TOLERANCE AND INTOLERANCE OF TREE GROWTH. 11 
52° 99’, the light intensity was 1.9, and an hour later, with the sun 
forming an angle of 51° 47’, 2.083. These measurements were taken 
on cloudless days. In Europe the highest light intensities measured 
were 1.5 near Vienna, at an elevation of 550 feet above sea level 
(Wiesner, 1896), between 1.5 and 1.6 at Kremsmunster at an elevation 
of 1,268 feet (F. Schwab, 1904), and 1.8 in Steiermark, at an eleva- 
tion of 4,550 feet (Thomas V. Weinzierl, 1902). 
OPTIMUM INTENSITIES. 
The optimum light intensity at which different species thrive best 
has not been fully determined, especially since this optimum varies 
during the life of the tree, and is subject to variations even in dif- 
ferent parts of the same vegetative season. In a general way it may 
be stated that with the majority of forest trees the optimum heght 
intensity at which the leaves function best and at which the pro- 
duction of flowers and fruits is most abundant lies nearer to the 
maximum amount of light available for the use of the tree than to the 
minimum light under which it can still exist. In some species this 
optimum coincides with the total daylight or with conditions that 
surround a tree grown in the open; in other species both the vegeta- 
tive and reproductive functions of the tree are most vigorous when 
the amount of ght is less than the total daylight; that is, when the 
trees are grown in a stand. 
TOLERANCE AND INTOLERANCE OF TREES. 
The ability of trees to endure shade is called tolerance of shade, 
or often merely tolerance. Trees that are capable of enduring shade 
are tolerant; those requiring full light intolerant. Some species are 
able to absorb enough light for assimilation even in the shade of a 
forest canopy. Thus, hemlock and spruce spring up and live for 
many years under other trees. Other species, such as tamarack, 
aspen, gray birch, and most of the yellow pines, require full light and 
can not endure shade from above. All trees, however, thrive in full 
hght, especially if they have it from the very start, and none requires 
shade except as a protection from drying or from frost. This does 
not mean, however, that all trees grown in the open absorb equally 
all the available light. Even in full light they need and use differ- 
ing amounts of light for their best growth. This is accomplished 
by a definite orientation of the foliage in relation to the source of 
hght, by the development of denser crowns which lessen the amount 
of hght that can penetrate into the interior, or by a change in the 
structure of the leaves so as to decrease the assimilative energy. In 
the open they are able to make these modifications for their best 
growth to better advantage than in the shade of a dense forest, and 
