Search

Partitioning the soil surface CO{sub 2} flux (R{sub S}) flux is an important step in understanding ecosystem-level carbon cycling, given that R{sub S} is poorly constrained and its source components may have different responses to climate change. Trenched plots are a classic method of separating the R{sub S} source fluxes, but labor-intensive and may cause considerable disturbance to the soil environment. This study tested if various methods of plant suppression in trenched plots affected R{sub S} fluxes, quantified the R{sub S} response to soil temperature and moisture changes, and estimated the heterotrophic contribution to R{sub S}. It was performed in a boreal black spruce (Picea mariana) plantation, using a complete randomized design, during the 2007 growing season (May-November). Trenched plots had significantly lower R{sub S} than control plots, with differences appearing {approx}100 days after trenching; spatial variability doubled after trenching but then declined throughout the experiment. Most trenching treatments had significantly lower (by {approx}0.5 {mu}mol m{sup -2} s{sup -1}) R{sub S} than the controls, and there was no significant difference in R{sub S} among the various trenching treatments. Soil temperature at 2 cm explained more R{sub S} variability than did 10-cm temperature or soil moisture. Temperature sensitivity (Q10) declined inmore » the control plots from {approx}2.6 (at 5 C) to {approx}1.6 (at 15 C); trenched plots values were higher, from 3.1 at 5 C to 1.9 at 15 C. We estimated R{sub S} for the study period to be 241 {+-} 40 g C m{sup -2}, with roots contributing 64% of R{sub S} after accounting for fine root decay, and 293 g C m{sup -2} for the entire year. These findings suggest that laborious hand weeding of vegetation may be usefully replaced by other methods, easing future studies of this large and poorly-understood carbon flux.« less

The purpose of this study was to quantify the effects of climate change (in the form of elevated air and soil temperatures) on transpiration and sap flux rates of black spruce ( Picea mariana (Mill.) BSP). Five treatments were established in northern Manitoba, Canada, with the experimental design consisting of heated and control blocks. Air and soil temperatures were maintained approximately 5 °C above control temperatures using greenhouse chambers. Two of the chambers also included controls to maintain ambient vapor pressure difference (VPD). Sap flux (JS), was not significantly different among treatments in the May or July time periods. However, JS was significantly greater for heated VPD controlled trees than for trees in all other treatments when averaged over the entire study period. JS was positively correlated to photosynthetic photon flux density for all trees. Lower photosynthetic photon flux density in chamber treatments resulted in the creation of models used to estimate JS and canopy transpiration (EC) values that were used in the analysis. Average daily and cumulative growing season EC values were significantly greater for the heated VPD controlled treatments than for other treatments. The results from this study suggest that EC of boreal black spruce will increase if VPD remains unchanged while air temperatures increase.

The effects of changing climate and disturbance on forest water cycling are not well understood. In particular bryophytes contribute significantly to forest evapotranspiration (ET) in poorly-drained boreal forests, but few studies have directly measured this flux and how it changes with stand age and soil drainage. We used large chambers to measure bryophyte evaporation (E) in Canadian Picea mariana forests of varying ages and soil drainages, as well under controlled laboratory conditions, and modeled daily E using site-specific meteorological data to drive a Penman-Monteith-based model. Field measurements of E averaged 0.37 mm day-1, and ranged from 0.03 (Pleurozium schreberii in a 77-year-old dry stand) to 1.43 mm day-1 (Sphagnum riparium in a 43-year-old bog). canopy resistance ranged from ~0 (at 25 °C, some values were

Bryophytes are the dominant ground cover vegetation layer in many boreal forests and in some of these forests the net primary production of bryophytes exceeds the overstory. Therefore it is necessary to quantify their spatial coverage and species composition in boreal forests to improve boreal forest carbon budget estimates. We present results from a small exploratory test using airborne lidar and multispectral remote sensing data to estimate the percentage of ground cover for mosses in a boreal black spruce forest in Manitoba, Canada. Multiple linear regression was used to fit models that combined spectral reflectance data from CASI and indices computed from the SLICER canopy height profile. Three models explained 63–79% of the measured variation of feathermoss cover while three models explained 69–92% of the measured variation of sphagnum cover. Root mean square errors ranged from 3–15% when predicting feathermoss, sphagnum, and total moss ground cover. The results from this case study warrant further testing for a wider range of boreal forest types and geographic regions.

The boreal forest is predicted to experience the greatest warming of any forest biome during the next 50-100 years, but the effects of warming on vegetation phenology are not well known. The objectives of this study were to (1) examine the effects of whole ecosystem warming on bud burst and annual shoot growth of black spruce trees in northern Manitoba, Canada and (2) correlate bud burst to cumulative degree-days (CDD). The experimental design was a complete randomized block design that consisted of four replicated blocks. Each replicate block contained four treatments: soil warming only (heated outside, HO), soil and air warming (heated inside, HI), control outside (no chamber, no heating, CO), and inside a chamber maintained at ambient conditions (no soil or air warming, control inside, CI). Bud burst was measured during the first and second years of the experiment, starting in 2004, and annual shoot growth was measured for the first 3 years (2004-2006) of the study. On average, shoot bud burst occurred 11 and 9 days earlier in 2004 and 2005, respectively, for HI than for other treatments. However, mean CDD required for bud burst for HI was within the standard deviation of CO for both years. In year 1 of the treatments, shoot bud burst occurred earlier for HI than other treatments (CI, CO, HO), but final shoot length of HI trees was less than in CO trees. In the second year of warming, final shoot length was not different for HI than CO. By the third year of warming final shoot length was significantly greater for HI than all other treatments. Empirical results from this study suggest that soil and air warming causes an earlier bud burst for all years of observation and greater shoot lengths by the third season of warming. A longer growing season and greater annual shoot growth should increase carbon uptake by boreal black spruce trees in a warmer climate.

