Growth and quality of young oaks (Quercus robur and Quercus petraea) grown in cluster plantings in central Europe: A weighted meta-analysis

Abstract

Cluster planting of oaks (Quercus robur and Quercus petraea) was introduced to central Europe in the 1980s and 1990s as an economic and ecological alternative to the traditional and costly row planting for re-afforestation of wind-thrown and clear-cut areas. Clusters comprising 20–30 seedlings are either ‘nests’ (nest planting) with very dense spacing of ca. 0.2m between trees, or ‘groups’ (group planting) with 1m between trees. Commonly, 100 groups or 200nestsha−1 were planted in uniform distribution. We used a comprehensive weighted meta-analysis to compare the growth and quality (stem form, crown shape, branch free bole length, number of potential future crop trees) of oaks grown in clusters with those planted in rows (e.g. 2×1m spacing). Data were obtained from 25 trials located in Germany, Switzerland and Austria. Tree survival, growth and quality were significantly lower in nests than in neighbouring row planting counterparts. Very low initial growing space was presumably one of the main reasons for low survival, unfavourable growth and quality development of oaks in nest plantings. In group plantings, survival, growth and tree quality were similar or superior to row plantings. Tree quality benefitted from the presence of trainer trees in group plantings. Based on this study, we recommend planting oak groups as an alternative to traditional row planting. Our study showed the usefulness of weighted meta-analysis to develop a synthesis from raw data collected from independent silvicultural trials established with comparable designs and similar research goals.