MESA VERDE Environment of Mesa Verde, Colorado Wetherill Mesa Studies

Mesa Verde is an imposing landmass that rises abruptly above the semiarid country of southwestern Colorado. It is a relatively flat tableland, between the high San Juan massif to the northeast and and the lower desert to the southwest. The mesa lies on the eastern edge of the Colorado Plateau physiographic province, "a land of gently folded sedimentary rocks eroded on a majestic scale into broad plateaus, precipitous mesas and buttes and dark canyons" (Hack, 1942, p. 3). This great landmass dips gently over a 15-mile stretch from an elevation of 8,500 feet at the northern escarpment to about 6,500 feet at the southern end. The mesa is not an unbroken tableland, as its Spanish name suggests. It is more a plateau than a mesa, but usage has firmly established the latter term. Moreover, scores of canyons dissect the once continuous surface into somewhat isolated segments, each of which is locally called a mesa (fig. 1).

Fig. 1 Mesa Verde National Park and vicinity, showing locations of environment measurement sites. (click on image for an enlargement in a new window)

Mesa Verde is fascinating not only because of its spectacular terrain and interesting geology and biology, but because of its prehistoric occupation by aboriginal man. For about 700 years, up to the close of the 13th century, Pueblo Indians lived and farmed successfully here under difficult climatic and soil conditions. Their impact on the landscape is still visible in numerous and varied dwelling sites, midden deposits (often supporting sagebrush where forest normally occurs), and agricultural check-dam systems. What other features are ascribable to the Indians' activities constitute a tantalizing problem for ecologists. Following its abandonment around A.D. 1300, the mesa was essentially undisturbed until white settlers began moving into the area in the 1870's.

Part of the mesa became Mesa Verde National Park in 1906, but the landscape is so rugged that few studies had been made of the soils, atmospheric factors, biota, and ecological processes within it. The Wetherill Mesa Project provided an opportunity to fill many gaps in our knowledge of the total Mesa Verde environment. This report is concerned primarily with quantitative data on the atmospheric factors and vegetation of representative stand ecosystems in Mesa Verde.

GEOLOGY AND SOILS

The Mesa Verde is composed of marine sediments of Upper Cretaceous age. These rocks form a discrete geological unit, the Mesaverde group, whose members include the Cliff House sandstone, the Menefee formation, and the Point Lookout sandstone. The resistant Cliff House sandstone caps the mesa and is underlain by the Menefee formation, a coal-bearing deposit that outcrops on the steep canyon slopes. The lowest and least conspicuous member is the Point Lookout sandstone, exposed along the North Rim and in the deeper canyons of the south and west extremities of the mesa. The top two layers are primarily responsible for the rugged canyon-mesa terrain so characteristic of the area.

Headward erosion of the canyons, perhaps during more moist climatic regimes, produced numerous large alcoves situated along the upper cliffs, many of which contain the ruins of cliff dwellings built by the prehistoric Indians. These alcoves have been sculptured by spring-sapping, a weathering process that weakens and undercuts the sandstone cliffs where they are in contact with the impervious shale strata. To a lesser degree, some of the shallower alcoves, called exfoliation caves, have resulted from release of confining pressures when erosion exposed rocks that were once deeply buried. (For further information on cave origins of this type, see Bradley, 1963.)

The regional and local geology have been described by Douglas Osborne (in Hayes, 1964). Osborne's discussion is based largely on the work of Hunt (1956) and Wanek (1959).

The soils are primarily wind-deposited loess in origin (Arrhenius and Bonatti, 1965), although along the upper reaches of Mesa Verde some residual soils are developed from weathered sandstone. Soil depth is extremely variable. Deep profiles occur at the heads of canyons, on alluvial-colluvial terraces, on the canyon floors, and on the broad mesa where the loess soil may reach depths of 15 feet. But soils are either very shallow or nonexistent in the mesa-top drainages and rim areas, on the canyon talus slopes, and on the narrow, more easily eroded ridges of the northern third of the mesa.

The carbonate content of the underlying sandstone, and consequently of the soils, also varies considerably. In the brush zone along the North Rim, the residual soils are darker colored and slightly acid, with little or no free lime (Roberts, MS.). They contrast sharply with the almost white, flaky soils encountered in the juniper-dominated woodland toward the southern end of the mesa. In this area the soil surface is highly calcareous, which may be due to different parent material, less precipitation, and higher temperatures.

Trewartha (1954, p. 286) states that "steppe (or semi-arid) lands are often the recipients of large amounts of fine dust or loess blown out of the drier and less well protected deserts." Because of the relatively meager rainfall, leaching is not a serious detriment to soil productivity. Mineral plant foods are usually abundant. "Yet, because of the low and variable rainfall in which they develop, and to which they largely owe their quality, (semiarid lands) are not extensively used for crop production. It is the old story of fruitful soils and prolific climates seldom being areally coincident" (ibid). Nonetheless, as will be shown later, the prehistoric Pueblo Indian farmers made use of the Mesa Verde soils to a remarkable extent.

