Dominant Spruce Research Articles

Tree regeneration potential in urban spruce-dominated forests is shaped by management history

With projected climate change, urban spruce-dominated forests are facing increasing risks of forest damages, encouraging management for mixed stand structures. In the major cities of southern Finland, forests are preferentially maintained continuously covered, and tree regeneration is mostly reliant on naturally emerging sapling stock. To understand the role of forest management in shaping natural tree regeneration in urban forests, we studied the occurrence and abundance of tree saplings in spruce-dominated tree stands on a management continuum ranging from partially logged to undisturbed near-natural stands. Study sites (N = 74) were located in three urban centers in southern Finland: Tampere, Lahti and Helsinki region. We analyzed the data by comparing tree regeneration potential between management classes reflecting the timing and intensity of past logging, and by relating the occurrence of saplings with current structures of the living tree stand.Compared to undisturbed stands, partially logged sites had increased regeneration of broad-leaved trees and repressed regeneration of spruce, while the most abundant sapling tree species across all management classes was rowan. We conclude that partial logging can be a successful way of directing tree regeneration towards lower dominance of spruce. However, when the canopy is only partially opened, created gaps may end up being filled primarily by rowans that are comparatively shade-tolerant. Because the ingrowth of rowan and many other broad-leaved tree species is strongly restricted by dense populations of large browsing animals in non-urban areas, urban forests may develop stand structures and tree species composition that are otherwise rare in the Fennoscandian boreal forests.

Evaluating the Legacy Effects of the Historical Predatory Seed Harvesting on the Species Composition and Structure of the Mixed Korean Pine and Broadleaf Forest from a Landscape Perspective

Adequate seed provenance is an important guarantee for the restoration of the mixed Korean pine (Pinus koraiensis) and broadleaf forest (MKPBF). However, the commercial harvest exclusion in natural forests has led to a sharp decline in economic income from timber. Given the economic value of Korean pine seeds, predatory seed harvesting (PSH) has become increasingly serious in the past 20 years and has significantly reduced the seed and seedling bank, which might seriously threaten the restoration and sustainable management of the MKPBF. How the historical PSH has affected the future of the MKPBF is unclear at the landscape scale. In this study, we quantified the effects of seed harvesting at the landscape scale by a forest landscape model LANDIS PRO, and then assessed the legacy effects of the historical PSH on the composition and structure of the MKPBF in the Xiaoxing’an Mountains, Northeast China. Our results showed that the historical PSH decreased the Korean pine basal area of all age cohorts, with an average decrease of 0.06 to 0.19 m2 ha−1 but insignificantly altered the age structure diversity of Korean pine throughout the simulation. Our results indicated that the historical PSH remarkably decreased the dominance of Korean pine by 11.1%, but significantly increased the dominance of spruce (Picea koraiensis and Picea jezoensis) and fir (Abies nephrolepis) by 3.8% and 4.5%, respectively, and had an insignificant effect on the other tree species over the simulation. We found that the historical PSH evidently changed the succession trajectories of the disturbed stands, which would result in the transition from the succession pattern dominated by Korean pine to that dominated by spruce and fir. The historical PSH decreased the importance value of Korean pine by 12.2% on average but increased it by 5.1% and 6.0% for spruce and fir, respectively, and resulted in an average 33.2% increase in the dissimilarity index compared with the initial state during the whole simulation period. Future forest management should strictly limit the intensity and rotation of seed harvesting to protect the seed provenance of Korean pine and consider how to ensure the recovery and sustainable management of the MKPBF through direct seeding or seedling planting.

