NATURAL REGENERATION OF DOUGLAS FIR. 7 
Douglas-fir forests, and so prevent Douglas fir from being the domi- 
nant tree for more than the first generation, except in those locali- 
ties which western red cedar or western hemlock do not reach by mi- 
gration until two or more generations of Douglas fir have occupied 
the ground. 
During the summer of 1913 meteorological readings were taken 
on the north slope, south slope, and flat of Warrens Gap in the 
Wind River Valley in southern Washington.* Some of the im- 
portant points shown by these readings are the maximum tempera- 
ture of the surface soil, soil moisture, and evaporation. The ex- 
tremes of all the factors mentioned occurred in July and August, 
which is the critical period for plants in this region. With a surface- 
soil temperature of 129.4° F. occurring on the south slope, while the 
north-slope maximum was 82.4° F., and with a surface-soil mois- 
ture of only 1 per cent on the south slope as compared with 6.5 per 
cent on the north slope, there is little need to conjecture about the 
failure of small, shallow-rooted seedlings on the south slope. 
_The comparatively short period of drought in this region, how- 
ever, ordinarily enables the Douglas-fir seedlings to become estab- 
lished.. Although the surface soil dries out to the point where seed- 
lings.can not survive, the water content of the soil at a depth of 6 
inches usually remains above their minimum need. The soil moisture 
remained at 11.2 per cent on the south slope and 17.5 per cent on. 
the north slope at a depth of 6 inches. This amount of water pro- 
duces favorable conditions for growth. Although the water content 
of the soil is above the minimum requirement of the plant, generally 
_ the drier sites are exposed to higher temperatures and greater tran- 
spiration. The striking effect of evaporation on these sites is shown 
by the evaporation from open-water surfaces exposed only to vertical 
radiation, as, for the most part, are masses of vegetation. On the 
south slope 15.1 inches of water evaporated from an open water tank 
during the month of August, while only 1.8 inches evaporated on the 
north slope and 6 inches on the flat. The effect of protection from 
the sun’s rays on plants is shown by the survival of seedlings under 
the protection of shrubs on these severe sites and by their failure in 
the open, even where the moisture content of the soil is about the 
same. The greater demand for moisture by seedlings in the open 
because of greater transpiration can not be supplied when the mois- 
ture content of the soil approaches the minimum requirement. 
A vegetative cover of either pea vine or brush has a noticeable 
effect on the moisture content of the soil. To determine this effect 
‘n a south exposure, an area was selected where the pea vine was 
very dense. One square rod was denuded of all vegetation, and the 
area beside the denuded plot was left intact. Readings of air tem- 
perature at the height of the crowns of seedlings, and of soil 
‘emperature at the surface and at depths of 6 and 12 inches, were 
taken each week on each area. The results are given in Tables 2 
and 3. : 
$Hofmann, J. V. ‘The Importance of Seed Characteristics in the Natural Reproduc- 
tion of Coniferous Forests. eindie= in the Biological Sciences, No. 2, University of 
| Minnesota. 1 
