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Aims Black spruce (Picea mariana [Mill.] B.S.P.) and white spruce (Picea glauca [Moench] Voss.) are congeneric species. Both are moderately shade tolerant and widely distributed across North American boreal forests. Methods To understand light effects on their ecophysiological responses to elevated CO2, 1-year-old seedlings were exposed to 360 µmol mol−1 and 720 µmol mol−1 CO2 at three light conditions (100%, 50% and 30% of full light in the greenhouse). Foliar gas exchanges were measured in the mid- and late-growing season. Important Findings Elevated CO2 increased net photosynthesis (P n) and photosynthetic water use efficiency, but it reduced stomatal conductance and transpiration. The stimulation of photosynthesis by elevated CO2 was greatest at 50% light and smallest at 100%. Photosynthesis, maximum carboxylation rate (V cmax) and light-saturated rate of electron transport (J max) all decreased with decreasing light. Elevated CO2 significantly reduced V cmax across all light treatments and both species in mid-growing season. However, the effect of elevated CO2 became insignificant at 30% light later in the growing season, with the response being greater in black spruce than in white spruce. Elevated CO2 also reduced J max in white spruce in both measurements while the effect became insignificant at 30% light later in the growing season. However, the effect on black spruce varied with time. Elevated CO2 reduced J max in black spruce in mid-growing season in all light treatments and the effect became insignificant at 30% light later in the growing season, while it increased J max later in the season at 100% and 50% light. These results suggest that both species benefited from elevated CO2, and that the responses varied with light supply, such that the response was primarily physiological at 100% and 50% light, while it was primarily morphological at 30% light. [ABSTRACT FROM AUTHOR]

Understanding tree vulnerability to freezing temperatures will help resource managers to mitigate the effects of climatic variability. To test the effectiveness of tissue dehardening curves to represent whole seedling responses, we tested the cold tolerance of 4 conifer species to a range of freezing temperatures, − 15, − 25, and − 35 °C in late March, and − 5, − 10, and − 15 °C in early May. Results show that, after a 30-min exposure to freezing temperatures, needle mortality started at − 25 °C in late March (early spring dehardening) before budbreak activation and at − 10 °C in early May (late spring dehardening) when pine buds had flushed and white spruce (Picea glauca) buds had started swelling. Freezing temperatures delayed the timing of budbreak; increased needle, terminal bud, and seedling mortality; and reduced shoot growth and number of top laterals. White spruce and lodgepole pine (Pinus contorta) were more sensitive to freezing in terms of needle, terminal bud, and seedling mortality and growth reduction. The effects of March freezing were generally detected in the first year, whereas those of the May freezing were still evident in the second year on the shoot growth of black spruce (Picea mariana) and jack pine (Pinus banksiana). Although they seem relatively hardier based on first year post-freezing assessments (i.e., less immediate physical damage from freezing), black spruce and jack pine may have sustained more internal or physiological shock/damage. Our study results support the use of tissue cold hardiness–forcing temperature relationships to assess the risk of freezing damage for conifer seedlings. [ABSTRACT FROM AUTHOR]

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Abstract: CO2 elevation stimulates plant growth, which in turn demands more nutrients to sustain. Since the increase of demand for nitrogen (N), phosphorus (P) and potassium (K) may be in different proportions, the optimal N–P–K ratios at elevated [CO2] are likely different from those at the ambient [CO2]. This study investigated the effects of various N supply levels with constant and variable (constant P and K concentrations) N–P–K ratios under ambient and elevated [CO2] on black spruce (Picea mariana Mill. BSP) seedlings. One-year-old seedlings were exposed to two [CO2] (370 vs. 720μmolmol−1), two nutrient ratio regimes (constant vs. variable N/P/K ratios) and six N concentrations (10, 80, 150, 220, 290 and 360μmolmol−1) in four environmentally controlled greenhouses for 3.5months. Growth response to N varied with [CO2] and N/P/K ratios: under the elevated [CO2], height growth increased with increasing N supply when P and K concentrations were kept constant across different N levels, but it only increased when increasing N from 10 to 150μmolmol−1, and started to decline with further increase in N supply when N/P/K ratios were kept constant at different N levels; at the ambient [CO2], height growth was greatest at 150μmolmol−1 N and was generally greater at 220–360 than at 10 and 80μmolmol−1 N in both nutrient ratio treatments. The foliage to root ratio, shoot mass ratio and total biomass generally increased with increasing N supply but root mass ratio decreased. The smallest specific leaf area occurred at the lowest N supply when N/P/K ratios were kept constant but at 220μmolmol−1 N when P and K concentrations were kept constant across different N supplies. The results of leaf nutrient concentrations suggest that the elevated [CO2] increased demand for N, P and K and the increase for N was greater than P and K, altering the relationship between growth and nitrogen supply. Under the elevated [CO2], high N supplies resulted in growth suppression by critical toxicity content only in the constant N/P/K ratios treatment but low N supplies led to growth suppression by critical deficiency content in both nutrient ratio treatments. At the ambient [CO2], in contrast, N/P/K ratio treatments did not affect growth suppression by critical deficiency or critical toxicity content. Because elevated CO2 causes unequal increases in N, P and K demands, N–P–K ratios should be considered when modeling plant growth responses to elevated CO2. [Copyright &y& Elsevier]

