12 
Many of the more northern forms named, though not all, will succeed 
in the more southern localities. 
Many more species, both native and introduced, may occur to the 
reader, but these have come within the observation of the writer and are 
classified from this standpoint. It must be remembered that light and 
moisture conditions will greatly influence the action of species, as well 
as resistance to cold. In general, it may be expected that as one goes 
westward the conditions affecting growth are more severe, so that 
species which succeed in the eastern part of the plains may fail entirely 
in eastern Colorado. 
Trees that are not quite hardy while young can sometimes be grown 
where success would otherwise be impossible by planting them among 
others of a larger size which may have been set a few years previously, 
thus affording a slight protection for the more delicate kinds. Several 
of our most useful species are thus subject to injury from frost during 
their infancy, becoming quite hardy after the first few years. Even 
the hardiest species will occasionally suffer badly or be entirely killed 
by late frosts. At Brookings, S. Dak., almost all the European Larch 
trees in a plat numbering several hundred were killed by a heavy frost 
after the leaves were one-third grown. The trees which survived 
required two years to recover their normal condition. Frost injury to 
foliage is common to all species in the Northwest, where even the Aspen 
is not exempt in early spring before the young growth becomes 
hardened. — 
A lack of heat affects the growth of trees principally in dwarfing 
them where they are not killed outright. The Russian Mulberry, which 
is little more than a shrub in the southern counties of South Dakota, 
where it is always killed back in cold winters, becomes a good-sized 
tree in the Arkansas Valley. Bur Oak, one of the largest of its genus 
as grown in Kentucky and Missouri, presents a stunted appearance in 
the forests of Minnesota. On the other hand, it has been observed 
that the Boxelder as grown at Yankton, 8. Dak., is larger and more 
vigorous than the same species at Hutchinson, Kans. The trees in 
both sites are in bottom land where they have a plentiful supply of 
moisture. This indicates that too much heat may have a dwarfing 
tendency the same as insufficient heat. This dwarfing may be turned 
to good advantage by the grower in his selection of species for mixed 
planting. Usually in the choice of nurse trees a species is selected 
that will grow rapidly during the first few years, so as the better to 
protect the youth of its more delicate nurslings, and then more slowly, 
so that the protected trees may forge ahead and have the full benefit 
of all the light for their crowns. It will be for such secondary species 
only that advantage can be taken of the effects of cold upon the tree 
growth, and even here other kinds can be found which are not thus 
harmed, and would presumably be equally good for the purpose in 
view. 
