James D. Stewart

University of Alberta, Edmonton, Alberta, Canada

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Publications (8)8.65 Total impact

  • James D. Stewart, Victor J. Lieffers
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    ABSTRACT: Five-week-old Pinuscontorta var. latifolia Engelm. seedlings were subjected to 6 weeks of preconditioning with four combinations of watering and nitrogen fertilization, followed by a second 6-week period to evaluate the continuing effects of the preconditioning treatments. In both phases, the seedlings were fertilized with nitrogen at either 3 or 6%•d−1 relative addition rate and were either subjected to drought or well watered. Preconditioning treatments were crossed with second-phase treatments to give a 24 factorial design. Nitrogen and watering preconditioning effects on biomass, water relations, nutrients, and metabolic constituents persisted at least 6 weeks regardless of the second-phase treatments. Interaction between nitrogen and watering treatments applied simultaneously was found only in relative growth rate and foliar potassium concentration. Interactions between preconditioning and second-phase treatments were found in foliar nitrogen, potassium, and α-amino nitrogen concentrations, root water content, and xylem pressure potential. Foliar starch concentration increased when either nitrogen or water were limiting, but showed no evidence of interaction or preconditioning effect. Preconditioning at the lower nitrogen rate resulted in elevated reducing sugar concentration in the leaves, which persisted during the second-phase period. We did not detect an effect of increased reducing sugars on osmotic potential. Preconditioning at the lower nitrogen rate increased subsequent root weight ratio and reducing sugars concentration, which could contribute to drought resistance. Turgor regulation occurred via changes in cell wall elasticity rather than osmotic adjustment.
    Canadian Journal of Forest Research 02/2011; 23(8):1663-1671. · 1.56 Impact Factor
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    ABSTRACT: One-year-old seedlings from two pairs of upland – lowland populations were subjected to three cycles of drought preconditioning. During a subsequent fourth cycle, preconditioned seedlings and seedlings that had been maintained under well-watered conditions were subjected to a final soil drought. During the drought periods, small but significant differences in gas exchanges and water relations were observed between upland and lowland seedlings but the differences were not consistent either within each lowland – upland pair or over the four cycles. During the final drought period, preconditioned seedlings maintained significantly higher net photosynthesis rate, stomatal conductance, and transpiration rate than unconditioned seedlings. In contrast, no significant differences were found in midday water potential, total sugar content, or water relations parameters between preconditioned and unconditioned seedlings. The results suggest an absence of ecotypic variation with respect to drought tolerance in black spruce. They also suggest that preconditioning in black spruce seedlings occurs mostly through the acclimation of stomatal and photosynthetic mechanisms to drought, without active osmotic adjustment. Key words: Picea mariana, ecotype, gas exchanges, water relations, osmoregulation, preconditioning.
    Canadian Journal of Botany 02/2011; 72(10):1511-1518. · 1.40 Impact Factor
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    ABSTRACT: The dispersal of white spruce (Picea glauca (Moench) Voss) seed through trembling aspen (Populus tremuloides Michx.) forests was investigated by releasing artificial seed (confetti) from different heights on a meteorological tower, and, secondly, by observing the distribution of spruce regeneration along transects radiating out from small isolated patches of mature spruce seed trees. Mean dispersal distance of confetti increased with height of release. Before leaf fall of the aspen canopy, most confetti landed close to and in all directions around the tower. After leaf fall, no confetti was observed upwind from the tower and the mean dispersal distance increased, with peak densities occurring at a distance of 15 m in the downwind direction. The rate of decrease in regeneration density with distance from patches of mature, seed-bearing white spruce was much less than that observed during confetti release experiments. Furthermore, regeneration densities were significantly greater in the prevailing downwind direction (toward the east). The results indicate that stronger than average winds, primarily from the northwest, west, and southwest, play a major role in the dispersal of white spruce seed. Simulation modelling of the observed distribution of regeneration suggests that long-distance (>250 m) dispersal may be an important mechanism for the persistence of white spruce in the fire-prone boreal forest of western Canada.Key words: seed dispersal, boreal forest, mixedwood, wind dispersal, artificial seed.
    Aspen Bibliography. 02/2011;
