42 CIRCULAR 3 3 3, U.S. DEPAETMEXT OF AGRICULTURE 



the wood, or callousing that distorts diameter growth; or in other 

 ways affect tree development adversely. The use of climbing irons 

 is dangerous for the climber, also, because bark does not always 

 hold and because occasionally something goes wrong with the sup- 

 porting apparatus. If climbing irons are used they should have 

 spurs sufficiently long and stout to give a good purchase and should 

 be adapted to use on the tree species involved. 



In dense stands of pole-wood size it is sometimes possible for a 

 climber to go up one tree and, by crossing over in the branches, 

 come down another, making measurements of both. 



Where many trees are to be measured and the lower branches of 

 the trees are within easy reach, a ladder may be used. The use of a 

 light ladder, with or without an extension device, saves much time. 

 Even for fairly tall trees, a 10- or 12-foot sectional ladder such as 

 is often used by painters in scaffolding work is a means of obtain- 

 ing measurements quickly and safely. "When a ladder of the latter 

 type is used, each separate section should in turn be lashed to the 

 bole. More than two sections of this ladder cannot be used safely 

 unless the ladder is in an almost vertical position. 



Standing trees up to 22 inches in diameter and 85 feet in height 

 are measured by the Central States Forest Experiment Station by 

 the use of special equipment, consisting of a 14-foot extension lad- 

 der, 150 feet of y 2 -inch hemp rope, a swing board, shown in figure 3, 

 an 8-foot bamboo pole marked in 1-foot lengths, and a telephone 

 climber's safety belt and strap. The climber uses the ladder to 

 a height of 24 feet and then climbs to the highest dependable crotch, 

 carrying with him one end of the rope and one end of a 100-foot 

 steel tape. The rope is fed through the crotch, hand over hand, until 

 it comes within reach of men on the ground, who then draw the 

 swing board up to the climber. The height of the tree above the 

 highest crotch reached by the climber is determined with the pole, 

 and that below with the tape. After taking all needed measure- 

 ments above the crotch the climber lowers himself, by means of a 

 special slipknot, to definite points at which he measures diameters 

 and bark thicknesses. Distances are measured directly along the 

 stem of the tree. This scheme is feasible only with trees that have 

 sturdy forks near the tops, and so cannot be used with some conifers. 



Various dendrometers have been developed by means of which 

 upper diameters of a tree can be measured from the ground. Of 

 those used in this country the Liljenstrom 23 has so far been judged to 

 be the most accurate. The Liljenstrom dendrometer is best suited for 

 measuring straight-boled trees, and is better adapted to conifers 

 than to hardwoods {79). 



In using any dendrometer. however satisfactory and reliable it 

 may be. the limitations of its accuracy for the work at hand must 

 be carefully observed. It should be remembered, also, that a special 

 study of bark thickness at various heights is necessary before the 

 data obtained with a dendrometer can be used in volume-table 

 construction. 



23 J. A. Amiln (3) describes this instrument in detail and gives the results of a 

 study of its accuracy. See also (35). 



