The Role of Artificial and Natural Regeneration in Increasing the Sustainability of Forest Ecosystems in Poland

0323-B1

Jan Lukaszewicz^[1], Witold Kopryk and Rafa Paluch

Abstract

Artificial and natural regeneration is reasonable under specific habitat, climatic and stand conditions and should be applied provided it serves to increase the relative sustainability of forest ecosystems.

In 2002, natural regeneration accounted for 7.9% of all restocking (reforestations and afforestations) in Poland. To obtain a young generation of trees from self-seeding a number of factors should be taken into consideration. Under the climatic and soil conditions of Poland, as well as high population pressure and the resultant potential species composition of stands, regeneration by planting is and should be the main form of forest restitution. Stand regeneration by planting, established in accordance with the ecosystem’s demands, imitates natural regeneration after major disturbances and over a longer time span ensures the sustainability and balanced development of forests in the right temporal and spatial order.

Natural regeneration should be applied where the probability of obtaining a young generation of trees from self-seeding and stable stands in the future is high. Unfortunately, natural regeneration that will ensure forest sustainability and species diversity on the most of the territory of Poland is not possible, mainly because of the habitat structure with predominant pine forests, the domination of species naturally arising after major disturbances, and low annual precipitation with frequent droughts during the germination period. These factors determine the choice of the clear-cut or partial cutting systems and associated regeneration by planting or by limited strictly controlled self-seeding.

Text

There are four ways of forest regeneration:

1. Artificial regeneration by planting seedlings raised in forest nurseries. Most bare-root seedlings in Poland are raised in nurseries established on forest soils in open-terrain surrounded by forests. Container-grown seedlings undergo micorrhization, which increases their performance in plantations. Seed regionalisation i. e. division of the country into seed regions, makes sure that seeds used come from of indigenous populations. Nearly 38 species of forest trees and 24 species of forest shrubs are produced in forest nurseries.

2. Artificial regeneration by direct sowing is not used in forest practice due to high costs, lack of related research works and silvicultural methods. Under suitable habitat and microclimatic conditions, using relevant sowing techniques this way of forest regeneration can be efficient especially when regenerating birch, oak, etc.

3. Controlled natural regeneration - we make use of seedlings from self-seeding of trees growing on the regenerated site or close to it. In this method species composition and seedling coverage is controlled. Stocking before and after self-seeding, seedling growth conditions their number and species composition are also controlled.

4. Uncontrolled (spontaneous) natural regeneration without man’s intervention and assistance. Species composition and amount of seedlings is often incidental and depends on soil weeding, density of the stand left for natural succession, species composition of a self-seeded stand and near-by stands, climatic conditions in the seed year and following years, ecosystem developmental stage (the richer habitat the higher is the likelihood of species rotation connected with their light demand and shade tolerance) and anthropopressure.

Each of the methods of forest regeneration is applicable under specified habitat, climatic and stand conditions provided it increases the relative sustainability of forest ecosystems.

Table 1 presents the rate of reforestation and afforestation in Poland. In 1990, natural regeneration accounted for 4.1% and in 2000 - 7.9% of all regenerated areas in the country. During the last decade the naturally restocked area increased by 60.6%.

Different species regenerate best and occupy the largest areas in the following habitats: pine - poor coniferous forest habitats, spruce - mixed coniferous and mixed deciduous forest habitats, fir and oak - mixed deciduous forest habitats and beech - deciduous forest habitats. Species from natural regeneration prefer more wet habitat, which are uncommon under Polish conditions. The amount of precipitation in the growing season, probability of spring and autumn frosts and the mean temperature in January are climatic factors, which account for the spatial variation in regeneration of different species in Poland. The largest regenerated areas occupy terrains of diversified morphology - highlands, uplands and northern lake districts. In the Carpathians where regeneration conditions are the best the share of stands with young regeneration or undergrowth is the highest. Natural regeneration is most closely connected with the amount of precipitation in the growing season. The areas with the highest precipitation deficiency, especially northern Mazowsze and western Wielkopolska are in fact deprived of natural regeneration (Zajaczkowski Jacek 1998). The most strongly linked with climate are species that have a distinct distribution limit (fir, spruce and beech) in Poland. Most often they regenerate within the compact range of their distribution. Natural regeneration of pine is widely distributed throughout Poland. In some regions e.g. Solska Primeval Forest conditions for natural regeneration are most favourable probably due to the moist sites - retaining water in the poorly permeable geological layers. However, most Polish forest soils are strongly permeable.

