Causes and effect of latest windthrow on further development of beech-fir stands in Slovenian forests

In the December 2017, 211 000 ha of Slovenian forests or 2 201 000 m³ were severely damaged by the windstorm. The damage was greatest in the adult beech-silver fir forest stands. In each management district (FMD) with different share of damaged forested area - Kočevje (69%), Postojna (22%) and Ljubljana (19%), research location were established to evaluate soil properties, root characteristics and ecophysiological regeneration response of predominating tree species in damaged stands as well as the economic effect of the windstorm on sensitive high karst sites. The restoration and regeneration efficiency of latest disaster (2017) was also compared with restoration efficiency of areas damaged in 2008, identifying possible specifics / differences and providing guidelines for their optimal recovery.

Timber database from Slovenian Forest Service with data about felling and felling reasons between Dec. 2017 and the end of 2019 was used to evaluate economic losses. The total amount from the database was multiplied by the shares of individual quality category, obtaining the quantitative assortment structure according to the actual felling. We used tables of assortment losses caused by wind to estimate the actual assortment loss and multiplied those with 2018 roundwood market prices. Estimated cumulative biomass loss amounted 205 855 m³ - 5.6% less, compared to regular felling, or 16.1 million €.

Physical soil properties between plots damaged by wind and control areas without damage did not confirm any significant difference in relation to weather and microsite conditions (rootstock, exposition, altitude, species structure of stands, microrelief). Characteristics of root systems of the most affected tree species - fir, spruce and beech did not differ between damaged and neighboring undamaged sites. An overview of fir and spruce windbreaks during the 1995 - 2018 period outlined the cumulative effect of multiple site conditions with strong wind, severely reducing the mechanical stability of fir and spruce; consequently, the following weather events with even mild wind intensities resulted in calamity.

Natural regeneration ability on all plots of recent windthrow shows successful regeneration of beech, maple, and fir in all light categories of damaged stands, but questionable regeneration of fir, which is in tight relation with ungulate browsing.  For comparison with regeneration after windthrow in 2017, we analyzed successional development in three windthrow areas from 2008. Seedling densities ranged from 5000 to 9000 ha-1, with the highest densities in the 150-300 cm height class. Within plots with natural regeneration, height and species diversity were better compared to planted plots. Shade-tolerant species increased in plots at lower elevations, while pioneer species still increased at higher elevations. Overall browsing was moderate, but silver fir and sycamore had difficulty establishing. The results indicate that regeneration spreads from areas that were covered with seedlings shortly after windthrow and that certain parts will not be covered with regeneration for a longer period.