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 therefore 
will grow far down among the oaks under suitable humidity con- 
ditions, but where the humidity follows the temperature rather 
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 
Mcntana and Idaho it grows out ot 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 growing 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 alse 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 ihe 
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 propar life zone. 
The accompanying map shows the life zones as actually worked our 
in the Great Plateau. 
Life zones were first clearly marked out by Humboldt and his 
names should stand. - 
The upper Hmi of the db life zone is about 60 degrees an- 
+ 
