Boreal forests are expected to shift as the result of persistent warming in the northern high latitudes, and some of these shifts may occur in and near the boreal (taiga) - tundra ecotone (TTE). However, the rates, patterns, and location of these shifts will likely reflect the heterogeneity in the environmental drivers of forest structure. Predictions of change should account for this heterogeneity across multiple scales. Here, we use a Landsat-scale map of the TTE forest structure patterns to identify different types of forest transition zones, target focal sites with high resolution forest model simulations in the zones, and present the simulation results for the focal TTE sites across the North American arctic/boreal domain. We analyzed forest structure and composition metrics across these focal sites to understand current forest patterns along topographic, soil and climatic gradients at 10 m pixel resolution. Our results provide some insight into the spatial variability in the direction, rate, and magnitude of some shifts in forest structure at the cold edge of the boreal.

As the climate of arctic-boreal regions changes, the ecotone response between boreal forest and tundra will be dynamic. The boreal component of the ecotone is typified by broadleaf and coniferous trees, whereas the tundra component is dominated by tall shrubs, dwarf shrubs, graminoids, mosses, and lichens. The two biomes come together both gradually and abruptly based on the complex environmental matrix of topography, soil characteristics, permafrost presence, light availability, temperature, moisture, and fire regimes. Using the boreal forest individual-based gap-dynamics model, SIBBORK-TTE, we can simulate this shifting ecotone with high spatial resolution; however, the intersection between boreal forest and tundra requires that shrub species and low-statured tundra plant types be integrated into the model that previously only included trees. Working within the domain of the NASA Arctic-Boreal Vulnerability Experiment (ABoVE), we integrated three tall shrub genera (Salix, Betula, and Alnus), and other tundra PFTs, into the SIBBORK model. Modeling shrub ramets within the tree canopy expands the simulation range capability of SIBBORK-TTE from boreal to boreal-transition sites, increasing the applicability of the model at the ecotone. The model was calibrated and tested for a number of key sites within the ABoVE domain of Alaska. The results highlight the importance of shrub-tree competitive dynamics in predicting treeline advancement and retreat within the ecotone. Because the dynamics of the latitudinal treeline are not only temperature driven, the integration of tall shrubs, and other tundra plant functional types, will provide an important tool to accurately predict how the boreal forest and tundra interact at their ecotone.

Alberta’s forestry is facing challenges associated with climate change, different populations vary in the response to a variety of climate conditions, and the possible maladaptation caused by climate change which may impact the expected benefit from tree improvement programs. Taking advantage of trials where seedlots of white spruce (Picea glauca (Moench) Voss) and lodgepole pine (Pinus contorta Dougl.) were moved to adjacent breeding regions for testing in both progeny and provenance trials, we explored a new method of analysis named the ‘Height Proportion Function (HPF)’. Merging with the top height equations in the Growth and Yield Projection System (GYPSY), the HPF was used to predict the effect of climate change on the growth and yield of improved and unimproved seedlots under three Representative Concentration Pathways (RCPs), including RCP26, RCP45 and RCP85.

The simulation results indicate that height growth is strongly related to the mean coldest month temperature (MCMT) for white spruce and mean annual precipitation (MAP) has the strongest effect in lodgepole pine. Regardless of RCPs, the white spruce seedlots originating from lower provenance MCMT in northern Alberta are expected to show an increase in height growth compared to seedlots with higher provenance MCMT, in the middle to southern Alberta. The lodgepole pine seedlots, however, are expected to show a decrease in height growth across Alberta regardless of provenance MAP under all RCPs. Regardless of RCPs, improved seedlots will be outgrown by unimproved seedlots for white spruce by 2090, while improved seedlots of lodgepole pine will retain their growth advantage over unimproved seedlots. These results highlight the necessity of incorporating climate change into estimating the benefits of tree improvement programs in Alberta, and also offer a quantitative tool in predicting the effect of climate change on the growth and yield of both improved and unimproved seedlots.

