1. Measuring Growing Stock of Trees
The definition of a forest includes all land areas with a tree crown cover of as low as 10% but this does not disclose very much about the abundance of timber in different countries or regions. Therefore other measures are needed when analyzing the availability of timber resources. In national forest management and enterprise-levels, the volumetric measurement of trees has always been the basis of forest planning.
What is the standard measure?
The standard measure is the volume of growing stock. Growing stock volume is the above-stump volume of living trees measured from the bark up to the treetops. It includes all living trees, the diameter of which at breast height (d.b.h.- or 1.3 m) is over 0 cm. It is the living tree component of the standing volume, which also includes dead trees (TBFRA 2000).
In 2005, the total global growing stock was estimated at 434 billion m3, which corresponds to an average of 110 m3 per hectare. The countries with the largest growing stock per hectare were found in central Europe and in some tropical areas.
Globally, total growing stock shows a slight overall downward tendency - mainly owing to a decrease in forest area. However, some regions show significant trends in growing stock per hectare (i.e., Europe shows an increase and Southeast Asia a decrease).
In forestry planning and assessment of the sustainability of forestry production, it is vital to know how much forests are growing. For that purpose, two types of increment measures are distinguished: gross increment and net increment.
1. Gross Increment
Gross increment is the average annual volume of increment (an increase of growing stock) that over the reference period of all trees, is measured by a minimum diameter breast height (d.b.h.) of 0 cm. It includes the increment of trees that have been felled or have died during the reference period. This increment is usually measured every 5 or 10 (although rarely) years and this annual increment becomes the average for that reference period.
2. Net Increment
Net annual increment is the average annual volume over the given reference period of gross increment but less than that of natural losses of all trees to a minimum diameter of 0 cm (d.b.h) (TBFRA 2000).
In order to monitor the annual production and sustainability in forestry, one also has to be aware of three other terms related to fellings and other losses in forests.
Annual fellings refer to the average annual standing volume of all trees that are felled during a given period. It includes the volume of trees or parts of trees that are not removed from the forest. It also includes silvicultural and pre-commercial thinnings.
These fellings are further divided into removals and logging residues. Removals represent those parts of fellings that are transported out of the forest. Logging residues are represented by that part of felled stemwood, which remain in the forest.
Together fellings and natural losses constitute the drain, meaning all wood material that has died from natural causes (natural loss) or has been taken by people from the growing stock.
All components related to increment and total drain compose a logical system (Figure F7) that monitors and analyses the different changes in the forest and also in the efficiency of forest management and utilization.
Figure F7: Components of gross increment and drain in Europe (Päivinen et al. 1999)
Gross increment indicates the total increase of growing stock during a given time period. When a natural loss is deducted, one gets the net increment. The relative share of natural losses is very high when forests are not used regularly for wood production (perhaps due to conservation or recreational interests or simply that their availability is not good for timber harvesting).
If fellings are lower than the net increment (as in Figure F7), the growing stock is increasing. If the relative share of logging residues from fellings is higher, it may indicate wasteful logging techniques, or that there is demand is for a selected species or timber assortment. This can also result from the poor quality of growing stock (i.e, abundance of rotten tree stems).
Removals (sometimes including other parameters) are often given without the bark, due to technical reasons that are related to wood processing needs. Removals (over or under the bark) are used for the production of different wood based products (i.e, sawn timber, pulp and paper) or are used as fuel wood or other minor purposes.
When the total drain (fellings and natural losses) during a given period (say 5 or 10 years) is the same as the gross increment, the difference, referred to as the "forest balance", is zero and the growing stock is stable. As in the conceptual European case (Figure F7) - if total drain is smaller than gross increment, then the forest balance is positive and the amount of growing stock is increasing.
If drain is larger than gross increment over a long period of time then the forest resources are diminishing. This has been the unfortunate situation for many decades.
Globally, growing stock has decreased annually between 1990 and 2005 by 570 million m3, where the annual rate of decrease was 0.15%. Commercial growing stocks decreased by 321 million m3 (0.2% per year). One reason for this is that substantial areas of natural forests (4.6 million hectares per year) have previously been allocated for productive purposes and have also been designated to other uses. At the same time productive forest plantations have increased almost 2.2 million hectares per year. Overall, total wood removals have decreased 3.2 million m3 annually (0.1 % per year) (FAO 2006).
The volume measures of forests form the basis of assessing the role of forests within the global carbon cycle. It is estimated that the world's forests store 283 gigatonnes (Gt) of carbon in their biomass alone. The carbon that is stored in forest biomass, dead wood, litter and soil is more than the amount of carbon in the atmosphere. For the world as a whole, carbon stocks in forest biomass have decreased by 1.1 Gt of carbon annually, owing to continued deforestation and forest degradation. This has been partly offset by forest expansion which includes planting and an increase of growing stock per hectare in some regions. Carbon stock per hectare in forest biomass has decreased during the period 1990 and 2005 only slightly, at a rate of 0.02 % (FAO 2006).