The integrated assessment of dark-forest ecosystems in the Southern Siberia mountains demonstrates the existence of damage foci on the windward slopes of the most mountain ranges at the elevation 650–700 a.s.l. At the north-east of Altai only the drying is observed everywhere. The beginning stages of biocenosis damage in low mountains and leeward south and east slopes and strong damage in high and middle mountains on the windward north and west ones are observed. The study is shown that Pinus sibirica drying process is characteristic for conifers type, but drying of Abies sibirica has the specific character. A description of declining factors has been presented and the basic hypotheses of fir forest declining is considered. A conclusion about the complex character of causes of the decline which has a regional specific has been made.