Wood use and forest management at the Late Bronze Age copper mining site of Prigglitz-Gasteil in the Eastern Alps – A combined anthracological, archaeological, and palynological approach

The Late Bronze Age mining site of Prigglitz-Gasteil provides a wealth of sources of information for the reconstruction of wood use, past vegetation, and forest management at a copper production facility. In this paper we have combined charcoal analyses from domestic and workshop contexts and the investigation of mining timbers found in the backfill of the opencast copper ore mine with the first results from pollen analyses from the nearby Saubachgraben mire. The complementary information from these different contexts allowed a reconstruction of the impact of settlement and mining activities on the natural mixed forest around the site, chiefly composed of beech, spruce, and fir. Forest clearing affected mainly spruce. Despite the dominance of spruce among the conifers, however, fir wood was exclusively selected for the production of mining timbers, which were used to support the opencast mine walls or for water management constructions. The gathering of firewood for domestic hearths followed the principle of least effort, leading to a secondary succession with pioneering species in the immediate surroundings of the mining settlement. The investigated wood and charcoal finds indicated an intentional and selective use of the locally available wood species. At the current state of the investigations, there is no evidence of scarcity or shortage in the wood supply.

Increasing fire and the decline of fire adapted black spruce in the boreal forest

Intensifying wildfire activity and climate change can drive rapid forest compositional shifts. In boreal North America, black spruce shapes forest flammability and depends on fire for regeneration. This relationship has helped black spruce maintain its dominance through much of the Holocene. However, with climate change and more frequent and severe fires, shifts away from black spruce dominance to broadleaf or pine species are emerging, with implications for ecosystem functions including carbon sequestration, water and energy fluxes, and wildlife habitat. Here, we predict that such reductions in black spruce after fire may already be widespread given current trends in climate and fire. To test this, we synthesize data from 1,538 field sites across boreal North America to evaluate compositional changes in tree species following 58 recent fires (1989 to 2014). While black spruce was resilient following most fires (62%), loss of resilience was common, and spruce regeneration failed completely in 18% of 1,140 black spruce sites. In contrast, postfire regeneration never failed in forests dominated by jack pine, which also possesses an aerial seed bank, or broad-leaved trees. More complete combustion of the soil organic layer, which often occurs in better-drained landscape positions and in dryer duff, promoted compositional changes throughout boreal North America. Forests in western North America, however, were more vulnerable to change due to greater long-term climate moisture deficits. While we find considerable remaining resilience in black spruce forests, predicted increases in climate moisture deficits and fire activity will erode this resilience, pushing the system toward a tipping point that has not been crossed in several thousand years.

Excluding Large Wild Herbivores Reduced Norway Spruce Dominance and Supported Tree Species Richness in a Young, Naturally Regenerated Stand

Large wild herbivores are important and natural components of forest ecosystems, but through their browsing activities have the potential to influence the structure and composition of forest communities, thus timber production and ecosystem dynamics. To examine the effects of browsing by wild herbivores on a young post-disturbance forest in the Kysuce region of northwestern Slovakia, we established two sets of 2 m radius plots, 15 within a fenced area (5.12 ha) that excluded large wild herbivores, and 15 within an adjacent unfenced area. In each plot, we recorded the species, tree height, stem base diameter, and mutual geographic positions of trees. When we compared tree community characteristics between the unfenced and fenced plots, we found fewer and smaller broadleaved tree species, except silver birch (Betula pendula Roth.) in the unfenced plots. Although common rowan (Sorbus aucuparia L.) was the dominant species within fenced plots, where some individuals were over 6.0 m tall, this species was rare outside the fenced area and usually did not exceed 1.5 m. In contrast, Norway spruce (Picea abies Karts L.) was more abundant and taller within the unfenced area, likely released from competition by suppression of broadleaved trees by herbivores. In addition, fenced plots also showed twice the tree species richness (Shannon index) of unfenced ones. Despite changes in tree communities, total aboveground biomass stock was only slightly but significantly lower in the unfenced than the fenced plots (29.6 kg per 10 m2 vs. 33.5 kg per 10 m2). Our study suggested that browsing pressure by large wild herbivores that focused on most broadleaved trees weakened interspecies competition and allowed the expansion of Norway spruce. As a consequence, converting spruce monocultures to mixed species stands is likely unrealistic when faced with heavy browsing pressure by wild large herbivores.

