V.I. Kharuk

Radiometric slope correction for forest biomass estimation from SAR data in the Western Sayani Mountains, Siberia

We investigated the possibility of using multiple polarization (SIR-C) L-band data to map forest biomass in a mountainous area in Siberia. The use of a digital elevation model (DEM) and a model-based method for reducing terrain effects was evaluated. We found that the available DEM data were not suitable to correct the topographic effects on the SIR-C radar images. A model-based slope correction was applied to an L-band cross-polarized (hv) backscattering image and found to reduce the topographic effect. A map of aboveground biomass was produced from the corrected image. The results indicated that multipolarization L-band synthetic aperture radar (SAR) data can be useful for estimation of total aboveground biomass of forest stands in mountainous areas.

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.

Landcover attributes from ICESat GLAS data in Central Siberia

NASAs ICESat Geoscience Laser Altimeter System (GLAS) was launched in January 2003 and collected lidar data during February and September of that year. Lidar is a laser altimeter that measures the distance from the instrument to the surface by measuring the time elapsed between the pulse emission and the reflected return. The returned signal may identify multiple returns originating from trees, buildings and other objects and permits the calculation of their height. Sampling the returns at discrete time intervals enables backscatter profiles to be constructed. Lidar data can provide estimates of other structural parameters such as biomass, stand volume and leaf area. This study used GLAS data acquired over our study sites in central Siberia to examine the signal as a source of information of forest stand characteristics. Example lidar profiles are presented and preliminary analysis is described. The results indicate that GLAS profile information may be useful for understanding MODIS landcover classes

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.

Characterization of Forests in Western Sayani Mountains, Siberia from SIR-C SAR Data

Abstract This paper examines the use of space-borne radar data to map forest types and logging in the mountainous Western Sayani area in central Siberia. L- and C- band HH-, HV-, and VV-polarized images from the Shuttle Imaging Radar–C instrument were used in the study. Techniques to reduce topographic effects in the radar images were investigated. These included radiometric correction using illumination angle inferred from a digital elevation model and reducing apparent effects of topography through band ratios. Forest classification was performed after terrain correction utilizing typical supervised techniques and principal component analyses. An ancillary data set of local elevations was also used to improve the forest classification. Map accuracy for each technique was estimated for training sites based on Russian forestry maps, satellite imagery, and field measurements. The results indicate that it is necessary to correct for topography when attempting to classify forests in mountainous terrain. Radiometric correction based on a digital elevation model improved classification results but required reducing the synthetic aperture radar resolution to match the digital elevation model. Using ratios of synthetic aperture radar channels that include cross-polarization improved classification and had the advantages of eliminating the need for a digital elevation model and preserving the full resolution of the synthetic aperture radar data.

Using MODIS and GLAS data to develop timber volume estimates in central Siberia

Mapping of boreal forests type, structure parameters and biomass are critical for understanding the boreal forests significance in the carbon cycle, its response to and impact on global climate change. The biggest deficiency of the existing ground based forest inventories is the uncertainty in the inventory data, particularly in remote areas of Siberia where sampling is sparse, lacking, and often decades old. Remote sensing methods can overcome these problems. In this study, we used the moderate resolution imaging spectroradiometer (MODIS) and unique waveform data of the geoscience laser altimeter system (GIAS) and produced a map of timber volume for a 10degx12deg area in Central Siberia. Using these methods, the mean timber volume for the forested area in the total study area was 203 m3/ ha. The new remote sensing methods used in this study provide a truly independent estimate of forest structure, which is not dependent on traditional ground forest inventory methods.

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.

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).