Sustainable mountain forest regeneration

Today's forest regeneration is the key to tomorrow's forest services. Regeneration dynamics are extremely variable in time and space (stem numbers can range from less than 500 ha^-1 to well over 100,000 ha^-1) which leads to large differences in regeneration density among tree species and size classes. Therefore, regeneration stocks are difficult to assess, and their quantification requires profound knowledge of the prevailing regeneration processes as well of potential influencing factors. These include various threats from adverse weather conditions (e.g., wet snow, drought), pathogens (e.g., snow mould) and wildlife (especially browsing ungulates). Since these influencing factors interact strongly and often have an impact on regeneration processes over decades, they make it difficult to reliably assess forest regeneration.

In recent decades, great progress has been made in important areas of mountain forest management, for example when assessing the protective effect of stand structures. However, there are still large gaps in knowledge regarding regeneration processes due to the major challenges in selecting and recording key parameters that accurately reflect regeneration dynamics. In addition, it is difficult to estimate how regeneration will develop, as both the mortality of individual plants and additional seeding can be subject to extreme fluctuations. This is especially problematic as sufficient, diverse forest regeneration is very often a decisive factor in the management of mountain forests and creates the basis for the forest of the future.

The long-term project goals (time horizon 2025-2030) are to:

• Create a scientific and technical basis so that natural regeneration can be effectively promoted through silviculture. In the long term, regeneration plants should become forest stands that conform to targets and provide important forest services (focus on protection forest).
• Clarify how climate change affects regeneration development. For example, in dry summers, seedlings and saplings of drought-sensitive tree species could be affected by increased mortality, or natural regeneration of tree species from lower elevations could become increasingly established.
• Elaborate a "Practical Guide to Assessing Natural Regeneration". This methodological guide should provide information on natural regeneration at the stand level that is meaningful, technically sound, and therefore comprehensible and robust (reproducible). An important application of this information is silvicultural decision-making in the NaiS framework (Sustainability and Success Control in Protection Forests).

The project focuses on spruce-fir forests. However, the project results are likely to be relevant for other forest types as well.

For the creation of the technical bases, we will determine:

1. The demographic dynamics (long-term development of tree populations) of natural regeneration in mountain forests.
2. The influence and relative importance of factors (site, stand, disturbances) that are important for natural regeneration (demographic development, stem numbers, size distribution, height increment, tree species composition).

In addition, accompanying studies on sub-processes are planned that will improve the mechanistic understanding of regeneration processes in mountain forests.

A total of ten experimental sites were established in the Swiss Alps (Table 1 and Figure 2). Important criteria for site selection were:

1. homogeneity within each site with respect to stand structure, relief, aspect and site type;
2. tree species composition: good representation of Norway spruce and silver fir in the regeneration pool, and a gradient in the proportion of silver fir in the mature stand;
3. environmental/site gradient: the main site types in fir-spruce forests were considered including differences in elevation and aspect. Experimental sites in the continental Central Alps were deliberately avoided because fir trees are only rarely found there.

All experimental sites are 1.1-1.5 ha in size, established as growth and yield plots (dbh threshold: 4 cm) and are contractually secured as long-term research plots.

Table 1: Characterisation of the experimental sites. Site types according to ARGE Frehner M, Dionea SA and IWA –Wald und Landschaft AG 2020: NaiS-NFI – Allocation of NFI-sampling plots to site types. Final report. Mandated by the Federal Office of the Environment BAFU, 68 p.

The study design allows for long-term observations of regeneration populations along site gradients. For this purpose, ten experimental sites (case studies) were specifically selected and silvicultural interventions (three treatments) were carried out in order to increase existing gradients of light availability. The design enables the assment of the development of regeneration  populations with a statistically meaningful number of plants under different environmental conditions. The effects of targeted silvicultural treatments on the development of microsite conditions (especially ground vegetation) and regeneration patterns will be  examined and quantified.