Changes in fire regimes are driving the carbon balance of much of the North American boreal forest, but few studies have examined fire-driven changes in evapotranspiration (ET) at a regional scale. This study used a version of the Biome-BGC process model with dynamic and competing vegetation types, and explicit spatial representation of a large (106 km2) region, to simulate the effects of wildfire on ET and its components from 1948 to 2005 by comparing the fire dynamics of the 1948-1967 period with those of 1968-2005. Simulated ET averaged, over the entire temporal and spatial modeling domain, 323 mm yr-1; simulation results indicated that changes in fire in recent decades decreased regional ET by 1.4% over the entire simulation, and by 3.9% in the last ten years (1996-2005). Conifers dominated the transpiration (EC) flux (120 mm yr-1) but decreased by 18% relative to deciduous broadleaf trees in the last part of the 20th century, when increased fire resulted in increased soil evaporation, lower canopy evaporation, lower EC and a younger and more deciduous forest. Well- and poorly-drained areas had similar rates of evaporation from the canopy and soil, but EC was twice as high in the well-drained areas. Mosses comprised amore » significant part of the evaporative flux to the atmosphere (22 mm yr-1). Modeled annual ET was correlated with net primary production, but not with temperature or precipitation; ET and its components were consistent with previous field and modeling studies. Wildfire is thus driving significant changes in hydrological processes, changes that may control the future carbon balance of the boreal forest.« less

This research quantified the canopy phenology, growing season and inter-annual dynamics of the overstory and understory vegetation in a boreal wildfire chronosequence in northern Manitoba, Canada. Optical measurements of the canopy radiation regime were made in the 2004–2006 growing seasons to evaluate the spatial and temporal variation in the effective leaf area index (Le) and fraction of intercepted photosynthetic active radiation (FIPAR) of the canopy.

We re-visited a seven-stand boreal chronosequence west of Thompson, Manitoba, Canada, in which coarse woody debris (CWD) and its instantaneous decomposition were measured in 2000. New CWD measurements were performed in 2007, and tree inventories updated to provide mortality and snag failure data. These data were used to model CWD changes, compare methods of estimating decomposition, and infer possible fragmentation rates. Measured CWD was between 9.7 (in both the 77- and 43-year-old stands) and 80.4 (in the 18-year-old stand) Mg ha−1 in 2007. Spatial variability was high; at most stands CWD levels had not changed significantly from 2000 to 2007. Tree mortality was a significant flux only in older stands, whereas snag fall rate varied by an order of magnitude, from 2.9% y−1 (0.2 Mg ha−1 y−1) in the 9-year-old stand to 9.8% y−1 (2.3 Mg ha−1 y−1) in the 12-year-old stand. A one-pool model based on these inputs underestimated actual 2000–2007 CWD decomposition in the younger stands, suggesting that fragmentation could be an important part of the carbon flux exiting the CWD pool. We compared three independent measures of annual decomposition (k): direct measurements of CWD respiration, rates based on the 7-year re-sampling effort described here, and rates inferred from the chronosequence design itself. Mean k values arrived at via these techniques were 0.06 ± 0.03, 0.05 ± 0.04, and 0.05 ± 0.05 y−1, respectively. The four-pool model suggested that the transition rate between decay classes was 0.14–0.19 y−1; the model was most sensitive to initial CWD values. Although the computed k values implied a problem with chronosequence site selection for at least one site, the overall CWD trend was consistent with a larger number of sites surveyed in the region.

A common hypothesis for northern ecosystems is that low soil temperatures inhibit plant productivity. To address this hypothesis, we reviewed how separate components of ecosystem carbon (C) cycling varied along a soil temperature gradient for nine welldrained, relatively productive boreal black spruce (Picea mariana Mill. [B.S.P.]) forests in Alaska, USA, and Saskatchewan and Manitoba, Canada. Annual soil temperature [expressed as soil summed degree days (SDD)] was positively correlated with aboveground net primary productivity (ANPP), while negatively correlated with total belowground carbon flux (TBCF). The partitioning of C to ANPP at the expense of root processes represented a nearly 1:1 tradeoff across the soil temperature gradient, which implied that the amount of C cycling through these black spruce ecosystems was relatively insensitive to variation in SDD. Moreover, the rate at which C accumulated in the ecosystem since the last stand replacing fire was unrelated to SDD, but SDD was positively correlated to the ratio of spruce-biomass:forest-floor-mass. Thus, plant partitioning of C and the distribution of ecosystem C were apparently affected by soil temperature, although across regions, precipitation co-varied with soil temperature. These two factors likely correlated with one another because of precipitation’s influence on soil heat balance, suggesting that a soil temperature‐precipitation interaction could be responsible for the shifts in C allocation. Nonetheless, our results highlight that for this boreal ecosystem, ANPP and TBCF can be negatively correlated. In tropical and temperate forests, TBCF and ANPP have been reported as positively correlated, and our results may reflect the unique interactions between soil temperature, forest floor accumulation, rooting depth, and nutrient availability that characterize the black spruce forest type.

