Airborne lidar measurements help forest owners detect changes

The study of Tartu Observatory’s Senior Research Fellow Mait Lang and his colleagues from the Estonian University of Life Sciences found out that changes in forests can effectively be detected by using bi-temporal airborne lidar measurements.

The Estonian Land Board has been doing airborne laser scanning (ALS) together with aerial photography since 2008. The same area is measured every second year, the spring or summertime pair of data is obtained after every four years. The laser impulses reflect from the soil surface and vegetation. The coordinates of the reflection spots (x, y, z) are registered and the three-dimensional point cloud describes the height of the terrain as well as the height and density of vegetation.

Airborne lidar measurements are characterised by the density of the reflection points. Point density in Estonian Land Board ALS data ranges from 0.2 to 2 points per square metre. Using a lidar enables to get much more accurate estimates about canopy cover and stand height than using a radar or multispectral satellite images. On the other hand, airborne lidar measurements are more expensive than other methods.

The goal of Lang and his colleagues' experiment was to test how well the summer- and springtime measurement pairs indicate forest disturbances and height growth.

Thinning was used as an example of the disturbances. Thinning as a type of forest management task includes designing the structure of the remaining forest stand in a way that enables to make the most use of the soil potential fertility. After thinning, the tree canopy will be sparser. The results of the study indicated that this change can also be detected in lidar measurements. At the same time, thinning did not decrease forest stand height increment.

Forest management is carried out according to forest management plans and database records are usually updated after thinnings. However, results of the study can be generalised and apply for the detection of disturbances that cause loss of canopy cover. That includes leaf- and needle-loss done by fungi or insects, storm damage, or accelerated natural mortality. Early detection of such events enables forest owners or monitoring specialists to make operational decisions, and helps better targeting of forest inventory as the detected disturbances point to the stands which need a field visit by a forest inventory staff.

More information: Mait Lang, Senior Research Fellow in Remote Sensing of Forests, mait.lang@ut.ee