Part 9

VEGETATIONAL ANALYSIS OF HABITATS

Detailed maps of vegetation within the trapping grid were needed to aid in analyzing distribution of mice within the grid. In preparing such maps, I recorded all plants within a 25 foot radius of each trapping station. The dominant and codominant plants in the overstory (trees or shrubs) were noted at each station. Next the three most abundant plants other than the dominant and codominants were rated for each station, where possible. Finally a listing was made of all remaining species of plants.

On the basis of this analysis, four vegetational maps were prepared. One shows associations of dominant overstory and understory plants. Individual maps are devoted to the first, second and third most abundant plants in the ground cover within the trapping grid (Figs. 5-8). Approximately seven man-hours were required to analyze each trapping unit, and 112 man-hours to analyze the entire grid.

The home range grid encompasses approximately one million square feet. At least four different vegetational stands occur within the grid: 1) pinyon-juniper woodland with various associations in the understory; 2) Artemisia tridentata (big sagebrush), or A. nova (black sagebrush); 3) Quercus gambelii (Gambel oak); and 4) mixed shrubs--Fendlera rupicola (fendlerbush), Amelanchier utahensis (Utah serviceberry), and Cercocarpos montanus (mountain mahogany).

Flora in the ground cover is regulated, at least in part, by the canopy cover; hence different associations of pinyon-juniper woodland and each of the stands mentioned above have different plants, or a different distribution of the same kinds of plants, in their ground cover.

Units A, B, E, and parts of D and G in the western third of the grid are in pinyon-juniper woodland (Fig. 5). A relatively pure understory of Poa fendleriana (muttongrass), is typical of such woodland on the middle parts of the mesas. Woodland on the western third of the grid differs somewhat in that, when the area occupied by each plant is considered, Artemisia tridentata is codominant there with Poa fendleriana. As far as individual plants are concerned, Poa far outnumbers Artemisia. The next most abundant plants in the ground cover are Solidago petradoria (rock goldenrod), Chrysothamnus depressus (dwarf rabbitbrush), and Penstemon linarioides (penstemon), in that order.

In unit E there is a large depression, about 200 by 60 feet, created by removal of soil (Fig. 8). Artemisia nova grows there, and pioneering plants adapted to early stages of succession are present.

A zone of woodland, where Artemisia nova replaces A. tridentata as an understory codominant with Poa fendleriana, borders the pinyon-juniper-muttongrass community to the east. The next most abundant plants in the ground cover are Solidago petradoria, Penstemon linarioides and Comandra umbellata (bastard toadflax). Koeleria cristata (Junegrass) is as abundant as Comandra, but probably is less important as a source of food for mice.

A small strip of the pinyon-juniper-muttongrass community with an understory of Artemisia nova and Purshia tridentata (bitterbrush) adjoins the above area to the east (Figs. 5-8). Solidago petradoria, Balsamorrhiza sagittata (balsamroot), and Comandra umbellata are the three most abundant plants in the ground cover. The terrain slopes eastward from this zone into a large drainage.

As the forest floor begins to slope into the drainage, the ground becomes rocky and shrubs assume more importance in the understory. Most of this shrubby zone is on the slope; on the western side this zone abuts pinyon-juniper woodland, and on the eastern side is bordered by Artemisia tridentata in the sandy bottom of the drainage. Shrubs become more abundant and pinyon and juniper trees become less abundant as one approaches the drainage. In the vegetation maps, this brushy zone is delimited on the east by a heavy line passing vertically through the middle of the grid (Figs. 5-8). The codominant shrubs in the understory of this zone are Amelanchier utahensis, Artemisia nova and Purshia tridentata. The three most abundant plants on the ground are Artemisia ludoviciana, Chrysothamnus depressus and Penstemon linarioides.

The drainage occupies most of unit N and parts of Units I, J and M. Unit N is at the head of the drainage; the ground slopes rapidly southward and the bottom of the drainage in unit J is approximately 50 feet lower than in unit N. The canopy cover of the drainage is Artemisia tridentata (Fig. 5). The same three plants that are most abundant in the ground cover of the slope are also most abundant in the drainage.

The eastern slope of the drainage is covered with oak chaparral (Quercus gambelii); this zone occupies parts of units J, L, M, and P. Artemisia ludoviciana, Solidago petradoria, and Viguiera multiflora (goldeneye), are the most abundant plants of the ground cover.

Mixed shrubs (Amelanchier, Cercocarpos, and Fendlera) form large islands in the oak chaparral, in units K, L and P. The brushy areas of oak and mixed shrub give way at the top of the slope to pinyon-juniper forest with an understory of Artemisia nova and Purshia tridentata. The three most abundant plants in the ground cover of the shrub zones are Solidago petradoria, Balsamorrhiza sagittata, and Comandra umbellata. The eastern part of unit O has Amelanchier utahensis in the understory, in addition to Artemisia nova and Purshia tridentata (Fig. 5). The northeastern corner of unit O is in pinyon-juniper woodland with an understory of Cercocarpos montanus.

