Grassland establishment in solar parks on former arable land - spontaneous succession and first findings of a highly divers seed-based approach

In recent years, landscapes in many countries have been transformed by efforts to fight global warming, specifically the shift towards renewable energies. Photovoltaics is one of the key technologies for reducing greenhouse gas emissions and achieving climate neutrality for Europe by 2050, which has led to the promotion of solar parks. Due to the ambitious goals for the expansion of photovoltaics, a multiple of the area used so far will be built on with solar parks. These parks are often built on arable land, can span up to several hundred hectares, and grassland vegetation is usually created between and under the panels. In most cases, this is done by spontaneous revegetation or by seeding species-poor mixtures dominated by grasses.

In a case study of 12 spontaneously planted solar parks in a structurally poor agricultural landscape in central Germany, we found an average of 90 vascular plant species. However, the number of target grassland species is only one third and the majority are ruderal species. A comparison of the three zones showed that there were the same number of target grassland species in the area between the panels and without panels, but significantly fewer under the panels. Surprisingly, the coverage of target grassland species was highest between the panels.

Establishing grasslands using highly divers native seed mixtures can significantly enhance target species richness. This finding has already been well documented by grassland restoration research. However, it has not yet been proven for solar parks with their special characteristics, such as small-scale modified site conditions. In two solar parks on ex-arable land, we seeded a site adapted seed mixture with high percentage of forb seeds (39 species, including 3 grasses). In the first year after sowing, the establishment success of the sown species in the areas between the PV panels were recorded. The establishment rate was 25-30 % in the unshaded sections, increased significantly in the partially shaded sections and fell sharply with increasing shade. The low establishment rate is probably related to the significantly below-average rainfall in 2021, while the partially shaded sections provided better establishment conditions due to higher soil moisture. Heavy shading hindered the establishment of light-demanding grassland species. An increasing establishment rate is expected in the further vegetation development and must continue to be monitored.