1918] HUTCHINSON—FOREST TREES , 469 
temperature as an ecological factor. The trend of nearly all experi- 
ment has been to show that water is of vastly greater importance.” 
BELL (2) states, “A great difference in the moisture of the air of 
two regions otherwise resembling each other in climatic conditions 
has also a powerful effect upon the growth of forests; and the dry- 
ness of the air in the western prairie and arid regions is, no doubt, 
the chief cause of the absence of timber.’’ 
There has been much recent research concerned with the water 
relation of plants (9). The greater number of investigators have 
selected the region of the great plains, a region where water is domi- 
nantly the limiting factor, as their field of investigation. TRAN- 
SEAU (22, 23), LivincsTon (16, 17), and FULLER (13) have shown 
that the water factor may be regarded as a complex depending 
primarily upon the amount of soil water available for the plant 
and the rate of evaporation. The amount of available soil water is 
dependent upon precipitation during the growing season and the 
physical properties of the soil, while evaporation depends chiefly 
upon the humidity, air currents, and temperature of the atmos- 
phere. Briccs and SHANTz (4) in their work on the wilting co- 
efficient have emphasized the specificity of tolerance in plants with 
respect to minimum soil water. The valuable experimental data 
recorded in these papers demonstrate that water frequently acts 
as a limiting factor; the converse, that there is a considerable 
range for any given species wherein water does not factor in a 
limiting capacity, is less frequently emphasized. 
It is significant that the limits of tree species such as Picea 
mariana, P. canadensis, Larix americana, Populus balsamifera, 
Abies canadensis, and Betula papyrifera, which extend north of the 
arid plains, are not deflected southward’ in the Manitoba-Minne- 
sota region, while almost invariably the more southern species are 
deflected when they come in contact with the region of diminished 
precipitation. The evidence would indicate that the water factor 
limits the westward extension of such species as Acer saccharum, 
Tsuga canadensis, Fagus americana, Thuja occidentalis, and Ulmus 
americana. 
It may be noted, also, that when this deflection takes place 
the order in which the tree limits occur is changed in many instances. 
