De I. VNER VEW D Forest Trees of the Pacific Slope

INTRODUCTION. 15
The occurrence of trees is also influenced by their tolerance-that
is, their ability to exist, for a part or the whole of their lives, in dense
shade or their requirement of various degrees of shade or of full light.
To what extent, however, tolerance-inherent or acquired-may be
accounted for by the amount of soil moisture a given species requires
can not be stated now. Finally, the characteristic habits and methods
of reproduction, by seed or by sprouts, most important factors in the
life history of a tree, have much to do with the occurrence of a species.
It may be said here, in passing, that dendrology, the botany of
trees, properly includes a study of the distinguishing characteristics
of tree species for the purpose of identification and, naturally, of the
affinities which determine their classification into orders and other
natural groups. The characteristics of a tree include the definition
of both external and internal form characters-the morphology of its
trunk, root, branches, twigs, buds, leaves, flowers, fruit, seed-as well
as of the anatomical structure of the tissues, including characteristic
secretions-gums, resins, etc.-of which these parts are composed.
A study of the physiological processes which characterize the life of
the tree organism are a part, too, of dendrology. It deals also with
the natural range-horizontal and vertical and its peculiar climatic
conditions, as well as with the habitat or occurrence-including the
character of site and soil the tree chooses either in pure or mixed
growths. What the forester has long called silvics, a study of the
habits and life history of trees in the forest, therefore falls naturally
under dendrology. Silvics, as the basis for all practical silvicultural
operations, deals with the factors which influence the life and growth
of trees in their natural or adopted habitat. In recent years the new
science of ecology, a study of plant associations, has included, in so
far as the life habits of trees are concerned, a part of dendrology as
one of its natural subdivisions. It appears logical, however, to consider
dendrology as still including the study of tree associations.
This leaves forest ecology in its proper place as a department of
general ecology, and at the same time preserves the identity of an
essential part of dendrology, a distinct division of general botany.
However this may be, the serious student of tree life-dendrologycan
make no mistake in taking the broadest view of the field and in
striving to familiarize himself with all that pertains to trees, from
a study of their distinguishing characteristics to their modes of life
and associations.
ACKNOWLEDGMENTS.
Grateful acknowledgment is here made to Dr. C. Hart Merriam,
who placed at the writer's disposal transcripts of his voluminous
notes on the distribution and occurrence of California trees. The unpublished
data thus made available is the result of over twenty years