zone during June and September. During July and August there is only a slight difference in the number of hours of growing temperatures in the three zones, although, of course, the highest and undoubtedly most effective temperatures are much more frequent in the lowest zone. Favorable temperatures occur in sufficient duration for inception of plant growth in the oak-brush zone about May 1 and sometimes earlier. Frequent heavy frosts and many hours of unfavorable temperature, however, retard the early development. Beginning about the middle of May, the temperature becomes very favorable for growth and continues favorable until about October 1, when frosts are apt to arrest further development. On the whole, the plants generally found in the oak-brush type require more heat units (potential growth hours) to complete their life cycle than do the plants in the aspen-fir and spruce-fir types. TABLE 2.-Average monthly precipitation, period 1914 to 1922, inclusive In the aspen-fir type favorable growing temperatures occur about May 15, but because of snow which lies on the ground until about May 25 the vegetation is delayed considerably in starting new growth. However, by the time the snow has disappeared the season has advanced, so that development is very rapid. Freezing temperatures sometimes check the new growth, but the hours of temperature unfavorable to growing which occur after the beginning of growth are much fewer in this zone than in the oak-brush zone, and consequently the plant development is more rapid. Except on the wind-swept ridges, snow usually lies on the ground in the spruce-fir zone until about June 20. The temperatures prevailing at so late a date are very favorable for plant growth and the vegetation is seldom delayed by freezing tempera tures. PRECIPITATION There is considerable variation in the average monthly and the average yearly precipitation in the three zones. The oak-brush has the lowest yearly average and the aspen-fir the highest, while that of the spruce-fir nearly equals that of the aspen-fir (Table 2, fig. 3). The lowest monthly average occurs in June and the highest usually in March for all types. Air temperature and precipitation are so closely related that it is impossible to segregate them and determine the specific influence of each on the development of vegetation. The growing season in the oak-brush zone is longer than that in either of the others, partly because air temperatures are favorable for plant growth both earlier and later, and partly because of the influence of the snow in the higher zones. Not only does precipitation influence temperature, but the amount of precipitation and the time of its occurrence also have an important influence on plant growth and FIG. 3.-Average monthly precipitation, 1914-1922, inclusive development. From November 1 to May 1 precipitation is usually in the form of snow, especially in the aspen-fir and spruce-fir types. Precipitation is relatively very high during these months and there is a much larger accumulation of snow in the two higher types than in the lower one. This partly explains the delay in the beginning of plant growth in the spring in these upper areas. Temperatures favorable for plant growth may occur early in May in the aspen-fir and spruce-fir types, but at that time they have little if any effect on plant development because of the deep covering of snow. On many of the steep north exposures in the upper reaches of the aspen-fir zone and in the spruce-fir zone snowdrifts may remain until the growing season is well advanced. In the sprucefir zone they often remain until the latter part of August, and in some years a few last almost throughout September. presence no doubt has some cooling effect on the air temperatures in these zones. Their Scarcity of moisture and high temperatures in the oak-brush zone are responsible for the presence there of drought-enduring vegetation which is less succulent and somewhat less palatable than that at the greater elevation. Insufficient moisture during the main growing season will in all zones result in a comparatively small yield of herbaceous forage. On the other hand, warmer air temperatures may cause greater volume production if sufficient moisture is present, especially in the spruce-fir and aspen-fir zones. 66 GRAZING PERIODS The proper time for grazing to begin on a range, or the time of range readiness," may be defined as the date in any one year when the range first reaches the condition in which there is sufficient feed to keep livestock in thrifty condition and when the stock may be admitted without serious impairment of the growth and reproductive processes of the more important forage plants. To determine when this condition is reached is a problem of prime importance in economical range management. RELATION OF HEIGHT OF HERBAGE TO RANGE READINESS EFFECT OF TIME OF FIRST CROPPING ON EARLY HEIGHT AND FORAGE YIELD An experiment was conducted on typical range plants to determine (1) the effect on early yield and vigor of growth of cropping at different stages of development, and (2) whether a certain average height growth of herbaceous vegetation is a reliable basis upon which to judge range readiness. Five different species of range forage plants were used, namely, Letterman needle grass (Stipa lettermani), violet wheat grass (Agropyron violaceum), mountain brome (Bromus polyanthus), wild geranium (Geranium viscosissimum), and mountain dandelion (Agoseris pumila). In each species 10 specimens for clipping and 10 for checks in each of three series were selected more or less at random on the range in the aspen-fir type at an elevation of 8,700 feet. Care was exercised in all cases to locate plants of nearly the same height and in as vigorous a condition as possible. Accordingly, comparatively young plants were chosen the bunch grasses, for instance, having a crown diameter usually not in excess of about 2.5 inches. Each specimen was staked and numbered. The herbage of those clipped was removed by a method somewhat resembling grazing, the leafage being cut about 1 inch above the ground's surface. The material removed from each plant was preserved separately and the dry weight was recorded. The first series of specimens were first measured and harvested on June 1, when the grass blades and the leafage of most other herbaceous plants were only 2 inches high or less. The second series were first harvested on June 10 and the third series on June 20. In each series the second harvesting was made 20 days after the first, the plants being allowed to grow uninterruptedly between croppings. TABLE 3.-Relation of height growth of native forage plants to yield and subsequent development The results obtained are summarized in Table 3 and Figure 4. The more important indications brought out are the weak growth 81467-26-2 |