Siberian Larch Forest Research Articles

Environmental drivers and remote sensing proxies of post-fire thaw depth in eastern Siberian larch forests

Abstract. Boreal fire regimes are intensifying because of climate change, and the northern parts of boreal forests are underlain by permafrost. Boreal fires combust vegetation and organic soils, which insulate permafrost, and as such deepen the seasonally thawed active layer and can lead to further carbon emissions to the atmosphere. Current understanding of the environmental drivers of post-fire thaw depth is limited but of critical importance. In addition, mapping thaw depth over fire scars may enable a better understanding of the spatial variability in post-fire responses of permafrost soils. We assessed the environmental drivers of post-fire thaw depth using field data from a fire scar in a larch-dominated forest in the continuous permafrost zone in eastern Siberia. Particularly, summer thaw depth was deeper in burned (mean=127.3 cm, standard deviation (SD) = 27.7 cm) than in unburned (98.1 cm, SD=26.9 cm) landscapes 1 year after the fire, yet the effect of fire was modulated by landscape and vegetation characteristics. We found deeper thaw in well-drained upland, in open and mature larch forest often intermixed with Scots pine, and in high-severity burns. The environmental drivers basal area, vegetation density, and burn depth explained 73.3 % of the measured thaw depth variability at the study sites. In addition, we evaluated the relationships between field-measured thaw depth and several remote sensing proxies. Albedo, the differenced normalized burn ratio (dNBR), and the pre-fire normalized difference vegetation index (NDVI) derived from Landsat 8 imagery together explained 66.3 % of the variability in field-measured thaw depth. Moreover, land surface temperature (LST) displayed particularly strong correlations with post-fire thaw depth (r=0.65, p<0.01). Based on these remote sensing proxies and multiple linear regression analysis, we estimated thaw depth over the entire fire scar. Our study reveals some of the governing processes of post-fire thaw depth development and shows the capability of Landsat imagery to estimate post-fire thaw depth at a landscape scale.

Controls over Fire Characteristics in Siberian Larch Forests

Controls over Fire Characteristics in Siberian Larch Forests

Winter soil temperature varies with canopy cover in Siberian larch forests

In the Arctic, winter soil temperatures exert strong control over mean annual soil temperature and winter CO2 emissions. In tundra ecosystems there is evidence that plant canopy influences on snow accumulation alter winter soil temperatures. By comparison, there has been relatively little research examining the impacts of heterogeneity in boreal forest cover on soil temperatures. Using seven years of data from six sites in northeastern Siberia that vary in stem density we show that snow-depth and forest canopy cover exert equally strong control on cumulative soil freezing degrees days (FDDsoil). Together snow depth and canopy cover explain approximately 75% of the variance in linear models of FDDsoil and freezing n-factors (n f; calculated as the quotient of FDDsoil and FDDair), across sites and years. Including variables related to air temperature, or antecedent soil temperatures does not substantially improve models. The observed increase in FDDsoil with canopy cover suggests that canopy interception of snow or thermal conduction through trees may be important for winter soil temperature dynamics in forested ecosystems underlain by continuous permafrost. Our results imply that changes in Siberian larch forest cover that arise from climate warming or fire regime changes may have important impacts on winter soil temperature dynamics.

Vegetation and glacier dynamics are sensitive to summer (not winter) warming and the evidence for larch refugia in the ‘Northern Pole of Cold’ inferred from sedimentary ancient DNA and geochemistry

