MESA VERDE Environment of Mesa Verde, Colorado Wetherill Mesa Studies

NORTHEAST EXPOSURE CANYON-SLOPE SITE, C—3

The northeast exposure site is directly across the canyon from C—1. Located on a 35° talus slope at an elevation of 6,500 feet, C—3 is about 100 feet above the canyon floor (fig. 14).

Fig. 14 C—3 instrument shelter on the east-northeast-facing talus slope in Navajo Canyon, directly across from C—1. There is a decided increase in the density of the pinyon-juniper forest and in the abundance of the brush understory.

The pinyon-juniper vegetation is more dense than that on the opposite slope, and two additional shrubs, oak and fendlerbrush, occur in the stand here (table 14). Other species of plants present are listed in table 2. The trees averaged 250 years old and the oldest individual was just over 260 years. The trees are older on this slope than on the opposite, southwest-facing (C—1) slope. This might be due to a sampling error, but the same relationship was found between the trees on the northeast- and southwest-facing slopes of Rock Canyon. These observations suggest that trees may actually live longer on northeasterly exposures.

TABLE 14.—TREE DATA AT C—3

The soil at this site is included in the Cliffhouse series, although it lacks strongly structured clay characteristic of the lower B horizon in this series. It is a moderately deep soil developing on the interbedded sandstone and shales of the Menefee formation. Cliffhouse soils are found primarily on the uplands or mesa tops and are not typical of the canyon's talus soils. Soil moisture samples and the soil profile described in table 15 were taken from a high, sloping bench slightly above the the C—3 site.

TABLE 15.—SOIL PROFILE UNDERLYING C—3 PINYON JUNIPER/MOUNTAIN BRUSH STAND*

*Soil classified as Cliffhouse fine sandy loam.

In the soil survey of Wetherill Mesa and adjacent canyons, the predominant soils of the talus situations were of the Mughouse-Rock Outcrop Complex. Since soils of the C—1 slope in Navajo Canyon are also in this category, it is reasonably safe to assume that the soils of the C—3 slope, as a whole, fall into the Mughouse series.

The data for growth-water storage listed in table 5 are somewhat conservative, since the nature of the underlying bedrock favors deep penetration and may carry some laterally moving water. Cracks in the bedrock also serve as moisture storage zones, as evidenced by the presence of roots, soil, and clay skins.

Environment data are presented in table 10 and the appendix. The local environment at C—3, like that of C—1, is strongly influenced by exposure to solar radiation. C—3 has slightly higher maximum air temperatures than C—1, but nocturnal temperatures on the two slopes are essentially identical because these temperatures are determined by the mass movement of cold air, which is independent of slope exposure.

Slope direction, which affects the duration of snow cover and its relationship to soil moisture during the growing season, is probably the most important factor controlling the stand vegetation. Although the frost-free period at C—3 was similar to that at C—1 (173 days in 1962 and 184 days in 1963), the duration of snow cover at the two sites was not the same. The C—3 slope kept a mantle of snow for several months, whereas C—1 had an ephemeral snow cover. In 1963, the late-lying snows were probably responsible for the delay in flowering on the C—3 slope until early April. The more favorable soil moisture conditions may account for the denser vegetation on the less-exposed slopes.

The stand at C—3 is a climax ecological unit whose character is influenced strongly by topography and soil. The presence of oak and fendlerbush indicates that this site is more mesic—that is, relatively more moist—than adjacent sites in the canyons and on the mesa top. These two shrubs suggest that C—3 is more-like M—3 than any of the other sites studied at Mesa Verde.