0322-B2
Tadeusz Zachara[1] and Wojciech Gil
Tending cuts, and thinning in particular, are very important issues for silviculture in Poland and other European countries. Systematic tending treatments facilitate the development of various functions of forests: ecological, production and social. Thinning operations (occasionally combined with pruning) aim to increase the technical quality of wood, improve the hygiene of forests and enhance forest resistance to biotic and abiotic stress factors. Under sustainable management of forests, which usually serves many functions (although in some cases a given function can predominate over others) the role of thinning is of particular importance.
The basis for thinning practice in this region is the Schädelin selective method, which favours individuals of the highest silvicultural value. Presently, the method is rarely used in its original form. It underwent a number of modifications under the impact of environmental, economic and social conditions. The tendency to simplify tending treatments by applying wider initial spacing, reducing the number of treatments and increasing their strength was due to economic reasons. Protective aspects, especially the need to prevent snow and wind damage, were the reason for modification of the criteria by which crop trees are chosen and the development of the so-called "group thinning" method.
Different criteria for the choice of crop trees and cutting methods should apply to forests affected by industrial emissions and to those growing on post-agricultural lands. Tending treatments are particularly important in forests under reconstruction of species composition or those being converted from the clearcut system to the selection system.
Tending cuts are one of the most important issues of silviculture oriented towards cultivation of healthy and resistant forest stands in the multifunctional and stable forest ecosystem. Although the formation of wood quality is the prime task of tending cuts they also influence the soil and atmospheric environments (through the regulation of the density) and improve a broadly understood health condition of forests.
The removal of trees during the stand growth is the interference in the process of natural diversification of stand spatial structure and subordinates these processes to the goals of forest management. Selective thinning is currently a commonly used method of tending stands in Poland and other neighbouring countries. Detailed solutions depend, however, on the specific natural and economic conditions, which decide about the direction of the upgrading and modification of the selective thinning method.
Forests occupy 9,047,000 ha of Poland or some 28.4% of its land area. Publicly-owned forests represent some 84% of the total, while private forests (mainly small-scale ownership) covering c. 1,500,000 ha.
The structure by dominant species shows a (76.8%, mainly Scots pine - 69%) preponderance of coniferous species, with broadleaves accounting for only 23.2% of the forest area. Although the area of broadleaved stands has been increasing steadily (by 13% since 1945), the share of these stands remains lower than the potential resulting from the site structure.
The classes of forest distinguished in Poland comprise the protective, the productive and the nature-reserve. The protective forests (now encompassing 3,520,000 ha) in turn consist of stands characterised as soil-protecting, water-protecting, mountain timberline, health-resort/climatic, green-belt, designated for mass recreation and tourism, landscape-enhancing, forests in and around towns and cities or in the zones of impact of industry. Finally, some of the area is in permanent seed stands supplying reproductive material of the highest quality (Michalak 2000).
Forests serve very diverse functions, which can be classified into three main groups (National Policy on Forests 1997):
Ecological (protective) functions connected with the enhancement of biological diversity, ensuring the protection of waters and soils, the stabilisation of the composition of the atmosphere and the purification of the atmosphere, etc.
Productive (economic) functions ensuring the production of timber and non-timber (forest berries, mushrooms, game) resources, as well as the development of tourism, the creation of places of work and the provision of income for state and local budgets by way of taxes.
Social functions connected with the shaping of favourable tourist and recreational conditions for society, the development of education, culture, science, the strengthening of national security, etc.
Many types of function of forest complement one another. There are also such which may limit other functions, thereby generating conflicts. Conflicts are revealed, first and foremost, in antagonisms between intensive economic or recreational use and some protective functions like the protection of biological diversity. The multifunctional forest model is an attempt to ease or prevent conflicts between different forest functions and to enhance their complementary character. It is the forest, which serves all forest functions on the basis of a compromise, (Bernadzki 1998) sporadically giving over the dominance to some of them, the forest that is managed in the sustainable way.
One of the priority goals of the Poland’s policy on forests is the reorientation of forest management towards the sustainable model that corresponds with the criteria formulated for Europe and the World by the Rio de Janeiro (Agenda 21, 1992) and Helsinki (1993) processes (Rykowski 1995). This requires a permanent perfecting of the concepts of forest management.