Climate seasonality critically influences the functioning and dynamics of ecosystems in continental areas. The ecological importance of winter temperatures on high-latitude vegetation changes has recently been argued to be largely overlooked in comparison to summer temperatures. The Oymyakon region from eastern Siberia, with its strong continentality of extremely cold winters and moderately warm summers, is ideally suited to study the response of past vegetation to seasonal temperature changes based on long ecological time-series. However, few paleorecords are available from this area. The history of regional glacial activity and potential plant refugia since Marine Isotope Stage (MIS) 3 is not well understood. Here, we present geochemical and plant DNA metabarcoding records retrieved from a sediment core from Lake Ulu in the Oymyakon region, which provides detailed information on glacier and vegetation dynamics over the last 43 cal. ka BP. Our results suggest that glacial fluctuations were primarily driven by summer insolation, and Lake Ulu was likely initiated by glacial retreat during MIS 3. The catchment experienced multiple glacial advance/retreat cycles until the Last Glacial Maximum, and the glaciers fully retreated by 20 cal. ka BP. In addition, a tundra-steppe landscape dominated by Dryas, Papaver, Saliceae, and Anthemideae occupied the catchment for most of the time and began to collapse around 19 cal. ka BP following the expansion of trees and shrubs such as Larix, Betula, Alnus, and Vaccinium. Postglacial plant assemblages in the Oymyakon region exhibit a high sensitivity to summer temperature variations, with minimal impact from winter temperatures. This can be explained by the dominance of summer insolation amplitude, extreme continentality, extended plant growing season, and plant genetic adaptation to cold. Notably, our ancient DNA record show the earliest postglacial expansion of larch in eastern Siberia (around 18.6 cal. ka BP), which is likely related to the presence of local refugia. This implies that the Oymyakon region may be one of the earliest sources for larch recolonization and that more research should be implemented to provide insights into larch expansion and migration, and to better predict the future scenarios for Siberian larch forests.

Fire‐Induced Carbon Loss and Tree Mortality in Siberian Larch Forests

AbstractClimate change is intensifying the fire regime across Siberia, with the potential to alter carbon combustion and post‐fire carbon re‐accumulation trajectories. Few field‐based estimates of fire severity (e.g., carbon combustion and tree mortality) exist in Siberian larch forests (Larix spp.), which limits our ability to project how an intensified fire regime will affect regional and global climate feedbacks. Here, we present field‐based estimates of fire‐induced tree mortality and carbon loss in eastern Siberian larch forests. Our results suggest that fires in this region result in high tree mortality (means of 83% and 76% at Arctic and subarctic sites, respectively). In both absolute and relative terms, aboveground carbon loss following fire is higher in Siberian larch forests than in North America, but belowground carbon loss is considerably lower. This suggests fundamental differences in wildfire behavior and carbon dynamics between dominant vegetation types across the boreal biome.

Mycorrhizal parameters of Larix sibirica fine roots across gradients of main environmental factors and altitude

Siberian larch (Larix sibirica) forests cover the largest areas in the Eurasian boreal zone, but the knowledge on its root system and ectomycorrhizas (EMs) are insufficient. The aim of this research is to find out whether the morphological parameters of L. sibirica EMs respond to the changes in altitude and main environmental factors. We measured the EM diameter, root diameter, mantle width and volume share, share of tannin cell layers, EM density and length of L. sibirica fine roots in four main types of habitats across the altitudinal gradient in the Urals. The diversity of fungal mantle types depended on the type of habitat. The minimal diversity of fungal mantles and the prevalence of plectenchymatous mantles were found in moister and unfertile habitats of sphagnous type. In other types of habitats, the diversity of fungal mantle types was approximately twice higher. A combination of factors (habitat type and altitude) significantly determined all quantitative parameters of the Larix EMs under study. We found a reduction in the EM linear dimensions with increasing altitude and a deterioration of environmental conditions, accompanied by a decrease in their functional parameters with a slight increase in the EM density and the share of nonmycorrhizal roots.

Shrubs Compensate for Tree Leaf Area Variation and Influence Vegetation Indices in Post‐Fire Siberian Larch Forests

AbstractIn post‐fire Siberian larch forests, where tree density can vary within a burn perimeter, shrubs constitute a substantial portion of the vegetation canopy. Leaf area index (LAI), defined as the one‐sided total green leaf area per unit ground surface area, is useful for characterizing variation in plant canopies. We estimated LAI with allometry for trees and tall shrubs (>0.5 and <1.5 m) across 26 sites with varying tree stem density (0.05–3.3 stems/m2) and canopy cover (4.6%–76.9%) in a uniformly‐aged mature Siberian larch forest that regenerated following a fire ∼75 years ago. We investigated relationships between tree density, tree LAI, and tall shrub LAI, and between LAI and satellite observations of Normalized Difference and Enhanced Vegetation Indices (NDVI and EVI). Across the density gradient, tree LAI increases with increasing tree density, while tall shrub LAI decreases, exhibiting no patterns in combined tree‐shrub LAI. We also found significant positive relationships between tall shrub LAI and NDVI/EVI from PlanetScope and Landsat imagery. These findings suggest that tall shrubs compensate for lower tree LAI in tree canopy gaps, forming a canopy with contiguous combined tree‐shrub LAI across the density gradient. Our findings suggest that NDVI and EVI are more sensitive to variation in tall shrub canopies than variation in tree canopies or combined tree‐shrub canopies in these ecosystems. The results improve our understanding of the relationships between forest density and tree and shrub leaf area and have implications for interpreting spatial variability in LAI, NDVI, and EVI in Siberian boreal forests.

