Studies on the Origin, Evolution, and Distribution of the Gramineae. V. The Subfamily Festucoideae

Abstract

Maps are presented showing the world distributions of the subfamily Festucoideae and the tribe Festuceae, based on data on relative specific frequency derived from about 500 widely-distributed floras and floristic lists. The maps show many points of similarity and, especially in N. America, there is a very high correlation between the numbers of species of the subfamily and tribe in regional floras.

Both the Festucoideae and the Festuceae attain their highest relative specific frequency in high latitudes or at high altitudes. There is a strong negative correlation between high specific frequency and midsummer temperature in the flora of the United States, and evidence of a similar, though less marked, relationship in other regions. The effect of rainfall is not apparent in the distribution maps.

Regions of high relative specific frequency of the tribes Agrostideae, Aveneae, Stipeae, and Triticeae are mapped, and discussed in relation to climate. In their geographical relationships, the tribes studied fall into two groups, viz. (1) Agrostideae, Aveneae, and Festuceae; (2) Stipeae and Triticeae.

The significance of the distribution patterns is discussed in relation to the evolutionary development of the subfamily Festucoideae and its constituent tribes. The phytogeographical evidence supports the view that each tribe is monophyletic, with the exception of the Aveneae (sensu str.), which is well developed in regions of diverse climate. A strongly developed enclave of high relative specific frequency of Festuceae in part of south-eastern Europe, which shows no obvious relationship to climate, is interpreted as evidence for the occurrence of a cycle of secondary evolution of the tribe in this region.