CXII HENRIK HESSELMAN. 
ground in a dense wood, but where it has not been so severe that the trees 
have been altogether swept away, one often finds a fine young forest under 
the older trees. Where the ground has not been burnt, but where the woods 
and other conditions are of the same nature as on other areas, one finds only 
weak half-stunted plants. As results of this action of fire one finds not in- 
frequently in Norrland a type of pine forest which one might perhaps call 
two-aged ones. . Under a fairly close stand of older trees there shoots up, 
despite overshadowing and competition with older trees, a fine young forest 
of well-growing pines (cf. figs. 23 and 24). Less often one finds something 
corresponding to this in spruce forests. Such a case is illustrated in fig. 25. 
As a result of the effect of fire on the ground, one can sometimes find the 
pine immigrating into a spruce forest and developing in its shade (see 
LR AO 
In the regrowth areas of herbulent spruce forests the young tree-plants 
have often to struggle against a rich vegetation of herbs and grasses. The 
competition is often too severe for the forest trees; the seeds find it difficult 
to reach the actual ground, and even if they succeed in germinating there the 
young plants are threatened with being pressed down, as it were smothered, 
by the herbs and the grasses when they wither in the autumn. Such places 
therefore are difficult to regenerate, especially in alpine districts (see, for in- 
stance, N. KR. BERLIN, 1915, K. E. KALLIN, 1916), although the formation of salt- 
petre on these grounds is fairly active (cf. fig. 3, page 933). In small gaps, 
on the other hand, where the herb and grass vegetation does not attain any 
very strong development, regeneration often takes place quite easily: fine 
examples of this are given in figs. 28, page 989 and 33, page 1003 and it 
is customary to recommend that in such forests small regeneration surfaces 
should be adopted (see, for instance K. E. KaALLIN, 1916, page 34). 
On the other hand, we have a sufficiently abundant experience that ground 
with a covering of raw humus is difficult to regenerate. This is due not only 
to the inferior conditions of germination for the seed, but also to the fact 
that the plant there finds it difficult to obtain the necessary food: it is 
customary, therefore, to delay the culture for some years, until the raw hu- 
mus covering has improved (see, for instance, HOLMGREN, I9I1, Pages 
34, 35)- On clearings where there is a rich vegetation of Azra jlexuosa, 
natural regeneration is associated with considerable difficulties and proceeds 
very slowly. The conversion of nitrogen, it is true, is more active than in 
an ordinary raw humus covering, but the decomposition of the organic nitrogen- 
ous compounds comes to a stop with the formation of ammonia (see page 
949). Nor do such celearings, as I shall show below, offer planted pine- 
plants the same favourable conditions as where the nitrogen is nitrified. 
In forests with a stronger covering of raw humus one usually finds spruce 
plants on mouldering timber, mouldering brushwood and the like (ef. fig. 
27). I have previously shown that on clearings and in gaps the mouldering 
timber readily produces a nitrification: possibly the mouldering trunks have a 
similar importance also when the wood is dense, although, of course, it is 
probable that the nitrification in such cases is very weak. 
If we go through our experience of the factors that favour regeneration, 
we find throughout that in a very notable manner they coincide with the nitri- 
fication of the humus nitrogen. Where the nitrogen ofthe humus cover- 
