The southern Siberian mountains stretch across 4.5 thousand km in the center of Eurasia and reach elevations of 4500 m. The mountains serve to intercept moisture-bearing air masses that create favorable conditions for growing of water-loving and shade-tolerant coniferous tree species, such as the Siberian pine (Pinus sibirica Du Tour) and Siberian fir (Abies sibirica Ledeb.). Damage and decline of these forests were observed at middle-to-high elevations from the1960s, which was expressed by necroses of branches with generative organs, by decreases in linear and radial growth, by chloroses, necroses and premature needle falls, and stem rot. These forests have been slowly declining from 1960-2000. From the 1990s to the mid-2000s, their dieback became catastrophic. The decline/dieback could be caused by anthropogenic climate-induced change (e.g., aridization) or environmental pollution (e.g., acid rain), followed by secondary biotic impacts such as disease and infestations on weakened trees. The goal is to evaluate whether climate-induced drought is significant enough to result in the decline and/or dieback of dark-needled conifers; and to determine if CMIP5 climates would be dry enough to cause decline/dieback of these forests by 2100.

Field examinations of dark-needled forest show damage occurred at middle-to-high elevations between 800-1200m on windward moist slopes. Climate observations for 1960-2019 demonstrate that at these sites moisture conditions have not changed much. Annual moisture index (AMI) at these elevations varied between 0.2-0.8 indicating very wet conditions. The high AMI limit for the dark conifers is 2.25. Drought index (SPEI) values <-1.5 characteristic of extreme droughts occurred only 3-4 times for 1961-2019. Trees survive occasional 1-2 drought years and then successfully recover. Thus, moisture conditions have remained sufficient to sustain Siberian pine and fir. This is also true for the extreme dry CMIP5 2100 scenario. We conclude the observed decline/dieback of dark conifers at middle-to-highlands across the southern Siberian Mts was not caused by drought stress.

The reported study was funded by RFBR, project number 20-05-00540.

Siberian pine (Pinus sibirica Du Tour) and fir (Abies sibirica Ledeb.) stands mortality in the Central Siberia has increased greatly in recent decades. We investigated these species decline and mortality based on satellite (MODIS, Landsat) time series and climatic variables [precipitation, thermal degree days (TDD = ∑(t > 0 °C), drought index SPEI and root zone moisture content (RZM)], topo features (elevation, slope steepness and exposure) and in situ data. Dendrochronology applied for analysis of the radial growth increment (GI).

The annual mortality rates were c. 0.4%/year for Siberian pine and c. 0.2%/year for fir stands (period covered: 2001–2018). The mortality of forest stands and trees was increasing with TDD and drought increase and decreasing RZM increase. Both species mortality occurred mainly within the southern part of the species ranges and decreased northward, correlated with latitudinal gradients of TDD and SPEI.

Mortality observed mostly at elevations < 1,000 m and decreased with increasing elevation, whereas the area of forests and GI of trees increased with elevation. In mountains, mortality observed primarily on south-facing slopes and it was increasing with slope steepness increase.

Siberian pine and fir forests mortality preceded by a reduction in the GI caused by elevated air temperatures, acute droughts and subsequent insect attacks. Since the onset of climate warming, GI increased until a breakpoint in the mid-1980s. Further temperature increase caused a reduction in GI owing to moisture stress and division of the tree population into “decliners” and “survivors”. Mortality caused by the combined impact of moisture stress and bark beetle attacks.

With the projected increase in drought, Siberian pine and fir trees are predicted to retreat from their southern low-elevation ranges and would be substituted by drought-tolerant species, such as Scots pine and larch, which still did not experience mortality under the warming Siberian climate.

