A field experiment was established to investigate whether a new site preparation technique by steam treatment could be used to control Deschampsia flexuosa vegetation. The experiment was conducted at a south Swedish clearcut dominated by Deschampsia. Growth and survival of planted Norway spruce seedlings in steam-treated plots were compared to seedlings planted in traditional soil scarified plots and in intact vegetation. A second experiment was conducted to study the duration of the heat in the soil during and after a steam treatment and its impact on vegetation reduction. Steam treatment efficiently killed the vegetation and duration for 60s or more reduced the vegetation to less than 25% of that in untreated plots for 2 years. Steam treatment also reduced the vegetation to a greater extent than soil scarification. After the third growing season the total seedling height was greatest for spruce seedlings grown in steam-treated plots and soil scarified plots. The number of spruce seedlings attacked by pine weevil was about the same in steam-treated plots as in intact plots, however, seedling survival was significantly lowest in intact plots. Pine weevil damage on seedlings grown in steam-treated plots may have affected seedling growth negatively.

Following boreal forest ecosystem disturbance,such as fire and clear-cutting, the ericaceousspecies Calluna vulgaris often becomescompletely dominant and may convert forestlandinto heathland. Calluna is known toeffectively exclude other vegetation and causes``growth check'' or stagnation of coniferseedlings to result in poor tree regeneration.In this study we investigated the use of steamtreatment as an alternative method forvegetation control of Calluna vulgaris.Scots pine (Pinus sylvestris L.)establishment and growth were recorded in botha planting and a seeding experiment. Steamtreatment was compared with mechanical soilscarification and intact vegetation in theplanting experiment and with intact vegetationin the seeding experiment. The results showthat the vegetation is still strongly reducedfive years after the treatment. The steamtreatment strongly favoured the establishmentand growth of both seeded and planted Scotspine. The planted seedlings grown in the steamtreated plots had a dry weight more than twiceas high when compared to scarified plots.Seedling height and basal diameter were alsohighest for seedlings grown in steamed plots.Seedlings planted in scarified plots did notgrow as well during the first years possiblydue to injuries caused by frost heaving. Theseeding experiment showed an increasedestablishment and growth of Scots pineseedlings in steam treated plots compared to inintact vegetation.

The purpose of this study was to determine whether Pteridium aquilinum (L. Kuhn) invades new ground or remain static under a closed forest canopy. It was also to study whether the species invades into new areas or revegetate previous occupied ground after clear-cutting in the hemiboreal zone. Two separate experiments were conducted over a six-year period: (1) The change in P. aquilinum frond density and cover under a closed tree canopy was studied on permanent plots; (2) P. aquilinum spread after clear-cutting was studied in terms of expanding fronts into new ground. The density of fronds did not change beneath the closed tree canopy during the study period. Expansion of the front of adjacent established P. aquilinum into one of two clear-cut sites was extremely rapid indicating an already present rhizome system, while expansion into the other site without rhizomes present was very slow. The results indicate that P. aquilinum does not increase its frond density or colonize new ground in the closed forest or invade new ground on clear-cut areas in the studied forest stands, but instead revegetates clear-cut areas in which it has previously been more abundant. However, forestry practices such as clear-cutting in areas in which P. aquilinum is sparsely distributed in the field layer in the closed forest prior to clear-cutting results in an increasing density of P. aquilinum which is sufficient to restrain forest regeneration.

In boreal forest, ground vegetation of bilberry (Vaccinium myrtillus L.) and feather mosses strongly suppress seed regeneration of Scots pine (Pinus sylvestris L.). We investigated a site preparation technique by steam treatment to control bilberry and moss vegetation after clear cutting. Sowing experiments were performed between 1993 and 1995 with Scots pine in four treatments: (1) untreated vegetation, (2) activated carbon added to the soil surface of otherwise intact ground vegetation, (3) steam treated vegetation, and (4) activated carbon added to the soil surface of steam treated vegetation. Activated carbon was added to adsorb possible phenolic compounds released from bilberry leaves and litter. Steam killed ground vegetation effectively and recolonisation of vascular plants and mosses was slow. Scots pine seedling establishment was improved by steam treatment especially when activated carbon was added to the soil surface. We interpret that activated carbon adsorbed and reduced the levels of phenolics from bilberry litter and humus, which otherwise may inhibit seed germination. Dry weight of four year old seedlings was three times higher in steam treated plots than in untreated plots. Seedling nutrient content was also strongly improved by steam treatment. We interpret the increased growth of seedlings in steam treated plots mainly as an effect of reduced resource competition from ground vegetation. Irrespective of treatment, almost all fine roots were colonised by ectomycorrhizal fungi. Using PCR-based molecular methods we were able to identify 28 taxa forming mycorrhiza on the seedlings. Steam treatment did not affect species richness and abundance of ectomycorrhizal inocculum. We conclude that steam treatment has the potential to be an efficient and environmentally acceptable method to reduce negative influence of bilberry vegetation on Scots pine seedling establishment and early growth.

