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Predicting photosynthetic acclimation to elevated C[O.sub.2] 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 C[O.sub.2] and temperature (ACT--400 [micro]mol*[mol.sup.-1] C[O.sub.2], current temperature; ECT--750 [micro]mol*[mol.sup.-1] C[O.sub.2], current + 4[degrees]C temperature). ECT reduced photosynthesis, maximum Rubisco carboxylation ([V.sub.cmax]), maximum electron transport rate ([J.sub.max]), 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 C[O.sub.2] 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 C[O.sub.2], ECT reduced [J.sub.max]/[V.sub.cmax], which is consistent with the response to warming but opposite to the response to elevated C[O.sub.2], 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. Key words: white birch (Betula papyrifera Marshall), elevated C[O.sub.2], photosynthetic acclimation, electron transport, leaf area, carbon gain Il est difficile de predire l'acclimatation photosynthetique a un taux eleve de C[O.sub.2] et au rechauffement, car ils ont des effets opposes. Nous avons etudie les reponses physiologiques et morphologiques du bouleau gris (Betula papyrifera Marshall) a une combinaison de C[O.sub.2] et de temperature (ACT : 400 [micro]mol*[mol.sup.-1] C[O.sub.2], temperature actuelle; ECT : 750 [micro]mol*[mol.sup.-1] C[O.sub.2], temperature actuelle + 4[degrees]C). L'ECT a reduit la photosynthese, la carboxylation maximale de la Rubisco ([V.sub.cmax]), la vitesse maximale de transport des electrons ([J.sub.max]), la photorespiration, la respiration diurne, l'azote foliaire, la conductance stomatique et mesophylle, mais a augmente la biomasse, la hauteur, la surface foliaire totale, le ratio de partage des electrons vers la carboxylation et l'oxygenation et le point de compensation du C[O.sub.2]. L'acclimatation photosynthetique est conforme a la theorie du gain optimal de carbone (le gain de carbone determine la coordination de la carboxylation, du transport d'electrons et de la respiration). Alors que l'acclimatation photosynthetique etait similaire a l'acclimatation au C[O.sub.2] eleve, l'ECT a reduit [J.sub.max]/[V.sub.cmax], ce qui est coherent avec la reponse au rechauffement mais oppose a la reponse au C[O.sub.2] eleve, suggerant que l'acclimatation thermique peut etre le mecanisme principal de l'acclimatation photosynthetique a l'ECT et que l'ECT a probablement modifie l'allocation de l'azote entre la machinerie pour la carboxylation et celle pour la regeneration du RuBP (<< ribulose-1,5- bisphosphate >>). L'augmentation de la surface foliaire totale par l'ECT a plus que compense l'effet negatif de la deregulation photosynthetique sur la sequestration du carbone, ce qui a entraine une croissance plus rapide et une plus grande biomasse sous l'ECT. [Traduit par la Redaction] Mots-cles: bouleau gris (Betulapapyrifera Marshall), C[O.sub.2] eleve, acclimatation photosynthetique, transport d'electrons, surface foliaire, gain de carbone, Introduction Atmospheric C[O.sub.2] is expected to continue to increase in the foreseeable future, along with increasing temperature (Kurepin et al. 2018). The associated climate change will have far-reaching impacts on [...]

Rising CO2, global warming, and N deposition create challenging environmental conditions to vegetation. Since elevated CO2 and rising temperature are coupled with each other, it is important to understand their combined effects on plants. We investigated the growth and photosynthetic responses of yellow birch to five levels of nitrogen supply under the current (cCT: current CO2 and temperature) and the predicted future CO2 and temperature conditions (fCT: elevated CO2, current + 4℃ temperature). The results show that fCT and high N supply increased seedling growth but fCT reduced photosynthetic capacity (e.g., maximum rate of Rubisco carboxylation-Vcmax, maximum rate of photosynthetic electron transport-Jmax)) and foliar N concentration. However, the magnitude of the fCT effect declined with increases in N supply. Furthermore, the fCT treatment significantly reduced the Jmax/Vcmax ratio, indicating a possible shift of N allocation from Jmax to Vcmax in the photosynthetic machinery. This result suggests that the photosynthesis of yellow birch may be more limited by electron transport under the predicted future climate condition. Both low N supply and fCT significantly increased photosynthetic N use efficiency (PNUE) and there was a negative relationship between PNUE and photosynthetic capacity. In general, yellow birch grew better under fCT than cCT, particularly above-ground growth. [ABSTRACT FROM AUTHOR]