Allometric equations for the estimation of above- and below-ground biomass for Larix sibirica Ledeb. in Northern Mongolia

The accurate estimation of tree above-ground (AGB) and below-ground (BGB) biomass components and their root/shoot ratio play key roles in stand and country-level forest biomass and carbon stock estimation. Nevertheless, site-specific and appropriate biomass equations and root/shoot ratio are hardly available for natural larch (Larix sibirica Ledeb.) forests in Mongolia. The present study aimed (1) to develop allometric equations to estimate the above- and below-ground biomass of L. sibirica trees, and (2) to estimate the root/shoot ratio applicable for estimating the root biomass based on above-ground biomass of natural larch forests in northern Mongolia. A total of 40 trees with DBH ranging from 6.8 to 40.8 cm were sampled for tree biomass analyses. For each biomass component, we calculated the proportion of biomass allocated to different components, and also tested four allometric equations based on diameter at breast height (DBH) and height (H) as independent variables. Our results, based on measurements of oven-dried biomass, revealed that stem biomass on average accounted for 44.5% and followed by branch (28.6%) and root (19.9%) biomass, respectively. Stem and branch biomass proportions were gradually increased with increasing DBH, while a contrary trend was observed for needles. The root/shoot ratio averaged 0.25. A comparison of the allocation of root biomass by diameter fractions showed an ever-growing trend of coarse roots with an increase in stem diameter, which often exceeded more than 50% of the total root biomass. However, biomass equations, which include both DBH and H were more precise than equations that are solely based only on DBH. Consequently, among the proposed allometric regression models for estimating the AGB and BGB, the equation y = aD b H c was selected as the best-fitted equation for estimating each biomass component in Siberian larch forests. These allometric equations are available to be used for the estimation of natural larch forest biomass and carbon stocks in the Khentii Mountains of Mongolia, where extreme continental climate conditions dominate.

Vulnerability of larch forests to forest fires along a latitudinal gradient in eastern Siberia

The predicted increase in frequency and intensity of boreal forest fires is considered a significant source of carbon dioxide emissions and linked with the degradation of permafrost covering more than half of Russia. Here we analyzed the stand structure and growth of East Siberian larch forests in response to fire severity. We measured 23 sites in the southern part of eastern Siberia along a latitudinal transect with a length of more than 1500 km. Live tree volume differed significantly across geographical regions ( p < 0.05), decreasing from south to north (76–250 m3·ha−1), with higher values in forests burnt with low severity. Similarly, volume of coarse woody debris decreased from south to north. The volume of dead standing trees, on the other hand, increased from south to north. The distribution of trees by diameter class in some areas showed clear evidence of fires, with small trees being absent to rare in forests burnt at high severity. The impact of severe fires on stand volume was negligible at the southern sites, potentially associated with rapid regeneration of birch. Birch is an important component of larch forests near the southern boundary of the permafrost, which may contribute to larch forests in the southern part of the study transect being less vulnerable to wildfires compared to northern larch forests.

