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Predicting photosynthetic acclimation to elevated CO2 and warming is difficult because they have opposite effects. We investigated physiological and morphological responses in white birch ( Betula papyrifera Marshall) to a combination of CO2 and temperature (ACT — 400 µmol·mol–1 CO2, current temperature; ECT — 750 µmol·mol–1 CO2, current + 4 °C temperature). ECT reduced photosynthesis, maximum Rubisco carboxylation ( Vcmax), maximum electron transport rate ( Jmax), photorespiration, daytime respiration, leaf N, and stomatal and mesophyll conductance, but increased biomass, height, total leaf area, electron partitioning to carboxylation and oxygenation ratio, and CO2 compensation point. The photosynthetic acclimation is consistent with the optimal carbon gain theory (carbon gain drives the coordination of carboxylation, electron transport, and respiration). While the photosynthetic acclimation was similar to acclimation to elevated CO2, ECT reduced Jmax/ Vcmax, which is consistent with the response to warming but opposite to the response to elevated CO2, suggesting that thermal acclimation may be the primary mechanism of photosynthetic acclimation to ECT and ECT probably altered N allocation between machinery for carboxylation and that for ribulose-1,5-bisphosphate regeneration. The increase in total leaf area by ECT more than offset the negative effect of photosynthetic downregulation on carbon sequestration, resulting in faster growth and greater biomass under ECT.

Elevated CO2 and warmer temperature occur simultaneously under the current climate change. However, their combined effects on the photosynthetic traits in boreal trees are not well understood. This study investigated the morphological and photosynthetic responses of yellow birch (Betula alleghaniensis Britt.) to a combined treatment of CO2 and temperature (ambient, ACT (400 μmol mol−1 CO2 and current temperature) vs elevated, ECT (750 μmol mol−1 CO2 and current +4 °C temperature)). It was found that ECT significantly reduced leaf-area based photosynthetic rate (An), maximum Rubisco carboxylation rate (Vcmax), photosynthetic electron transport rate (Jmax), leaf nitrogen concentration, respiration and mesophyll conductance. There were two interesting findings: first, the primary mechanism of photosynthetic limitation shifted from Ribulose-1,5-bisphosphate (RuBP) carboxylation (related to Vcmax) to RuBP regeneration (related to Jmax) in response to ECT, leading to decreased transition point (Ci-t and An-t) from RuBP carboxylation to regeneration; second, the increase in total leaf area in response to ECT more than compensated for the downregulation of leaf-area based photosynthesis, leading to greater biomass in ECT than in ACT. We proposed a new protocol for evaluating photosynthetic limitations by comparing the relative relationship between the transition point (Ci-t and An-t) and the photosynthetic rate at growth CO2 (Ci-g and An-g). Furthermore, we found that Jmax (RuBP regeneration) was the primary limitation to An under ECT.

Although the effects of nitrogen deposition on tree water relations are studied extensively, its impact on the relative sensitivities of stomatal and xylem hydraulic conductance to vapor pressure deficit and water potential is still poorly understood. This study investigated the effects of a 7-year N deposition treatment on the responses of leaf water relations and sensitivity of canopy stomatal conductance to vapor pressure deficit (VPD) and water potential, as well as the sensitivity of branch hydraulic conductance to water potential in a dominant tree species (Quercus wutaishanica) and an associated tree species (Acer mono) in a temperate forest. It was found that the N deposition increased stomatal sensitivity to VPD, decreased stomatal sensitivity to water potential, and increased the vulnerability of the hydraulic system to cavitation in both species. The standardized stomatal sensitivity to VPD, however, was not affected by the N deposition, indicating that the stomata maintained the ability to regulate the water balance under nitrogen deposition condition. Although the increased stomatal sensitivity to VPD could compensate the decreased stomatal sensitivity to water potential to some extent, the combined response would increase the percentage loss of hydraulic conductivity (PLC) when 50 % loss in stomatal conductance occurred, particularly in the dominant species Q. wutaishanica. The result indicates that N deposition would increase the risk of hydraulic failure in those species if the soil and/or air becomes drier under future climate change scenarios. The results of the study can have significant implications on the modelling of ecosystem vulnerability to drought under the scenario of atmospheric nitrogen deposition.

Climate change is predicted to cause northward migration of boreal tree species. However, the success of such a migration will be determined by trees’ ability to acclimate to the environmental conditions at the potential new site. This study was aimed to assess the interactive effects of changes in photoperiod associated with northward migration and soil warming on the ecophysiological traits of jack pine (Pinus banksiana Lamb.) under different carbon dioxide concentrations [CO2]. We examined the responses of foliar gas exchange and seedling cold hardiness after exposing 1-year old seedlings to two [CO2] (400 and 950 µmol mol−1), two soil temperatures (growing season average at seed origin and 5 °C warmer) and three photoperiod regimes (seed origin, and 5° and 10° further north) for one growing season. We found that the CO2 elevation significantly increased photosynthetic rate measured at treatment CO2, photosynthetic water use efficiency, maximum rate of carboxylation and triose phosphate utilization. Some of the effects were modified by photoperiod. Furthermore, the CO2 elevation and soil warming both reduced seedling cold hardiness, as indicated by increases in injury index. The lack of interactions between photoperiod and other treatments (elevated [CO2] and soil temperature) suggests that young jack pine seedlings may have the ability to acclimate to the new photoperiod regimes at higher latitudes north of the seed origin.

