Studies on the origin, evolution and distribution of the Gramineae. IV. The genus Poa L

Abstract

Studies of the regional and global distribution of the genus Poa L, show that it reaches its highest relative specific differentiation in regions of high latitude and high altitude. The genus is absent or rare in the tropics, except in mountainous regions, but constitutes more than 15% of the grass flora of Alaska, Iceland, Kamchatka, and the high Pamir Mountains.

There is a close association between the occurrence of high percentage frequencies of species of Poa and cool summer temperatures. In the United States the 75DF mid-summer (July) isotherm effectively demarcates those regions in which Poa spp. form more than 5% of the grass flora from those in which they constitute a smaller percentage. A similar relationship between percentage frequency and mid-summer temperature is found in most other parts of the world. Climatic factors other than temperature seem to have little influence on the distribution of Poa.

It is concluded that the genus has attained a high stage of development in nearly all parts of the world to which it is climatically adapted. The evidence does not permit firm conclusions about the centre of origin of the genus, but it has a major centre of species differentiation in the mountain and plateau region of central Asia. It is probably monophyletic, and has had a long evolutionary history in the course of which it has spread through regions now separated by topographical or climatic barriers.