Carbon loss from boreal forest wildfires offset by increased dominance of deciduous trees.

In boreal forests, climate warming is shifting the wildfire disturbance regime to more frequent fires that burn more deeply into organic soils, releasing sequestered carbon to the atmosphere. To understand the destabilization of carbon storage, it is necessary to consider these effects in the context of long-term ecological change. In Alaskan boreal forests, we found that shifts in dominant plant species catalyzed by severe fire compensated for greater combustion of soil carbon over decadal time scales. Severe burning of organic soils shifted tree dominance from slow-growing black spruce to fast-growing deciduous broadleaf trees, resulting in a net increase in carbon storage by a factor of 5 over the disturbance cycle. Reduced fire activity in future deciduous-dominated boreal forests could increase the tenure of this carbon on the landscape, thereby mitigating the feedback to climate warming.

Carbon cycling after boreal forest fire

Fire Ecology Wildfire activity has been increasing in the boreal forests of the Northern Hemisphere, releasing carbon into the atmosphere from biomass and soil, with potential feedback to climate warming. In a long-term study, Mack et al. analyzed wildfire impacts on the carbon balance of boreal forest in Alaska, with particular focus on forest-regeneration patterns. After fire, the species composition in most of the study sites changed from black spruce to a mixture of conifers and deciduous broadleaf tree species. The stands that had shifted to deciduous dominance stored fivefold more soil carbon than stands that returned to black spruce dominance. Therefore, the functional traits of deciduous trees compensated for the combustion loss of soil carbon, pointing to a potential mitigation of the feedback effect of boreal forest fire to climate warming. Science , this issue p. [280][1] [1]: /lookup/doi/10.1126/science.abf3903

Vegetation dynamics and Fire History in Färnebofjärden National Park, Central Sweden

Palaeoecological studies can identify past trends in vegetation communities and processes over long time scales. Pollen, plant macrofossils and charcoal analyses are used to reconstruct vegetation over the last 6400 years and provide information about former human impact and disturbance regimes in Färnebofjärden National Park, Central Sweden. Three specific conservation planning topics were addressed: (1) the changing ratio of conifers to broadleaved trees; (2) the origin and history of the river meadows and the biodiverse Populus tremula meadows; (3) the role of fire in the maintenance of biological values. Early diverse mixed broadleaved forest assemblages with pine were followed by significant declines of the more thermophilic forest elements prior to the expansion of spruce in the Iron Age. The rise to dominance of spruce was a ‘natural’ process that has been exaggerated by anthropogenic disturbance to artificially high levels today. The initial river meadow communities were facilitated by fire and frequent flooding events, but subsequent dynamics have more definitely been supported by human activities. Rural abandonment during the last 100 years has led to woody successions. Fire has been a continual disturbance factor with an influence on conservation issues such as Picea abies dominance and the maintenance of diverse, non-forest communities. Present occurrence of fire is unusually low, but natural fire frequencies are increasing in the region.

Persistence of balsam fir and black spruce populations in the mixedwood and coniferous bioclimatic domain of eastern North America.

The boreal ecocline (ca 49°N) between the southern mixedwood (dominated by balsam fir) and the northern coniferous bioclimatic domain (dominated by black spruce) may be explained by a northward decrease of balsam fir regeneration, explaining the gradual shift to black spruce dominance. 7,010 sample plots, with absence of major disturbances, were provided by the Quebec Ministry of Forest, Fauna, and Parks. The regeneration (sapling abundance) of balsam fir and black spruce were compared within and between the two bioclimatic domains, accounting for parental trees, main soil type (clay and till) and climate conditions, reflected by summer growing degree‐days above 5°C (GDD_5), total summer precipitation (May–August; PP_MA). Parental trees and soil type determined balsam fir and black spruce regeneration. Balsam fir and black spruce, respectively, showed higher regeneration in the mixedwood and the coniferous bioclimatic domains. Overall, higher regeneration was obtained on till for balsam fir, and on clay soils for black spruce. GDD_5 and PP_MA were beneficial for balsam fir regeneration on clay and till soils, respectively, while they were detrimental for black spruce regeneration. At a population level, balsam fir required at least 28% of parental tree basal area in the mixedwood, and 38% in the coniferous bioclimatic domains to maintain a regeneration at least equal to the mean regeneration of the whole study area. However, black spruce required 82% and 79% of parental trees basal area in the mixedwood and the coniferous domains, respectively. The northern limit of the mixedwood bioclimatic domain was attributed to a gradual decrease toward the north of balsam fir regeneration most likely due to cooler temperatures, shorter growing seasons, and decrease of the parental trees further north of this northern limit. However, balsam fir still persists above this northern limit, owing to a patchy occurrence of small parental trees populations, and good establishment substrates.

