Crowding indices : Experimental methodology and predictive accuracy

Mitchinson, Pelham James (1999) Crowding indices : Experimental methodology and predictive accuracy. University of Southampton, Doctoral Thesis.

Abstract

Crowding indices are simple empirical models used in research to quantify the effects of neighbours on the growth environment of a plant. They have usually been used to predict the final size reached by a plant after a period of growth and interaction with neighbours.

Crowding indices are based on the theory that the influence of neighbouring plants is highly dependent on their proximity. They make use of information such as size, angular dispersion, and proximity of neighbours in order to make their predictions.

Comparing measured final sizes with predicted sizes in a single experiment has, in many cases, been the only assessment of the predictive ability of these indices. In this study, the validity of the use of such measures of predictive ability is questioned. Theoretical arguments are described proposing reasons to expect predictive accuracy to be highly dependent on experimental methodology. Experimental evidence in this study had led to the rejection of the null hypothesis that predictive accuracy was independent of methodology. In the experiments conducted, predictive accuracy and optimum parameters of a broad range of indices were dependent on the species studied, the planting density and the environment conditions of the experiment. One of the experiments did show, however, a long period between maturity and death in which predictive accuracy was not significantly related to population age at measurement.

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