Searching for the X factor driving the decline/dieback of the Siberian fir forests across the National Park "Krasnoyarsk Stolby" in the East Sayan Mts, south-central Siberia

The problem of forest decline/dieback has attracted the attention of foresters/ecologists globally. The decline/dieback of the Siberian fir (Abies sibirica) forests in the National Park "Krasnoyarsk Stolby" have been started half a century ago. Three major reasons are considered: anthropogenic pollution, including acid rain; insect pests and phytopathogens; climate aridization. Our goal was to examine the two hypotheses of fir forest decline – due to climate droughts and the potential impact of urban sulfur dioxide emissions from the industrial metropolis of Krasnoyarsk bordering the Nat'l Park. In our analyses, we used meteodata of two weather stations close to the Nat'l Park for calculating SPEI droughts and wind dynamics; data from MERRA2, OMI, and TROPOMI instruments for determining concentrations of sulfur dioxide, SO₂. Values of SPEI < –1.5 that characterized severe droughts happened here once in 8-10 years. Moreover, trees are capable of surviving 1-2 year severe droughts. We found no convincing evidence of climate droughts as a driver of the National Park's fir forests decline. The highest SO₂ concentrations were recorded in the winter months (2–10 mg/m²) and twice less in the warmer months (1–4.5 mg/m²). Peak concentrations in May and June posed a particular threat to the developing photosynthetic apparatus of trees. Additionally, the montane forests over the South Siberian mountains are subject to the westerlies which bring air pollution from large industries along the 50-56 latitudes from Europe, the Urals, and South Siberia. A comparison of wind patterns between the 1991–2020 and the baseline 1966–1990 periods revealed no significant changes. The number of days with calm conditions that facilitate accumulating pollutants in the city and its surroundings remained constant. Plausible causes of the decline/dieback of the dark coniferous forests in the Nat'l Park and South Siberian Mountains need additional studies. The study was supported by Basic Project FWES-2024-0023.