Boreal Black Spruce Forest Research Articles

Decomposition of soil organic matter from boreal black spruce forest: environmental and chemical controls

Black spruce forests are a dominant covertype in the boreal forest region, and they inhabit landscapes that span a wide range of hydrologic and thermal conditions. These forests often have large stores of soil organic carbon. Recent increases in temperature at northern latitudes may be stimulating decomposition rates of this soil carbon. It is unclear, however, how changes in environmental conditions influence decomposition in these systems, and if substrate controls of decomposition vary with hydrologic and thermal regime. We addressed these issues by investigating the effects of temperature, moisture, and organic matter chemical characteristics on decomposition of fibric soil horizons from three black spruce forest sites. The sites varied in drainage and permafrost, and included a “Well Drained” site where permafrost was absent, and “Moderately well Drained” and “Poorly Drained” sites where permafrost was present at about 0.5 m depth. Samples collected from each site were incubated at five different moisture contents (2, 25, 50, 75, and 100% saturation) and two different temperatures (10°C and 20°C) in a full factorial design for two months. Organic matter chemistry was analyzed using pyrolysis gas chromatography-mass spectrometry prior to incubation, and after incubation on soils held at 20°C, 50% saturation. Mean cumulative mineralization, normalized to initial carbon content, ranged from 0.2% to 4.7%, and was dependent on temperature, moisture, and site. The effect of temperature on mineralization was significantly influenced by moisture content, as mineralization was greatest at 20°C and 50–75% saturation. While the relative effects of temperature and moisture were similar for all soils, mineralization rates were significantly greater for samples from the “Well Drained” site compared to the other sites. Variations in the relative abundances of polysaccharide-derivatives and compounds of undetermined source (such as toluene, phenol, 4-methyl phenol, and several unidentifiable compounds) could account for approximately 44% of the variation in mineralization across all sites under ideal temperature and moisture conditions. Based on our results, changes in temperature and moisture likely have similar, additive effects on in situ soil organic matter (SOM) decomposition across a wide range of black spruce forest systems, while variations in SOM chemistry can lead to significant differences in decomposition rates within and among forest sites.

Introduction. The boreal forest and global change

The boreal forest is the second largest biome in the world containing 33% of the Earth's forest cover ([FAO 2001][1]) of which approximately 25% is natural. It is circumpolar and shares similar taxa across its range. It has approximately 20 300 identified species. Along with the tropics, the

Dissolved Organic Carbon in Alaskan Boreal Forest: Sources, Chemical Characteristics, and Biodegradability

The fate of terrestrially-derived dissolved organic carbon (DOC) is important to carbon (C) cycling in both terrestrial and aquatic environments, and recent evidence suggests that climate warming is influencing DOC dynamics in northern ecosystems. To understand what determines the fate of terrestrial DOC, it is essential to quantify the chemical nature and potential biodegradability of this DOC. We examined DOC chemical characteristics and biodegradability collected from soil pore waters and dominant vegetation species in four boreal black spruce forest sites in Alaska spanning a range of hydrologic regimes and permafrost extents (Well Drained, Moderately Well Drained, Poorly Drained, and Thermokarst Wetlands). DOC chemistry was characterized using fractionation, UV–Vis absorbance, and fluorescence measurements. Potential biodegradability was assessed by incubating the samples and measuring CO2 production over 1 month. Soil pore water DOC from all sites was dominated by hydrophobic acids and was highly aromatic, whereas the chemical composition of vegetation leachate DOC varied significantly with species. There was no seasonal variability in soil pore water DOC chemical characteristics or biodegradability; however, DOC collected from the Poorly Drained site was significantly less biodegradable than DOC from the other three sites (6% loss vs. 13–15% loss). The biodegradability of vegetation-derived DOC ranged from 10 to 90% loss, and was strongly correlated with hydrophilic DOC content. Vegetation such as Sphagnum moss and feathermosses yielded DOC that was quickly metabolized and respired. In contrast, the DOC leached from vegetation such as black spruce was moderately recalcitrant. Changes in DOC chemical characteristics that occurred during microbial metabolism of DOC were quantified using fractionation and fluorescence. The chemical characteristics and biodegradability of DOC in soil pore waters were most similar to the moderately recalcitrant vegetation leachates, and to the microbially altered DOC from all vegetation leachates.

