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94 REACTIONS.
parent trees without changing the light values. In the first summer the
growth of the seedlings within the area much exceeded that of those outside,
while a totally new and vigorous herbaceous layer developed. He also determined
the holard of soils with and without living roots, and found the latter
to contain 2 to 6 times as much water. This emphasizes the influence of watercontent
in the later stages of succession and the degree to which competition
can modify it. It also makes it plain that the more obvious effects of light in
these same stages must be checked by the quantitative study of the water
relations.
(19) Reaction upon humidity, temperature and wind.—These three factors
are necesBarily linked together because of their direct effect upon the plant
through transpiration and the indirect effect through the evaporation of soil-
moisture. The plant community reacts directly upon each factor, and these
act upon each other, but the response of the plant is controlled by humidity.
The reaction of a sparse pioneer population is more or less negligible, but the
increasing density and height of the individuals bring about a measurable
result, which becomes significant in most closed associations, especially those
of shrubs and trees. In layered forests the j-eaction is greatopt in the ground
layer or beneath it, where it consists"of herbs. Himiidi'fy is directly increased
by transpiration, but the effect is cumulative because the moisture-laden air
is not carried away. The heat rays are absorbed or reflected, and the lower
temperature that results causes an increase in relative humidity. The capac-
ity of the air for moisture is correspondingly decreased and both transpiration
from the plants and evaporation from the soil-surface are reduced. The final
effect is to make the water-content more efficient and thus essentially to
increase it. The general effect of the reaction is the same as that of increasing
humus, and the two are indistinguishable as a rule. The reduced evaporation
from the surface soil, and perhaps from the seedlings as well, is a critical factor
in the ecesis of many seedlings, especially those of trees.
..(20) Reaction upon local climate.—Plant commvmities react upon the air
above them by transpiration and by lowering the temperature. As a consequence,
they receive more soil-moisture as dew and rain than do bare areas.
This reaction of vegetation is measurable only in the case of forest and scrub,
but probably occurs in some degree in all vegetation, particularly in the
formation of dew. The effect of wooded areas upon rainfall has long been
a subject of controversy, but the evidence in favor of a positive reaction is now
available from so many sources that it seems conclusive. Zon (1912 : 205) has
made the most recent summary of the evidence that forests increase rainfall.
At Nancy the average increase in forested areas for 33 years was 23 per cent,
while in Germany and India it was computed to be 12 per cent. A four years'
experiment to check out the possible error due to faulty instruments yielded
an excess of 6 per cent for the forest. Observations in the north of Germany
indicate that the influence of forest increases rapidly with the altitude. At
elevations less than 300 feet the effect was negligible, while at altitudes of
2,000 to 3,000 feet it ranged from 19 per cent to 84 per cent. Denuded mountains
often fail to cause moisture-laden winds to precipitate their moisture,
as Angot has shown to be the case in Spain. A similar influence is often exerted
by the hot, dry gravel ridges about Pike's Peak upon the local showers in midsimuner.