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The fire history of the northern larch forests within the permafrost zone in a portion of northern Siberia (∼66 • N, 100 • E) was studied. Since there is little to no human activity in this area, fires within the study area were mostly caused by lightning. Fire return intervals (FRI) were estimated on the basis of burn marks on tree stems and dates of tree natality. FRI values varied from 130 to 350 yr with a 200 ± 50 yr mean. For southerly larch dominated communities, FRI was found to be shorter (77 ± 20 yr at ∼61 • N, and 82 ± 7 at 64 • N), and it was longer at the northern boundary (∼71 • ) of larch stands (320 ± 50 yr). During the Little Ice Age period in the 16th-18th centuries, FRI was approximately twice as long those as recorded in this study. Fire caused changes in the soil including increases in soil drainage and permafrost thawing depth, and a radial growth increase to about twice the background value (with more than six times observed in extreme cases). This effect may simulate the predicted warming impact on the larch growth in the permafrost zone.

A fire return intervals (FRI) for zone of larch dominance and “larch‐mixed taiga” ecotone was studied. Extreme fire events were connected with summer air temperature deviations. Average FRI determined from stem fire scar dating was 82 ± 7 years for the zone of larch dominance, and 77 ± 20 for the “larch‐mixed taiga” ecotone. For the zone of larch dominance FRI on north‐east facing slopes was 86 ± 11 years, for south‐west facing slopes at 61 ± 8 years, for flat terrain at 68 ± 14 years, and for bogs 139 ± 17 years. FRI decreased from 101 years in the 19th century to 65 years in the 20th century, for the zone of larch dominance, and from 97 years to 50 years for the “larch‐mixed taiga” ecotone. A climate and anthropogenic impact on this phenomenon was analyzed. The decrease of FRI may interfere with climate‐driven migration of competitor species into zone of larch dominance, affecting biodiversity at high latitudes.

The expansion of so-called evergreen conifers (EGCs), including Siberian stone pine, spruce, and fir, along the transect oriented from the boundary of the larch-dominated zone (LDZ; mixed forests of the Yenisei Ridge) to its center has been studied. The normalized dispersal coefficient calculated as K i = ( n i -N i )/( n i + N i ) , where n i and N i are the relative numbers of the i th species in the undergrowth and the upper layer, respectively, serves as an indicator of the expansion. It has been found that the K i values for EGCs (and birch) are higher than the K i of larch even in the zone absolutely dominated by larch, where the relative numbers of EGCs in the upper layer is less than 1%. The EGC undergrowth has mainly been formed during the past 20-30 years, which is correlated with the trend of summer temperatures The spread of EGCs in the LDZ depends on the frequency of forest fires. The decrease in the time intervals between fires in the 20th century to 65 years (versus 100 years in the 19th century) may have prevented the expansion of competing species in the LDZ. The results obtained indicate that EGCs and birch penetrate into the zone traditionally dominated by larch, which is related to climatic changes during the past three decades. At the same time, tree stand density is increasing in the forest-tundra ecotone, and larch is spreading further into the tundra zone.

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a b s t r a c tThe Trans-Baikal (or Zabailkal'e) region includes the forest-steppe ecotones south and east of Lake Baikal in Russia and has experienced drought for several years. The decline and mortality of birch (Betula pendula) stands within the forest-steppe ecotone Trans-Baikal region was studied based on a temporal series of satellite data, ground measurements, and tree ring analysis. During the first decade of the 21st century birch stands decline and mortality were observed on about 5% of the total area of stands within our 1250 km 2 study area. Birch forest decline and mortality occurs mainly at the margins of stands, within the forest-steppe ecotone on slopes with direct insolation. During the first decade of the 21st century summer (June-August) precipitation was about 25% below normal. Soil water content measurements were lowest within dead stands and highest within healthy stands and intermediate within damaged stands. Drought impact on stands was amplified by an increase in summer air temperatures (+0.9°C) in comparison with the previous decade. Tree ring data of ''surviving'' and ''dead'' tree groups showed a positive correlation with summer/annual precipitation and negative correlation with summer air temperatures. Temperature and precipitation extreme anomalies tend to occur in the region with a period of about 27 years. The observed anomaly was the most severe since the beginning of meteorological observations in the year 1900. Data for the other sites showed a positive climate impact on the growth and expansion of Siberian forests. That is, the same species (B. pendula) showed considerable increase (1.4 times both in height and stem volume) during 20th-21st centuries as temperature increased but precipitation remained at adequate levels.

Fire history within the northern larch forests of Central Siberia was studied (65+°N). Fires within this area are predominantly caused by lightning strikes rather than human activity. Mean fire return intervals (FRI) were found to be 112 ± 49 years (based on fire scars) and 106 ± 36 years (based on fire scars and tree natality dates). FRI were increased with latitude increase, and observed to be about 80 years at 64°N, about 200 years near the Arctic Circle, and about 300 years nearby the northern range limit of larch stands (~71°+N). Northward FRI increase correlated with incoming solar radiation (r = − 0.95). Post Little Ice Age (LIA) warming (after 1850) caused approximately a doubling of fire events (in comparison with a similar period during LIA). The data obtained support a hypothesis of climate-induced fire frequency increase.

A fire history of northern larch forests was studied. These larch forests are found near the northern limit of their range at ~71°N, where fires are predominantly caused by lightning strikes rather than human activity. Fire-return intervals (FRIs) were calculated based on fire scars and dates of tree natality. Tree natality was used as an approximation of the date of the last fire. The average FRI was found to be 295±57 years, which is the longest reported for larch-dominated stands. Prior studies reported 80–90-year FRIs at 64°N and ~200 years near the latitude of the Arctic Circle. Comparing data from fires that occurred in 1700–1849 (end of the Little Ice Age, LIA) and 1850–1999 (post-LIA warming) indicates approximately twice as many fires occurred during the latter period. This agrees with the hypothesis that observed climatic warming will result in an increase in fire frequency. Our results also indicate that fires that did not leave visible fire scars on the tree stem may be identified based on the date of growth release revealed from dendrochronology.