The methodologies regarding the exploitation and regeneration of the stands are known as silvicultural treatments. From regeneration to exploitability (or physiological death), the forest goes through several successive stages. At the stage of regeneration, on one hectare there are several thousand saplings in the case of artificial regeneration, respectively tens of thousands of saplings in the case of natural regeneration. Until the moment of final exploitability, this number is reduced to only a few hundred trees per hectare.

The decrease of the number of specimens per hectare takes place, first of all, following the natural selection, which, in turn, occurs due to the gradual reduction of the growth space. This process is long, the growth of the trees is slow, especially in conditions where the space is small. In addition, in most cases, the specimens that remain after the process of natural selection do not correspond to the expectations of the parties involved:

• the owner’s expectations (economically: species, quality, etc.)
• the expectations of the population (from a social point of view: with a recreational function, used as firewood, etc.)
• the biodiversity needs (from an ecological point of view: the dynamic equilibrium of the ecosystem, the presence of invasive species and non-native species, etc.).

The process of reducing the number of trees can be accelerated by forestry interventions (felling), so that the rotation is shortened and the overall production of wood is increased. The selection is controlled so that the ecological, economic and social functions of the forest are fulfilled simultaneously.

Depending on the characteristics of the stand (especially the size of the trees), various types of cuttings are carried out: tending operations (cleaning-respacing, thinning, salvage cuttings, etc.) and silvicultural systems (silvicultural systems with successive fellings, silvicultural systems with continuous fellings, clear-fellings, coppice systems, etc.).

Tending operations

Cleaning-respacing

The cleaning-respacing operations are silvicultural interventions carried out repeatedly in the young and dense stands. These interventions are considered mass selection works, with a periodicity of 2-5 years. First are extracted all the diseased, broken specimens, those attacked by pests, as well as the deformed ones. Also, specimens from non-valuable and invasive species are extracted, and until the optimal density is reached, other specimens are extracted as well, regardless of species and quality. The wood material resulting from cleaning works is used only in special conditions, for logs, braids, Christmas trees, brooms, etc. In most cases, it is left in the forest for decomposition, thus enriching the soil.

Thinning

Thinning works are carried out repeatedly over a long period of time (approx. 50-60 years) in each stand. An intervention is necessary once every 4-10 years, depending on the characteristics of the stand (species, stand density, management goals, and other aspects). The individual and positive selection of trees is recommended, so that the growth and development of healthy, well-formed and valuable specimens (in the future) to be stimulated, by removing the disturing specimens that compete with them. The intensity of the thinning works must always be correlated with the stand structure, so that the effect of thinning is positive on the remaining individual trees, as well as on the forest stand. The stability and quality of the stand must not be affected after the intervention. There are 4 types of thinning works:

• low thinning – negative selection, by cutting trees with lower vitality, lagging behind with growth;
• high thinning – positive selection, by cutting tall trees that compete with future trees (with superior qualities);
• mixed thinning – felling specimens from both those with low vitality and those with high vitality;
• schematic thinning – it is carried out in regular stands (in rows), by felling certain specimens in a row (e.g. every second tree) or by removing some rows completely.

The wood material resulting from thinning is usually marketable (intermediate revenues).

Salvage cuttings

Salvage cutting, sanitation cuttings or hygiene cuttings are forestry interventions are carried out as needed, in any stand, regardless of age, height or diameter of the trees. They aim to ensure a proper phytosanitary condition, by urgently extracting specimens affected by diseases or pests (sources of contamination). In addition, because dry, broken or fallen trees are also a source of infection inside the stand, they must be removed periodically.

Silvicultural systems

The methodologies regarding the exploitation and regeneration of the stands are known as silvicultural systems. In the high forests, there are different treatments involving successive fellings, continuous fellings, and clear-fellings. When choosing the right treatment, several ecological, economic and social aspects are taken into account. The composition of the stand (existing species) is the main aspect. The environmental requirements of the seeds for sprouting and the environmental requirements of the seedlings for optimal development differ by species.

The growth and development of the seedlings can be stimulated by applying specific fertilizers. Additionally, the occurrence of some diseases, as well as the mass multiplication of some pests must be detected in time. Prevention and control treatments can be carried out with specific fungicides and insecticides. Pheromone traps, trap trees, adhesive boards and trap boards can be used, depending on the pest.

Silvicultural systems with successive fellings and regeneration under shelter

This category includes the uniform shelterwood system and the group shelterwood system. In both cases, the extraction of mature trees is done gradually, through several interventions related to the natural seeding (mast years, when the seed production is generous). Most forests managed with these silvicultural systems are exploited at the age of 100-120 years, depending on the species (beech, oaks, mixed stands) and on the country (laws).

In the case of the uniform shelterwood systems, the stand is thinned evenly, in order to steer the seedlings under the shelter of old trees. 3-4 successive cuttings should be carried out in the stand:

• preparatory felling (if necessary); 
• seeding felling; 
• secondary felling; 
• final felling. 

In the case of group shelterwood systems (uneven thinning of the stand, forming bigger light gaps), 3-4 successive cuttings are also carried out:

• preparatory felling (if necessary); 
• fellings for opening or forming the gaps;
• fellings for gap widening;
• final felling (when all mature trees are exploited).

Clear-cuttings

It involves cutting all mature trees, that reached the age of exploitability, by a single cutting.

In the case of certain forest species (e.g. spruce, poplars, black locust and others), regeneration inside the stand after repeated cuttings is very difficult or impossible, due to the necessities of the seedlings in terms of environmental conditions (especially light). The most suitable method for the regeneration of these species is on open ground, after the clear-fellings are carried out.

Additionally, clear-fellings are carried out in stands heavily affected by wind, snowfalls, or mass drying of trees, due to biotic and abiotic factors. In the case of stands with very low productivity, with species that do not use the productive potential of the habitat, restoration-substitution works are carried out. These works involve the removal of low productive trees, followed by artificial regeneration, with species of economic, ecological and social interest.

Continuous cutting treatments

It is applied in forest stands with all-aged structure, where selected trees are extracted, depending on a series of well-established criteria. After the exploitation of some specimens from the stand, the regeneration takes place naturally, in the remaining gaps.

In these stands, there are at the same time trees of all ages and of all sizes, and thus several forestry interventions are carried out simultaneously: assisting the natural regeneration and the development of seedlings, tending operations of young trees, regeneration methods and ssilvicultural systems, etc.

Additionally:

• The greatest biodiversity in a forest is found after the extraction of some trees. The released land is quickly occupied by various grass-like and shrubby plants, including fruit species. These plants attract insects, herbivorous species (e.g. deer) and omnivores (e.g. bears, wild boars). And, finally, carnivores are attracted by the presence of herbivores. The exploitation interventions thus offer indispensable food and shelter to many species;
• Forest exploitation is conditioned by norms (Laws, Technical Norms), depending on the country. These should be formulated in such a way that the ecological function (biodiversity) of the forests to be sustained.

In some countries, for a better control of the work, the trees which will be felled are marked before the intervention. The marking consists of:

• choosing the specimens of trees to be exploited;
• carving the bark at the base of the chosen trees and applying a distinctive sign (code) with the marking hammer and with paint of a certain color;
• measuring the breast height diameter (diameter at 1,3 m height) of the trees to be extracted and recording each value in the database;
• measuring the height of the chosen trees and recording them in the database (the height of a statistical community of trees is measured);
• the visual assessment of the quality of the chosen trees and recording the corresponding quality class in the database;
• carving the trunk and recording the corresponding current number on each marked tree.