INTRODUCTION. 



I. THE DISTRIBUTION OF VEGETATION IN GENERAL. AS RELATED 



TO CLIMATIC CONDITIONS. 



This publication constitutes an attempt to correlate the distribution 

 of the vegetation of the United States with the distribution of some 

 of the climatic conditions that appear to be most important to plants. 

 It has long been a matter of common information that such a correla- 

 tion exists, and some of its most obvious features have commanded 

 popular attention from the earliest settlement of the country. The 

 influence of a low and uncertain rainfall in inhibiting the occurrence 

 of trees in certain portions of Kansas and Nebraska, for example, and 

 the influence of the high winter precipitation of the Pacific Northwest 

 in permitting the occurrence of a heavy forest in that region, are mat- 

 ters that have come to the attention of every one familiar with those 

 regions. It has been the aim of our work to make a somewhat thor- 

 ough investigation of such correlations as these by bringing together 

 a carefully elaborated set of climatological data and a representative 

 set of data with respect to the occurrence of certain characteristic 

 species of plants, in addition to the facts of the distribution of typical 

 vegetations. We have sought, by appropriate means, to ascertain the 

 extremes of each climatic feature for each of the vegetational or dis- 

 tributional areas. In short, we have determined the maximum and 

 minimum values of each cUmatic feature for such well-known regions 

 as the Great Plains, the Gulf pine-belt, or for such well-known species 

 as the Sitka spruce {Picea sitchensis), the sage-brush (Artemisia tri- 

 dentata), and the small cane (Arundinaria tecta). 



Our desire is not only to set forth the basal facts upon which we have 

 worked and such features of correlation as we have been able to dis- 

 cover, but also to clarify some of the conceptions fundamental to such 

 work and to stimulate a greater interest in it. It is particularly desir- 

 able that our work should be regarded as a preliminary and extremely 

 general investigation of this subject for the United States, and that 

 more exact studies of smaller areas should be carried out in order to 

 study more thoroughly the relations with which we have dealt. It will 

 of course be possible to use the climatological data which we have 

 gathered for the study of other subdivisions of the vegetation than 

 those that we have used and for the determination of the climatic con- 

 trols for other species than those we have selected. It is also to be 

 hoped that the United States Weather Bureau and other agencies will 

 make it possible, at no distant date, to draw other chmatic maps than 

 those we have been able to construct from the data now available. 



