M. L. Dvinskaya

Expansion of evergreen conifers to the larch-dominated zone and climatic trends

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 Ki = (ni − Ni)/(ni + Ni), where ni and Ni are the relative numbers of the ith species in the undergrowth and the upper layer, respectively, serves as an indicator of the expansion. It has been found that the Ki values for EGCs (and birch) are higher than the Ki 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.

The spatiotemporal pattern of fires in northern taiga larch forests of central Siberia

The periodicity of fires in larch forests of Evenkia and their relationship with landscape elements have been studied. Cross-sections with “burns” in them caused by past fires have been analyzed in 72 test plots; the fire chronology encompassed the period from the 15th to the 20th century. The between-fire intervals (BFIs) have been calculated by two methods: (I) on the basis of burns alone and (II) on the basis of burns and the start of growth of the new generation of larch after the earliest fire. The BFI depends on local orographic features; it is 86 ± 11 (105 ± 12), 61 ± 8 (73 ± 8), 139 ± 17 (138 ± 18), and 68 ± 14 (70 ± 13) years for northeastern slopes, southwestern slopes, bogs, and flatlands, respectively. The mean BFIs calculated by methods I and II are 82 ± 7 and 95 ± 7 years, respectively. The permafrost horizon rises at a mean rate of 0.3 cm per year after a forest fire. It has been shown that the number of fires regularly peaks at periods of 36 and 82 years. There is also a temporal trend in fire frequency: the mean BFI was approximately 100 years in the 19th century and 65 years in the 20th century.

The Potential Impact of CO2 and Air Temperature Increases on Krummholz Transformation into Arborescent Form in the Southern Siberian Mountains

Abstract Trees in the southern Siberian Mountains forest-tundra ecotone have considerably increased their radial and apical growth increments during the last few decades. This leads to the widespread vertical transformation of mat and prostrate krummholz forms of larch (Larix sibirica Ledeb) and Siberian pine (Pinus sibirica Du Tour). An analysis of the radial growth increments showed that these transformations began in the mid-1980s. Larch showed a greater resistance to the harsh alpine environment and attained a vertical growth form in areas where Siberian pine is still krummholz. Upper larch treeline is ≥10 m higher than Siberian pine treeline. Observed apical and radial growth increment increases were correlated with CO2 concentration (r  =  0.83–0.87), summer temperatures (r  =  0.55–0.64), and “cold period” (i.e. September–May) air temperatures (r  =  0.36–0.37). Positive correlation between growth increments and winter precipitation was attributed to snow cover protection for trees during wintertime.

Wildfire Dynamics in Mid-Siberian Larch Dominated Forests

The long-term wildfire dynamics, including fire return interval (FRI), in the zone of larch dominance and the “larch-mixed taiga” ecotone were examined. A wildfire chronology encompassing the fifteenth through the twentieth centuries was developed by analyzing tree stem fire scars. Average FRI determined from stem fire scar dating was 82 ± 7 years in the zone of larch dominance. FRI was found to be dependent on site topography. FRI on north-east facing slopes in the zone of larch dominance was 86 ± 11 years. FRI was significantly less on south–west facing slopes at 61 ± 8 years and flat terrain at 68 ± 14 years. For bogs FRI was found to be much longer at 139 ± 17 years. The FRI decreased from 101 years in the nineteenth century to 65 years in the twentieth century. Connection of this phenomenon with natural and anthropogenic factors was analyzed. The relationship of extreme fire events with summer air temperature deviations at the regional and sub-continental levels was presented. Wildfire impact on permafrost thawing depth was analyzed. The implications of the observed trends on the larch community are discussed.