New data on age and conditions of buried organic-rich sediments formation in the сentral Ob River

This article presents the results of a comprehensive geochronological study of gyttja deposits from a section of a terrace above a floodplain located on the right bank of the Ob River near the town of Kolpashevo. According to the 230Th/U dating, the age of the gyttja could be dated in the range of 110–95 kyr. The results of spore–pollen analysis suggest that the gyttja deposit was formed in the period of a wide distribution of middle taiga forest landscapes with the dominance of spruce; the climatic conditions were similar to modern ones. These events probably took place in the interstadial time during MIS-5c or at the end of Kazantsevo interglacial period.

Importance of the first thinning in young mixed Norway spruce and European beech stands

Experimental results from the first thinning in mixed stands are not broadly experienced by forestry practice. To extend the experience with the thinning of a mixed stand, we studied thinned and unthinned mixtures of Norway spruce with European beech on two study sites in the Czech Republic, which represented different conditions: Všeteč (age of 19–35 years) – originally beech dominated site at 440 m a.s.l. and Deštné (age of 17–33 years) – originally spruce with beech site at 990 m a.s.l. Spruce and beech were mixed individually or in small groups. As the for number of trees, mixtures were 35–54% beech and 46–65% spruce at a lower altitude and 7–30% beech and 70–93% spruce at a higher altitude. In the period 1997–2013, we observed annually: mortality, diameter at breast height of all trees and height of trees (minimum 30 individuals) that represented diameter distribution. Results showed that the growth and development of young mixed spruce/beech stands were positively influenced by the first pre-commercial thinning on both locations. The most pronounced effect of thinning consisted in a decreased amount of basal area of dead trees. On control plots, salvage cut accounted for 34 and 46%, while on thinned plots it reached only 7–8% (thinned from above) and 18% (thinned from below) of basal area periodic increment during the 16-year study period. In contrast, diameter distribution was still relatively wide (i.e. an important amount of thin trees was left) at the end of observations on all plots of both study sites. Thinned stands also showed the better static stability (expressed as an h/d ratio) of dominant spruces compared to unthinned stands on both locations. Additionally, thinning supported the spruce share at a lower altitude and the beech share at a higher altitude.

Optimal multi-product management of stands producing timber and wild berries

Northern wild bilberries (Vaccinium myrtillus) and cowberries (V. vitis-idaea) are increasingly demanded, and forest owners are interested in their contribution to the total income obtained from the forest. In this study, bilberry and cowberry yield models were included in a stand growth simulator and the joint production of timber and berries was optimized by maximizing soil expectation value (SEV) with 3 % discount rate, assuming that 75 % of the berry yield is harvested. The berry prices and picking costs were included in calculations. Management was optimized for the following three stand types: Scots pine stand on Myrtillus site, mixed stand of Scots pine, Norway spruce and birch on Myrtillus site, and Scots pine stand on Vaccinium site. Management was optimized separately for an average and a good berry stand. With the current berry prices and picking costs, the optimal management of average berry stands changed very little from the timber-oriented stand management. When a pine stand growing on Vaccinium site produced good cowberry yields, thinnings were heavier and the rotation length was a few years longer than obtained for average berry stands. When both bilberry and cowberry yields were included in optimization, the optimal rotation length was 36 years longer than in timber-oriented management. In pine stands and mixed stands of pine, spruce and birch growing on Myrtillus site and having good bilberry yields, about 20 years longer rotation and a heavier first thinning were optimal to reduce canopy shading to a level that is favorable for bilberry crops. With increasing berry prices, it was more profitable to apply still longer rotation lengths and higher thinning intensities and convert the mixed stand into a pine–birch mixture—instead of spruce dominance—toward the end of the rotation. With the current berry prices, the SEV calculated with berries was more than twice as high as the SEV calculated without berries. Valuing forest berries may remarkably change the optimal management of stands, where the berry yields are known to be or can be assumed to become high.

