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Climate change is likely to affect the growth, development and regeneration of the black spruce stands across the boreal forest. Regeneration failures cause gaps in the dense black spruce-feathermoss (SM) mosaic increasing the landscape proportion of open lichen woodland (LW). The aims of the study are to determine whether the contrasting characteristics of SM and LW induce different maximum carboxylation rate (Vcmax), maximum electron transport rate (Jmax) and light-saturated maximum photosynthesis (Amax) in black spruce trees across a latitudinal or seasonal gradient. Results show that the Vcmax and Jmax were higher in SM than in LW in western Quebec, at the ecotone of the closed-crown and open forest. Vcmax and Jmax were different between SM and LW mainly because nutrient acquisition seems different between stand types. Latitude affects values of Vcmax and Jmax, but the effect could be explained by soil and vegetation composition between experimental plots rather than by latitude. Physiological capacities do not match Amax values for stand types and latitude. Indeed, Amax rates suggest that black spruce in LWs perform as well as those in SMs at the needle scale because Amax would be limited by CO2 concentration which prevents saturation of Rubisco. Despite the lack of difference between the Amax of SM and LW stands, future increases in CO2 concentration and temperature could induce a gap between their respective photosynthesis rates because of their different physiological capacities. [ABSTRACT FROM AUTHOR]

Climate change is likely to affect the growth, development and regeneration of the black spruce stands across the boreal forest. Regeneration failures cause gaps in the dense black spruce-feathermoss (SM) mosaic increasing the landscape proportion of open lichen woodland (LW). The aims of the study are to determine whether the contrasting characteristics of SM and LW induce different maximum carboxylation rate (Vcmax), maximum electron transport rate (Jmax) and light-saturated maximum photosynthesis (Amax) in black spruce trees across a latitudinal or seasonal gradient. Results show that the Vcmax and Jmax were higher in SM than in LW in western Quebec, at the ecotone of the closed-crown and open forest. Vcmax and Jmax were different between SM and LW mainly because nutrient acquisition seems different between stand types. Latitude affects values of Vcmax and Jmax, but the effect could be explained by soil and vegetation composition between experimental plots rather than by latitude. Physiological capacities do not match Amax values for stand types and latitude. Indeed, Amax rates suggest that black spruce in LWs perform as well as those in SMs at the needle scale because Amax would be limited by CO2 concentration which prevents saturation of Rubisco. Despite the lack of difference between the Amax of SM and LW stands, future increases in CO2 concentration and temperature could induce a gap between their respective photosynthesis rates because of their different physiological capacities.

The increase in open-crown forest stands in the closed-crown boreal forest of Quebec over the last 50 years prompts us to identify and understand the drivers responsible for stand opening. To do so, we studied 37 jack pine plots with varying degrees of canopy opening in the Eastern Canadian boreal forest to answer four questions: (1) Does stand opening result from a deficit in pine regeneration, from poor tree growth, or from both processes simultaneously? (2) In the event that pine stand opening results at least in part from poor tree growth, how early following stand initiation does the tree growth divergence occur between unproductive and productive plots? (3) Is poor tree growth in the unproductive plots related to water stress? Finally, (4) are there predisposing site factors and, if so, what are their contributions versus non-permanent factors such as disturbance history, vegetation, and soil dynamics? In the study area, jack pine stand openings resulted from both a poor regeneration density and weak tree growth. Tree growth divergence between productive and unproductive plots occurred very early during the post-disturbance forest succession and is not likely to result from water limitation during the early development of the trees as revealed by δ13C analysis of tree rings. Low-productivity plots were exclusively found on substrates with low base cation reserves. However, because plots of higher productivity were also found on these substrates, we conclude that stand susceptibility to regeneration failures may be greater on sites with such conditions. Variations in tree cover were mainly related to non-permanent environmental variables, suggesting that restoration of forest productivity is theoretically possible in the low-productivity sites investigated. [ABSTRACT FROM AUTHOR]