Changes in Forest Conditions in a Siberian Larch Forest Induced by an Extreme Wet Event

The taiga forest, a semi-arid and nitrogen-limited ecosystem on permafrost, has changed under extreme wet events. This study aims to understand the changes that occurred in a larch forest in Eastern Siberia after the wet event of 2006–2007. In the summer of 2018, studies were conducted at the Spasskaya Pad Experimental Forest Station near Yakutsk, Russia, where a transect (60 m × 510 m) with 34 plots (30 m × 30 m) was set. It included intact sites and affected sites with different levels of forest damage, owing to the extreme wet event. We observed spatial variations in the normalized difference vegetation index (NDVI) calculated from Landsat satellite-observed data, and the foliar δ13C, δ15N, and C/N (carbon/nitrogen) ratio obtained from the needle samples of 105 mature larch trees. Our results reveal that the affected plots had a lower NDVI than the intact plots, resulting from a difference in tree stand density. In addition, the stand density is suggested to be a controlling factor in the spatial variations in the foliar C/N and δ13C values based on their significant relationships with the NDVI in June. We concluded that the larch trees from the regenerating forests in the affected areas have a higher nitrogen level and light availability (relatively low C/N and high δ13C) because of the slight competition for resources, owing to a low-stand density. This may lead to further succession of the larch forests after the extreme wet event.

Spatial patterns of unburned refugia in Siberian larch forests during the exceptional 2020 fire season

AbstractAimWildfire is an essential disturbance agent that creates burn mosaics, or a patchwork of burned and unburned areas across the landscape. Unburned patches, fire refugia, serve as carbon sinks and seed sources for forest regeneration in burned areas. In the Cajander larch (Larix cajanderi Mayr.) forests of north‐eastern Siberia, an unprecedented wildfire season in 2020 and little documentation of landscape patch dynamics have resulted in research gaps about the characteristics of fire refugia in northern latitude forests, which are warming faster than other global forest ecosystems. We aim to characterize the 2010 distribution of fire refugia for these forest ecosystems and evaluate their topographic drivers.LocationNorth‐eastern Siberia across the North‐east Siberian Taiga and the Cherskii‐Kolyma Mountain Tundra ecozones.Time period2001–2020.Major taxa studiedCajander larch.MethodsWe used Landsat imagery to define burned and unburned patches, and the Arctic digital elevation model to calculate topographic variables. We characterized the size and density of fire refugia. We sampled individual pixels (n = 80,000) from an image stack that included a binary burned/unburned, elevation, slope, aspect, topographic position index, ruggedness, and tree cover from 2001 to 2020. We evaluated the topographic drivers of fire refugia with boosted regression trees.ResultsWe found no substantial difference in fire refugia size and density across the region. The fire refugia size averaged 7.2 ha (0.09–150,439 ha). The majority of interior burned patches exceed the potential wind dispersal distance from fire refugia. Topographic position index and terrain steepness were important predictors of fire refugia.Main conclusionsUnprecedented wildfires in 2020 did not impact fire refugia formation. Fire refugia are strongly controlled by topographic positions such as uplands and lowlands that influence microsite hydrological conditions. Fire refugia contribute to postfire landscape heterogeneity that preserves ecosystem functions, seed sources, habitat, and carbon sinks.

Contrasting 20-year trends in NDVI at two Siberian larch forests with and without multiyear waterlogging-induced disturbances

The fate of a boreal forest may depend on the trend in its normalized difference vegetation index (NDVI), such as whether the NDVI has been increasing significantly over the past few decades. In this study, we analyzed the responses of two Siberian larch forests at Spasskaya Pad and Elgeeii in eastern Siberia to various waterlogging-induced disturbances, using satellite-based NDVI and meteorological data for the 2000–2019 period. The forest at Spasskaya Pad experienced waterlogging (i.e. flooding events caused by abnormal precipitation) during 2005–2008 that damaged canopy-forming larch trees and increased the abundance of water-resistant understory vegetation. By contrast, the forest at Elgeeii did not experience any remarkable disturbance, such as tree dieback or changes in the vegetation community. Significant increasing NDVI trends were found in May and June–August at Elgeeii (p < 0.05), whereas no significant trends were found at Spasskaya Pad (p > 0.05). NDVI anomalies in May and June–August at Elgeeii were significantly associated with precipitation or temperature depending on the season (p < 0.05), whereas no significant relationships were found at Spasskaya Pad (p > 0.05). Thus, the 20 year NDVI trend and NDVI–temperature–precipitation relationship differed between the two larch forests, although no significant trends in temperature or precipitation were observed. These findings indicate that nonsignificant NDVI trends for Siberian larch forests may reflect waterlogging-induced dieback of larch trees, with a concomitant increase in water-resistant understory vegetation.