Excessively high temperatures and droughts after winter dormancy breaking can affect the growth and mortality of seedlings. An open-field experiment was conducted to understand the growth and mortality of Larix kaempferi seedlings to spring warming and drought treatments, and further to explore if seedlings could recover the growth capability when the treatment ceased. One-year-old seedlings were subjected to two temperature levels (ambient temperature and infrared heater warming of 4 °C compared to ambient temperature) and two precipitation levels (ambient precipitation and drought) for four weeks. Warming and drought treatments decreased the height and root collar diameter of seedlings throughout the period. After the cessation of treatments, mortality rates continued to increase in the drought treatment until the end of the growing season in November; the combination of warming and drought treatments had the highest mortality rates, followed by drought treatment, control, and warming treatment. However, accumulation of leaf, shoot, and the total biomass was greater in the combination of warming and drought treatments and further increased the height and root collar diameter of seedlings. This indicates that the reduced number of seedlings per plot due to the increased mortality may reduce the negative effects of warming and drought on seedling growth through alleviating resource competition among seedlings. This study shows that under the warmer and drier conditions the growth of Larix kaempferi seedlings could decline, and that such effects are likely to be mitigated by the decreased density due to the increased mortality rates.

Forest advance into low-profile vegetation biomes (i.e. tundra and grasslands) is expected to increase under a warming climate, impacting several ecosystem processes. Expansion depends on successful establishment, but also requires conditions that promote reproductive potential. Periods of high tree establishment are often assumed to align with periods of high vegetative growth, but some studies suggest reduced tree performance during periods of high seed production and also suggest that climatic conditions fostering seedling survival may not necessarily be conducive to growth in mature trees. We compared the timing of establishment pulses of two contrasting tree species (white spruce and trembling aspen) into grassland openings in southwest Yukon Territory with the radial growth of their respective source forests over the past century. We also consider potential time lags within these processes. White spruce, a seed-bearing conifer, revealed no or a slightly negative connection between growth and pulses of high establishment. Trembling aspen, an often clonal deciduous tree, revealed negative correlations with annual growth at the time of establishment, but positive correlations in periods prior and subsequent to these pulses. We found that the climatic drivers of growth and establishment are different for both species, but that the Pacific Decadal Oscillation is an important influence of regional climate variability. These results suggest we must consider establishment and growth independently when forecasting forest change and that increased growth based on ring width of mature individuals will not necessarily translate into establishment of new individuals leading to forest advance.

The ‘Rate of living theory’ (Speakman, 2005) suggests that as an organism reaches its maximal size it also approaches its life limit. In the last decade, several publications have evaluated the similar “Grow fast -- die young” hypothesis based on tree-ring data, usually based on sampling in extreme alpine habitats of low site index. Conclusions drawn from those studies suggest that the carbon sequestration of forests decreases as the longevity of individual trees decreases as climate warms due to a greater growth rate and more rapid achievement of maximum size. We focus on Larix sibirica and challenge those conclusions using inventory data of Siberian larch forests of southern Siberian mountains.

Based on these data, it is clear that both tree growth rate and maximum height increase as site quality increases. This is a normal expectation in forest mensuration. Tree growth regression models are applicable only within the limits of conditions in which the models were constructed. It is also inappropriate to apply inferences from separate trees to forest stands in more productive habitats, such as those of many Siberian larch forests. The results of these studies, therefore, are not sufficient to conclude that the vast global forests stretching across diverse environments will have limited capacity to sequester carbon in a warming climate. Potential tree growth in favorable warmer climates can never be extrapolated from data on carbon sequestration by slow-growing trees and stands in extreme habitats. Stating that fast-growing trees sequester less carbon, the above studies do not address the consequences or management applications of

their findings: should afforestation at alpine tree line be promoted to sequester carbon? Should reforestation of regularly logged lowland fast-growing forests be stopped if they do not sequester carbon? What are the logical implications of their results for understanding effects of warming climate on carbon sequestration?

The reported study was funded by RFBR, project number 20-05-00540.

