Sergey T. Im

Forest-tundra larch forests and climatic trends

Climate-related changes that occurred in the Ary-Mas larch forests (the world’s northernmost forest range) in the last three decades of the 20th century have been analyzed. An analysis of remote-sensing images made by Landsat satellites in 1973 and 2000 has provided evidence for an increase in the closeness of larch forest canopy (by 65%) and the expansion of larch to the tundra (for 3–10 m per year) and to areas relatively poorly protected from wind due to topographic features (elevation, azimuth, and slope). It has also been shown that the radial tree increment correlates with summer temperatures (r = 0.65, τ = 0.39) and the amounts of precipitation in summer (r = −0.51, τ = 0–41) and winter (r = −0.70, τ = −0.48), decreases with an increase in the closeness of forest canopy (r = −0.52, p > 0.8; τ = −0.48, p > 0.95), and increases with an increase in the depth of soil thawing (r = 0.63, p > 0.9; τ = 0.46, p > 0.9). The density of undergrowth depends on temperatures in winter (τ = 0.53, p > 0.8) and summer (r = 0.98, p > 0.99, τ = 0.9, p > 0.99) and the date of the onset of the growing period (r = −0.60, p > 0.99; τ = −0.4, p > 0.99) and negatively correlates with the amount of precipitation in summer (r = −0.56, p > 0.99, τ = −0.38, p > 0.99).

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.

Response of Pinus sibirica and Larix sibirica to climate change in southern Siberian alpine forest–tundra ecotone

Abstract A warming climate provides competitive advantages to Siberian pine (Pinus sibirica Du Tour) in areas with sufficient precipitation. The warmer temperatures observed in central Siberia over the past three decades appear to have had a noticeable effect on growth of Siberian pine and larch (Larix sibirica Ledeb.) in the south Siberian Mountain forest–tundra ecotone. Larch is more tolerant of harsh climates and exhibits an arboreal growth form, whereas Siberian pine is in krummholz form. Larch also has an advantage at the upper tree limit and in areas with low precipitation. Since the mid-1980s there have been measurable increases in growth increments, stand densification, regeneration propagation into the alpine tundra and transformation of krummholz into arboreal forms. Warming winter temperatures have been sufficient for increased survival of regeneration. Regeneration responded to temperature increase of 1°C by migration to areas 10–40 m higher in elevation. Regeneration has propagated into the alpine tundra at the rate of ~1.0–2.0 m year−1. Siberian pine and larch regeneration surpassed their upper historical limit by 10–80 m in elevation. While increased tree growth and migration into alpine tundra areas affect the regional carbon balance, it will also decrease albedo, which may increase warming at the regional level.

Siberian silkmoth outbreak pattern analysis based on SPOT VEGETATION data

The spatial pattern of Siberian silkmoth outbreak in south Siberian mountains was analysed based on SPOT VEGETATION data. A digital elevation model (DEM) was also used to relate outbreak area dynamics with topographic elements (elevation, azimuth and slope steepness). To avoid bias of spatial pattern data, areas with a given damage category and with given azimuth, slope steepness and elevation were referenced to the areas with similar parameters within the entire study area. The outbreak began between the elevations of ∼430–480 m and on south‐west slopes with steepness <5°; these conditions appear to be the most favourable pest habitat. As the pest searched for food it moved up and down slope, resulting in an elevation distribution split within a range of ∼390–540 m and slope steepness up to 15°. In the final phase the azimuth distribution of damaged stands became even, showing that pests at this phase settle in non‐optimal habitat. The final outbreak area was ∼20 000 ha, which is in good agreement with on‐ground data. The correlation between the initial phase of infestation and topographic features can be used to prioritize pest monitoring. Data obtained show that the SPOT VEGETATION sensor is applicable for monitoring taiga landscapes vulnerable to Siberian silkmoth outbreaks.

Climate-induced mountain tree-line evolution in southern Siberia

Abstract The elevational tree-line change within the transitional zone between boreal forest and Mongolian steppes was quantified for the last millennium. The basic approach included studies along transects and measurements of tree-line positions to identify current, historical, refugee and regeneration tree lines. Tree mortality and natality were determined based on dendrochronology analysis. Tree mortality in the sixteenth to eighteenth centuries coincided with the Little Ice Age, while tree establishment was stimulated by warming at the end of nineteenth century. Downward shifts in tree line varied by an order of magnitude. The current tree-line position reoccupied the historical tree line in some transects, and was below or above the historical line in others. The regeneration line surpassed the historical tree line by 91±46 m (mean± SD). Such a heterogeneous response was attributed to local topoclimatic conditions and sapling recruitment efficiency. A mean annual 1°C increase in temperature was associated with an upward shift of the tree line by about 70 m. The upward migration rate of the current tree line was about 0.8 m year−1 during the last century. The regeneration migration rate was about 2.3 m year−1 over the past three decades. Finally, the transformation of krummholz forms of larch and Siberian pine into arborescent form was documented.