The relationship between photosynthesis and leaf nitrogen concentration is often used to model forest carbon fixation and ratios of different nutrient elements can modify this relationship. However, the effects of nutrient ratios on this important relationship are generally not well understood. To investigate whether N/P/K ratios and CO2 concentration ([CO2]) influence relationships between photosynthesis and nitrogen, we exposed one-year-old black spruce seedlings to two [CO2] (370 and 720 μmol·mol-1), two N/P/K ratio regimes (constant (CNR) and variable (VNR) nutrient ratio) at 6 N supply levels (10 to 360 μmol·mol-1). It was found that photosynthesis (Pn) was more sensitive to nitrogen supply and N/P/K ratios under the elevated [CO2] than under ambient [CO2]; under the elevated [CO2], Pn declined with increases in N supplies above 150 μmol·mol-1 in the CNR treatment but was relatively insensitive to N supplies of the same range in the VNR treatment. Further, our data suggest that the nutrient ratio and the CO2 elevation effects on photosynthesis were via their effects on the maximum rate of carboxylation (Vcmax) but not electron transport (Jmax) or triose phosphate utilization (TPU). The results suggest that the CO2 elevation increased the demand for all three nutrient elements but the increase was greater for N than for P and K. The CO2 elevation resulted in greater photosynthetic use efficiencies of N, P and K, but the increases varied with the nutrient ratio treatments. The results suggest that under elevated [CO2], higher net photosynthetic rates demand different optimal N-P-K ratios than under the current [CO2].

In response to global warming, trees are expected to shift their distribution ranges to higher latitudes. The range shift will expose them to novel environmental conditions, such as new photoperiod regimes. These factors can interact with rising atmospheric CO2 ([CO2 ]) to affect trees' physiology and growth. This study simulated future environmental conditions to investigate photosynthetic responses to changes in photoperiod regimes (seed origin [48°N], 52, 55, and 58°N) and [CO2 ] (ambient 400 vs. elevated 1000 μmol mol-1 ) in white birch (Betula papyrifera Marsh.) seedlings. Our results show that elevated [CO2 ] stimulated leaf photosynthesis (Pn ) at the two lower latitudes (48 and 52°N). However, this stimulation by elevated [CO2 ] was lost in the two higher latitudes (55 and 58°N). Elevated [CO2 ] led to the downregulation of maximum Rubisco activity (Vcmax ) for the two higher latitudes, and maximum electron transport rate (Jmax ) and triose phosphate utilization (TPU) at 58°N, while it enhanced Jmax and TPU for the two lower latitudes. Increased instantaneous water-use efficiency (IWUE) for the two lower latitudes was primarily attributed to the CO2 stimulation of Pn while the higher IWUE under the photoperiod regimes of 55 and 58°N latitudes was explained by reduced water loss. Photoperiod effects varied with [CO2 ]: Pn increased at the photoperiod regimes of 55 and 58°N in ambient [CO2 ] while it tended to decline under these photoperiods in elevated [CO2 ]. Our study suggests that the photosynthesis of white birch will likely respond negatively to northward migration or seed transfer in response to climate change.

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 (Pn) 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 (Vcmax) and light-saturated rate of electron transport (Jmax) all decreased with decreasing light. Elevated CO2 significantly reduced Vcmax 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 Jmax 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 Jmax 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 Jmax 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.

Aims Soil lead contamination has become increasingly serious and phytoremediation can provide an effective way to reclaim the contaminated soils. This study aims to examine the growth, lead resistance and lead accumulation of mulberry (Morus alba L.) seedlings at four levels of soil lead contamination with or without biochar addition under normal or alternative partial root-zone irrigation (APRI). Methods We conducted a three-factor greenhouse experiment with biochar (with vs. without biochar addition), irrigation method (APRI vs. normal irrigation) and four levels of soil lead (0, 50, 200 and 800 mg·kg−1). The performance of the seedlings under different treatments was evaluated by measuring growth traits, osmotic substances, antioxidant enzymes and lead accumulation and translocation. Important Findings The results reveal that mulberry had a strong ability to acclimate to soil lead contamination, and that biochar and APRI synergistically increased the biomass and surface area of absorption root across all levels of soil lead. The seedlings were able to resist the severe soil lead contamination (800 mg·kg−1 Pb) by adjusting glutathione metabolism, and enhancing the osmotic and oxidative regulating capacity via increasing proline content and the peroxidase activity. Lead ions in the seedlings were primarily concentrated in roots and exhibited a dose–effect associated with the lead concentration in the soil. Pb, biochar and ARPI interactively affected Pb concentrations in leaves and roots, translocation factor and bioconcentration. Our results suggest that planting mulberry trees in combination with biochar addition and APRI can be used to effectively remediate lead-contaminated soils.

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.

Past research suggests climate change will cause the climate envelopes of various tree species to shift to higher latitudes and can lead to a northward migration of trees. However, the success and scope of the migration are likely affected by factors that are not contained in the climate envelope, such as photoperiod and interactive effects of multiple environmental factors, and these effects are currently not well understood. In this study, we investigated the interactive effects of CO2 concentrations ([CO2]), photoperiod and soil moisture on the morphological and physiological traits of Populus tremuloides Michx. We grew seedlings under two levels of [CO2] (ambient [CO2] (AC) 400 vs elevated [CO2] (EC) 1000 μmol mol−1), four photoperiod regimes (growing season photoperiods at 48 (seed origin), 52, 55 and 58°N latitude) and two soil moisture regimes (high soil moisture (HSM) vs low soil moisture (LSM), −2 MPa) for two growing seasons in greenhouses. Both morphological and physiological responses were observed. Low soil moisture reduced leaf size, total leaf area and height growth by 33, 46 and 12%, respectively, and increased root/shoot ratio by 20%. The smaller leaf area and increased root/shoot ratio allowed the seedlings in LSM to maintain higher the maximum rate of Rubisco carboxylation (Vcmax) and the maximum rate of electron transport for RuBP regeneration (Jmax) than control seedlings (55 and 83% higher in July, 52 and 70% in August, respectively). Photoperiod and [CO2] modified responses to LSM and LSM altered responses to photoperiod and [CO2], e.g., the August photosynthetic rate was 44% higher in LSM than in HSM under EC but no such a difference existed under AC. The increase in Vcmax and Jmax in response to LSM varied with photoperiod (Vcmax: 36% at 52°N, 22% at 55°N, 47% at 58°N; Jmax: 29% at 52°N, 21% at 55°N, 45% at 58°N). Stomatal conductance and its reduction in response to LSM declined with increasing photoperiod, which can have significant implications for soil moisture effect on northward migration. This study highlights the need to consider the complex interactions of [CO2], photoperiod and soil moisture when planning assisted migration or predicting the natural migration of boreal forests in the future.

