of yellow on the legs also varies. In one female there is a small yellow spot on the pleurae.

Stizus godmani lineatus (Cameron).

Stizus lineatus Cam., Biol. Centr. Am. (Hym.), Vol. II, p. 108, Pl. V, fig. 7, a, b.

The amount of black on the abdomen varies. The amount of yellow on the pleurae varies; in one specimen the pleurae are entirely pale, while in another they are largely black. The pale lines on the mesonotum vary in strength, in one specimen they are very faint. There is one specimen with the lines on the mesonotum very small, and very dark. Although it has a larger number of characters in common with this variety it resembles very closely godmani godmani. The lines on the mesonotum are very faint, the pleura is mainly black, and the abdomen is as in godmani godmani.

Stizus godmani flavus (Cameron).

Stizus flavus Cam., Biol. Centr. Am. (Hym.), Vol. II, p. 103, Pl. V, fig. 9, a, b, c. Stizus flavus Fox, Proc. Acad. Nat. Sci. Phil., p. 268, June, 1895.

Stizus flavus var. subalpinus Ckll., Proc. Dav. Acad. Nat. Sci., Vol. VII, p. 142, 1898. Stizus subalpinus Johnson and Roh., Ent. N., p. 374, October, 1908.

The black is usually entirely absent in the Boulder specimens. The three basal joints of the flagellum are sometimes yellow.

The following table may show the relative abundance of the three forms at Boulder. It is made from the specimens which were collected July 31, 1908, at the nesting site spoken of above, by sweeping with a net at random during the warmest part of the day.

The finding of these three forms together and in the proportions given above suggests that they are Mendelian, and that godmani flavus is recessive while godmani godmani is dominant. It would be very interesting to obtain a large series from the same nest, and see if the different forms do exactly follow the Mendelian ratios. It is the writer's desire to carry on such experiments next season.

THE SILVA OF COLORADO'

IV. FOREST FORMATIONS AND FOREST TREES

BY FRANCIS RAMALEY

Forest formations.—It is a simple matter to classify the forests of Colorado. They fall into two groups, those of the plains region and those of the mountain districts. The former are of slight economic importance at the present time but their existences suggests the possibility of tree-planting which may in time become highly profitable. River-bottom forests and pine-ridge forests occur in the plains region to the east of the continental divide, while in the western part of the state there are large areas with a growth of quaking aspen and of pinyon pine which it is difficult to classify. They might be termed either pineridge or foothill forests. In the mountain districts there are no trees of importance except the "evergreens."

Forest formations of mountain districts.-These are composed of coniferous trees, the various species of pine, spruce and fir and the single species of Douglas spruce. Generally the foothills support an open formation of rock pine2 while in the moister canyons there are Douglas spruces.3 In southern Colorado the pinyon pine4 forms a conspicuous part of the foothill forest and in places in the southern and western parts of the state there are some oaks of consequence. In the montane region a closer forest exists, composed largely of lodgepole pine,5 with some Colorado blue spruces along the streams. The best timber, however, is in the sub-alpine spruce forests. These consist essentially of Engelmann spruce' with some admixture of limber pine, sub-alpine and white fir9 and, in places, bristle-cone pine. 1o

10

8

Previous articles in this series, all published in the UNIVERSITY OF COLORADO STUDIES, are as follows: "Trees of the Pine Family in Colorado," Vol. IV, pp. 109-122, 1907; "The Poplars, Aspens and Cottonwoods," Vol. IV, pp. 187-197, 1907; "Woody Plants of Boulder County," Vol. V, pp. 47-63, 1907.

Forest formations of the plains region.-The river-bottom and pine-ridge forests occupy essentially different habitats. Along the water courses there is found a good growth of cottonwoods and willows with occasional box-elders, hackberries, mountain maples and alders. Where the stream has not cut a deep bed, and the ground slopes gently away at the sides, these river-bottom forests may be a quarter or halfmile in width. More often, however, they are much narrower. Along

FIG. 1.-MAP OF COLORADO. The continental divide is shown by the heavy dotted line, the front range of foothills by the short horizontal lines. East of the foothills lie the great plains.

the smaller streams there is frequently a single row of cottonwoods or willows. The pine-ridge forests are on the high ground extending between adjacent streams where they grow in coarse, frequently rocky soil. Good examples of pine-ridge forests are seen along the divide between the South Platte and the Arkansas and also at the edge of the "high plains" in the northeastern part of the state. The trees of these ridges are rock pines and cedars which form scattered groups hardly to

be dignified by the name of forest. Near Pawnee Buttes in northeastern Colorado there are limber pines instead of rock pines. To the south, and in certain parts of the western area of the state, the pinyon pine is added to the rock pine and cedar.

Classification of forests. From a botanical standpoint forests may be distinguished as mesophytic and xerophytic. The former are those made up of trees requiring a moderate amount of moisture, the latter are dry-country forests capable of enduring drought; they occur in regions of slight rainfall or in situations where the run-off is rapid. Mesophytic trees in Colorado are confined to areas where there is considerable seepage water, except in the case of Engelmann spruces in the sub-alpine zone where there is much more rainfall than at lower altitudes. It may be convenient to list the principal forest formations of Colorado in tabular form.

The pinyon pine is found only in the warmer and drier parts of the state. Oaks, chiefly shrubby species, enter into the foothill forest formation in certain places. They are entirely absent from the foothills north of Denver.

Distribution of species.-The tree flora of the state embraces fiftyfive species belonging to twenty genera. Most of the individuals, as well as the species in general, are found in the foothill, montane and sub-alpine zones of plant life. Timberline, at about 11,000 or 12,000 feet above the sea, is the upper limit of tree growth, although some shrubby willows occur in the alpine zone above. As noted in the preceding pages, the trees of the plains are to be found only along water courses or on the pine ridges of the high divides between streams. In the following table the general distribution of each tree is stated in relation to the continental divide, which extends north and south through the state, dividing it into nearly equal parts. It will be noticed that nearly all of the trees occur both east and west of the divide, somewhere in their distribution. As a matter of fact the same species does not necessarily occur at opposite points east and west of the main range. Those oaks, for example, which are present in the northwestern part of the state do not cross to the foothills of the eastern slope in northern Colorado but are found at Colorado Springs and southward. The distribution of the pinyon pine is similar. Many of the high-altitude trees extend far north to British Columbia and even Alaska. A considerable number range to the Pacific coast, but only a few show a distribution far eastward.

Limits of tree distribution.—The upper limit of tree growth, or timberline, is variable as to altitude. On north exposures the trees do not extend so high up the mountain side as they do on the warmer and less windy south slope. Cold, wind and dryness all combine to limit the upward extension of forests on mountains. At timberline are found scattered clumps of wind-blown trees often contorted into fantastic shapes. At the lower limit of forest growth, on the foothills and mesas adjoining the plains, the trees are unable to extend out to the plains largely because of the fine soil which supports a thick covering of grasses and does not permit trees to become started. Differences in climate are also important. The plains have colder winters and warmer summers, than the adjacent foothills. In the mountain parks, which are

2

This subject is discussed with some completeness by Mr. W. S. Cooper in an article referred to in the bibliography. From 11,000 to 14,000 feet may be considered the usual limit of trees.

• A comparison of the climates of plains and foothills is made by the present writer in the article referred to in the bibliography: "Studies in Mesa and Foothill Vegetation."