PRESENT PROBLEMS IN PLANT ECOLOGY: a PROBLEMS OF LOCAL DISTRIBUTION IN ARID REGIONS." By Prof. VOLNEY M. SPALDING, The physical conditions prevailing in arid regions are such as render it unsafe to admit without further investigation generalizations regarding their plant life which have been drawn from studies conducted elsewhere. This is sufficient justification of an attempt to analyze certain problems which confront the student of desert ecology in his efforts to apply knowledge or principles drawn from previous experience. These problems have the advantage of a certain clearness of definition, which corresponds in a way with the sharp features of the desert and its characteristic vegetation. Their solution may involve great difficulties, and some of them, with our present methods, may be incapable of solution, but they are, at all events, capable of clear statement. In the attempt to present such a statement, which may or may not prove successful, I shall for the present limit the discussion to the desert country of the southwestern United States, for the sufficient reason that my own studies have been conducted in that region; and I shall omit all consideration of the higher elevations of the mountains, which, though in the desert, are not of it; so that whatever is said at this time will be understood to apply to the floor of the desert, that is, the great plateaus and valleys which from Texas to California lie between the mountain peaks and ranges, together with the long slopes and low hills which border them on every hand and form the natural approach to the mountains. Proceeding in a manner that will be indirectly a record of personal experience, one of the first questions presented to a student of desert botany is this: What are the conditions that determine the successful occupation of a desert habitat by certain plants, but prevent its occupation by others? "A series of papers presented before the Botanical Society of America, at the Baltimore meeting, by invitation of the council. Reprinted by permission from The American Naturalist, vol. 43, No. 512, August, 1909. It will be necessary at the outset to understand what is meant by a desert habitat, since on this point the popular conception-and possibly that of some botanists-is not clear. There is as much difference between habitats in the desert as in any other region, possibly more, and their definiteness of location and relative sharpness of demarcation form one of the most striking.and characteristic features of arid regions. The rivers of the valley trough, such as the Santa Cruz, the Gila and Salt rivers in Arizona, though inconstant, are none the less the main drainage channels between the adjacent watersheds. Along their banks water-loving willows, cotton woods, and arrow weed find a congenial home. The adjacent flood plain, with its water table within reach of their roots, is the natural habitat of the mesquite and some other semimesophytic species. Within its limits the areas known as salt spots are inhabited by various halophytes, especially by species of Atriplex and Suada. Just beyond the flood plain is the long slope, a most characteristic feature of desert topography, which rises slowly to the foot of the mountains, often miles away, its soil and drainage conditions presenting a sharp contrast to those of the flood plain, and its vegetation being correspondingly different. The low outlying hills, in their turn, present quite as marked peculiarities of soil, and furthermore introduce differences of aspect which are correlated with marked differences of vegetation. In short, the habitats of such a desert region as that of southern Arizona, as far as edaphic relations are concerned, present conditions which vary all the way from distinctly hydrophytic to extreme xerophytic, and all these may be in close proximity. For all these habitats the fact is to be emphasized that the general climatic conditions are the same, and it is important to note that not a few of the plants which grow where a sufficient or even abundant water supply is assured are nevertheless marked, as a rule, as plants of an arid region by their coriaceous, hairy or otherwise xerophilous leaf structure. The point to be specially noted here is that while plants of the arid or semiarid southwest grow in a great variety of habitats, some of which are by no means dry, all are subject to the severe conditions of a desert climate, especially intense insolation, low percentage of atmospheric moisture, and drying winds. The problem, therefore, of the occupation of any one of these habitats is successfully met only by those plants that are already adapted, or are capable of individual adjustment to the dry air and hot sun in which they must live; all others inevitably fail. This will be made clear by reference to the introduction, or attempted introduction, of various cultivated plants, a subject which presents a most instructive history. The yards of Arizona cities. constitute an experiment station in which year by year, at private instead of public expense, the availability of one species after another for desert planting is being determined. From the great number of plants successfully cultivated there seems, at first sight, to be sufficient justification for the reiterated assertion that anything will grow here if you only give it water enough, but closer attention to the actual facts of the case makes it evident that this statement is true only in part, and that there are many plants that will grow only indifferently or not at all under the atmospheric conditions which prevail here, especially in the summer time. To give a few examples, geraniums, the universal easily raised plants of moister regions, are very uncertain, some varieties accommodating themselves fairly well to the desert air, while others fail altogether. Cannas and gladioli, which grow side by side in the east, part company here, the former making a good growth in Arizona gardens, the latter failing altogether. Those who have handled roses for a period of years have learned what varieties may be expected to do well in the dry air of the desert, and what ones may be counted out, and so on through a long list of plants which, by knowledge gained in the costly school of experience, are coming to be depended on, or are being rejected one after another, as they are found to be unsuited to the environment into which they have been brought. Thus, in a purely empirical way, it has been found that many plants successfully cultivated in regions of greater atmospheric humidity make an entirely normal growth in the desert, if their roots are well supplied with water, but that others, however well cared for in this respect, either fail completely, or come short of making a healthy growth, and that this is especially true in the summer months when desert conditions are most pronounced. With the accumulation of such facts the more evident does it become that a very complicated problem is here presented. Why