Topographically Varied. As the Mountains Rise, Glaciers and Rejuvenated Rivers Sculpture Them and Provide Habitats with Contrasting Exposures

VOL. 53, 1965 BOTANY: J. CLAUSEN 1315

or parallel position. Plastocyanin is thought to function after the second light reaction and before cytochrome f. We thank Prof. A. Trebst for his many helpful comments, for providing us with his most recent results, and for supplying samples of DAD. Abbreviations: NADP, nicotinamide adenine dinucleotide phosphate; ATP, adenosine tri- phosphate; DAD, 2,3,5,6-tetramethyl-p-phenylenediamine; TMPD, N,N,N',N'-tetramethyl-p- phenylenediamine. * Dr. Urbach's work was supported by a fellowship from the Charles F. Kettering Foundation. 1 de Kouchkovsky, Y., and D. C. Fork, these PROCEEDINGS, 52, 232 (1964). 2 Trebst, A., Z. Naturforsch., 18b, 817 (1964). 3 Trebst, A., and E. Pistorius, Z. Naturforsch., 20b, 143 (1965). 4 Duysens, L. N. M., Science, 120, 353 (1954). 5 Ibid., 121, 210 (1955). 6 Goedheer, J. C., Biochim. Biophys. Acta, 66, 61 (1963).

MICROCLIMA TIC AND VEGETATIONAL CONTRASTS WITHIN A SUBALPINE VALLEY BY JENS CLAUSEN DEPARTMENT OF PLANT BIOLOGY, CARNEGIE INSTITUTION OF WASHINGTON, STANFORD, CALIFORNIA Read before the Academy April 28, 1965 Most high mountain systems in the world are geologically young, precipitous, and topographically varied. As the mountains rise, glaciers and rejuvenated rivers sculpture them and provide habitats with contrasting exposures. The plant habitats of such mountains constitute a climatic mosaic with highly contrasting temperature zones in close juxtaposition. An example of such a geologically active region is the Slate Creek Valley of the Harvey Monroe Hall Natural Area, Mono County, California, where the Timber- line transplant station of our department is located. It is a recently glaciated, subalpine valley with predominantly north- and south-facing steep slopes that range in altitude between 10,000 and nearly 13,000 feet (3000-4000 m). Location and Geologic History.-The Slate Creek Valley is located on the east side of the Yosemite National Park, the summits of three Sierran peaks forming the west boundary to the park. It is in a region where great uplifts have occurred during the later part of the Quaternary period. Its west end is within the granitic batholith of the Sierra Nevada, and its east end within ancient folded and highly metamorphosed sediments. Slate Creek, which gives the valley its name, joins Lee Vining Creek that in crossing the Sierran uplift region drops 2500 feet within about 4 miles. This drainage is impounded in highly saline Mono Lake at 6400 feet of elevation on the western edge of the Great Basin. Slate Creek Valley is approximately 2 miles long from west to east and is a typ- ically U-shaped glaciated valley. As the glaciers retreated through the post- glacial period, they left behind about 11 terminal moraines, now populated by sub- Downloaded by guest on September 27, 2021 1316 BOTANY: J. CLAUSEN PROC. N. A. S.

alpine forest.3 The last two moraines are still very raw. Permanent snow patches lie within the Slate Creek cirque between White Mountain and Mt. Conness, and the Conness Glacier is still active on the north face of the mountain. Glacial polish and striations indicate that the Slate Creek glacier at one time was at least 700 feet deep, and erratic granitic boulders within the eastern metamorphic part of the valley bear witness to the transporting power of the glacier. One must assume that as the surrounding mountain areas were lifted in steps to their present altitude they carried their vegetation in position. These events must have caused greatly changed selective pressures. They need not have been catastrophic, however, because the highly varied exposures that obtain within subalpine to alpine valleys of young mountains provide temporary refugia, permitting time for selection and for evolution to change subalpine into alpine races. Rare alpine individuals can still be found among the Potentilla (cinquefoil) and Achillea (yarrow) populations from present subalpine regions at 7500 to 8500 feet.4 5 They constitute a part of the immense variability that exists as an evolutionary potential within all natural populations. One must assume that similar fore- runners were present in small numbers among the subalpine populations that were lifted to alpine heights during the earlier glacial periods. Relationships and Origins of the Present Vegetation.-In the Slate Creek Valley between the 10,000- and 13,000-foot levels one now finds a surprisingly rich vegetation. Approximately 350 species of flowering plants and ferns grow around and above tree line, making the vegetation of this valley one of the richest for such high altitudes. A local endemic vegetation of about 28 species has been supple- mented by three great vegetations of the world that appear to have converged toward this region. The local endemics, all high-altitude plants, could have existed on the ice-free mountain peaks during the period of maximum glaciation. Approximately 74 species of the Slate Creek Valley vegetation are members of circumboreal to circumpolar species complexes that are common to high latitudes in Eurasia and North America, or have close relatives there. These are exemplified by woody plants such as pines, hemlock, juniper, willows, mountain ash,2 arctic heathers such as Cassiope and Phyllodoce, and herbs including the arctic sorrel, Oxyria. These arctic and boreal species appear to have come south through the mountain ridges of western North America, and have their southern limits within the Sierra Nevada. Another group of 69 species constitutes the Pacific Coast element, invading from the west. They are members of species complexes that have evolved series of climatic races that enabled them to ascend the mountains, cross the passes eastward, and develop minute alpines within the Slate Creek Valley. These are exemplified by Eriogonum nudum, Lupinus polyphyllus superbus, Streptanthuss tortuosus, and the shrubby Penstemon newberryi. The largest element of the Slate Creek Valley flora is composed of about 182 species complexes that are native to the arid Great Basin and the Rocky Mountains, and appear to have entered from the east side, although some of them spill over to the west side. The species from the arid regions, such as the sagebrushes (Artemisia), the rabbit brushes (Chrysothamnus), and less arid plants such as the paintbrush (Castilleja miniata), seek the screes or talus of the steepest south slopes. Downloaded by guest on September 27, 2021 VOL. 53, 1965 BOTANY: J. CLAUSEN 1317