A shadow fraction method for mapping biomass of northern boreal black spruce forests using QuickBird imagery

We have developed and tested a method for mapping above-ground forest biomass of black spruce ( Picea mariana (Mill.) B.S.P.) stands in northern boreal forests of eastern Canada. The method uses QuickBird images and applies image processing algorithms to extract tree shadow fraction (SF) as a predictive variable for estimating biomass. Three QuickBird images acquired over three test sites and 108 ground sample plots (GSP) were used to develop and test the method. SF was calculated from the fraction of tree shadow area over the area of a reference square overlaid on the images. Linear regressions between biomass of GSP and SF from the images for each test site resulted in R 2 in the range from 0.85 to 0.87 (except one case at 0.41), RMSE of 11 to 18 t/ha and bias of 2 to 5 t/ha. Statistical tests demonstrated that local regressions for the three test sites were not statistically significantly different. Consequently, a global regression was calculated with all GSP and produced R 2, RMSE, and bias of 0.84, 14.2 t/ha and 4.2 t/ha, respectively. While generalization of these results to extended areas of the boreal forest would require further assessment, the SF method provided an efficient means for mapping biomass of black spruce stands for three test areas that are characteristic of the northern boreal forest of eastern Canada (boreal and taiga shield ecozones).

Predicting the impacts of forest management on woodland caribou habitat suitability in black spruce boreal forest

Forest managers are under increasing pressure to conduct sustainable forestry while balancing multiple land use objectives. In the boreal forest, conflicts between forestry and conservation of woodland caribou Rangifer tarandus caribou (Gmelin) require that managers understand the effects of harvesting timber on the long-term viability of caribou. We hypothesized that large contiguous patches of suitable habitat are needed to ensure the long-term occurrence of caribou on managed forests. We developed a resource selection function describing the probability of caribou occurrence using satellite telemetry data from adult female woodland caribou. We used the Patchworks stochastic optimization model to predict the effects of three forest management strategies on caribou occurrence, harvest volume, and transportation cost for a 120-year planning period. The Patchworks model spatially allocates forest harvesting and generates a near-optimal solution to the constraints defining various management objectives. Management strategies that included caribou constraints produced greater probabilities of caribou occurrence than a strategy lacking explicit caribou objectives. No strategy predicted a substantial loss to forest harvest volume. The strategy with only aspatial habitat supply targets produced probabilities of caribou occurrence similar to, or greater than, the strategy containing spatial constraints relevant to caribou. The spatially constrained management strategy forecasted higher costs for building forest access roads. Our findings apply to seasonal range suitability for occupancy of caribou and further investigation is required to evaluate forest harvesting strategies in relation to annual range requirements. A combination of management targets that reflect the habitat needs and spatial requirements of woodland caribou should be used to guide conservation of caribou populations in boreal forests.

Sphagnum community change after partial harvest in black spruce boreal forests

In eastern Canada, boreal forests develop structural diversity in association with time since stand replacing fire. In some regions, this is associated with significant changes in the bryophyte community ( Sphagnum moss invasion) and paludification (thick waterlogged forest floor development). The bryophyte community responds to opening of the canopy, and increasing moisture by replacement of slow growing species by faster growing Sphagnum spp. (e.g. magellanicum, fallax) that are dependent on constant hydration. Within a forest management context, partial harvest systems have been proposed as a strategy to maintain structural diversity, which is currently not accomplished with low retention systems. However, it is unknown whether these interventions will effectively accelerate community succession. The questions addressed in this study were: (1) is the composition of Sphagnum colonies in partially cut stands more similar to old-growth communities than in control, and low retention cut stands, (2) what aspects of harvest disturbance drive these changes, and (3) is the growth rate of Sphagnum capillifolium (an early successional shade tolerant species) different in partial versus low retention harvest systems? After harvest, Sphagnum patch size was reduced by 19.8% and 11.7% after low retention and partial harvest, respectively. While trends were not constant across three separate partial cut trials, the proportion of Sphagnum magellanicum, Sphagnum fallax and Sphagnum fuscum increased compared to controls and low retention 1–2 years after harvest. Models of percent Sphagnum cover indicated machinery track cover, percent cover of vascular plants, and patch depth were positive factors, while the influence of open canopy varied among species. Despite the inclusion of individual disturbance variables, the summary variable ‘treatment’ was significant in all models. Growth of S. capillifolium in partial cuts was intermediate to growth rates in control and low-retention cuts. Growth was positively influenced by slash cover and, contrary to the patch level, negatively influenced by track cover. These results indicate that partial harvest does represent an intermediate level of disturbance, as direct and indirect harvest effects were reduced, as was Sphagnum death. Change in composition 1 and 2 years after harvest indicates that partial harvests may effectively shift the bryophyte community towards an older community type and may thus be used to create landscape diversity. Long term trends and entire community compositions need to be assessed before this can be stated definitively. However, as paludified stands are less productive, the capacity of these partially harvested sites to produce merchantable timber is questioned.

