This indicator shows the degree to which current forest composition differs from the natural one. It is calculated by comparing the percentages of actual growing stock of individual tree species in a forest community with natural tree composition. Forest communities represented in a given forest sub-compartment are taken into account.

Based on the percentage of non-native tree species on the site, four categories are distinguished:

• preserved forests (up to 30% of foreign tree species),
• altered forests (31–70%),
• heavily altered forests (70–90%),
• completely altered forests (above 90%).

According to FAO, forests are divided according to their naturalness as follows:

• Primary forest: Forest / Other wooded land (OWL) of native species, where ecological processes are undisturbed by human activities.
• Modified natural forest: Forest / OWL of native species or naturally regenerating introduced species, where ecological processes have been disturbed by human activities, including forests established through natural and/or assisted natural regeneration.
• Semi-natural forest: Forest / OWL of native species or naturally regenerating introduced species established through natural or assisted natural regeneration.
• Productive plantation: Forest / OWL of introduced and, in some cases, also native species, established through planting or seeding, used mainly for production of wood or non-wood goods.
• Protective plantation: Forest / OWL of introduced species established through planting or seeding mainly for provision of forest services.

The two methodologies are not directly comparable, and it should be noted that plantations do not count as forests in Slovenia.

The (un)naturalness of forests is a result of human activity that has altered their natural tree composition. Owing to the mountainous landscape of Slovenia and inaccessible karst terrain, many forests are difficult to access, which is why human activity in Slovenia has affected forests less severely than in most Central European countries. As a consequence, forests are relatively well preserved, particularly with regard to the variety of natural tree species composition and (vertically as well as horizontally) the structure of stands.

Today the percentage of natural forests exceeds 50%, while only about one tenth of the forests are heavily altered (mostly anthropogenic spruce forests and completely altered forests). The most common reason for derogations from naturalness is the increase in the number of conifers on sites unsuitable for their growth. The degree of naturalness is related to the accessibility of forest land, the production capacity of forest sites, the history of forests and the owners’ interest in managing stands.

Derogations from the natural state of a forest are a consequence of past inappropriate management. Most often, it is a result of spruce planting in the distant past (following the model of the German forest management school), especially in the Štajerska, Koroška and Gorenjska regions, and black pine planting and its further spread by means of natural propagation in the Kras region. Heavily and completely altered forests also include pioneer forests and shrubs. In the long term, altered tree composition is considered to be the most critical because it often indicates reduced stability of stands.

Altered tree composition is the reason for the reduced resistance of Slovenian forests and, consequently, greater damage (especially to the conifers, such as fir and spruce) due to air pollution. Also, the number of natural disasters that threaten less stable forests is increasing. A higher percentage of black pine in the Kras region, coupled with droughts during the summer months, increases the probability of forest fires. Tree species that are inappropriate for certain sites (e.g. spruce) are under a lot of stress during dry and warm years and are less resistant to bark beetle attacks.

By means of the systematic increase of the percentage of deciduous trees in spruce stands and gradual indirect changes, the condition of stands is gradually improving as regards natural tree composition. According to the FAO Global Forest Assessment, the percentage of natural forests (categories Primary and Modified natural) is much higher in Slovenia than in the rest of Europe.

Thanks to the naturalness and diversity of our forests, they can be considered a valuable natural asset, important not only for Slovenia but also for Europe as a whole. By means of natural forest management, Slovenian forestry protects and maintains this important asset, thus performing an important cultural role as well.

The changing naturalness of forests is a natural and long-term process, which makes its short-term monitoring difficult. Consequently, the data we have shown are not adequate to make conclusions regarding the direction of this indicator’s development. We can assume, however, that in the following decades, systematic sustainable forest management will have a positive impact on the naturalness of forests in Slovenia. We can expect the share of the category of heavily altered forests to reduce significantly over the following decades. Our assumption is that the trend will develop towards the achievement of the qualitative objective.