Alkaline soils, sparse ground vegetation, and low altitude positively influence post-windthrow tree regeneration in temperate and boreal forests of Switzerland (Central Europe)

Background/Question/Methods

Wind is the dominant cause of canopy disturbances in temperate forest ecosystems throughout Central Europe. Severe wind conditions are expected to increase as a result of climate change, thus catastrophic winterstorms like Vivian (1990), Lothar (1999) or Kyrill (2007) are likely to become more frequent in the future. Consequently, forest stands may increasingly be affected by wind damage. Little is known about regeneration dynamics on large windthrow areas followed by salvage-logging or left with no intervention. Few case studies after Vivian (4) and Lothar (7) in Switzerland suggest that mountain forests regenerate slowly in comparison to lowland forests, but results are confounded with soil conditions and local forest management. In a new sample covering 90 windthrow sites resulted from storms Vivian and Lothar and located in the northern lowland and northern Alps of Switzerland, regeneration status was assessed in 2010 and 2011 in order to better quantify factors influencing post-windthrow tree regeneration, i.e. number of tree individuals per area. In addition to tree regeneration, the following factors were assessed: soil pH (2.9-7.2), organic layer (0-23 cm), post-windthrow treatment (salvage-logged or no intervention), altitude (350-1800 m a.s.l.), aspect, slope, and predominant vegetation cover (9 types). Data were collected on six circular sample plots (20 or 50 m²) per windthrow site, which allowed scale dependent statistical analyses at the level of sample plots (n=540) and windthrow sites (n=90), respectively.

Results/Conclusions

GLM analyses of both samples showed that soil pH is the most important factor positively influencing tree recruitment, followed by small coverage of ground vegetation and altitude, i.e. higher regeneration densities in the milder climate at lower altitudes. The analysis at the level of windthrow sites showed that the smaller the investigation unit (all sites – sites affected by the same storm event – sites with the same principal tree species) the higher the percentage of variation explained by the assessed variables (from 40% up to 73%). Results may be important for management decisions after severe windthrow damage, especially in zones where the protective function of forests is transiently questioned.