Soil surface carbon dioxide (CO 2 ) flux (Rs) was measured for 2 years at the Boreal Soil and Air Warming Experiment site near Thompson, MB, Canada. The experimental design was a complete random block design that consisted of four replicate blocks, with each block containing a 15 m x 15 m control and heated plot. Black spruce [Picea mariana (Mill.) BSP] was the overstory species and Epilobium angustifolium was the dominant understory. Soil temperature was maintained (∼ 5 °C) above the control soil temperature using electric cables inside water filled polyethylene tubing for each heated plot. Air inside a 7.3-m-diameter chamber, centered in the soil warming plot, contained approximately nine black spruce trees was heated ∼ 5 °C above control ambient air temperature allowing for the testing of soil-only warming and soil + air warming. Soil surface CO 2 flux (Rs) was positively correlated (P

This study analyzed the spatial dependencies of soil moisture and temperature in a six-stand chronosequence of boreal black spruce (Picea mariana (Mill.) BSP) stands. Spatial variability of soil temperature (TSOIL) was evaluated twice during the growing season using four transects in each stand, employing a cyclic sampling design with measurements spaced 2–92 m apart. Soil moisture (θg) was measured on one occasion. A spherical model was used to analyze the geostatistical correlation structure; θg and TSOIL at the 7- and 21-year-old stands did not exhibit stable ranges or sills. The fits with stable ranges and sills modeled the spatial patterns in the older stands reasonably well, although unexplained variability was high. Calculated ranges varied from 3 to 150 m for these stands, lengths probably related to structural characteristics influential in local-scale energy transfer. Transect-to-transect variability was significant and typically 5%–15% of the mean for TSOIL and 10%–70% for θg. TSOIL and θg were negatively correlated for most stands and depths, with TSOIL dropping 0.5–0.9 °C for every 1% rise in θg. The results reported here provide initial data to assess the spatial variability of TSOIL and θg in a variety of boreal forest stand ages.

This study examined the nitrogen (N) dynamics of a black spruce (Picea mariana (Mill.) BSP)-dominated chronosequence in Manitoba, Canada. The seven sites studied each contained separate well- and poorly drained stands, originated from stand-killing wildfires, and were between 3 and 151 years old. Our goals were to (i) measure total N concentration ([N]) of all biomass components and major soil horizons; (ii) compare N content and select vegetation N cycle processes among the stands; and (iii) examine relationships between ecosystem C and N cycling for these stands. Vegetation [N] varied significantly by tissue type, species, soil drainage, and stand age; woody debris [N] increased with decay state and decreased with debris size. Soil [N] declined with horizon depth but did not vary with stand age. Total (live + dead) biomass N content ranged from 18.4 to 99.7 g N m−2 in the well-drained stands and 37.8–154.6 g N m−2 in the poorly drained stands. Mean soil N content (380.6 g N m−2) was unaffected by stand age. Annual vegetation N requirement (5.9 and 8.4 g N m−2 yr−1 in the middle-aged well- and poorly drained stands, respectively) was dominated by trees and fine roots in the well-drained stands, and bryophytes in the poorly drained stands. Fraction N retranslocated was significantly higher in deciduous than evergreen tree species, and in older than younger stands. Nitrogen use efficiency (NUE) was significantly lower in bryophytes than in trees, and in deciduous than in evergreen trees. Tree NUE increased with stand age, but overall stand NUE was roughly constant (∼ ∼150 g g−1 N) across the entire chronosequence.

The spatial and temporal dynamics of soil temperature (TSOIL) strongly influence a wide range of biotic and abiotic processes in boreal forests. Relatively few spatial and temporal TSOIL measurements have been made in these systems; in addition, not all ecosystem models take into account the effect of changing stand structure during stand development on TSOIL dynamics. The goals of this study were to measure TSOIL and its spatiotemporal variability in a boreal forest chronosequence, develop and test a computationally simple empirical model to predict TSOIL, and quantify the effects of different approaches to deriving TSOIL on simulated ecosystem processes. TSOIL was measured hourly at six depths (0‐100 cm) for 3‐4 years, and sampled in a spatial grid monthly during one growing season, in a black spruce (Picea mariana (Mill.) BSP)-dominated boreal forest chronosequence. We report annual and daily TSOIL patterns, air‐soil hystereses, and TSOIL changes at the freeze-thaw transition. An empirical model predicting TSOIL as of the weighted sum of past air temperatures generally accounted for 90‐95% of temporal TSOIL variability at shallow depths, and 77‐83% at 50‐100 cm. A variety of stand structural characteristics affected the model parameters, with leaf area index (LAI) usually the most significant. Spatial TSOIL correlation, measured at depths of 2 and 10 cm, was generally constant between 5 and 30 m. The new empirical model that accounts for changes in canopy structure greatly improved the prediction of TSOIL for stands with high LAI in the Biome-BGC process model; the broader implications of this change are discussed. This model has simple data requirements ‐ only air temperature and LAI ‐ although the parameters given here should only be used for boreal stands with similar soil types. # 2005 Elsevier B.V. All rights reserved.

We quantified the effect of stand age and tree species composition on canopy transpiration ( E C ) by analysing transpiration per unit leaf area ( E L ) and canopy stomatal conductance ( G S ) for boreal trees comprising a five stand wildfire chronosequence. A total of 196 sap flux sensors were used on 90 trees consisting of Betula papyrifera Marsh (paper birch; present in the youngest stand), Populus tremuloides Michx (quaking aspen), Pinus banksiana Lamb. (jack pine), and Picea mariana (Mill.) (black spruce). While fine roots were positively correlated with stand E C ; leaf area index, basal area, and sapwood area were not. Stands less than 70 years old were dominated by Populus tremuloides and Pinus banksiana and stands greater than 70 years old were composed almost entirely of Picea mariana. As Populus tremuloides and Pinus banksiana increased in size and age, they displayed an increasing sapwood to leaf area ratio ( A S : A L ), a constant minimum leaf water potential (Y L ), and a constant proportionality between G S at low vapour pressure deficit ( DjG Sref ) and the sensitivity of G S to D (‐d ). In contrast, A S : A L , minimum Y L , and the proportionally between ‐d and G Sref decreased with height and age in Picea mariana . A G S model that included the effects of D , A S : A L , tree height, and for Picea mariana an increasing soil to leaf water potential gradient with stand age, was able to capture the effects of contrasting hydraulic properties of Picea mariana , Populus tremuloides and Pinus banksiana during stand development after wildfire.

