Response of Net Ecosystem Productivity of Three Boreal Forest Stands to Drought

In 2001–03, continuous eddy covariance measurements of carbon dioxide (CO₂) flux were made above mature boreal aspen, black spruce, and jack pine forests in Saskatchewan, Canada, prior to and during a 3−year drought. During the 1st drought year, ecosystem respiration (R) was reduced at the aspen site due to the drying of surface soil layers. Gross ecosystem photosynthesis (GEP) increased as a result of a warm spring and a slow decrease of deep soil moisture. These conditions resulted in the highest annual net ecosystem productivity (NEP) in the 9 years of flux measurements at this site. During 2002 and 2003, a reduction of 6% and 34% in NEP, respectively, compared to 2000 was observed as the result of reductions in both R and GEP, indicating a conservative response to the drought. Although the drought affected most of western Canada, there was considerable spatial variability in summer rainfall over the 100−km extent of the study area; summer rainfalls in 2001 and 2002 at the two conifer sites minimized the impact of the drought. In 2003, however, precipitation was similarly low at all three sites. Due to low topographic position and consequent poor drainage at the black spruce site and the coarse soil with low water-holding capacity at the jack pine site almost no reduction in R, GEP, and NEP was observed at these two sites. This study shows that the impact of drought on carbon sequestration by boreal forest ecosystems strongly depends on rainfall distribution, soil characteristics, topography, and the presence of vegetation that is well adapted to these conditions.     

Carbon sequestration in boreal jack pine stands following harvesting

A large area of boreal jack pine (Pinus banksiana Lamb.) forest in Canada is recovering from clear‐cut harvesting, and the carbon (C) balance of these regenerating forests remains uncertain. Net ecosystem CO₂ exchange was measured using the eddy‐covariance technique at four jack pine sites representing different stages of stand development: three postharvest sites (HJP02, HJP94, and HJP75) and one preharvest site (OJP). The four sites, located in the southern Canadian boreal forest, Saskatchewan, Canada, are typical of low productivity jack pine stands and were 2, 10, 29, and 90 years old in 2004, respectively. Mean annual net ecosystem production (NEP) for 2004 and 2005 was ?137±11, 19±16, 73±28, and 22±30 g C m^?2 yr^?1 at HJP02, HJP94, HJP75 and OJP, respectively, showing the postharvest jack pine stands to be moderate C sources immediately after harvesting, weak sinks at 10 years, moderate C sinks at 30 years, then weak C sinks at 90 years. Mean annual gross ecosystem photosynthesis (GEP) for the 2 years was 96±10, 347±20, 576±34, and 583±35 g C m^?2 yr^?1 at HJP02, HJP94, HJP75, and OJP, respectively. The ratio of annual ecosystem respiration (R) to annual GEP was 2.51±0.15, 0.95±0.04, 0.87±0.03, and 0.96±0.03. Seasonally, NEP peaked in May or June at all four sites but GEP and R were highest in July. R at a reference soil temperature of 10 °C, ecosystem quantum yield and photosynthetic capacity were lowest for the 2‐year‐old stand. R was most sensitive to soil temperature for the 90‐year‐old stand. The primary source of variability in NEP over the course of succession of the jack pine ecosystem following harvesting was stand age due to the changes in leaf area index. Intersite variability in GEP and R was an order of magnitude greater than interannual variability at OJP. For both young and old stands, GEP had greater interannual variability than R and played a more important role than R in interannual variation in NEP. Based on year‐round flux measurements from 2000 to 2005, the 10‐year stand had larger interannual variability in GEP and R than the 90‐year stand. Interannual variability in NEP was driven primarily by early‐growing‐season temperature and growing‐season length. Photosynthesis played a dominant role in the rapid rise in NEP early in stand development. Late in stand development, however, the subtle decrease in NEP resulted primarily from increasing respiration.     

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

Although 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.     

Competition and facilitation between tree species change with stand development

Processes governing tree interspecific interactions, such as facilitation and competition, may vary in strength over time. This study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana, trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not differ with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecific interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecific interactions may differ with soil nutrient availability.     

