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     

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.     

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.     

Effects of increasing saturation vapour pressure deficit on growth and ABA levels in black spruce and jack pine

 Plant responses to saturation vapour pressure deficit (SVPD) were studied by subjecting black spruce [Picea mariana (Mill) B.S.P.] and jack pine seedlings (Pinus banksiana Lamb.) to humid (0.3 – 0.8 kPa) or dry (2.0 – 2.5 kPa SVPD) regimes for 4 weeks using a computer-controlled environmental system to control diurnal variation in SVPD. Dry matter accumulation in needles was not altered by increasing SVPD. However, root growth declined by 60% which increased shoot to root ratio and reduced total seedling dry weight in both black spruce and jack pine. Relative growth rate of jack pine also declined to about half the rate of plants grown under humid conditions. In situ root marking studies showed that the decline in root growth of jack pine under the high SVPD was the result of reduced lateral root initiation, whereas root elongation was unaffected by humidity. A 4-week exposure to dry air increased abscisic acid (ABA) levels in needles, but not roots, of jack pine whereas ABA levels in black spruce were not altered. A short (3-day) exposure failed to increase needle ABA levels in either species. These results suggest that the responses of conifers to dry air were not the result of ABA accumulation. Received: 24 March 1996 / Accepted: 30 May 1996     

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.     

New Plant Growth Regulators Protect Photosynthesis and Enhance Growth Under Drought of Jack Pine Seedlings

To determine whether natural plant growth regulators (PGRs) can enhance drought tolerance and the competitive ability of transplanted seedlings, 1.5-year-old jack pine (Pinus banksana Lamb.) seedlings were treated with homobrassinolide, salicylic acid, and two polyamines, spermine and spermidine, triacontanol, abscisic acid (ABA), and the synthetic antioxidant, Ambiol. PGRs were fed into the xylem for 7 days and plants were droughted by withholding water for 12 days. ABA, Ambiol, spermidine, and spermine at a concentration of 10 μg L⁻¹ stimulated elongation growth under drought, whereas ABA, Ambiol, and spermidine maintained higher photosynthetic rates, higher water use efficiency, and lower Ci/Ca ratio under drought compared with control plants. The damaging effects of drought on membrane leakage was reversed by Ambiol, ABA, triacontanol, spermidine, and spermine. Because ABA, Ambiol, and both polyamines enhanced elongation growth and also reduced membrane damage in jack pine under drought, they show promise as treatments to harden seedlings against environmental stress. The protective action of these compounds on membrane integrity was associated with an inhibition of ethylene evolution, with a reduction in transpiration rate and an enhancement of photosynthesis, which together increased water use efficiency under drought. Although most of the tested compounds acted as antitranspirants, the inhibition in membrane leakage in ABA-, Ambiol-, and polyamine-treated plants appeared more closely related to the antiethylene action. Received December 30, 1998; accepted October 14, 1999     

Ecophysiological adaptations of black spruce (<Emphasis Type="Italic"> Picea mariana</Emphasis>) and tamarack (<Emphasis Type="Italic"> Larix laricina</Emphasis>) seedlings to flooding

Black spruce [ Picea mariana (Mill.) B.S.P.] and tamarack [ Larix laricina (Du Roi) K. Koch] are the predominant tree species in boreal peatlands. The effects of 34 days of flooding on morphological and physiological responses were investigated in the greenhouse for black spruce and tamarack seedlings in their second growing season (18 months old). Flooding resulted in reduced root hydraulic conductance, net assimilation rate and stomatal conductance and increased needle electrolyte leakage in both species. Flooded tamarack seedlings maintained a higher net assimilation rate and stomatal conductance compared to flooded black spruce. Flooded tamarack seedlings were also able to maintain higher root hydraulic conductance compared to flooded black spruce seedlings at a comparable time period of flooding. Root respiration declined in both species under flooding. Sugar concentration increased in shoots while decreasing in roots in both species under flooding. Needles of flooded black spruce appeared necrotic and electrolyte leakage increased over time with flooding and remained significantly higher than in flooded tamarack seedlings. No visible damage symptoms were observed in flooded tamarack seedlings. Flooded tamarack seedlings developed adventitious roots beginning 16 days after the start of flooding treatment. Adventitious roots exhibited significantly higher root hydraulic conductivity than similarly sized flooded tamarack roots. Flooded black spruce lacked any such morphological adaptation. These results suggest that tamarack is better able to adjust both morphologically and physiologically to prolonged soil flooding than black spruce seedlings.     

