In the Global Forest Assessment 2005, published by FAO, the following definitions of subindicators are listed:

Growing stock: Volume of all living trees with bark that have more than a certain diameter (X cm) at breast height (or above any irregularities in the form of the trunk). It includes the stem from ground level or stump height up to a given top diameter (Y cm), and may also include branches above the minimum diameter (W cm). In Slovenia, the minimum (X) is 10 centimetres, so all trees thicker than 10 cm are counted as growing stock. The minimum diameter (Y) is 5 centimetres. Branches are not included in the growing stock.

Increment: The average annual volume of increment over the reference period of all trees measured to a minimum diameter breast height (or above any irregularities in the form of the trunk). It includes the increment on trees which have been felled or died during the reference period.
Growing stock and increment are calculated based on the measurements of trees at permanent sample plots taken during the Forest Management Plan renewal. In some forests (if the site’s annual production capacity is less than 4 m³ per hectare), the growing stock and increment volume are estimated based on ocular assessment. The bases for determining the increment are two consecutive measurements within a given period (10 years).

Fellings: Average annual volume of all trees, living or dead, measured overbark to a minimum diameter that are felled during the given reference period, including the volume of trees or parts of trees that are not removed from the forest.
The method of taking inventory of fellings and the method of acquiring data on the growing stock and increment are defined in the Rules on the forest management and silviculture plans: Rules on the forest management and silviculture plans, Official Gazette of the Republic of Slovenia, no. 5/1998 and Rules amending the Rules on forest management and silviculture plans, Official Gazette of the Republic of Slovenia, no. 7/2006.

We are pleased to note that from the viewpoint of growing stock and increment, the Slovenian forests have been improving for decades. This is mainly due to systematic forest management, and partly due to the changes in the methodology of determining the growing stock (ocular estimation – control sample method).

The increment percentage in Slovenian forests is kept at a steady level compared to the growing stock. The allowable cut, whose realisation was only 75% in the last decade, is also following this trend.

In the article Forest Area Management Plans for the period 2001 – 2010, published in the Journal of Forestry, several simulations of the development of Slovenian forests and the annual allowable cut are shown (multi-purpose forests and forests with a special purpose where forest management measures are allowed are included) bearing in mind the valid data on forests (Figure 1).

The growing stock and increment values have increased significantly in the majority of Slovenian forests with the use of more suitable methods of identification. However, due to poor baseline data on the condition of growing stock and increment, the prognoses for their development and consequently the prognoses for the development of the allowable cut in Slovenian forests for the following decades, outlined in the Slovenian Forest Development Programme, are inadequate.

At the time of preparation of this projection, the growing stock was determined based on the 1998 Rules in less than half of the forest management units. Because the methodology has changed, and a significant increase in growing stock was recorded in Slovenian forests (which corresponds to the authors’ predictions; the simulations are also available in this article), the growing stock planned for 2010 was exceeded already in 2006.

Throughout the history, growing stock has been one of the main indicators of the condition of forests. Information on growing stock is of vital importance for the understanding of the dynamics and the production capacity of forests. It serves as a basis for the development of national policies and strategies for sustainable use of forest resources.

Together with the increment, fellings, the forest area and the allowable cut, it forms a basic group of indicators used in monitoring the development of forests. These indicators serve as a basis for estimating the amount of biomass and carbon in a specific forest. The information about how these indicators have been changing through time is of vital importance.

In the last five decades, the growing stock per hectare has increased by 108%, and the increment by 111%, while the fellings varied between 2,000,000 m³ (in 1991) and 3,700,000 m³ (in 2006), which was 100% increment in the 1950’s and less than 40% in the beginning of the 1990’s. Today, the fellings amount to approximately 50% of the increment. The reason for such large accumulation of growing stock in Slovenia and for such variation in increment lies in the rational management of forests.

When speaking of growing stock, increment or fellings, one must also indicate the structure by tree species and broader structure (coniferous and deciduous trees). The ratio between the coniferous and deciduous trees is changing constantly and was increasing in favour of coniferous trees until 1961, when the percentage of conifers was 58%. After 1961, the percentage of conifers began to decrease and in 2000 it dropped under 50%.

Fellings reflect the economic and social use of forest resources in the national economy and the dependent local communities. They contribute to the monitoring of sustainable use of forest resources by comparing the actual removal with the sustainable one.

According to the FAO Global Forest Assessment Study, the growing stock and increment in Slovenian forests in 2005 were much larger than in the rest of Europe. (Figure 2).
In Slovenia as well as in Europe, the growing stock and increment are increasing.

In 2000, the fellings in Europe were reported to be 614,000,000 m³, and in 2005 they were 672,000,000 m³. The positive trend is present in the majority of European countries, and the reason for this lies in the increase of forest areas and the increment. Fellings are strongly influenced by the market, which explains the significant variation between individual years. The ratio between fellings and increment is low and does not vary.

Assessment of the trend:

The assessment of the trend in Europe for the coming decades (TBFRA 2005) is the following:
• The forest surface area will continue to grow steadily;
• The trend of increment increase will stop because of the change in the age structure of forests, the change of species composition and the decrease in afforestation;
• Growing stock shall continue to increase;
• The trend of fellings is uncertain and depends on market-related factors as well as the supply situation. Based on the historical development in the last 50 years, a moderate increase is expected.

Similar assessments also apply to Slovenia. We expect the development to follow the guidelines set out in the draft National Forest Programme. Continued systematic forest management will help us come closer to the optimal average annual growing stock of 320-330 m³/ha. To achieve this, we will need to continue to accumulate the increment selectively and with moderation. Our estimate is that the indicator shall develop towards reaching the set objective.