MESA VERDE Environment of Mesa Verde, Colorado Wetherill Mesa Studies

HIGHEST MESA-TOP SITE, M—3

The M—3 site (fig. 7) was located at Park Point, an 8,575-foot ridge that is the highest point in Mesa Verde. This unique setting, providing a spectacular view of the surrounding country, made it our windiest site. As shown in figure 1, this site was on the upper escarpment, or North Rim, of Mesa Verde, about 5 miles north of and 1,500 feet higher than park headquarters and the area of the M—2 site.

The mountain brush zone so prevalent along the North Rim is really a patchwork of dense thickets of oak and serviceberry, interspersed with openings dominated by black sagebrush. Within this patchwork many conifers are gradually becoming established. These are pinyon, primarily, and some junipers (Juniperus osteosperma and J. scopulorum) and Douglas-fir (Pseudotsuga menziesii).

The vegetation of the openings consists of a large number of species, and the foliage cover is extensive. Black sagebrush is the dominant species in the community, but a few trees and several saplings and seedlings of pinyon are established in the sample opening. Black sagebrush is a long-lasting successional dominant like big sagebrush, but it seems to be better adapted to shallow soils than the latter. At present, it covers the roadbeds along the North Rim that were abandoned in the early 1930's.

Because the study area is a complex of shrub stands, rather than being a continuous forest as in the other areas, the vegetation data consist only of a list of plants (table 2) present in the two types of local stands—in other words, a black sagebrush opening (fig. 8) and an oak thicket (fig. 9).

Fig. 8 A black sagebrush-grass community typical of the openings in the mountain brush zone near M—3, In addition to several pinyon trees (one at left), many seedlings and saplings have become established here, Oak thicket at right.

Fig. 9 An oak thicket in the mountain brush vegetation near M—3, This thicket, about 10 feet tall, recovered after a fire swept over the ridge in the mid-19th century. The major understory species are mutton grass and snowberry.

Soils at M—3 (table 9) are classified as Mughouse stony loam. The Mughouse series includes well-drained, noncalcareous Brown soils which are developing in residual materials derived from the underlying sandstone and interbedded shales. These are the typical soils of the upland hills and ridges along the North Rim. The soils at Park Point are somewhat darker colored than average and a hint of A₂ horizonation may be found under the older stands of oak brush. Soil depth is quite variable, ranging from 18 inches for the weather station profile (table 9) to 43 inches in the opening shown in figure 8. This, however, approximates the range in depth to bedrock for the series (20 to 40 inches). A considerable portion of the station site is covered by a pavement of sandstone slabs and channery. The profile in table 9 was described from the station site, where the ridge slopes south from 5 to 10 percent. Under the oak thickets there would be a thicker A₁ horizon with a deep litter cover.

TABLE 9.—SOIL PROFILE UNDERLYING M—3 MOUNTAIN BRUSH VEGETATION*

*Soil classified as Mughouse stony loam.

In spite of the relatively low storage capacity of growth water in these soils, their shallow depth and heavy charge of rock fragments probably induce quite a favorable moisture regime.

The large amount of coarse rock and gravel in these soils creates more pore space and increases their permeability to rainfall. In addition, much of the underlying parent material is an unconsolidated, weakly cemented residuum which allows deeper wetting. Thus the underlying bedrock may be an important source of moisture, acting as a temporary reservoir. These coarse-textured soils also favor root penetration.

One rarely visits the Park Point ridge without experiencing a conspicuous feature of this site—strong wind. Wind velocities at the Park Point station were almost twice those measured at the other stations, and they seem to be less patterned than those occurring at the other stations. For reasons we cannot explain, the early part of 1962 was exceptionally windy at the North Rim site; however this was not the case at the lower elevations (fig. 19).

Although gusts of wind up to 40 m.p.h., were recorded at M—3, little is known about their duration or frequency. As Daubenmire (1959, p. 269) stresses, mean velocities for long intervals of time can be very misleading; winds of gale proportions may blow for a few minutes, yet not be indicated in the weekly or monthly record.

Wind has an indirect bearing on the precipitation data at M—3. Measurement of precipitation is difficult at any site, but where wind velocities are high, as at Park Point, the difficulties are compounded. According to Conrad and Pollak (1950, pp. 14-15), "the amount of water or snow collected in the ordinary gage depends, to a certain degree, upon the wind velocity and upon the resistance offered by the air to the particles of precipitation. The stronger the wind and the greater the air resistance, the smaller is the amount caught in the gage compared with that collected under otherwise similar conditions during absolute calm." Daubenmire (1959, p. 92) points out that precipitation gages with large diameters tend to deflect wind upward as it strikes the instrument, a phenomenon which may reduce the actual amount of moisture by more than half. Because snow would be deflected even more easily than rain, the winter record is probably in greater error than the summer one. Thus it is necessary to take into account the effect of wind on the overall moisture record at M—3.

One important effect of wind on plants is to increase transpiration. Our evaporation data give information on this point. Evaporation at M—3 during summer months was the highest of all sites measured, even though the air temperatures were comparatively low.

Wind also has a significant effect on the length of the frost-free period. Since M—3 is higher than the other mesa-top sites, one would expect it to have a shorter frost-free season. But this was not the case. In 1963, this period was 171 days, identical to that of the other mesa-top stations, In 1962, it was 162 days, again very similar to the other sites. According to Geiger (1957, p. 110), higher wind velocity means increased convection, and increased convection results in decreased temperature gradients. Consequently, temperatures are lower during the day and higher at night. It is the moderating effect of wind at night that is responsible for the extended frost-free period at this site.

The similarity in the frost-free periods between sites does not mean that the growing seasons of plants are the same at all the sites. Actually, plant growth begins long before the last spring frost. In the spring of 1963, flowering of the indigenous plants at M—3 did not begin until late April, 2 to 3 weeks later than at M—2. The delay in growth at M—3 may have been due to a late snow cover and to a lag in the warming of the soil. Snow cover, though variable because of drifting, persisted from late November through March. The soil temperature records for 1963 show that substantial thawing and heating of the ground did not occur before May. Mutton grass, which began flowering early in April at the lower elevations did not flower until the first week in May at M—3. Similarly, serviceberry started to flower at the canyon stations the last week of April, but did not begin flowering along the North Rim until a month later. In general, therefore, a 3- to 4-week delay in flowering was evident between the northern and southern ends of Mesa Verde.

The M—3 vegetation is undergoing stages of changes from pioneer stands that developed after fires to the relatively stable climax cover of a pinyon and juniper forest. Burned stumps and charcoal throughout the area indicate that only small islands of the landscape have escaped burning.

Through the use of dendrochronological techniques, both on the burned specimens and modern increment cores from the present stand of scattered trees, it appears that a fire swept over the ridge about 1840. An earlier fire may have occurred in the 1600's, as the oldest pith date from the sample collection is 1710, with several more specimens giving approximately the same date. Moreover, these trees were relatively young when they burned.

Shrubs have fared better because they are able to produce new shoots from adventitious buds on the root crowns that survive fire. The local trees lack this regenerative capacity. Trees are now reproducing from seed throughout the area, so we do not doubt that forest will eventually replace the shrublands if fires are suppressed.

That the M—3 shrub vegetation is successional does not in itself indicate the area has soils or atmospheric factors different from the other stands we studied. In the absence of a true climax stand, however, we are unable to characterize the type in detail. The lushness of the shrubs and the more favorable water complex suggest that the forest would be more mesophytic, except on exposed sites where wind would produce more arid conditions.