Tree-Line Structure and Dynamics at the Northern Limit of the Larch Forest: Anabar Plateau, Siberia, Russia

Abstract The goal of the study was to provide an analysis of climate impact before, during, and after the Little Ice Age (LIA) on the larch (Larix gmelinii) tree line at the northern extreme of Siberian forests. Recent decadal climate change impacts on the tree line, regeneration abundance, and age structure were analyzed. The location of the study area was within the forest-tundra ecotone (elevation range 170–450 m) in the Anabar Plateau, northern Siberia. Field studies were conducted along elevational transects. Tree natality/mortality and radial increment were determined based on dendrochronology analyses. Tree morphology, number of living and subfossil trees, regeneration abundance, and age structure were studied. Locations of pre-LIA, LIA, and post-LIA tree lines and refugia boundaries were established. Long-term climate variables and drought index were included in the analysis. It was found that tree mortality from the 16th century through the beginning of the 19th century caused a downward tree line recession. Sparse larch stands experienced deforestation, transforming into tundra with isolated relict trees. The maximum tree mortality and radial growth decrease were observed to have occurred at the beginning of 18th century. Now larch, at its northern boundary in Siberia, is migrating into tundra areas. Upward tree migration was induced by warming in the middle of the 19th century. Refugia played an important role in repopulation of the forest-tundra ecotone by providing a seed source and shelter for recruitment of larch regeneration. Currently this ecotone is being repopulated mainly by tree cohorts that were established after the 1930s. The last two decades of warming did not result in an acceleration of regeneration recruitment because of increased drought conditions. The regeneration line reached (but did not exceed) the pre-LIA tree line location, although contemporary tree heights and stand densities are comparatively lower than in the pre-LIA period. The mean rate of tree line upward migration has been about 0.35 m yr-1 (with a range of 0.21–0.58), which translates to a tree line response to temperature of about 55 m °C-1.

Forest–tundra ecotone response to climate change in the Western Sayan Mountains, Siberia

Abstract Tree response to climate trends is most likely to be observed in the forest–tundra ecotone, where mainly temperature limits tree growth. On-ground observation and multitemporal Landsat data were used in the analysis of forest–tundra ecotone dynamics (from 1976 to 2000) in the Western Sayan Mountains, Siberia. Observations showed an increase in forest stand crown closure, upward tree-line and regeneration shift and the transformation of Siberian pine and fir krummholz into arboreal forms. Closed stands were increasing in the area at a rate of 0.8% year−1 and advancing their upper boundary at an altitudinal rate of 0.6 m year−1; these changes were shown mainly by the transformation of sparse stands into closed stands. The altitudinal rate of regeneration propagation was estimated at 1.2 m year−1. It was also found that these changes correlated positively with temperature trends. The response of tree vegetation to air temperature increase was dependent on topographic relief features (azimuth and slope steepness).

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.

Characterization and Monitoring of Tundra-Taiga Transition Zone with Multi-sensor Satellite Data

Monitoring the dynamics of the circumpolar boreal forest (taiga) and Arctic tundra boundary is important for understanding the causes and consequences of changes observed in these areas. Because of the inaccessibility and large extent of this zone, remote sensing data can play an important role for the purposes. In this study, climate-related changes that occurred in the Ary-Mas larch forests (the world’s northernmost forest range) in the last three decades of the twentieth century were analyzed. An analysis of Landsat images in 1973 and 2000 has provided evidence for an increase in the closeness of larch forest canopy by 65% and the expansion of larch to the tundra for 3–10 m per year and to areas relatively poorly protected from wind due to topographic features (elevation, azimuth, and slope). It was found that a tundra-taiga transitional area can be characterized using multi-spectral Landsat ETM+ summer images, multi-angle MISR red band reflectance images, RADARSAT images with larger incidence angle, or multi-temporal and multi-spectral MODIS data. Because of different resolutions and spectral regions covered, the transition zone maps derived from different data types were not identical, but the general patterns were consistent.

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.