Society’s expectations towards the diverse benefits resulting from forests entail the need to differentiate the elements of forest management including tending cuts. Management goals being in line with site conditions economic, ecological and social needs are formulated for each stand. Each stand is managed in a particular manner depending on the goal, so it also refers to tending cuts, which are being made from the beginning of the stand life till the mature stage.
In Poland’s forest practice, tending cuts which are applied at the earliest developmental stages of a stand are called cleanings, and a felling applied to an immature stand in older age classes is called thinning.
The major tasks of cleanings are: the regulation of species composition, canopy closure and growth rate of trees and stand improvement.
Thinning is a felling mainly made for the purpose of improving the growth and quality of a stand, increasing its resistance and volume increment of high quality wood. They promote natural regeneration and enhance the protective function of a stand, if such a function occurs.
The goals of tending cuts, their intensity and strength largely depend on the dominating function in a given forest complex.
Tending cuts applied in soil-protecting forests and forests growing at high elevations above sea level in the mountains aim to favour the strongest (with longer and wider crones) and the most deeply rooted individuals. A healthy and stable forest cover enhancing the resistance of a stand against wind damaging effects is, in this case, a more important task than the production of the high-quality timber (Bernadzki et al. 1999).
In water-protecting forests tending cuts are made for the purpose of favouring the best shaped individuals which in the future will provide seeds for natural regeneration; this is the most efficient way of forest renewal under conditions when the exposing of the ground (using clear cuts) is not desirable. The protective aim can combine here with the production of high-quality timber (Bernadzki et al. 1999).
The tending of stands in health resort regions is oriented towards the shaping of appropriate species composition and multi-layered stand structure, the preservation of good sanitary condition of a stand and high silvicultural value i.a. due to the prolonged felling age (the average felling age of pine as the most common species in Poland equals 100 years (£onkiewicz 1997).
In the forests intensively penetrated by tourists, the application of tending cuts to admixture species whose role is growing over time is equally important (Bernadzki et al. 1999).
In the forests growing in the regions affected by industrial pollution the greatest attention is given to the adequate species composition (promoting species resistant to air pollution) and the best sanitary condition of a stand (Bernadzki et al. 1999).
In the forests under legal protection (with high degree of naturalness) tending cuts are made for the purpose of preservation of the greatest number of characteristics and elements of the natural forest (suitable species composition, age structure, form of mixture and vertical structure). Tending cuts should also serve in the enhancing of natural regeneration (Tichonov 1995; Soko³owski 1997), especially for beech and fir.
Tending cuts on afforested post-agricultural lands aim to accelerate the process of creating forest habitats and prevent the spreading of pathogenic fungi. In these stands positive selection is not applied, cuts have the character of the low thinning.
In the forests under industrial impact the situation is similar. During thinning operations weakened and dying trees are removed to retain the most vital individuals. Negative selection prevails.
Each of the above-mentioned forest categories requires tending cuts and abandoning these treatments can negatively influence the fulfilment of the dominating function of the forest. Still more important is a systematic tending of the multifunctional forest in which neither of the functions is superior. To abandon thinning operations or to perform them insufficiently often affects the productive function (poorer quality of wood) and results in poorer fulfilment of other forest functions due to the increased threat from the damage-causing agents.
Tending cuts are based on two major methods of selection: positive and negative. Positive selection favours the best trees whilst negative selection pays attention to defected trees, which are removed from the stand (Bernadzki et al. 1999). Historically older, negative selection method being in line with the method of natural selection was widely applied in the past. At present, it is generally applied at the stage of tending cuts in young plantations and thickets (cleanings), and in certain cases in older stands (e.g. in the stands growing in industrial regions).
The development of silvicultural-grading systems has led to selecting trees of the highest value and to favouring them in the course of tending operations (positive selection). Like in the Schädelin system (Schädelin 1942) most commonly adopted in Europe, the tending cuts, which are carried out in a stand at any stage of its life, are the "combined" methods that is such which combine thinning principles laid down for the low and crown thinnings in their pure forms.
The recent methods of tending cuts (thinning) applied in older stands in Poland are based on the above-mentioned Schädelin system. The main rule is gradual selection of the decreasing number of future crop as a result of fortuitous events causing that trees are removed from the stand or loose their silvicultural value (Leibundgut 1984). The cutting methods differ in the criteria of the choice of crop trees (connected with biometric parameters of trees and their position in a stand), and these are chosen with regard to the accepted management goal.
Various conditions of managing forests in different European countries have caused that the Schädelin cutting system underwent various modifications, which supported by long-term studies and experimental works were implemented in forest practice in most of the countries. However, they have a number of features in common.