In the boreal forest, open lichen woodlands have been described as an alternative stable state to closed-crown feather moss forest. In this study, we addressed the role of terricolous lichens in stabilizing open woodlands by hindering tree regeneration and/or growth. Based on field and greenhouse experiments, we compared germination and growth of jack pine (Pinus banksiana) on feather mosses (primarily Pleurozium schreberi) and lichens (primarily Cladonia stellaris), using bare mineral soil as a control. Drivers were investigated by (1) manipulating nutrient supply, (2) simulating shade of a closed canopy on the ground layer with the assumption this would mitigate lichen influence on pine growth, and (3) examining pine root ectomycorrhizal colonization and diversity as indicators of pine ability to take up nutrients. Total growth of 6-month-old greenhouse and 2-3-year-old field seedlings, as well as belowground growth of 2-year-old greenhouse seedlings, was significantly greater in moss than in lichen. Seed germination was not affected by ground cover type. Although field phosphorus and base cation availability was greater in mosses than in lichens, fertilization did not entirely compensate for the negative effects of lichens on pine growth in the greenhouse. Ground layer shading had no impact on pine growth. Lichens were associated with reduced abundance and modified composition of the root ectomycorrhizal community. By suggesting that terricolous lichens constitute a less favorable growth substrate than mosses for pine, our results support the hypothesis that lichens contribute to open woodland stability in the potentially closed-crown feather moss forest. [ABSTRACT FROM AUTHOR]

The subject of conifer regeneration failure after canopy removing disturbance leading to expansion of ericaceous heaths in boreal forests received considerable attention. However, despite seedbed quality being a key factor in tree regeneration, our understanding of the role of lichen and moss dominated seedbeds on tree seedling regeneration after forest fires remains unclear. Objective of this study was to investigate the effect of time since fire and post-fire cryptogamic (lichen and moss) seedbeds with variable organic matter thickness on black spruce regeneration. We conducted black spruce seeding experiment in black spruce – Kalmia forests burned 11, 17 and 37 years ago. In each site we applied three seedbed manipulation treatments (mat-intact, mat-mixed and mat-removed) to test if seedbed manipulation improves spruce regeneration. This experiment was extended to three moss-dominated seedbeds to compare seedbed moisture and temperature effects on spruce regeneration. We also tested potential allelopathic effect of two common lichen and moss species in laboratory bioassay. Seed regeneration was low in all seedbeds. Black spruce germination and seedling growth was lowest in presence of Cladonia cristatella followed by C. stellaris , Polytrichum juniperinnum and Pleurozium schriberi . HPLC analysis of C. cristatella indicated the presence of usnic acid, a common germination inhibiting allelochemical. Moss seedbeds were relatively favourable for spruce regeneration. We conclude that (i) lichen seedbeds inhibit seedling regeneration due to adverse biophysical and chemical (allelopathic) effects, (ii) moss seedbeds facilitate black spruce regeneration by maintaining favourable moisture and temperature, and (iii) seedbed manipulation treatments produce mixed results depending on the dominant cryptogam, organic matter thickness and seedbed moisture and temperature. [ABSTRACT FROM AUTHOR]

Field observations using chronosequences are helpful to study vegetation succession. This method allows to establish comparisons based on soil composition, stand structure, micro- and macrofossil remains from sites of different ages but on similar edaphic and topographic conditions. In the boreal forest, post-fire succession through time is triggered by climate, disturbance history (insect epidemics, fire and logging), latitude and altitude. The main objective of this research is to identify the main patterns of early post-fire succession, including similarities and differences in vegetation composition and attributes, of three contrasted ecosystems distributed along an altitudinal gradient. To do so, we have monitored the successional development of the alpine, subalpine and boreal lichen-woodland sites during the first 21 years (1991 to 2011) of post-fire sequence in eastern Canada 1991 to 2011. Each site was characterized by a different functional group that became established following fire. A rapid resurgence of ericaceous shrubs and lichens was observed in the lichen woodland and subalpine sites. Bryophyte and lichen species were not an important component of vegetation communities during the earlier stages of post-fire succession. For all three sites monitored, lichens were the last functional group to establish in the chronosequences. Herbs and mosses characterized the post-fire succession in alpine areas, the latter functional group established late in the chronosequence to cover >25% of the site after 15 years. Post-fire chronosequences in the three contrasted environments indicate that plant succession is a repetitive process often involving similar resilient plant assemblages. [ABSTRACT FROM AUTHOR]