The supporters of natural regeneration regard forest nurseries as the necessary evil. While they can tolerate afforestation of post-agricultural lands by planting they do not accept regeneration of forest areas by planting treating it as an anachronism. Who is right? Would the stands regenerated several decades or several hundred years ago appear to be unstable and species-poor forest ecosystems or on the contrary or may be the truth lies in the middle? We should ask the question about the future of our forests, the role they should play and analyse consequences of overpromoted natural regeneration of forests in Poland, Europe and in the world. The main aim of the natural and artificial regeneration of forests is a sustainable, balanced development of forest ecosystems. Diametrically opposite way to attain the goal is a controlled regeneration by self-seeding or planting on small, medium or large cutting areas prepared prior to regeneration. The challenge of forest science is to answer the question under which soil, climatic and microclimatic conditions of the changing environment the applied methods of forest regeneration will be optimal and which method will bring about the desired ecological effect which is sustainable and stable stands satisfying most of the roles they play today and will play in the future.

The analysis presented in this paper relate to forest regeneration in Poland, however, most strategies are typical of forests throughout temperate countries being under strong human impact on forests during several or even dozen or so hundreds of years. The objective of this paper is to underline the role of artificial regeneration in the past years and in the future as the main method of enhancing biodiversity and sustainability of forest ecosystems and in many cases ensuring the existence forest formations in space and time. Where natural methods of natural regeneration fail or are unrealistic planting ensures the attainment of the main goal - sustainability of forest ecosystems. Under climatic and soil conditions, as well as high anthropopressure impact and the resultant potential species composition of Polish stands regeneration by planting is a rational method of forest management ensuring the coverage of forest areas with stands having species composition close to natural. Stand regeneration by planting established in accordance with biological and habitat demands of tree species ensures a sustainable and balanced development of a multifunctional forest. In this context, the decision whether to apply natural regeneration of pine often means increased silvicultural risk (Bernadzki 1996, 2001, Zajaczkowski Jan 2001). This particularly refers to pine stands growing on soils with water deficits. Spring and summer droughts are frequent in our climate. They can inhibit germination and early growth of seedlings. Light-demanding natural regeneration of pine requires careful light management. Young generation of pine growing under canopy are often damaged by snowfalls (mostly in overdensed undergrowth). Under-canopy seedlings are more frequently exposed to fungal diseases than plantations established in large well-aired cutting areas.

From the beginning of the world, forest regenerated on poor soils with water deficit after major disturbances such as forest fires, floods, windstorms or pest outbreaks. It is also characteristic of Scots pine the main species occurring in Poland. An example of such natural ecological disasters in Poland are forest fires (e.g. 1992 in the Rudy Raciborskie Forest District -10 thousand ha), floods (e.g. in the Oder valley - 1997 - 19.3 thousand ha of land was flooded) and recent windthrows in the forests of the Regional Directorate of State Forests in Bialystok (nearly 12 thousand ha of pine forests in all age classes were completely destroyed).

We can take decisions whether damaged forest areas after windthrows, fires, floods, pest outbreaks or declining old stands with insufficient natural regeneration should be left untouched for natural succession or to replant them in accordance with the forest knowledge obtained by generations. The basis for decision-making is the question whether one, two or even three human generations should be deprived of forests. The poorest soil and the more severe climate this period is longer. For example in Poland trees are being replaced by grasses and fern, and sometime by rowan clumps as a result of broadleaved forest decline in the high-mountain zone. After several decades, new generation of spruces occupy decaying dead trees and after next several decades they take over the dominance (Holeksa 1998).

