Reforestation after Calamity Events with Direct Sowing of Pellet Seeds by Drone

Forest calamity events such as wind breakage, forest fires, and pest infestations have increased in the past due to anthropogenic climate change. These disturbances, while natural succession processes, now occur with unprecedented frequency and severity. To prevent subsequent degradation spirals involving soil erosion, humus loss, and carbon release, rapid ecosystem restoration through reforestation has become crucial, with emphasis on establishing both pioneer vegetation and target tree species. The direct sowing of pelletised seeds by drone is an inexpensive and effective method for reforestation, in contrast to the frequently practised planting. Even areas that are difficult to access can be reached through a targeted species-specific composition and biodiversity can be increased. However, the low germination and establishment rates of the seedlings are challenging. Water availability and other site characteristics have been identified as decisive factors for success. But harsh conditions after calamities are difficult to control. 

In the PICO we present a study aiming to reduce the negative impacts through improved germination and establishment rates under dry environmental conditions. To achieve this, different pellet compositions were analysed in a factorial germination experiment for three tree species (Pinus sylvestris, Ulmus laevis, Alnus glutinosa) on undisturbed topsoil samples. The six weeks experiment has three replications for every pellet composition. The pellets exposed to different a) moisture, b) soils, c) sowing depth, and d) to herbaceous vegetation as additional seeds. The biomass, the number and the survival rate of the seedlings were used as variables from 300 individual samples. The influencing variables were identified using generic multivariate models (Random Forest and Boosted Regression Tree). The experiment complemented with data from field trials for a wide range of species and locations for evaluation.

Using generic multivariate models, their supporting effect is confirmed. Moisture and site characteristics were identified as essential influencing variables. The positive effect of hydrogel and magnesium lime in the pellet composition is evident at different moisture levels at the locations. Dry climatic phases can be specifically overcome. The results corroborate the improvement of germination and establishment of seedlings in direct sowing by drones in reforestation. They also point to site-specific optimisation options for direct sowing of pellet seeds by drone.