Patterns of Cross-Continental Variation in Tree Seed Mass in the Canadian Boreal Forest

Seed mass is an adaptive trait affecting species distribution, population dynamics and community structure. In widely distributed species, variation in seed mass may reflect both genetic adaptation to local environments and adaptive phenotypic plasticity. Acknowledging the difficulty in separating these two aspects, we examined the causal relationships determining seed mass variation to better understand adaptability and/or plasticity of selected tree species to spatial/climatic variation. A total of 504, 481 and 454 seed collections of black spruce (Picea mariana (Mill.) B.S.P.), white spruce (Picea glauca (Moench) Voss) and jack pine (Pinus banksiana Lamb) across the Canadian Boreal Forest, respectively, were selected. Correlation analyses were used to determine how seed mass vary with latitude, longitude, and altitude. Structural Equation Modeling was used to examine how geographic and climatic variables influence seed mass. Climatic factors explained a large portion of the variation in seed mass (34, 14 and 29%, for black spruce, white spruce and jack pine, respectively), indicating species-specific adaptation to long term climate conditions. Higher annual mean temperature and winter precipitation caused greater seed mass in black spruce, but annual precipitation was the controlling factor for white spruce. The combination of factors such as growing season temperature and evapotranspiration, temperature seasonality and annual precipitation together determined seed mass of jack pine. Overall, sites with higher winter temperatures were correlated with larger seeds. Thus, long-term climatic conditions, at least in part, determined spatial variation in seed mass. Black spruce and Jack pine, species with relatively more specific habitat requirements and less plasticity, had more variation in seed mass explained by climate than did the more plastic species white spruce. As traits such as seed mass are related to seedling growth and survival, they potentially influence forest species composition in a changing climate and should be included in future modeling of vegetation shifts.     

The effects of Laccaria proxima and fibrous pulp waste on the growth of nine container-grown conifer seedling species

 Japanese larch (Larix kaempferi), white spruce (Picea glauca), black spruce (Picea mariana), red spruce (Picea rubens), jack pine (Pinus banksiana), mugo pine (Pinus mugo), red pine (Pinus resinosa), Japanese black pine (Pinus thunbergii) and Douglas-fir (Pseudotsuga menziesii var. menziesii), were inoculated to test the effective host range of the ectomycorrhizal fungus Laccaria proxima and the possibility of utilizing pulp waste as a potting medium for containerized seedling production. Laccaria proxima tended to improve the container growth of Japanese black pine and white spruce, and significantly improved that of jack pine, mugo pine, black spruce, red spruce and Douglasfir. The growth of red pine and Japanese larch were only slightly improved with L. proxima. Pulp waste (33% by volume) had negative effects on tree seedling growth, except for Douglasfir (no significant effect). The interactions of Laccaria proxima and pulp waste varied; the hosts were significantly positive (P<0.01) in the case of jack pine and black spruce, but there was no significant effect for the rest. Negative effects were found with Japanese black pine. Use of pulp waste in seedling production of jack pine, black spruce, mugo pine, red spruce and Douglasfir inoculated with L. proxima and of Japanese black pine both with and without L. proxima is feasible, but further research is necessary to determine the optimal percentage of pulp waste that can be utilized in seedling production of tree species and the field performance of these seedlings. Accepted: 30 August 1995     