Net primary production (NPP) was measured in seven black spruce (Picea mariana (Mill.) BSP)-dominated sites comprising a boreal forest chronosequence near Thompson, Man., Canada. The sites burned between 1998 and 1850, and each contained separate well- and poorly drained stands. All components of NPP were measured, most for 3 consecutive years. Total NPP was low (50‐100gCm � 2 yr � 1 ) immediately after fire, highest 12‐20 years after fire (332 and 521gCm � 2 yr � 1 in the dry and wet stands, respectively) but 50% lower than this in the oldest stands. Tree NPP was highest 37 years after fire but 16‐ 39% lower in older stands, and was dominated by deciduous seedlings in the young stands and by black spruce trees (485%) in the older stands. The chronosequence was unreplicated but these results were consistent with 14 secondary sites sampled across the landscape. Bryophytes comprised a large percentage of aboveground NPP in the poorly drained stands, while belowground NPP was 0‐40% of total NPP. Interannual NPP variability was greater in the youngest stands, the poorly drained stands, and for understory and detritus production. Net ecosystem production (NEP), calculated using heterotrophic soil and woody debris respiration data from previous studies in this chronosequence, implied that the youngest stands were moderate C sources (roughly, 100gC m � 2 yr � 1 ), the middle-aged stands relatively strong sinks (100‐300gCm � 2 yr � 1 ), and the oldest stands about neutral with respect to the atmosphere. The ecosystem approach employed in this study provided realistic estimates of chronosequence NPP and NEP, demonstrated the profound impact of wildfire on forest‐atmosphere C exchange, and emphasized the need to account for soil drainage, bryophyte production, and species succession when modeling boreal forest C fluxes.

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.

The objective of this study was to compare the carbon (C) budgets of two similar-aged boreal black spruce (Picea mariana [Mill.] BSP) forest communities: closed-canopy black spruce with feathermoss ground cover (BSFM) on moderately drained soils and open-canopy black spruce with Sphagnum ground cover (BSSP) on poorly drained soils. C content, soil surface carbon dioxide (CO 2 ) flux (R s ), heterotrophic respiration (R H ), net primary production (NPP), and net ecosystem production (NEP) were measured or estimated. The total C content for the two communities (BSFM, 113 Mg C ha -1 ; BSSP, 107 Mg C ha -1 ) did not differ significantly (P = 0.95). However, annual R s was significantly greater (P < 0.0001) for the BSFM (564 g C m -2 y -1 ) than the BSSP (319 g C m -2 y -1 ) community. The greater R H in the BSFM (440 g C m -2 y -1 ) than the BSSP community (264 g C m -2 y -1 ) and the higher peat C content for the BSSP (84 Mg C ha -1 ) than the BSFM (34 Mg C ha -1 ) community provided evidence that the BSSP community had a lower decomposition rate than the BSFM community. NEP was significantly (P = 0.04) more negative for the BSFM community (-222 ± 35 g C m -2 y -1 ) than the BSSP community (-128 ± 14 g C m -2 y -1 ), but both communities were C sources to the atmosphere. The results from this study illustrate the influence of small differences in edaphic conditions on ecosystem C accumulation.

Knowledge of foliar surface area is important in many fields, but estimating the area of nonflat conifer needles is difficult. The primary goal of this study was to use optical scanning and immersion methods to test and refine the standard cross-sectional geometries assumed for black spruce (Picea mariana (Mill.) BSP) and jack pine (Pinus banksiana Lamb.) needles. Projected leaf area (PLA, measured using a flatbed scanner), and hemisurface leaf area (HSLA, estimated from water immersion) were compared for conifer samples from a 37-year-old even-aged stand in northern Manitoba, Canada. The HSLA–PLA relationship was used to infer information about needle cross-sectional geometry after assuming a basic form (rhombus for black spruce and hemiellipse for jack pine). The cross section of black spruce needles was best approximated as a rhombus with a major/minor diagonal ratio of 1.35. Jack pine needles were best described by a hemiellipse with major/minor axis ratio of 1.30. Minor but incorrect assumptions of needle cross-sectional geometry resulted in foliar area errors of 6–8% using scanning methods and 1–2% using immersion methods. Simple equations are presented to calculate hemisurface needle area from volume or projected needle area based on these refined parameters.

Specific leaf area (SLA) and leaf area index (LAI) were estimated using site-specific allometric equations for a boreal black spruce (Picea mariana (Mill.) BSP) fire chronosequence in northern Manitoba, Canada. Stands ranged from 3 to 131 years in age and had soils that were categorized as well or poorly drained. The goals of the study were to: (i) measure SLA for the dominant tree and understory species of boreal black spruce-dominated stands, and examine the effect of various biophysical conditions on SLA; and (ii) examine leaf area dynamics of both understory and overstory for well- and poorly drained stands in the chronosequence. Overall, average SLA values for black spruce (n = 215), jack pine (Pinus banksiana Lamb., n = 72) and trembling aspen (Populus tremuloides Michx., n = 27) were 5.82 +/- 1.91, 5.76 +/- 1.91 and 17.42 +/- 2.21 m2 x kg-1, respectively. Foliage age, stand age, vertical position in the canopy and soil drainage had significant effects on SLA. Black spruce dominated overstory LAI in the older stands. Well-drained stands had significantly higher overstory LAI (P0.001), but lower understory LAI (P = 0.022), than poorly drained stands. Overstory LAI was negligible in the recent (3-12 years old) burn sites and highest in the 70-year-old burn site (6.8 and 3.0 in the well- and poorly drained stands, respectively), declining significantly (by 30-50%) from this peak in the oldest stands. Understory leaf area represented a significant portion (40%) of total leaf area in all stands except the oldest.