Using pre-classification to improve the accuracy of species-specific forest attribute estimates from airborne laser scanner data and aerial images

The aim of this study was to examine whether pre-classification (stratification) of training data according to main tree species and stand development stage could improve the accuracy of species-specific forest attribute estimates compared to estimates without stratification using k-nearest neighbors (k-NN) imputations. The study included training data of 509 training plots and 80 validation plots from a conifer forest area in southeastern Norway. The results showed that stratification carried out by interpretation of aerial images did not improve the accuracy of the species-specific estimates due to stratification errors. The training data can of course be correctly stratified using field observations, but in the application phase the stratification entirely relies on auxiliary information with complete coverage over the entire area of interest which cannot be corrected. We therefore tried to improve the stratification using canopy height information from airborne laser scanning to discriminate between young and mature stands. The results showed that this approach slightly improved the accuracy of the k-NN predictions, especially for the main tree species (2.6% for spruce volume). Furthermore, if metrics from aerial images were used to discriminate between pine and spruce dominance in the mature plots, the accuracy of volume of pine was improved by 73.2% in pine-dominated stands while for spruce an adverse effect of 12.6% was observed.

Role of forest fires in Holocene stand-scale dynamics in the unmanaged taiga forest of northwestern Russia

Fossil pollen, conifer stomata, and charcoal records for the last 10,000 years were studied from three small hollow sites (Larix Hollow, Mosquito Hollow, and Olga Hollow) located at the modern western range limit of Siberian larch ( Larix sibirica) in northwestern Russia to investigate the role of forest fires in stand-scale dynamics of taiga vegetation. Wavelet coherence analysis was utilized to reveal the significance of fire on the vegetation composition at different timescales by assessing the phase and strength of the relationship between forest fires and most common boreal tree taxa in a time–frequency window. Pollen and stomata data show that all of the modern-day common tree taxa, including Norway spruce ( Picea abies) and Siberian larch, have been present in the study region since the early Holocene. The absence of charcoal layers at Mosquito Hollow suggests that this site has acted as a fire-free refugium with continuous dominance of spruce throughout the Holocene. Meanwhile, the Larix Hollow record indicates frequent local fire events and as a consequence, a more variable tree species composition. The wavelet coherence results show that the impact of forest fires on vegetation varies from short-term (<200-year periods) changes in individual tree taxa to long-term (400–800 years) changes in forest composition, such as the expansion of spruce population after local high-intensity fires around 7500–7000 cal. yr BP and the increase in abundance of birch and alder during periods of high fire frequency. Our results suggest that Holocene fire histories can be markedly different within a small geographical area, demonstrating the importance of site-specific factors in the local fire regime in the unmanaged taiga forest.