Sphagnum establishment and expansion in black spruce (Picea mariana) boreal forests

Boreal forest bryophyte communities are made up of distinct colonies of feathermosses that cover the forest floor. In some black spruce ( Picea mariana (Mill.) BSP) boreal forests, Sphagnum spp. establish colonies on the forest floor 30–40 years after the feathermosses, and ultimately expand to dominate the community. The mechanisms that permit the Sphagnum spp. to establish and expand are unknown. The objectives of this study were to examine the establishment and expansion substrates of Sphagnum spp., and the conditions correlated with colony expansion. Forty colonies, in six stands, of Sphagnum capillifolium (Ehrh.) Hedw. were dissected to determine their substrates, and the environmental conditions in which all colonies present were growing were measured. Coarse woody debris was the dominant establishment and early expansion substrate for Sphagnum capillifolium colonies. With age as the control factor, large colonies showed a significant partial correlation with canopy openness, and there were fewer individuals per cm3 in large colonies than there were in small colonies. These results suggest that Sphagnum establishment in these communities is dependent on the presence of coarse woody debris, and expansion is linked to the stand break-up, which would allow an increase in light intensity, and rainfall to reach the colony. Consequently the community change represented by Sphagnum establishment and expansion is initially governed by a stochastic process and ultimately by habitat availability and species competition.

Comparison of carbon dioxide fluxes over three boreal black spruce forests in Canada

AbstractAlthough mature black spruce forests are a dominant cover type in the boreal forest of North America, it is not clear how their carbon (C) budgets vary across the continent. The installation of an eddy covariance flux tower on an Old Black Spruce (OBS) site in eastern Canada (EOBS, Québec) provided a first opportunity to compare and contrast its annual (2004) and seasonal C exchange with two other pre‐existing OBS flux sites from different climatic regions located in Saskatchewan [Southern OBS (SOBS)] and Manitoba [Northern OBS (NOBS)]. Although there was a relatively uniform seasonal pattern of net ecosystem productivity (NEP) among sites, EOBS had a lower total annual NEP than the other two sites. This was primarily because warmer soil under a thicker snowpack at EOBS appeared to increase winter C losses and low light suppressed both NEP and gross ecosystem productivity (GEP) in June. Across sites, greater total annual GEP and ecosystem respiration (R) were associated with greater mean annual air temperatures and an earlier beginning of the growing season. Also, GEP at all three sites showed a stronger relationship with air temperature in spring and early summer compared with later in the growing season, highlighting the importance of springtime conditions to the C budget of these boreal ecosystems. The three sites had different parameter estimates describing the responses of R and GEP at the half hour time scale to near surface temperature and light, respectively. On the other hand, the responses of both R and GEP to temperature at the monthly scale did not differ among sites. These results suggest that a general parameterization could be sufficient at coarse time resolutions to model the response of C exchange to environmental factors of mature black spruce forests from different climatic regions.

A long‐term record of carbon exchange in a boreal black spruce forest: means, responses to interannual variability, and decadal trends

