The agrostrological index: A phytogeographical approach to the problems of pasture plant introduction

Abstract

In continuation of a previous study of grass distribution, a modified normal distribution spectrum is presented, giving the average percentage frequency of grass species of the five largest tribes in the eastern and western hemispheres. This modified spectrum forms the basis of iso-agrostological maps delimiting areas in which each tribe is present in more than normal frequency. An iso-agrostological map of the United States shows a general agreement with the distribution of temperature zones, but little correlation with vegetation. More useful comparisons between grass distribution in different countries can be made through the agrostological index, in which the percentage of grasses in each tribe is included in one of 10 frequency groups. In general, areas in different parts of the world with identical agrostological indices have similar latitude, climate, and topography, though some exceptions are noted. Where accurate floristic data are available, the agrostological index may be used to show grass distribution in small areas, and the relationship of this distribution to climatic and other environmental factors. Regions with similar agrostological indices have similar cultivated pasture plants, and hence the index may be of value in pasture plant introduction.