From the above investigations we learn that forest soil fauna comprises a series of different types closely attached to certain types of forest soil, chiefly as these present them-selves to our view by means of the soil flora and through our examinations of the forest soil profiles.
The difference between flora types and fauna types is very striking, for while among flora types totally different species of plants occur, fauna types mostly display quanti-tative differences.
The plants of the forest soil are distinguished for their constancy of habitat. They are chiefly perrennial plants, fre-quently with root-stocks (rhizoma), and one or more species
[149] 149 of plants, in competing with other species, will seize upon the ground so as to make it difficult, if not impossible, for other species to intrude upon their domains, unless a change in the prevailing conditions take place. The species particu-larly adaptable to a certain locality will be the predominating ones, and will maintain their hold most vigorously.
In the case of fauna things are different. The life time of the animals is limited, and it is out of the question, as in the case of plants, for a few suitable species to keep a place all to themselves; moreover, the number of individuals often undergoes fluctuations from year to year without any ade-quately assignable cause. Further, animals are locomotive, so that they can seek out the most favourable places in the soil, and thus be more independent of the conditions offered them.
Finally, it seems as if many kinds of animals are more ad-aptive concerning the exigencies of life than plants.
The consequence then is that, while we could characterize flora types by certain species, our characterization of the fauna types is essentially of a relative nature. Earthworms, to some extent, constitute an exception the species of which are more or less markedly circumscribed by the qualities of the soil. We can distinguish between the following main types of fauna:
T h e fauna of deciduous forest mull or M u l l s o i l f a u n a : It consists chiefly of earthworms (50—80 per cent, of the weight), riddling the topsoil and mixing it with organic matter from the surface of the ground. The characteristic animal is the turgid worm (Allolobophora iurgida), in the best mull soil also the common earthworm (Lumbricus terrestris), (see further p. 103). Of arthropods, the chief species are the common mil-liped, the Trichoniscus, and some ground-beetle species. The slower the decomposition the lower the total weight of animals per square meter, and the greater the number of arthropods; click-beetle larvae, diptera larvae, collembola, and mites will increase in number in proportion as earthworms grow fewer.
T h e fauna of spruce forest mull or S u r f a c e m u l l f a u n a : Earthworms constitute only about one half the weight of ani-mals, and comprise such species as the reddish worm (Lum-bricus rubellus), purple worm (L. castaneus), and the
Dendro-150 [150]
baena species, all inhabiting the upper h u m i c mull, to which they convey but little mineral soil. Turgid worm, and other pronounced mull soil species, are wanting. T h e arthropods are mostly mites, millipeds, diptera larvae, and click-beetle larvae.
D e c i d u o u s f o r e s t r a w h u m u s f a u n a : Earthworms (hardly any but Dendrobaena species) constitute from a few per cent, to about twenty per cent, of the weight, and inhabit the layer of h u m u s , which is but slightly mixed with mineral soil.
The fauna mainly consists of arthropods, of which, chiefly, diptera larvae, click-beetle larvae, millipeds, collembola, and mites predominate. The two latter orders are equally represented.
B e e c h f o r e s t s o i l p o o r i n m u l l , of the Polytrichum type, is particularly poor in fauna. Earthworms are very rare.
We here find an arthropod fauna comprising some diptera larvae and click-beetle larvae; collembola abound, and are far in ex-cess of mites.
S p r u c e f o r e s t r a w h u m u s f a u n a : Earthworms are sparse (rarely exceeding ten per cent, of the weight), and confined to the octagonal worm (Dendrobaena ocioedra).
It is an arthropod fauna, characterized by the absence of Trichoniscus; millipeds are rare, while mites abound and far exceed collembola in number. Diptera larvae, and especially click-beetle larvae, predominate in weight. A peculiarity, more-over, is the great number of Geophilidae. In inferior spruce localities, e. g. heath plantations, mites play a still more import-ant part than in superior soil.
In the superior mull, where decomposition was particu-larly brisk, we found the weight of animals preponderating, but their number small; in soil where the process of de-composition is slowr and accumulation of h u m u s takes place, we found the greatest number of animals, but these were minute, and their total weight was lower, yet exceeding that of the in-ferior mull forms. As a standard for the activity of fauna in the forest soil, weight is more reliable than numbers, but neither of these standards is satisfactory. A far better standard is that of respiration. Calculations of animal respiration go to show that it evidently constitutes quite a considerable part of the total respiration of the forest soil.
