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Development of tree quality, productivity, and diversity in oak (Quercus robur and Q. petraea) stands established by cluster planting

concluded 12/2012



Pedunculate oak (Quercus robur) and sessile oak (Q. petraea) are likely to become increasingly important in Central Europe owing to their stability, tolerance of relatively warm climates, and their valuable timber. Although natural regeneration is often the preferred option for oak stand establishment in many regions of Central Europe, planting and seeding still play a major role in the reforestation of oak-dominated forests. Artificial regeneration is the only way to establish oak stands in situations where acorn sources are lacking. In particular, this is the case where coniferous stands are to be converted to oak forests, where competition from herbaceous and woody plants hinders the natural regeneration of oak, or where oak stands are to be established in cleared areas following storms or other disturbances. Planting oak seedling in rows with a high initial density (e.g. 5,000 – 7,000 seedlings ha-1) is often used for artificial regeneration of oaks. However, high costs associated with site preparation (particularly in wind-thrown areas), planting, fencing and successive tending measures remain a matter of concern for conventional row planting of oaks. These considerations may apply equally to the artificial regeneration of other hardwood species.
Such factors motivated foresters and researchers to seek alternatives to the establishment of oak stands. Low-density planting, where the artificial regeneration of the desired species is complemented by natural regeneration of additional species is one such approach to reduce costs and to maintain successional processes and increase biodiversity at the same time. For oaks, low density planting in the form of widely spaced clusters with two different designs has been developed in Europe. Clusters comprising 20 to 30 seedlings are either ‘nests’ (nest planting) with very dense spacing of ca. 0.2 m between trees, or ‘groups’ (group planting) with 1 m between trees. In contrast to nests plantings, clusters in group plantations are encircled with a varying number of individuals of a trainer tree species (e.g. Tilia cordata, Carpinus betulus). Commonly, ca. 100 groups or 200 nests ha-1 were planted in uniform distribution. This new oak regeneration technique became popular in the 1980s and 1990s to reforest wind-thrown area created by catastrophic winter hurricanes in the 1990s (“Vivian” and “Wiebke” in 1990 and “Lothar” in 1999). Although many cluster planting trials were established since then, no comprehensive analysis had been carried out to study growth and quality attributes related to timber production in oaks grown in cluster planting. In addition, natural regeneration in the space between the clusters, stand productivity and the influence of naturally regenerated trees on growth and stem quality of oaks grown in clusters had never been studied. Therefore, the objectives for this study were as follows: 1) to compare comprehensively survival, growth (diameter at breast height or DBH, height), stability (height-to-DBH ratio), and quality (stem form, crown shape, branch free bole length, potential future crop tree) of oaks grown in clusters when compared to conventional row planting; 2) to assess and compare tree species diversity and stand productivity in stands established through cluster and row planting; and to test further whether stand productivity in cluster planting stands may be influenced by species richness and density of the naturally regenerated and planted trees; and 3) to quantify influences from intraspecific and interspecific interactions on growth and quality of oaks in mixed stands established through cluster planting.
The first objective was addressed by synthesizing original forest inventory data gathered and collected from 25 trial pairs consisting of cluster and respective neighbouring row planting sites (ca. 5,000 seedlings ha-1) located in Germany, Switzerland and Austria and carrying out a meta-analysis. The second and third objectives were addressed by analysing data from 7 cluster trials located in Baden-Württemberg and Hessen, Germany. Again, row plantings counterparts were used for the comparison of the investigated stand establishment methods.
The comprehensive mixed effect meta-analysis revealed that tree survival, growth and quality were significantly lower in nests than in neighbouring row planting counterparts. Intense intraspecific competition due to very low initial growing space (only 0.04 m2 per seedling) was presumably one of the main reasons for low survival, unfavourable growth and quality development of oaks in nest plantings. However, in group plantings which provided larger initial growing space (1 m2 per seedling), survival, growth and tree quality were similar or superior to row plantings. The meta-analysis also showed that tree quality benefitted from the presence of trainer trees in group plantings.
Species richness and diversity were significantly higher in cluster plantings than in row plantings. Basal area of naturally regenerated trees (e.g. Betula pendula, Populus spp., Salix spp., Acer pseudoplatanus, Sorbus aucuparia) contributed to ca. 43% of total stand basal in cluster plantings and was significantly higher than in row plantings. As a result total stand basal area did not differ significantly between the analysed stand establishment methods. Productivity of stand established through cluster planting was significantly related to density of naturally regenerated species.
Competition from mid- and late successional tree species had a stronger negative impact on growth of target oak trees than competition from oaks and early successional tree species. Intraspecific competition was sufficient to promote self-pruning in oaks grown in clusters. Additional, interspecific competition did not further advance the branch-free bole length of target oaks. Oaks grown in the inner part of groups showed higher probability to develop into potential future crop trees than oaks grown in the periphery of groups. This study also showed that in the majority of groups (80%) in a 20-year old stand at least one potential future crop tree developed.
Based on this study, it can be concluded that owing to high mortality, poor growth and inferior stem quality, oak nest planting should not be pursued further to establish oak stands. In contrast, oak group plantings can be recommended as a suitable alternative to conventional row planting. Significant environmental (high species richness and productivity), silvicultural (quality development in oaks) and economic (e.g. low site preparation and plantings cost) gains can be achieved with low-density planting of oaks in groups.


Supervisors: Prof. Dr. Jürgen Bauhus,   Prof. Dr. Ullrich Kohnle (FVA Freiburg)
Researcher: Somidh Saha
Funding: DAAD Fellowship, State Forest Department of Rhineland Palatinate (Landesforst Rhineland Pfalz), Georg-Ludwig-Hartig Stiftung, Graduate School "Environment, Society and Global Change of the University of Freiburg, Müller-Fahnenberg Foundation,
Duration: 1/10/2008  -  31/07/2012


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