Wood harvesting/net increment ratio
The annual wood increment is contrasted with wood removals. To ensure sustainable forest management, the balance between the increment, i.e. wood growth, and removals, i.e. logged and dying wood, as measured by tree generations, must be maintained over the longer term. While growth – depending on forest structure and climate development – changes rather slowly, losses are subject to short-term fluctuations and, due to disturbances, are observed more frequently and intensely than in recent decades. The volume of wood harvested depends primarily on the age structure of the forests, the tree species composition and the timber market. Mortality and unplanned forced utilization are caused by natural events such as storms and drought combined with their associated consequences, such as insect infestation.
medium
impossible to evaluate
In the period between NFI4 and NFI5, the ratio of net growth (excluding mortality) to wood utilization was largely balanced on a national scale. From the perspective of sustainable management of limited resources, planned utilization to the extent of wood growth is desirable in the long term. However, unplanned forced utilizations due to disturbances are increasingly influencing the recorded utilization volume (share of forced utilizations in the National Forest Inventory NFI3-NFI4 period: 13.9%, NFI4-NFI5: 29%).
Clear differences in the net increment ratio to harvesting can be observed between the various regions of Switzerland. For instance, the increment exceeds the harvest in the Alps and Southern Alps, whereas the harvest is higher in the Jura and the Central Plateau.
However, harvesting is only one factor in assessing sustainability. Other forest services, such as protection against natural hazards, rainwater filtration for drinking water use, biodiversity and habitat for plants and animals, as well as recreational areas for people, must also be considered. To ensure the provision of these services, appropriate silvicultural measures are necessary. Depending on the objectives, these can reduce the timber stock and still increase the overall sustainability of the forest, e.g. when a forest stand is adapted to climate change. In this sense, this indicator is not a control variable and consequently cannot be evaluated during the long-term adaptation of the forest to climate change.
To conserve and strengthen the productive functions of forests, the Ministerial Conference for the Protection of Forests in Europe (Forest Europe) postulates that a balance be maintained between the annual net increment and harvesting in forest areas that are available for wood production. The comparison of wood increment, harvesting and mortality reflects this postulate. With an average of 347 m3/ha (source: NFI5), the wood supply in Switzerland is one of the highest in Europe.
Net increment and wood harvesting data are provided by aerial images, the continuous sample survey carried out in forests and subsequent modelling. Data are derived from an annual survey of forest owners regarding the volume of timber harvest sold, by assortment and wood species group.
To calculate the ratio, harvesting is divided by net increment.
Definition of increment (net growth): annual increase in the stemwood volume of all trees (with ingrowth) minus annual mortality.
Definition of harvesting (wood harvesting): stemwood volume of all trees harvested between two consecutive forest inventories. While wood harvesting is based on figures from the National Forest Inventory, timber harvesting figures come from forestry statistics.
Definition of mortality: stemwood volume of all trees that died or disappeared naturally between two consecutive inventories.
Evaluation unit: The increment is measured on the basis of the forest area (accessible forest without brushwood) according to the respective inventory.
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