Artificial regeneration by planting on a small cutting area imitate natural regeneration after major disturbances and for Scots pine such a disturbance occur on average every 120 years which results from the felling age of this species. Artificial regeneration of forests played a dominant role in conserving forest resources and providing woody vegetation cover of most of the forest areas in Poland. If silvicultural methods applied to stands established in such a way was in accordance with the laws of Nature, the stability and sustainability of these forests was often higher than sustainability of the so-called natural stands. Without intervention and assistance of man species richness of a stand and biodiversity of the forest floor in many natural stands of the strict reserve in the Bialowieza Primeval Forest would decrease. The abundance of oligotrophic and light-demanding species decrease in favour of eutrophic species (Paluch 2001, Sokolowski 1991). The number of the appearing species is not as high as to compensate the number of disappearing ones. The reason for these changes is the expansion of hornbeam, a species that does not tolerate any other plant under its canopy. In the fragments of the Bialowieza Primeval Forest established by planting and controlled natural regeneration managed in the clear-cutting and selection cutting systems the species-richness is sustained and adjusted to habitat conditions.

One of the examples of stable forest ecosystems are stands established by planting pine from Tabórz (North Poland) and rationally managed for hundreds of years. During Napoleonic campaigns it was discovered and widely promoted as "Bois du Tabre". At the World Exhibition in Paris at the end of the 19^th century it was an object of particular interest not only for foresters (Dziekonski 1994, 1998). The existence and development of these stands are owed to the close-to-nature silvicultural system used for the past 250 years. The history of stands established on rich soils of the Tabórz Primeval Forest dates back to about 1750, the time of the controlled forest economy. The principles of forest management were based on the division of a forest into compartments, age classes and wide use of clear-cutting or shelterwood cutting system. Most pine stands from Tabórz was the effect of using clear-cutting system with clear-cuts of 70 m wide started from northeast. Stumps were pulled out and corn was grown for two years following tree removal from the regenerated areas. In the third year, the cutovers were regenerated with pine by "hand sowing". Planting many-year and later one-year seedlings raised in the nurseries established close to the regenerated areas was infrequent. On some cutting areas seedlings were planted out in the cornfields. Pure pine plantations and thickets grown on fertile soils were regularly and intensively tended.

Pure pine stands which were established by sowing or planting in the period from the end of the 18^th century until 20^th century underwent transformations as a result of intensive silvicultural operations and major disturbances both biotic and abiotic. By bringing in broadleaved species such as beech, oak, hornbeam, as well as spruce under pine canopy by way of natural succession or artificial regeneration pine monocultures were converted into stable, two-storied broadleaved-pine stands. Regeneration of these stands requires man’s intervention and assistance. Controlled natural regeneration (self-seeding) is used where competing grasses make it possible but successful regeneration is achieved only by planting.

Stands with predominant share of Scots pine occupy 70% of forest area in Poland. Scots pine, our main forest species is, as a rule, artificially regenerated with high quality seedlings raised in forest nurseries. It should be remembered that forest regionalisation of seed and seedlings distribution, i. e division of the country into seed regions has been developed in Poland to prevent negative effects of uncontrolled transfer of different tree populations. The binding principle of seed collection is to minimise the risk of genetic depletion of plantations and prevent introducing undesirable gene pool. Natural regeneration of Scots pine is preferred in protective forests growing under suitable habitat conditions. However, it bears greater silvicultural risk and requires strong discipline and knowledge from foresters who have to apply adequate silvicultural treatments such as tending of undergrowth or a gradual removal of trees in the upper storey including removal cutting. Researches have demonstrated that opening-up can cause heavy losses to the undergrowth approximating even to 50-70% of the initial state (Sobczak et al. 1990).