AbstractWe present a decadal (1994–2004) record of carbon dioxide flux in a 160‐year‐old black spruce forest/veneer bog complex in central Manitoba, Canada. The ecosystem shifted from a source (+41 g C m−2, 1995) to a sink (−21 g C m−2, 2004) of CO2 over the decade, with an average net carbon balance near zero. Annual mean temperatures increased 1–2° during the period, consistent with the decadal trend across the North American boreal biome. We found that ecosystem carbon exchange responded strongly to air temperature, moisture status, potential evapotranspiration, and summertime solar radiation. The seasonal cycle of ecosystem respiration significantly lagged that of photosynthesis, limited by the rate of soil thaw and the slow drainage of the soil column. Factors acting over long time scales, especially water table depth, strongly influenced the carbon budget on annual time scales. Net uptake was enhanced and respiration inhibited by multiple years of rainfall in excess of evaporative demand. Contrary to expectations, we observed no correlation between longer growing seasons and net uptake, possibly because of offsetting increases in ecosystem respiration. The results indicate that the interactions between soil thaw and water table depth provide critical controls on carbon exchange in boreal forests underlain by peat, on seasonal to decadal time scales, and these factors must be simulated in terrestrial biosphere models to predict response of these regions to future climate.

Sphagnumspore availability in boreal forests

The role of propagule availability in determining community composition is poorly understood, and is infrequently investigated for bryophytes. In addition the extent to which spore availability is limited by dispersal is unknown. If spore availability is not dispersal-limited, local and regional spore dispersal and wind availability may affect spore availability at any point. In this study, the abundance of Sphagnum spores was investigated within the context of a successional sequence where Sphagnum spp. invade a feather moss community in black spruce boreal forests of northwestern Québec, Canada. Spores were trapped and grown in a greenhouse to protonemal stage to estimate the abundance of spores within three sites that varied in Sphagnum abundance, and stand density (a surrogate for wind intensity). Sporophyte production was also investigated in one site where individual Sphagnum colonies could be distinguished. Spores were less abundant in sites with less ground cover of Sphagnum present in the community, although spores were trapped in all sites. Spore abundance was inversely correlated with local stand density, indicating that wind intensity may play a role in limiting dispersal. Sporophytes were produced in colonies that were larger and had greater access to light. These results suggest that Sphagnum invasion into young dense forests may be partially limited by spore dispersal, although the availability of germination substrates may also play an important role.

Edge influence on forest structure in large forest remnants, cutblock separators, and riparian buffers in managed black spruce forests

:Remnants of old forests left on the landscape following forest harvesting, especially corridors, provide benefits of connectivity and facilitation of movement or dispersal, which may be hindered by the presence of edges. Our objective was to determine the extent of edge influence on forest structure in these forest remnants in black spruce boreal forest. We sampled canopy cover and the density of trees, snags, and logs along clearcut edge–forest gradients in large forest patches, cutblock separators, and riparian buffers. The distance-of-edge influence was determined by comparing values at different distances from the edge to values in interior forest using randomization tests. Forest remnants had lower live tree density and canopy cover and higher mortality and windthrow than interior forest. Distance-of-edge influence on forest structure extended 10–30 m from the edge, and was slightly more extensive into cutblock separators where two edges are in close proximity, but was less extensive in riparian buffers, possibly due to the presence of stable internal edges near the stream. Because of edge influence, structure near the edges of forest remnants and across narrow corridors is modified; wider corridors would be required to provide a core habitat of interior forest conditions.

Partitioning sources of soil respiration in boreal black spruce forest using radiocarbon

AbstractSeparating ecosystem and soil respiration into autotrophic and heterotrophic component sources is necessary for understanding how the net ecosystem exchange of carbon (C) will respond to current and future changes in climate and vegetation. Here, we use an isotope mass balance method based on radiocarbon to partition respiration sources in three mature black spruce forest stands in Alaska. Radiocarbon (Δ14C) signatures of respired C reflect the age of substrate C and can be used to differentiate source pools within ecosystems. Recently‐fixed C that fuels plant or microbial metabolism has Δ14C values close to that of current atmospheric CO2, while C respired from litter and soil organic matter decomposition will reflect the longer residence time of C in plant and soil C pools. Contrary to our expectations, the Δ14C of C respired by recently excised black spruce roots averaged 14‰ greater than expected for recently fixed photosynthetic products, indicating that some portion of the C fueling root metabolism was derived from C storage pools with turnover times of at least several years. The Δ14C values of C respired by heterotrophs in laboratory incubations of soil organic matter averaged 60‰ higher than the contemporary atmosphere Δ14CO2, indicating that the major contributors to decomposition are derived from a combination of sources consistent with a mean residence time of up to a decade. Comparing autotrophic and heterotrophic Δ14C end members with measurements of the Δ14C of total soil respiration, we calculated that 47–63% of soil CO2 emissions were derived from heterotrophic respiration across all three sites. Our limited temporal sampling also observed no significant differences in the partitioning of soil respiration in the early season compared with the late season. Future work is needed to address the reasons for high Δ14C values in root respiration and issues of whether this method fully captures the contribution of rhizosphere respiration.

