J. FOR. SCI., 49, 2003 (7): 313–320 313 JOURNAL OF FOREST SCIENCE, 49, 2003 (7): 313–320 Introskeletal erosion (ISE) is defned as vertical down- wards gravitation movement and/or percolation of small organic as well as inorganic solid particles into spaces between stones – soil skeleton – of weathered rock cover. The soil body is lost from the surface, the mass moves to- wards the basis of weathering zone on block felds (ŠACH 1990). Another phenomenon is the mineralization of the surface humus after a change in microclimatic conditions on the soil surface. The process of ISE is mostly initialized by a clear-cut of forest stand and is strengthened by cutting and logging operations. These ecological processes appear even in the forest decline and thinning phases in sensitive mountain localities at extreme skeleton and sunny sites (air pollution, bark beetle attack, etc.). At progressive ISE, a decrease in the thickness of soil profle and its losses oc- cur, block appearance on the soil surface is observed and the extremity of the site increases. To a small extent, block felds are an interesting natural phenomenon, especially on the timber line, but a larger increase in their surface represents a heavy problem in forest sustainability. Pro- cesses of ISE often become limiting factors for reforesta- tion of more extreme sites in exposed localities – with all the undesirable effects on the non-production functions of forest ecosystems (production function is very often minor there). For their importance, they have been studied in the Czech Republic since the early 80ies. Research is concen- trated on the extent and dynamics studies: soil depth, sur- face stones, site microrelief, mineralization intensity, herb layer dynamics – herb and moss layers (ŠACH, PAŠEK 1996; PODRÁZSKÝ 1996; V ACEK et al. 1999). METHODS Differentiated potential of ISE danger was determined on the basis of feld survey and site analyses. The study was performed in particular Forest Natural Regions (FNR – geomorphologically and phytogeographically defned natural areas with differentiated forest history, growth conditions and dynamics): – Šumava Mts. (FNR 13), – Krkonoše Mts. (FNR 22), – Hrubý Jeseník Mts. (FNR 27). This study was supported by Research Projects Infuence of Environment on Forest Regeneration (S. V ACEK) Grant Agency of the Czech Republic No. 206/03/1583 Nutrient Cycles in Mountain Forest Ecosystems Saturated by Nitrogen: Past, Present and Future State of Water, Soil and Norway Spruce Stands (V. V. PODRÁZSKÝ). Introskeletal erosion threat in mountain forests of the Czech Republic S. V ACEK 1 , V. V. PODRÁZSKÝ 2 , M. MIKESKA 3 , W. K. MOSER 4 1 Forestry and Game Management Research Institute, Research Station Opočno, Czech Republic 2 Czech University of Agriculture, Faculty of Forestry, Prague, Czech Republic 3 Forest Management Institute, Branch Hradec Králové, Czech Republic 4 Missouri Department of Conservation, Columbia, USA ABSTRACT: Summarization of the potential of introskeletal erosion (ISE) was done on the basis of feld surveys in the Šumava Mts., Krkonoše Mts. and Jeseníky Mts. areas. The results give a rough estimation of the ISE extent in mountain forests of the Czech Republic. They indicate that in the mountainous areas of the Czech Republic there are 46,535 ha of forest lands endangered by these processes, which represents 10.1% of mountain forests and 1.7% of the total forest area in the country. The worst situation is in the Krkonoše Mts., where the considerable ISE potential is on 30.5% of forest area (in the Jeseníky Mts. 16.6%, in the Novohradské hory Mts. 13.3%, in the Šumava Mts. 8.2%). Because these areas are relevant production and water protection areas, the ISE pro- cesses have to be studied carefully and forest lands have to be protected, also using special technologies for forest protection and restoration. Keywords: mountain forests; spruce stands; immissions; bark beetle; introskeletal erosion; protection; reforestation