Biostimulation Effect of the Seaweed Extract (Ecklonia maxima Osbeck) and Plant Growth Promoting Bacteria (Bacillus subtilis Ehrenberg) on the Growth of the European Beech (Fagus sylvatica L.) Seedlings

In forest tree nursery production, bioactive compounds are sought to promote plant growth in response to changing environmental conditions. This study evaluated the effects of the Ecklonia maxima (Osbeck) seaweed extract and the bacterium Bacillus subtilis (Ehrenberg) and their combination on the growth and development of beech (Fagus sylvatica L.) seedlings. A two-factor experiment was conducted in 2023 at the Didactic and Research Station, Department of Ecology and Silviculture, University of Agriculture in Krakow. The nursery experiment investigated the effects of foliar applications of varying doses: Ecklonia maxima at 960 and 1920 cm3·ha−1 and Bacillus subtilis at 112 and 224 g·ha−1 on European beech (Fagus sylvatica L.) seedlings. The application of E maxima seaweed extract a dose of 960 cm3·ha−1 promoting beech seedlings increase in the area of the root system (101.0 ± 17.8 cm2). Seedlings sprayed with B. subtilis at a dose of 112 g·ha−1 were characterized by the longest root system, the largest root collar diameter, and the highest DQI (Dickson Quality Index) values (22.9 ± 3.4 cm, 6.00 ± 0.4 mm, and 0.74 ± 0.2, respectively). Applying B. subtilis at 224 g·ha−1 resulted in an increased SQ (Sturdiness Quotient) value (from 6.33 ± 0.7 for the control variant to 6.62 ± 0.7) and the lowest SHI (Seedling Health Index) value (5.59 ± 0.9). Applying higher doses of Ecklonia maxima (1920 cm3·ha−1) and B. subtilis (224 g·ha−1) increased the SQ index value but decreased the root system area. The best DQI and SHI values were observed with the highest dose combinations of E. maxima and B. subtilis. Selecting suitable biological substances promoting growth can support the sustainable production of beech seedlings while improving the adaptability of forest tree seedlings.