In the Slate Creek Valley plant species having their close relatives in northern- most Canada, Alaska, Siberia, and Lapland meet with alpines that are conspecific with plants of the mild west coast of California and with relatives of the desert plants of the Great Basin. All can live so close together because the high-altitude valleys provide multitudes of niches that differ in climate and soils. Evidence of Local Climatic Differences.-A dramatic example of the existence of a local warm pocket was observed within a 1000-foot-wide cirque representing the last step of glaciation within Slate Creek Valley. In the bottom of the cirque at 10,800 feet, snow lies until September, and open areas are sparsely populated by low alpines. A morainal island, 100 feet high and 200 feet wide, in the center is covered by a luxuriant population of tall subalpines including Veratrum californi- cum, Castilleja miniatum, Senecio triangularis, and Calamagrostis canadensis growing above their normal altitude. The highly frost-sensitive species among them were in September undamaged by frost. This morainal island appears to be in the focal center of heat radiations from rock walls facing east to south. Prompted by similar observations, Dr. Malcolm A. Nobs and I made brief periods of recordings of temperature that indicated that great differences may exist between the north- and south-facing slopes within the Slate Creek Valley. On the steep south slopes the rays of the sun strike almost perpendicularly during the summer, causing intense heating. On north slopes the rays arrive at low angles, comparable to regions near the Arctic Circle. During quiet nights the cold air from the higher points on the slopes drains to the bottom of the valley, causing sharp temperature inversions. Although these principles are well known, few observations have been reported on the extent of the differences. During 1962 seven weeks of the brief alpine growth period were spent in the Slate Creek Valley making population studies on conifers.3 This also afforded an opportunity to make regular temperature readings. Two Bendix temperature re- corders were installed together with Taylor minimum and maximum thermometers as a check. They were placed on the steep rocky slopes about 4 cm above the rock in settings that protected the thermometers from the direct rays of the sun, but permitted ventilation. The south-slope installation was at 10,600 feet, 100 feet above the tree line of Pinus murrayana3 and in the region of the only population of Mimulus lewisii within the valley. The north-slope installation was at 10,200 feet of altitude, also 100 feet above the tree line of P. murrayana on the that slope, also 100 feet above the tree line of P. murrayana on that slope. It was below a local elfinwood colony of Pinus albicaulis in a region where the subalpine and alpine races of that species in- tercross and mingle. The thermometers on the slopes were alternately read at 3 and 4 days' intervals and compared with daily readings of Taylor minimum and maximum thermometers in installations in a transplant garden at 10,000 feet in the meadow bottom of the valley, and in a south-slope garden at 10,050 feet. The readings at the 10,050- foot south slope consistently fell between readings in the meadow and on the 10,600-foot south slope. Readings on the north slope were consistent with readings the previous year over a briefer period at the same locality and at another location at that altitude on the same slope but 800 feet farther east. Figure 1 shows the range of the minimum-maximum temperatures within two- Downloaded by guest on September 27, 2021 1318 BOTANY: J. CLAUSEN PROC. N. A. S. Slate Creek Valley Temperature in C°: August 5 - September 17,1962 Noilro exposure IV-lliey floor acu- exr.s e 10,200ff 0000 IC 6^ Aug Sept Aug Sept Aug Sep' C° -17 5 -17 5 --- a-- T --T- T- - 1-- 30 ------Meor- CY