Spatial and temporal variations in boreal forest fire frequency in northern Alberta

Abstract. Spatial and temporal variations in fire frequency in the boreal forest of Wood Buffalo National Park (WBNP) were assessed using forest stand age, fire scar and historical data. I test the hypotheses that (1) fire frequency is higher in jack pine forests and aspen forests than in black spruce forests and white spruce forests, (2) these variations in fire frequency can be related to the mean waterbreak distance (MWD) around a site and (3) fire frequency has changed over the past 300 years. The fire cycles (the time required to burn an area equal in size to the entire study area) in jack pine forests (39 years) and in aspen forests (39 years) were significantly shorter than those in black spruce forests (78 years) and in white spruce forests (96 years). The length of the fire cycle varies inversely with the MWD around a site, and the MWD was significantly higher in jack pine and aspen forests than in black or white spruce forests. It is suggested that covariations between soil type and the MWD influence, respectively, variations in forest dominant and fire frequency. A change in fire frequency at 1860 was apparent in the fire history for all of WBNP, the black spruce dominated stands, and the near and medium MWD classes. The fire cycle estimates for these classes were all significantly shorter during the period 1750 to 1859 (fire cycles = 25–49 years) than they were in the period 1860 to 1989 (fire cycles = 59–89 years). The possible roles of changes in climate and aboriginal burning practices in causing the temporal change in fire frequency are discussed.     

Soil CO2 efflux in contrasting boreal deciduous and coniferous stands and its contribution to the ecosystem carbon balance

Similar nonsteady‐state automated chamber systems were used to measure and partition soil CO₂ efflux in contrasting deciduous (trembling aspen) and coniferous (black spruce and jack pine) stands located within 100 km of each other near the southern edge of the Boreal forest in Canada. The stands were exposed to similar climate forcing in 2003, including marked seasonal variations in soil water availability, which provided a unique opportunity to investigate the influence of climate and stand characteristics on soil CO₂ efflux and to quantify its contribution to the net ecosystem CO₂ exchange (NEE) as measured with the eddy‐covariance technique. Partitioning of soil CO₂ efflux between soil respiration (including forest‐floor vegetation) and forest‐floor photosynthesis showed that short‐ and long‐term temporal variations of soil CO₂ efflux were related to the influence of (1) soil temperature and water content on soil respiration and (2) below‐canopy light availability, plant water status and forest‐floor plant species composition on forest‐floor photosynthesis. Overall, the three stands were weak to moderate sinks for CO₂ in 2003 (NEE of ?103, ?80 and ?28 g C m^?2 yr^?1 for aspen, black spruce and jack pine, respectively). Forest‐floor respiration accounted for 86%, 73% and 75% of annual ecosystem respiration, in the three respective stands, while forest‐floor photosynthesis contributed to 11% and 14% of annual gross ecosystem photosynthesis in the black spruce and jack pine stands, respectively. The results emphasize the need to perform concomitant measurements of NEE and soil CO₂ efflux at longer time scales in different ecosystems in order to better understand the impacts of future interannual climate variability and vegetation dynamics associated with climate change on each component of the carbon balance.     

Hydraulic adjustment in jack pine and black spruce seedlings under controlled cycles of dehydration and rehydration

Drought adjustments were compared in black spruce ( Picea mariana [Mill] B.S.P), and jack pine ( Pinus banksiana [Lamb.]) by subjecting seedlings to five cycles of dehydration and rehydration. A computer-controlled root misting chamber system, supplied low (−1.5 MPa), moderate (−2.0 MPa), and severe (−2.5 MPa) dehydration, respectively, in cycles 1, 3 and 5. Although cell water relations failed to adjust to chronic dehydration, there was limited osmotic adjustment in black spruce (cycle 3), and water was re-allocated from the apoplast to the symplast in jack pine (cycles 1 and 3). Dehydration postponement was more important than dehydration tolerance. Jack pine was better able to postpone dehydration than black spruce. Specific conductivity, the hydraulic conductivity per unit stem cross-sectional area, was lower in jack pine and slower to decline during chronic dehydration. When specific conductivity was corrected for the greater leaf area in black spruce, the leaf-specific conductivity did not differ in the two species. There was no increase in needle leakage in jack pine and stomata in jack pine seedlings reopened fully after rehydration. Black spruce was more of a 'water spender', and less water stress (−2.0 MPa, cycle 3) was required to lower specific conductivity, compared to jack pine (−2.5 MPa, cycle 5). Leakage from needle membranes increased in black spruce, and stomata failed to reopen after rewatering (cycles 3 and 5). A greater needle area, smaller root system, and a higher specific conductivity lowered the water stress threshold for cavitation in black spruce, which is confined to moister sites in the boreal forest. Jack pine had a larger root system, smaller needle area and lower specific conductivity than black spruce. Because of these static features, jack pine is more drought tolerant and it is often found on sites that are too hot and dry for black spruce.     