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    ABSTRACT: Gas exchange and water relations were measured on seedlings from two pairs of upland and lowland black spruce (Picea mariana (Mill.) BSP) populations. The seedlings were planted under natural atmospheric conditions in exterior sand beds. Measurements were carried out on eight separate days during the summer of 1992. Diurnal and seasonal responses of gas exchange and water relations parameters to natural changes in environmental factors were analyzed for possible ecotypic differences. Differences were found among the four populations and within one of the two upland–lowland pairs. However, no differences could be linked significantly to an upland–lowland ecotypic differentiation. The physiological responses are discussed with respect to diurnal and seasonal changes in environmental factors and to similarities with results from an earlier experiment involving mature trees from the same four populations. Key words: Picea mariana, water stress, net photosynthesis, stomatal conductance.
    Canadian Journal of Botany 02/2011; 73(5):716-722. · 1.40 Impact Factor
  • Pierre Y. Bernier, James D. Stewart, Antonio Gonzalez
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    ABSTRACT: Containerized black spruce seedlings, grown in Sphagnum peat at two degrees of humification (light and moderate) and two artificially created textural grades (with or without particles <1.3 mm), were subjected to controlled degrees of water stress using polyethylene glycol (PEG) at 0, 20, 75 and 175 g PEG kg−1 H2O, with and without dialysis membranes. After a 4‐day exposure, shoot water potential and water potential at the soil‐root interface were strongly affected by the humification of the peat at the highest PEG concentration. Differences in fine particle content of the peats had little effect on the physiological variables measured. Needle conductance was affected greatly by the presence of the dialysis membrane itself. The results suggest that high aeration, one of the most desirable physical properties of a growing medium for nursery production, may become undesirable once the seedlings are outplanted.
    Scandinavian Journal of Forest Research 01/1995; 10(1-4):184-189. · 1.07 Impact Factor
  • James D. Stewart, Victor J. Lieffers
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    ABSTRACT: The pH of the nutrient solution bathing the roots of four-month-oldPinus contorta var.latifolia Englm. seedlings was monitored continuously between additions of nutrients. Nitrogen was supplied in the form of NH4NO3, and was added three times per week in amounts relative to seedling fresh weight. No pH change was associated with the nutrient addition cycle; however, extinguishing of the lights at night resulted in a decrease in pH of almost half a pH unit in the first hour. The pH reverted to normal within a few hours. Re-illumination resulted in a pH increase of a smaller magnitude, but over a similar time span. Estimation of the proton extrusion rate gave values of about 17 µmol (g FW root)-1 h-1.
    Plant and Soil 01/1994; 162(2):299-302. · 3.24 Impact Factor
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    ABSTRACT: This study investigated the establishment and survival of naturally and artificially seeded white spruce (Picea glauca), as well as three sizes of planted white spruce stock, on different types of scarification beneath mature aspen (Populus tremuloides) in northern Alberta. White spruce seed rain, natural and hand- seeded regeneration establishment and survival, and understory vegetation response were monitored for 4 yr onscarified strips (light blading, heavy blading, and ridged) and on undisturbed controls. Despite a heavy seed rain in 1993, seedling establishment on control plots was almost nil. On scarified plots, the number of seeds required to produce,a live seedling after 3 growing seasons ranged from 15 to 37 and up to 68 in one site. Most mortality occurred during the summer. Germination rate on the hand-seeded plots ranged from 19 to 28% on the scarified strips. There was no significant difference in survival from seed (15%) after three growing seasons among,the three scarification treatments. Survival of planted stock was 98% on scarified strips and 96% on control strips. Diameter growth was least on control plots but was not significantly different among,the scarified treatments. In contrast, height increment was greatest on the light blading treatment and differed little among the other two treatments and control. These results suggest that spruce regeneration in aspen forests can be promoted,by scarificationor underplanting. West. J. Appl. For. 15(4):177‐182. White spruce (Picea glauca) can be difficult and expensive
    Aspen Bibliography.
  • Pierre Y. Bernier, James D. Stewart, Gary D. Hogan
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    ABSTRACT: Inside growth chambers, experimenters and plant material can modify the concentration of CO 2 by physiologically significant amounts. Mathematical equations are derived to account for chamber leak rate, net CO 2 exchange by plants, and respiration by experimenters. These equations can be used to predict the duration and intensity of expected changes in CO 2 concentration inside the growth chambers. Illustrations for two cases are given.