Tree migration to higher latitudes may occur in response to future changes in climate, exposing the trees to higher concentrations of carbon dioxide ([CO2]), new photoperiods, different levels of soil moisture, and other new conditions. These new conditions can influence the physiology, survival, and growth of trees. This study examined the interactive effects of [CO2], photoperiod, and soil moisture on the morphology and resistance to xylem cavitation in trembling aspen (Populus tremuloides Michx.). One-year-old seedlings, in greenhouses, were exposed to two [CO2] (ambient [CO2] 400 μmol·mol−1 or an elevated [CO2] 1000 μmol·mol−1), four photoperiod regimes corresponding to latitudes 48°N (seed origin), 52°N, 55°N, and 58°N, and two levels of soil moisture (60%–75% and 13%–20% of field capacity) for one growing season. Seedling growth, leaf size, specific leaf area, biomass allocation, and xylem resistance to cavitation (water potentials for 20%, 50%, and 80% loss of hydraulic conductivity) were assessed. The seedlings under the longest photoperiod regime (58°N latitude) had greatest height and biomass but smallest specific leaf area. Under the elevated [CO2], however, the longest photoperiod regime significantly reduced xylem resistance to drought-induced cavitation compared with the photoperiod corresponding to 48°N. These results suggest that when migrating to higher latitudes, trembling aspen may grow faster but could become less resistant to drought and more prone to hydraulic failure during a drought spell.

Climate change affects timing and variability of spring phenology, as well as occurrence of spring frosts, and therefore influences forest structure, function, and management practices. In this study, we evaluated changes in budburst timing, sequential order of budburst, budburst temperatures, and frosts, from 1981–2010 baseline to different future climates (+2, +4, and +6 °C for moderate, high, and extremely high warming) for selected boreal and temperate species in eastern Canada. We used species-specific budburst models that are developed from large-scale forcing experiments. Budburst averaged 10–15 days earlier per 2 °C increase of temperatures, except for temperate yellow birch and eastern white pine that slowed down to 5–7 days in budburst advancement from +4 to +6 °C. Earlier budburst was associated with greater annual and interspecific variations in budburst timing, lower budburst temperatures, more frosts, and less annual variations in sequential order of budburst. Compared to temperate trees, boreal trees had greater budburst advances and annual variations, but less interspecific variations in timing and sequential order of budburst. Early flushing species had greater phenological changes and annual variations, as well as more frosts. Our results suggest that budburst advance that has occurred will continue with greater among years/species variability but is unlikely to converge between boreal and temperate regions in eastern Canada under anticipated levels of climate warming.

Bud development influences shoot branching and the plasticity and adaptability of plants. To explore the differences of post-embryonic development of different types of buds, shoots originated from adventitious buds and axillary buds of cuttings in two populations of balsam poplar (Populus balsamifera L.) were investigated for differences in leaf morphology, photosynthetic and growth characteristics, and the effects of a carbonic anhydrase (CA) inhibitor on CA activity, photosynthesis and mesophyll conductance (gm ). The results showed that axillary buds produced ovate first few leaves and longer shoots while adventitious buds produced lanceolate first few leaves with higher specific leaf area (SLA). There were no significant differences in leaf area-based photosynthetic rate (An ), maximum carboxylation rate (Vcmax ), and maximum electron transport rate (Jmax ) between shoots originated from the two bud types. Based on the principal component analysis, shoots of adventitious bud origin grouped on daytime respiration and SLA, while cuttings from axillary buds clustered toward the opposite direction of quantum yield and light saturation point. Shoots originated from different types of buds had different growth rates and biomass, but the direction of the differences varied with the population of the mother tree. The two populations differed in An , gm , and relationships between CA, An , and gm . There were differences in post-embryonic growth traits of shoots from axillary buds and those from adventitious buds, which may be an adaptive strategy for regeneration under different light conditions.

Measuring foliar gas exchange (photosynthesis, respiration, and transpiration) has become popular in a wide range of disciplinary fields because to the invention of portable gas exchange equipment. There are three common types of portable gas exchange systems: closed system, semi-closed system and open system. Modern instruments are simple to use and require little or no calibration by the operator. Indeed, some equipment can automatically create photosynthetic response curves to light and carbon dioxide. The ease of use and expanded capabilities of gas exchange instruments have substantially increased user confidence and the usage of gas exchange measurements both in field studies and research in enclosed environments. Measurement and calculation errors, however, are still possible. One paper submitted to a peer-reviewed journal, for example, reported net rates of photosynthesis with intercellular CO2 concentrations higher than the ambient CO2 concentration at the time of the observations. In my own lab, erroneous measurements have been noticed, particularly by inexperienced students. Errors can be caused by a variety of factors, including faulty equipment, inappropriate procedures, leaking leaf chambers, leaking or obstructed gas lines, or inadequate measurement conditions. Errors can also occur if the measurements are taken in the wrong order or at the wrong time for various treatments within the same study. Furthermore, even when studying the same species, data from multiple research are not necessarily comparable due to changes in measuring techniques or bases on which results are expressed (e.g., projected, hemi-surface and total surface leaf area). In this paper, I'll go over some aspects of foliage gas exchange measurements that are either confusing or require special attention, including different types of instruments and their pros and cons.

