Silvicultural systems
According to the definition, silvicultural system is a set of basic management measures applied in individual forest stands in order to regulate their spatial and age structure as well as their tree species composition. Its most important part is the specification of regeneration felling character and subsequent establishment of a new generation.
Silvicultural system as a term is widely internationally recognised and used. However, in different countries it is known under different names depending on the importance of main core forestry subjects in individual countries. Therefore, we are talking about forest management systems as well as silvicultural systems and harvesting systems. All these expressions contain the word system in them, which is in Slovakia due to tradition replaced by "method."
Except for silvicultural system we also use the term silvicultural system form. This term refers to spatial organisation of the silvicultural system chosen.
Silvicultural systems as being defined both in Slovakia and abroad say nothing about the restoration method. In theory each of them allows both natural and artificial restoration, yet in the eyes of forestry public each of them is own to a certain restoration method.
Slovak legislation currently recognises 3 silvicultural systems:
It is often being forgotten that in relation to the existence of 3 main silvicultural systems there are also other silvicultural systems, frequently even "worse" than clear cutting silvicultural system so much being criticised by general public. Defining some of them is just a question of understanding the topic correctly, while others exist whether we admit it or not, despite legislation directly prohibiting them. If the result of such management is a certain specific look of forest stands we without hesitation talk about a "peculiar" silvicultural system.
It is necessary to mention at least some of the other silvicultural systems such as:
- So-called "marginal" cutting is in many countries considered part of a shelterwood silvicultural system. Restoration is limited to the forest stand margins, that is to say to the edges of individual felling coupes (most frequently belts) and is organised in both directions, downwards as well as outwards of parent stands. Its main advantage is a great variability of ecological conditions on a very limited area, which gives origin to forest stands formed by shade demanding as well as light-demanding tree species. Most often, natural regeneration is expected. Not so long ago, this silvicultural system dominated Slovak forestry practice. It has been banned because of its possible misuse - if it was not planned realistically and natural regeneration thus did not appear, its overall result resembled that of a small-scale clear cutting. Its abandonment has been widely criticised as it forces cruisers to prescribe clear cutting method anytime "typical" shelterwood system can not be used due to skidding difficulties. On the other hand, many official clear cuttings are in reality nothing else but marginal restoration. It is important to realise that ecological differences between marginal, shelterwood and clear cutting are related to the shape of felling coupes rather than their size. Each of them can be also realised in belts, which are by conservationists automatically considered to be a form of clear cutting.
- High grading "method" is basically no silvicultural system at all. In reality it can be described as a selection of the most economically lucrative trees and leaving all less "attractive" behind. Conservationists highly favour this approach, consider it ideal and make no differences between creaming and selective system. However, individual selection of only a certain tree type leads to gradual changes in tree species composition and worsening quality of the remaining forest stands as space is given for lower quality tree specimens to develop while high quality specimens are taken out. Lowered quality of forest stands goes hand in hand with lowered quality standards of customers and the whole process can repeat itself following pretty much the same scenario. This process finally results in shapeless forest stands mainly being formed by so called commercially unimportant species. Another foreign term used to describe this "system" is creaming, in Slovakia we use term "stealing the stands out."
- Seed tree "method" essentially means clear cutting with leaving a small number of trees behind (they are called seed trees) in order to provide enough seeds to establish a new forest stand. This method originated following the practice of using truly large-area clear cuttings (representing at times several hundreds of hectares) where natural regeneration was simply impossible. In terms of Slovak forestry practice, where maximal width of individual cuts should almost never exceed two tree lengths, has this method absolutely no logical justification.
- "Intervention free" method: According to the main principle of this method, forest stands are left "to their own devises." Though it is not customary to recognise this method as a separate silvicultural system, it undoubtedly deserves full acknowledgment as a separate silvicultural system. It results in forest stands with natural tree species composition and structure. Therefore this system is very popular and demanded by conservationists. However, it is necessary to realise the following facts:
- So-called "natural" tree species composition does not mean historically original state, but only the state correspondent with current site potential. This can be very different as a result of changed ecological conditions.
- Natural forest stands are usually characterised by complicated long-lasting developmental cycles, which are possible only in stands with suitable tree species composition and structure. Nevertheless, majority of our forests as far back as in the Middle Ages underwent a major restructuring resulting in a simple even-aged structure, very often accompanied by altered original tree species composition. As these forest stands can be changed to natural ones only over a longer period of time (as to without help of foresters) and because individual interim stages are for conservationists fairly unattractive (for example, vast areas of standing dead trees with pioneering tree species underwood), this method can not be really recommended for forestry practice.
