Characteristics of forests in the spruce vegetation zone
The Slovak Republic is one of the most forested countries in Europe. Forest covers about 20,000 km2 (41%) of the total area of the country, a substantial part of which is occupied by the mountains of the Carpathian Arch (highest peak: Gerlachovsky Peak, 2655 m). Forests in Slovakia have commercial functions as well as functions of benefit to the public: timber production, water management, soil erosion control, avalanche control, nature conservation, tourism, and aesthetic value. Many rivers that are important for neighboring countries spring from the Slovak mountains; Slovakia is therefore sometimes called the roof of Central Europe.
All forest ecosystems found in the spruce vegetation zone are protective forests. The most important forest ecosystem in terms of benefit to the public is spruce forests, located at an altitude of 1250–1550 m in the so-called spruce vegetation zone (SVZ). In this zone, total annual precipitation ranges between 1000 and 1300 mm, mean annual temperature ranges between 2°C and 4°C, and the vegetation period lasts 70–100 days. The SVZ forests cover about 40,000 ha or 2% of the total forest area and are located in the central and northern parts of the country, some of them in national parks.
The original SVZ forests were made up mostly of sparse stands or groups of trees with Norway spruce as a dominant species. Some forests also have European larch, European beech, mountain ash, and individual stands of dense mountain pine. Silver fir, cembra pine, and sycamore maple can also be found. The most frequent forest-type groups (original species composition before human influence) are Sorbeto-Piceetum (mountain ash-spruce) and Lariceto-Piceetum (larch-spruce).
Main problems in the SVZ
Research into forest ecosystems in the SVZ (also referred to as high-mountain forests) has mainly focused on the decreasing health of trees. Monitoring of forest health in certain areas within the SVZ showed that about 90% of SVZ forest can be considered to be affected by air pollution. The typical situation involves a low air pollution level, with existing growth conditions allowing spruce trees to survive about 60 more years. A rise in ozone concentration with altitude has been proven within the SVZ. Furthermore, a significant decrease of soil pH values was recorded (0.5–1.0 unit since the 1960s). Although sulfur and nitrogen emissions were considerably reduced during the past decade, these substances are still accumulated in the soils.
Another negative ecological factor significantly influencing the health of high-mountain forests, especially in the last 2 decades, is unfavorable climatic conditions (lack or unsuitable distribution of annual precipitation, temperature extremes, etc). Formerly, the Slovak high-mountain forests were considered to have sufficient precipitation and favorable soil moisture. However, recent studies showed a dramatic change in the water regime in mountain forest soils, especially in sparse spruce stands. Soil acidification and lack of soil moisture are considered the most negative factors—worsening, or on some sites even disabling, natural regeneration of high-mountain forests.
In addition, these forests have been seriously damaged by storms. Trees damaged by wind or physiologically weakened by climatic extremes create favorable conditions for bark beetle outbreaks. Whereas in the past such outbreaks occurred only up to 1000 m, presently this limit is at 1300 m and in certain areas even at the timberline. All these factors cause weakening or even collapse of forest ecosystems. Forest stands become sparse and fragmented. This phenomenon is most evident on mountain ridges at 1300–1600 m.
Recent research efforts
Studies located mainly in the Tatra National Park (Figure 1) have explored the possibilities of restoration (through reforestation) of mountain pine stands that had been reduced by livestock grazing in previous centuries. Further research has focused on reclamation measures such as liming and fertilizing in spruce forests weakened by air pollution or other agents. Since 1999, a 4-year research project at the Zvolen Forest Research Institute has been studying methods for high-mountain forest management based on principles of sustainable development. The project is financed by the Slovak Ministry of Agriculture and is the largest project in the history of Slovak forestry.
Main outputs of the Zvolen research project
Until now, the project has consisted of 6 subprojects on the following topics:
Ecological change in forest environments and its influence on the functional potential of high-mountain forests.
Silvicultural methods in high-mountain forests based on principles of sustainable development.
Protection of high-mountain forests against main kinds of harmful agents.
Research on and development of methods, techniques, and technologies for silviculture and harvesting in high-mountain forests.
Efficient management of high-mountain forests.
Planning and management in high-mountain forests using ecological approaches.
