Abiotic ecosystem characteristics

Physical and chemical state characteristics

Organic soils

Organic soils are of natural importance in forest ecosystems. Bog woodland habitats are characterised by their distinctive hydrological regime as well as characteristic stands and moss cover. The aim is to restore and maintain the habitats in representative areas not only nationally but also internationally.

Such woodlands are important and valuable at all stages of forest succession. Their naturally degraded habitat is usually transformed into other natural forest types. At the same time, they are characterised by their exceptional scope of accumulation of peat layer and therefore play an important role in the greenhouse gas cycle. Bog woodlands, as a natural forest habitat, obviously contribute to a stable ecological condition, and therefore the proportion of organic soils, in which they grow, in a spatial unit is a valuable indicator describing the physical state of the abiotic ecosystem.

Drained organic soils

Unlike organic soils, drained organic soils are exclusively associated with human economic activity. Land reclamation refers to measures that change the hydrological, air, thermal and nutrient regimes of the soil as well as natural conditions for long periods. In order to maintain natural habitats and stable ecological environment, land reclamation, i.e. the draining of soils, obviously cannot be considered a desirable practice.

Average of total soil dissolved organic carbon (DOC)

In the context of climate change, methodological tools that help to calculate carbon migration and accumulation have become particularly important. In 2002, Lithuania ratified the United Nations Framework Convention on Climate Change (UNFCCC) and assumed its obligations regarding the reduction of greenhouse gas (GHG) emissions and the accounting thereof. The Kyoto Protocol to the UNFCCC identifies soils as one of the main sources of carbon accumulation and requires not only protection thereof but also, as far as possible, increase in the amount of soil carbon content. The accumulation of carbon in long-living forms in soil has a positive effect on quality of the soil and the ecosystem as a whole, and is also a promising way to reduce climate change and mitigate the consequences of global warming. The total amount of soil dissolved organic carbon (DOC) stored in the forest floor and soil (depth range – 0 to 30 cm) is about 2 times higher than in the atmospheric layers and as much as 3 times higher than in plant biomass.

Biotic ecosystem characteristics

The group of the biotic ecosystem characteristics covers 3 classes: compositional, structural and functional state characteristics. In this group, 10 indicators were selected for the assessment of forest condition: 3 forest condition indicators were attributed to compositional, 5 – structural, 2 – functional state characteristics.

Compositional state characteristics

The following indicators were attributed to describe the compositional state: proportion of mixed stands, natural stands and artificially regenerated stands in a spatial unit.

Mixed stands

Lithuania is situated in the zone of temperate-boreal forests which are not characterised by pure (i.e. single-species) stands but by mixed stands. Mixed stands are stands in which the volume of trees of dominant species in the canopy accounts for less than 86 per cent of the volume of the stand’s canopy. Mixed stands are valuable from a natural point of view – they are more biologically diverse than pure stands. The aim of increasing the area of mixed (mixed-species) stands is provided in the General Plan of the Territory of the Republic of Lithuania (2030). Mixed stands are more resilient to climate change and related elements such as windthrow, invasions by European spruce bark beetles, etc.

Natural stands

Natural stands naturally regenerate themselves. They can be likened to the type of natural and semi-natural habitats. Such naturally regenerating forests are of great value because they are the most adapted to soil and climatic conditions, they are rich in biodiversity, they often age faster (trees in naturally regenerating forests tend to reach natural maturity in a shorter period of time), and they are subject to rotting processes, which, in terms of biodiversity, reinforces the ecological role of the forest even more. Many experts in ecology believe that the aim should be to increase natural or semi-natural regeneration of forests.

Artificially regenerated stands

Artificially regenerated stands. During artificial reforestation, experts (foresters) select target tree species (the forest habitat is taken into account), as well as plan the target composition of stands to be achieved at forest maturity age. Such stands are valuable from an economic point of view, as tree species such as pine or spruce are usually planted for their valuable wood. Furthermore, these species have a longer lifespan than some others (e.g. grey alder reaches natural maturity at the age of 51 years, while spruce – 121 years, pine – 171 years, oak – 201 years). Properly selected tree species adapt well to climate change conditions. It is possible to develop stands that are adapted to climate change and resilient to natural disasters based on scientific studies.

Structural state characteristics

The following indicators were attributed to structural state characteristics of biotic ecosystem: average stocking level of stands, proportion of low density stands, proportion of modal density (stocking level) stands, annual volume of naturally dying trees, proportion of old stands in a spatial unit.

Average stocking level of stands

Stocking level is one of the indicators characterizing Lithuanian forest stands (see the definition of forest in the Law on Forests of the Republic of Lithuania), which in relative terms can be treated as an indicator describing tree cover density. According to the Forest Management Manual (FMM), the stocking level is calculated based on the ratio of the sums of diameters of the measured and normal stand or the volume of tree stems (with bark) which are selected according to the dominant tree species and the average height of the layer. A normal stand can relatively be considered such a stand in which the tree crowns are completely closed. The relative stocking level of such stand is equated to 1. In the saplings where the average height of trees does not exceed 5 m the stocking level is determined as follows: in meadows – by calculating the ratio of the number of trees per 1 ha to the theoretical maximum (10,000 trees per 1 ha), in plantations – by calculating the ratio of the normative number of planting sites to the number of surviving viable trees. The stocking level is determined visually or instrumentally, expressed in coefficients ranging from 0.3 to 1 (the stocking level may exceed 1 when the sum of the diameters exceeds the norm). In the absence of empty clearings in the stand crowns, the relative stocking level of the stand may be equated to 1, if the clearings cover half of the crown area – to 0.5, etc. According to the Law on Forests of the Republic of Lithuania, the minimum stocking level of stands growing on forest land is 0.3. If the stocking level of the stand does not exceed 0.3, the tree-covered area is a low density stand and the plot is considered to be non-forested land.