La domination de la vegetation au sol par les ericacees decourage fortement l'etablissement et la croissance de la regeneration arbustive dans les forets boreales. Les tentatives pour controler cette vegetation suite a une coupe ont conduit a l'utilisation a grande echelle et souvent critiquee de la scarification et des herbicides. Nous avons etudie une nouvelle technique de preparation de terrain en utilisant la vapeur pour controler la vegetation au sol dominee par Empetrum hermaphroditum Hagerup dans un site en foret boreale au nord de la Suede. Les essais d'ensemencement furent realises entre 1990 et 1993 avec le pin sylvestre (Pinus sylvestris L.) dans quatre traitements: (1) traitement de la vegetation a la vapeur, (2) addition de charbon active a la surface du sol apres avoir applique la vapeur, (3) aucun traitement et (4) addition de charbon active a la surface du sol ou la vegetation etait intacte. Le charbon active a ete ajoute pour absorber les substances produites par les feuilles et la litiere de E. hermaphroditum qui inhibent la germination. Le traitement a la vapeur a tue les especes de champ et celles de l'etage inferieur pendant les premiers 5 ans apres le traitement. L'etablissement des semis de pin sylvestre a ete grandement ameliore a toutes les annees d'ensemencement par le traitement a la vapeur lorsque du charbon active etait ajoute au sol. Le poids sec des semis âges de 5 ans etait 260% plus eleve dans les parcelles traitees a la vapeur comparativement a celles qui ne l'avaient pas ete. Dans les parcelles ou il n'y avait pas eu de traitement, les semis etaient peu nombreux et avaient accumule peu, ou meme perdu, d'importants macroelements comparativement au contenu original des graines. La meilleure croissance des semis dans les parcelles traitees a la vapeur serait principalement attribuable a la diminution de l'interference par la vegetation au sol. Les resultats suggerent que le traitement a la vapeur serait un outil potentiellement efficace pour reduire l'influence negative de la competition par les ericacees sur la croissance des semis des especes arborescentes.

The Vaccinium myrtillus L. - feather moss vegetation community immobilizes nutrients in surface organic layers and suppresses growth of coniferous seedlings in northern boreal forests. On a site dominated by this type of vegetation, a new site preparation technique, involving steam treatment to kill ground vegetation, was tested and compared with conventional site preparation techniques such as soil scarification and burning. Steam treatment was as efficient as burning and soil scarification in reducing competing vegetation. After 4 years, Scots pine (Pinus sylvestris L.) seedlings planted in the burned and scarified treatments had lower growth, needle dry weight, and nitrogen contents compared with seedlings in sites treated with steam. Soil microflora recovered quickly after steaming, suggesting that steaming does not directly cause long-term soil sterilization. We interpret the superior growth of P. sylvestris seedlings in steamed plots as being due to both strongly reduced resource competition and enhanced release of nutrients in the remaining humus. We conclude that steam treatment has the potential to be an efficient and environmentally acceptable method to reduce the negative influences that ericaceous ground vegetation has on the growth of planted coniferous seedlings.

The nature of interference of bracken with Scots pine and Norway spruce seedling establishment was considered in three field experiments. In a seeding experiment, it was found that Scots pine germination was highest on exposed mineral soil and lowest when intact bracken litter and humus were present, suggesting adverse effects of litter and humus on pine regeneration probably due to phytotoxicity. In a second experiment, smothering by bracken caused high mortality of Scots pine seedlings while Norway spruce seedlings were relatively unaffected. Mortality for both Scots pine and Norway spruce seedlings was low when planted in a adjacent Scots pine-bilberry stand with no bracken. Annual shoot growth of Norway spruce was higher in bracken than in Scots pine-bilberry vegetation while no differences in shoot growth between these two vegetation types occurred for Scots pine. In a third experiment, activated carbon was added to the ground under Norway spruce seedlings planted in bracken to adsorb possible phytotoxic compounds released by bracken. The addition of carbon had no effect on seedling mortality or growth rate, indicating that the seedlings were not susceptible to allelochemicals released by bracken. Since large Norway spruce seedlings were relatively unaffected by bracken interference in this study, artificial regeneration with containerized Norway spruce seedlings is suggested to achieve an acceptable conifer tree establishment on clear-cuts invaded by bracken.

SummaryThe comparative effects of lime, sulphuric acid, wood ash, glyphosate and ammonium nitrate on Pteridium aquilinum (L.) Kuhn growth and development were evaluated in a Scots pine (Pinus sylvestris L.) clear‐cut stand in south‐east Sweden over a 4‐year period. P. aquilinum growth was effectively controlled following the application of glyphosate, but the other treatments generally had little effect on growth in terms of shoot density and length. Shoot length was positively related to the number of shoots on plots treated with ammonium nitrate but negatively correlated with the number of P. aquilinum shoots found on wood ash‐treated and control plots. In a bioassay aspen (Populus tremula L.) seedling emergence and pH were significantly lower in humus treated with sulphuric acid, glyphosate, ammonium nitrate and in untreated humus as compared with the humus treated with lime and wood ash. The negative effect of humus on emergence of aspen seedlings may be caused by phytotoxic compounds at a lower pH level. Forest regeneration might be performed with shade‐tolerant species such as Norway spruce (Picea abies (L.) Karst.) planted beneath a thin canopy of trees to prevent the rapid increase of P. aquilinum.