Evaluating the Differenced Normalized Burn Ratio for Assessing Fire Severity Using Sentinel-2 Imagery in Northeast Siberian Larch Forests

Fire severity is a key fire regime characteristic with high ecological and carbon cycle relevance. Prior studies on boreal forest fires primarily focused on mapping severity in North American boreal forests. However, the dominant tree species and their impacts on fire regimes are different between North American and Siberian boreal forests. Here, we used Sentinel-2 satellite imagery to test the potential for using the most common spectral index for assessing fire severity, the differenced Normalized Burn Ratio (dNBR), over two fire scars and 37 field plots in Northeast Siberian larch-dominated (Larix cajanderi) forests. These field plots were sampled into two different forest types: (1) dense young stands and (2) open mature stands. For this evaluation, the dNBR was compared to field measurements of the Geometrically structured Composite Burn Index (GeoCBI) and burn depth. We found a linear relationship between dNBR and GeoCBI using data from all forest types (R2 = 0.42, p &lt; 0.001). The dNBR performed better to predict GeoCBI in open mature larch plots (R2 = 0.56, p &lt; 0.001). The GeoCBI provides a holistic field assessment of fire severity yet is dominated by the effect of fire on vegetation. No significant relationships were found between GeoCBI components (overall and substrate stratum) and burn depth within our fires (p &gt; 0.05 in all cases). However, the dNBR showed some potential as a predictor for burn depth, especially in the dense larch forests (R2 = 0.63, p &lt; 0.001). In line with previous studies in boreal North America, the dNBR correlated reasonably well with field data of aboveground fire severity and showed some skills as a predictor of burn depth. More research is needed to refine spaceborne fire severity assessments in the larch forests of Northeast Siberia, including assessments of additional fire scars and integration of dNBR with other geospatial proxies of fire severity.

Modelling the productivity of Siberian larch forests from Landsat NDVI time series in fragmented forest stands of the Mongolian forest-steppe

The monitoring of the spatial and temporal dynamics of vegetation productivity is important in the context of carbon sequestration by terrestrial ecosystems from the atmosphere. The accessibility of the full archive of medium-resolution earth observation data for multiple decades dramatically improved the potential of remote sensing to support global climate change and terrestrial carbon cycle studies. We investigated a dense time series of multi-sensor Landsat Normalized Difference Vegetation Index (NDVI) data at the southern fringe of the boreal forests in the Mongolian forest-steppe with regard to the ability to capture the annual variability in radial stemwood increment and thus forest productivity. Forest productivity was assessed from dendrochronological series of Siberian larch (Larix sibirica) from 15 plots in forest patches of different ages and stand sizes. The results revealed a strong correlation between the maximum growing season NDVI of forest sites and tree ring width over an observation period of 20 years. This relationship was independent of the forest stand size and of the landscape’s forest-to-grassland ratio. We conclude from the consistent findings of our case study that the maximum growing season NDVI can be used for retrospective modelling of forest productivity over larger areas. The usefulness of grassland NDVI as a proxy for forest NDVI to monitor forest productivity in semi-arid areas could only partially be confirmed. Spatial and temporal inconsistencies between forest and grassland NDVI are a consequence of different physiological and ecological vegetation properties. Due to coarse spatial resolution of available satellite data, previous studies were not able to account for small-scaled land-cover patches like fragmented forest in the forest-steppe. Landsat satellite-time series were able to separate those effects and thus may contribute to a better understanding of the impact of global climate change on natural ecosystems.

Сравнительная оценка таксационных показателей лесных культур лиственницы сибирской (Larix sibirica) при интродукции в нижегородской области