Of the most striking significance in the forest soil are the
[151] 151 larger earthworm species (Lumbricus terrestris, L. rubellus, and Allolobophora turgida; moreover, Eisenia rosea, Allolobophora trapezoides, A. chlorotica, Oclolasium lacteum, and others), riddling the topsoil through a n d through and mixing it with deciduous organic matter. The deep and friable mull soil, as we find it in deciduous forests, is contingent upon the activity of these animals.
Where the above-mentioned earthworms are wanting, we shall in most favourable cases have a surface mull, as we quite commonly find it in spruce forests. There is indeed a number of smaller earthworms (Lumbricus rubellus, L. castaneus, and the Dendrobaena species), conveying some mineral soil up into the h u m u s layer, but they do not carry on the intensive mixing of mineral soil and h u m u s performed by the larger species.
Arthropods are of the greatest importance to raw h u m u s soil, where organic matter is deposited on the surface of the mineral soil without being intermixed with same. They here perform a great work by gradually decomposing and consuming all deciduous leaves, etc., and in this way trans-forming same into an upper layer of h u m u s (the mouldering layer) in which a brisk process of decomposition takes place, releasing a great a m o u n t of ammonia. The raw h u m u s is distinguished from the surface mull by an incompleted process of decomposition, resulting in the gradual accumulation on the top of the mineral soil of a h u m u s layer not easily trans-mutable. The difference may, to some extent, be inherent in the geological qualities of the soil, but may also be due to the fact that earthworms of the surface mull carry mineral soil into the h u m u s layer and mix it with same, a process conducive to the propagation of bacteria, which are re-quisite for carrying the processes of decomposition to their completion.
On the basis of our investigations we are justified in con-cluding that fauna is of very great importance to the pro-cess of decomposition in forest soil, indeed constitutes a prominent feature of this process, and is a constituent prin-ciple in the determining of the soil we get. It therefore be-hoves the forester, when fulfilling his duties, always to keep in view whatever may be conducive to the progress of fauna, and to avoid everything that may be detrimental to same.
152 [152]
Of particular importance is the promotion of the large earthworm species, which seem to be requisite for the devel-opment and preservation of the good mull soil and the type of soil called Brown Forest Soil (RAMANN'S Braunerde); other-wise the topsoil will easily become compact and be exposed to leaching. Foliferous trees, especially those bearing easily decomposable leaves; a soil flora of herbs; and good shelter — all these factors are propitious to earthworms. An interspersion of light-demanding trees, such as oak, ash, maple, and larch, in beech stands will prove favourable to the soil flora and to bushes as well as to lower trees and underwoods, af-fording shelter against the wind. Conifers are apt to scare away the most important species of earthworms, and an inter-spersion of foliferous trees, therefore, will be profitable. T h e heavy and frequent thinnings — every second year —, intro-duced by forest officer Mr. E. MOLDENHAWER at Frijsenborg in Jutland, will, at any rate on superior soil, result in a herb flora under the spruce trees, which must prove for the benefit of earthworms. In poorer conifer soil, as in heath plantations, we may perhaps not be able altogether to check the raw h u m u s formations, but our investigations in localities 23 and 32 show that heavy thinnings in spruce stands result in a moss covering and a subsequent abundance of fauna, especially octagonal worms and diptera larvae, giving the h u m u s a far looser and more pleasing character. If in these forests we can pass from raw h u m u s to surface mull, it will be of great ad-vantage, but possibly we shall be able to learn how to culti-vate the forest so as attain to a genuine mull soil. To this end, an exact knowledge of animal requirements will be nec-essary, and we shall therefore have to study it thoroughly.
T h e provisions mentioned above, the intermingling of various species of trees and the carrying out of heavy and frequent thinnings, fully accord with the ideas of modern Danish sylviculture. To these must be added endeavours to procure shelter in the outskirts of the woods, light-demanding trees with an undergrowth of brushwood being requisite for this purpose. With a more luxurious vegetation, better shelter and cover in the forest, a more abundant bird fauna, too, will flourish, aiding us in checking the spread of noxious insects.
[153] 153 In conclusion we shall merely glance at the problems presenting themselves for future study. We have seen that different fauna types occur, each affecting the forest soil in its own particular way, and we have gained a little insight into the life and habits of some of the chief species of animals. But we cannot stop h e r e ; we shall have to know the nutriments and habits of all the chief species of animals, and a study along these lines will often be possible by quite simple ex-pedients. Further, we must subject the nematoda and protozoa faunas to a thorough investigation. W e must also follow u p our investigations about the influence on the fauna, of our syl-vicultural measures, particularly those of thinning and regene-ration. A number of problems thus present themselves for our consideration, problems that must be solved before the forester can reap full benefit of the activities carried on by fauna in forest soil.
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