Paludification in black spruce ( Piceamariana) forests of eastern Canada: Potential factors and management implications

Over time boreal black spruce forests on fine-textured soils in western Quebec, Canada develop very thick forest floors composed of poorly decomposed litter created by the tree and understory layers. These paludified soils are typically waterlogged and cold, and in this fire-mediated landscape, are at least partially consumed by stand replacing fires, which facilitates the establishment of the next generation of trees. Within a context of ecosystem-based management, forest harvest should mimic the dual effects of high severity fire on tree and forest floor biomass. This study was designed to investigate potential factors of forest floor thickness in order to determine the impact of removing only a tree layer, and to suggest strategies to limit paludification in this important forestry region. Forest floor thickness, fire severity, basal area, canopy closure, cover of Sphagnum spp. and ericaceous spp. were measured in black spruce stands across a chronosequence from 50 to 350 years after fire. Fire severity was determined to be a key factor in determining forest floor thickness by path analysis. After high severity fires forest floor thickness was primarily dependant on stand age, but was also positively influenced by Sphagnum spp. cover and negatively influenced by the presence of trembling aspen ( Populus tremuloides). These results suggest that forest interventions that do no remove the organic layer may be mimicking low severity fires and promoting poor tree growth and regeneration. Forest floor thickness may be limited by avoiding interventions that open the canopy and may promote the presence of Sphagnum spp. and ericaceous spp., and or by practicing mixed silviculture of trembling aspen and black spruce. However, a balance needs to be maintained between the application of these techniques and the preservation of paludified forests in the landscape.

Structural development following fire in black spruce boreal forest

We investigated stand development along a chronosequence on organic, clay and sand sites in black spruce boreal forest in northwestern Quebec, Canada. Our objectives were: (1) to describe trends and stages of structural development following fire; (2) to compare trends and stages of development both in isolation from and in conjunction with species replacement. We tested the hypothesis that although trends in structural development are similar among site types, productivity and composition affect the timing of developmental stages. Data on live trees, snags and logs were collected at 91 sites. Trends with time since fire were analyzed using segmented piecewise linear regression. On organic sites, tree basal area and density increased continuously with time since fire, while deadwood abundance decreased and then increased. Live tree basal area, tree density and deadwood abundance generally followed expected S-, N- and U-shaped trends, respectively, on clay sites, but often with decreases in later stages due to paludification. Fewer trends were significant on sand sites, although tree basal area decreased likely due to a change in species composition. Older forests on all site types were more structurally diverse. To estimate the timing of the stages of structural development, we introduce a new analysis technique which uses the breakpoints of the piecewise regressions. On organic sites, only three stages of stand development were evident, whereas a four-stage stand development model was appropriate for both clay and sand sites. We found that local conditions affected not only the timing of developmental stages, but also the number of stages and the trends themselves. We attributed these differences to changes in species composition and productivity. We refine the theory of structural development by representing patterns in both live and deadwood as two-stage trends with two possible outcomes for each stage. Our new method of determining the timing of the developmental stages using empirical data can be used to develop management practices that emulate structural development in order to conserve biodiversity on a landscape scale.

Controls on moss evaporation in a boreal black spruce forest

Mosses are an important component of the boreal forest, but little is known about their contribution to ecosystem carbon, water, and energy exchange. We studied the role of mosses in boreal forest evapotranspiration by conducting two experiments in a black spruce forest in Fairbanks, Alaska. Moss evaporation was measured using lysimeters filled with Hylocomium splendens or Sphagnum capillifolium. Microclimate and moisture content were varied by placing the lysimeters in different habitats (dense forest, open forest, bog), and by manipulating the water supply (no water, natural rainfall, water added). Moss evaporation rates between 1 June and 8 September averaged 0.3, 0.9, and 1.5 mm day−1 in the dense forest (Hylocomium), open forest (Hylocomium and Sphagnum), and bog (Sphagnum) respectively. Assuming a total forest evapotranspiration rate of 2 mm day−1, this study shows that moss evaporation contributes considerably to boreal black spruce forest evapotranspiration. Moss evaporation rates depended strongly on the openness of the forest and to a lesser degree on the density of the vascular plant canopy and on moss species. The strong influence of habitat suggests that microclimate is the primary factor determining moss evaporation rates. Hylocomium evaporation reacted strongly to experimental water additions, indicating that precipitation frequency is an important factor in addition to microclimate for this species. The large moss evaporation rates in this study suggest a potential cooling effect of mosses, of Sphagnum in particular.

