A Calculation of Expected Plant Frequency


Steven R. Helm

Additional Authors:


June 2010


ABSTRACT A calculation is presented that converts plant frequency per plot size sampled to expected frequency per any plot size of interest so as to compare studies that used different plot sizes or to assess degrees of spatial randomness in individual studies that used multiple plot sizes. Expected frequency is exponentially related to measured frequency, being dependent on plot size of interest relative to plot size from which measured frequency was obtained. Expected frequency per plot size of interest (Fe) may be expressed as Fe 5 1 2 (1 2 Fs)r, where Fs is measured frequency per plot size sampled and r is the ratio of plot size of interest to plot size sampled. The calculation assumes that plants are randomly distributed, or approximately so, and criteria for presence of plants in plots are consistent in studies being compared. INTRODUCTION The binomial proportion, referred to as ‘‘frequency’’ or ‘‘percent frequency’’ in the field of plant ecology, is an often used measure of abundance (Mueller-Dombois and Ellenburg 1974) and is defined as the fraction of plots sampled in which at least one individual of a species, or other taxon of interest, is present and is expressed from zero to one (or zero to 100%) per plot area; all plots of a particular study being of equal area. Frequency sampling was developed by Raunkiaer (1909–10) (English translation in Raunkiaer 1934) and is relatively easily undertaken as it relies only on observations of presence or absence within sample plots. Frequency sampling has been used in such varied habitats as alpine areas, grazed meadows, marshes, and forests. It has been the most commonly applied quantitative parameter for the analysis of forest undergrowth and herbaceous communities in North American descriptive studies (Mueller-Dombois and Ellenburg 1974). Frequency sampling is also commonly used for monitoring rhizomatous species and invasions of undesirable species and offers an advantage over some other techniques, such as cover sampling, in that once plants have germinated, frequency measures remain fairly stable throughout the growing season