THE ALTITUDINAL LIMITS OF VASCULAR PLANTS

The alpine flora of the great mountain systems of the earth has attracted the interest of botanists for many years, especially since the pioneering work of Humboldt at the beginning of the 19th century. Humboldt not only collected plants at the highest altitude yet attained (on Chimborazo, in the Ecuadorian Andes) but also initiated the science of plant geography with his essays on the effect of latitude and altitude on the distribution of plants. A by-product of the mountaineering movement led by Whymper and Conway in the latter part of the 19th century was the accumulation of collections of high-alpine plants from the Andes and Himalayas. Since World War II a great many expeditions to the Himalayas and other high mountains have tremendously increased our knowledge of the plant life in some of the most inaccessible terrain on this planet. Hemsley (1902) and Pampanini (1930) reviewed the records of the flora of the uppermost levels of the Himalayan-Tibetan region, Heilprin (1892) analyzed the flora of the Mexican volcanoes, and Fries (1905) provided a valuable discussion of high-altitude plants in the Andes. Recently, Hedberg (1951, 1957) summarized the records from the high African mountains, and Reisigl and Pitschman (1959) presented an exhaustive analysis of the high altitude flora in the Alps. The only mountainous region with a true high-alpine flora which is extremely poorly known is New Guinea; the fragmentary records are discussed by Van Steenis (1935). To my knowledge, however, no one has attempted to bring together and correlate the high-altitude records for the entire earth. My interest in high-alpine flora developed during a Harvard Mountaineering Club expedition to the Karakoram Range, north Pakistan, in the summer of 1955. When 2 members of the climbing team, Dr. John Humphreys and Mr. Craig Merrihue, brought down 2 flowering plants from rock crevices at an altitude of over 19,000 ft. it appeared that these plants-Draba altaica and Waldheirnia glabra-might represent an altitudinal record for the angiosperms. Consequently, a search through the literature was made to check on previous reports. In the older literature Schlagintweit is credited with finding flowering plants well above 19,000 ft (up to 6038 m, or c. 19,800 ft), but as Hemsley (1902) remarked, it is probable that his altitudinal figures are erroneous. Hemsley regarded Saussurea tridactyla, collected in Tibet by Thorold at 19,000 ft, as the highest authenticated record; and Pampanini (1916) did not find any Himalayan plants which exceeded this level. However, very recently N. Polunin (1960) has reported not less than 9 species (not all identified) from Mohala Bhanjyang, Nepal, at an altitude of 5944 m (19,500 ft). Polunin appears to be correct in considering this as the highest altitude yet reported. He also quotes 0. Polunin as having observed flowering plants as high as 21,000 ft (6400 m), but so far no specimens have been taken at such heights. At any rate, we may expect that as exploration proceeds in Nepal and other high areas in the Himalayas the present record will be exceeded and flowering plants will be found at altitudes of over 6000 m (19,685 ft). To facilitate discussion, the data on high-alpine records has been organized into 4 tables. An attempt has been made to exert some critical scrutiny on literature reports, but this is very difficult. It is not certain that either the determinations of the plants or the altitudes stated are correct, although reports showing obvious inconsistencies have been eliminated. Undoubtedly a considerable number of reports has been overlooked, since the literature is extremely scattered. Nevertheless, the tables give a general idea of what is known about the distribution of high-level vascular plants, and at the same time suggest lacunae which need to be filled. Table I indicates in a general way the relationship between latitude and altitudinal limits for alpine plants. As might be expected, the upper elevational limits parallel those of the tree-line (Daubenmire 1943) and snowline, but the correlation is not exact. In some instances apparent discrepancies may simply be due to the fact that certain mountains (such as the Alps) have been more exhaustively studied. There is also reason to question the accuracy of some of the statements in the literature. Many writers speak of the upper limits for all vascular plants as though they coincided with the snow-line, but this is by no means necessarily so (Heilprin 1892, et al.). Furthermore, the level of the snowline may vary several hundred feet on the same mountain. It is obvious that the relation of latitude to altitudinal limits is not simple. For instance, it is certain that vascular plants reach higher altitudes in the Mt. McKinley, Alaska, area than in the Torngat Mountains, Laborador, even though the latter are 40 farther south. As Abbe (1936) points out, the depression of the alpine flora in the Torngats is probably related to the unfavorable local climate (i.e., low temperature due to winds off the Labrador current). On the other hand, it is surprising that the upper limits for vascular plants are over 1000 m lower on the Mexican volcanoes such as Citlaltepetl (Orizaba) than they are in the Himalayas (Masherbrum), even though the latter peak is about 170 closer to the North Pole. This depression of the Mexican alpine flora relative to the Himalayan has not been explained, and is rather difficult to understand in view of the fact that in the Andes elevations comparable to the Himalayas are attained (e.g., 5800 m in Nevado de Cachi, at 250 S). Possibly this may be attributed to the effect of size of mountain masses on temperature, as suggested by Daubenmire (1947). The smaller size of the Mexican volcanic belt, according to this view, would be correlated with a lower firn line than that in the Andes or Himalayas. The available data (Table I) indicate that the highest alpine plants occur at intermediate latitudes (28-36? in the Himalayas, 16-25? in the Andes) whereas at lower latitudes-as in the African mountains-altitudinal limits are somewhat depressed. This apparent difference may not be significant, however, because there are not enough records and it is not certain that many of these reflect the true limits. The region which best shows the latitudinal effect is the Andes, and it is still rather poorly known. The addition of alpine records from such mountains as Santa Marta (110 N), Huascarain (9? S), and