Low density stands

Low density stands refer to forest areas that do not meet the definition of a normal stand. As mentioned, the Law on Forests of the Republic of Lithuania defines a forest as an area of land covered with trees with a minimum stocking level of 0.3 (which is the minimum, but not the optimum, stocking level). Low density stands include forest and non-forest land areas with the stocking level ranging from 0.1 to 0.4. Low density stands that create ‘edge effect’ (attracting more invertebrate and vertebrate species) are valuable in terms of biodiversity. Such areas receive more sun and moisture and therefore are often characterised by abundant herbaceous vegetation, which also attracts a variety of animals and insects.

Modal density (stocking level) stands

Modal density (stocking level) stands are those with the optimum stocking level (ranging 0.6 to 0.8), which are the most adapted to climate change, are suitable for biodiversity and recreation, and have good protective functions.

Annual volume of naturally dying trees

Dead wood (dead trees, branches, slashes, debris) is one of the indicators of natural, human-untouched forest habitats (i.e. natural forest succession). Dead wood is valuable from a natural point of view. According to various sources, one third of forest animal species (mainly invertebrates) are associated with dead wood. There is no detailed and consistent inventory of dead wood in Lithuanian forests.

However, the national forest inventory (periodically carried out by sampling methods) includes annual volume of naturally dying trees, which does not reveal the total deadwood accumulation but shows the annual volume of trees that die from natural causes. The volume of dead wood in Lithuanian forests is higher than estimated (there may be dead wood from previous years that has not yet decayed). It should also be borne in mind that dead wood can be removed from forests through sanitary felling. Therefore, the estimated amount is essentially only an indication of the potential for deadwood accumulation.

Old stands

Old stands are those that have reached and surpassed the minimum age of the main felling in commercial forests. The minimum felling age is a relative indicator of the age at which the wood is most economically valuable. Once a tree reaches and surpasses the minimum felling age, it no longer grows as rapidly, and once it reaches natural maturity, it ages – the processes of rotting start and take place. Old stands are valuable from a biodiversity point of view: if economic benefits are not reaped in time, the main functions of the forest become ecological and/or recreational.

Functional state characteristics

Growing stock volume

Wood is one of the main resources provided by forest. Growing stock volume per 1 ha is one of the main indicators for describing a forest. The growing stock volume depends on many factors, but it is mainly influenced by anthropogenic effects, i.e. human work – quality regeneration, education, protection against diseases and pests, etc. The total volume of growing stock shows the amount of wood accumulated in Lithuanian forests.

Mass of organic carbon in living forest biomass

Lithuania, like many countries, has ratified the United Nations Framework Convention on Climate Change (UNFCCC). Under the Kyoto Protocol to the UNFCCC, Lithuania is committed to accounting for greenhouse gas (GHG) from all sources and sinks in all sectors, including land use, land-use change and forestry (LULUCF). The Lithuanian State Forest Service under the Ministry of the Environment is responsible for GHG accounting in the LULUCF sector in Lithuania. Therefore, this indicator, which is classified as a biotic ecosystem function indicator, is not only of national but also of international importance.

Landscape characteristics

Landscape characteristics include 3 forest indicators: rate of forest area increase during 10 years, proximity to the nearest forest, forest edge density.

Rate of forest area increase during 10 years

Based on the data of the Lithuanian State Forest Service, as of 1 January 2020, the Lithuanian forest coverage made up 33.7 per cent. The General Plan of the Territory of the Republic of Lithuania aims at increasing the total forest coverage to 38 per cent. Although Lithuania’s total forest coverage is stable and steadily increasing (according to the data of the Lithuanian State Forest Service, it made up 30.7 per cent in 2000, i.e. an increase of 3 per cent over the last 20 years), it varies considerably between different territorial units of Lithuania. Agrarian, karst regions and urbanised areas have low forest coverage and bio-ecologically poor landscapes. Therefore, the General Plan of the Territory of the Republic of Lithuania (2030) aims at increasing forest coverage by differentiating it according to the natural basis of the morphological areas, the nature and intensity of economic use and the degree of ecological risk. The aim is to provide tangible ecological benefits in highly agricultural and urbanised areas, and to ensure that the scenic quality of the landscape is not deteriorated in forested hilly areas. Although forest coverage is probably the most important indicator characterizing the landscape, due to the unequal distribution of its values in the spatial units under consideration, an average indicator of change in forest coverage, closely related to forest coverage, has been selected.

Proximity to the nearest forest

To assess the spatial structure of Lithuanian forests, the Euclidean distance calculation can be used, which produces a geographical matrix expressing the minimum distance of the grids to the forest. The higher the average minimum distance to the forest in the selected area of the territory, the greater the fragmentation of forests in the landscape and the lower the concentration of forests in large forest massifs (the value of the minimum distance of the grid in the forest to the forest is equal to zero (0 m)). This indicator is a graphical representation of forest fragmentation in the landscape and can be considered as one of the indicators for assessing the spatial structure of Lithuanian forests.

Forest edge density

From a natural point of view, forest edges are important for ecosystems of forests (and not only them). According to various literature sources, more than half of the total number of different plants and animals can be found in forests. Many species of insects and various small organisms choose forest edges as safe habitats. This indicator was chosen as an additional indicator to describe the fragmentation of Lithuanian forests in the landscape and to assess the spatial structure of the country’s forests.