Elevated CO2 and warmer temperatures represent the future environmental conditions in the context of global change. A good understanding of plant response to their combined effects is, therefore, critically important for predicting future plant performance. This study investigated the photosynthetic acclimation of Amur linden (Tilia amurensis) seedlings (current year, about 60 cm tall), a shade-tolerant tree species in the temperate broadleaf deciduous forest, to the combination of current CO2 concentration and temperature (CC) and the combination of the predicted future CO2 concentration and temperature (FC). The results show that FC promoted aboveground growth, but reduced photosynthetic capacity (V cmax: maximum rate of RuBP carboxylation and J max: maximum photosynthetic electron transport rate). However, the photosynthetic rate measured under the corresponding growth CO2 concentration was still higher under FC than under CC. FC depressed the photosynthetic limiting transition point (C i-t, A n-t) from Rubisco carboxylation to RuBP regeneration, i.e. A n-t decreased without a change in C i-t. FC did not change leaf N concentration but increased the total leaf N content per tree and photosynthetic nitrogen utilization efficiency. This suggests that N utilization, rather than photosynthetic capacity, may play an important role in the acclimation of the species to future climatic conditions. This study provides new insights into the photosynthetic acclimation of Amur linden and can be used to predict its possible performance under future climatic conditions. [ABSTRACT FROM AUTHOR]

Tree migration to higher latitudes may occur in response to future changes in climate, exposing the trees to higher concentrations of carbon dioxide ([[CO.sub.2]]), 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 [[CO.sub.2]], 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 [[CO.sub.2]] (ambient [[CO.sub.2]] 400 [micro]mol*[mol.sup.-1] or an elevated [[CO.sub.2]] 1000 [micro]mol*[mol.sup.-1]), four photoperiod regimes corresponding to latitudes 48[degrees]N (seed origin), 52[degrees]N, 55[degrees]N, and 58[degrees]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[degrees]N latitude) had greatest height and biomass but smallest specific leaf area. Under the elevated [[CO.sub.2]], however, the longest photoperiod regime significantly reduced xylem resistance to drought-induced cavitation compared with the photoperiod corresponding to 48[degrees]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. Keywords: climate change, tree migration, morphology, cavitation resistance. Un jour, les arbres pourraient migrer a des latitudes plus elevees en reponse aux changements clima-tiques, les exposant des concentrations plus elevees de [CO.sub.2] ([[CO.sub.2]]), a de nouvelles photoperiodes, a une humidite du sol differente et a d'autres conditions nouvelles. Les nouvelles conditions peuvent influencer la physiologie, la survie et la croissance des arbres. Cette etude s'est penchee sur les effets interactifs de la [[CO.sub.2]], de la photoperiode et de l'humidite du sol sur la morphologie et la resistance a la cavitation du peuplier faux-tremble aspen (Populus tremuloides Michx.). Des plants ages d'un an ont ete exposes a deux [[CO.sub.2]] ([[CO.sub.2]] ambiante de 400 [micro]mol*[mol.sup.-1] et [[CO.sub.2]] elevee de 1000 [micro]mol*[mol.sup.-1]), quatre regimes de photoperiodes correspondant a des latitudes de 48[degrees] (origine de la semence), 52[degrees]N, 55[degrees]N et 58[degrees]N, et deux conditions d'humidite du sol (60-75% et 13-20% de la capacite du champ) pendant une saison de croissance en serres. La croissance des plants, la taille des feuilles, la surface specifique des feuilles, les repartitions de la biomasse et la resistance du xyleme a la cavitation (potentiels hydriques pour 20, 50 et 80% de perte de conductivite hydraulique) ont ete evaluees. Les plants soumis a la plus longue photoperiode (latitude 58[degrees]N) presentaient la hauteur et la biomasse les plus elevees, mais la surface foliaire specifique la plus petite. À une [[CO.sub.2]] elevee, cependant, la photoperiode la plus longue reduisait significativement la resistance du xyleme a la cavitation induite par la secheresse comparativement a la photoperiode correspondant a une latitude de 48[degrees]N. Les resultats suggerent que lors de sa migration vers des latitudes plus elevees, le peuplier faux-tremble pourrait croître plus rapidement, mais devenir moins resistant a la secheresse et plus enclin a une insuffisance hydraulique durant un episode de secheresse. [Traduit par la Redaction] Mots-cles : changement climatique, migration des arbres, morphologie, resistance a la cavitation., Introduction The projected increases in global temperatures and changes in precipitation patterns will likely induce a northward migration of boreal trees by the end of this century (McKenney et al. [...]