Northern forests are at risk due to genetically adapted traits that are becoming maladaptive under climate change. With disproportionate warming in polar regions, it is essential to assess the potential vulnerability of northern tree populations to climate shifts and variability. Analyzing growth responses in a multi-decade provenance trial of lodgepole pine (Pinus contorta var. latifolia) indicated that northern populations could be sensitive to climatic extremes like drought and shoulder-season frosts. Notably, when trees sourced from central British Columbia (BC) were transferred to northern regions, they did not appear to be impacted by extremes more severely than local seed sources. Since these central BC provenances are adapted to climates that are expected to materialize in northern environments, it appears that assisted migration may help mitigate declines of northern forests. However, the poor performance of trees sourced from regions further south highlight the importance of seed transfer limits. Altogether, assisted migration in northern regions, judiciously applied, may outweigh the risks of status quo reforestation.

The study focuses on investigating drivers of the ground vegetation dynamics as a part of research on simulating productivity and composition of boreal forests. The dynamics of the vegetation of the ground layer depends on the heterogeneity of the conditions under the forest canopy. The largest contribution to the dynamics of elements is given by the dominant species of plants with a cover of more than 50%. Most of these are long-rhizomatous plants. They form extensive clones with an area of 4-5 m2 and more. The mosaic of their location under the canopy is determined by the environmental factors. Studies of dwarf shrubs bilberry (Vaccinium myrtillus L.) and lingonberry (Vaccinium vitis-idaea L.) in even-aged single-species Scots pine forests without undergrowth on sandy soils and in uneven-aged mixed stands (Pinus sylvestris L., Picea abies (L.) Karst., Betula pendula Roth, Tilia cordata Mill.) on sandy loam soils in the Moscow region were carried out. Aboveground shoots of dwarf shrubs are absent when less than 7% of photosynthetically active radiation is transmitted below the canopy. The single shoots survive at the illumination from 7 to 10%. When the illumination is more than 10%, the coverage of dwarf shrubs is 60-80%. Also, plant species differ in relation to soil moisture and soil richness in nitrogen. Bilberry is more resistant to high soil moisture, which determines the position of its parcels in lower parts of microrelief, while lingonberry occupy higher patches. The reason for this is the deeper location of rhizomes and root systems of lingonberry. The results of a series of simulation experiments using the EFIMOD - CAMPUS-S - Romul_Hum model system made it possible to analyze the influence of spatial heterogeneity on the dynamics and species structure of the forest ground vegetation as well as on C and N stocks in forest soils.

We considered the phenomenon of «tree waves” (hedges and ribbons) formation within alpine tundra in the Siberian Mountains and its response to warming. We used time series of high- and medium resolution satellite scenes, dendrochronology methods, climate variables and on-ground studies.

At the treeline larch (Larix sibirica), Siberian pine (Pinus sibirica) and birch (Betula tortuosa) forms hedges on windward and on upper part of leeward slopes, whereas ribbons observed on the leeward ones. Hedges oriented along prevailing winds, whereas ribbons were perpendicular. Hedges always linked with microtopography features. Ribbons formation dependent to snow accumulation/melting processes.

Developed hedges has a dense “aerodynamic friendly” shape due to tree crown overlapping. Once establishes, hedges provided positive feedback for seedlings establishment due to wind blocking, soil formation and enrichment by biogenic elements, and moisture improvement due to snow accumulation.

During the last five decades, trees GI in both hedges and ribbons strongly increased. Trees GI and migration rate positively correlated with warmer winter and warmer fall (September-October) temperatures. That was attributed to longer vegetation period for evergreen (Siberian pine) as well as for deciduous larch and birch. In last case, GI stimulation attributed to the non-leaf (bark) photosynthesis.

Alongside with temperature, winds in April-May and September-October periods negatively influenced trees upper boundary migration rate and trees GI. We attribute that to combine effect of cooling and tissues desiccation with consequent water stress and cells damage.

Geographically, trees uphill migration rate varied from 0.5 m/yr (northern mountains) to 4.0 m yr/yr. (southern mountains).

New hedges and ribbons are forming within the treeline zone, whereas downhill within both, the ribbons and hedges zones, gaps between ribbons and hedges were filled in with established trees and expanding trees crowns, and by that turning into closed forests. Thus, climate-driven trees migration into alpine tundra occurred, alongside with “trees diffusion”, also by the “wave pattern”.