Ectomycorrhizal colonization on black spruce and jack pine seedlings outplanted in reforestation sites

Six-week-old, mycorrhiza-free, bareroot jack pine and black spruce seedlings were outplanted in ten reforestation sites, situated between 45–48° latitude N and 69–74° longitude W, within the province of Quebec, representing diverse operational forestry disturbances and ecological conditions. Two months after outplanting, root systems of black spruce seedlings had fewer mycorrhizae than those of jack pine seedlings. Ectomycorrhizal colonization on black spruce seedlings did not vary significantly with the reforestation site. Percent mycorrhizal colonization for these seedlings was positively correlated with seedling dry weight while with the jack pine seedlings, mycorrhizal colonization varied significantly with the outplanting site and there was no correlation between mycorrhizal formation and seedling dry weight. Multiple linear regressions showed pH to be a determinant soil factor for mycorrhizal colonization for the two species. Drainage was the other influential factor affecting colonization of black spruce while organic matter accumulation was more important for jack pine. Inoculation with selected ectomycorrhizal fungi could be more important for black spruce than for jack pine seedlings.     

Comparison of polyethylene glycol 3350 induced osmotic stress and soil drying for drought simulation in three woody species

 Drought simulation usually involves either soil drying or the use of an osmoticum, such as high molecular weight (>3000) polyethylene glycol (PEG). Although easy to apply, PEG absorption and toxicity remain a concern. This study compared the effects of soil drying and use of an osmoticum (PEG 3350). Osmotic stress and soil drought were applied to 5-month-old seedlings of jack pine (Pinus banksiana Lamb.) and black spruce [Picea mariana (Mill) B.S.P.] , which are both coniferous species from cold, boreal regions of North America, and flooded gum (Eucalyptus grandis W. Hill ex Maiden), a hardwood species growing in warmer, sub-tropical regions of Australia. Results showed that PEG 3350 was absorbed by roots, transported to shoots, and deposited on the leaves of both flooded gum and jack pine (but not black spruce). PEG lowered relative water content and damaged leaf tissues in both species, and also damaged stomata of flooded gum. Although 12 days of PEG-induced osmotic stress produced a decline in water potentials that was similiar to soil drying, it also caused significantly higher membrane injury and reduced net photosynthesis and stomatal conductance in leaves of all three species. Recovery of net photosynthesis and stomatal conductance in PEG-treated jack pine and black spruce was also slower after stress alleviation. Even a short exposure to PEG 3350 adversely affected seedlings compared to soil drought. These results confirmed that drought effects may vary, depending on the species and the method of stress induction. Received: 6 March 1996 / Accepted: 17 September 1996     

Net ecosystem productivity of boreal jack pine stands regenerating from clearcutting under current and future climates

Life cycle analysis of climate and disturbance effects on forest net ecosystem productivity (NEP) is necessary to assess changes in forest carbon (C) stocks under current or future climates. Ecosystem models used in such assessments need to undergo well-constrained tests of their hypotheses for climate and disturbance effects on the processes that determine CO₂ exchange between forests and the atmosphere. We tested the ability of the model ecosys to simulate diurnal changes in CO₂ fluxes under changing air temperatures (T_a) and soil water contents during forest regeneration with eddy covariance measurements over boreal jack pine (Pinus banksiana) stands along a postclearcut chronosequence. Model hypotheses for hydraulic and nutrient constraints on CO₂ fixation allowed ecosys to simulate the recovery of C cycling during the transition of boreal jack pine stands from C sources following clearcutting (NEP from −150 to −200 g C m⁻² yr⁻¹) to C sinks at maturity (NEP from 20 to 80 g C m⁻² yr⁻¹) with large interannual variability. Over a 126-year logging cycle, annualized NEP, C harvest, and net biome productivity (NBP=NEP–harvest removals) of boreal jack pine averaged 47, 33 and 14 g C m⁻² yr⁻¹. Under an IPCC SRES climate change scenario, rising T_a exacerbated hydraulic constraints that adversely affected NEP of boreal jack pine after 75 years. These adverse effects were avoided in the model by replacing the boreal jack pine ecotype with one adapted to warmer T_a. This replacement raised annualized NEP, C harvest, and NBP to 81, 56 and 25 g C m⁻² yr⁻¹ during a 126-year logging cycle under the same climate change scenario.     

