372 



COMMUNITY ECOLOGY: 



thoughout time, it displays moderate to great changes 

 in different geological periods. 



The evolution of many geofloras through time re- 

 sults in new species, new communities, new com- 

 munity elements, and even new geofloras. This evolu- 

 tion can be extremely confusing unless one realizes 

 that each of these ecological units through lime is of 

 changing size and is evolving in some areas and not 

 in others. In addition, evolution can be extremely 

 rapid in some localities and imperceptible in others. 

 Because of these possibilities, a single element or 

 even smaller unit could give rise to a geoflora in one 

 part of its range and to additional elements in other 

 parts, but still maintain its identity elsewhere. 



In reading the following account, one must be pre- 

 pared for change — change in the shape of the con- 

 tinents, in the topography of the land, in climate, in 

 the distribution of geofloras, and change in smaller 

 ecological units, down to and including the individual 

 species. 



A group of maps are provided to aid the reader in 

 following the changes. These maps imply no more 

 than general trends. They are far from being as ac- 

 curate as maps of the present distribution of vegeta- 

 tion. At the present time there is insufficient informa- 

 tion for such accuracy. However, the maps could be 

 much more accurate on the basis of the information 

 available. For example, no serious attempt was made 

 to include the available information of discontinu- 

 ous distributions or altitudinal distributions. Also, 

 very crude approximations were made to select and 

 simplify the limits of distribution of the various floras. 

 The generalization of distribution is believed to serve 

 best as an overview of what has happened. 



GEOFLORAS AND ELEMENTS 



The history of North American vegetation relates 

 the evolution and dispersal of three major geofloras. 

 Two of these major floras were present some 75 million 

 years ago, and the other appeared shortly thereafter. 

 Since the time of their origin each geoflora was sub- 

 jected to great changes in composition and to major 

 movements over vast areas of our continent. How- 

 ever, no geoflora has lost its essential identity as 

 Neotropical-Tertiary, Arcto-Tertiary, or Madro- 

 Tertiary. 



Neotropical-Terfiary Geoflora. The Neotropical- 

 Tertiary Geoflora has been the southernmost unit. 



mostly broad-leaved evergreen plants of subtropical 

 to tropical climate. Although it ranged over wide 

 areas of the United States and even up to Canada on 

 occasion, at the present time it is mostly in the sub- 

 tropical forests from southern Mexico to Panama and 

 in eastern Asia, areas now having relatively uniform 

 climate and over 80 inches of rainfall per year. In the 

 United States, the only living remnants of the geo- 

 flora are in southern Florida. The geoflora includes 

 such plants as cycads, palms, magnolias, figs, laurels, 

 avocados, and cinnamon. 



Arcfo-Terfiary Geoflora. This, the northernmost of 

 the three geofloras but a temperate climate flora, at 

 first contained mostly temperate deciduous plants. 

 However, some primarily nondeciduous present-day 

 elements probably were derived from this geoflora. 

 Possible derivative now include: 



1 . Tundra, the northernmost flora, ranging from 

 Alaska, across the northern edge of the continent, 

 and to Labrador, is a cold-climate element having 

 lichens, mosses, grasses, sedges, other herbs, and a 

 few shrubs. 



2. Taiga is a savanna flora just south of the Tun- 

 dra, extending across the continent from Alaska 

 through Canada almost to the Great Lakes, and then 

 on to Vermont. It is composed of scattered conifers, 

 including black spruce, white spruce, hemlock, white 

 pine, jack pine, balsam fir, and a few deciduous trees 

 such as aspens and birches, among a ground cover of 

 shrubs and lichens. This element often is not sepa- 

 rated from the Northern Coniferous Forest, and prob- 

 ably it should not be. 



3. Boreal Forest, a group of elements, is a dense for- 

 est of mostly conifers that range south of the Taiga 

 approximately to the United States border in the 

 east and down the mountain ranges of the west. 

 Within this coniferous forest, four major subdivisions 

 can be recognized. The .\orthern Conijerotis Forest is 

 little more than a consolidation of Taiga trees. The 

 .Suhalpine Forest is a western mountain (Cascades, 

 Sierras, and Rockies) counterpart of the Northern 

 Coniferous Forest. The .Montane Forest, also of the 

 western mountains but just below the Subalpine 

 Forest, is typified by yellow pine, lodgepole pine, firs, 

 western larch, and some Douglas fir. The C^ast Forest 

 of the Pacific Coast, especially the Coast Ranges, 

 from Alaska to central California contains Sitka 

 spruce, western hemlock, Douglas fir, western white 

 pine, redwood, western white cedar, arborvitae, and 

 yellow pine. 



