THE LIFE ZONE MAP
It is realized that the life zone concept is open to serious criticism when used for intensive studies of small areas, and that when used for any purpose it may be open to charges of vagueness. Although the present treatment cannot hope to escape altogether such charges, an effort has been made to designate the limits of the zones as definitely as possible.
It was only after weighing all the factors involved that the decision was made to describe the Colorado River Basin in terms of life zones rather than by plant associations comparable to those used by most ecologists today. The deciding factor was that of simplicity for the benefit of the average nonscientific reader. It was believed that loss of critical definition, resulting from the use of these more generalized zones, would be more than compensated for, in semipopular treatise, by the reduction in number of classifications.
The life zone map of the Colorado River Basin is like nearly all maps of large areas that have ever been made, in that no matter how great has been the expenditure of care and energy to insure its accuracy, someone more familiar with local areas can discover errors of various degrees of importance. In the case of the present map, much less time was available for its preparation than originally had been anticipated, owing to circumstances beyond the control of the Colorado River Survey project. In fact, the life zone study never could have been completed if it had not been for the air surveys, which made possible a fairly comprehensive coverage of the basin within a period of only 9 months. Even so, the final product would have been much less accurate had it not been for the generous help, elsewhere acknowledged in full, of many persons having an extensive technical knowledge of various parts of the basin.
As a first step, all available botanical, zoological, and ecological publications were consulted in order to establish the average elevation boundaries of all of the life zones in the various parts of the basin. By no means all of these publications were specifically concerned with life zones, but since the major plant indicators and association equivalents of the zones have long been recognized, any information on the distribution and elevation of these indicators in various parts of the basin could be translated fairly easily into terms of life zone boundaries. These preliminary data were summarized as follows:
Life ZonesLower Limits
1 Bailey, Vernon, Mammals of the Grand Canyon National Park, U. S. Department of Interior, MS.
A transparent tracing-cloth base map of the Colorado River Basin was laid over the appropriate Regional Aeronautical Charts (Department of Commerce), which show topographic relief at 1,000-foot intervals. Provisional life zone boundaries, based on the foregoing tabulated data, were drawn on the tracing cloth, using the contours shown on the aeronautical charts as general guidelines. The topographic accuracy of these charts, which are on a scale of 16 miles to the inch, is for the most part quite high even for the more remote, unsettled regions. Contact prints of the completed tracing-cloth map, which measured 34 by 57 inches, were then taken into the field and compared, area for area, with conditions on the ground.
Nearly all of the field checking was done in a two-place light plane at altitudes varying between five hundred and three thousand feet above ground level. Planes of this type, which can cruise up to 6 hours without landing and give approximately 16 miles per gallon of fuel are receiving increasing use in many types of regional surveys and certainly will have an important and permanent place in such work.
Corrections were made as carefully as possible on the field copy of the life zone map while in the air, together with any pertinent notations. Occasionally the changes also were indicated on a set of Sectional Aeronautical Charts having a scale of 8 miles to the inch. Approximately 10,500 linear miles were flown on this reconnaissance project, which pretty thoroughly covered the basin. (Plate 2, in pocket.)
Ground studies are an indispensable supplement to air surveys and should follow them for purposes of verification, but the air surveys should come first because they provide at the outset a visual and mental grasp of the entire region which could be attained only by months or years of ground work. This is particularly true of large unfamiliar regions having a rugged and complicated topography which can be glimpsed only intermittently and at long range by ground travel, but it holds good in any type of country.
A disadvantage of air surveys is that some landscape features, such as potential development sites, appear deceptively small and are in danger of receiving inadequate consideration. With practice, however, supplemented by a checkup on the ground, one learns to make the proper correction for scale. Moreover, the required correction becomes less as one makes the surveys from lower altitudes.
Other disadvantages of air observation, such as the impossibility of making stationary observations are relative (light aircraft usually being easy to maneuver in small circles), and with practice can be greatly reduced. However, the viewpoint of persons on the ground with respect to scenery, and the feeling for the type of recreational use to be made of the area by persons confined to the ground can best be appreciated while one is on the ground. This probably represents the most important advantage of ground studies over air surveys.
The air surveys showed that for many areas the provisional life zone map was surprisingly correct even in minor details, which demonstrates the accuracy of the aeronautical charts and the published ecological studies already mentioned. In a few localities actual conditions on the ground differed widely from assumed conditions on which the map had been based. This was particularly true of the Upper Sonoran Zone in the Little Colorado River Basin, which on the first draft of the map was taken from the life zone maps of Merriam (1890) and Swarth (1914) instead of from the altitude table (pp. 220 and 221), with which the maps were in conflict. These early life zone maps had been drawn up before the life zone concept had been fully developed and before detailed studies had been made of this particular basin. Apparently, these writers based the lower limit of this zone on the lower limit of the piñon-juniper belt in the basin, but subsequent studies have shown that this is not the true lower boundary of the zone in this particular area. Lack of moisture is the operating factor at the lower boundary of the piñon-juniper belt in the Little Colorado River Basin, and on the basis of temperature conditions the Upper Sonoran Zone actually extends to a much lower level in this area, as demonstrated by the presence of other Upper Sonoran life zone "indicators" of more elastic moisture requirement.
Some errors were found to have resulted through having taken data from some of the more recent vegetation type maps, but these were few. They arose, no doubt, from the physical impossibility encountered by the type mappers of covering all of the terrain on foot, which occasionally led to misidentification of distant vegetation belts.
By far the commonest source of error in the provisional map arose from assigning, on the basis of topographic data, a higher life zone to an isolated mountain region than actually occurred there. Although special care had been taken to avoid this mistake, the warming effect of large surrounding desert regions often was even more extreme than had been anticipated. This was exemplified in the Kaiparowits Plateau, the Abajo, La Sal, Virgin, New York, Cedar, and other mountain masses.
Important assistance has come from the many persons who have made more or less intensive biological or botanical observations in various parts of the Colorado River Basin and who have generously contributed their time and special knowledge of local conditions in checking various portions of the present life zone map, as well as the biological information presented in the text. Of these persons, special thanks are due Dr. Angus M. Woodbury, Dr. William H. Behle of the Department of Biology, and Dr. Seville Flowers of the Department of Botany, of the University of Utah; Dr. Harold S. Colton and Edwin D. McKee of the Museum of Northern Arizona; Dr. Alden H. Miller and Dr. Seth B. Benson of the Museum of Vertebrate Zoology, University of California; A. J. van Rossem, Department of Zoology, University of California; Dr. H. C. Bryant, superintendent, Grand Canyon National Park; N. H. Frost, Field Technician, Arizona Fish and Game Commission; Joseph S. Dixon, Dr. Adolph Murie, Dr. James W. Moffett, Clifford C. Presnall, and Dr. Walter P. Taylor, of the Fish and Wildlife Service; Dr. Forrest Shreve of The Desert Laboratory, Tucson; Dr. Charles T. Vorhies, Department of Entomology and Economic Zoology, and Dr. Robert A. Darrow and Dr. Walter S. Philipps, Agricultural Experiment Station, University of Arizona; Dr. E. Raymond Hall, Museum of Natural History, University of Kansas; A. E. Borell and Dr. R. M. Bond, Soil Conservation Service; Dr. Alfred M. Bailey, Colorado Museum of Natural History; Dr. W. G. McGinnies, Rocky Mountain Forest and Range Experiment Station, and A. A. Nichol, Emergency Rubber Project, U. S. Forest Service; Dr. A. O. Weese, Department of Animal Biology, University of Oklahoma; and Dr. John W. Scott, Department of Zoology, University of Wyoming.