We tested how nutrient supply and interspecific belowground competition affect ecophysiological and morphological responses to elevated CO 2 in black ( Picea mariana (Mill). B.S.P) and white spruce ( Picea glauca [Moench] Voss.). It is found that belowground competition and high nutrient greatly enhanced the relative performance of black spruce over white spruce at elevated CO 2 . We have previously found that interspecific belowground competition reduce growth, whole seedling photosynthesis, and biomass allocation to leaf and that belowground competition and nutrient supply affect responses to elevated CO2 in the above two species, but we did not examine the physiological and morphological mechanisms of the responses. To examine the interactive effects of belowground competition, nutrient supply and elevated CO2 on root morphology, photosynthetic rate, and biochemical and photochemical capacity of photosynthesis in black spruce (Sb, Picea mariana [Mill.] B.S.P.) and white spruce (Sw, Picea glauca [Moench] Voss.). Seedlings were grown in individual containers (no belowground competition) or in a common container (belowground competition) under 380 vs. 720 µmol mol−1 CO2 and high vs. low nutrient supply in the greenhouse for one growing season. Elevated CO2 stimulated photosynthesis and nutrient use efficiency to a much greater degree in black than white spruce when they were grown in the same container, particularly under high nutrient supply. The ability to produce a greater length of roots per unit volume of soil was associated with the response of black spruce. The synergistic effects of elevated CO2 and belowground competition on the physiology and root morphology of black spruce suggest that elevated CO2 will likely increase the relative competitiveness of black spruce over white spruce.

Trees may migrate northward in response to climate change and become exposed to new photoperiod and soil moisture regimes. This study assessed the impacts of photoperiod and its interaction with soil moisture and carbon dioxide concentration ([CO2]) on the hydraulic conductivity in jack pine (Pinus banksiana Lamb.) and its vulnerability to xylem embolism. Seedlings were exposed to 400 vs. 950 μmol·mol−1 [CO2], 60%–70% vs. 30%–40% (of field capacity) soil moisture, and photoperiods of seed origin and 5° and 10° north of seed origin in greenhouses. Cavitation vulnerability curves were measured for determining the xylem pressure at which 50% hydraulic conductivity was lost (ΨPLC50). It was found that elevated [CO2] significantly increased hydraulic conductivity, whereas low soil moisture decreased it. Under elevated [CO2], the xylem became progressively more vulnerable to embolism with changes in photoperiod regime from the seed origin to 10° north of the seed origin, as indicated by the progressively less negative ΨPLC50. However, no such a trend was detected under the ambient [CO2]. The results suggest that the species may become less resistant to drought as the atmospheric [CO2] increases, hindering the northward migration or seed transfers. Even within its current natural distribution range, trees near its northern boundary of the range may be more vulnerable to embolism as the atmospheric [CO2] increases even without any change in moisture conditions.

Conifer winter damage results primarily from loss of cold hardiness during unseasonably warm days in late winter and early spring, and such damage may increase in frequency and severity under a warming climate. In this study, the dehardening dynamics of lodgepole pine (Pinus contorta Dougl. ex. Loud), jack pine (Pinus banksiana Lamb.), white spruce (Picea glauca (Moench) Voss), and black spruce (Picea mariana (Mill.) B.S.P.) were examined in relation to thermal accumulation during artificial dehardening in winter (December) and spring (March) using relative electrolyte leakage and visual assessment of pine needles and spruce shoots. Results indicated that all four species dehardened at a similar rate and to a similar extent, despite considerably different thermal accumulation requirements. Spring dehardening was comparatively faster, with black spruce slightly hardier than the other conifers at the late stage of spring dehardening. The difference, however, was relatively small and did not afford black spruce significant protection during seedling freezing tests prior to budbreak in late March and early May. The dehardening curves and models developed in this study may serve as a tool to predict cold hardiness by temperature and to understand the potential risks of conifer cold injury during warming–freezing events prior to budbreak.

Trees may migrate northward in response to climate change, which would expose them to different environmental conditions, such as [CO2], photoperiod, soil moisture. Changes in these environmental conditions will likely influence tree phenology and growth. We grew Populus tremuloides Michx. from seeds for two growing seasons under two [CO2] (400 vs 1000 μmol mol−1), four photoperiod regimes (corresponding to 48 °N (seed origin), 52 °N, 55 °N, and 58 °N latitude), two soil moistures (60 %–75 % vs 13 %–20 %) and examined phenological responses. We found that longer photoperiods extended growing season by delaying autumnal phenology and increased growth. Low moisture also delayed autumnal phenology but reduced growth, and the effects were reduced by elevated [CO2]. Surprisingly, low moisture reduced cold hardiness of aspen. Elevated [CO2] delayed bud break in the spring and the effects were magnified by the low moisture. The interactive effects of photoperiod, CO2, and soil moisture on phenology, cold hardiness, growth, and growing season indicate the complex nature of northward migration that has been predicated under the scenario of climate change. Our results suggest that these interactions should be considered when planning latitudinal seed transfers and predicting the migration of boreal trees in response to climate change.