Dynamics of tree layer composition, tree age and large diameter trees in Habitats Directive Annex I forest habitats in Estonia on the basis of monitoring data collected from 2010–2012

Abstract The protection of late-succession forest habitats and continuous woodlands is the primary goal in Estonian forest nature conservation. Therefore ecological qualities of almost all forest stands in Estonian protected areas are described. Forest patches will be mapped and protected as Annex I forest habitats if they have enough structural and functional qualities. The member states of the European Union report in every six years about the conservation status of European Habitats Directive Annex I habitats. Changes in area as well as changes in habitat type’s structure and functions are also declared. In the present paper the dynamics in tree species composition, stand age and larger diameter trees of monitored Annex I forest habitats is analyzed. The mean age of stands is older than the Estonian mean value, but it is only slightly higher than the cutting age of the corresponding tree species. The mean diameter of larger living trees is in general larger than dead wood, which is indicative of the forest succession and historical low-intensive forest management. Currently, conservation activities support natural succession in forests that formerly were managed economically. Therefore, changes in the composition of tree layer species are to be expected. The dominance of spruce and the abundance of broad-leaved trees are increasing in non-managed forest habitats. In contrast, the proportion of pine, birch and aspen decreases significantly. In general the number of tree species in habitat patches will increase; thereby the dominance of certain species will decline. According to the Estonian interpretation of the forest types in the Manual of the Habitats Directive, some stands with changed main tree species will need to be reclassified. Therefore the area of some forest habitat types may increase in the future. On contrary the area and functional quality of some other habitat types may decrease.

Replacing Norway spruce with European beech: A comparison of biomass and net primary production patterns in young stands

An alteration of species composition in temperate forests – both managed and natural – is one of the expected effects of environmental change. Present forest tree species ranges will be altered by changing environmental conditions. By a combination of continuous and destructive sampling, we compared biomass stocks and annual NPP in naturally regenerated stands of Norway spruce and European beech. We purposely selected a site where future environmental conditions are predicted to favour beech over presently dominant spruce. We found no difference in overall productivity, but biomass allocation differed significantly between the two species. Beech allocated more assimilates to stem and roots than spruce. There was no significant difference between the species in NPP of the fast turnover biomass pool comprising foliage and fine roots. Maximum height growth occurred about a month earlier than in spruce, potentially changing the timing of carbon (C) flow into the soil pools. We show that the replacement of spruce by beech will result in changes in forest biomass allocation and in alterations of belowground C cycle. Such changes will affect forest ecosystem function by modifying the magnitude and timing of certain C fluxes, but also by potentially changing the species composition of forest biota dependent on them.

Expected impacts of climate change on forests: Czech Republic as a case study

  We provide fundamental information about the future development of selected climate elements in relation to anticipated threat to forests in the Czech Republic. All analyses were carried out in relation to four elevation zones with specific potential forest vegetation – up to 350 m a.s.l. (oak dominance), 350–600 m a.s.l. (beech dominance), 600–900 m a.s.l. (beech-fir dominance), 900–1,100 m a.s.l. (spruce dominance). We found out that while the projected increase in mean annual air temperature is almost constant over the Czech Republic (+3.25–3.5°C in the distant future), the frequency of heat spells at lower elevations is expected to increase dramatically compared to higher elevations. The precipitation totals during the vegetation season are projected to increase in the near future by up to 10% and to decrease in the distant future by up to 10% over all vegetation zones. In general, drought is presumed to become a key limiting factor at lower elevations, while increased temperature along with the prolonged vegetation season at higher elevations can be beneficial to forest vegetation. Consequently, northward progression of forest tree species and retraction of the species lower distribution range are a generic response pattern. Such impacts are presumed to be accompanied by changes in the distribution and population dynamics of pests and pathogens. Mainly the impacts on two key forest pests, Ips typographus and Lymantria dispar, are discussed.  

Biosynthesis of the Major Tetrahydroxystilbenes in Spruce, Astringin and Isorhapontin, Proceeds via Resveratrol and Is Enhanced by Fungal Infection