Forest structure and composition at young fire and cut edges in black spruce boreal forest

We compared structure and composition at forest edges created by wildfire and clear-cutting in black spruce (Picea mariana (Mill.) BSP) dominated boreal forest in northwestern Quebec. Forest structure and plant species composition were sampled along transects perpendicular to eight 3- to 4-year-old fire edges and eight 2- to 5-year-old cut edges. Significance of edge influence was assessed by comparing mean values at different distances from the edge to the range of variation in interior forest. The influence of clearcut edges was minimal, generally extending only 5 m from the edge, and included greater log density and different species composition, compared with interior forest. At fire edges, prominent responses to edge creation included increased snag density and lower moss cover, compared with interior forest, extending up to 40 m into the forest. This initial structural change was likely due to partial burning extending into the forest. Overall, fire edges had more snags and a different species composition than cut edges. Our hypothesis that edge influence is more extensive at fire edges than at cut edges was supported for overstory and understory structure, but not for species composition. We suggest that there is a need for management to consider the cumulative effect of the loss of fire edges on the landscape.

Modeling topographic effects on net ecosystem productivity of boreal black spruce forests.

Current regional estimates of net primary productivity (NPP) of boreal black spruce overlook the large variation in NPP caused by small-scale topographic effects on soil water, temperature and nutrient availability. Topographic effects on black spruce NPP could likely be modeled by simulating the lateral and vertical movement of water, and its effects on soil nutrient transformation and uptake, through three-dimensional watersheds defined by aspects and slopes of their topographic positions. To examine this likelihood, the ecosystem model 'ecosys' was run for 120 years on a transect that included upper- and lower-slope positions and a basin in which a basal water table was set 0.5 m below the soil surface. For the run, we used soil properties and weather conditions recorded at the 115-year-old BOREAS Southern Old Black Spruce site. Short-term model performance was tested by comparing diurnal and annual carbon (C) transfers simulated under 1994 weather conditions during the 115th year of the model run with those measured at this site during 1994 by eddy covariance, surface chambers and allometry. After 115 years, annual spruce NPP simulated at the upper-slope positions was twice that at the basin (350 versus 170 g C m-2), whereas accumulated wood C was almost three times as large (6.8 versus 2.4 kg C m-2). In the model, increases in NPP and wood growth in upper-slope positions were caused by lower soil water contents, higher soil temperatures, and more rapid O2 uptake that accelerated heterotrophic respiration and hence nutrient mineralization and uptake. Modeled differences in wood growth with topographic position were quantitatively consistent with measurements of boreal black spruce at several research sites differing in water table depth. Modeled differences also agreed with differences in wood growth rates derived from allometric measurements at boreal black spruce sites differing in productivity indices as a result of differences in subsurface hydrology. The magnitude of these differences clearly indicates the importance of accounting for subsurface hydrology in regional estimates of boreal forest productivity.

Year‐round observations of the energy and water vapour fluxes above a boreal black spruce forest