Compared with the effects of spring frosts on opening buds or newly flushed tissues, winter freezing damage to conifers, owing to temperature fluctuations prior to budbreak, is rare and less known. In this study, changes in cold hardiness (measured based on electrolyte leakage and needle damage) and spring budbreak were assessed to examine the responses of four boreal conifer species — black spruce (Picea mariana (Mill.) B.S.P.), white spruce (Picea glauca) (Moench) Voss), jack pine (Pinus banksiana Lamb.), and lodgepole pine (Pinus contorta Dougl. ex. Loud.) — to different durations of experimental warming (16 °C day to –2 °C night with a 10 h photoperiod, except for night temperatures during November warming (+2 °C)). Seedlings showed increased responses to warming from November to March, while the capacity to regain the cold hardiness lost to warming decreased during the same period. This suggests an increasing vulnerability of conifers to temperature fluctuations and freezing damage with the progress of chilling and dormancy release from fall to spring. Both lodgepole pine and jack pine initiated spring growth earlier and had greater responses to experimental warming in bud phenology than black spruce and white spruce, suggesting a greater potential risk of frost/freezing damage to pine trees in the spring.

Responding to the predicted shift in climate envelope jack pine, (Pinus banksiana Lamb.) might migrate 10° northward between 2071 and 2100 and will be exposed to a different photoperiod regime. Successful migration of the species might depend on the initial acclimating capability to the conditions of new location. The impacts of elevated carbon dioxide concentration [CO2], soil temperature and photoperiod on the phenological traits, growth and biomass responses in jack pine seedlings were investigated. Seedlings were grown in greenhouses under two [CO2] (400 and 950 μmol•mol-1), two soil temperatures (ambient soil temperature at seed origin and 5°C warmer) and three photoperiod regimes (photoperiods at seed origin, 5° north of the seed origin and 10° north of the seed origin). Budburst and bud setting time were recorded and the seedling height (Ht), root collar diameter (RCD), root biomass, stem biomass and leaf biomass were measured after six months of treatment. It was observed that under elevated [CO2], ambient Tsoil and photoperiods associated with 10° northward migrations budburstis advanced by 10 days. Photoperiods toward north significantly prolonged the bud setting time. However, tri-factor interactive effect on bud set was not statistically significant. Elevated [CO2] significantly (P 2] significantly increased the projected leaf area, it had no significant effect on specific leaf area. Elevated [CO2] significantly reduced the shoot to root ratio, which indicated higher biomass allocation in roots under elevated [CO2]. However, all these growth and biomass responses were statistically insignificant under tri-factor interactive effects. The results suggest that climate change induced northward migration will not affect the growth of jack pine. However, a long distance migration (e.g. 10° north) will expose the species to late-spring frost damage.

The carbon dioxide concentration ([CO2]) in the atmosphere has been increasing since the start of the industrialization. The impact of increasing [CO2] 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 CO2 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 CO2 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 [CO2] (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, CO2 elevation and its interactions did not significantly affect seedling growth. The CO2 elevation significantly increased total seedling photosynthesis while root interaction reduced it and the reduction was greater under the elevated CO2. The photosynthetic response to CO2 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. CO2 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 CO2 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.

Past studies indicate that narrower conduits such as those in diffuse-porous species are less vulnerable to freeze-thaw (FT) induced embolism and also facilitate the refilling of embolized xylem conduits early in the spring, resulting in an earlier bud break. In this study, we investigated if a novel environmental condition associated with climate change-induced northward migration will affect the vulnerability to FT-induced embolism and spring phenology in white birch. Seedlings were grown under ambient (400 μmol mol−1) or elevated CO2 concentration (1000 μmol mol−1), and four photoperiod regimes corresponding to 48 (seed origin), 52, 55, and 58 °N latitude. We found that the longest photoperiod (corresponding to 58 °N latitude) significantly increased the maximum specific hydraulic conductivity of the stem. CO2 concentration ([CO2]) and photoperiod had no significant impact on the vulnerability to FT-induced embolism. The treatment of 5 freeze-thaw cycles (+5 to −20 °C) led to an 11 % loss of hydraulic conductivity in dormant seedlings that had been stored at −4 °C for 3 months while the effect of such a treatment in the fall was much smaller. This result suggests that freeze-thaw events in late winter or spring can impair the hydraulic conductivity of the xylem which in turn may negatively affect the physiology of the trees. Indeed, the FT treatment in this study delayed budburst and leaf expansion in the spring. It is interesting to note that photoperiods had the opposite effect on budburst under different [CO2]: longer photoperiods led to earlier budburst in the spring under elevated [CO2], but delayed budburst under ambient [CO2]. The synergistic effect of longer photoperiods and CO2 elevation suggests that the growing season for white birch may be longer than what we predict from either factor alone at a migration site in the future when [CO2] will be much higher.

Global warming induced northward migration will expose trees to longer photoperiod regimes during the growing season. The phenotypic ability of trees to take advantage of the longer photoperiods and elevated atmospheric CO2 concentration ([CO2]) will likely be a critical factor for determining their success and performance at the new locations. We investigated how growth, biomass, and biomass allocations respond to the interactive effects of photoperiod regimes and [CO2] in white birch (Betula papyrifera Marsh.). Seedlings were exposed to ambient (400 μmol mol−1) or elevated concentration (1000 μmol mol−1) [CO2], and four photoperiod regimes corresponding to 48 (seed origin), 52, 55, and 58°N latitude for two growing seasons. Our results show that growth, biomass, and biomass allocation were affected by photoperiod regime but not by [CO2]. Seedling growth and biomass were stimulated by the three photoperiod regimes north of the seed origin. Plants under the photoperiod regime of 52°N were 15% higher in growth and 18% higher in biomass than under the photoperiod regime of the seed origin (48°N). However, increases in photoperiod regimes beyond that of 4° north of the seed origin did not lead to an additional increase in growth and biomass. The differences in biomass components among the three longer photoperiods were statistically insignificant, but the leaf biomass and stem biomass were higher under the longer photoperiods relative to the seed origin. While the differences between two adjacent photoperiods were not always statistically significant during the two growing seasons, biomass allocated to roots showed a general decreasing trend with increases in photoperiod from the seed origin to 58°N latitude. Our results suggest that despite the limited plasticity in growth and biomass displayed in much higher latitudes, white birch will likely benefit from the longer photoperiod regimes during the growing season associated with migration or seed transfer to higher latitudes.

