SITE PREPARATION BY STEAM TREATMENT - EFFECTS ON FOREST VEGETATION CONTROL AND ESTABLISHMENT, NUTRITION, AND GROWTH OF SEEDED SCOTS PINE

Ericaceous-dominated ground vegetation in boreal forests strongly supp resses tree seedling establishment and growth. Attempts to control thi s vegetation have led to the large-scale, and often criticized, use of soil scarification and herbicide treatment to control vegetation afte r cutting. We investigated a new site preparation technique by steam t reatment to control ground vegetation dominated by Empetrum hermaphrod itum Hagerup in a north Swedish boreal site. Sowing experiments were p erformed between 1990 and 1993 with Pinus sylvestris L. in four treatm ents: (1) steamed vegetation, (2) activated carbon added to the soil s urface of steamed vegetation, (3) untreated vegetation, and (4) activa ted carbon added to the soil surface of otherwise intact ground vegeta tion. Activated carbon was added to adsorb germination inhibitors from E. hermaphroditum leaves and litter. Steam treatment killed field- an d bottom-layer species during the first 5 years after treatment. Scots pine seedling establishment was much improved for all seed years by s team treatment when activated carbon was added to the soil. Dry weight of 5-year-old seedlings was 260% higher in steam-treated plots than i n untreated plots. Seedlings in untreated vegetation were few and had gained little or even lost important macronutrients when compared with the original seed supply. We interpret the increased growth of seedli ngs in steamed plots mainly as an effect of reduced interference of fo rest floor vegetation. We suggest that steam treatment has the potenti al to be an effective tool to reduce negative influence of competitive ericaceous ground vegetation on tree seedling growth.