How Windthrow Impact Can Be Assessed With UAV Intervention
High Resolution UAV Images As Basis To Determine Stem Volume Loss
North England experiences some of the windiest and wettest weather in Europe. North English forests are regularly hit by high winds and when these are accompanied by heavy rainfall they pose a big threat to the stability of forest stands. Windthrow is one of the greatest threats to forest stands in North England and assessing the economic impact of windthrows is a main part of the forestry in this region. Our study shows that QuestUAV images are a cheap and effective tool to rapidly determine windthrow extents over up to 500 hectares per day at a flight altitude of 400ft. Freely available Geo-Information Systems (GIS) allow a forester to create UAV-based windthrow maps and to quantify the loss in stem volume and hence the financial loss.
Our study was carried out at Matterdale, a forest stand situated within the Lake District National Park in the county of Cumbria. The forest extends to 290 hectares and is planted with Sitka Spruce (Picea sitchensis) and Lodgepole Pine (Pinus contorta) of different age groups.
Getting ready for takeoff (Pre-flight checks on the QuestUAV Q-200 Surveyor Pro)
Early 2015, a winter storm hit the forest and caused windthrows over the whole stand with significant impact on the forest stability and economic rotation cycles. Shortly after the storm event QuestUAV, in close cooperation with the Forestry Commission England, flew Matterdale forest with the objective to cover the whole stand with high resolution, up-to-date images, in order to map the extent of windthrows and determine the lost stem volume.
Orthomosaic and Digital Surface Model (DSM) of Matterdale Forest
We flew the forest with the QuestUAV surveying drone Q-200 Surveyor-Pro, equipped with a gimballed Sony A6000 camera, capturing high definition images with 2.9 cm GSD resolution at 400ft. The raw images were processed with Pix4Dmapper Pro. The resulting image products (figure below) provide the basis for windthrow maps and stem volume calculations, which were conducted in the open-source GIS software, QGIS.
Results and Conclusions
The graphic below shows how windthrows can easily be identified in a UAV orthomosaic. Affected areas were mapped out quickly and with highest accuracies. Our study revealed that windthrows occurred in two sub-stands of Matterdale forest. Three areas with a total size of 3.5 hectares were destroyed by the storm, which is 1.7 % of the total sub-stand area.
By combining the windthrow extent with forest inventory data on species composition, tree age, mean tree height and stem volume, we determined the lost stem volume in cubic meters (table below). In summary, 1,945 m3 stem volume has been lost over 3.5 hectares during the storm event.
Our study reveals that QuestUAV drones allow a quick targeted response to monitor forests after hazardous events. Drone-based maps are a quick and cheap alternative when compared to conventional methods on assessing the scale of an event on the ground.