Data from airborne laser scanning and digital aerial photogrammetry reveals new possibilities for estimating forest productivity

Sustainable forest management requires accurate knowledge about the state of the forest. In Norway as well as many other countries, forest management inventories are implemented periodically, typically every 10-15 years, with the aim of providing data for tactical and strategic planning of individual forest properties.

“Foresters require accurate forest information to be able to do their job,” PhD candidate Lennart Noordermeer says.

Manual interpretation

Forest productivity is most commonly represented by site index (SI), expressed as the expected dominant height at a given index age for a given species. Errors in SI may lead to sub-optimal decisions regarding the timing of final harvest and regeneration method, and potentially to financial losses for the forest owner.

In Norwegian commercial forest management inventories, SI is commonly estimated at stand-level by means of manual interpretation of aerial images. This interpretation is associated with great levels of uncertainty. The information from the aerial pictures is often supplemented with field surveys and information from previous registrations.

“This is resource-intensive work,” Noordermeer says.

“In addition, as mentioned, there is a lot of uncertainty related to the process.”

Technological revolution

Airborne laser scanning (ALS) and digital aerial photogrammetry (DAP) have revolutionized the field of forest inventory in recent decades and are now commonly used in forest inventory. Typical information that such technology can provide is timber volume, crown area, tree height, species composition and number of individuals.

“However operational practices for SI estimation have remained unchanged since the 1970s,” Noordermeer comments.

To date, it has been challenging to use remote analysis to estimate the forest's capacity to produce timber. Until now.

Noordermeer has in his PhD developed practical methods of SI estimation using data derived from ALS and DAP.

Repeated surveys

Data from airborne laser scanning is nowadays widely used in modern forestry. As a natural consequence of this, we are now witnessing a new development: areas that were surveyed 10-15 years ago are now being surveyed for the second time using the same technology. This opens up for a range of possibilities for forest planners. By combining ALS data with aerial imagery, forests can be surveyed in a whole new way.

“By using three-dimensional ALS information from two points in time, we can now estimate the growth rate of forests” Noordermeer explains.

Automatic, detailed and over large areas

In four studies, Noordermeer has developed methods to predict, estimate and map timber production at sub-stand level automatically.

He has, among other things, looked at practical methods of SI estimation; the direct method, in which models are applied for direct prediction of SI from ALS metrics from two points in time, and the indirect method in which the SI is derived indirectly from estimates of canopy height development over time.

“Both methods provided reliable SI estimates, however the direct method was most accurate,” he says.

He also found that operational use of the methods requires that the forest area be classified according to whether significant forest disturbances have occurred.

In forest areas, therefore, DAP data is mainly limited to the structure of the upper crown layer, while ALS data also provides information on the height of the lower vegetation and the terrain below due to the laser pulses penetrating the vegetation.

Well suited

Noordermeer’s results show that bitemporal ALS and DAP data are highly suitable for the estimation of SI.

“The methods I have worked with can be used to predict, estimate and map SI at sub-stand level automatically over large areas of forest. They are practically applicable and cost-efficient.”

The methods have already been implemented by several forest owners cooperatives. In addition to estimating site index, the results point to the possibility of determining the age of dominant trees based on the estimated height and site index.

“Field measurements from validation plots showed that age calculated from repeated laser acquisitions is as reliable as the age calculated with today's operational methods,” he concludes.

 Noordermeer defends his PhD thesis «Large-area forest productivity estimation using bitemporal data from airborne laser scanning and digital aerial photogrammetry» Wednesday 27 May, 2020.

Published 25. May 2020 - 20:03 - Updated 25. May 2020 - 20:09