20 F MenMean Ma Mean Meo-~Mea o~~~~~~~oc-I --t --- mn _ ---____ m; m

eon P ne -- jT~~~~Mean

0 -

610,600 South ft North \expose exposure 10,200ff o

. v 10.000 ft Volley floor FIG. 1.-Temperature amplitudes in 0C at three locations of different exposures and altitudes, based on minima and maxima observed at 3- and 4-day intervals during 7 weeks of the growing season, 1962.

week intervals of the growth period at the three sites. It will be noticed on the north-facing slope that although the maximum temperatures were low, the mini- mums were 3-50C above freezing, and the amplitudes only about 110C. In the valley bottom, on the other hand, night frosts occurred during any half-week period, but the day temperatures were on an average 50C higher than on the north slope, causing diurnal amplitudes of about 20'C. On the south-facing slope, 600 feet above the valley floor, only one night during the 7-week period did the minimum temperature touch the freezing point, and the maximum temperatures ranged between 25 and 290C. The diurnal amplitude was some 25"C, about 21/2 times as much as on the north slope. It is clear from these observations that local climatic differences of great magni- tude exist between sites that are within 500-800 m distance from each other. It is therefore reasonable that plant groups of such diverse origin as mentioned above can find congenial habitats within the Slate Creek Valley. Certain species of arctic relationship, such as Oxyria digyna and Luzula spicata, are limited to north-facing slopes. The lodge pole pine, Pinus murrayana, is most vigorous and abundant on the high-day-temperature south slopes. On the north slope it does not thrive beyond the gently sloping 10,100-foot limit, but the Downloaded by guest on September 27, 2021 VOL. 53, 1965 B3O'TANY: J. CLAUSEN 1319

whitebark pine, P. albicaulis, produces large trees here despite the low day temperatures.3 Discussiom.-Temperature differences like those observed within short distances inside the Slate Creek Valley far exceed those generally observed between neighboring climatic zones. One must remember, however, that the differences reported above were observed within the short growth period, but this is the period where food is being produced to keep the plant surviving during 10 months of severe winter. Plants differ in the efficiency with which they are able to utilize various combina- tions of day and night temperatures. Went6 and Hiesey5 showed the importance of the proper relation between day and night temperatures for the growth of different races and varieties of plants. The differences between neighboring sites within the Slate Creek Valley are of this order. The findings in the Slate Creek Valley confirm the observations by Billings' con- cerning temperature inversions in the pinyon-juniper zone of the Virginia Moun- tains at 6600 ft of altitude southeast of Reno, Nevada. Billings found only slight differences between maximum temperatures on south and north slopes, possibly because the slopes were gentler than the 450 slopes of the thermometer sites in the Slate Creek Valley, and the altitudes were also lower. Billings attributes the warm- er minima on both slopes to loss of cold air to the valley bottom. It is suggested that the heat capacity of rocks may be significant in the storage of heat during the day on the south slope and is aided by the loss of cooled air through drainage during the night. On the north side the input of heat during the day is low, but the buffering effect of the rocks prevents freezing at night. Summary.-Great contrasts between climatic niches have been observed within the west-east-oriented, recently glaciated, topographically highly varied subalpine-alpine Slate Creek Valley east of Yosemite National Park. Temperature regimes obtain on opposite very steep slopes of the valley that simulate differences in latitude. During the brief 2-month growing season the daily temperature amplitudes varied between 4 and 15'C on north slopes, and 2 and 280C on south slopes, whereas in the bottom of the valley temperature inversions cause frost most of the nights. The highly varied microclimates and edaphically distinct habitats are reflected in an unusually rich vegetation that consists of elements of arctic circumpolar, Pacific coastal, Great Basin-Rocky Mountain vegetations that have converged in this region. They have joined an endemic fell field vegetation that probably existed during the period of heaviest glaciation. These highly diverse elements find favorable habitats within the microclimatic and edaphic niches of this valley. 'Billings, W. D., "Temperature inversions in the pinyon-juniper zone," Butler Univ. Bot. Studies, 11, 112-118 (1954). 2 Clausen, Jens, "Tree lines and germ plasms," these PROCEEDINGS, 50, 860-868 (1963). 3 Clausen, Jens, "Population studies of alpine and subalpine races of conifers and willows," Evolution, 19, 56-68 (1965). 4Clausen, Jens, David D. Keck, and Wm. M. Hiesey, Effect of Varied Environments on Western North American Plants, Carnegie Institution Pub. 520, vii + 452 pp. (1940). 5 Clausen, Jens, David D. Keck, and Wm. M. Hiesey, Environmental Responses of Climatic Races of Achillea, Carnegie Institution Pub. 581, iii + 129 pp. (1948). 6 Went, Frits W., "Thermoperiodicity, in growth and fruiting of the tomato," Am. J. Bot., 31, 135-150 (1944). Downloaded by guest on September 27, 2021