More detailed edaphic information is available in the soil reconnaissance reports of Wetherill Mesa by Roberts (MS.) and White (MS.). A standard soil survey of the area, made by Orville A. Parsons, will be published as a report of the Wetherill Mesa Project.

VEGETATION

The plants of Mesa Verde are part of the Sierra Madrean flora that occupies the Great Basin, Colorado Plateau, and the Sierra Madre of northern Mexico (Benson, 1957, p. 598). Vegetal material identified from the Wetherill Mesa Project's excavations and earlier collections indicate a prehistoric flora similar to that of the present day (Welsh, MS.).

Ecologically, the mesa is in the pinyon-juniper climax region that forms the lowest forest zone in the Rocky Mountains and is the only zone present on many of the low ranges of the Great Basin. The total geographic range of the climax region extends from eastern Oregon and southern Idaho southward along western Colorado, northeastern Arizona, and New Mexico. The trees at Mesa Verde grow relatively tall (up to 35 feet) and close together, producing a pinyon-juniper type unlike the scrubby trees in an open "pygmy woodland," which are characteristic of the climax region generally.

The almost continuous mantle of pinyon-juniper trees over the mesa gives an impression of monotonous homogeneity when viewed from the air. Walking through the forest, however, you find a surprising variety of plants. These plants are organized into several different stand-types because of subtle variations in slope, altitude, soil, the effects of fires, and the influences of prehistoric man.

Pinyon pine (Pinus edulis) and Utah juniper (Juniperus osteosperma) are the dominant trees, but there are some small stands of Douglas-fir (Pseudotsuga menziesii), aspen (Populus tremuloides), and ponderosa pine (Pinus ponderosa). Cottonwood (Populus fremontii) and Rocky Mountain juniper (Juniperus scopulorum) are scattered throughout the area, the latter common along the northern escarpment. Douglas-fir, the most abundant of these less common trees, occurs in sheltered side canyons, seep areas, and at the higher elevations. It is the source of the names "Spruce Canyon" and "Spruce Tree House," probably through an error in identification. Aspen is restricted to the coves of the North Rim and some of the more moist and protected canyon sites. There are only a few scattered stands of ponderosa pine, and reproduction is limited. Gambel oak (Quercus gambelii) and Utah service-berry (Amelanchier utahensis) are the dominants in a shrub zone caused by recurrent fires along the higher parts of the mesa, while big sagebrush (Artemisia tridentata) is common on the sandy loam terraces along the canyon floors and also on some prehistoric trash deposits on top of the mesas.

Fire has had an important effect on the vegetation. A reconnaissance of the larger burns and a study of aerial photographs show that fires have been more common in the higher, north part of the mesa than in the lower areas. Possibly the prehistoric Indians deliberately burned these upper parts of the mesa, which are of marginal farming value, in order to maintain the shrub vegetation, which supports a heavier game population than a pinyon-juniper forest approaching climax. Many fires, however, were undoubtedly started by lightning.

The mountain brush vegetation in the North Rim area is relatively unstable. Pinyon reproduction has increased as a result of stringent fire-protection measures by the park, and the area is gradually becoming reforested.

The present-day vegetation probably closely resembles that which flourished before the prehistoric Indians entered the area. To be sure, today's dense oak thickets on abandoned terraces and on reservoirs behind check dams, as well as concentrations of sagebrush on many midden sites, were lacking in preoccupation time. But these vegetation units occupy only a small part of the mesa. As long as the Indians were active in the area, the vegetation was probably a patchwork of forest, agricultural fields, and brushland. Sagebrush may have been more abundant than it is today. It would have grown rapidly in abandoned fields and burned areas, and it was used in quantity for roofing material in the prehistoric structures. After the Indians left, the vegetation pattern changed through natural ecological processes and in due course most of the region returned to preoccupation conditions.

CLIMATE

According to Koppen's classification of world climates based on annual and monthly means of temperature and precipitation, Mesa Verde has a cold, middle latitude, semiarid climate (BSk of the Koppen system. see Trewartha, 1954, pp. 225-226). Trewartha (op. cit., p. 268) observes: "In general, the steppe (or semiarid type of dry climate) is a transitional belt surrounding the real desert and separating it from the humid climate beyond." Though it lies in an area of dry climate, Mesa Verde is closer to a humid than to a desert climate because of the proximity of the San Juan Mountain massif to the northeast and of the rest of the southern Rocky Mountains farther east.

The following statements by Trewartha (op. cit.) are pertinent to the intermountain region of which Mesa Verde is a part:

Dry climates in the middle latitudes usually are found in the deep interiors of the great continents, far from the oceans, which are the principal sources of the atmosphere's water vapor. Further intensifying the aridity of the deep continental interiors is the fact that in both Eurasia and North America these locations are commonly surrounded by highlands that block the entrance of humid maritime air masses and of rainproducing storms (p. 280).

The essential feature of a dry climate is that potential evaporation from the soil surface and from vegetation shall exceed the average annual precipitation. In other words, during a normal year the capacity of the atmosphere to acquire water evaporated from the soil surface and transpired from plants is greater than the water added to the soil through precipitation. In such a climate there is a prevailing water deficiency and a constant ground water supply is not maintained, so that permanent streams cannot originate within such areas. It may be possible, however, for permanent streams to cross areas with dry climates . . . provided they have their sources in more humid regions (p. 267).

