PRESENT PROBLEMS IN PLANT ECOLOGY 1

III. VEGETATION AND ALTITUDE

PROFESSOR CHARLES H. SHAW

THE MEDICO-CHIRURGICAL COLLEGE, PHILADELPHIA

IN the study of the relation of plants to environment, there are few problems of greater interest than those presented by the vegetation of mountains. The general facts are somewhat familiar, and reference to them is here necessarily brief. Whatever the vegetation of the surrounding country, mountains are usually forested; the forest is often composed of several zones in which different kinds of trees successively predominate; higher up the forests finally cease and give place to grassland of perennial herbs and low shrubs-these are some of the more general facts of mountain vegetation. There is bound up with them not only strangeness and beauty, but also a series of most interesting problems in the ecology of plants.

If I understand rightly the reason for this symposium, my duty is to state, as well as in brief compass I may, the present state of our knowledge in regard to these phenomena. In general, they rest back upon physical environment. In so saying, however, it must be kept in mind that biotic factors early modify the primitive physical ones; the reason for the occurrence or absence of a species may be the conditions created by other species.

The important factors which vary with altitude seem to be heat, light, precipitation, evaporation, and a factor made up of several of those named, namely, length of season. Let us consider these severally, and try how far A series of papers presented before the Botanical Society of America, at the Baltimore meeting, by invitation of the council.

1

we may at present determine the part played by each in connection with the plant life of mountains.

1. Heat.- From the days of Humboldt to the present, the vast importance of temperature in this connection has been recognized. In spite of this, our knowledge is rather scanty and vague. It has been usual to attribute great importance to the severe cold of mountain tops. This view, however, is entirely open to challenge. Mountains which rise up from warm plains certainly can not be invaded by plants which are killed by frost, but is there warrant in any physiological knowledge we now possess for supposing that extreme low temperatures are of any especial significance for plants which endure freezing? There is reason for believing that some woody plants are cracked by severe cold, but I believe the general question must be answered in the negative. That treeless mountain tops are not so because of great cold is shown by two facts: First, in cases that have been investigated, timber lines do not bear any direct relation to isothermal lines; and furthermore, forests do exist in the coldest districts on the globe. Herbaceous plants, too, taken even in the active condition, are known to survive extremely low temperatures, undisturbed.

We may note in passing the obvious fact that, for most low growing mountain plants, deeply buried as they are under snow, extreme winter temperatures do not come in for consideration at all.

That the lower temperatures of air and soil during the growing season are factors of great importance is not to be doubted. More specific information is not easy to obtain. Direct observation avails nothing, for we can hardly point to a single feature of anatomy or histology which is called forth by or conditioned upon heat or cold. In order to make further progress, recourse is naturally had to instrumental study. In the use of this method, many pitfalls await the worker. It is relatively easy to secure data of temperature, but vastly difficult to interpret them.

It seems to me that, seeing a good

many ecologists are making use of temperature data, this point will bear some emphasis. Feeling that ecology is new, and exact instrumental work is the kind that counts, it is very easy, when one has secured a fine series of readings, or better still, a complete thermographic tracing for a growing season, to entertain the impression that he has accomplished something of note; whereas the fact in the matter simply is that if he has done his work well, it is of a quality with the routine work of a weather bureau. To relate the physical data to the manifold activities of a living plant is another matter, and calls for all the power of critical thought and all the knowledge of physiology which any man can command.

For our purpose, it seems possible to do little more than to point out some difficulties to be surmounted. To begin with, any method which assigns increasing values to higher temperatures must go astray as soon as the plant's optimum is passed, and for most of the plants we are dealing with, we do not know where that is. If, also, as there is some reason to believe, the growth-temperature curve has more than one maximum, a still further difficulty would be brought in.

Furthermore the ecologic optimum is made up of many harmonic optima, and may vary in different life phases of the same plant. In experiments in forcing fruit trees, it has been found that the optimum for blooming is markedly lower than for other periods for the plant's activity. Finally the temperatures recorded are for the soil or air, whereas the ones wanted are those that prevail within the plant. Leaves and shoots are warmed by sunshine and cooled by the evaporation of water. In this way temperatures may be brought about which differ materially from those recorded by a thermometer alongside. In the case of an Alpine plant, sheltered in some sunny angle of rock, how widely the temperature within its leaves may differ from that shown by a thermometer near by, properly set up for air temperatures in shade!

On the positive side, there seems less to be said. It

can not be doubted that the plant life of mountains is in no small degree modified and controlled by temperature, but who can put his finger on definite facts and say "this temperate forest is on this mountain in the tropics because of temperature," or "this Alpine plant is a dwarf because of cold." Strongly as we may suspect such points, we must be cautious about including them within the realm of our positive knowledge.

The peculiarities of Alpine plants have been thought to be due, in part, to great daily variation in temperature; that owing to the greater clearness and rarity of the atmosphere, the plant is more exposed to heating by intense sunshine by day, and cooling by more rapid radiation at night. Since growth takes place particularly at night, it seems evident that marked night cooling would lead to reduced size. That this is actually true has been proven by placing growing plants each night in the ice chest. At present more light is chiefly needed as to what temperatures are really experienced by Alpine plants over night. Air temperatures, at any rate, are more equable at high altitudes. In connection with the work in the Selkirks during the past summer, two complete thermographic records were taken, one meter from the surface, same hillside and exposure, at altitudes of 800 m. and 1,700 m., respectively. The daily maxima recorded at the upper station were notably less than at the lower; the nightly minima only slightly so. was not recorded for many weeks.) well with the general results of meteorologists. refer of course only to air temperatures.

(Freezing point Such data accord They

Briefly summarizing, we may say that our knowledge of the relation of heat to mountain vegetation is not great. The importance of extreme low temperatures has been much overestimated. The significance of moderate temperatures is not yet capable of exact interpretation. The hypothesis of night cooling as a cause of alpine dwarfing needs further physical facts as a basis.

2. Precipitation.-That mountains

are islands of

greater precipitation and that the vegetation of mountains is largely dependent upon atmospheric water are ideas familiar to every one. Schimper has further suggested that light showers at high altitudes favor the development of grassland rather than forest. This idea, to be tenable, requires the further assumption that the soil in the grassland zone is deficient in water supply. This may be true for lower latitudes, but it scarcely seems admissible for northern mountains where the late melting snows leave the soil supplied with all the water it can hold.

In quite a different way, too, precipitation in the form of snow becomes a decisive factor for vegetation. In a paper read before this society, a year ago, I showed that the timber line of the Selkirks was due to the heavy snow beds at those altitudes. By a coincidence, Cowles showed elsewhere, at the same time, that in a number of places in North America snow beds and timber line were causally related.

3. Closely connected with the question of heat and precipitation is another factor of prime importance in some mountains, namely, Length of Season.

In respect to this factor at least two points of view are necessary. For trees and plants growing on wind-swept spots, length of season is a question of temperature. For the vast majority of low-growing plants, on the other hand, length of season is also a question of emergence from snow beds to air and sunshine.

Taking up, first, the question of forests, it has been said that the total heat available at high altitudes is not sufficient for the maturing of new wood, and more particularly, that the season is too short for the ripening of good seed. Both of these ideas must stand or fall simply upon evidence, and so far as I am aware, none of a reliable character has been brought forward. It may be remarked that seedling trees usually seem abundant enough at timber line. It would be of interest to gather seeds of balsams, etc., growing at timber line and learn