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 the increment changes rather slowly depending on the forest structure and climate development, removals are normally subject to short-term fluctuations. The volume of wood harvested depends primarily on the age structure of the forests, the tree species composition and the timber market. Mortality is caused by natural events such as storms, avalanches and drought combined with their associated consequences, such as insect infestation.
The net increment was very much in balance with wood harvesting in the period between 2007 and 2013. From the perspective of forest management, the volume of wood harvested should match the volume of wood grown.
Clear differences in the ratio of wood increment to harvesting can be observed between the various regions of Switzerland. For instance, the increment exceeds the harvest in the Pre-Alps, Alps and Southern Alps, whereas the ratio between the two processes is balanced in the Jura. In contrast, the increment is lower than the harvest in the Central Plateau.
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.
Net increment and wood harvesting data are provided by aerial images, the continuous sample survey carried out in forests and subsequent modelling. 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 NFI1 inventory. The assessment is based on the network (list items) according to NFI4 (2009-2013).
For the 1985-1995 (NFI1-NFI2) period, only data on the ratio between removals (harvesting and mortality) and the increment are available.