Black and white spruce are largely sympatric, suggesting both species have similar climate requirements. The two species, however, are segregated across the landscape with black spruce most common on cold, nutrient poor sites and white spruce more common on warm, productive sites. Because site conditions influence climate-growth responses, it is difficult to compare white and black spruce climate-growth responses as these responses are confounded by the differences in site conditions in which the two species naturally occur. As the climate warms, understanding how a changing climate and associated changes in permafrost and fire regimes will interact to shape future species composition in the boreal forest is critical. We examined the climate-growth responses of black and white spruce growing in the same sites. This approach eliminated the confounding factor of site conditions and facilitates comparison of the two species. We included standardized thaw depth of the active layer in our analysis as a representation of permafrost, which is a key factor delineating these two species’ habitat preferences and is actively warming and thawing. We found the climate growth-responses of the two species, but especially white spruce, hinged on the thaw depth. Specifically, with increasing June-July temperatures white spruce radial growth increased in areas with no near surface permafrost, but strongly decreased when growing in areas with near surface permafrost. Black spruce radial growth was less sensitive to June-July temperature than white spruce but more sensitive to summer precipitation. These findings point to a primary mechanism potentially driving the positioning of these species within the landscapes of boreal interior Alaska and imply thawing of permafrost may foster expansion of white spruce in this region at the expense of black spruce, but that in a wetter climate black spruce may gain competitive advantage over white spruce in some landscape positions.

Plot-based studies of post-disturbance forest recovery are typically limited by sample size, and spatial and temporal extent, precluding a comprehensive analysis of recovery across a range of site types, forest types, and disturbance magnitudes. Spectral recovery, as measured using a time series of optical satellite data, provide quantitative insights on the return of vegetation following disturbance. Satellite-derived spectral recovery assessments are therefore useful as they provide a consistent, large-area, spatially explicit framework for assessing recovery that can be integrated with field plots and other sources of reference data such as airborne laser scanning (ALS) data. Our research generated a national assessment of recovery following wildfire and harvest using Landsat time series for Canada’s forested ecosystems. Subsequent research has used measures of forest height and cover derived from ALS data as well as field plot data. Results corroborate the use of spectral recovery metrics to provide a spatially exhaustive and retrospective assessment of recovery, and to provide baseline data on the potential for natural regeneration at a given location. Results of recent spatially-explicit analyses that highlight areas with slower rates of spectral recovery relative to a regionally-specific baseline will be presented.

Investigation of relationships between biomass of different plant organs (compartments) is important, because their varying turnover rates affect the amount and composition of aboveground and belowground litter, which has a direct effect on nutrient cycling and carbon accumulation in soil. A study of biomass partitioning among organs of the bilberry (Vaccinium myrtillus L.), a common clonal dwarf shrub in boreal coniferous forests, was carried out at eleven study sites in Finland. We hypothesize that the biomass partitioning of bilberry follows a common pattern across a variety of edaphic conditions and geographic range, and this pattern can be modelled by the rank distribution of organ biomasses. We found that the highest proportion of the total biomass belonged to rhizomes, which were followed by shoot stems, fine roots and leaves. Furthermore, this pattern was observed regardless of the latitudinal zone (northern or southern Finland), site fertility level and dominant tree species (Scots pine or Norway spruce) forming the canopy. However, the exact ratios between the biomass compartments varied between different sites. In particular, we found that the ratio between above- and belowground biomass was dependent on the C/N ratio of the soil organic layer (exponential model, R2 = 0.443), showing that bilberry allocates more biomass to belowground parts in the nutrient-poor than fertile sites. The results suggested that there are uniform regularities in the structural organization within plant species representing the clonal growth form of woody dwarf shrubs. This observation enables us to use the rank distributions of organs in modelling biomass partitioning of bilberry in a wide range of environmental conditions, although the mechanisms underlying such regularities requires further investigation. The research was supported by the Academy of Finland, projects 322972, 312912, and EU Life+ FutMon project, and the Russian Science Foundation, project #18-14-00362.

