MESA VERDE Environment of Mesa Verde, Colorado Wetherill Mesa Studies

MIDDLE MESA-TOP SITE, M—2

The M—2 site was selected as representative of a type of stand that occurs about midway between the northern and southern extremities of Chapin Mesa. It was at 7,150 feet elevation, about 1 mile north of park headquarters and within one-half mile of the Weather Bureau station. Instruments were placed in a clearing that had served as a dirt tennis court between 1937 and 1942 (fig. 5).

The vegetation is a pinyon-juniper/mutton grass community in which trees and grass are conspicuous and dominant, while the shrubs common in other parts of Mesa Verde are absent from most stands of this type (fig. 6). Mutton grass (Poa fendleriana) is the understory dominant, but the swordlike leaves of yucca (Yucca baccata) occasionally break the monotony of the predominantly tree-grass landscape. There are fewer plant species in this stand than in any of the others studied. Data on the two species of trees are given in table 7. The trees are the largest and the vegetation is the most dense of the six stands investigated. Trees are up to 30 feet tall, about twice as high as those at M—1, and the largest pine and juniper were 17 and 26 inches in diameter, respectively. The 10 largest pinyon trees were an average of 265 years old, the oldest being 362 years old (pith date, A.D. 1601). These data illustrate the problem of estimating age from size: a 16-inch-diameter tree was 320 years old, whereas a 17-inch neighbor was almost 100 years younger.

Fig. 6 The pinyon-juniper/mutton grass community characteristic of M—2. Mutton grass and broad-leafed yucca are the only conspicuous understory plants. The forest here attains a height of about 35 feet, the maximum height on Mesa Verde.

TABLE 7.—TREE DATA AT M—2

The surface soils at M—2 are loams, with underlying clay loams, and are part of an extensive soil type found at Mesa Verde, probably covering the greatest area on the mesa tops at this elevation. As can be seen from table 8, the upper 3 feet of the profile are noncalcareous, or only slightly calcareous, and consist of an older soil buried by water-deposited and eolian sediments of uniform texture. This soil has been classified as Witt loam, although at M—2 it is somewhat less well developed and the underlying sandstone bedrock occurs only 4 feet below the surface. The profile at the type location on Chapin Mesa is nearly 10 feet deep, and similar soils at pithouse excavations rest on the Cliff House sandstone at depths ranging from 4 to 15 feet (Roberts, MS.).

TABLE 8.—SOIL PROFILE UNDERLYING M—2 PINYON-JUNIPER/MUTTON GRASS STAND*

*Soil classified as Witt loam.

The soil moisture constants for M—2 are given in table 5.

The high silt and clay content of the soil probably favors establishment of grasses over the deeper-rooted shrubs, because fine-textured soils tend to retard root penetration. The shrubby plants on Mesa Verde are generally most abundant on the shallower soils along the mesa rims, in the coarse soils of the canyon talus slopes, or on the narrow ridges to the north.

Environment data are given in table 6 and the appendix. The maximum wind gust measured was 36 m.p.h. A comparison of the M—2 data with that from the Weather Bureau station is made in Chapter 4.

One important finding is the quantitative effect of cloudiness on the amount of solar radiation that reaches the earth's surface. The influx of solar radiation was 644 gm. cal./cm.²/day in August of 1962, a month which had relatively little cloudiness, and it dropped to 514 (a 20-percent decrease) in August of 1963, a period of considerable cloudiness.

Fig. 7 M—3, on the North Rim, at 8,575 feet elevation, looking south to Mesa Verde tableland, in late winter.

The frost-free period at M—2, lasting from the latter part of May through October, was 161 days in 1962 and 171 days in 1963. It is interesting that in 1963 the frost-free season was the same at both M—1 and M—2, although there is an elevational difference of 500 feet between the sites. Snow covered the ground from mid-December to the latter part of March during the winters of l961-62 and 1962-63.

Flowering of the early perennials began in April of 1963, several weeks later than at M—1.

The total character of this forest stand suggests that the site provides near optimum growing conditions for both pinyon and juniper. This stand type is an important climax ecosystem on the mesa, because it occurs on the broad mesa tops in an altitudinal belt across the entire Mesa Verde landscape where there is a thick mantle of soil. It is a relatively tall, dense forest compared to the M—1 type and those in most other parts of the pinyon juniper climax region of the Southwest, and it thereby gives ecological uniqueness to Mesa Verde.