AbstractFluxes of energy and water vapour were measured continuously over an ∼120‐year‐old black spruce (Picea mariana (Mill.) B.S.P.) forest in northern Saskatchewan, Canada, from 6 May 1999 to 5 May 2001 using the eddy covariance technique as part of the Boreal Ecosystem Research and Monitoring Sites (BERMS) program. These results demonstrate that long‐term eddy covariance fluxes can reliably be measured during the harsh boreal winter. Typical daytime bulk surface conductance values were about 5–8 mm s−1 during the growing season. Surface conductance sharply declined after midday in response to increasing vapour pressure deficit. The monthly mean values of energy balance components showed that, in spring (March–June), partitioning of available energy flux was mainly in the form of sensible heat, which changed to almost equal proportions of sensible and latent heat in the summer (July–October). In winter (November–February), most of the net radiation was balanced by sensible heat flux. The monthly mean values of net radiation, sensible heat and latent heat flux were about −20 to 20 W m−2, −10 to 25 W m−2 and 5 to 10 W m−2 respectively in winter. The average mid‐day Bowen ratios were approximately 3·5, 1·7 and 5·2 in the spring, summer and winter seasons respectively. The maximum daily evaporation was about 3·5 mm day−1 in summer and 0·1–0·25 mm day−1 in winter. Over the 2 year period, the accumulated precipitation was 835 mm; this compared with 711 ± 70 mm of evapotranspiration, which showed that more than 85% of water was returned to the atmosphere through evapotranspiration. This study reports the first complete annual cycles of energy and water vapour fluxes at this black spruce site. Since black spruce is the dominant tree species in the North American boreal forest, these results have significance for regional and global energy and water cycles. Copyright © 2003 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.

Structure, composition, and diversity of old-growth black spruce boreal forest of the Clay Belt region in Quebec and Ontario

Old-growth black spruce (Picea mariana) boreal forest in the Clay Belt region of Ontario and Quebec is an open forest with a low canopy, quite different from what many consider to be "old growth". Here, we provide an overview of the characteristics of old-growth black spruce forest for three different site types on organic, clay, and coarse deposits. Our objectives were (1) to identify the extent of older forests; (2) to describe the structure, composition, and diversity in different age classes; and (3) to identify key processes in old-growth black spruce forest. We sampled canopy composition, deadwood abundance, understorey composition, and nonvascular plant species in 91 forest stands along a chronosequence that extended from 20 to more than 250 years after fire. We used a peak in tree basal area, which occurred at 100 years on clay and coarse sites and at 200 years on organic sites, as a process-based means of defining the start of old-growth forest. Old-growth forests are extensive in the Clay Belt, covering 30–50% of the forested landscape. Black spruce was dominant on all organic sites, and in all older stands. Although there were fewer understorey species and none exclusive to old-growth, these forests were structurally diverse and had greater abundance of Sphagnum, epiphytic lichens, and ericaceous species. Paludification, a process characteristic of old-growth forest stands on clay deposits in this region, causes decreases in tree and deadwood abundance. Old-growth black spruce forests, therefore, lack the large trees and snags that are characteristic of other old-growth forests. Small-scale disturbances such as spruce budworm and windthrow are common, creating numerous gaps. Landscape and stand level management strategies could minimize structural changes caused by harvesting, but unmanaged forest in all stages of development must be preserved in order to conserve all the attributes of old-growth black spruce forest. Key words: boreal forest, old growth, paludification, Picea mariana, structural development, succession.

Comparison of Net Primary Production and Light-use Dynamics of Two Boreal Black Spruce Forest Communities

Black spruce (Picea mariana [Mill.] BSP) is the most dominant forest ecosystem in the North American boreal region. There are at least two contrasting boreal black spruce forest communities: open-canopy black spruce overstory with Sphagnum ground cover (BSSP) and closed-canopy black spruce overstory with feathermoss ground cover (BSFM). The objectives of this study were (a) to compare net primary production (NPP) and light-use efficiency (LUE) for the two contrasting black spruce communities, and (b) to quantify the contribution of different vegetation strata (bryophytes, understory, overstory, and belowground) to total NPP and LUE in a mature black spruce forest stand in central Saskatchewan, Canada.Total NPP for the BSFM community (446 g biomass m -2 y -1 ) was significantly greater (P = 0.04) than the BSSP community (276 g biomass m -2 y -1 ). Bryophyte and under-story NPP together comprised the greatest fraction of total (overstory, understory, bryophytes, and belowground) NPP in the BSSP community (51%), whereas overstory NPP comprised the largest fraction of total NPP (86%) for the BSFM community. Overstory LUE was similar (P = 0.87) for the two communities (0.20-0.21 g MJ -1 annual absorbed photosynthetic active radiation [APAR]), but ecosystem LUE was significantly greater (P = 0.03) for the BSFM community (0.20 g MJ -1 annual APAR) than the BSSP community (0.15 g MJ -1 annual APAR). The results from this study demonstrate that all vegetation components must be measured to accurately estimate NPP and LUE of boreal black spruce ecosystems, and community-specific LUE coefficients increase the accuracy of LUE models.