Modeling analysis of primary controls on net ecosystem productivity of seven boreal and temperate coniferous forests across a continental transect

Process‐based models are effective tools to synthesize and/or extrapolate measured carbon (C) exchanges from individual sites to large scales. In this study, we used a C‐ and nitrogen (N)‐cycle coupled ecosystem model named CN‐CLASS (Carbon Nitrogen‐Canadian Land Surface Scheme) to study the role of primary climatic controls and site‐specific C stocks on the net ecosystem productivity (NEP) of seven intermediate‐aged to mature coniferous forest sites across an east–west continental transect in Canada. The model was parameterized using a common set of parameters, except for two used in empirical canopy conductance–assimilation, and leaf area–sapwood relationships, and then validated using observed eddy covariance flux data. Leaf Rubisco‐N dynamics that are associated with soil–plant N cycling, and depend on canopy temperature, enabled the model to simulate site‐specific gross ecosystem productivity (GEP) reasonably well for all seven sites. Overall GEP simulations had relatively smaller differences compared with observations vs. ecosystem respiration (RE), which was the sum of many plant and soil components with larger variability and/or uncertainty associated with them. Both observed and simulated data showed that, on an annual basis, boreal forest sites were either carbon‐neutral or a weak C sink, ranging from 30 to 180 g C m^?2 yr^?1; while temperate forests were either a medium or strong C sink, ranging from 150 to 500 g C m^?2 yr^?1, depending on forest age and climatic regime. Model sensitivity tests illustrated that air temperature, among climate variables, and aboveground biomass, among major C stocks, were dominant factors impacting annual NEP. Vegetation biomass effects on annual GEP, RE and NEP showed similar patterns of variability at four boreal and three temperate forests. Air temperature showed different impacts on GEP and RE, and the response varied considerably from site to site. Higher solar radiation enhanced GEP, while precipitation differences had a minor effect. Magnitude of forest litter content and soil organic matter (SOM) affected RE. SOM also affected GEP, but only at low levels of SOM, because of low N mineralization that limited soil nutrient (N) availability. The results of this study will help to evaluate the impact of future climatic changes and/or forest C stock variations on C uptake and loss in forest ecosystems growing in diverse environments.     

Regeneration of planted conifers across climatic moisture gradients on the Canadian prairies: implications for distribution and climate change

The influence of dry climates on white spruce ( Picea glauca (Moench) Voss)) regeneration was examined by conducting surveys of seedlings and small trees that had regenerated naturally at 100 farm shelterbelts and plantations in southern Saskatchewan, Canada. The sites surveyed were located along a climate moisture gradient extending from the relatively moist boreal forest, across the aspen parkland, to the semi-arid prairie grasslands. Natural regeneration was greatest at sites in the boreal forest and northern aspen parkland, decreased in the southern aspen parkland, and was negligible in the grassland zone. Furthermore, the few seedlings found in the drier zones were usually in poor condition. Similar results were obtained for the introduced Colorado spruce ( Picea pungens Engelm.) and Scots pine ( Pinus sylvestris L.). It is concluded that the present climate of the southern parkland and grassland is too dry to permit natural regeneration of white spruce and other conifers. If increases in atmospheric CO₂ levels lead to a drier future climate in the southern boreal forest of western Canada, the ability of conifers to regenerate naturally may be significantly reduced.     

Historical and projected carbon balance of mature black spruce ecosystems across North America: the role of carbon–nitrogen interactions

