TUNDRA AND CONIFEROUS FORESTS 67 



and Arkansas." Similar opinions have also been expressed by Cole- 

 man, Matthew, Dawson, Leverett, Upham, and Davis. 



The same period was early recognized in Scandinavia by Blytt a6 

 and has been generally accepted by most European phytogeographers. 

 Of the numerous papers on the subject published in the same volume, 

 those of Andersson 35b " c seem especially conclusive. He decides that 

 during the early postglacial stages the climate of Scandinavia had 

 winters similar to those of the present, while the summers were longer 

 and about 2.5° C. warmer. Later the climate became wetter, while 

 remaining equally warm, while for the modern period there is evidence 

 of a slow lowering of the temperature. It is also worthy of note that 

 Andersson lays great stress on the value of relic colonies as evidence 

 of past distribution. 



The term xerothermic period has long been used by Europeans for 

 this period of mild and drier climate, and may well be extended to the 

 same type of climate, probably contemporaneous, in America. 



A xerothermic period, occurring in comparatively recent times, and 

 without any subsequent geological episodes to modify greatly the trend 

 of plant migrations, would certainly have left an impress on the dis- 

 tribution of vegetation which would still be visible. Among the effects 

 which might be expected is a great extension of the prairie flora toward 

 the east, taking advantage of the favorable climate. This should now 

 be evidenced by relic prairie colonies and by isolated stations of western 

 species at the east and by a deficiency of hydrophytic and mesophytic 

 boreal relics at the west. Both of these results are actually demon- 

 strable at the present time, as will be shown below. A third effect 

 should be seen in the migrations of the deciduous forests from the 

 southeastern center, in which the more mesophytic species would be 

 limited at first to a northward migration only, passing to the east 

 of the area affected by xerothermic conditions, while the more xerophy- 

 tic species would migrate both north and west, accompanied by other 

 species of edaphically moist floodplains, such as now form the western- 

 most groves of Kansas and Nebraska. Present distribution indicates 

 that this was also a fact, although it is less clearly evident at the 

 present time. 



Tundra and Coniferous Forest Migration. — The northern 

 boundary of the tundra vegetation overlying the glacial ice extended 

 toward the north with the retreat of the ice. At its southern boundary, 



35b Andersson, Gunnar. Das spiitquartare Klima, erne zusammenfassende Ueber- 

 sieht. xii-lvi. 



35c Andersson, Gunnar. Swedish climate in the late-quaternary period. 247-294. 



36 Blytt, Axel. Dia Theorie der wechseluden kontinentalen und insurlaren 

 Klimate. Engler's Bot. Jahrb. 2 : 1-50. 1881. 