Open lichen woodlands (LWs) are degraded stands that lack the ability to regenerate naturally due to a succession of natural and/or anthropogenic disturbances. As they represent both interesting forest restoration and carbon sequestration opportunities, we tested disc scarification and planting of two sizes of containerized black spruce (Picea mariana Mill. (BSP)) seedlings for their afforestation. We compared treatment of unproductive LWs to reforestation of harvested, closed-crown black spruce-feathermoss (BSFM) stands. After one year, seedling survival and nutritional status were equivalent among stand types but despite higher root elongation index (REI), planted seedlings in LWs had lower relative growth rate, smaller total biomass and stem diameter than those in BSFM stands. Soil fertility variables, soil temperature, nor seedling water potential, helped at explaining this early growth response. Disc scarification significantly improved seedling first-year survival, biomass and foliar nutrient concentrations of P, Ca, and Mg. Smaller planting stock showed higher REI, higher shoot water potential, and higher foliar nutrient concentration of all but one of the measured nutrients (N, P, K and Mg). Hence, preliminary results suggest that planting of smaller containerized black spruce stock, combined with disc scarification, shows potential for afforestation of unproductive LWs. The impact of the lichen mat and other potential growth limiting factors on afforestation of these sites requires further investigation.

Field observations using chronosequences are helpful to study vegetation succession. This method allows to establish comparisons based on soil composition, stand structure, micro- and macrofossil remains from sites of different ages but on similar edaphic and topographic conditions. In the boreal forest, post-fire succession through time is triggered by climate, disturbance history (insect epidemics, fire and logging), latitude and altitude. The main objective of this research is to identify the main patterns of early post-fire succession, including similarities and differences in vegetation composition and attributes, of three contrasted ecosystems distributed along an altitudinal gradient. To do so, we have monitored the successional development of the alpine, subalpine and boreal lichen-woodland sites during the first 21 years (1991 to 2011) of post-fire sequence in eastern Canada 1991 to 2011. Each site was characterized by a different functional group that became established following fire. A rapid resurgence of ericaceous shrubs and lichens was observed in the lichen woodland and subalpine sites. Bryophyte and lichen species were not an important component of vegetation communities during the earlier stages of post-fire succession. For all three sites monitored, lichens were the last functional group to establish in the chronosequences. Herbs and mosses characterized the post-fire succession in alpine areas, the latter functional group established late in the chronosequence to cover >25% of the site after 15 years. Post-fire chronosequences in the three contrasted environments indicate that plant succession is a repetitive process often involving similar resilient plant assemblages.

Over the last few decades, shrub species have expanded rapidly in open tundra environments due to climate change. Previous experimental studies in this environment have shown that nutrient addition and, to a lesser extent, warming, had positive effects on shrub growth. However, the response of shrub species in open forested ecosystems such as lichen woodland is still largely unknown. The main objective of this study was to evaluate the performance of Betula glandulosa (Michx., dwarf birch) seedlings subjected to direct (warmer temperature, increased precipitation) and indirect (increased nutrient availability) effects of climate change in a lichen woodland (25 % tree cover). The study took place 10 km south of the subarctic treeline in western Québec (Canada). At the end of the second growth season, seedling leaf, woody stem and root biomass along with leaf area had increased significantly in response to nutrient addition. Moreover, seedlings exposed to nutrient addition had greater nitrogen, phosphorus and potassium concentrations in their leaves. Warming treatment also had a significant but weaker impact on leaf and woody stem biomass, while increase in precipitation had only a slight impact on seedling root biomass. Our results indicate that B. glandulosa response to simulated changes in the abiotic environment is similar to that observed in open tundra, suggesting that this species could also become more widespread in the forested ecosystems of the forest-tundra ecotone. [ABSTRACT FROM AUTHOR]