Drought tolerance, abscisic acid and electrolyte leakage in fast-and slow-growing black spruce (Picea mariana) progenies

To investigate the role that drought tolerance plays in growth, abscisic acid (ABA) accumulation and electrolyte leakage during water stress were compared in fast- and slow-growing black spruce ( Picea mariana [Mill.] B. S. P.) progenies. Changes in the ABA content of the needles were quantified using an indirect enzyme-linked immuno-sorbent assay validated by gas chromatography electron capture detection. Needle electrolyte leakage was estimated using a conductivity bridge. Seedlings were stressed using (1) osmotic stress, induced by a stepwise increase in concentrations of polyethylene glycol 3 350 (PEG) for ABA study and (2) air drying for electrolyte leakage study. Progenies did not differ in ABA levels under unstressed conditions, but progeny differences were observed under osmotic stress. Needle ABA content increased up to 500% under osmotic stress. Slow-growing black spruce progenies (25 and 46) accumulated more ABA under moderate (18% PEG), but not severe (25% PEG), osmotic stress. The slow-growing progenies also leaked more electrolytes under moderate to severe water stress and lost 50% electrolytes at a higher xylem tension, suggesting they suffered more injury and were less dehydration tolerant. Our previously-published results showed that slow-growing progenies lost their photosynthesis and stomatal conductance more quickly during osmotic stress and recovered more slowly after rehydration. Therefore, tolerance of dehydration leading to a maintenance of physiological integrity during drought stress could explain the fast growth rates of more vigorous black spruce progenies.     

Microtubule pattern and the occurrence of pre-prophase bands in embryogenic cultures of black spruce (Pieca mariana Mill.) and non-embryogenic cultures of jack pine (Pinus banksiana Lamb.)

Summary The organization of microtubules during interphase and prophase in embryogenic cultures of black spruce (Picea mariana) was investigated by indirect immunofluorescence. Somatic embryos of black spruce possessed an extensively branched and interconnecting network of fine interphase cortical microtubules. The development of pre-prophase bands (PPBs) in embryogenic black spruce cultures was compared with that in non-embryogenic cell cultures of jack pine (Pinus banksiana). PPBs in both species were initially arranged as a very broad array of microtubules, later (early to mid-prophase) becoming narrower and more intensely fluorescent. The occurrence of pre-prophase bands in relation to the number of phragmoplasts (i.e. PPB index) of black spruce somatic embryos was significantly higher (p<0.01) than that found for jack pine cells.     

Gas exchange and growth responses of ectomycorrhizal Picea mariana, Picea glauca, and Pinus banksiana seedlings to NaCl and Na2SO4

Abstract: Black spruce (Picea mariana), white spruce (Picea glauca), and jack pine (Pinus banksiana) seedlings were inoculated with Hebeloma crustuliniforme or Laccaria bicolor and subjected to NaCl and Na₂SO₄ treatments. The effects of ectomycorrhizas on salt uptake, growth, gas exchange, and needle necrosis varied depending on the tree and fungal species. In jack pine seedlings, ectomycorrhizal (ECM) fungi reduced shoot and root dry weights and in the ECM white spruce, there was a small increase in dry weights. Sodium chloride treatment reduced net photosynthesis and transpiration rates in the three studied tree species. However, NaCl-treated black spruce and jack pine colonized by H. crustuliniforme maintained relatively high photosynthetic and transpiration rates and needle necrosis of NaCl-treated black spruce seedlings was reduced by the ECM fungi. Higher concentrations of Na+ were found in shoots compared with roots of the three examined conifer species. ECM fungi reduced the concentrations of Na+ mainly in the shoots and this reduction was greater in plants treated with NaCl compared with Na₂SO₄. Shoots contained generally higher concentrations of Cl^- compared with roots. In the NaCl-treated black spruce and white spruce, both ECM species significantly reduced Cl^- concentrations. Our results point to overall greater phytotoxicity of NaCl compared with Na₂SO₄ and support our earlier findings which demonstrated beneficial effects of ECM fungi for woody plants exposed to NaCl stress.     

