‘I913] COOPER—ISLE ROYALE 27 
within a period of six years, 9-14 years after the windfall occurred. 
There is only one that clearly antedates it. 
Upon inquiring as to the cause of these facts, the factor of radiant 
energy immediately suggests itself (see p. 20). The older balsams, 
now fallen, when living were close enough to cast a dense shade over 
the area which they controlled, and there can have been no young 
trees beneath them, since if there had been, the present generation 
would antedate the windfall. It was not until 10 years after that 
event that young balsams began to appear in the area. Radiant 
energy being the principal factor involved, this interval of a number 
of years is entirely to be expected, since some time would elapse 
before the disintegration of the tangle of branches with their 
persistent needles would allow a large amount of the energy to 
reach the ground. Evidence in support of this hypothesis was 
found in every quadrat and in every considerable part of the forest. 
In no other case was it possible to determine the exact age of the 
windfall, but the general relation between the older and younger 
generation was usually plainly to be seen. Frequently the only 
sign of windfall is in the rotting moss-covered logs, but the close 
group of even-aged trees, sometimes 50 years old or more, tells the 
story plainly. 
In quadrat 1 two other windfall areas are shown, one (c) quite 
recent, the other (6) older. The greater range of age among the 
trees in these areas suggests that those of the former generation 
did not all fall together. This type of windfall is commoner than 
that represented by group a. ‘The fall of the first trees gives the 
wind a better chance to reach others. This slow process may be 
extended over a long period, even until the new generation has 
begun to fill in the gaps first made. In contrast to the dense group- 
ing just described is the remaining area of the quadrat, where the 
older. The part not included in the three groups comprises two- 
thirds of the area of the quadrat, yet it contains only 18 trees, 7 of 
which are over 60 years; while the other third of the quadrat con- 
tains 45, only 5 of them being over 60 years. The fewer trees in the 
larger part nevertheless produce a dense shade, and there is very 
little young growth beneath them. 