CO2–light interactions can influence the competition among boreal plants, but are poorly understood. We investigated the effect of such interactions on the growth and biomass of 1-year-old black spruce (Sb) (Picea mariana (Mill.) BSP) and white spruce (Sw) (Picea glauca (Moench) Voss) grown with CO2 concentrations ([CO2]) of 360 and 720 µmol·mol–1 under 30%, 50%, and 100% light, in greenhouses. There were significant two-way and three-way interactions. Root collar diameter (RCD) of Sw decreased with decreasing light, while in Sb, there was no significant difference in RCD for plants grown under 50% or 30% light. Height was greater for plants grown under 100% light than if the plants were shaded. Elevated [CO2] increased RCD by 33% and enhanced stem volume by 67%, 98%, and 84% under 100%, 50%, and 30% light, respectively. The CO2 enhancement of total biomass was relatively higher under lower light, and greater for Sb than Sw. Elevated [CO2] decreased specific leaf area under 50% light only. Root mass was generally higher under 100% light than when shaded. Elevated [CO2] increased the root mass of Sb under 100% light, but decreased it under 30% light. Elevated [CO2] decreased the shoot/root ratio under 100% light, but increased it under 30% light. Our data suggest that raising [CO2] will likely increase species competitiveness under low light conditions, and that the increase will be greater in species that are relatively shade tolerant. [ABSTRACT FROM AUTHOR]

One-year-old seedlings of black spruce (Picea Mariana (Mill.) B.S.P.), white spruce (Picea glauca (Moench) Voss.), jack pine (Pinus banksiana Lamb.) and aspen (Populus tremuloides Michx.) were exposed to 5, 10, 15, 20, 25, 30 and 35 °C soil temperatures for 4 months. The rate of net photosynthesis (A), stomatal conductance, mid-day xylem water potential, the internal to ambient CO2 concentration ratio (Ci/Ca), and specific leaf area (SLA) were evaluated 3 months after the initiation of the treatments. The responses of the above variables generally showed a bell-shaped curve except the Ci/Ca ratio. The relationships between soil temperature and the ecophysiological variables (except Ci/Ca) were modeled using third-order polynomials. The models suggest that jack pine generally had the highest optimal soil temperature among the four species, followed sequentially by black spruce, trembling aspen and white spruce. Aspen was generally more sensitive to low soil temperatures than the conifers while black spruce was more sensitive to high soil temperatures than other species. Jack pine and trembling aspen generally showed different response patterns than black and white spruce. The photosynthesis in aspen and jack pine was progressively less limited by stomatal conductance (relative to mesophyll limitation) as soil temperature increased from 5 to 35 °C whereas in the spruces, stomatal conductance was the least limiting to photosynthesis at moderate soil temperatures (10–15 °C for white spruce, 15–20 °C for black spruce). The xylem water potential of jack pine and aspen was more sensitive to low soil temperatures while that of the spruces was more sensitive to high soil temperatures. The specific leaf area of jack pine and aspen showed large variations with changes in soil temperature while that of the spruces was relatively constant throughout the range of soil temperatures. Most of the conifer seedlings in the 35 °C soil temperature treatment died at the end of the experiment while all aspen seedlings survived. [Copyright &y& Elsevier]