Carbon dioxide flux from coarse woody debris (CWD) is an important source of CO2 in forests with moderate to large amounts of CWD. A process-based understanding of environmental controls on CWD CO2 flux (RCWD) is needed to accurately model carbon exchange between forests and the atmosphere. The objectives of this study were to: (1) use a laboratory incubation factorial experiment to quantify the effect of temperature (TCWD), water content (WC), decay status, and their interactions on RCWD for black spruce [Picea mariana (Mill.) BSP] CWD; (2) measure and model spatial and temporal dynamics in TCWD for a boreal black spruce fire chronosequence; and (3) validate the RCWD model with field measurements, and quantify potential errors in estimating annual RCWD from this model on various time steps. The RCWD was positively correlated to TCWD (R2=0.37, P

Allometric equations were developed relating aboveground biomass, coarse root biomass, and sapwood area to stem diameter at 17 study sites located in the boreal forests near Thompson, Man. The six species studied were trembling aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera Marsh.), black spruce (Picea mariana (Mill.) BSP), jack pine (Pinus banksiana Lamb.), tamarack (Larix laricina (Du Roi) Koch.), and willow (Salix spp.). Stands ranged in age from 4 to 130 years and were categorized as well or poorly drained. Stem diameter ranged from 0.1 to 23.7 cm. Stem diameter was measured at both the soil surface (D0) and breast height (DBH). The relationship between biomass and diameter, fitted on a log–log scale, changed significantly at ~3 cm DBH, suggesting that allometry differed between saplings and older trees. To eliminate this nonlinearity, a model of form log10 Y = a + b(log10 D) + c(AGE) + d(log10 D × AGE) was used, where D is stem diameter, AGE is stand age, and the cross product is the interaction between diameter and age. Most aboveground biomass equations (N = 326) exhibited excellent fits (R2 > 0.95). Coarse root biomass equations (N = 205) exhibited good fits (R2 > 0.90). Both D0 and DBH were excellent (R2 > 0.95) sapwood area predictors (N = 413). Faster growing species had significantly higher ratios of sapwood area to stem area than did slower growing species. Nonlinear aspects of some of the pooled biomass equations serve as a caution against extrapolating allometric equations beyond the original sample diameter range.

Boreal black spruce forests typically have a dense ground cover of bryophytes. The two main bryophyte groups in boreal black spruce forests, feathermoss and Sphagnum, have ecophysiological characteristics that influence the biogeochemical cycles of black spruce forests differently. The objective of this study was to examine the environmental controls of ground cover composition and net primary production (NPP) of feathermoss and Sphagnum in a boreal black spruce forest in central Saskatchewan. The fraction of Sphagnum ground cover was positively correlated to canopy photosynthetically active radiation (PAR) transmittance (r2=0.48, P=0.03), but the fraction of feathermoss ground cover was negatively correlated to canopy PAR transmittance in plots where Sphagnum was present (r2=0.87, P

Estimates of leaf area index obtained with indirect measurement techniques, which are replacing more arduous destructive sampling methods, are frequently questioned due to light interception by woody elements and a non-random distribution of foliage elements. Usually, branches are assumed to be positioned randomly with respect to leaves or shoots in the canopy. However, in this study of boreal forest architecture, branches are shown to be preferentially shaded by other non-woody elements (e.g. shoots or leaves) in both coniferous and deciduous species of the boreal region. A new instrument called a Multiband Vegetation Imager (MVI) is used to capture two-band (Visible, 400–620 nm and Near-Infrared, 720–950 nm) image pairs of contrasting Canadian boreal forest canopies during the BOReal Ecosystem-Atmosphere Study (BOREAS). The spatial relationship of branches and photosynthetically active foliage is studied to estimate the fraction of the effective branch hemi-surface area index ( B e ) that is masked by leaves and shoots. We suggest an approach that corrects indirect LAI measurements using the LAI-2000 or a similar instrument by correcting for the following biases: (1) the effective canopy branch hemi-surface area that is not masked by leaves or shoots in the canopy, (2) the amount of stem hemi-surface area beneath crowns, (3) leaf (or shoot) (Ω e ( θ )) and branch (Ω b ( θ )) non-random spatial distributions in the canopy, and (4) the fraction of maximum LAI resulting from defoliation in the canopy. In boreal aspen, MVI image analysis shows that 95% of the effective branch hemi-surface area is masked by other foliage in the canopy. In jack pine and black spruce forests, 80–90% of the effective branch hemi-surface area is masked by other foliage in the canopy. These estimates suggest the fraction of indirect LAI that consists of branches intercepting light is less than 10%. Therefore, branches generally do not intercept a significant amount of beam radiation in boreal forests, and do not significantly bias indirect LAI measurements. However, stems, which comprise 30–50% of the total woody area in this study, may not be preferentially shaded by leafy foliage. Therefore, stem contribution to indirect LAI estimates measured with the LAI-2000 or a similar instrument cannot be overlooked. MVI estimates of the total branch hemi-surface area index agree to within 10–40% of direct measurements made in similar species; however, the error between indirect and direct measurements may be due largely to difficulties associated with obtaining adequate sampling so that the error may fall within the noise level of measurements.

We used eddy covariance; gas-exchange chambers; radiocarbon analysis; wood, moss, and soil inventories; and laboratory incubations to measure the carbon balance of a 120-year-old black spruce forest in Manitoba, Canada. The site lost 0.3 ± 0.5 metric ton of carbon per hectare per year (ton C ha −1 year −1 ) from 1994 to 1997, with a gain of 0.6 ± 0.2 ton C ha −1 year −1 in moss and wood offset by a loss of 0.8 ± 0.5 ton C ha −1 year −1 from the soil. The soil remained frozen most of the year, and the decomposition of organic matter in the soil increased 10-fold upon thawing. The stability of the soil carbon pool (∼150 tons C ha −1 ) appears sensitive to the depth and duration of thaw, and climatic changes that promote thaw are likely to cause a net efflux of carbon dioxide from the site.