Stilbenes are dibenzyl polyphenolic compounds produced in several unrelated plant families that appear to protect against various biotic and abiotic stresses. Stilbene biosynthesis has been well described in economically important plants, such as grape (Vitis vinifera), peanut (Arachis hypogaea), and pine (Pinus species). However, very little is known about the biosynthesis and ecological role of stilbenes in spruce (Picea), an important gymnosperm tree genus in temperate and boreal forests. To investigate the biosynthesis of stilbenes in spruce, we identified two similar stilbene synthase (STS) genes in Norway spruce (Picea abies), PaSTS1 and PaSTS2, which had orthologs with high sequence identity in sitka (Picea sitchensis) and white (Picea glauca) spruce. Despite the conservation of STS sequences in these three spruce species, they differed substantially from angiosperm STSs. Several types of in vitro and in vivo assays revealed that the P. abies STSs catalyze the condensation of p-coumaroyl-coenzyme A and three molecules of malonyl-coenzyme A to yield the trihydroxystilbene resveratrol but do not directly form the dominant spruce stilbenes, which are tetrahydroxylated. However, in transgenic Norway spruce overexpressing PaSTS1, significantly higher amounts of the tetrahydroxystilbene glycosides, astringin and isorhapontin, were produced. This result suggests that the first step of stilbene biosynthesis in spruce is the formation of resveratrol, which is further modified by hydroxylation, O-methylation, and O-glucosylation to yield astringin and isorhapontin. Inoculating spruce with fungal mycelium increased STS transcript abundance and tetrahydroxystilbene glycoside production. Extracts from STS-overexpressing lines significantly inhibited fungal growth in vitro compared with extracts from control lines, suggesting that spruce stilbenes have a role in antifungal defense.

The long-term effects of logging residue removal on forest floor nutrient capital, foliar chemistry and growth of a Norway spruce stand

This study compared the nutrient capital of the forest floor, the nutrient status of trees, and the growth of a stand of Norway spruce (Picea abies L. (Karst.)) planted on a whole-tree harvesting treatment (needles left on site) with a conventional stem-only harvesting treatment 30 years after clearcutting. No significant treatment effects were detected in the amount of organic matter, the amounts of nutrients in the forest floor, or the concentrations of foliar nutrients. The results indicate that whole-tree harvesting with the needles left on the site did not reduce the long-term nutrient capital of the forest floor or the nutritional status of trees. Whole-tree harvesting significantly reduced the height of dominant spruce compared to stem-only harvesting, however the stem volume of dominant spruce did not differ between the harvesting treatments. The greater height growth of the dominant trees in the stem-only harvesting treatment could be attributed to indirect factors other than changes in site resource availability (e.g. protection against frost damage), and hence the effect of whole-tree harvesting on potential site productivity was inconclusive due to the confounding effect of site factors. In the whole-tree harvesting treatment, the total stem volume of the stand and, consequently, the actual site productivity, was lower when compared to stem-only harvesting due to the lower density of naturally regenerated seedlings.

The Origin and Dynamics of Subalpine White Spruce and Balsam Fir Stands in Boreal Eastern North America

Associations among the few tree species in the North American boreal landscape are the result of complex interactions between climate, biota, and historical disturbances during the Holocene. The closed-crown boreal forest of eastern North America is subdivided into two ecological regions having distinct tree species associations; the balsam fir zone and the black spruce zone, south and north of 49°N, respectively. Subalpine old-growth stands dominated by trees species typical of the balsam fir forest flora (either balsam fir or white spruce) are found on high plateaus, some of which are isolated within the black spruce zone. Here we identified the ecological processes responsible for the distinct forest associations in the subalpine belt across the eastern boreal landscape. Extensive radiocarbon dating, species composition, and size structure analyses indicated contrasted origin and dynamics of the subalpine forests between the two ecological regions. In the black spruce zone, the subalpine belt is a mosaic of post-fire white spruce or balsam fir stands coexisting at similar elevation on the high plateaus. With increasing time without wildfire, the subalpine forests become structurally similar to the balsam fir forest of the fir zone. These results concur with the hypothesis that the subalpine forests of this area are protected remnants of an historical northern expansion of the fir zone. Its replacement by the fire-prone black spruce forest flora was caused by recurrent fires. In the subalpine belt of the fir zone, no fire was recorded for several millennia. Harsh climate at high altitude is the primary factor explaining white spruce dominance over balsam fir forming a distinct subalpine white spruce belt above the balsam fir dominated forest.