There are two relatively pure stands of sagebrush in the grid: one is in unit N, and the other in unit F and part of unit G. As figures 5 to 8 show, unit N has a relatively pure stand of Artemisia tridentata (big sagebrush), with Artemisia ludoviciana, Agropyron smithii (western wheatgrass), and Koeleria cristata (Junegrass), being most abundant in the ground cover. Artemisia tridentata and Artemisia nova form the overstory in unit F and part of G. The three most abundant plants in the ground cover there are Chrysothamnus depressus, Solidago petradoria, and Penstemon linarioides (Figs. 6-8).

MICROCLIMATES OF DIFFERENT HABITATS

Four microclimatic stations were established in units D, F, L and M of the trapping grid to record air temperatures and relative humidities at ground level. These sites were chosen as being representative of larger topographic or vegetational areas within the grid. Belfort hygrothermographs were installed on June 10, 1964, and were serviced once each week through October 31, 1964, at which time the stations were dismantled. Each station consisted of a shelter 18 by 9 by 11.5 inches, having a false top to minimize heating (Fig. 9). The shelters were painted white. Several rows of holes, each one inch in diameter, were drilled in all four sides of each shelter, to provide circulation of air. The holes were covered by brass window screening to prevent entry of insects and rodents. Preliminary tests with several U. S. Weather Bureau maximum and minimum thermometers, suspended one above the other, from the top to the bottom of the shelter, revealed that there was no stratification of air within the shelters. Nevertheless, each shelter was placed so that the sun did not strike the sensing elements of the hygrothermograph inside it.

Accuracy of the hair elements was checked by means of a Bendix-Friez battery driven psychrometer, in periods when humidity conditions were stable (on clear days the relative humidity is at its lowest limits and is "stable" for several hours during early afternoon).

The four microclimatic stations were in the following places: 1) a stand of big sagebrush near Far View Ruins; 2) a pinyon-juniper-muttongrass association; 3) a stand of big sagebrush at the head of a drainage; and 4) a stand of Gambel oak on a southwest-facing slope of the drainage. Table 4 shows monthly averages of maximum and minimum air temperatures and relative humidities at each of the four sites. Vegetation and microclimates of the sites are discussed below.

Far View Sagebrush Site, 7,650 feet elevation

The shelter housing the hygrothermograph was next to the stake of station F4a in the trapping grid (Fig. 10), in a stand of big sagebrush on the flat, middle part of the mesa top, approximately 100 yards southwest of Far View Ruins. The sagebrush extends approximately 200 feet in all directions from the station (Fig. 5). Pinyon pine and Utah juniper trees are encroaching upon this area, and scattered trees are present throughout the sagebrush. This area is one of the habitats of P. maniculatus.

Sagebrush tends to provide less shade for the ground than pinyon-juniper woodland, and therefore the surface temperatures of the soil rise rapidly to their daily maximum. In mid-June, air temperatures rise rapidly from 6 A. M. until they reach the daily maximum between 2 and 4 P. M. Shortly after 4 P. M. the air temperatures decrease rapidly and reach the daily low by about 5 A. M.

Relative humidities follow an inverse relationship to air temperatures; when air temperatures are highest, relative humidities approach their lowest values. Thus, on clear days, humidities decrease during the day, reaching a minimum slightly later than air temperatures attain their maximum. Unless it rains, the highest humidities of the day occur between midnight and 6 A. M.

Drainage Site, 7,625 feet elevation

This site was in the bottom of the drainage that runs through the eastern side of the trapping grid, and through parts of units M, N, I, and J. The site was at station M4d on a level bench at the head of the drainage (Fig. 11). Southward from the station the drainage deepens rapidly, and the bottom loses approximately 25 feet in elevation for every 200 feet of linear distance. P. maniculatus lives here.

The microclimate of the drainage differs markedly from that of other stations. The major difference is attributable to the topography of the drainage itself. Nocturnal cold air flows from the surrounding mesa top to lower elevations. A lake of cold air forms in the bottom of the drainage; the depth of the lake depends in part upon the depth of the drainage. The same phenomenon occurs in canyons and causes cooler night time temperatures on the floor of canyons than on adjacent mesa tops (Erdman, Douglas, and Marr, in press). Drainage of cold air into lower elevations affects both nocturnal air temperatures and relative humidities. Table 4 shows that maximum air temperatures in the drainage did not differ appreciably from those at other stations. Mean minimum temperatures, however, were considerably lower in the drainage than at the other sites. This phenomenon is reflected also in the mean air temperatures at this station.

The drainage site had the highest humidities of all stations each month in which data were collected (Table 4). Relative humidities of 90 to 100 per cent were common in the drainage, but occurred at other stations only in rainy periods. For example, in the month of August, 26 of the daily maximum readings were between 95 and 100 per cent at the drainage site, but at the other stations relative humidities were above 95 per cent for an average of only nine nights. Minimum humidities were about the same for all stations, since they are affected by insolation received during the day, and not by the drainage of cold air at night.

Oak Brush Site, 7,640 feet elevation

The station was in an oak thicket at trapping station L4a, 250 feet south and 50 feet east of the drainage site on a southwest-facing slope of about 30 degrees (Fig. 12). The station was on the lower third of the slope, approximately 15 feet higher than M4d, the station in the bottom of the drainage. P. truei and P. maniculatus occur together in this area.