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Slope exposure as a factor in the distribution of Pseudotsuga 
taxifolia in arid parts of Washington 
GOTE TURESSON 
It is a well-known fact that the vegetation on hills and moun- 
tain slopes differs according to the exposure of the locality to 
different quarters of the compass. A flora of mainly xerophytic 
character inhabits the south-facing slope, while the northern 
declivity, less exposed to insolation, is covered with a more or less 
mesophytic vegetation. Also the southwest and southeast slopes 
are generally occupied by a xerophytic flora, the northeast and 
northwest by a mesophytic. 
The great differences in the distribution of plant-communities 
as well as individuals due to variations in insolation may be seen 
in every part of the world. Warming (10, 11) tells us how in 
Greenland the southern slopes of a mountain chain may have an 
open xerophytic vegetation ‘‘appearing as if burnt up,’”’ while the 
northern slopes are at the same time covered by a dense, green, 
mossy carpet, with many flowering plants. The same may be 
observed in the Mediterranean countries, and even in the proximity 
of the equator we find a marked distinction between the flora of 
the northern and southern slopes. 
But slope exposure is an important factor not only in the de- 
termining of different types of vegetation over wide areas; it also 
affects the vegetation within a very limited area. Giltay (3) 
has shown how a great difference may exist between the temper- 
ature and atmospheric humidity on the northern and southern 
slopes respectively of the sand-dunes in Holland, even when only 
a few paces apart. More recently Stenstrém (4) investigated 
the difference in the flora of the embankments of railways, showing 
how some plants occupy the southern embankment, others the 
northern, etc. 
While studying the influence of slope exposure upon the dis- 
tribution of the various plants on the banks of Spokane River in 
astern Washington the writer’s attention was particularly directed 
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