Shelterwood silvicultural system
This system is characterised by the regeneration taking place under the protective shelter of parent stand. Thus we generally talk about shelterwood cutting. Parent forest stand is gradually being reduced to allow the establishment and development of understorey representing a new generation stand. The nature of this reduction varies from country to country, and somewhere it even includes the cutting of narrow strips to create canopy gaps. In the most typical case, this system is realised in 4 subsequent interventions: preparatory cut, establishment cut, liberation cut and overstorey removal cut. Reduced shelterwood cutting or in extreme cases only overstorey removal cut are applied in forest stands with canopy reduced by harmful agents.
This silvicultural system is most often linked to natural regeneration. However, artificial regeneration via so called underplanting and undersawing is also possible and accepted. Actually the only vital requirement of this system is that a new generation should be established under the shelter of parent forest stand prior to its total removal eliminating thus the phase of intact open area characteristic for clear cutting. This fact is very important from the point of view of soil protection.
Shelterwood system results in even-aged forest stands where more complicated age and spatial structure is reached by application of its small-scale area approach. This approach limits the maximum size of the integral intervention area to 3 ha (commonly the area is much smaller). Individual forest stand is gradually being renewed by adding such fragments next to each other over a longer period of time (sometimes up to 50 years). This approach results in the forest stand being formed by groups or belts of trees differing in age. Its age span is given by the renewal duration. Large-scale form does not possess such advantages, but is recommended and justified in cases when significant shortening of regeneration period is required.
Apart from its advantages there are also some disadvantages to this system. The most important are:
- difficult transport manageableness on steeper slopes (skidding of felled timber damages new generation as well as left standing mature trees),
- complicated accessibility of forest stands (skid roads),
- weed overgrowth danger (weed overgrowth in forest stands with open canopy inhibits their natural regeneration)
- requires high professional standard of growers' skills (difficulties in securing the regeneration of all required tree species while preventing the regeneration of undesirable species).
Selection silvicultural system
This silvicultural system is characterised by the absence of both larger integral open area and larger compact young forest stand. Felling is evenly distributed across the whole area of forest stand with individual exploitable tree specimens or their small groups (up to 0.20 ha in size) being selected. This criterion divides selection system into the following methods:
- single tree selection method that results in uneven-aged stands having tree specimens of various diameter and age closely to each other,
- group selection method that in reality represents a very fine shelterwood system thus resulting in forest stands formed by several groups differing in age.
As this silvicultural system does not provide for open area formation nor does it lead to significant changes of biomass volume, it is very convenient from the point of view of ensuring permanent fulfilment of all forest functions. Equally we can say that final forest stands are very stable. Selection silvicultural system almost exclusively rests on natural regeneration, underplanting is acceptable only in the case of new tree species introduction.
Selection system is the system most closely associated with the forest stand structure. It is practically applicable only in the forest stands with uneven-aged structure which allows permanent replacement of felled mature trees by the new generation ones or in the stands with possible scope for transformation to this system. Therefore, the countries, where forest stands have been over the centuries changed to even-aged ones (in Slovak terms we are talking about the period from Middle Ages onwards), face serious difficulties in their attempts to increase the proportion of selection forests. Genuine selection forest stands are very rare in Slovakia.
This silvicultural system is highly favoured by conservationists. With relation to this fact it is necessary to realise some other disadvantages on top of the so-called " historic problem."
- Selection structure is not natural, can not be found in any of the primeval forests of Central Europe region. Thus this structure is sustainable only thanks to regular felling. Once left to "its own devises," forest stands tend to develop towards more even-aged ones.
- Selection system is very accessibility demanding (requiring a thick network of forest roads).
- Biodiversity of single tree selection stands is due to light insufficiency (partially also due to dead matter presence) lower than in the forests managed by other silvicultural systems.
- Light-demanding tree species are unable to regenerate (including their mixtures with shade-demanding tree species).
Clear cutting (clear cut) silvicultural system
This system is characterised by single removal (felling) of entire forest stand or most often by its part so the subsequent restoration takes place on barren, standing tree-free, land. The main characteristic feature of this system serving to differentiate it from the shelterwood system is non-existent natural regeneration at the time of parent stand felling. As soon as there is at least one vigorous seedling to be found on the felled land with size 1m2, we are talking about a simplified shelterwood cutting instead of clear cutting silvicultural system.