The Mancos River, which borders Mesa Verde and drains its many canyons, has its headwaters in the La Plata Mountains to the north. Within the mesa proper the only natural water supply available to man and other animal life comes from springs and seeps in the canyons.

A climatic summary of the immediate region is excerpted from Gittings (1941, p. 808):

The distinct climatic feature of the western section [of Colorado] . . . is the comparative uniformity of the weather from day to day. Severe cold waves, common on the eastern plains, are comparatively rare. . . . There is a tendency for a high-pressure area to form in western Colorado in winter and to remain stationary for several days. When such a pressure distribution controls the weather, the sky is clear, the day temperatures are moderately high and uniform, and the nights are cold, though seldom excessively so except when the ground is covered with snow and where the air drainage is poor. Night temperatures depend largely on the topography, air drainage exerting a greater control than does the actual elevation. . . . In western sections of the State the most important part of the precipitation occurs in winter and early spring; January, February, and March are the months of heaviest snowfall. . . . In southwestern counties there is a marked tendency toward drought in late spring and early summer; June is practically rainless.

The following account of the recent climate of Mesa Verde is taken from the 41-year record (in part, shown in table 1) of the U.S. Weather Bureau station near the Mesa Verde National Park headquarters, and from a discussion by Erdman (MS.). The approximate location of the station is lat. 37° 12' N. and long. 108° 29' W. Its position and elevation (7,070 feet) make it as representative of the Mesa Verde physiographic unit as possible. Since 1923, the average annual precipitation has been 18 inches. The late winter months constitute one of the "wet seasons," February being one of the wettest months of the year with almost 2 inches of moisture. Most of the moisture during this period occurs as snow. Winters seem to be relatively mild, perhaps because of the predominantly sunny days. January, the coldest month, has a mean temperature of 29° F. and 19 inches of snowfall. The coldest recorded temperature (-20° F.) occurred during the severe nationwide cold wave of January 1963. According to Trewartha (1954, p. 181), in this climate "the winter season has many more large and steep-gradient cyclones and anticyclones than summer, so that the cooler seasons have more variable weather than the warmer periods of the year." This proved to be the case in our own observations: temperatures were more variable during the winter than during the summer months.

TABLE 1.—WEATHER DATA, MESA VERDE NATIONAL PARK
United States Weather Bureau station, Chapin Mesa, elevation 7,070 feet. Deviation of the 1962 and 1963 annual summaries from the 41-year record (1923-63)

Winter moisture is a critical factor as it determines the vegetational aspect of the landscape in late spring and early summer, typically the driest period of the growing season. Annuals and some perennials are highly dependent upon the surface moisture during these periods of low rainfall.

Although July is the hottest month of the year, with a mean temperature of 72° F. and a maximum of 102° F., the heat is tempered by rains which normally begin about this time of year. Rainfall reaches its peak in August (average of 2 inches) and decreases gradually into the autumn. During the late summer months the days begin with cloudless skies, but by noon, because of intense air turbulence, cumulus clouds develop and thunderstorms are common. Precipitation is usually localized and intense for a short period of time. Consequently, runoff is high and the precipitation is not nearly so effective as winter and spring precipitation in controlling the growth of indigenous plants.

During 1962 and 1963, Mesa Verde experienced subnormal rainfall and above-average temperatures, the latter occurring especially throughout the autumn months (table 1). Although the amount of precipitation between the 2 years did not differ appreciably, its pattern varied greatly. A sustained dry period prevailed throughout most of 1962, broken finally by heavy rains in the fall. The precipitation record approached the normal pattern the following year, although May and June were unusually dry and August was abnormally wet. Lightning-induced fire is a perennial threat to parched vegetation. Prehistoric burns in the northern third of Mesa Verde National Park and on several large tracts in the southern part of the park were undoubtedly caused by lightning. More recently, in July 1934, a lightning fire burned about 5,000 acres on the northern third of Wetherill Mesa and adjacent areas (Watson, 1934, pp. 16-17). And in July 1959, lightning ignited a fire that burned 3,043 acres of forest and brush in Morfield Canyon in the southeastern part of Mesa Verde. Fire as a byproduct of summer storms has been an important ecological factor in the area.

In considering the relationship of man to his environment, a subject of no small importance in the prehistoric and present-day occupation of the region, Trewartha (1954, p. 283) makes this comment:

Because of the greater precipitation than in deserts, the steppes are somewhat better fitted for human settlement, but this, together with the unreliable nature of the rainfall, also makes them regions of greater economic catastrophe. A succession of humid years may tempt settlers to push the agricultural frontier toward the desert, but here also drought years are sure to follow, with consequent crop failure and ensuing disaster.

In essence, then, and as we might well expect, the semiarid, broken landmass we call Mesa Verde suffers from erratic rainfall that affects not only plant and animal life but man, especially when his main sustenance derives from the soil itself.