The practice of using Siberian larch in artificial reforestation on the territory of the Nizhny Novgorod region in the forests of the Bogorodsk district is considered. It should be noted that the relevance of the development of forest plantations with the diversification of their species composition is due to the prospects for the expansion of the forest industry, which is determined by the State Program “Forestry Sector Development in the Russian Federation”. At the same time, despite of a wide range of possibilities of economic use of Siberian larch in the Nizhny Novgorod region, it is practically not used here when developing artificial stands for various purposes and structures. Scots pine is mainly used in the development of forest plantations in the Nizhny Novgorod region. Although the positive effect of the introduction of Siberian larch into the stand composition is confirmed by a comparative assessment of valuation indicators in forest plantations on the territory of the Bogorodsk district forestry. Based on the research results, 3 plots with the same types of forest site conditions were examined. High-quality Siberian larch stands with various species composition were formed on the plots. The work was carried out by the field methods; the principle of single logical distinction was their methodological framework. The characteristics of the trial plots were given on the basis of an on-site survey. All research results were statistically significant. A relationship has been established between the species composition and the stock of these stands. In larch plantations mixed in species composition, a larger stock has been accumulated than in pure stands. Also, the study revealed a relationship between the age of stands and the variability of the tested valuation indicators. The research results confirmed the effectiveness of the development of Siberian larch forest plantations in the Nizhny Novgorod region and the acceptability of using the forestry fund areas with forest site conditions of category C2 for these purposes. The obtained results will be useful in development of Siberian larch plantations in the Middle Volga region, since it has been revealed that the plantations in the Bogorodsk district of the Nizhny Novgorod region achieve the high productiveness at the age of 54.

Simulating interactions between topography, permafrost, and vegetation in Siberian larch forest

In eastern Siberia, topography controls the abundance of the larch forest via both drought and flooding stresses. For the reconstruction of these topographical effects, we modified a dynamic vegetation model to represent soil water relocation owing to within-grid heterogeneity of elevation, over-wet-kill of trees, and air temperature differences within-grid. After calibration, the model reasonably reconstructed the geographical distributions of observation-based-estimates of fundamental properties of plant productivity and thermo-hydrology. Thus, the model appropriately responded to environmental gradients in eastern Siberia. The modified model also partially reconstructed the topography control on tree abundance and thermo-hydrology status in eastern Siberia, although its geographical distribution was not always good. In the modified model, soil water redistribution increased the risk of over-wet-kill in lower elevation classes, whereas it reduced the risk of over-wet-kill for larch trees in higher elevation classes. We demonstrated that without considering the latter effect, forest collapse due to over-wet stress would happen throughout eastern Siberia under a forecasted climatic condition during the 21st century, which will deliver a much moister environment throughout eastern Siberia. Therefore, modeling the over-wet-kill of trees without considering topographical heterogeneity would result in the overestimation of forest collapse caused by the over-wet-kill of trees under an expected climate trend in eastern Siberia.

Relationships between low-temperature fires, climate and vegetation during three late glacials and interglacials of the last 430 kyr in northeastern Siberia reconstructed from monosaccharide anhydrides in Lake El'gygytgyn sediments

Abstract. Landscapes in high northern latitudes are assumed to be highly sensitive to future global change, but the rates and long-term trajectories of changes are rather uncertain. In the boreal zone, fires are an important factor in climate–vegetation interactions and biogeochemical cycles. Fire regimes are characterized by small, frequent, low-intensity fires within summergreen boreal forests dominated by larch, whereas evergreen boreal forests dominated by spruce and pine burn large areas less frequently but at higher intensities. Here, we explore the potential of the monosaccharide anhydrides (MA) levoglucosan, mannosan and galactosan to serve as proxies of low-intensity biomass burning in glacial-to-interglacial lake sediments from the high northern latitudes. We use sediments from Lake El'gygytgyn (cores PG 1351 and ICDP 5011-1), located in the far north-east of Russia, and study glacial and interglacial samples of the last 430 kyr (marine isotope stages 5e, 6, 7e, 8, 11c and 12) that had different climate and biome configurations. Combined with pollen and non-pollen palynomorph records from the same samples, we assess how far the modern relationships between fire, climate and vegetation persisted during the past, on orbital to centennial timescales. We find that MAs attached to particulates were well-preserved in up to 430 kyr old sediments with higher influxes from low-intensity biomass burning in interglacials compared to glacials. MA influxes significantly increase when summergreen boreal forest spreads closer to the lake, whereas they decrease when tundra-steppe environments and, especially, Sphagnum peatlands spread. This suggests that low-temperature fires are a typical characteristic of Siberian larch forests also on long timescales. The results also suggest that low-intensity fires would be reduced by vegetation shifts towards very dry environments due to reduced biomass availability, as well as by shifts towards peatlands, which limits fuel dryness. In addition, we observed very low MA ratios, which we interpret as high contributions of galactosan and mannosan from biomass sources other than those currently monitored, such as the moss–lichen mats in the understorey of the summergreen boreal forest. Overall, sedimentary MAs can provide a powerful proxy for fire regime reconstructions and extend our knowledge of long-term natural fire–climate–vegetation feedbacks in the high northern latitudes.