The role of carbon (C) and nitrogen (N) interactions on sequestration of atmospheric CO₂ in black spruce ecosystems across North America was evaluated with the Terrestrial Ecosystem Model (TEM) by applying parameterizations of the model in which C–N dynamics were either coupled or uncoupled. First, the performance of the parameterizations, which were developed for the dynamics of black spruce ecosystems at the Bonanza Creek Long-Term Ecological Research site in Alaska, were evaluated by simulating C dynamics at eddy correlation tower sites in the Boreal Ecosystem Atmosphere Study (BOREAS) for black spruce ecosystems in the northern study area (northern site) and the southern study area (southern site) with local climate data. We compared simulated monthly growing season (May to September) estimates of gross primary production (GPP), total ecosystem respiration (RESP), and net ecosystem production (NEP) from 1994 to 1997 to available field-based estimates at both sites. At the northern site, monthly growing season estimates of GPP and RESP for the coupled and uncoupled simulations were highly correlated with the field-based estimates (coupled: R ²= 0.77, 0.88 for GPP and RESP; uncoupled: R ² = 0.67, 0.92 for GPP and RESP). Although the simulated seasonal pattern of NEP generally matched the field-based data, the correlations between field-based and simulated monthly growing season NEP were lower (R ² = 0.40, 0.00 for coupled and uncoupled simulations, respectively) in comparison to the correlations between field-based and simulated GPP and RESP. The annual NEP simulated by the coupled parameterization fell within the uncertainty of field-based estimates in two of three years. On the other hand, annual NEP simulated by the uncoupled parameterization only fell within the field-based uncertainty in one of three years. At the southern site, simulated NEP generally matched field-based NEP estimates, and the correlation between monthly growing season field-based and simulated NEP (R ² = 0.36, 0.20 for coupled and uncoupled simulations, respectively) was similar to the correlations at the northern site. To evaluate the role of N dynamics in C balance of black spruce ecosystems across North America, we simulated historical and projected C dynamics from 1900 to 2100 with a global-based climatology at 0.5° resolution (latitude × longitude) with both the coupled and uncoupled parameterizations of TEM. From analyses at the northern site, several consistent patterns emerge. There was greater inter-annual variability in net primary production (NPP) simulated by the uncoupled parameterization as compared to the coupled parameterization, which led to substantial differences in inter-annual variability in NEP between the parameterizations. The divergence between NPP and heterotrophic respiration was greater in the uncoupled simulation, resulting in more C sequestration during the projected period. These responses were the result of fundamentally different responses of the coupled and uncoupled parameterizations to changes in CO₂ and climate.     

Cross‐scale controls on carbon emissions from boreal forest megafires

Climate warming and drying is associated with increased wildfire disturbance and the emergence of megafires in North American boreal forests. Changes to the fire regime are expected to strongly increase combustion emissions of carbon (C) which could alter regional C balance and positively feedback to climate warming. In order to accurately estimate C emissions and thereby better predict future climate feedbacks, there is a need to understand the major sources of heterogeneity that impact C emissions at different scales. Here, we examined 211 field plots in boreal forests dominated by black spruce (Picea mariana) or jack pine (Pinus banksiana) of the Northwest Territories (NWT), Canada after an unprecedentedly large area burned in 2014. We assessed both aboveground and soil organic layer (SOL) combustion, with the goal of determining the major drivers in total C emissions, as well as to develop a high spatial resolution model to scale emissions in a relatively understudied region of the boreal forest. On average, 3.35 kg C m^?2 was combusted and almost 90% of this was from SOL combustion. Our results indicate that black spruce stands located at landscape positions with intermediate drainage contribute the most to C emissions. Indices associated with fire weather and date of burn did not impact emissions, which we attribute to the extreme fire weather over a short period of time. Using these results, we estimated a total of 94.3 Tg C emitted from 2.85 Mha of burned area across the entire 2014 NWT fire complex, which offsets almost 50% of mean annual net ecosystem production in terrestrial ecosystems of Canada. Our study also highlights the need for fine‐scale estimates of burned area that represent small water bodies and regionally specific calibrations of combustion that account for spatial heterogeneity in order to accurately model emissions at the continental scale.     

Drought causes reduced growth of trembling aspen in western Canada

Adequate and advance knowledge of the response of forest ecosystems to temperature‐induced drought is critical for a comprehensive understanding of the impacts of global climate change on forest ecosystem structure and function. Recent massive decline in aspen‐dominated forests and an increased aspen mortality in boreal forests have been associated with global warming, but it is still uncertain whether the decline and mortality are driven by drought. We used a series of ring‐width chronologies from 40 trembling aspen (Populus tremuloides Michx.) sites along a latitudinal gradient (from 52° to 58°N) in western Canada, in an attempt to clarify the impacts of drought on aspen growth by using Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI). Results indicated that prolonged and large‐scale droughts had a strong negative impact on trembling aspen growth. Furthermore, the spatiotemporal variability of drought indices is useful for explaining the spatial heterogeneity in the radial growth of trembling aspen. Due to ongoing global warming and rising temperatures, it is likely that severer droughts with a higher frequency will occur in western Canada. As trembling aspen is sensitive to drought, we suggest that drought indices could be applied to monitor the potential effects of increased drought stress on aspen trees growth, achieve classification of eco‐regions and develop effective mitigation strategies to maintain western Canadian boreal forests.     