One-year old seedlings of trembling aspen (Populus tremuloides Michx.), black spruce (Picea mariana (Mill.) B.S.P.), white spruce (Picea glauca (Moench) Voss), and jack pine (Pinus banksiana Lamb.) were subject to seven soil temperatures (5, 10, 15, 20, 25, 30 and 35 °C) for 4 months. All aspen seedlings, about 40% of jack pine, 20% of white spruce and black spruce survived the 35 °C treatment. The seedlings were harvested at the end of the fourth month to determine biomass and biomass allocation. It was found that soil temperature, species and interactions between soil temperature and species significantly affected root biomass, foliage biomass, stem biomass and total mass of the seedling. The relationship between biomass and soil temperature was modeled using third-order polynomials. The model showed that the optimum soil temperature for total biomass was 22.4, 19.4, 16.0 and 13.7 °C, respectively, for jack pine, aspen, black spruce and white spruce. The optimum soil temperature was higher for leaf than for root in jack pine, aspen and black spruce, but the trend was the opposite for white spruce. Among the species, aspen was the most sensitive to soil temperature: the maximum total biomass for aspen was about 7 times of the minimum value while the corresponding values were only 2.2, 2.4 and 2.3 times, respectively, for black spruce, jack pine and white spruce. Soil temperature did not significantly affect the shoot/root (S/R) ratio, root mass ratio (RMR), leaf mass ratio (LMR), or stem mass ratio (SMR) (P>0.05) with the exception of black spruce which had much higher S/R ratios at low (5 °C) and high (30 °C) soil temperatures. There were significant differences between species in all the above ratios (P<0.05). Aspen and white spruce had the smallest S/R ratio but highest RMR while black spruce had the highest S/R but lowest RMR. Jack pine had the highest LMR but lowest SMR while aspen had the smallest LMR but highest SMR. Both LMR and SMR were significantly higher for black spruce than for white spruce. [Copyright &y& Elsevier]

To examine the effects of soil temperature on a coupled photosynthesis–stomatal conductance model, seedlings of trembling aspen (Populus tremuloides Michx.), jack pine (Pinus banksiana Lamb.), black spruce (Picea Mariana (Mill.) B.S.P.) and white spruce (Picea glauca (Moench) Voss) were exposed to soil temperatures ranging from 5 to 35 °C for 4 months. Light and CO2 response curves of foliar gas exchange were measured for model parameterization. The effects of soil temperature on four key model parameters, Vcmax (maximum rate of carboxylation), Jmax (maximum rate of electron transport), α (energy conversion efficiency or quantum efficiency of electron transport) and Rd (daytime dark respiration), were modeled using two third-order polynomial equations and a modified Arrhenius equation. In all species, Vcmax and Jmax increased with soil temperature up to an optimum, and then decreased with further increases in soil temperature. In the conifers, α showed a similar response to soil temperature as Vcmax and Jmax, but soil temperature had no significant effect on α in aspen. Soil temperature had no significant effect on Rd in any species. The three equations described the relationships between soil temperature and the model parameters reasonably well, but performed best for Vcmax and worst for α. No significant relationships were identified between soil temperature and the parameters of the stomatal conductance model. [ABSTRACT FROM PUBLISHER]

Effects of shoot water potential (Ψ) and leaf-to-atmosphere vapor pressure difference (VPD) on gas exchange of jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana (Mill.) B.S.P.), and aspen (Populus tremuloides Michx.) were investigated at the northern edge of the boreal forest in Manitoba, Canada. Laboratory measurements on cut branches showed that net photosynthesis (An) and mesophyll conductance (gm) of jack pine and gm of black spruce did not respond to Ψ until a threshold Ψ was reached below which they decreased linearly. Photosynthesis of black spruce decreased slowly with decreasing Ψ above the threshold and declined more rapidly thereafter. The threshold Ψ was lower in black spruce than in jack pine. However, stomatal conductance (gs) of black spruce decreased continuously with decreasing Ψ, whereas gs of jack pine showed a threshold response. Mesophyll limitations were primarily responsible for the decline in An at low Ψ for jack pine and black spruce in the middle of the growing season, but stomatal limitations became more important later in the season. Field measurements on in situ branches on warm sunny days showed that both conifer species maintained Ψ above the corresponding threshold and there was no evidence of Ψ limitation on An of jack pine, black spruce or aspen.Vapor pressure difference was important in regulating gas exchange in all three species. An empirical model was used to quantify the gs response to VPD. When parameterized with laboratory data for the conifers, the model also fit the corresponding field data. When parameterized with field data, the model showed that stomata of aspen were the most sensitive of the three species to VPD, and stomata of black spruce were the least sensitive. For jack pine and aspen, stomata of foliage in the upper canopy were significantly more sensitive than stomata of foliage in the lower canopy. Vapor pressure difference had a greater impact on An of aspen than on An of the conifers as a result of aspen's greater stomatal sensitivity to VPD and greater slope of the relationship between An and intercellular CO2 concentration (Ci). During the 1994 growing season, VPD averaged 1.0 kPa, corresponding to ratios of Ci to ambient CO2 of 0.77, 0.71 and 0.81 for jack pine, black spruce and aspen, respectively. We conclude that increases in VPD at the leaf surface in response to climate change should affect the absolute CO2 and H2O fluxes per unit leaf area of the aspen component of a boreal forest landscape more than those of the conifer component. [ABSTRACT FROM AUTHOR]

