yields and relative growth rates were significantly reduced by shading (P 0-05). The depressing effect of shading on growth rate was very slightly less than on unit leaf rate, the difference being due to slight increases in leaf area ratio. This, in turn, was tracted entirely to small increases in specific leaf area. Leaf weight ratio was not affected significantly, nor were substantial effects recorded in root/shoot ratio or in fresh weight/dry weight ratio. We conclude that the vegetative growth of Pyrola has a low capacity for adjustment to both natural and experimental shading; its growth rate in the field will tend to be lower under a dense leafy canopy than in less shaded habitats. The inherently low growth rate might, however, be an advantageous feature under shade conditions. The evident success of the species in shaded situations is certainly due to factors other than a simple shade requirement for vegetative growth.