Countrywide characterization of forest edge structure from airborne laser scanning data

Forest edges represent the transition zone between the open country side and the forest interior. Ideally, the edge zone consists of a shrub belt (vegetation height < 4 m) and a shelterbelt with a distinct height gradient towards the forest interior. Forest edges provide several ecological functions. They offer habitats for plant and animal species, regulate fluxes of nutrients and pollutants between surrounding agricultural areas and the forest, or regulate the microclimate. Repeated assessment of forest edge conditions will help forest owners the maintenance of these edge functions considering the increased pressure due to e.g., intensified agriculture.

The aim of the ongoing project is to provide a map of forest edge structure characterization for entire Switzerland (total forest edge length 186’773 km). We used the latest freely available full coverage airborne laser scanning (ALS) data, which provides point densities of 15-20 points/m² and a forest mask provided by the Swiss National Forest Inventory (NFI). The high point densities allowed to assess both the horizontal and the vertical structure of the forest edges. On the one hand, we extracted information on the edge composition which is closely related to parameters extracted within NFIs. These comprise detailed information on the shelterbelt composition including its slope, presence or absence of the shrub layer and detection of overhanging trees. Furthermore, we computed the vegetation height distribution and the number of vegetation layers within the edge zone. On the other hand, ALS data was used to compute additional features such as the horizontal canopy cover and canopy gaps, the vertical canopy density, and the 3D light availability within the forest edge zone.

We followed a sampling-based approach and characterized the forest edge structure for discrete sampling points at the edge of the forest mask. The forest edge zone covers an area of +/- 25 m along the forest mask from the sample point and +/- 30 m into and outside of the forest, respectively, measured perpendicular to the forest mask edge. Validation of the derived parameters is based on more than 300 terrestrial NFI plots comprising forest edges. This discrete sampling-based forest edge characterization map could potentially be transferred into a quasi-continuous forest edge description by increasing the number of sampling-points on the forest edge.

Repeated six-yearly ALS acquisitions by the Federal Office of Topography swisstopo will enable to produce regular forest edge characterization data sets as a basis for the monitoring of the forest edge development at a countrywide extent. This will help to identify forest edge areas which are at severe threat of degradation and thus require treatment intervention. Thereby, the quality of forest edges can be preserved or improved for important ecosystem services.