The predicated changes in precipitation and temperature associated with the continued elevation of atmospheric CO2 concentration will trigger the northward shift of the Climate Envelopes for 130 North America tree species by as much as 10 degrees. However, climate envelope models do not take into account changes in other factors that may also influence the survival and growth of plants at the predicted new locations, such as photoperiod and nutrient regimes. This study investigated how photoperiod and nitrogen supply would affect the ecophysiological traits of black spruce (Picea mariana (Mill) B. S. P.) that are critical for survival and growth at new locations predicted by climate envelope models. We exposed black spruce seedlings to the photoperiod regime at the seed origin (PS) and that 10° north of the seed origin (PNM) as predicted by climate envelope models under the current and doubled atmospheric CO2 concentration and different levels of N supply (30 vs. 300 μmol·mol-1 N). We found that the PNM and the 30 μmol·mol-1 N supply both had negative impact on the development of seedling cold hardiness in the fall, and led to earlier burst of the terminal bud and greater rate of mortality in the following growing season. While the PNM stimulated seedling growth in the first growing season, the effect was not sustained in the second growing season. Our results suggest that the photoperiod regimes and poor nutrient conditions at higher latitudes will likely constrain the scope of the northward migration or seed transfer of black spruce.

Climatic warming may increase temperature variability, especially in winter months, leading to increased risk of early loss of cold hardiness and therefore freezing damage. In this study, changes in cold hardiness (measured based on electrolyte leakage), budbreak, and survival were used to indicate the responses of seedlings of 3 boreal broadleaf species [Formula: see text] trembling aspen (Populus tremuloides Michx.), balsam poplar (P. balsamifera L.), and white birch (Betula papyrifera Marsh.) [Formula: see text] to experimental warming. Seedling responses were greater in winter (January) and spring (March) than fall (November), and were greater in trembling aspen and balsam poplar than white birch. Warming for 5 or 10 days at 16 °C day/–2 °C night with a 10-h photoperiod in winter and spring generally reduced cold hardiness. Combined with freezing temperatures in the postwarming ambient environment, this reduction increased seedling mortality and stem dieback and extended time to budbreak. Cold hardiness increased somewhat 10 days after seedlings were returned to the outdoor environment following warming in spring, when ambient temperatures were less damaging. The resistance of white birch to warming, likely because of its greater thermal requirement for budbreak and slower natural dehardening, suggests that this species is better suited to withstand increasing winter temperature variability that might occur under climate change. To improve the accuracy of phenological modelling, the effects of winter freezing on budbreak should be factored in.

Predicting the future distribution of plants requires a mechanistic understanding of how plants cope with their new environmental conditions under a changed climate. We examined the effects of elevated carbon concentration ([CO2]) and elevated Tsoil on the physiological responses to light of a shade-tolerant woody species, mountain maple (Acer spicatum L.). Seedlings were exposed to ambient and elevated [CO2] (392 μmol mol−1 vs. 784 μmol mol−1), current and elevated soil temperatures (Tsoil 17 vs. 22 °C) at high and low (100% vs. 30%) light conditions for one growing season. It was found that elevated Tsoil stimulated net photosynthesis (A) by 18% in the low-light treatment but tended to reduce A in the high-light treatment. Elevated [CO2] increased A by 100% under the elevated Tsoil and by 48% at the current Tsoil. The effect of elevated Tsoil also varied with [CO2]: it increased A by 13% under the elevated [CO2] but reduced A by 17% under the ambient [CO2]. We observed a significant increase in the instantaneous water-use efficiency of photosynthesis (IWUE) in response to the high-light treatment under the elevated [CO2] but not under the ambient [CO2]. Our data indicate that the elevated [CO2] markedly increased nitrogen (N) allocation to the photochemical apparatus of A in seedlings grown under the high-light condition. These results suggest that the predicted increases in atmospheric [CO2] will likely enhance the growth of mountain maple in canopy gaps in the forest, while the predicted elevated Tsoil will negate the benefit of high-light conditions.

Phosphorus (P) is a common limiting nutrient element to plants and its supply and uptake by plants are strongly influenced by soil temperature. However, the interactive effects of the two factors on the physiological responses of plants to global change are poorly understood. In this study, we examined how P supply and Tsoil interacted in affecting physiological responses in white birch (Betula papyrifera) to [CO2]. We exposed seedlings to 7˚C, 17˚C and 27˚C Tsoil, 0.1479, 0.3029 and 0.5847 mM P2O5, and 360 and 720 µmol∙mol −1 [CO2] for four months. We have found that both the low soil temperature and CO2 elevation resulted in photosynthetic down regulation but the specific mechanisms of the down regulation were different between the two treatments, particularly the relative contributions of biochemical and photochemical capacity, mesophyll conductance and sink strength for carbohydrate utilization to the down regulation. Furthermore, our data suggest that morphological adjustments, such as reduced leaf size and total leaf area, were the primary form of responses in white birch to low phosphorus supply and no significant physiological acclimation to P supply was detected. Our results suggest that white birch will likely enhance water use efficiency under the projected future climate conditions with doubled carbon dioxide concentration, particularly at warmer soil temperatures. Although a trade-off between water use efficiency and nutrient use efficiency is widely accepted, our results suggest that there does not have to be a trade-off between the two, for instance, CO2 elevation increased both use efficiencies and low soil temperature and reduced nitrogen efficiency without affecting water use efficiency under elevated CO2.

