Introduction 



Southwestern United States, as we have come to know it, is a vast and complex 

 region that includes practically any ecosystem that can be found in the world 

 today, exclusive of polar regions. The area studied extends from the warm- 

 temperate mesophytic forests and Gulf Coastal Plain of southeastern Texas, and 

 the subtropical Rio Grande Valley, to the alpine summits of the Rocky Mountains 

 in New Mexico and Arizona, and the Sonoran Desert of southwestern Arizona. 



For the most part, our region is one of high evaporation which, even in a single 

 season, can greatly affect the composition of various plant communities. This 

 high rate of evaporation that causes drastic fluctuations in the water level of a 

 water body can quickly alter or change entirely its ecology. Through desiccation, 

 with the lowering of the water level, much of the vegetation occupying the 

 marginal zone can perish, while the submersed and floating vegetation may be 

 adversely affected by lack of light and oxygen sufficient to carry on the life 

 processes. 



In addition to this drastic evaporation from open water surfaces and land, 

 vast quantities of water are transpired from plants. Our work, which we con- 

 sider to be a water-economy oriented botanical treatise, is concerned primarily 

 with these plants that have the greatest impact upon our water resources. For 

 this reason not only those plants that live in open water or marsh areas are treated 

 but also those plants that are known as phreatophytes, or those plants whose 

 roots tap the ground water. These latter plants are considered by some authorities 

 to pose a definite threat to the meager water resources in some parts of south- 

 western United States. Many government and private foresters consider that such 

 plants as the salt cedars (Tamarix) that grow especially in alkaline or saline 

 floodplains, about lakes and on streams and river banks use water wastefully and 

 are of little or no benefit. These foresters advocate the cutting and rooting out 

 of these plants. This, of course, would be the simplest and probably the costliest 

 procedure. We believe, however, that more consideration should be given to a 

 long-range, more permanent control. 



Surface waters of southwestern United States are almost entirely utilized, and 

 ground water is being pumped at a rate that exceeds the estimated recharge. 

 In some areas in this vast region the average depth to ground water has been 

 found to be increasing at an annual rate in excess of 20 feet. In the light of 

 such frightening statistics we should realize that we should delay no longer in 

 learning all we can about our water resources and every factor that may have 

 any kind of influence upon them, no matter how trivial such may seem to be. 



Our decision as to what plants should be included in this work has been 

 influenced as much by practical and utilitarian factors as by strictly biological 

 considerations. In respect to interpretations, we have found that the most 

 exasperating and frustrating part of the work is that which involved decisions as 

 to what species to include; in other words, what should the limits be? After being 

 certain that all strictly aquatic and wetland species have been included, we found 

 that the periphery of inclusion had a tendency to spread to the margin of 



I 



