Diameter Growth in Box Elder and Blue Spruce

It is the common belief that trees grow throughout the greater part of the vegetative period, the commonly called "growing season," which is roughly defined as being limited to the period from the last killing frost in the spring to the first killing frost in the fall, when the broad-leaved hardwoods such as maple, ash, and aspen show their autumnal colors. In the Rocky Mountain forest region this period comprises from 75 to ioo days. Until recently little was known as to the exact time at which trees actually make their diameter growth. Neither has the course of growth nor the formation of the annual rings or cones of wood been measured until recently. About twenty-five years ago FRIEDRICH (2), an Austrian forester, devised instruments capable of measuring the daily growth in diameter of trees. Somewhat later he improved his accretion autograph, as it was called, by adding an electric attachment which recorded the growth in the investigator's office. He would entertain his visitors by telling them that while they could not hear grass grow, they could not only see but hear the tree in the park grow, the increase in the periphery being announced by the sounding of a signal. In i9i8 MACDOUGAL devised a new instrument for recording in minute detail the daily and seasonal growth and all changes in the size of the trunk of a tree. The instrument is called a dendrograph. The essential part of it consists of a yoke composed of slotted bars of some alloy, such as bario or invar, which has a very low temperature coefficient. This is held in place by upright pieces of brass wire seated in small brass plates, which afford flexible supports capable of uniformly adjusting any tension which may be developed. These plates are clamped to a belt of wooden blocks hinged together and fastened securely to the tree, serving