The relative contribution of elastic and osmotic adjustments to turgor maintenance of woody species.

To determine how tissue water relations vary and contribute to turgor maintenance in species from contrasting ecological zones, seedlings of jack pine ( Pinus banksiana Lamb.), black spruce ( Picea mariana [Mill] B.S.P.) and flooded gum ( Eucalyptus grandis W. Hill ex Maiden) were subjected to an 8 day drought stress by water withholding with and without prior mild water stress conditioning. Jack pine, a deep-rooted species from dry, sandy boreal sites, lost turgor at the lowest relative water content (75–65%) and water potential, and had lowest maximum bulk elastic modulus (E_max of 5.2–5.8 MPa). Although this suggests a high inherent dehydration tolerance, jack pine did not further adjust its elasticity when repeatedly stressed. Black spruce, a shallow-rooted species from predominantly moist sites in the boreal region, lost turgor at intermediate relative water content (86–76%) and water potential, but could adjust its elasticity to maintain turgor in repeatedly stressed tissues. Flooded gum, a deep-rooted species from moist, warm temperate-subtropical regions, had a low inherent drought tolerance since it lost turgor at higher relative water content (88–84%) and water potential, but was capable of some adjustment when the stress was repeated. Elastic adjustment (<3.7 MPa) was more important for turgor maintenance than osmotic adjustment (<0.13 MPa), which was statistically nonsignificant. Maximum bulk modulus of elasticity, but not osmotic potentials at full turgor, was significantly correlated with the relative water content and water potential at zero turgor in droughted seedlings. These results highlight the importance of tissue shrinkage for dehydration tolerance. Both the inherent capacity for turgor maintenance of a species under drought and its ability to adjust to repeated drought should be considered in genetic selections for drought tolerance.     

Soil and tree chemistry reflected the cumulative impact of acid deposition in Pinus banksiana and Populus tremuloides stands in the Athabasca oil sands region in western Canada

The spatial variability of soil chemistry and Ca/Al ratios of soil solution and fine roots were investigated in jack pine (Pinus banksiana) and trembling aspen (Populus tremuloides, aspen) stands to assess the impact of chronic acid deposition on boreal forest ecosystems in the Athabasca oil sands region (AOSR) in Alberta, Canada. Available SO₄²⁻ (as the sum of soluble and adsorbed SO₄²⁻) accumulated in the soil near tree boles of both species, reflecting the influence of canopy intercepted SO₄²⁻. In jack pine stands, pH and soluble base cation concentrations decreased towards tree boles due to increased SO₄²⁻ leaching; the reverse was found in aspen stands due to deposition of base cations leached from the canopy. As a result, Ca/Al ratios in the soluble fraction in soils near jack pine boles were 5–20 times lower than that near aspen boles. The Ca/Al ratio did not reach the critical limits of 1.0 for soil solution (ranged from 1.0 to 4.1) or 0.5 for fine roots (0.7–7.9) in the studied watersheds. However, Alⁿ⁺ concentrations in the soil solution ranged from 0.2 to 4.1 mg L⁻¹ in NE7 and from 0.1 to 8.5 mg L⁻¹ in SM8 that can inhibit the growth of white spruce (Picea glauca) seedlings that commonly succeed aspen in upland sites in the AOSR. We suggest that the spatial variation caused by tree canopies/stems will affect forest regeneration and the effect of acid deposition on forest succession in the AOSR should be further studied.     

