°0 LIGHT IN RELATION TO TREE GROWTH. 
system are based on this principle of allowing more light to the 
remaining trees in order to stimulate seed production. 
In general, the more vigorous the individual, the greater is its 
tolerance. Any factor which reduces its vitality reduces its tolerance. 
trees from seed, all other conditions being equal, stand shade much 
better than sprouts, and in artificial plantations nursery stock is 
more tolerant than trees which have been started by direct seeding. 
MEASURE OF TOLERANCE. 
Since tolerance is affected by so many different factors, it is evident 
that the tolerance of any particular species is necessarily variable 
and exceedingly difficult to measure. Until recently, therefore, it 
has been customary not to attempt to measure tolerance of shade in 
positive, but only in relative terms. 
SCALES OF TOLERANCE. 
The tolerance of different species is usually compared in lists or 
scales, the most tolerant being usually placed first and the least 
tolerant last. The position which a species occupies in relation to 
these extremes is the expression of its tolerance. Broad and rather 
indefinite terms have been used, such as * very tolerant,” “ tolerant,” 
“ intermediate,” “ intolerant,” “ very intolerant.” The following are 
examples of scales of tolerance prepared by European authorities, 
beginning with the most tolerant: 
GAYER (1898). BUHLER (Morozov, vol. 2: 1286-1295). 
Taxus baccata (English yew). Tazrus baccata (English yew). 
Abies pectinata (silver fir). Abies pectinata (silver fir). 
Fagus (beech). Fagus (beech). 
Picea (Norway spruce). Picea (Norway spruce). 
Carpinus betulus (European horn- Pinus strobus (white pine). 
beam). Ulmus montana (wych elm). 
Pinus strobus (white pine). Tilia (linden). 
Tilia parvifolia=T.: cordata (small- Alnus incana (hoary alder). 
leaf linden). Quercus (oak). 
Alnus incana (hoary alder). Acer platanoides (Norway maple). 
Acer (maple). Pinus montana (Swiss pine). 
Ulmus montana (wych elm). Pinus sylvestris (Scotch pine). 
Frazinus excelsior (European ash). Populus (poplar). 
Quercus pedunculata (oak). Betula (birch). 
Populus tremuloides (aspen). Lariz europea (larch). 
Pinus sylvestris (Scotch pine). 
Betula verrucosa (birch). 
Betula pubescens (birch). 
Larix europea (European larch). 