Open lichen woodlands (LWs) are degraded stands that lack the ability to regenerate naturally due to a succession of natural and/or anthropogenic disturbances. As they represent both interesting forest restoration and carbon sequestration opportunities, we tested disc scarification and planting of two sizes of containerized black spruce (Picea mariana Mill. (BSP)) seedlings for their afforestation. We compared treatment of unproductive LWs to reforestation of harvested, closed-crown black spruce-feathermoss (BSFM) stands. After one year, seedling survival and nutritional status were equivalent among stand types but despite higher root elongation index (REI), planted seedlings in LWs had lower relative growth rate, smaller total biomass and stem diameter than those in BSFM stands. Soil fertility variables, soil temperature, nor seedling water potential, helped at explaining this early growth response. Disc scarification significantly improved seedling first-year survival, biomass and foliar nutrient concentrations of P, Ca, and Mg. Smaller planting stock showed higher REI, higher shoot water potential, and higher foliar nutrient concentration of all but one of the measured nutrients (N, P, K and Mg). Hence, preliminary results suggest that planting of smaller containerized black spruce stock, combined with disc scarification, shows potential for afforestation of unproductive LWs. The impact of the lichen mat and other potential growth limiting factors on afforestation of these sites requires further investigation. [ABSTRACT FROM AUTHOR]

Black spruce-lichen woodlands (LW) are naturally occurring unproductive low tree density stands within the eastern North American closed-crown boreal forest. Natural reforestation in LWs is impeded by the lichen mat and ericaceous shrubs that inhibit seedling establishment. Disk scarification is a mechanical site preparation method that creates furrows where lichens and shrubs are removed and mineral soil is exposed, which is the preferred seedbed for black spruce KlJ natural regeneration. The objective of this study was to quantify the impact of disk scarification on black spruce establishment in LWs by natural seeding. Disk 13 scarification was performed amid scattered seed trees in six study sites located in the central area of boreal Québec's spruce-moss biodimatic domain. Newly 9 established black spruce seedlings were significantly more abundant (ca. 81%; x2 = 28.72, P < 0.001) in the furrows of scarified plots even though the ·jjfl proportion of disturbed soil was small (ca. 20%). Seedling establishment occurred for at least 3 years following scarification, with a peak in the first year. The El distribution and density of seed trees (112-363 stems ha"1) did not limit natural seedling establishment in this study. Five years after scarification, observed densities and stocking levels of newly established black spruce seedlings were sufficient to expect afforestation without planting in scarified LWs. [ABSTRACT FROM AUTHOR]

Expansion of deciduous shrub species in open subarctic landscapes has been reported at several locations around the circumpolar region. However, few data are available to evaluate the response of shrub species in forested ecosystems. For this study, we conducted a dendrochronological analysis to compare the establishment and growth of Michx. individuals prior to and after an experimental removal of tree cover (1987) in a lichen woodland located at the boreal forest - tundra ecotone. Tree removal was followed by a B. glandulosa recruitment pulse. It is likely that the newly established seedlings benefited more from heavy trampling during tree removal, which destroyed the lichen cover, than from tree removal itself. A strong growth increase after tree removal suggests that the black spruce tree cover suppressed B. glandulosa growth prior to 1987. Moreover, while radial growth for the period prior to tree removal was not a function of the number of growing degree-days during the growing season, the same climatic variable explained 46% of the growth variability in the second period. These results suggest that the presence of a tree cover might limit the response of shrub species to climate change in forested ecosystems. [ABSTRACT FROM AUTHOR]

