Long-term development of natural regeneration in irregular, mixed stands of silver fir and Norway spruce

Abstract

The timely establishment of natural regeneration of the preferred species after the death or removal of mature trees is essential in continuous-cover forestry. In the context of the gradual shift from even-aged and monospecific to uneven-aged and/or mixed forest stands, the limited availability of statistical models to predict seedling establishment, survival, and growth has increasingly become a bottleneck for forest management planning and, ultimately, a potential limitation to a wider adoption of alternative silvicultural approaches.We investigated the development of top height and density of natural regeneration in 19 uneven-structured, mixed silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) H. Karst.) stands in southwestern Germany using long-term observations (35 years) from permanent plots. We used linear mixed-effects models to analyze the influence of overstory and understory-related variables on regeneration of fir and spruce.The height of the five tallest juvenile trees per regeneration subplot and species increased significantly with diminishing canopy cover and increasing structural diversity of the overstory. However, competition exerted by tall juveniles substantially impacted the development of smaller neighbors, which were less able to profit from favorable overstory conditions. These results indicate that canopy cover and structural diversity need to be taken into account when modeling height development of juvenile trees in irregular stands. Importantly, these results also demonstrate the potential of silvicultural interventions to shorten the time period when terminal shoots are vulnerable to browsing.Densities of juvenile trees displayed a unimodal relationship with the mean height of the regeneration. Fir and spruce densities culminated at a mean height of approx. 50 cm and decreased rapidly afterwards. This pattern indicates an early onset of competition within the regeneration layer. For both species, juvenile densities were unrelated to overstory structural diversity, yet they showed positive relationships with overstory density and site productivity.Overall, fir juveniles developed faster in height than spruce juveniles. Even rather rapid group-shelterwood cutting regimes with complete canopy removal within two decades still favored fir regeneration. In addition, a high proportion of fir in the regeneration cohort had a stronger negative effect on spruce juvenile density than vice versa. Since spruce is less shade-tolerant than fir, it is likely that fir will dominate the future stand composition.Overall, our models provide the basis to predict natural regeneration dynamics in structurally complex stands dominated by fir and spruce and to further evaluate alternative treatment scenarios.