The Desert Laboratory of the Carnegie Institution of Washington

Forrest Shreve

Some of the earliest and most important steps in civilization were taken in the deserts of Asia and Africa, thousands of years ago. Now, in this twentieth century, we find ourselves in the desert again, trying to learn how to adjust ourselves to its conditions, how to make possible a life which shall be rich and fruitful even if it is not like the life of our forefathers, and how to make the desert feed and clothe and shelter us, at the same time that its broad vistas, rugged mountains, unique plants, and matchless coloring afford us some of the inspiration which life requires.

In the days of Babylon, Nineveh and Carthage, man was the slave of the desert, for it controlled his life. The people of European ancestry who are now entering the deserts of the Southwest are armed with resources and knowledge and with the distinctly modern habit of investigating their surroundings and giving a rational and organized basis to their experience.  The possibilities of the newer desert civilization are large. If its material advantages can be realized, at the same time that it preserves some of the traditions of the old desert civilization and of the old Southwest, there should be a bright future for the American Desert. Far-seeing men, interested in the desert country, aware of its potentiality, but fully realizing the necessity of knowing it better, are the ones responsible for the existence of the Desert Laboratory. One of the earliest projects presented to the Carnegie Institution of Washington was for the establishment of such a laboratory, primarily for the investigation of plant life of the arid regions. The suggestion resulted in the appointment of an advisory board, charged with the selection of the best location and the outlining of plans for work. The members of the board were Dr. D. T. MacDougal, then Assistant director of the New York botanical Garden, and Mr. F. V. Coville, Chief Botanist of the U. S. Department of Agriculture. These men, already familiar with the whole arid region, made a further examination of promising localities, and selected the Tucson region as the best place for the Desert Laboratory, not because it is the most desert spot to be found, but because it has the richest and most diversified vegetation of any area in the arid part of the United States, and also because of the varied types of country that are easily accessible from Tucson in every direction.

In 1903 the first building was erected, two miles west of Tucson on the slopes of Tumamoc Hill, and Dr. W. A. Cannon was appointed Resident Investigator. Two years later the Carnegie Institution organized a Department of Botanical Research, placed Dr. D. T. MacDougal at its head, and made Tucson the headquarters of the department. In 1906 the laboratory was enlarged and a staff of workers was brought together. In 1928 the growth of the biological work  of the Carnegie Institution in the western states led to the formation of the Division of Plant Biology, of which the Desert Laboratory is one of the centers of operation.

At the time of the establishment of the Laboratory, a tract of over 800 acres of hill and plain surrounding the buildings was acquired by purchase and lease. This area has been effectively fenced since 1907, and the long period without disturbance has brought the plant life back to virgin desert conditions such as one can find only in the most remote and ungrazed parts of Arizona. The grounds of the Laboratory have been of great importance for its work, giving a chance for continued observation of the growth, seasonal habits and reproduction of the plants, a handy source of living material, and an opportunity for conducting instrumental and experimental work under outdoor conditions without danger of interference. The location of the Laboratory buildings, 335 feet above the valley of the Santa Cruz River, places them in the midst of the hill type vegetation, which is particularly rich and diversified on the heavy clay soil of Tumamoc Hill and other basaltic hills in the vicinity. The location of the Laboratory also gives the workers freedom from interruption, clean air, and an inspiring view of the valley and surrounding mountains.

During the past twenty years eleven investigators have worked at the Desert Laboratory for long periods, over forty men an women have worked from two to twelve months, and several hundred scientists from home and abroad have made short visits, to secure some particular plant or animal material for study elsewhere, to prepare exhibits for museums, to secure data, to consult with members of the staff, or merely to see and examine the unique plants in their natural setting.

 

In the patio of the Desert Laboratory the natural vegetation has been preserved
and supplemented with plants for study and experimentation.

The results of work done at the Desert Laboratory have been printed as publications of the Carnegie Institution, and as articles in a number of scientific journals. Up to the present time about 355 books and articles have appeared describing work done there. Many other papers have been based on material supplied by the Laboratory or on work done elsewhere by members of the staff. Four splendidly illustrated volumes describing all of the members of the cactus family were prepared by Britton and Rose, on the initiative of Dr. MacDougal, and financed and published by the Carnegie Institution.

