Mountain forest regeneration (phase 2)

Today's forest regeneration is the key to tomorrow's forest services. It is extremely variable in time and space (stem numbers from less than 500/ha to well over 100,000/ha) and differentiated by tree size and species, making it difficult to assess and quantify. Threats from adverse weather conditions (e.g., wet snow, drought), pathogens (e.g. snow mold) and wildlife (especially browsing ungulates) are manifold and often last for decades. This makes it difficult to assess forest regeneration reliably.

In important areas of mountain forest management, great progress was made in recent decades, for example, in assessing the protective effect of stand structures. However, in the area of regeneration, there are still large gaps in knowledge. This is despite the fact that sufficient, diverse forest regeneration is very often a decisive factor in mountain forest management. For example, it is difficult to predict how the regeneration cohort will develop, how great the mortality is and how much additional seedlings can be expected to establish.

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

• Create a technical basis so that natural regeneration can be effectively promoted by silviculture. In the long term, the regeneration should grow into 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 silver fir-Norway spruce (Abies alba Mill. and Picea abies (L.) Karst.) forests. However, the project results are likely to be relevant for other forest types as well.
For the creation of the professional bases are determined:

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, number of stems, size distribution, height increment, tree species composition).

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

The study design is based on long-term monitoring of regeneration populations on environmental (site) gradients. For this purpose, forests were selected as case studies and silvicultural experiments were set up (manipulation), which further increase existing microsite gradients. The design enables the assessment of the development of regeneration populations with a statistically meaningful number of plants under different environmental conditions. Silvicultural treatments increase local environmental variability and allow treatment effects on microsite (primarily vegetation) and regeneration development to be tested and quantified. In order to demonstrate treatment effects, stand condition (stand structure, regeneration) will be assessed by means of an inventory prior to silvicultural interventions. Each experimental site is then divided into three plots. In one plot there is no intervention (control), in one there is a weak intervention (removal of about 20% of the growing stock) and in one there is a strong intervention (removal of about 30% of the growing stock). Four small fences will be placed in each plot to demonstrate the influence of wild ungulates on regeneration.

In total, ten experimental sites were identified in the Swiss Alps (Figure 1 and Table 1). Important criteria for the plot 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, 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, with differences in elevation and aspect. 
In addition, we have considered the following criteria: Simplicity of design, feasibility of silvicultural treatments (exclusion of stands that have been recently treated or are very unstable), walkability and accessibility, browsing situation.
All sites are 1.5 ha in size, established as growth and yield research plots (calipering 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.

Regeneration processes are monitored in approximately 75 sample plots (SP) per experimental site, i.e., 24-25 per plot (Figure 2, left). A SP consists of three concentric circular rings (Figure 2, right): 10 m2 for regeneration plants in size class 1 (1 year old to 9.9 cm tall), 20 m2 for size class 2 (10 cm to 39.9 cm tall), and 50 m2 for size class 3 (40 cm tall to 3.9 cm BHD). Germinants are not recorded.

The regeneration inventory consists of three parts: (1) recording of sample plot information and microsite characteristics and regeneration presence at 12 points per SP; (2) recording of single individuals, their microsite and development (mortality, plant height), among others the 1st quadrant is used for this purpose (Figures 3 and 4); (3) counting of the number of regeneration plants on the concentric sample circle rings according to tree species and size class.

In addition, the thickness of the organic layer, the humus form and the light conditions are determined at 4 of the 12 points per SP using a solariscope (Behling engineering, Hermannsburg, Germany).

The field manual for the regeneration inventory will be provided upon request.

Brang P, Nikolova P, Gordon R, Zürcher S (2017) Auswirkungen grosser Verjüngungslücken im Gebirgswald auf Verjüngungs und Holzzuwachs. Schlussbericht des Projektes Eingriffstärke und Holzzuwachs im Gebirgswald. Birmensdorf, Eidgenössische Forschungsanstalt WSL. 48 p.

Kalt T, Nikolova P, Ginzler C, Bebi P, Edelkraut K, Brang P (2021) Kurzes Zeitfenster für die Fichtennaturverjüngung in Gebirgsnadelwäldern. SZF (in print)        

Zaugg A, Lässig A, Nikolova P, Brang P. 2020. Projekt Gebirgswaldverjüngung: Dokumentation der Flächenauswahl. Interner Bericht. Birmensdorf, Eidg. Forschungsanstalt WSL, 9 S. + Anhang.

ZHAW Wädenswil