36 BULLETIN 1402, U. S. DEPARTMENT OF AGEICULTUKE 
Large poles and seed trees are rarely injured and never destroyed 
by tractors, for the machines lack the power to overcome such ob- 
stacles. As in big- wheel yarding, the roads avoid as many obstruc- 
tions as possible. 
Tractor logging as at present used thus ranks as one of the most 
satisfactory methods, both from the standpoint of logging efficiency 
and from that of forest productivity. On ground of moderate 
steepness, such as is characteristic of practically the entire east slope 
of the Sierras, tractors are preferable to donkeys because of greatly 
reduced fire danger, and materially lower damage to advance growth 
and seed trees. 
OTHER METHODS OF YARDING 
Yarding with "bummers" or spool carts, with animal power, is 
perhaps the characteristic system in the Blue Mountains, but it is 
practically unknown in the California pine region. It is used on 
moderate slopes with success, and is said to be more efficient than 
donkey yarding in the timber of that region, which differs very 
slightly from that in the California pine region. This method, too, 
is very satisfactory, causing minimum damage to advance repro- 
duction and practically none to seed trees. 
It is remarkable that in two near-by regions, with essentially sim- 
ilar timber and topography, such as are found in the Blue Moun- 
tains and in the east side of the Sierras, such radically different log- 
ging methods should have been accepted by operators. In general, 
it appears that while the northern operators have tested and re- 
jected donkey in favor of animal yarding, the California operators, 
having tried out only the big- wheel system of animal yarding, have 
tended toward a general adoption of donkey logging even on easy 
ground, a most unfortunate thing from the standpoint of forest pro- 
ductivity. Tractor logging is likely to reverse this tendency to some 
degree. 
COMPARATIVE COSTS OF DIFFERENT LOGGING METHODS 
Two general questions have been presented in the preceding dis- 
cussion that require study : ( 1 ) What are the relative costs of animal 
and steam logging on ground suitable for animal logging? (2) 
What additional cost must be recognized if a method, which has been 
shown to be absolutely inimical to forest production, such as the 
high lead, is discontinued? 
ANIMAL VERSUS STEAM LOGGING 
The use of steam logging on ground suitable for animal logging 
has been found unfortunate from the standpoint of the forest, be- 
cause of the greater destruction of young growth and seed trees. 
Even low-speed ground yarding, although not in the same class 
as high -lead yarding as a destructive agency, is still in general more 
damaging than wheels. What then is the relative cost of the two 
principal methods where used under comparable conditions? 
On one large operation with excellent cost records, it was found 
that over a period of five years the cost per thousand board feet on 
easy ground was from 33 to 59 cents per thousand less for wheels 
than for donkey logging, reckoning direct cost exclusive of over- 