Clear cutting system is usually related to the idea of artificial regeneration. However, natural regeneration is also possible in quite a few cases. Its extent depends on the clear-cut area size (grossly on its width). Extensive clear-cut areas traditionally offer extreme microclimatic conditions suitable only for so-called pioneering tree species (pine, birch, alder, aspen, goat willow, up to a point also spruce). However, once this area is decreased to a certain limit, margins of neighbouring forest stands function as a kind of shelter, soothing thus harsh climatic conditions characteristic for large open areas. In Slovakia, law strictly limits the maximal size of a clear cut. We recognize two categories of clear cut based on their size:
- Small-scale area form is limited by 3 ha, while the width of individual clear cut can not exceed 2 average lengths of the stand being restored. It results in not entirely uniform even-aged forest stands which are formed by 3 or even 4 generations (theoretically even more) differing up to 10 years in age.
- Large-scale area form is limited by 5 ha, while its width can not exceed 100 metres. This form is allowed only in legally specified individual cases, for example, on poplar plantations, and its application leads to utterly even-aged forest stands.
Clear cuts provide in terms of Slovakia acceptable conditions for natural regeneration of beech, noble hardwoods and other tree species. The only genuinely "troublesome" tree species for natural regeneration of clear-cut areas is probably fir. Nevertheless, this tree species can be artificially restored. Natural regeneration is many times impossible because of weed growth. According to law, grower is obliged to secure reforestation of the clearing within 2 years of its creation.
When applying clear cut system to forest stand restoration, the whole process starts with clearing off belts of forest equaling 2 average tree lengths while the gap between these belts must represent at least 2 belt widths. This way the stand is actually being felled in 3 or 4 consecutive interventions. A new belt can be added to the old one only when newly developing young forest is sufficiently thick and stable. Young forests must be "secured" within 5 years of the first reforestation. During this period, young forest is intensively protected against harmful agents and dead seedlings are permanently being replaced. The whole process of stand restoration thus takes between 20 and 30 years.
Clear cut system brings along openings, very often stripped off any vegetation. This approach always represents a certain risk of soil erosion, however, we can say that the cases of serious erosion problem related to clear cut method being used in Slovakia over hundreds of years are very rare. The fact is that clear cut system is not really nowadays favoured by our forestry authorities, the reverse is actually true. Despite its notoriously negative image it also has some advantages and is very often used as the last resort when all other systems are inapplicable. Its advantages are mainly good transportation manageability (they require the fewest skidding roads) and a low dependence on the parent stand quality (this system is, for instance, the only one suitable for transformation of spruce only-forests into more suitable ones).
From the point of view of public, clear cut system is very badly accepted and over the years many mythbusters about it have been created. We would like to try to reason some of them:
- Certain public group (consisting of very rare forest visitors) principally does not see difference between clear cut system and deforestation. However, clear cut system application also brings along binding provisions to reforest felled areas as we already mentioned above. In defense, we would like to point out to the fact that the total forested land acreage has not been decreasing, but quite on contrary, it has almost doubled since World War I.
- Conservationists have tendency to include every felled "strip" of forest into clear cut category. However, as it is clear from previous sections, so called "belt cuts" are not characteristic only for clear cut system, but are also used in shelterwood system. The only important difference is whether natural regeneration has been left behind once parent stand has been cut off. Within first few years natural regeneration might not be clearly identifiable amidst opening vegetation. Many openly criticized clear cuts in reality never been ones and on principle the main reason for widespread criticism is only their aesthetic value
- Conservationists are even "allergic" to the term clear cut itself. However, it is essential to realise that 'harmfulness" of clear cut lies not in its name but in its area. Our legislation does not help a great deal to "improve" already badly shaken clear cut reputation as it does not define its minimal area, that is to say the area when opening takes on properties not characteristic for a typical clear cut. Our experience prove that for instance clear cuts up to 1 tree length in diameter are not from the ecological viewpoint clear cuts at all. Natural regeneration takes place very fast while typical opening vegetation is absent. Furthermore, even our traditional clear cut up to 2 tree lengths in diameter is nothing to compare with hundreds of hectares of clear cuts known from some countries abroad.
- Even more "enlightened" part of public admitting certain advantages to clear cut system still show the tendency to regard it as something strange and unfriendly to nature. Yet it is necessary to realise that clear cut system has been used on the territory of Slovakia since the Medieval Ages. This fact has profoundly been reflected in the state of our forests, which are considered ones of the nicest in Europe, and in the general public "forestry" awareness. Terms like forest stand age, young growth stand, etc would not exist if there were not hundreds of years of documented forest exploitation history behind. Many of our " semi-natural forests" were created via clear-cut system application, even true to sense large-scale area clear cuts have been "realised" and we even know when exactly.
- Public often mistake clear cuts with windfall areas. Wind and other harmful natural elements can cause serious damage when opening vast areas of forest, much bigger than legally permitted. In connection to this fact it is important to agnize that clear cuts in mountainous regions of Slovakia always take the form of belts and any other opening of different shape is never a result of intentional felling.