• Both nutrient supply and root interaction between seedlings influenced responses to CO 2 elevation. • CO 2 stimulation to photosynthesis was much greater under low than high nutrient supply. • CO 2 elevation substantially reduced low nutrient limitation to photosynthesis. • The degrees of responses were very different between black spruce and white spruce. The carbon dioxide concentration ([CO 2) in the atmosphere has been increasing since the start of the industrialization. The impact of increasing [CO 2 on the physiology and growth of plants and on the composition, dynamics and productivity of plant communities have been studied extensively in the past few decades. However, the responses to CO 2 elevation can be altered by other environmental and biological factors. Such interactions, particularly those involving multiple factors, are complex and not well understood. In this study, we investigated the interactive effects of CO 2 elevation, nutrient availability and root interaction between plants on the total seedling photosynthesis, growth, and biomass allocation of two boreal conifer tree species, black spruce and white spruce. Seedlings were exposed to two [CO 2 (380 vs. 720 µmol mol−1) and two nutrient levels (low vs. high) with or without root interaction. We found that each of the three treatments influenced the effects of others and the two species also responded differently to those interactions. Root interactions reduced seedling growth only when nutrient supply was high. White spruce was more sensitive root interaction than black spruce while black spruce was more sensitive to the negative impact of low nutrient supply than white spruce. Surprisingly, CO 2 elevation and its interactions did not significantly affect seedling growth. The CO 2 elevation significantly increased total seedling photosynthesis while root interaction reduced it and the reduction was greater under the elevated CO 2. The photosynthetic response to CO 2 elevation was more sensitive to nutrient availability in white spruce than in black spruce. Seedling biomass was more sensitive to the negative impact of root interaction and low nutrient supply in black spruce than white spruce. Root interaction reduced biomass allocation to leaf and the impact was reduced by low nutrient. Biomass allocation to leaf was more sensitive to low nutrient supply in black spruce than white spruce. CO 2 elevation reduced but root interaction increased root/leaf mass ratio, partially offsetting each other when both treatments were applied. The root/leaf ratio of white spruce was more sensitive to CO 2 elevation while that of black spruce was more sensitive to root interaction and nutrient supply. Low nutrient supply and root interaction both had positive effect on root/leaf ratio and their effects were confounded when both were applied simultaneously. Therefore, their effects were observed only when one of them was absent. [ABSTRACT FROM AUTHOR]

Abstract: This paper will discuss TRIPLEX-Flux, a process-based model that estimates net ecosystem production (NEP) as well as analyzing the level of sensitivity of the model''s response by simulating CO2 flux in an existing old black spruce BOREAS site in central Canada. The research objectives were: (1) to test the TRIPLEX-Flux model simulations against flux tower measurements; and (2) to examine parameter and input variable effects on model response via sensitivity analysis. Validation of NEP data at 30min intervals derived from tower and chamber measurements showed that the NEP data from the model corresponded well with the measured NEP from the BOREAS site (R 2 >0.65). Sensitivity analysis demonstrated different levels of sensitivity between morning and noon periods and from the current to doubled atmospheric CO2 concentrations. Additionally, the comparison of different algorithms to calculate stomatal conductance showed that the NEP predicted by the model using the iteration algorithm was consistent with the results using a constant C i/C a of 0.74 for current and 0.81 for doubled CO2 concentrations. Varying parameter and input variable values by ±10% resulted in a similar model response between morning and noon periods (less than or equal to 27.6% and 27.4%, respectively). Most parameters were more sensitive at noon than they were in the morning except for those that were correlated with air temperature, suggesting that air temperature has considerable influence over model sensitivity for these parameters/variables. Air temperature effect was greater under doubled than it was under current atmospheric CO2 concentrations. In contrast, model sensitivity to CO2 decreased under doubled CO2 concentrations. [Copyright &y& Elsevier]

Abstract: In process-based models of forest ecosystems, carbon allocation is a key process that partitions the net primary productivity (NPP) among different biomass components and thus affects forest growth patterns at different stages of the forest''s growth. Common allocation models usually express the carbon allocation as linear or monotonic functions of time. These models are acceptable for a limited growth period, but inadequate for simulating long-term development of a forest. To improve the usefulness of these models and simplify the model structure, we developed an analytical solution to partition NPP that considers the effects of litterfall and growth rate. We derived NPP allocation functions based on functions for the net growth rate (dry mass) and based on empirical growth curves estimated from normal yield tables for northern Ontario, Canada. The improved allocation reflects growth restrictions that arise in overmature stands according to different site classes across landscape. The details of the derivation and parameterization are presented and discussed. We validated the new NPP allocation functions for black spruce (Picea mariana) in the boreal forest of northern Ontario. The improved NPP allocation simulated the NPP dynamics of forest ecosystems more accurately over the course of stand development, and the results were biologically realistic. [Copyright &y& Elsevier]