The growing labour costs in forest management have since generated the tendency towards simplification of tending treatments. In the past, the possibility of rational tending of young stands was perceived in applying the line cutting method. Numerous studies (Abetz 1967; Bernadzki 1969; Liebeneiner 1969; Hamilton 1976) pointed to negative silvicultural effects of this method such as the declining quality of stands and the increasing threat of wind and snow damage.
At present, efforts are taken to find the solution by widening the initial spacing, reducing the frequency of treatments and increasing their intensity (Abetz 1967; Huss 1993) in comparison to the traditional selective thinning (Schädelin 1942; Leibundgut 1984; Schober 1990). As regards the productive function of forests the intensity of the thinning has no significant effect on volume increment (Erteld 1961), but it causes that the greatest increment occurs in the trees of the highest silvicultural value (Huss 1999). Own studies carried out in pine forests (Zachara 1999) demonstrate that to attain this goal the intensity of the first thinning should not be lower than 20-30% of the basal area.
On the other hand, the increased intensity of thinning enhances the risk of damage from wind and snow in the first years after the treatment (Fries 1969). Too strong cuttings can also reduce stand volume increment by reducing the number of potential crop trees (Paøez 1980). This is the sufficient reason that a very strong thinning should not be treated as a widespread method of stand tending and in multifunctional stands in particular. Strong thinning cuts can be recommended in forest plantations in which the wood function dominates over other forest functions. In some cases, strong thinning can be considered as a correct method of silvicultural procedure if the aim is the change from the pure, one-storeyed to the multi-species, multi-storeyed stand (Bergmann 1995). The use of the increment thinning essentially accelerates the growth of the understorey (Lockow 1998). The tending strategy with strong thinning necessitates the pruning of crop trees (Pechmann 1974) otherwise the quality of wood can be unsatisfactory.
Today group thinning is a commonly accepted modification of the classical selective thinning method. In the group thinning the requirement of even distribution of crop trees in a stand (Zaj¹czkowski 1990; Otto 1994a; Otto 1994b) is abandoned to retain the thickest trees during the felling. Stable groups of trees in a stand are points of the resistance against snow and wind, which for pine and spruce are of particular importance. This method, which is known as the „quality group thinning", is also recommended for beech. In comparison with stands thinned with the classical Schädelin method it increases the share of trees of the high value in a stand (Kato and Mûlder 1998).
The cuts, which do not disturb natural regularities of stand development, enhance natural regeneration, increase the resistance of a stand to environmental stress factors and stand diversity. So, the cuts simultaneously fulfil several functions important from the point of view of the multifunctional forest the strengthening the production functions and protective functions of forests.
Selection structure thinning is a special variant of tending cuts (Kynast and Kaiser 1994), which are applicable under European conditions in the stands with the high share of fir. This variant is used in uneven-aged forest stands. They based on an intervention into all the strata of a multi-layer stand; they also serve in the improvement of the quality of trees in the overstorey and enhance natural regeneration. They can be useful in the process of conversion from the clearcut or shelterwood system to the selection cutting system.
Besides the choice of a suitable method of forest regeneration, tending is a primary consideration for silviculture. Thinnigs aim to increase technical quality of wood, improve the hygiene of forests and enhance forest resistance to the biotic and abiotic stress factors.
The role of thinnings is of particular importance under sustainable forest management principles.
The Schädelin selective method being in line with the principles of the multifunctional forest should be the basis of the tending strategy undergoing local modifications depending on the specificity of a given forest stand.
The choice of the method of tending has to take account of the variety of functions: productive, ecological and social that forests serve under the sustainable management.
The tendency towards simplification of tending treatments by applying wider initial spacing, reducing the number of treatments and increasing their strength was due to economic reasons. Combining the natural (protection of forest environment) and economic (wood production) goals is impossible in forest plantations in which a stand established from the extreme low number of trees does not require tending cuts.
Protective aspects were the reason for modification of the criteria of the choice of crop trees and the development of the group thinning method.
Different criteria of the choice of crop trees and cutting methods should apply to forests affected by industrial emissions and to those growing on post-agricultural lands. Tending treatments play a particular role in the forests being under reconstruction of species composition or conversion from the clearcut system to the selection system.
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| [1] Forest Research Institute,
ul. Bitwy Warszawskiej 1920 r. No. 3. Warsaw, Poland. Email: [email protected] |