Aims Some shade-tolerant understory tree species such as mountain maple (Acer spicatum L.) exhibit light-foraging growth habits. Changes in environmental conditions, such as the rise of carbon dioxide concentration ([CO 2]) in the atmosphere and soil warming, may affect the performance of these species under different light environments. We investigated how elevated [CO 2] and soil warming influence the growth and biomass responses of mountain maple seedlings to light availability. Methods The treatments were two levels of light (100% and 30% of the ambient light in the greenhouse), two [CO 2] (392 µmol mol −1 (ambient) and 784 µmol mol −1 (elevated)) and two soil temperatures (Tsoil) (17 and 22°C). After one growing season, we measured seedling height, root collar diameter, leaf biomass, stem biomass and root biomass. Important findings

In the spring of 2012, conifers in a large area in northwestern Ontario exhibited severe needle browning prior to budbreak, affecting more than 250 000 ha of forests north and west of Thunder Bay. Examination of weather data suggests that damage was caused by a combination of warm temperatures in March resulting in dehardening followed by freezing temperatures in April that were below a critical value. Damage was similar in nature to that observed in 2007 in northeastern Ontario, but in this case occurred earlier in the year and affected a larger area. Areas of northern Ontario where trees were affected were easily separated from those where no damage was observed using daily minimum temperature and cumulative growing degree day data. We suggest that a new term, winter freezing damage, be used to describe conifer needle and bud damage prior to budbreak when a period of warm temperatures in late winter/early spring followed by a period of sufficiently cold freezing temperatures causes damage to forest stands.

CO 2 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 [CO 2 ] are likely different from those at the ambient [CO 2 ]. 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 [CO 2 ] on black spruce ( Picea mariana Mill. BSP) seedlings. One-year-old seedlings were exposed to two [CO 2 ] (370 vs. 720 μmol mol −1 ), two nutrient ratio regimes (constant vs. variable N/P/K ratios) and six N concentrations (10, 80, 150, 220, 290 and 360 μmol mol −1 ) in four environmentally controlled greenhouses for 3.5 months. Growth response to N varied with [CO 2 ] and N/P/K ratios: under the elevated [CO 2 ], 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 150 μmol mol −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 [CO 2 ], height growth was greatest at 150 μmol mol −1 N and was generally greater at 220–360 than at 10 and 80 μmol mol −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 220 μmol mol −1 N when P and K concentrations were kept constant across different N supplies. The results of leaf nutrient concentrations suggest that the elevated [CO 2 ] 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 [CO 2 ], 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 [CO 2 ], in contrast, N/P/K ratio treatments did not affect growth suppression by critical deficiency or critical toxicity content. Because elevated CO 2 causes unequal increases in N, P and K demands, N–P–K ratios should be considered when modeling plant growth responses to elevated CO 2 .

Comparative stress resistance of 1-year-old white birch (Betula papyrifera Marsh.), balsam poplar (Populus balsamifera L.), and trembling aspen (Populus tremuloides Michx.) seedlings was evaluated after exposure to freezing or defoliation. Photosynthesis in leaves surviving freezing (−5 °C) declined immediately after treatment, but nearly fully recovered within 3 weeks. Defoliation did not significantly increase photosynthesis in the remaining leaves. Refoliation occurred after freezing that killed terminal shoots and released current buds from apical dominance, while new leaves of larger size were produced through continuous growth of terminal shoots in 50% or 100% defoliation. Freezing and complete defoliation significantly reduced diameter and height growth in all species, whereas 50% defoliation did not affect growth. These results indicate some of the physiological and morphological responses to foliage loss in broadleaved boreal species that can help to maintain growth and productivity under a warming climate, which may result in more frequent damaging spring frosts and insect defoliation.

Northern white-cedar (Thuja occidentalis L.) regeneration originating from layering is generally considered to be more shade tolerant than that of seed origin. In this study, we examined photosynthetic light-use efficiency, an important component of shade tolerance, of these two forms of regeneration using light-response curves of 1-year-old rooted cuttings of layers and seedlings grown under low (20% of full sun) and high (90% of full sun) light conditions. Northern white-cedar showed a strong potential for photosynthetic acclimation to light availability, as indicated by significant differences in the light compensation point (LCP), dark respiration (R d ), apparent quantum efficiency (PE), and light saturation point (Q sat ) but not light-saturated rate of net photosynthesis (A max ) between cuttings grown under high and low light. Values of Rd, LCP, and PE did not differ between layers and seedlings grown under high and low light, but layers had higher Amax and Qsat than seedlings when grown under high light. This result may reflect a comparatively lower capacity for photosynthetic acclimation of seedlings to high light conditions, perhaps because of their younger physiological age. A lower capacity for photosynthetic acclimation to light in seedlings could limit the repair and recovery of their photosynthetic systems from damage associated with the more extreme microenvironments of open, recently disturbed sites.

