100 BU-LLETIX 1333, U. S. DEPARTMENT OF AGRICULTURE. 



The writer believes, however, that there are situations occupied 

 by limber pine in which the winter soil temperatures go considerably 

 below those characteristic of yellow-pine sites, permitting freezing 

 of the soil for long periods. The ability of limber pine to exist under 

 such circumstances, and with extreme wind exposure, points to a 

 marked difference in adaptation, between this species and yellow 

 pine, which may be quite independent of their apparently similar 

 neat requirements. 8 There has come to the writer's notice what may 

 be called an adaptation of outward form, by which during the winter 

 the needles of limber pine become very closely appressed. This 

 gives the limbs a club-like appearance quite different from the 

 'feathery 7 ' one of summer. 



It is therefore quite evident that while limber pine has heat re- 

 quirements similar to those of western yellow pine, which are satisfied 

 by the conditions resulting from the insolation of bare, exposed soils. 

 it~ has also a faculty for resisting winter-drying, which permits the 

 species to range almost from the lower limits of yellow pine to points 

 practically equivalent to timber line, not reached by the yellow pine. 

 This heat requirement makes it possible for the limber pine to be a 

 forerunner of Douglas fir or spruce on gravel slides too strongly 

 insolated to admit the other species, and on burns and cut-over 

 areas, but excludes it from a prominent place in the ultimate forest. 



That limber pine is nowhere more abundant nor more widely dis- 

 tributed than in the Pikes Peak region is, perhaps, due to the pres- 

 ence of a young, denuded soil and to other soil properties which will 

 be discussed in the section on soil moisture. 



(3) The conditions brought out by the Douglas fir sites are amply 

 illustrative of the soil-temperature variations which are possible in 

 different localities. These conditions give the impression that both 

 winter and summer soil temperatures may have a bearing on forest 

 composition, but that at times the conditions are not fully expressed 

 by the soil temperatures alone — that atmospheric conditions also 

 must be taken into account. 



The data for Douglas fir, considered in connection with the quasi- 

 yellow-pine sites, on the one hand, and the conditions in Douglas 

 fir sites which seem to encourage spruce reproduction, on the other, 

 may be summarized as follows: 



(a) Sites on which Douglas fir reproduction will about maintain the predomi- 

 nant position of this species (F— 14, 15) are characterized by an annual mean 

 soil temperature of about 37° F., a growing-season mean of 51°, and a winter 

 mean of 26°, with the soil at 1 foot frozen for about 140 days. 



(6) Greater insolation and radiation, secured on a similar site (F-7-S), by 

 complete removal of the cover, create summer soil temperatures about 3° higher, 

 and winter temperatures 2° lower, the freezing period being somewhat earlier. 

 The summer conditions undoubtedly encourage more germination of western 

 yellow and limber pines, but the winter conditions will probably keep the pines 

 to a very subordinate position in the ultimate composition of the stands. 



(c) Sites in the Pikes Peak region (F-4 and 12) better insolated than the 

 last, but having their summer extremes modified by fairly heavy cover, favor 

 western yellow and limber pines with the fir, but appear to be progressing steadily 

 toward pure fir stands. 



(d) A site (W-A2) having much more insolation than the last because of a 

 south exposure and a scant cover, scarcely warmer than the "normal Douglas 

 fir" sites in summer, but 4° warmer in winter, and not freezing to a depth of 



8 An exception was noted early in July, 1917, when with air and soil unusually dry, limber pines in an 

 open stand on a southeast slope showed foliage injury exactly corresponding to winter-killing, while 

 yellow pines on the same s^ite escaped without apparent injury, " 