Measurements of soil-surface CO2 fluxes are important for characterizing the carbon budget of boreal forests because these fluxes can be the second largest component of the budget. Several methods for measuring soil-surface CO2 fluxes are available: (1) closed-dynamic-chamber systems, (2) closed-static-chamber systems, (3) open-chamber systems, and (4) eddy covariance systems. This paper presents a field comparison of six individual systems for measuring soil-surface CO2 fluxes with each of the four basic system types represented. A single system is used as a reference and compared to each of the other systems individually in black spruce (Picea mariana), jack pine (Pinus banksiana), or aspen (Populus tremuloides) forests. Fluxes vary from 1 to 10 μmol CO2 m−2 s−1. Adjustment factors to bring all of the systems into agreement vary from 0.93 to 1.45 with an uncertainty of about 10–15%.

Leaf area index (LAI) is a key structural characteristic of forest ecosystems because of the role of green leaves in controlling many biological and physical processes in plant canopies. Accurate LAI estimates are required in studies of ecophysiology, atmosphere-ecosystem interactions, and global change. The objective of this paper is to evaluate LAI values obtained by several research teams using different methods for a broad spectrum of boreal forest types in support of the international Boreal Ecosystem-Atmosphere Study (BOREAS). These methods include destructive sampling and optical instruments: the tracing radiation and architecture of canopies (TRAC), the LAI-2000 plant canopy analyzer, hemispherical photography, and the Sunfleck Ceptometer. The latter three calculate LAI from measured radiation transmittance (gap fraction) using inversion models that assume a random spatial distribution of leaves. It is shown that these instruments underestimate LAI of boreal forest stands where the foliage is clumped. The TRAC quantifies the clumping effect by measuring the canopy gap size distribution. For deciduous stands the clumping index measured from TRAC includes the clumping effect at all scales, but for conifer stands it only resolves the clumping effect at scales larger than the shoot (the basic collection of needles). To determine foliage clumping within conifer shoots, a video camera and rotational light table system was used. The major difficulties in determining the surface area of small conifer needles have been largely overcome by the use of an accurate volume displacement method. Hemispherical photography has the advantage that it also provides a permanent image record of the canopies. Typically, LAI falls in the range from 1 to 4 for jack pine and aspen forests and from 1 to 6 for black spruce. Our comparative studies provide the most comprehensive set of LAI estimates available for boreal forests and demonstrate that optical techniques, combined with limited direct foliage sampling, can be used to obtain quick and accurate LAI measurements.

Root biomass, net primary production and turnover were studied in aspen, jack pine and black spruce forests in two contrasting climates. The climate of the Southern Study Area (SSA) near Prince Albert, Saskatchewan is warmer and drier in the summer and milder in the winter than the Northern Study Area (NSA) near Thompson, Manitoba, Canada. Ingrowth soil cores and minirhizotrons were used to quantify fine root net primary production (NPPFR). Average daily fine root growth (m m(-2) day(-1)) was positively correlated with soil temperature at 10-cm depth (r(2) = 0.83-0.93) for all three species, with black spruce showing the strongest temperature effect. At both study areas, fine root biomass (measured from soil cores) and fine root length (measured from minirhizotrons) were less for jack pine than for the other two species. Except for the aspen stands, estimates of NPPFR from minirhizotrons were significantly greater than estimates from ingrowth cores. The core method underestimated NPPFR because it does not account for simultaneous fine root growth and mortality. Minirhizotron NPPFR estimates ranged from 59 g m(-2) year(-1) for aspen stands at SSA to 235 g m(-2) year(-1) for black spruce at NSA. The ratio of NPPFR to total detritus production (aboveground litterfall + NPPFR) was greater for evergreen forests than for deciduous forests, suggesting that carbon allocation patterns differ between boreal evergreen and deciduous forests. In all stands, NPPFR consistently exceeded annual fine root turnover and the differences were larger for stands in the NSA than for stands in the SSA, whereas the difference between study areas was only significant for black spruce. The imbalance between NPPFR and fine root turnover is sufficient to explain the net accumulation of carbon in boreal forest soils.

A strong argument can be made that there is a greater need to study the effect of warming on boreal forests more than on any other terrestrial biome. Boreal forests, the second largest forest biome, are predicted to experience the greatest warming of any forest biome in the world, but a process-based understanding of how warming will affect the structure and function of this economically and ecologically important forest biome is lacking. The effects of warming on species composition, canopy structure and biogeochemical cycles are likely to be complex; elucidating the underlying mechanisms will require long-term whole-ecosystem manipulation to capture all the complex feedbacks (Shaver et al. 2000, Rustad et al. 2001, Stromgren 2001). The DOE Program for Ecosystem Research funded a three year project (2002-2005) to use replicated heated chambers on soil warming plots in northern Manitoba to examine the direct effects of whole-ecosystem warming. We are nearing completion of our first growing season of measurements (fall 2004). In spite of the unforeseen difficulty of installing the heating cable, our heating and irrigation systems worked extremely well, maintaining environmental conditions within 5-10% of the specified design 99% of the time. Preliminary data from these systems, all designed and builtmore » by our laboratory at the University of Wisconsin, support our overall hypothesis that warming will increase the carbon sink strength of upland boreal black spruce forests. I request an additional three years of funding to continue addressing the original objectives: (1) Examine the effect of warming on phenology of overstory, understory and bryophyte strata. Sap flux systems and dendrometer bands, monitored by data loggers, will be used to quantify changes in phenology and water use. (2) Quantify the effects of warming on nitrogen and water use by overstory, understory and bryophytes. (3) Compare effects of warming on autotrophic respiration and above- and belowground net primary production (NPP) budgets. Autotrophic respiration budgets will be constructed using chamber measurements for each tissue and NPP and standard allometry techniques (Gower et al. 1999). (4) Compare microbial and root dynamics, and net soil surface CO2 flux, of control and warmed soils to identify causes that may explain the hypothesized minimal effect of soil warming on soil surface CO2 flux. Fine root production and turnover will be quantified using minirhizotrons, and microbial dynamics will be determined using laboratory mineralization incubations. Soil surface CO2 flux will be measured using automated soil surface CO2 flux systems and portable CO2 analyzers. The proposed study builds on the existing research programs Gower has in northern Manitoba and would not be possible without in-kind services and financial support from Manitoba Hydro and University of Wisconsin.« less

