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Peter Brang

Forschungsanstalt WSL

Swiss Federal Research Institute WSL
Stand Dynamics and Silviculture
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Article

Author(s): Christoph Angst (external author)
Editorial office: WSL, Switzerland
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Diverse forest dynamics on experimental windthrow sites

Viele Tiere profitieren vom erhöhten Angebot an Licht und Wärme.
Fig. 1 - After the hurricane: many animal species took advantage of the increased light and temperature.
Photos: Thomas Reich (WSL)
 
Sich in belassenen Windwurfflächen zu bewegen, ist ein Balance-Akt
Fig. 2 - Moving around on an uncleared windthrow is a constant balancing act.
Photo: Thomas Reich (WSL)

Within the framework of an interdisciplinary project, researchers studied forest dynamics on large windthrow sites with and without salvage logging, which were established after the hurricane Lothar in 1999. This article gives an overview of the evolution of tree regeneration, ground vegetation and fauna.

In the year 2000, WSL scientists established experimental treatments at seven sites on the Swiss central plateau to gain a better understanding of forest dynamics after hurricanes. Each plot consisted of one cleared and one uncleared area. Several groups of scientists from different fields were involved with these studies for three years.

High species diversity due to light and warmth

Due to the sudden loss of the canopy, the microclimate altered radically, speeding up decomposition processes in the upper soil. The released nutrients enhanced growth of numerous new light-demanding plant species. Plant species richness was higher in cleared than on uncleared treatments, whereas the neighboring intact forest showed the poorest species diversity. Initial species richness significantly decreased in uncleared areas three years after the hurricane, and on cleared areas one year after it. Among other reasons, this is due to the strong increase of highly competitive blackberry and raspberry species.

On uncleared areas, the researchers found up to three times more insect species than in the intact forest. The diversity of small mammal species varied to a smaller degree. On the large storm sites studied, seven species were found, while only five were found in the forest. The most common species, the bank vole and the wood mouse, definitely preferred the windthrow areas.

Young trees compete with blackberries

In contrast to the results presented above, species richness of mycorrhizal fungi declined on windthrow sites during the first four years after Lothar. This was caused by the extended loss of trees. Nevertheless, the remaining micorrhizal fungi still managed to cover completely the roots of establishing Norway spruce seedlings. The ability of mycorrhizal fungi to infect young trees thus remains intact for a long time after a storm event.

In general, the so-called advance regeneration, which already existed before the storm damage, and the mast seed crop of beech in 1999 were much more important for the regeneration of the climax tree species than the regeneration from seed that germinated after the storm. In contrast to current knowledge, even the seeds of pioneer tree species such as willow, poplar and birch, which are easily dispersed by wind, or of rowan which is dispersed by birds, up until now played only a minor role in forest succession in six out of seven sites. However, the advance regeneration may be significantly damaged during timber harvesting. During the next few years, the rapidly growing blackberry cover may negatively affect the development of the natural regeneration. On most of the study sites, however, a sufficient natural regeneration can be expected.

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