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Aim The spruce–moss forest is the main forest ecosystem of the North American boreal forest. We used stand structure and fire data to examine the long-term development and growth of the spruce–moss ecosystem. We evaluate the stability of the forest with time and the conditions needed for the continuing regeneration, growth and re-establishment of black spruce ( Picea mariana) trees. Location The study area occurs in Québec, Canada, and extends from 70°00′ to 72°00′ W and 47°30′ to 56°00′ N. Methods A spatial inventory of spruce–moss forest stands was performed along 34 transects. Nineteen spruce–moss forests were selected. A 500 m2 quadrat at each site was used for radiocarbon and tree-ring dating of time since last fire (TSLF). Size structure and tree regeneration in each stand were described based on diameter distribution of the dominant and co-dominant tree species [black spruce and balsam fir ( Abies balsamea)]. Results The TSLF of the studied forests ranges from 118 to 4870 cal. yr bp. Forests < 325 cal. yr bp are dominated by trees of the first post-fire cohort and are not yet at equilibrium, whereas older forests show a reverse-J diameter distribution typical of mature, old-growth stands. The younger forests display faster height and radial growth-rate patterns than the older forests, due to factors associated with long-term forest development. Each of the stands examined established after severe fires that consumed all the soil organic material. Main conclusions Spruce–moss forests are able to self-regenerate after fires that consume the organic layer, thus allowing seed regeneration at the soil surface. In the absence of fire the forests can remain in an equilibrium state. Once the forests mature, tree productivity eventually levels off and becomes stable. Further proof of the enduring stability of these forests, in between fire periods, lies in the ages of the stands. Stands with a TSLF of 325–4870 cal. yr bp all exhibited the same stand structure, tree growth rates and species characteristics. In the absence of fire, the spruce–moss forests are able to maintain themselves for thousands of years with no apparent degradation or change in forest type. [ABSTRACT FROM AUTHOR]

Paleoecological analysis using complementary indicators of vegetation and soil can provide spatially explicit information on ecological processes influencing trajectories of long-term ecosystem change. Here we document the structure and dynamics of an old-growth woodland before and after its inception 1000 years ago. We infer vegetation and soil characteristics from size and age distributions of black spruce (Picea mariana (Mill.) B.S.P.), soil properties, plant fossils, and paleosols. Radiocarbon ages of charcoal on the ground and in the soil indicate that the fire return interval was ∼300 years between 2750 and 1000 cal. yr BP. No fire evidence was found before and after this period despite the presence of spruce since 4200 cal. yr BP. The size structures of living and dead spruce suggest that the woodland is in equilibrium with present climate in absence of fire. Tree establishment and mortality occurred regularly since the last fire event around 950 cal. yr BP. Both layering and occasional seeding have contributed to stabilize the spatial distribution of spruce over the past 1000 years. Since initial afforestation, soil development has been homogenized by the changing spatial distribution of spruce following each fire. We conclude that the history of the woodland is characterized by vegetation shifts associated with fire and soil disturbances and by millennial-scale maintenance of the woodland's structure despite changing climatic conditions. [ABSTRACT FROM AUTHOR]

Aim Our two main goals are first to evaluate the resilience of the boreal forest according to latitude across the closed-crown forest zone using the post-disturbance distribution and cover of lichen woodlands and closed-crown forests as a metric, and second to identify the disturbance factors responsible for the regeneration and degradation of the closed-crown forest according to latitude since the 1950s. Location The study area extends between 70°00′ and 72°00′ W and throughout the closed-crown forest zone, from its southern limit near 47°30′ N to its northern limit at the contact with the lichen woodland zone at around 52°40′ N. Methods Recent (1972–2002) and old (1954–1956) aerial photos were used to map the distribution of lichen woodlands across the closed-crown forest zone. Forest disturbances such as fire, spruce budworm ( Choristoneura fumiferana (Clemens)) outbreak, and logging were recorded on each set of aerial photos. Each lichen woodland and stand disturbance was validated by air-borne surveys and digitized using GIS software. Results Over the last 50 years, the area occupied by lichen woodlands has increased according to latitude; that is, 9% of the area that was occupied by closed-crown forests has shifted to lichen woodlands. Although logging activities have been concentrated in the same areas during the last 50 years, the area covered by logging has increased significantly. Outbreaks by the spruce budworm occurred predominantly in the southern (47°30′ N to 48°30′ N) and central (48°53′ N to 50°42′ N) parts of the study area, where balsam fir stands are extensive. In the northern part of the study area (51°–52°40′ N), extensive fires affected the distribution and cover of closed-crown forests and lichen woodlands. Main conclusions Over the last 50 years, the area occupied by closed-crown forests has decreased dramatically, and the ecological conditions that allow closed-crown forests to establish and develop are currently less prevalent. Fire is by far the main disturbance, reducing the ability of natural closed-crown forests to self-regenerate whatever the latitude. Given the current biogeographical shift from dense to open forests, the northern part of the closed-crown forest zone is in a process of dramatic change towards the dominance of northern woodlands. [ABSTRACT FROM AUTHOR]