We measured light-saturated photosynthesis (A(net)), foliage respiration (R(fol)) and stem respiration (R(stem)) of black spruce (Picea mariana (Mill.) B.S.P.) in heated (+5 degrees C) and control plots during 2005, 2006 and 2007 in Thompson, MB, Canada. Large greenhouses and soil-heating cables were used to maintain air and soil temperature 5 degrees C above ambient air and soil temperature. Each greenhouse contained approximately nine black spruce trees and the majority of their fine root mass. Treatments were soil and air warming, soil-only warming, greenhouses maintained at ambient air temperature and control. Gas exchange rates ranged 0.71-4.66, 0.04-0.74 and 0.1-1.0 micromol m(-)(2) s(-)(1) for A(net), R(fol) and R(stem), respectively. Treatment differences for A(net), R(fol) and R(stem) were not significant in any of the 3 years of measurements. The results of this experiment suggest that in a warmer climate, black spruce may not have significant changes in the rate of photosynthesis or respiration.

This study used the Biome Biogeochemical Cycles (Biome-BGC) process model to simulate boreal forest dynamics, compared the results with a variety of measured carbon content and flux data from two boreal chronosequences in northern Manitoba, Canada, and examined how model output was affected by water and nitrogen limitations on simulated plant production and decomposition. Vascular and nonvascular plant growth were modeled over 151 years in well-drained and poorly drained forests, using as many site-specific model parameters as possible. Measured data included (1) leaf area and carbon content from site-specific allometry data, (2) aboveground and belowground net primary production from allometry and root cores, and (3) flux data, including biometry-based net ecosystem production and tower-based net ecosystem exchange. The simulation used three vegetation types or functional groups (evergreen needleaf trees, deciduous broadleaf trees, and bryophytes). Model output matched some of the observed data well, with net primary production, biomass, and net ecosystem production (NEP) values usually (50–80% of data) within the errors of observed values. Leaf area was generally underpredicted. In the simulation, nitrogen limitation increased with stand age, while soil anoxia limited vascular plant growth in the poorly drained simulation. NEP was most sensitive to climate variability in the poorly drained stands. Simulation results are discussed with respect to conceptual issues in, and parameterization of, the Biome-BGC model.

Biogeochemical process models are increasingly employed to simulate current and future forest dynamics, but most simulate only a single canopy type. This limitation means that mixed stands, canopy succession and understory dynamics cannot be modeled, severe handicaps in many forests. The goals of this study were to develop a version of Biome-BGC that supported multiple, interacting vegetation types, and to assess its performance and limitations by comparing modeled results to published data from a 150-year boreal black spruce (Picea mariana (Mill.) BSP) chronosequence in northern Manitoba, Canada. Model data structures and logic were modified to support an arbitrary number of interacting vegetation types; an explicit height calculation was necessary to prioritize radiation and precipitation interception. Two vegetation types, evergreen needle-leaf and deciduous broadleaf, were modeled based on site-specific meteorological and physiological data. The new version of Biome-BGC reliably simulated observed changes in leaf area, net primary production and carbon stocks, and should be useful for modeling the dynamics of mixed-species stands and ecological succession. We discuss the strengths and limitations of Biome-BGC for this application, and note areas in which further work is necessary for reliable simulation of boreal biogeochemical cycling at a landscape scale.

We quantified the contributions of root respiration (RC) and heterotrophic respiration to soil surface CO 2 flux (R S ) by comparing trenched and untrenched plots in well-drained and poorly drained stands of a black spruce (Picea mariana (Mill.) BSP) fire chronosequence in northern Manitoba, Canada. Our objectives were to: (1) test different equations for modeling R S as a function of soil temperature; and (2) model annual R S and RC for the chronosequence from continuous soil temperature measurements. The choice of equation to model R S strongly affected annual R S and RC, with an Arrhenius-based model giving the best fit to the data, especially at low temperatures. Modeled values of annual R S were positively correlated with soil temperature at 2-cm depth and were affected by year of burn and trenching, but not by soil drainage. During the growing season, measured RC was low in May, peaked in late July and declined to low values by the end of the growing season. Annual RC was < 5% of R S in the recently burned stands, ∼40% in the 21-year-old stands and 5-15% in the oldest (152-year-old) stands. Evidence suggests that RC may have been underestimated in the oldest stands, with residual root decay from trenching accounting for 5-10% of trenched plot R S at most sites.

[1] Understanding the effects of wildfire on the carbon (C) cycle of boreal forests is essential to quantifying the role of boreal forests in the global carbon cycle. Soil surface CO2 flux (Rs), the second largest C flux in boreal forests, is directly and indirectly affected by fire and is hypothesized to change during forest succession following fire. The overall objective of this study was to measure and model Rs for a black spruce (Picea mariana [Mill.] BSP) postfire chronosequence in northern Manitoba, Canada. The experiment design was a nested factorial that included two soil drainage classes (well and poorly drained) × seven postfire aged stands. Specific objectives were (1) to quantify the relationship between Rs and soil temperature for different aged boreal black spruce forests in well-drained and poorly drained soil conditions, (2) to examine Rs dynamics along postfire successional stands, and (3) to estimate annual soil surface CO2 flux for these ecosystems. Soil surface CO2 flux was significantly affected by soil drainage class (p = 0.014) and stand age (p = 0.006). Soil surface CO2 flux was positively correlated to soil temperature (R2 = 0.78, p < 0.001), but different models were required for each drainage class × aged stand combination. Soil surface CO2 flux was significantly greater at the well-drained than the poorly drained stands (p = 0.007) during growing season. Annual soil surface CO2 flux for the 1998, 1995, 1989, 1981, 1964, 1930, and 1870 burned stands averaged 226, 412, 357, 413, 350, 274, and 244 g C m-2 yr−1 in the well-drained stands and 146, 380, 300, 303, 256, 233, and 264 g C m−2 yr−1 in the poorly drained stands. Soil surface CO2 flux during the winter (from 1 November to 30 April) comprised from 5 to 19% of the total annual Rs. We speculate that the smaller soil surface CO2 flux in the recently burned than the older stands is mainly caused by decreased root respiration.

