REGENERATION MEASURES AND THE FORMATION OF SALTPETRE CXIII 
inesrustitranstormed. into, nitrict acid, the megeneration proceeds 
easily and the young pine and spruce plants develop well, pro- 
vided that they do not have to compete with an uncommonly lux- 
uriant grass and herb vegetation. Where the nitrogen ofthe humus- 
covering is not nitrified, natural regeneration is rendered difficult, 
and the spruce and pine plants grow slowly. 
Where the humus nitrogen is nitrified, regeneration goes on with a scantier 
access of light than on other ground: of these good examples are given both 
by the recently mentioned two-aged pine-forest and also by judiciously han- 
dled herbulent spruce forests (see fig. 28). Under such circumstances spruce 
and pine plants have lower demands for light. In a treatise published some 
years ago GUNNAR ANDERSSON and the present writer (1907, pages 100, 101) 
have shown that the stunted pine and spruce plants that are encountered in 
great numbers in old pine stands in no wise suffer from any deficient assimila- 
tion of carbonic acid gas: the assimilating cells in pine and spruce needles 
contain an abundant quantity of starch, but despite this the coniferous tree- 
plants cannot develop. For the work of assimilation itself the supply of 
light is clearly sufficient: and the cause of the suppressed development must 
be sought elsewhere. It is a fact long since known that a tree's need of light 
changes with external circumstances, and that the need of light is greater on 
a poor than on a good ground (RAMANN, 1893, pages 299, 300). Easily access- 
ible nitrogen is usually found on forest land in insufficient or small amounts: 
in such lands it is usually the minimum factor that determines the fertility 
of the soil. After a rapid fire the nitrogen of the humus-covering becomes 
easily accessible, the ground is substantially improved with regard to the minimum 
factor, and because of this the pine-plants can shoot up in the shade of the older 
stand. In order to understand the importance of the nitrification of nitrogen for the 
regeneration of pine and spruce forests, however, it is necessary to discuss in 
somewhat greater detail the nitrogen problem of the pine and spruce, that is 
to say the question of the form in which these trees best absorb nitrogen. 
For a normal development of the pine-plant, as of the spruce-plant, so far 
as we can judge, access is required to latent assimilable nitrogen, that is to 
say, chiefly nitric acid or ammoniac compounds. In connection with the account 
which I have previously given concerning the different value ofthe nitritive com- 
pounds (see HESSELMAN, 1917, page 384), one would incline to suppose that 
the pine and the spruce were ammonia plants: they live preferably on a soil 
with an acid reaction, that is to say, they would prefer the physiologically acid 
salts of ammonia to the nitrates that in physiological respects work basically. 
My previous account of the nitrification of humus nitrogen in Swedish forest 
lands (HESSELMAN, 1917) shows with all desirable elearness that no nitrifica- 
tion, or at least no nitrification of importance, occurs in our mossy conifer- 
ous forests, not even in such as show an unusually high productivity, for in- 
stance the often mentioned Jönåker woods. Thus for the older pine or spruce 
forests ammonia, or possibly organic compounds of nitrogen, form a quite 
satisfactory source of nitrogen. But the question is somewhat complicated: 
for the younger plants, so far as we can judge, the case is altered. No fully 
decisive experiments seem to exist in forest literature; but what we already 
know for certain would seem, when combined with my own observations, to 
fully illustrate this question. 
VIII. — Meddel. från Statens Skogsförsöksanstalt. 
