How we work

Statistics from SLU's National Forest Inventory are part of Sweden’s official statistics and primarily aim to describe the condition and changes in Swedish forests.

The statistics are based on approximately 20,000 sample plots, of which around 12,000 are surveyed in the field, where up to 120,000 trees are measured annually.

Each field season, around 50 people are hired to carry out the field inventory. Staff at the Division of Forest Statistics within the Department of Forest Resource Management in Umeå handle the other organizational components—planning, training of field personnel, data collection, data quality assurance, data processing, and reporting.

From using paper field forms during the first inventory in 1923, methods for registering field data have evolved alongside technological developments. In the mid-1960s, punch cards were introduced, and since 1983, handheld data collectors have been used. Today, we use Allegro handheld computers on sample plots, utilizing wireless communication for synchronization and backup. Data is stored in databases directly in the handheld devices and is then transferred to the office in Umeå, where data from all inventory teams is compiled into an annual database. After quality assurance of this database, it is used to update the reporting database, which stores inventory data from 1983 onwards with the most recent year’s data.

 
Work Continues at the Office
Once the field data has been collected, further review, post-processing, and storage are carried out before results from the National Forest Inventory can be produced and reported.

 
Quality Control
After field data collection, an extensive quality control process begins. Several routines are used: in-field plausibility checks are performed directly in the data collector, special control teams perform re-surveys to verify the work of field teams, and additional checks are conducted at the office before data is finally entered into the database.

 
Tree-Ring Measurements
Sample trees are aged at breast height in the field. The core samples are brought to the Tree-Ring Measurement Office for further analysis. There, the tree-ring growth on the cores is measured using precision instruments, and the results are stored in a database. These data form the basis for estimating forest growth.

 
Calculated Variables
Once field data is quality-assured, a large number of additional variables are calculated. To estimate timber volume and annual growth, the volume and growth of trees in the sample are assessed. Tree volume is calculated using functions based on diameter and height. Growth is estimated from the tree-ring width data. Tree biomass is also calculated using established allometric functions.

 
Database
Data from the inventories—both raw field data and calculated variables—from 1983 to 2023 is stored in a database that is used to generate results and support data retrievals.

 
Analysis and Reporting
Results from the National Forest Inventory are reported in many forms, ranging from simple figures provided over the phone to extensive analyses requiring several person-years of work. Certain key statistics are published annually in the Skogsdata report, which presents averages from the most recent five years.