Competition for light in a plant monoculture modelled as a spatial stochastic process

In a glasshouse experiment closely spaced plants were regularly sub-irrigated with nutrient solution and it was assumed that competition between individuals was for light. Interaction between neighbours is modelled as a spatial process in relation to differences in plant height. The influence one plant has on another is calculated as the ratio of the angle β between their apices, and an angle θ which represents the intensity of competition in the population as a whole. When β <θ suppression is considered to occur, i.e. tall plants suppress short plants but not vice versa, competition is one-sided. The status of each plant within the population is calculated by multiplication of its individual plant neighbour interaction terms where β <θ. This need not be restricted to nearest neighbours. Plants with status = 1 are free from competition, those with status = 0 are totally suppressed. It is acknowledged that other processes than that modelled as competition may result in variation in relative growth rates. To account for this plant status is used to modify a relative growth rate drawn from a population of mean μ and standard deviation σ. Plants for which status μ is less than an estimated φ are considered to die. Parameter values were estimated for a sequence of fortnightly harvests of glasshouse grown Tagetes patula. There was a decrease in μ and θ, φ increased and plants came to compete with their second nearest neighbours as well as their nearest as the community developed. The greatest σ was at an intermediate stage, suggesting that more frequent assessments of the competition process are required at some stages than at others. Model parameters fitted to one data set were used in a Monte Carlo testing procedure with a second, independent data set. The importance of this technique is stressed because plants in a single community are not independent realization of the competition process. Simulations with the model reproduced both bimodality in the frequency distributions of plant size and an even spatial distribution of large or surviving plants, features which have been observed in a range of plant monocultures. To achieve these features it was essential that plant interaction be modelled as a one-sided process.