Underplanting of beech under spruce – site preparation and plant type

Underplanting of beech unter spruce – site preparation and plant type

For the introduction of broadleaves under Norway spruce canopy a pathway has to be chosen from a variety of preparative measures, regeneration techniques and plant distribution patterns. Preparative measures commonly associated with forest regeneration are fertilization and site preparation. Fertilisation is a costly measure, implicating negative side-effects such as enhanced growth of ground vegetation or increased risk of nutrient leaching. At the same time the effect of fertilization on the growth of newly planted trees is limited by their small root systems and retarded root growth. On degraded acidified soils, however, growing conditions can be improved considerably by mixing lime into the mineral soil on planting spots. Spotwise application confines the risk of nutrient losses and ground water pollution. The positive effects of site preparation on survival and growth of broadleaved seedlings have been demonstrated in numerous studies.

Site preparation can be performed mechanically or by use of herbicides or prescribed burning but the latter two methods are restricted by law or certification standards in many European forest districts. For mechanical site preparation a variety of techniques is available. Site preparation creates soil beds that facilitate root growth and water uptake, it removes or represses ground vegetation that competes for nutrient and water with the seedlings and it may improve the microclimate or the chemical soil conditions. However, when intensive methods are applied (e.g. rotary cutter) there is a risk of increased nutrient leaching. In addition, when used in shelterwoods, damages to the roots and stems of the shelter trees can occur. Little is known about the influence from site preparation on the root rot infection of spruce shelterwoods.

Practical experience with advance-plantings of broadleaved seedlings has shown that medium sized bare root seedlings (40–80 cm shoot length) generally provide high survival rates and satisfactory growth at acceptable costs (Figure 4). Tall seedlings (> 120 cm shoot length) may be advisable in cases of severe browsing pressure or where competitive ground vegetation excludes the use of smaller plants. Wild seedlings can provide cheap and vital planting stock if basic requirements are met concerning genetics, technique of extraction, transport, storage and plant attributes. Professional logistics and careful handling of seedlings are essential for successful plantation establishment. Planting stock should be delivered just-intime for planting. Protection against desiccation must be provided during plant transport and storage. Root pruning shall be avoided . To facilitate planting, moderate cutback of individual extensive lateral roots is acceptable if it does not significantly reduce fine root mass. Slit-planting methods are suitable for most seedlings and transplants (< 80 cm shoot length). On sandy soils with larger plants (60 – 120 cm shoot length), planting by semicircular planting-spade has proved superior to slit-planting in terms of survival and root development. For tall planting stock (> 120 cm shoot length) planting holes are required.

Direct seeding has the potential of being a regeneration method with low costs. With oak it has successfully been carried out in clear-cuts, on abandoned farm lands and under sparse shelterwoods of oak or pine. However, birds and rodents, which eat seeds and newly emerged seedlings, are a major problem when attempting forest regeneration from seeds. More damage from rodents can be expected when natural vegetation is present at the regeneration site. In recent years, techniques that are specific for direct-seeding of beech under Norway spruce canopies have been developed (Figure 4). Direct-seeding of beech reduces costs to about 50 – 70 % compared to planting and creates high plant densities which can promote future timber quality. Nevertheless, at present most forest managers prefer planting. A major drawback of directseeding besides the biotic and abiotic risk is the general lack of experience with this method. Results from seeding experiments and practical experience indicate that direct-seeding of beech below spruce canopies can be successful if basic requirements are met. From theses studies it can be concluded that it is necessary to use seeds of high quality (germination capacity over 70 %) and to bring out high seed quantities (30-60 kg per hectare net seeding area). For sufficient seedling growth, the Norway spruce canopy should be opened to approximately 70 % of full stocking. Spring seeding between April and middle of May is recommended and the seeding technique must provide removal of vegetation and humus layer. Seeds must be completely covered to avoid predation by birds. Direct seeding of beech is not recommended on sites where strong competition from ground vegetation is expected or advance regeneration of spruce is abundant. Finally, fencing is often needed for both direct seeding and planting to avoid damage by wild boars, hare and deer.

In plantations established for timber production, the desired plant density depends on the chosen tree species and stock type. Furthermore it is influenced by the presence of pioneer trees as well as the duration and degree of canopy closure. Investigations show that even under open field conditions the speed of selfpruning in beech plantations with initial stand densities of 6.000 – 7.000 seedlings per hectare is comparable to denser plantings and natural regenerations. However, extremely low plant densities slow down the selfpruning process inducing a severe deterioration of timber quality. Use of tall planting stock, and admixture of pioneer trees and a long period of shelter justify a moderate reduction of plant density. If the production of high quality timber is not intended, plant numbers can be reduced significantly. Admixture can be achieved by artificial or natural regeneration. The admixture of Norway spruce is of great economical importance in most broadleaf plantations under spruce canopies (Figure 5). For this reason, an adequate proportion of naturally regenerated spruce is desired on sites where spruce is stable. However, if a vital spruce regeneration already exists at the time of broadleaf introduction and species are mixed, it can become a serious competitor. Therefore, competition between spruce and beech should be controlled by spatial separation of species and careful management of light conditions by means of a slow harvesting progress. It is not feasible to define simple pathways for the conversion of Norway spruce forests. Even under comparable circumstances various silvicultural options exist. Specified silvicultural goals such as “timber production” and restoration of natural forest communities"ask for specific silvicultural approaches. Resource input levels may change during the process of regeneration. High input for site preparation can induce low input for plantation establishment. The success of conversion very much depends on a precise definition of its goals and the harmonisation of the single steps of the regeneration process.

Marian Kazda, coordination
University Ulm, Department of Systematic Botany and Ecology
Albert Einstein-Allee 11, D-89081 Ulm