Natural regeneration should take account of local habitat and climatic conditions, for example frequent heavy snowfalls increase the risk of mechanical damage to pine undergrowth. The risk is strengthened by unpredictable weather events which can substantially affect the growth and survival of seedlings (e.g. droughts or too wet summers) (Bernadzki 1996, Andrzejczyk, Twaróg 1997). Mierzejewski (1975) has indicated that pine can successfully regenerate in those regions of the country where mean precipitation sums are higher than 550 mm per year and 340 mm during the growing season (May - September). Optimum conditions for natural regeneration of pine occur in more wet coniferous and mixed coniferous habitats with the dead, moss or green ground cover on brown and podsolic soils and ground water level at a depth of 1-3 m (Mierzejewski 1975, Heinsdorf 1994, Barzdajn et al. 1996).

Seed crop and meteorological conditions decide about the initiation of natural regeneration. In the second phase, canopy shelter of the parental stand is fundamental for seedling growth. Moisture and light are regarded as the most important factors for seedling performance in the field (Heinsdorf 1994, Barzdajn et al. 1996). A precondition for the emergence of seedlings is soil preparation whose aim is to uncover the mineral layer and reduce competition of weeds. Mierzejewski (1975) has demonstrated that the number of one-year-old seedlings in the soil prepared with a plough was higher by several times than that without soil preparation.

Besides artificial regeneration pine successfully regenerates from seeding from an adjacent stand (marginal trees). Light demanding, resistant to frost and water deficit Scots pine grows well in open terrain. The success of such regeneration is when seedling density approximates to 10 seedlings/1m² and in a zone of 50-60 m in width. Natural seeding under shelterwood should not be widely used. Ecological requirements of Scots pine under natural conditions of Poland are the reason that multi- and two-storied pine stands occur on a limited scale (Zajaczkowski Jan 1996). Man-made two-storied pine monocultures in large areas are hazardous to forest sustainability (increasing the risk of damage by abiotic factors - wind, snow or fire). A relatively high uncertainty of obtaining desirable effects of natural regeneration of pine makes us believe that artificial regeneration is the most successful method of Scots pine regeneration in Poland. The share of natural regeneration of this species should not be greater that 10% of the regenerated areas (Zajaczkowski Jan 2001). Many limitations to natural regeneration in Poland make us believe that total rejection or significant limitation of artificial regeneration will inhibit sustainability of forest ecosystems. On the other hand regeneration of shade-tolerant species (fir, beech, spruce) including natural regeneration in Poland is widely applied. A variety of cutting systems are adopted to satisfy ecological requirements of individual species. Fir is almost exclusively naturally regenerated.

To summarise, to obtain a new generation of trees by natural means many elements should be taken into consideration including those which are unpredictable and independent of man (atmospheric factors) and predictable the knowledge of which is indispensable to take decision which stand should be select for natural regeneration (health status, stand quality, habitat) and those silvicultural operations which are entirely dependent on man and decide of the success.

Under the climatic and soil conditions of Poland, as well as high anthropopressure impact and the resultant potential species composition of stands regeneration by planting is and should be the main form of forest restitution. Stand regeneration by planting established in accordance with ecosystem’s demands imitate natural regeneration after major disturbances and in a longer time span ensures the sustainability and balanced development of forests in the right temporal and spatial order. Natural regeneration should be applied where probability of obtaining young generation of trees from self-seeding and stable stands in the future is high. Unfortunately, natural regeneration ensuring forest sustainability in the present state or in the form of potential species diversity on the most of the territory of Poland is not possible, and mainly due to the habitat structure with predominant pine forests, domination of species naturally arising after major disturbances, low annual precipitation with frequent droughts during the germination period. These factors determine the choice of the clear-cut or partial cutting systems and associated regeneration by planting or by limited strictly controlled self-seeding.

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