From the text... 'The migration of trees is slowed by the relatively long interval of maturation, often as much as 10 to 20 or more years between dispersals. Mass migration of tree species, and hence of forest, would proceed at a much slower rate than isolated colonizations due to occasional long dispersals. The recurrent fire and drought on the* wind-swept plains would be sufficient delay indefinitely the centripetal migration of trees from peripheral, sheltered sites. Hence, it seems reasonable to suppose that scarp-restriction of forest or woodland vegetation throughout the region of grassy plains in the central part of the continent originated as a consequence of regional forest and prairie fires, and that subsequent annual conflagrations helped to maintain a treeless condition by sweeping the seasonally dry grasslands on the smooth surface of the Plains for great ditances, until the fires were stopped by an abrupt topographic break. When all the facts are admitted, the generalization that the treeless grasslands of the Great Plains are climatically determined seems simplistic. Obviously, climate is a factor, as is always true in plant geography, but there are others: the regional flatness and continuity of the physiography; the smooth, relatively undissected mantle of unconsolidated Pleistocene sediments; the fuel-producing, annual dieback of grasses characteristic of the herbaceous way of life; and the sine qua non, fire. Radiocarbon-dated macrofossil and pollen records from the Plains region of central North America indicate that areas now occupied by grassland or desert vegetation were wooded during the Wisconsin glacial. In the northern part of the Great Plains, boreal conifers characteristic of the existing Canadian taiga occurred at widely scattered localities from southern Saskatchewan south to Kansas and from Minnesota south to Missouri. On the warmer and more arid Plains of the Llano Estacado in the southern part of the Texas panhandle, several pollen profiles from Wisconsin-age sediments show very high contents of pine pollen (more than 90 percent) and low contents of spruce pollen (10 percent or less). Abundant macrofossil evidence has been obtained at the southwestern border of the Plains. In what is now one of the most arid sectors of the Chihuahuan Desert, an open, xerophilous woodland of piñon pine (Pinus cembroides var. remota), live, oaks, and juniper prevailed during Wisconsin time. But there is no indication that treeless grassland shifted southward into what is now the arid Chihuahuan Desert during the Wisconsin glacial, when much of the Great Plains / south of the ice sheet was occupied by coniferous forest, woodland, or possibly parkland.'