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 ([C[O.sub.2]]) on the hydraulic conductivity in jack pine (Pinus banksiana Lamb.) and its vulnerability to xylem embolism. Seedlings were exposed to 400 vs. 950 ([micro]molX[mol.sup.-1] [C[O.sub.2], 60%-70% vs. 30%-40% (of field capacity) soil moisture, and photoperiods of seed origin and 5[degrees] and 10[degrees] north of seed origin in greenhouses. Cavitation vulnerability curves were measured for determining the xylem pressure at which 50% hydraulic conductivity was lost ([[PSI].sub.PLC50]). It was found that elevated [C[O.sub.2]] significantly increased hydraulic conductivity, whereas low soil moisture decreased it. Under elevated [C[O.sub.2]], the xylem became progressively more vulnerable to embolism with changes in photoperiod regime from the seed origin to 10[degrees] north of the seed origin, as indicated by the progressively less negative [[PSI].sub.PLC50]. However, no such a trend was detected under the ambient [C[O.sub.2]]. The results suggest that the species may become less resistant to drought as the atmospheric [C[O.sub.2]] 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 [C[O.sub.2]] increases even without any change in moisture conditions. Keywords: climate change, tree migration, jack pine, hydraulic conductivity, embolism. Les arbres peuvent migrer vers le nord en reponse aux changements climatiques et devenir exposes a de nouveaux regimes de photoperiode et d'humidite du sol. Cette etude a evalue les impacts de la photoperiode et ses interactions avec l'humidite du sol et la concentration de dioxyde de carbone ([C[O.sub.2]]) sur la conductivite hydraulique du pin gris (Pinus banksiana Lamb.) et sa vulnerabilite a l'embolie du xyleme. Des plants ont ete exposes a 400 vs. 950 ([micro]molX[mol.sup.-1] [C[O.sub.2]], 60-70 % vs. 30-40 % (de la capacite du champ) d'humidite du sol et a des photoperiodes correspondant a l'origine des semences, et a 5[degrees] et 10[degrees] au nord de l'origine des semences lors d'experiences en serres. Les courbes de vulnerabilite a la cavitation ont ete mesurees pour determiner la pression du xyleme a laquelle 50 % de la conductivite hydraulique etait perdue ([[PSI].sub.PLC50]). Les auteurs ont trouve qu'une [C[O.sub.2]] elevee accroissait significativement la conductivite hydraulique alors que l'humidite du sol la diminuait. Dans des conditions de [C[O.sub.2]] elevee, le xyleme devenait progressivement plus vulnerable a l'embolie avec les changements de regime de photoperiode correspondant a l'origine des semences vers 10[degrees] au nord de l'origine des semences, comme indique par la [[PSI].sub.PLC50] progressivement moins negative. Cependant, une telle tendance n'etait pas detectee dans des [C[O.sub.2]] ambiantes. Les resultats suggerent que l'espece peut devenir moins resistante a la secheresse en fur et a mesure de l'augmentation de la [C[O.sub.2]], genant la migration boreale ou le transfert des semences. Meme a l'interieur de leur distribution naturelle actuelle, les arbres situes pres de la frontiere nord de leur etendue peuvent etre plus vulnerables a l'embolie en fonction de l'augmentation de la [C[O.sub.2]] atmospherique meme sans changement des conditions d'humidite. [Traduit par la Redaction] Mots-cles : changements climatiques, migration des arbres, pin gris, conductivite hydraulique, embolie., Introduction Global warming will increase the frequency and severity of droughts by altering the amount, intensity, frequency, and type of precipitation (Christensen et al. 2007). Rainfalls are likely to increase [...]

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. Key words: conifer winter injury, dehardening curves, budbreak, relative electrolyte leakage, visual assessment. Les dommages causes par le gel hivernal chez les coniferes sont surtout dus a la perte de tolerance au froid lors des journees anormalement chaudes a la fin de l'hiver et tot au printemps. La frequence et la severite de ces dommages pourraient augmenter a cause du rechauffement du climat. Dans cette etude, la dynamique du desendurcissement du pin tordu (Pinus contorta Dougl. ex Loud.), du pin gris (Pinus banksiana Lamb.), de l'epinette blanche (Picea glauca (Moench) Voss) et de l'epinette noire (Picea mariana (Mill.) B.S.P.) a ete etudiee en lien avec l'accumulation thermique lors d'un desendurcissement artificiel en hiver (decembre) et au printemps (mars) en ayant recours a la perte relative d'electrolytes et a l'examen visuel des aiguilles de pin et des pousses d'epinette. Les resultats indiquent que le taux et le degre de desendurcissement ont ete sensiblement les memes chez les quatre especes malgre des exigences d'accumulation thermique tres differentes. Au printemps, le desendurcissement etait comparativement plus rapide; l'epinette noire etait legerement plus tolerante au froid que les autres coniferes au stade avance de desendurcissement printanier. Cependant, la difference etait relativement faible et l'epinette noire n'a pas beneficie d'une tres grande protection lors des tests de gel des semis avant le debourrement, a la fin du mois de mars et au debut du mois de mai. Les courbes de desendurcissement et les modeles developpes dans le cadre de cette etude peuvent servir d'outils pour predire la tolerance au froid en fonction de la temperature et comprendre les risques potentiels de dommages causes par le froid chez les coniferes durant les episodes de rechauffement suivi d'un gel avant le debourrement. [Traduit par la Redaction] Mots-cles: dommages causes par le gel chez les coniferes, courbes de desendurcissement, debourrement, perte relative d'electrolytes, examen visuel., Introduction The ability of boreal conifers to survive winter cold depends not only on the level of cold hardiness that they can potentially achieve, but also on their responses to [...]