The results from the first 5 subprojects have been synthesized and used to elaborate planning and management principles for high-mountain forests (ie, the sixth subproject), covering the following issues: basic management decisions, desired tree species composition, desired stand structure, target stocking, and management principles.
Basic management decisions
Basic decisions in the Slovak forest management practice concern forest category (commercial, protective, special purposes), silvicultural system (clear-cutting, shelterwood, selective cutting), rotation, and regeneration period. Basic decisions were elaborated for 3 classes—primeval, natural, and man-made forests—that reflect the intensity of human influence on the forest ecosystems (Table 1).
Desired tree species composition
Both in the original (primeval) and in current high-mountain forests, Norway spruce is the dominant species. It tolerates the harsh living conditions in the SVZ better than all other species. Other species have been proposed depending on the site and climatic conditions, with the main aim of ensuring the forests' public-benefit functions.
Desired stand structure
The threshold values of selected indicators for desired stand structure were derived from data collected on research plots classified in the first class (primeval forest). They characterize the most original SVZ forest stands and were therefore considered as a benchmark for the desired stand structure. Primeval forests have 3 development stages—growth, maturity, and decline—characterized by adjusted average values of the following indicators: degree of diameter dispersion (to assess tree diameter variability); share of canopy level (to assess tree height variability); ratio between crown length and tree height, and tree height and tree diameter; and mosaic of stand clusters.
However, it will not be possible to reach the desired stand structure even in the next generation because of large areas of artificially formed stands where management has been neglected. The characteristics of a realistic target stand structure were therefore derived from the data representing the second degree of naturalness (natural forest).
Target stocking was derived on the basis of an original procedure that takes into account requirements related to soil and water protection, stability, and natural regeneration. Relationships between stocking, indicators of stability, and preconditions for natural regeneration were analyzed. Combining these aspects, the value of 0.7 (0.6 at the upper forest limit) proved optimal for stand stocking.
In a large area of the SVZ the objective of forest management is restoration, improvement, or maintenance of the forest's self-regulating abilities. To achieve this goal, functionally effective stand structures need to be formed. The more the stand structure approaches the status of primeval or natural forests, the better these forests are able to develop through internal self-regulating processes. Restoration or improvement of the self-regulating abilities of these forests also has great economic significance: the higher the forest's self-regulating ability, the fewer the management interventions that will be needed and the more effective the performance of forests' public-benefit functions.
With this in mind, we propose to plan and carry out any measures in these forests only on the basis of their actual “naturalness” class, which has to be the decisive criterion for determining the urgency of proposed measures. Additional criteria should include an assessment of static stability, natural regeneration, health condition, and stocking, as an indicator of fulfillment of ecological functions (mainly soil and water protection).
Basically, it can be stated that the forest stands classified in the first “naturalness” class can be left as is. In such stands, natural regeneration usually fully corresponds to the actual stand structure, and both static stability and health condition are excellent. Forest stands that do not meet these criteria—mostly man-made, even-aged, vertically and horizontally little-differentiated forests, but also natural forests with various development stages whose natural regeneration ability is insufficient—require concrete measures. These measures can be classified according to the degree of urgency, based on the forest's actual status.
Perspectives for the future
Better management of high-mountain forests will require building a comprehensive net of forestry roads that are ecologically adapted to the terrain. It will be necessary to adapt all forestry activities in high-mountain forests to ecological standards and to introduce the most recent techniques and technologies. Clear-cutting is forbidden in the SVZ and has been fully replaced by shelterwood and selection systems. On sites with deteriorated soils, recovery measures such as area-wide application of dolomitic limestone by airplane or helicopter, addition of dolomitic limestone and NPK fertilizers in holes, or application of mulching cloths when planting will create suitable growth conditions for subsequent forest stands.
Generally, natural regeneration is preferable (Figure 2). However, on certain sites tree species diversity will be enhanced by planting desired tree species. Mixed stands (especially of Norway spruce, European beech, silver fir, Scots pine, sycamore maple, European larch, and mountain ash) will gradually substitute pure spruce plantations, thus enhancing the ecological stability of the forests (including resistance to ongoing climatic change). The health status of forests and occurrence of harmful agents will continue to be monitored. In the field of forest protection, preventive methods will be given preference over suppressive methods.