树木年龄和断面积对加拿大北方林树木死亡率的影响

以加拿大北部的杨树(Populus spp.)、斑克松(Pinus banksiana)、黑云杉(Picea mariana)为对象,采用长期定位试验,对134块固定样地的活立木及枯死木进行调查,并运用线性回归的方法研究树木年龄、断面积和林分类型对3种树木死亡率的影响.结果表明:随着树龄和断面积的增加,林木的死亡率呈上升趋势.杨树在斑克松林中的死亡率较高,而在黑云杉林中死亡率较低.在黑云杉林中,树龄是影响斑克松死亡率的主要因子;而在杨树林中,断面积是影响斑克松死亡率的重要因子;不同林分类型中树龄对黑云杉死亡率的影响显著.树种组成对树种的死亡率有显著影响;树木年龄、断面积和林分类型之间的交互效应对各树种的死亡率均有显著影响;不同林分类型中同一树种的死亡状况有明显差异.     

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.     

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.     

Effects of NaCl on responses of ectomycorrhizal black spruce (Picea mariana), white spruce (Picea glauca) and jack pine (Pinus banksiana) to fluoride

Black spruce ( Picea mariana ), white spruce ( Picea glauca ) and jack pine ( Pinus banksiana ) were inoculated with Suillus tomentosus and subjected to potassium fluoride (1 m M KF and 5 m M KF) in the presence and absence of 60 m M NaCl. The NaCl and KF treatments reduced total dry weights in jack pine and black spruce seedlings, but they did not affect total dry weights in white spruce seedlings. The addition of 60 m M NaCl to KF treatment solutions alleviated fluoride-induced needle injury in ectomycorrhizal (ECM) black spruce and white spruce, but had little effect in jack pine seedlings. Both KF and 60 m M NaCl treatments reduced E values compared with non-treated control seedlings. However, with the exception of small reductions of K_r by NaCl treatments in black spruce, the applied KF and NaCl treatments had little effect on K_r in ECM plants. Chloride tissue concentrations in NaCl-treated plants were not affected by the presence of KF in treatment solutions. However, shoot F concentrations in ECM black spruce and white spruce treated with 5 m M KF + 60 m M NaCl were significantly reduced compared with the 5 m M KF treatment. The results point to a possible competitive inhibition of F transport by Cl. We also suggest that the possibility that aquaporins may be involved in the transmembrane transport of F should be further investigated.     

Characterization of tree macroremains production in a recently burned conifer forest in northern Québec,Canada

The production of plant macroremains was studied in a conifer forest twomonths after it burned in 1996 in northern Québec. The proportions ofvarious types of charred and uncharred pieces (needles, cones andwood) produced by black spruce (Piceamariana) and jack pine (Pinusbanksiana) were determined by sampling around individual trees.Both species produced equivalent masses of charred material, but pieces of woodcharcoal from jack pine are generally larger that those of black spruce. Theproportion of charred versus uncharred needles is the bestindicator of the species dominance in the forest. Although the fall of uncharredneedles is delayed from the time of a fire, they contribute to more than half ofall remains produced. Jack pine cones remain on the tree for a long time after afire, while charred cones of black spruce are dehiscent (cones come off thebranches easily). Trees are poor wood charcoal producers compared toundergrowth shrubs. As a result, the macroremains assemblage associated with afire event is made up of large amount of uncharred material from trees and alarge proportion of charred pieces produced by undergrowth vegetation. Modernassemblages of plant macroremains indicate that in order to reconstruct pastvegetation associated with fire disturbance, it is important to distinguishbetween the various types of remains, because wood charcoal is mainly producedby material that is already dead.     

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

In 2000–03, 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. The online version of the original article can be found under doi:     

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.     

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.