Effects of extreme drought and wet events for tree mortality: Insights from tree‐ring width and carbon isotope ratio in a Siberian larch forest

AbstractRecent studies suggest that forest vulnerability to tree mortality has increased as a result of extreme climate events such as severe drought and heavy rain. However, little is known about the mortality processes, particularly when they are induced by heavy rain. A dendroecological analysis was conducted on living and dead trees in a Siberian larch forest using the chronologies of 30 years of radial growth and carbon isotope (Δ13C) discrimination. The analysis demonstrates the relationship between heavy rain and larch tree mortality. Radial growth and Δ13C patterns in dead trees at a wet subsite diverged from those of living trees only during the heavy rain period in 2005–2007. On the other hand, the radial growth and Δ13C patterns in dead trees at the dry subsite diverged from those of living trees after severe drought events in 1998 and in 2002–2003. At the wet subsite, the heavy rain was intense enough to push the larch trees past their mortality threshold, whereas previous drought conditions also contributed to tree mortality at the dry subsite. Our results demonstrated that the process of larch tree mortality differs with site‐specific topographical soil moisture conditions, even when synchronous tree death occurs in a specific year of climatic extremes.

Strong and stable relationships between tree-ring parameters and forest-level carbon fluxes in a Siberian larch forest

The tree-ring width index (RWI) and satellite-derived vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), are used as long-term indicators of the past forest carbon uptake. However, fundamental questions remain with respect to what is represented by the RWI and NDVI at the ecosystem level. To address this question, we compared tree-ring parameters (RWI and the carbon isotope ratio: δ13C) and NDVI products with forest ecosystem CO2 fluxes estimated using the eddy covariance method, at a larch forest in eastern Siberia. The RWI and tree-ring δ13C correlated well with the ecosystem gross primary production (GPP), and their temporal stabilities were high during 2004–2014. However, the NDVI products did not show any temporally stable relationship with the GPP. This could be ascribed to significant changes in the understory vegetation in this forest, i.e., from dense cowberry to shrubs and moisture-tolerant grasses, because of an excessively moist environment during 2007–2008. Changes in the understory vegetation could be reflected by the NDVI products but not by the GPP. Our results indicate that it is more feasible to study forest carbon uptake using tree-ring parameters than using satellite-derived vegetation indices in such larch-dominated forest ecosystems in eastern Siberia.

Vegetation Indices Do Not Capture Forest Cover Variation in Upland Siberian Larch Forests

Boreal forests are changing in response to climate, with potentially important feedbacks to regional and global climate through altered carbon cycle and albedo dynamics. These feedback processes will be affected by vegetation changes, and feedback strengths will largely rely on the spatial extent and timing of vegetation change. Satellite remote sensing is widely used to monitor vegetation dynamics, and vegetation indices (VIs) are frequently used to characterize spatial and temporal trends in vegetation productivity. In this study we combine field observations of larch forest cover across a 25 km2 upland landscape in northeastern Siberia with high-resolution satellite observations to determine how the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) are related to forest cover. Across 46 forest stands ranging from 0% to 90% larch canopy cover, we find either no change, or declines in NDVI and EVI derived from PlanetScope CubeSat and Landsat data with increasing forest cover. In conjunction with field observations of NDVI, these results indicate that understory vegetation likely exerts a strong influence on vegetation indices in these ecosystems. This suggests that positive decadal trends in NDVI in Siberian larch forests may correspond primarily to increases in understory productivity, or even to declines in forest cover. Consequently, positive NDVI trends may be associated with declines in terrestrial carbon storage and increases in albedo, rather than increases in carbon storage and decreases in albedo that are commonly assumed. Moreover, it is also likely that important ecological changes such as large changes in forest density or variable forest regrowth after fire are not captured by long-term NDVI trends.