By withdrawing only the quantity thus determined, the forest may be exploited without imperilling its sustained production, provided the exploitation be equably diffused both as to time and space.
With a view to this there is prepared — and it may have been in course of preparation in the forest while some of these data were being prepared in the office — a survey, trigonometrical or other, of the forest and of all the plots or patches of which it is composed, varying in any way from what is adjoining, with a diagram of the whole representing each of these, accompanied by a report specifying the particulars of each. And with these before him, together with the other data, the forest official entrusted with the wrork proceeds to what is called I'assiette des coupes, the allotting of the different portions to be felled in successive periods, which is done with provision for subsequent rectification, if, through anticipated contingencies or unforseen incidents, this should prove desirable.
Let us suppose the forest under consideration is to be subjected to the regime of timber forest, with a revolution fixed at 120 years, it may be divided into four approximately equivalent portions to be exploited successively in four successive periods of thirty years, but in any or all of which there may be carried on extraordinary operations which occasion may call for, the produce of which are reckoned on as a component part of the annual yield of the forest.
The portion allotted for exploitation in the first period of thirty years is then on a like principle subdivided into approximately equivalent portions for exploitation during successive periods of say five years. And there is prepared a provisional scheme of annual operations throughout the first sub-period of five years, which, when sanctioned, are followed for a time; but from time to time the scheme is reviewed, proceeds are compared with estimates, and if necessary the scheme of operations is modified.
These operations include not only the fellings of certain portions, but the thinnings of others repeated from time to time, and the removal of some of the baliveaux left for seed, and shelter, and shade, a sufficient number of trees being left in each felling to resow the ground and give shelter to the seedlings. By these reserved trees there is secured the natural reproduction of the forests in connection with the sustained production of the same, and by them shelter required by the seedlings is supplied; but as these advance in growth their being overtopped would prove a hindrance to their growth. All that is desirable is such shade as might be supplied by the shadow of the tree passing over them as the sun advances in his course; and all the baliveaux excepting such as might sufficient to yield this are removed. Subsequently these also may be removed with benefit to the new repeuplement, and they also are removed. To the new repeuplement it is also advantageous that from time to time they should be thinned; and this is done either at the time that baliveaux are being felled, or at other times. By these successive operations there is combined with the natural reproduction a progressive amelioration of the forest; and the products of these operations are taken into account along with the products of the regular fellings in reckoning the periodic produce.
Besides these extraordinary products of thinnings, &c., added to the ordinary products of the regular fellings, it occasionally happens that a storm or a fire occasions such devastation that some, or many, or all of the trees must be cleared away, and an estimated average of the additional wood thus supplied for the market enters into the estimate of the periodic produce.
The skill of the forest agent entrusted with the work is seen in his so apportioning and allotting the fellings in the division assigned to the period, and the thinnings over the whole area of the forest, so as best to secure the three objects aimed at — sustained production, progressive amelioration, and natural reproduction. While this operation supplies ample scope for the exercise of skill, instruction is given in the School of Forestry of the country in regard to the means of doing what is required. The instructions I cite not here, but content myself with reporting what is done in such a way that a definite idea may be formed of what the improved system of exploitation is, and of wherein it differs from Jardinage, felling a tree here and there as required, and from Exploitation h tire et aire, the progressive felling of adjacent areas.