31 



by observation of all the species of a region, and then tie these limits 

 as far as possible to known temperature curves. No two species of 

 plants are equally sensitive to temperature and humidity, and so 

 they will not be bound by the same limitations. For example the 

 aspen is more sensitive to humidity than temperature and thereforo 

 will grow far down among the oaks under suitable humidity con- 

 ditions, but where the humidity follows the temperature rathei* 

 closely the aspen conforms to the usual life zone limits well, such as 

 in the Great Basin and Colorado, New Mexico and Sierras, but in 

 Montana and Idaho it grows out ol its true life zone among the 

 oaks, because of humid conditions. A species that stops short off 

 at one life zone limit nearly always feathers out on the other. So 

 the real life zone limits have to be determined by careful observa- 

 tion of all the species gi'owing in it. In this search for zonal limits 

 there are usually some species that conform pretty closely to them 

 and they become very valuable in directing attention as we approach 

 the edges of the zones. 



The alpine regions are naturally limited by the upper edge of 

 trees, and should as well include the meadows that lie between the 

 tongues of trees commonly called subalpine meadows. The great 

 forested region of the west is naturally limited by the spruce, the al- 

 pine fir and for the most part the aspen, and best of all the upper 

 limit of the sagebrush and the deciduous oak. The forested region is 

 the Upper Temperate life zone of which the Alpine and Arctic form 

 only a subdivision. The great forested region of deciduous trees 

 does not belong to the Upper Temperate life zone, but is mostly of 

 the Middle Temperate. The Spruce zone is for the most part a 

 region of evergreen, coniferous forests, though the western yellow 

 pine, Pinus ponderosa, belongs In the Middle Temperate. 



The Middle Temperate life zone is well defined by the distribu- 

 tion of the deciduous oak, the upper limit of the sagebrush and the 

 lower limit of the aspen in the southern regions. 



The Lower Temperate life zone is well defined by the distribu- 

 tion of the white cedar or juniper, Juniperus Utahensis, the pinon, 

 Pinus monophylla and edulis and the Mexican pinons, and also by 

 the lower limit of the deciduous oaks, ,the upper limits of the live 

 oaks, and the creosote (Larrea) and mesquit (Prosopis) bushes, as 

 well as the lower limits of the sagebrush. 



The Tropical life zone is well defined by the distribution of the 

 Creosote bush, mesquit, acacias, Parkinsonias, and barrel cactuses. 

 (Echinocactus) and most of the live oaks. An exhaustive examina- 

 tion of the flora (see Cont. 13) shows that the Tropical life zone extends 

 farther north than is commonly supposed. 



A high humidity tends to throw the lower limits of the life zones 

 below normal, particularly along streams, even so far as to go 

 completely through the adjoining zone where you have species of the 

 two zones almost side by side in canons, the upper zone plants along 

 the bottom and the lower zone plants along the sides. A low humidity 

 tends to throw the upper limits of a zone higher than normal as is 

 shown along the exposed and dry slopes of mountains, where tongues 

 of the lower zones run far up into the upper zone. These considera- 

 tions have to be noted in placing species in their jiropc^r life zone. 

 The accompanying map shows the life zones as actually wonted our 

 1:1 the Great Plateau. 



Life zones were first clearly marked out by Humboldt and his 

 names should stand. 



The upper limit of the Tropical life zone is about GO degrees an- 



