In northern boreal forests the warming winter climate leads to more frequent snowmelt, rain-on-snow events and freeze-thaw cycles. This may be harmful or even lethal for tree seedlings that spend even a half of the year under snow. We conducted a snow cover manipulation experiment in a natural forest to find out how changing snow conditions affect young Scots pine (Pinus sylvestris L.) seedlings. The ice encasement (IE), absence of snow (NoSNOW) and snow compaction (COMP) treatments affected ground level temperature, ground frost and subnivean gas concentrations compared to the ambient snow cover (AMB) and led to the increased physical damage and mortality of seedlings. The expression responses of 28 genes related to circadian clock, aerobic and anaerobic energy metabolism, carbohydrate metabolism and stress protection revealed that seedlings were exposed to different stresses in a complex way depending on the thickness and quality of the snow cover. The IE treatment caused hypoxic stress and probably affected roots which resulted in reduced water uptake in the beginning of the growing season. Without protective snowpack in NoSNOW seedlings suffered from cold and drought stresses. The combination of hypoxic and cold stresses in COMP evoked unique transcriptional responses including oxidative stress. Snow cover manipulation induced changes in the expression of several circadian clock related genes suggested that photoreceptors and the circadian clock system play an essential role in the adaptation of Scots pine seedlings to stresses under different snow conditions. Our findings show that warming winter climate alters snow conditions and consequently causes Scots pine seedlings various abiotic stresses, whose effects extend from overwintering to the following growing season.

Recent climatic changes significantly affected forest ecosystems in northern Eurasia. Trees growing in Siberia are very sensitive to climate change due to strong temperature limitation of their growth. Our study covers high-latitude (northeastern Yakutia, eastern Taimyr, central Evenkia) and high-altitude (Russian Altai) zones in Eurasia, where tree-ring parameters (tree-ring width, cell-wall thickness, and maximum latewood density) mainly record summer air temperature variations. To reveal the impact of moisture changes (e.g., amount of precipitation, vapor pressure deficit, relative humidity and potential evapotranspiration) on tree growth in Siberian forest ecosystems, we evaluated delta C-13 in tree-ring cellulose over the past century. We found that at all the study sites mainly June-July precipitation and June-July evapotranspiration affect larch radial growth, while the strongest influence of vapor pressure deficit on the delta C-13 was observed in northeastern Yakutia. Further increase of vapor pressure deficit and rise of air temperature in the coming decades in Siberian regions will probably lead to drought and related forest mortality even under additional source of water due to permafrost thaw.

Conifer mountain forests influence numerous human populations by providing a host of critical economic, sociological, and ecosystem services. Although the causes of the elevational, transitional boundaries of these forests (i.e., upper and lower timberlines) have been questioned for over a century, these investigations have focused predominately on the growth limitations of saplings or mature trees at the upper alpine boundary. Yet, the elevational movement of timberlines is dependent initially on new seedling establishment in favorable microsites that appear to be generated by ecological facilitation. Recent evidence suggests that this facilitation is critical during the initial 1-2 years of growth when survival may be less than a few percent, only cotyledons are present, and survival occurs only in favorable microsites created by inanimate objects (e.g., boulders, dead stems), microtopography, or already established vegetation. Dramatic changes in tree form (e.g., krummholz mats) across the timberline ecotone also plays an important role in generating microsite facilitation. These favorable, facilitated microsites have been characterized broadly as experiencing low sky exposure during summer (day and night) and leeward wind exposure during winter that generates protective snow cover, all of which are needed for new seedling survival. Thus, determining the specific microclimate and edaphic characteristics of favorable microsites, and their frequency at timberline, will provide a more mechanistic understanding and greater predictability of the future elevation and extent of conifer mountain forests. In addition, although the ecophysiological advantages of a needle-like leaf morphology is well established for adult conifer trees, the advantage of this phylogenetically unique trait in emergent seedlings has not been thoroughly evaluated. Understanding seedling ecophysiology and the functional morphology that contributes to survival, plus the nature and frequency of favorable microsites at timberline, will enable more reliable estimates of future elevational shifts in conifer mountain forests. This approach could also lead to the development of a valuable and sensitive tool for forest managers interested in evaluating future changes in these forests under increased large-scale infestation and drought mortality, as well as for current scenarios of predicted climate change.

The boreal forest accounts for approximately 22% of the Northern Hemisphere landmass with nearly 40% of this huge biome growing on continuously frozen soils. Projected climate change leading to degradation of permafrost and increasing drought situation at high latitudes in Eurasia will seriously affect productivity of forests on permafrost. Here we present the results of an on-going research of tree radial growth in the midst of the permafrost zone in Siberia, Russia (Tura region, 64 degrees N, 100 degrees E, 140-610 m a.s.1.). Tree-ring width and density chronologies of Gmelin larch and Siberian spruce from a great variety of sites characterized by different thermo-hydrological regime of soils are analyzed. The obtained results reveal that current tree radial growth and tree-ring structure in permafrost region in Siberia are largely dependent on local site conditions and may be constrained by low air and soil temperatures as well as soil water availability. Varying climatic responses and seasonal radial growth of trees at different habitats indicate a range of possible scenarios of further development of northern larch stands. Forest fire is another important factor strongly affecting tree stand dynamics and forest ecosystem functioning in the continuous permafrost zone. Analysis of tree-ring parameters indicate that post-fire dynamics of tree-ring structure is in accordance with the changes in habitat conditions caused by removal by fire and then gradual recovery of ground vegetation resulting in an alteration in soil active layer depth. In general, the results of this multi-proxy analysis for trees growing under various conditions in the continuous permafrost zone in Siberia allow assumptions about changes in tree productivity, stand dynamics and therefore carbon uptake under projected climate change and permafrost degradation.