Factors influencing tree regeneration after windthrow in Swiss forests

Severe wind damage is one of the most important disturbance agents in European forest ecosystems. In a biodiversity context, wind disturbance is viewed as an enrichment, because increased light availability and modified microclimatic conditions promote plant diversity in the years following the disturbance. However, severe damage to mountain forests transiently reduces the protective effect of forests against natural hazards like avalanches or rockfall, and generally leads to economic loss. The frequency of wind disturbance, most often in the frame of European winter storms, has increased in the recent years, and thereby the damaged forest area. The natural regeneration ability of windthrown forests and the factors explaining the variability of regeneration are, therefore, of considerable interest (Priewasser 2013: Dissertation, Chapter I).

The quantity of lying and standing deadwood is changed strongly by windthrow and after subsequent salvage logging. It is an important component of forest ecosystems, since many deadwood-associated species depend on the presence of deadwood. Also for tree regeneration success, deadwood can be crucial. Yet, little is known about deadwood quantity and quality after windthrow in Central Europe and how much deadwood is left after salvage logging (Priewasser 2013: Dissertation, Chapter II).

Another factor clearly changing after windthrow is the cover of ground vegetation. Due to increased light availability, fast growing species can rapidly form dense vegetation cover that significantly suppresses tree regeneration. One example of such a species is bracken (Pteridium aquilinum L.). It is assumed that bracken suppresses tree regeneration especially through the release of phytotoxic compounds (allelopathy). Regeneration failure of beech (Fagus sylvatica L.) and sycamore (Acer pseudoplatanus L.) in dense bracken stands has not been investigated for allelopathy yet, although beech forests are frequent in Europe (Priewasser 2013: Dissertation, Chapter III).

Chapter I.

Natural tree regeneration was recorded in 2010 and 2011 in 90 windthrow gaps (each ≥ 3 ha) in Switzerland. The forest gaps were caused by two severe storms in 1990 (Vivian) and 1999 (Lothar). Soil pH, vegetation cover, post-storm treatment (‘salvage-logging’ or ‘no timber harvesting’) and elevation were the four most important predictors explaining the variability of sapling density. Although general trends could be detected, the heterogeneity among forest gaps was large. In contrast to conventional wisdom on forest succession after disturbance, late-successional tree species dominated in a majority of gaps 10 to 20 years after wind disturbance regarding sapling density.

Chapter II.

Deadwood amount and quality were investigated on the same gap sample as the natural tree regeneration in Chapter I. A special focus was on the post-storm treatments ‘salvage-logging’ or ‘no timber harvesting’. The study revealed surprisingly high deadwood volumes (74.6 m3 ha-1) in salvage-logged forest gaps. This value distinctly exceeds the proposed minimum deadwood volumes for forest stands in a conservation context. Additionally, a wide variety of decay stages and diameter classes (10 to ≥ 70 cm) was found in both salvage-logged and unharvested gaps, suggesting considerable habitat diversity for deadwood-associated species irrespective of the treatment. Since it takes a few decades until deadwood turns into a suitable seedbed, the storm- induced deadwood pieces that were up to a maximum of 20 years old did not provide an appropriate substrate for tree establishment yet.

Chapter III.

To test causes of regeneration failure in dense bracken stands, germinating seeds of beech and sycamore were exposed to bracken leachates in a greenhouse experiment. Moreover, seedlings exposed to bracken leachates in the greenhouse experiment were compared with seedlings grown in shadow or with bracken rhizomes in a common garden experiment. Except for a significantly but only slightly reduced germination rate of beech in the bracken treatment, no other allelopathic influences of bracken were found in either experiment. In contrast, strong evidence for light competition was detected as probably one of the most important factors causing regeneration failure of beech and sycamore in dense bracken stands during the first vegetation period.

Overall, the two studies based on the data inventory of the 90 forest gaps (Chapter I and II) revealed general driving forces of natural tree regeneration, as well as the insight that current logging practices in Switzerland leave enough and diverse deadwood ensuring habitat diversity for deadwood-associated species. Moreover, in contrast to the assumption that bracken causes tree regeneration failure through the release of phytotoxic compounds, the third chapter showed that mainly light competition led to lower growth performance in beech and sycamore.