Climate change is driving large changes in forest ecosystems around the globe. It is likely that increases in temperature due to climate change will drive significant changes to forest productivity, mortality and recruitment in the boreal biome. Some of these changes will occur rapidly while others are expected to unfold gradually. For example, tree-level productivity can decline decades before mortality and associated landscape-level changes occur. The ability to predict these changes would be invaluable to both researchers and land managers, as it would allow time to address or mitigate impending changes to forests and their ecosystem services. In this study, we present a modeling framework to predict future changes in forest biomass at sites across Canada and Alaska. We integrate several long-term satellite vegetation indices along with soil characteristics, long-term climate variables and species composition. A range of machine learning methods including Random forest and extreme gradient boosted regression models were trained on over 10,000 forest inventory plots with repeat measurements covering a wide range of forest types across boreal North America. We present results of our predictive models, cross-validated using a withheld subset of the forest inventory plots. Using these models, we investigate the limits of prediction of future biomass change. The models developed by this study has the potential to improve our understanding of long-term and large-scale drivers and changes in boreal forest biomass as well as to provide scientific and management communities with a novel tool for monitoring.

Climate and acute or chronic drought stress are factors leading to the weakening of forest ecosystems; the same factors are also driving extensive bark beetle infestations. Siberian spruce (Pícea obováta) forests of the Dvinsko-Pinejsky reserve in the Arkhangelsk region, Russia have been subject to unprecedented tree cover loss caused by Eurasian spruce bark beetle (Ips typographus L.) attacks in the last two decades. This is the first recorded case, when such an extensive outbreak occurred in such high longitude. We obtained data on climate and tree cover loss due to natural factors by remote sensing to calculate annual tree loss change over a 14-year period (2001-2014). Using nonlinear regression models revealed that a combination of average temperature and precipitation by year, summer month temperature and precipitation, and tree mortality caused by natural factors best explained the annual tree loss change in the study area.

In many regions of northern Canada the boreal forest - arctic tundra ecotone is a constellation of discrete forest patches that become smaller and more isolated with increasing latitude. Understanding the potential response of the ecotone to global climate change requires an understanding of these constituent patches. We examined stand dynamics of twelve forest patches spanning a 75 km section of the ecotone in a remote area of the Northwest Territories. Sampling transects were established that extended from center to perimeter of each patch. We used dendroecological techniques to date trees and generate age distributions. These were compared against patterns expected to occur if forest patches were changing in response to recent climate warming, including gradual or episodic expansion or infilling. Results were highly varied; some patches showed evidence of recent expansion, some of recent infilling, and others experienced no substantive change at all. We found no consistent pattern associated with patch location along the ecotone; patches located further south were just as likely to experience infilling, expansion, or stability as patches located further north. Nearly 60% of all stems established within the last 100 years, but there was no evidence to suggest a more focused period of establishment. The results lead us to conclude that forest-tundra change is highly localized in this region of central Canada. While climate warming acts a broad-scale driver of treeline advance, there are finer-scale variables that may interact to mediate change.

Surrounding pollination in plus-tree seed orchards of Scots pine may affect the frost hardiness (FH) of the progenies and thus affect their use in forest regeneration in Finland. In order to avoid surrounding pollination, seed orchards of Finnish plus trees have been established in Ukraine, i.e. far from their natural distribution. It is not known whether the pollination site affects the FH of the progenies, however. The study consisted of the progenies of plus-tree seed orchards in Finland and Ukraine, in addition to the progenies from natural stands in Finland, with three lots of seeds from each site. FH was examined by whole-plant-freezing tests twice during cold acclimation in controlled conditions.

Among the progenies, the FH of needles varied between −40 °C and −80 °C by relative electrolyte leakage (REL), and between −25 °C and −50 °C by chlorophyll fluorescence (CF). The FH remained approximately the same (REL) or decreased (CF) with prolonged cold acclimation. No immediate root damage could be found by impedance loss factor at −30 °C. In the growth tests, a clear threshold of FH was found between −8 °C and −16 °C for roots, shoots and biomass. To conclude, no consistent results were found concerning the effects of the pollination sites on the FH of the progenies.