15 Dec 2022

Light availability, tree growth and sustainability in permanent beech forests

WHFF Project 2017.15

Project management: Harald Bugmann

 

The most important facts in brief

  • In order to better quantify growth patterns on beech sites with respect to mixing options in permanent forests, beech (Fagus sylvatica L.) and sycamore (Acer pseudoplatanus L.) trees of the thicket and polewood classes were subjected to stem disc analysis for the present research project.
  • While both height and diameter development of both tree species were strongly dependent on light availability, the height increment of beech was found to be greater at dense stands.
  • Sycamore has a longer crown, which should be developed with care, but already showed reduced height increment with age at small diameter.
  • In the demographic equilibrium, which in the permanent beech forest lies at a basal area of 21 m2 to 25 m2, the growth of sycamore into the crown layer is therefore strongly questioned, especially in small gaps (<270 m2) with less than 10% of open space light availability.
  • To ensure substantial proportions of sycamore in the mix, it should be rejuvenated in gaps of at least 400 m2.
  • Spatially heterogeneous interventions and group removals can create a diverse mosaic of regeneration niches in the permanent forest. This creates opportunities for the preservation or creation of tree species mixtures.

 

Project description

In the last three decades, the manifold demands on near-natural silviculture and economic drivers have helped single-tree oriented permanent forest management to gain much attention. Of particular importance are the occurrence of severe disturbance events, climate change, the difficult economic situation of many forest enterprises, and – increasingly important – ecocentric considerations.

The sustainably established permanent forest continuously offers a wide range of timber assortments and a correspondingly high operational reliability. Since utilization and maintenance take place simultaneously on the same area, in combination with biological rationalization, the effort required for the first production stage can be greatly reduced.

Today, many forestry operations are faced with the question: How can the natural regeneration of tree species requiring more light be ensured under permanent forest management? Especially in old-growth stands rich in tree species, the preservation of the existing tree species diversity is often not ensured by permanent forest thinning. This is because the composition of the regeneration under the given diverse seed supply is directly determined by the intensity of the initial regeneration cut. At the same time, however, against the background of a changing climate, mixed stands are of crucial importance for future stands and their adaptability and resilience.

In this research project, the authors therefore investigated key questions to better understand growth dynamics in forests managed according to permanent forest principles on beech sites. Which tree species mixtures are possible on these sites under the strong competition of beech by permanent forest management is still subject of controversial discussions. Due to the lack of significant proportions of light-demanding tree species in the regeneration of permanent forests on beech sites, the focus in this project was on the mixing possibilities of beech (Fagus sylvatica L.) and sycamore maple (Acer pseudoplatanus L.).

The work focused on the following questions:

  1. How do the diameter and height increment of beech and sycamore of the thicket and polewood classes behave along light availability gradients?
  2. How large must the number of stems in the ingrowth be to achieve a sustainably built demographic structure in permanent forests on beech sites?
  3. Under what conditions can it be ensured that more light-demanding tree species are maintained?

A total of 14 forest areas on medium beech sites with mixed stands managed by regular permanent forest thinning were studied. Mean annual temperatures in the study areas ranged from 8 to 11°C, and mean annual precipitation ranged from 890 to 1300 mm.

 

Conclusions

The permanent beech forest can be brought into demographic equilibrium with a stock between 21 and 25 m2/ha. To maintain this equilibrium, about 90-120 trees per hectare are needed in the breast height diameter (DBH) class 3 (8-11.9 cm). With this basal area, sycamore maple can only be retained in small proportions because under typical light conditions of less than 10% of open air radiation, it accounts for less than 25% of the stem number in the initiation of regeneration in the permanent forest, while remaining suppressed for too long to take advantage of its growth strategy.

Sycamore regeneration can be promoted in gaps of >400m2 and, unlike beech, must be both favored by maintenance and promoted with timely expansions of the gap. In smaller gaps, it takes more than 30 years for sycamore to reach 4cm DBH. At this age, the sycamore’s height growth potential is already greatly reduced and regrowth into the canopy layer is severely challenged.

To maintain or create tree species mixtures with tree species of different shade tolerance on beech sites, a diverse mosaic of different regeneration niches is needed. In the permanent forest, this requires a shift from the advantages of single-tree harvesting to group harvesting. In order to create forests that are as diverse and richly structured as possible, the free combination of different cutting methods over small areas is suitable, depending on the regeneration objective.

 

The project was supported by the Swiss Forest and Wood Research Promotion WHFF-CH of the Federal Office for the Environment FOEN and the Conference for Forests, Wildlife and Landscape KWL of the cantons.

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