                The aim of the work at the Desert Laboratory has been to learn as much as possible about the natural behavior of plants, the character and fluctuations of the conditions of climate and soil, and the relation of the latter phenomena to the former. The desert Environment has been studied, particularly with reference to rainfall, evaporation and the moisture of the soil. Work on the physiology of plants has been chiefly concerned with their relations to water supply and water loss, and with the influences exerted on both succulent and non-succulent plants by conditions of scanty and irregular water supply. Field work and laboratory experimentation have gone hand in hand in an effort to learn the process which enable plants to persist under the extreme conditions of the desert. Each of the investigators who has worked at the Desert Laboratory has contributed a few lines to the growing picture which will one day enable us to understand the character and the limitations of desert life.

It has often been remarked that the finest bits of desert in the Tucson Region seem like some immense botanical garden. The impression comes from the widely dissimilar plants that may be found growing together. The giant saguaro, the green-barked palo verde, the thorny ocotillo, the graceful creosote bush, the shining cholla, the acacia, the barrel cactus, a dozen of other types, are mingled with each other in matchless landscape effects. These plants, which are so unlike in appearance, in structure, in the seasonal habits, and in their physiological behavior, are nevertheless growing together on the same soil in the same climate. To what extent these plants have developed identical behavior in spite of their different appearance? Are the spots identical in which they grow, even if they look alike? Are there any other soils or other climates in which some of the members of this group would not be found? What are some of the ways in which these plants have worked out different solutions to the same problems? A good deal has been learned that would help to answer these questions, but the answers are by no means yet complete.

                An interesting example of the way in which the same problem may have more than one solution is to be found in the comparison of cacti with other desert plants which are perennial, but not succulent, and still others which are short lived. Each of these groups has a different method of securing enough water to maintain life, to grow and to produce seeds. The cacti have root systems which are widely extended through the uppermost layers of soil. Very soon after a heavy rain their absorbing roots become active, and in a few days they take from the ground an amount of water which will last them as many weeks. The larger cacti contain a great deal of water all the time, varying from 70 to 90% of their weight. They secure their water quickly at times when it is abundant and lose it slowly, at a rate which depends on the heat, dryness, sunshine and wind. The trees and shrubs, such as mesquite, palo verde, acacia and creosote bush (often improperly called “greasewood”) all have a limited amount of water in them at any time. Their roots penetrate the soils to a depth of 6 to 3o feet. They are able to secure some of the water in the upper layers of the soil, but in the dry seasons they depend on the water which has penetrated to deeper levels. Unlike the cactus they must secure from the soil each day an amount of water very closely equal to what they have lost on that day. It has been found that night is very important for the non-succulent plants. In the cool, dark hours they lose much less water than in the daytime, and this gives them a chance to catch up. The third group of short-lived plants are not really desert plants at all so far as their structural features are concerned. Their appearance is confined to the rainy seasons and it is only their seeds which have to withstand the dry periods. Many of the short-lived plants every resemblance to the plants of moist regions, but nevertheless they are able to live in the desert alongside the cactus and the mesquite.

There is surely no plant in all Arizona which attracts more attention than the state flower, the sahuaro. It is a weird surprise to the newcomer and a cherished symbol of home to the Arizonian.  Along with other kinds of massive cacti found in Mexico, it has many interesting features of structure and behavior. The commonest botanical question asked in Arizona is, “how old are the biggest sahuaros?” The answer to this is based on indirect evidence, through a study of its rate of growth at different heights, for there are no annual rings to count. It takes from 15 to 25 years for them to reach a height of one foot, unless they are given some extra water. They grow very slowly at first, and then speed up to a growth of three to four inches per year in the best localities. The age of the very large ones is between 150 and 175 years. The showy white flowers of the sahuaro appear in may, forming a crown at the top of the trunk and at the tip of each branch. The juicy red fruits come to maturity in June, just as the dryest time of the year, and the pulp and seeds are eagerly sought by birds. The fruits and the woody skeleton are the only parts of the sahuaro that are useful to man, for it contains extremely little starch, sugar or other substance of value.