is it that one plant, properly watered, does well in the desert, while another, though treated in the same way, makes a poor growth or fails altogether? At first thought it would seem as though there must be a difference in the capacity of the root systems of the two plants for absorption, and that this may be a sufficient explanation of their different behavior; but it is evident on consideration, that with precisely the same capacity for root absorption, a plant in which transpiration is successfully regulated may thrive in an atmosphere in which one subject to excessive transpiration will perish. The most elaborate experiments and the most exact determinations of rate of absorption-assuming that such determinations are possible-would be very likely to throw no light on the problem. Comparisons of the transpiration rate of the plants in question appear more promising, but the same difficulty arises in an attempt to pursue the investigation along this line, for there is no reason to suppose that two plants of widely different rates of transpiration 45745°- -SM 1909- -30 can not successfully occupy the same habitat if their capacity for root absorption differs in the same ratio. But supposing that with infinite patience and with a reasonable approach to accuracy both sets of physiological data have been determined, we are still, quite possibly, entirely in the dark as to the real cause of the different behavior of the plants under investigation. It may be in their case that the whole matter of absorption, conduction, and transpiration is beside the mark, and that certain plants can not succeed in the desert because the intense insolation exerts directly a prejudicial influence to which they have not become inured. The intricate nature of the subject is apparent, and it is also evident that there is little encouragement for any one to take it up who has not had extended training and thorough equipment for physiological research. Yet with all its difficulties the problem is an attractive one, and the abundance of material to be had in any desert city, together with the great mass of data that has accumulated in the hands of horticulturists and at the experiment stations, offers the best of opportunities for extended and fruitful work. If, as we have seen, the different deportment in the desert of plants growing, or having the opportunity to grow, side by side in wellwatered ground, is an exceedingly complicated matter, by how much are the difficulties increased when we pass from a habitat of uniform and highly favorable conditions to the various and often extremely trying conditions which prevail in different neighboring habitats, such as the dry slopes underlaid by caliche, the salt spots, and others. If the case of a plant growing in well-watered soil may become desperate because of the scorching winds or the intense insolation to which its top is exposed, what hope is there for one that essays to grow where both dry air and dry soil present the supreme test of endurance? As a matter of fact only relatively few species meet the test successfully, yet there are some that do, and they present some of the most instructive data yet derived from the study of desert plants. But little reflection is needed to arrive at the conclusion that the classical question regarding the relative importance of physical constitution and chemical composition of the substratum to plant growth-though like the poor it promises to be always with us-does not and can not reach the heart of the problem. For every plant which successfully holds its place in a true desert habitat there is a delicate balancing of the regulation of transpiration, the power of absorption, the capacity of the conducting system, the presence or absence of storage tissues, and, we may well believe, the possession of protoplasmic properties which contribute to its powers of endurance. This being the case, it would seem that in future, investigations of the habitat relations, of desert species especially, must be directed mainly to the plant itself. The advantage of a thorough knowledge of soils is too obvious to call for comment, but it must be remembered that we are as yet only at the threshold of a greater and more promising work; namely, the investigation of the physiological requirements and capabilities of plants that can grow in a true desert habitat as compared with those that can not. In such comparative study lies, as it seems, the hope of real progress. It is impracticable for any investigator at the present time to mark out a straight path for others to pursue, and it would very properly be regarded as an impertinence were he to attempt this; yet there are certain obvious suggestions that may be offered. In the first place, important results have already followed the simplest experiments and observations when these have been conducted with exactness and with a definite end in view. To refer to a specific case, Professor Thornber, of the University of Arizona, undertook a few years ago to compare the habits of certain desert plants as regards germination. It was found that while the seeds of some species germinated at a given temperature, others could not be made to do so until they had been subjected to temperatures approaching the freezing point. These latter were seeds of winter annuals, and by this method a fundamental physiological difference between them and the summer annuals was established. Doubtless an indefinite amount of instructive and necessary work remains to be done in this direction, but the key to the situation was found in carrying out the simple experiments described. Again, partly as a relief from severer work, Doctor Cannon undertook, in the midst of his investigations at the Desert Laboratory, to map the distribution in the soil of the roots of some of the plants growing in the vicinity. Hardly was the work well in hand, and the root topography of less than half a dozen species mapped, when it was found that the clue to certain facts of distribution, blindly observed up to that time, had been discovered. I have spoken of this in more detail in another connection. Obviously it is indispensable that determination of physiological data and of those belonging to the physical environment should proceed step by step together; and nowhere is this more strikingly true than in the investigation of soil relations. To refer to one more case of recent experience, within the past year Doctor Livingston has determined the percentage of soil moisture present in soils obtained from each of the topographic areas of the Desert Laboratory domain and the adjacent flood plain of the Santa Cruz River. His studies were conducted independently, though naturally not in ignorance of ecological studies which were being carried out at the same time on the same ground. It now appears that a well-nigh perfect correspondence exists between the two sets of facts obtained by independent workers, so perfect, in truth, that a causal relation offers |