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Successful tree regeneration is a key process in ensuring forest sustainability and one of the most crucial investments made in silviculture. In the case of silver fir stands, optimum growth performance and stand health are strictly linked to a diverse vertical structure. This structure is usually achieved by natural regeneration and more close���to���nature, complex silvicultural systems. However, there are also stands where silver fir is missing for various historical reasons, and its restoration by artificial regeneration is required. We present results of the application of two planting methods in the Bieszczady Mountains (SE Poland) to convert pure grey alder stands into silver fir dominated mixed stands. Since 2001 in the Cisna Forest District, where a traditional row planting method is often used, a novel, group planting method (which is similar to the oak cluster planting) has also been applied: i) to reduce damage caused by deer, ii) to facilitate weed control, and iii) to promote natural regeneration of additional tree species among groups. We established a grid of 18 sample plots (each plot consisting of 4 groups of 17 seedlings in each) in three circa 20 year���old silver fir stands, as well as 15 plots (15 rows of 16 seedlings in each) representing one traditional stand (row method) in close proximity. For each tree (silver fir or admixed species) we determined the diameter at breast height, the total height and crown base height. We also assessed the silvicultural quality based on the current health status (including fungi and deer damage), as well as crown shape in the vertical and horizontal profile. To quantify the differences in growth performance we used a linear mixed���effects model approach, available in the R Statistics package (nlme). We found that height and diameter growth were higher and more uniform for the row method, but there were no statistical differences in the height of living crown base or tree slenderness. The silvicultural quality was only slightly better when using the row method. The tree species diversity (the number of admixed tree species present and their abundance) was significantly higher in stands regenerated using the group planting method. Therefore, we conclude that the group planting method could be recommended for tree species conversion on eutrophic sites, because it is likely to decrease establishment costs and creates a more diverse tree species composition and vertical structure, although it may lead to slower growth rate of silver fir up to 20 years. Due to this response, we expect higher resilience of such forest stands to the future biotic and abiotic disturbances under observed environmental changes., Sylwan 165 (7): 542-553

Recent studies have reported superior tree quality and comparable tree growth of oaks planted in group compared with row planting. However, a comparative assessment of the potential future crop trees (PFCTs) between group and row planting is still lacking. Here, we compared the density and tree quality of planted oaks and other naturally regenerated trees between group and row planting. We assessed whether the distribution of PFCTs fulfills the goal of maintaining a spatially homogenous distribution of such crop trees at the stand level by geospatial analysis. We selected 10 group and row planting stands that were either 14 to 15 or 21 to 22 years old. Tree density in group plantings was significantly higher than in row plantings. Stand basal area was higher in row planting in younger stands but comparable to group planting in older stands. The proportion of trees with straight stems and monopodial crowns was higher in groups than in rows. The density and species’ richness of the PFCTs was significantly higher but the number of oak PFCTs was lower in group than in row plantings. In group plantings, naturally regenerated PFCTs contributed to 43% of total PFCTs, but to only 19% in row plantings. Also, the spatial distribution of PFCTs in group plantings was more uniform than in row plantings. Uniform and homogeneous distribution of the PFCTs in group planting stands can facilitate design and conduct of tending operations. Thus, the group planting technique offers not only cost savings in the establishment phase but also additional benefits, including spreading of risks through increased tree species diversity.

Recent studies have reported superior tree quality and comparable tree growth of oaks planted in group compared with row planting. However, a comparative assessment of the potential future crop trees (PFCTs) between group and row planting is still lacking. Here, we compared the density and tree quality of planted oaks and other naturally regenerated trees between group and row planting. We assessed whether the distribution of PFCTs fulfills the goal of maintaining a spatially homogenous distribution of such crop trees at the stand level by geospatial analysis. We selected 10 group and row planting stands that were either 14 to 15 or 21 to 22 years old. Tree density in group plantings was significantly higher than in row plantings. Stand basal area was higher in row planting in younger stands but comparable to group planting in older stands. The proportion of trees with straight stems and monopodial crowns was higher in groups than in rows. The density and species’ richness of the PFCTs was significantly higher but the number of oak PFCTs was lower in group than in row plantings. In group plantings, naturally regenerated PFCTs contributed to 43% of total PFCTs, but to only 19% in row plantings. Also, the spatial distribution of PFCTs in group plantings was more uniform than in row plantings. Uniform and homogeneous distribution of the PFCTs in group planting stands can facilitate design and conduct of tending operations. Thus, the group planting technique offers not only cost savings in the establishment phase but also additional benefits, including spreading of risks through increased tree species diversity.

Low density plantings complemented by natural regeneration is an increasingly common reforestation technique to ensure growth of a sufficient number of trees from desired species while maintaining natural processes such as succession. One such form of low density planting that aims at lowering establishment costs—oak clusters—has been developed as an alternative to row planting since the 1980s in central Europe. However, whether cluster planting provides higher species richness and productivity than high density row planting has not previously been analyzed. Here, we compare tree species richness and productivity (measured as stand basal area) between oak cluster plantings and conventional row planting in young (10–26 years old) forest stands at seven study sites in Germany. Tree species richness was significantly higher in cluster plantings than in row plantings, whereas total basal areas were comparable. Naturally regenerated trees contributed on average to 43% of total stand basal area in cluster plantings, which was significantly higher than in row plantings. Total stand basal area in cluster planting was significantly related to the density of naturally regenerated trees. In turn, tree species diversity, density and basal area of naturally regenerated trees were increased with the size of unplanted area between clusters. Our results demonstrate that the admixture of naturally regenerated, early and mid-successional tree species compensates for a possible loss in productivity from planting fewer oaks. Low density cluster plantings can offer significant environmental benefits, at least for the first few decades of stand development, without compromising productivity.