Development of the land area of Sink Forests in Finland since 1921

The definition of Forestry land in Finland is based on the volume growth of existing, living trees in the selected area - or on the potential volume growth in same area if there has been for instance a temporary forest clear cut in the area. In the standard silviculture the area is waiting for replanting or for natural recovery, in a few (less than 10) years.

The Forestry land has been divided into four categories (Figure 1). First, when the average volume growth (stems only) is more than one solid cubic meter per hectare per annum, the land area is called Forest land.

Second, when the average volume growth is between 0.1 - 1.0 m3/ha/a the land area is called Poorly productive forest land.

Third, when the average growth is less than 0.1 m3/ha/a the land area is still included in the main category of forestry land but it has been nominated as Unproductive (forest) land.

Fourth, under the main Forestry category there is still a minimal forestry land class called Forest roads and depots.

In addition, there is the fifth category in the total land area, the Other land area. It includes agricultural lands, the land area in cities, towns and communities, roads and other by areas minority lands.

The total Forestry land area in Finland, based on the last, the 13th National Forest Inventory (NFI 13 - VMI 13, median year 2017), is 26.251 million hectares, the total land area is 30.392 mill. ha. 

Figure 1. Shares of Finnish land area (total 30.392 mill. ha) into different classes, based on 13th National Forest Inventory. The Sink Forests (deep blue + red) cover 22.660 mill. ha. 

The Sink Forests

From the point of view of Carbon sequestration it is logical to define the area of Sink Forests as the sum of the Forest land (productive) and the Poorly productive forest land. They both have created a measurable Carbon sink in the recent years. Based on their annual growth they maintain a sustainable Carbon sink flow into the Finnish forest biomass. Unproductive (forest) lands and Forests road et depots do not create similar forest Carbon sink flows.

Using the Finnish Carbon Multiplier (CM, from solid cubic meters to Carbon tons) of 0,35535 the land area is defined as Sink Forest when the annual Carbon flux is at least 35,5 kg C/ha/a. The Poorly productive forest land has the Carbon sink flux between 35,5 - 355 kg C/ha/a. The productive Forest land has the Carbon sink flux of at least 355 kg C/ha/a.

In the latest National Forest Inventories the area of Sink Forests is at the level of 22.8 million hectares. The development of the area of Sink Forests in Finland from 1921 is shown in Figure 2.

Figure 2. Development of the area of Sink Forests in Finland from 1921

The development of Sink Forests area in Finland has three phases. The first phase before the second world war belonged to the era of clearing Finnish forests forests for arable fields.

After the war there was the phase of intensified silviculture. It lasted to the beginning of 1970s. The drainage of slowly growing peatland forests was an essential part of the silviculture of that period. The annual drainage was in its maximum in 1969.

From 1970s onwards the third phase started. The national drainage program was completed. The intensive silviculture continued mainly on productive Forest lands.

The Sink Forests area turned, however, into diminishing trend around mid 1970s. This was mainly caused by the industrialization of the societies, by building the towns and the cities and by opening new roads and highways through the forests.

Since 1970s the diminishing trend of the Finnish Sink Forests area  has been relatively linear (Figure 3), as compared to the previous decades.

Figure 3. Development of the area of Sink Forests in Finland from 1971

The linear regression equation for the Sink Forests area (sf, mill. ha) development is

(1)     sf(t) = -0.012695 * t + 48.355

R2 = 95.2 %

By derivating the equation we get the average annual reduction of forest area in Finland. Since 1970s the loss has been on average of 12'695 hectares Sink Forests in a year.

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The original text (3.5.2020) was based on the 12th National Forest Inventory data. The current text (11.4.2021) is based on the 13th National Forest Inventory data (November 2020). The concept of Sink Forests was also included in this newer version.