Another striking plant of the desert is the ocotillo, which looks like a cluster of loosely held wands, beset with thorns. After every rainy period it bears leaves, which are large and thin as compared with other desert leaves, and are destined to turn yellow and fall as soon as the soil begins to dry out. There may be half a dozen crops of leaves on the ocotillo in a single year, but its flowers are born only once, in the early spring. The brilliant scarlet racemes, appearing on the ends of the branches, are among the showiest flowers of the desert. Just because it has thorns, the ocotillo is often supposed to be a cactus, but is not.

Among the various pieces of field work that have been carried out at the Desert Laboratory in past years has been investigations of the root systems of a number of the larger perennials, studies of the changes which resulted from the recession of the Salton Sea, in southern California, the study of the distribution of plants in relation to the physical texture of the soil, and the investigation of the relation between the vertical differences of vegetation and of climatic conditions on mountains. Visits have been made from the Laboratory to the deserts of Egypt, Algeria, South Africa and Australia, for the comparative study  of desert plants and conditions.

The investigation of environmental conditions is of fundamental importance for all research in the physiology and ecology of plants and animals. It is not only desirable to know the average conditions of rainfall, temperature, humidity, etc., but also to know the extreme conditions, for it is under them that the plant and animal life is subjected to the greatest risks.  Ever since the establishment of the Laboratory, continuous records have been kept of the most important climatic factors, and for shorter periods data have been secured on light, soil moisture, soil temperature and other conditions. For Tucson the average rainfall from 1868 to 1925 was 11.70 in. Since 1905 the annual rainfall at the Laboratory has been about 1 in. greater than it has been in Tucson. Over half of the precipitation falls in July, August and September. Since 1908 there have been six times when there was no rain at all for ninety days or more, the longest of these periods extending from January 11th to June 1st, 1910. The most extreme temperatures that have been recorded in the past 25 years are a maximum of 112 degrees and a minimum of 6 degrees. Greater extremes can be secured by using improperly exposed thermometers, or by taking observations in special localities.

Several small areas have been laid out on the grounds of the Desert Laboratory for observing the changes that take place in the vegetation over long periods. Accurate maps of these areas were made in 1906, showing the location of every perennial plant. After twenty-three years without disturbance the plant populations of some of these areas has greatly increased. On two areas located on the slopes of Tumamoc Hill there has been little change in the population. In all cases there are many of the plants of 1906 which are now gone, which is even true of the most long-lived ones like the sahuaro and the palo verde. Certain species of plants are now to be found in some of the areas which were not represented there in 1906. The continued studies of these areas will determine the average length of life of the plants, and will give a vivid picture of the ceaseless changes in nature.

                A great deal of importance for the study of desert plants attaches to the dry hot period which extends from late March or early April to the commencement of the summer rains about the second week in July. This is a period of dry soil, lengthening days, almost continuous sunshine, and increasing heat. The condition of the soil at this season, the movement and loss of water by plants, and the mechanism by which plants secure water and control its loss, are all features which have been given considerable attention and are now one of the uppermost groups of problems at the Laboratory.

                Almost immediately after the first heavy rain of the mid-summer there is a striking transformation of the desert. The lean cacti become turgid, the trees begin active growth, the dormant shrubs come into leaf, herbaceous annuals spring up in great abundance, and the insect life is increased a thousand fold. The physiological phenomena which accompany the rapid change in the behavior of the perennials at this time have been studied very little, and are one of the promising fields for future work.

                 One of the striking things about the ephemeral plants which appear after each of the rainy seasons is that the group appearing in the later winter and those growing in the summer are entirely distinct. The seeds of the winter ephemerals lie in the moist soil of summer without germinating, and the seeds of the summer plants do the same in the winter. This is known to be due to a difference in the temperature required for germination in the two groups, but the reasons for such a great difference in temperature response have not been investigated.

                 The energies of the men who have worked at the Desert Laboratory have been wholly devoted to research, leaving them no time for education work, and no opportunity to help in the popularization of the knowledge that they have acquired about the plants of the arid regions. The effort is being made to attack the fundamental problems, and to pursue them consecutively, so that the work of the Laboratory can be of some help in swelling the great fund of knowledge about the environment of man, and of some use to those who are directly concerned with the practical problems of the plant industries.

 

Reprinted from Progressive Arizona and the Great Southwest
(this booklet was not dated; however, judging from the material presented it dates from around 1930.)
 
Forrest Shreve died in 1950 at the age of 72)