Comparative Reaction of Larch (Larix sibirica Ledeb.) Radial Increment on Climate Change in the Forest Steppe and Highlands of Southern Siberia

The influence of climate changes on larch (Larix sibirica Ledeb.) radial increment under conditions of a limited (forest steppe) and sufficient (high-altitude Kuznetsk Alatau, floodplain stands) humidification is considered. The relationship between growth index of larch trees (N = 257) and ecological and climatic variables is analyzed. In the forest steppe, with the onset of warming, a decrease in the aridity of the climate, an increase in the duration of the growing season (1980s), and an increase in the larch growth index followed by its depression in the 1990s have been observed. Radial-increment depression is caused by an increase in vapor-pressure deficit and arid climate due to a rising air temperature. In the 2000s, radial-increment fluctuations with average values not exceeding those before the beginning of current climate warming period occurred. In the highlands, since the 1970s, there has been a general increase in the larch radial increment closely associated with the main limiting factor of growth—air temperature. At the same time, in arid years, the radial-increment depression of larch trees in highland and floodplain larch forests is also noted. When implementing “hard” climate scenarios (RCP 6.0 and RCP 8.5), it is likely that the larch growth index in a forest steppe will decrease further and its increase in areas of sufficient moisture will be observed.

Siberian Pine and Larch Response to Climate Warming in the Southern Siberian Mountain Forest: Tundra Ecotone

The tree response to climate trends is most likely observable in the forest-tundra ecotone, where temperature limits tree growth. Here we show that trees in the forest-tundra ecotone of the mid of the south Siberian Mountains responded strongly to warmer temperatures during the past two decades. There was a growth increment increase, stand densification, regeneration propagation into the alpine tundra, and transformation of prostrate Siberian pine, larch and fir into arboreal forms. A temperature increase of 1°C allows regeneration to occupy areas ∼40–100 m higher in elevation, depending on the site. Siberian pine and larch regeneration and arboreal forms now occur at elevations up to 200 m higher in comparison with the known location of the former tree line. These species surpass their upper historical boundary of 10–80 m elevation. Regeneration is propagating into the alpine tundra with the rate of 0.5–2.0 m/year. The observed winter temperature increase is significant for regeneration survival. Measurements of the radial and apical growth increments indicates an acceleration of krummholz transforming into arboreal forms in the mid-1980s. Larch surpasses Siberian pine in cold resistance, and has an arboreal growth form where Siberian pine is in krummholz form. Improving climate provides competitive advantages to Siberian pine in the areas with sufficient precipitation amount. Larch, as a leader in harsh environment resistance, received an advantage at the upper front tree line, and in the areas with low precipitation. Observed tree migration into the alpine stony tundra will decrease albedo, providing a positive feedback to global warming at the regional level.

Evidence of Evergreen Conifers Invasion into Larch Dominated Forests During Recent Decades

Dark needle coniferous (DNC: Siberian pine, spruce, fir) expansion into larch dominated area was investigated along transects, oriented from the west and south borders of the larch dominated communities to its centre. The expected invasion of DNC into larch habitat was quantified as an increase of the proportion of those species both in the overstory and regeneration. Abundance and invasion potential was expressed using the following variables: (1) N i and n i – the proportion of a given species in the overstory and regeneration, respectively, and (2) K i – “the normalized propagation coefficient” defined as K i = (n i − N i )/(n i + N i ). The results show that Siberian pine and spruce have high K i values both along the margin and in the centre of zones of absolute larch dominance, where their presence in the overstory is <1%. There is a tendency of K i to increase for DNC and birch from south to north and from west to east. The age structure of the regeneration showed that it was formed mainly during the last 2–3 decades. Regeneration number correlates with winter temperature increase, showing winter temperatures importance for regeneration survive. The DNC invasion into larch habitat is wildfire dependant. Fires promote an invasion of DNC due to better ecological conditions on the burns. On the other hand observed climate-induced fire retune interval reduction may complicate DNC invasion into larch habitat, because larch regenerates better after fire than DNC since larger seed-trees amount. The results obtained indicate DNC and birch invasion into the larch habitat and its relation to the climatic changes for the last 3 decades. At the same time larch stand crown closure and larch invasion into tundra observed in the northern forest-tundra ecotone.

Fire cycling in the larch-dominated communities

The fires return intervals and fire-danger cycling in the Siberian larch-dominated communities, with their relevance to the landscape elements were investigated. The goal of this study is to investigate wildfire cycling, and to formalize burns distribution over the different landscape elements.