Climate‐diameter growth relationships of black spruce and jack pine trees in boreal Ontario,Canada

To predict the long‐term effects of climate change – global warming and changes in precipitation – on the diameter (radial) growth of jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana [Mill.] B.S.P.) trees in boreal Ontario, we modified an existing diameter growth model to include climate variables. Diameter chronologies of 927 jack pine and 1173 black spruce trees, growing in the area from 47°N to 50°N and 80°W to 92°W, were used to develop diameter growth models in a nonlinear mixed‐effects approach. Our results showed that the variables long‐term average of mean growing season temperature, precipitation during wettest quarter, and total precipitation during growing season were significant (alpha = 0.05) in explaining variation in diameter growth of the sample trees. Model results indicated that higher temperatures during the growing season would increase the diameter growth of jack pine trees, but decrease that of black spruce trees. More precipitation during the wettest quarter would favor the diameter growth of both species. On the other hand, a wetter growing season, which may decrease radiation inputs, increase nutrient leaching, and reduce the decomposition rate, would reduce the diameter growth of both species. Moreover, our results indicated that future (2041–2070) diameter growth rate may differ from current (1971–2000) growth rates for both species, with conditions being more favorable for jack pine than black spruce trees. Expected future changes in the growth rate of boreal trees need to be considered in forest management decisions. We recommend that knowledge of climate–growth relationships, as represented by models, be combined with learning from adaptive management to reduce the risks and uncertainties associated with forest management decisions.     

Seasonal variability of forest sensitivity to heat and drought stresses: A synthesis based on carbon fluxes from North American forest ecosystems

Climate extremes such as heat waves and droughts are projected to occur more frequently with increasing temperature and an intensified hydrological cycle. It is important to understand and quantify how forest carbon fluxes respond to heat and drought stress. In this study, we developed a series of daily indices of sensitivity to heat and drought stress as indicated by air temperature (T_a) and evaporative fraction (EF). Using normalized daily carbon fluxes from the FLUXNET Network for 34 forest sites in North America, the seasonal pattern of sensitivities of net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (RE) in response to T_a and EF anomalies were compared for different forest types. The results showed that warm temperatures in spring had a positive effect on NEP in conifer forests but a negative impact in deciduous forests. GEP in conifer forests increased with higher temperature anomalies in spring but decreased in summer. The drought‐induced decrease in NEP, which mostly occurred in the deciduous forests, was mostly driven by the reduction in GEP. In conifer forests, drought had a similar dampening effect on both GEP and RE, therefore leading to a neutral NEP response. The NEP sensitivity to T_a anomalies increased with increasing mean annual temperature. Drier sites were less sensitive to drought stress in summer. Natural forests with older stand age tended to be more resilient to the climate stresses compared to managed younger forests. The results of the Classification and Regression Tree analysis showed that seasons and ecosystem productivity were the most powerful variables in explaining the variation of forest sensitivity to heat and drought stress. Our results implied that the magnitude and direction of carbon flux changes in response to climate extremes are highly dependent on the seasonal dynamics of forests and the timing of the climate extremes.     

Influence of nutrients,disturbances and site conditions on carbon stocks along a boreal forest transect in central Canada

The interacting influence of disturbances and nutrient dynamics on aboveground biomass, forest floor, and mineral soil C stocks was assessed as part of the Boreal Forest Transect Case Study in central Canada. This transect covers a range of forested biomes–-from transitional grasslands (aspen parkland) in the south, through boreal forests, and into the forested subarctic woodland in the north. The dominant forest vegetation species are aspen, jack pine and spruce. Disturbances influence biomass C stocks in boreal forests by determining its age-class structure, altering nutrient dynamics, and changing the total nutrient reserves of the stand. Nitrogen is generally the limiting nutrient in these systems, and N availability determines biomass C stocks by affecting the forest dynamics (growth rates and site carrying capacity) throughout the life cycle of a forest stand. At a given site, total and available soil N are determined both by biotic factors (such as vegetation type and associated detritus pools) and abiotic factors (such as N deposition, soil texture, and drainage). Increasing clay content, lower temperatures and reduced aeration are expected to lead to reduced N mineralization and, ultimately, lower N availability and reduced forest productivity. Forest floor and mineral soil C stocks vary with changing balances between complex sets of organic carbon inputs and outputs. The changes in forest floor and mineral soil C pools at a given site, however, are strongly related to the historical changes in biomass at that site. Changes in N availability alter the processes regulating both inputs and outputs of carbon to soil stocks. N availability in turn is shaped by past disturbance history, litter fall rate, site characteristics and climatic factors. Thus, understanding the life-cycle dynamics of C and N as determined by age-class structure (disturbances) is essential for quantifying past changes in forest level C stocks and for projecting their future change.