White birch (Betula papyrifera Marsh.) seedlings were grown under two carbon dioxide concentrations ([CO2]) (360 vs 720 μmol mol−1), three soil temperatures (Tsoil) (5, 15, 25°C initially, increased to 7, 17, 27°C, respectively, one month later), and three moisture regimes (low: 30–40%, intermediate: 45–55%, high: 60–70% field water capacity) for four months in environment-controlled greenhouses. The dry mass of stem, leaves, and roots was measured after 2 and 4 months of treatment. Low Tsoil decreased stem, leaf and total biomass in both measurements, however, the decrease was significantly greater in the elevated than ambient [CO2] after 4 months. Intermediate Tsoil increased root biomass in both measurements. Low moisture reduced stem, leaf, root and total biomass after both 2 and 4 months of treatment. There was a significant Tsoil-moisture interactive effect on leaf, root, and total biomass after 4 months of treatment, suggesting that the magnitude of biomass enhancement in warmer Tsoil was dependent on the moisture regime. For instance, the increase in total biomass from the low to high Tsoil was 22, 50, and 47% under the low, intermediate and high moisture regimes, respectively. In contrast, the Tsoil×moisture effect on stem biomass was significant after 2 months, but not after 4 months of treatment. High Tsoil increased leaf mass ratio (LMR) after 4 months of treatment, but decreased both root mass ratio (RMR) after both 2 and 4 months, and root:shoot ratio (RSR) after 4 months of treatment. The low moisture regime decreased LMR after 2 and 4 months of treatment, but increased RSR after 4 months of treatment. There were no significant [CO2] effects on biomass allocation or [CO2]×Tsoil×moisture interactions on biomass production/allocation.

In spring 2007, young planted and natural conifers suffered extensive needle and bud injury near Hearst and Kapuskasing in northeastern Ontario. Damage was observed on all species of conifers up to 20 years old and 8 m in height. Taller trees, especially those in the overstory, and young understory trees protected by a closed canopy had less damage. The damage was caused by earlier than normal loss of cold hardiness followed by late spring frosts according to damage observations, weather station data, and calculated thermal time requirements for budbreak and cold hardiness of conifers. During May 19 to 21, 2007, minimum temperatures were recorded between -8°C and -9°C, and before this period, temperatures were warm enough to induce early dehardening and loss of trees' cold hardiness. According to the historical weather data for Kapuskasing, estimated budbreak time has become earlier since 1918 and freezing temperatures during budbreak and shoot elongation (between growing degree days 100 and June 15) have become more frequent since 1980. If the trend towards earlier budbreak is due to climate change, then the type of frost damage we observed in northeastern Ontario in 2007 could become more common. We discuss implications of such events and suggest research needed to understand the risk of frost damage with climatic warming and to reduce damage. Key words: conifer frost damage, mature needle and bud (or needle/bud) mortality, early dehardening and budbreak (or dehardening/budbreak), climatic warming

Boreal mixedwoods are an important element and the most productive forest type in the Canadian boreal forests. However, they experience frequent disturbances. In order to better understand the responses of boreal mixedwoods to different combinations of anthropogenic and natural disturbances, we investigated the natural regeneration of boreal mixedwoods that were previously subjected to three different harvesting treatments (clearcut, partial-cut and uncut control) and naturally regenerated, but subsequently burnt by a severe natural fire 6 years later. The study was conducted 8 years following the fire. Significant interactions were found among harvesting method, species and block in several regeneration variables. There were a total of 12 woody species (trees and shrubs) regenerated, but not all the species were present in all the sites. In general, the species richness and species diversity of the new stands were lowest on clearcut sites while the differences between partial-cut and control varied with blocks. However, the combined total density for all species was lowest on uncut control sites. Density and regeneration index data show that trembling aspen was the predominant tree species in all stands except at one uncut control site where jack pine was the dominant species. The density of trembling aspen generally declined from clearcut to partial to the uncut control. Pincherry, beaked hazel and mountain maple were the dominant shrub species in the new stands, but no general patterns were found in terms of variations in density with harvesting methods for any of the shrub species. Jack pine and white birch were the tallest tree species in the clearcut treatment while white birch was taller than jack pine in the partial-cut and control. The results suggest that active measures are necessary to restore the complex structure of the initial mixedwoods.

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 30 min 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 (R2 > 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 Ci/Ca 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.

To investigate genetic variation in the response of white spruce (Picea glauca (Moench) Voss) to CO2 eleva- tions, one-year-old seedlings of 11 provenances from Ontario were grown at 360, 530 and 700 � mol mol -1 CO2 concentra- tions. After 60 days of treatment, CO2 assimilation rate (A) at growth (CO2) was 32% and 74% higher in the 530 and 700 �mol mol -1 CO2 treatment, respectively, than in 360 �mol mol -1 (CO2). After 90 days, A under 530 and 700 �mol mol -1 (CO2) was 141% and 223% greater, respectively, than under 360 �mol mol -1 (CO2). When measured at a common (CO2) (360 �mol mol -1 ), however, A under 530 and 700 �mol mol -1 (CO2) treatment was 39% and 53% lower than in the 360 � mol mol -1 (CO2) treatment, respectively, after 60 days of treatment, and 22 and 35% less, respectively, after 90 days. The CO2 elevations also significantly suppressed Vcamx, Jmax and TPU after 60 days and 90 days of treatment. The (CO2) eleva- tions significantly reduced Stomatal conductance and transpiration after 60 days of treatment, but not after 90 days. gs was 28% and 35% lower under 530 and 700 � mol mol -1 (CO2), respectively, than under 360 � mol mol -1 (CO2) after 60 days of treatment. E was 23 and 33% lower under 530 and 700 �mol mol -1 , respectively, than under 360 �mol mol -1 (CO2). Pho- tosynthetic water use efficiency (WUE) was 80% and 157% higher after 60 days of treatment and 145% and 196% higher after 90 days, respectively, under 530 and 700 � mol mol -1 (CO2) than under 360 � mol mol -1 (CO2). There were no signifi- cant between-provenance differences in any of the above responses; nevertheless, the ecophysiological performance of seedlings was closely correlated with the temperature and precipitation regimes of the seed origin. In addition, provenance significantly affected seedling total biomass, shoot mass, shoot/root ratio and height growth.