Abstract: Black spruce-lichen woodlands are low tree density stands within the closed crown, North American boreal forest that represent a diverging post-fire type of black spruce forest. As natural densification of lichen woodlands has never been observed, afforestation remains the only way of shifting these stands to closed canopy stands. The objective of the study was to evaluate site preparation effects on growth and water relations of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) seedlings in black spruce-lichen woodlands (LW), compared to managed black spruce-feathermoss stands (BSFM). Site preparation treatments in LW were no preparation (LWno), patch scarification (LWps), and disk scarification (LWds). The operationally managed BSFM stands stood for the control. Results from the third growing season indicate that soil water availability in intact or lightly prepared patch scarified LW is a limiting variable for seedling growth for both black spruce and jack pine seedlings. However, once LW are prepared with disk scarification, this planting check factor is significantly reduced, to the point of being equal to BSFM in terms of water availability and seedling water relations. The significant seedling growth difference, in favor of BSFM, might be a consequence of the lower level of pre-plantation disturbance in LW, compared to BSFM. Our results also suggest that jack pine, through higher water stress tolerance, could constitute a wise silvicultural choice over black spruce, especially with LW established on coarse drought-prone material. [Copyright &y& Elsevier]

• Boreal forest is resilient in the face of repeated disturbances during the Holocene. • Moderate fire frequency leads to a recurrence dynamics. • Resilience remains precarious as shown by lichen woodlands formation. • Climate change is expected to amplify the forest/woodland phenomenon. • Charcoals in mineral soils allow reconstruction of past local tree assemblages. The dynamics of the boreal forest has followed several successional trajectories during the Holocene caused by recurrent stand-scale, abiotic and biotic disturbances. Under stable environmental conditions, site disturbances should favor the regeneration of forest communities enabling the recovery process to produce forests similar to the pre-disturbance states. However, a failure in the post-disturbance recovery process can also occur to shift the forest communities to alternative states. Although fire is the main disturbance factor affecting the dynamics of the boreal forest, long-term resilience of most forest ecosystems remains poorly known because of lack of stand-scale paleoecological evidence. To evaluate the resilience ability of boreal forest ecosystems, we reconstructed their Holocene fire history and documented their successional pathways at the stand scale in two climatically-contrasted regions (western continental versus eastern humid climates of boreal Québec, Canada) based on botanically-identified and 14C-dated soil charcoal remains. Since mid-Holocene, western closed-crown conifer forests were resilient to fire disturbance, but hardwood trees declined significantly. Yet in the eastern region, closed-crown conifer forests similar to that observed today seems to have remained in place since its origin. In both regions, moderate fire frequency leads to a recurrence dynamics that favors renewal of forests stands with the same composition. However, balsam fir – paper birch stands follow a successional pattern characterized by the establishment of paper birch, with subsequent increasing abundance of balsam fir. This allows the maintenance of forests composed of these species, whose dominance fluctuates according to the time elapsed since the last fire. The succession from black spruce to balsam fir stands seems to be an uncommon process that requires a prolonged fire-free period. Although most closed-crown forests are resilient postfire ecosystems which have persisted over several millennia, their resilience is precarious as evidenced by the transformation of some forests into lichen woodlands after fire. [ABSTRACT FROM AUTHOR]