Measurements of the photosynthetic response to midsummer irradiance were made for 11 species representing the dominant trees, understory shrubs, herbaceous plants and moss species in an old black spruce (Picea mariana (Mill.) B.S.P.) boreal forest ecosystem. Maximum rates of photosynthesis per unit foliage area at saturating irradiance, A(max), were highest for aspen (Populus tremuloides Michx.), reaching 16 micromol m(-2) s(-1). For tamarack (Larix laricina (Du Roi) K. Kock) and P. mariana, Amax was only 2.6 and 1.8 micromol m(-2) s(-1), respectively. Values of A(max) for understory shrubs and herbaceous plants were clustered between 9 and 11 micromol m(-2) s(-1), whereas A(max) of feather moss (Pleurozium schreberi (Brid.) Mitt.) reached only 1.9 micromol m(-2) s(-1). No corrections were made for differences in shoot structure, but values of photosynthetic light-use efficiency were similar for most species (70-80 mmol CO2 mol(-1)); however, they were much lower for L. laricina and P. mariana (15 mmol CO2 mol(-1)) and much higher for P. schreberi (102 m;mol CO2 mol(-1)). There was a linear relationship between Amax and foliage nitrogen concentration on an area basis for the broad-leaved species in the canopy and understory, but the data for P. mariana, L. laricina and P. schreberi fell well below this line. We conclude that it is not possible to scale photosynthesis from leaves to the canopy in this ecosystem based on a single relationship between photosynthetic rate and foliage nitrogen concentration.

Author(s): Gower, ST; Hunter, A; Campbell, J; Vogel, J; Veldhuis, H; Harden, J; Trumbore, S; Norman, JM; Kucharick, CJ | Abstract: The objective of this study was to compare nutrient concentration, distribution, and select components of nutrient budgets for aspen (Populus tremuloides), jack pine (Pinus banksiana), and black spruce (Picea mariana) forest ecosystems at the BOReal Ecosystem Atmosphere Study (BOREAS), southern and northern study areas near Candle Lake, Saskatchewan and Thompson, Manitoba, Canada, respectively. The vegetation (excluding fine roots and understory) in the aspen, black spruce, and jack pine stands contained 70-79%, 53-54%, and 58-67% of total ecosystem carbon content, respectively. Soil (forest floor and mineral soil) nitrogen (N), calcium (Ca), and magnesium (Mg) content comprised over 90% of the total ecosystem nutrient content, except for Ca and Mg content of the southern black spruce stand and Ca content of the southern aspen stand which were less than 90%. Annual litterfall N content was significantly greater (p l 0.05) for trembling aspen (30-41 kg N ha-1 yr-1) than for jack pine (5-10 kg N ha-1 yr-1) or black spruce (6-7 kg N ha- yr-1), and was generally greater, but not significantly, for the southern than for the northern study area. Aboveground net primary production was positively correlated (R2 = 0.91) to annual litterfall N content for the BOREAS forests, and for all boreal forests (R2 = 0.57). Annual aboveground nutrient (N, Ca, Mg, and K) requirements (sum of the annual increment of nutrient in foliage, branches, and stems) were significantly greater (p l 0.05 ) for trembling aspen than for jack pine or black spruce forests. Annual aboveground N requirements ranged from 37-53, 6-14 and 6-7 kg N ha-1 yr-1 for trembling aspen, jack pine, and black spruce forests, respectively. The greater nutrient requirements of decidous than evergreen boreal forests was explaianed by a greater annual production of biomass and lower use efficiency of nutrients. Nutrients cycling characteristics of boreal forests were influenced by climate and forest type, with the latter having a greater influence on litterfall N, annual nutrient requirements, nutrient mean residence time, and nutrient distribution.

[1] This study examined the distribution and respiration dynamics of woody debris (WD) in a black spruce-dominated fire chronosequence in northern Manitoba, Canada. The chronosequence included seven stands that burned between 1870 and 1998; each stand contained separate well-drained and poorly drained areas. The objectives of this study were to (1) quantify the distribution of WD, by diameter and decay class, in well-drained and poorly drained stands across the chronosequence, (2) measure the evolution of CO2 from WD samples and model the effects of moisture, size and decay on respiration, and (3) model annual WD respiration and compute decay constants for each site. Coarse woody debris biomass ranged from 1.4 Mg ha−1 to 177.6 Mg ha−1, generally declining in the older stands of the chronosequence. More decayed WD had significantly (α = 0.05) higher moisture, lower density, and higher respiration rates than less decayed WD. Moisture and decay class were significant predictors of respiration when moisture was below 43%. Above this level, moisture was not significant, but stand soil drainage was significant, with drier sites having higher WD respiration. Year of burn was not significant in the respiration models. Modeled annual carbon emissions from WD ranged from 0.11 to 1.92 Mg C ha−1 yr−1. Modeled annual decay rates, between k = 0.01 and k = 0.06, changed across the age sequence. Thus a single-exponential decay model may not be appropriate for the age sequence. Calculating k directly from wood respiration measurements, as done here, may be useful in allowing examination of year-to-year changes in k.