Sowing by drone on calamity areas

In the course of the B79 “Emergency mixture” project, seed mixtures of herbaceous, shrubby and arboreal pioneer plants have been distributed on calamity areas in the Bavarian Limestone Alps since 2020. The aim is to recolonise disaster areas as quickly as possible with fast-growing pioneer plants that do not choke other vegetation, in order to minimise nutrient losses and quickly establish a forest-like soil climate. The sudden exposure of an area is followed by increased solar radiation, erosion and rapid encroachment by grass - and the latter in particular makes it difficult for seedlings to become established. The emergency mixture is intended to bring about the rapid development of cover and thus reduce humus decomposition and overgrowth with grass.

There are lots of advantages to using seed rather than young plants: The roots can develop more naturally, the seedlings adapt better to the site, and in comparison with planting, the seed is easier to handle. The planting of shrubs and perennials in particular would also be impracticable, as there is a lack of appropriate planting material and the workload would be unreasonably high. Especially in steep terrain, sowing by drone is a safe and cost-effective method that can be used over large areas in a short time.

The seed is usually sown on its own or with soil conditioners such as vermiculite. Since the seed of pioneer plants is often very small and light, they need to be made larger and heavier for targeted sowing. One way of doing this is pelleting, a process in which the seed is coated with natural additives to increase its weight. The resulting pellets are then easier to distribute by hand or drone.

An example area for pioneer sowing by drone was established at the Scharitzkehlalm mountain refuge near Berchtesgaden. This is a windthrow area at an altitude of about 1,220 m, on a southwest-facing slope. Two strips measuring 10 x 40 m were established here and sown in the late autumn of 2021. In addition, control plots were established. In spring 2022, 10 x 10 m fences were erected on both strips. The two strips are characterised by root plates, tree stumps and rocks (Figure 2).

To evaluate the success of the sowing measure, the strips were stratified according to soil conditions:

• Mineral soil
• Humus
• Brushwood
• Grass

On the 0.5 m² observation plots of the designated strata, the seedlings of the plant species sown that had germinated were counted in the summer of 2022 (Figure 6). If we look at the success of the germination, two things are striking: Firstly, only naturally seeded seedlings of red elderberry and raspberry are found on the control plots, i.e. no pioneer tree species are to be found there. Secondly, there is practically no evidence of successful germination on grassy parts of the sown areas.

In the group of trees, rowan seedlings were only found on the “mineral soil” and “brushwood” strata. The germination was most successful on brushwood, with an average of 4.5 seedlings on 20% of the plots. The whitebeam was more likely to have germinated, as it was also present on the “humus” stratum. Here, too, germination was most successful on brushwood, with an average of 3.5 seedlings on 20% of the plots.

No birch seedlings were found on the observation plots, although birch seeds were included in the seed mixture applied. Like other seeds, birch seeds can remain germinable in the soil for one to five years. It cannot therefore be ruled out that birch seedlings will still be found in the coming years - this has been observed on other trial plots.

Red elderberry, the only shrub species distributed, had the highest number of plots with seedlings in the “humus” stratum, with an average of 2.9 seedlings on 35% of the established observation plots. The red elderberry seed distributed also germinated on the "mineral soil” and “brushwood” strata. Red elderberry seedlings were also found in smaller quantities on the control plots in mineral soil and humus.

Raspberry was the only plant to occur on all strata of the seeded area and on the control area. The highest number of plots with seedlings was on the “humus” stratum on both the sown and control areas. Overall, the seed plots had a higher number of seedlings per stratum than the control areas. The second perennial included in the seed mix was rosebay willowherb, which germinated on all strata except grass. For this plant, too, humus proved to be the best germination bed, with an average of 3.7 seedlings on 28% of the observation plots.

If we extrapolate the germination success of the sown trees (rowan, whitebeam) and shrubs (red elderberry) that have actually developed on the basis of Figure 6 with the area shares of the strata to one hectare, the result is seedling numbers of 3,700 - 5,100 plants per ha (Figure 7).

It is clear that only the red elderberry germinates without active sowing, and it does so only on a very small proportion of the area. In the trees group, the rowan sown produces approx 3,700 plants per ha, with the "brushwood” stratum being by far the most important. With approx. 4,100 seedlings per ha, the whitebeam shows the best results from sowing of all the trees - its share can thus be reduced significantly in future sowing measures. With just under 2,400 plants per hectare, red elderberry has a somewhat lower emergence success on “brushwood” than the two tree species. However, as the red elderberry also germinates well in mineral soil and humus, it produces the highest number of seedlings among the sown species.

Overall, this resulted in more than 5,000 plants per ha. If we compare these figures with planting densities of spruce, which are given in Burschel and Huss (1987) at 2,500-5,000 plants per ha with standing room of 2-4 m² or in Dengler (1990) at 4,444 plants per ha for a square planting association with standing room of 2.25 m², the seedling numbers given are among the highest numbers.