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 (Piceaglauca) (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. Key words: winter freezing, dehardening, rehardening, pine and spruce seedlings. Comparativement au dommage cause par les gelees printanieres aux bourgeons emergents ou aux tissus nouvellement sortis, le dommage cause aux coniferes par le gel hivernal lors de fluctuations de temperatures avant l'eclosion est rare et moins connu. Dans cette etude, les changements de robustesse au froid (mesures a partir de la perte d'electrolyte et le dommage aux aiguilles) et l'eclosion printaniere ont ete evalues afin d'examiner les reponses de quatre especes boreales de coniferes--l'epinette noire (Picea mariana (Mill.) B.S.P.), l'epinette blanche (Piceaglauca)(Moench) Voss), le pin gris (Pinus banksiana Lamb.) et le pin lodgepole (Pinus contorta Dougl. Ex. Loud,)--a differentes durees de rechauffement experimental (16°C jour/-2°C nuit, photoperiode de 10 h, hormis la temperature nocturne du rechauffement de novembre (+2°C)). Les plants presentaient des reponses accrues au rechauffement de novembre a mars, alors que la capacite de recouvrer la robustesse au froid perdue lors du rechauffement diminuait durant la meme periode. Cela suggere une vulnerabilite accrue des coniferes aux fluctuations de la temperature et au dommage par le gel en fonction de la progression du refroidissement et la levee de la dormance de l'automne au printemps. Le pin lodgepole et le pin gris amorcaient tous deux leur croissance printaniere plus tot et reagissaient davantage au rechauffement experimental que l'epinette noire et l'epinette blanche quant a la phenologie des bourgeons, suggerant que le risque potentiel de dommage au gel est plus eleve chez les pins au printemps. [Traduit par la Redaction] Mots-cles: gel hivernal, de-acclimatation, re-acclimatation, plants de pins et d'epinettes., Introduction Global temperatures are expected to continue to rise in association with the predicted change of climate (Christensen et al. 2007). However, climate warming will not be uniform, being generally [...]

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 (A n ), maximum Rubisco carboxylation rate (V c max), photosynthetic electron transport rate (J max), 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 V c max) to RuBP regeneration (related to J max) in response to ECT, leading to decreased transition point (C i-t and A n-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 (C i-t and A n-t ) and the photosynthetic rate at growth CO2 (C i-g and A n-g ). Furthermore, we found that J max (RuBP regeneration) was the primary limitation to A n under ECT. [ABSTRACT FROM AUTHOR]

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--trembling aspen (Populus tremuloides Michx.), balsam poplar (P. balsamifera L.), and white birch (Betula papyrifera Marsh.)--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. Key words: cold hardiness, winter warming, dehardening, temperature, boreal broadleaves. Il est possible que le rechauffement climatique accentue les variations de temperature, particulierement durant les mois d'hiver, augmentant ainsi les risques de perte hative de la resistance au froid et par consequent de dommages dus au gel. Dans cette etude, les changements dans la resistance au froid (mesuree via la perte d'electrolytes), le debourrement et la survie ont ete utilises pour connaitre la reaction des semis de trois especes feuillues boreales, le peuplier faux-tremble (Populus tremuloides Michx.), le peuplier baumier (P. balsamifera L.) et le bouleau blanc (Betula papyrifera Marsh.), a un rechauffement experimental. La reaction des semis etait plus forte en hiver (janvier) et au printemps (mars) qu'a l'automne (novembre) et plus forte chez le peuplier faux-tremble et le peuplier baumier que chez le bouleau blanc. Un rechauffement pendant 5 a 10 jours a 16°C le jour et -2°C la nuit, avec une photoperiode de 10 heures en hiver et au printemps, a generalement reduit la resistance au froid. Combinee a des temperatures sous le point de congelation dans le milieu ambiant apres le rechauffement, cette reduction a augmente la mortalite des semis ainsi que le deperissement de la tige et a prolonge la duree de la periode qui precede le debourrement. La resistance au froid s'est quelque peu intensifiee 10 jours apres que les semis eurent ete replaces a l'exterieur a la suite d'un rechauffement au printemps lorsque les temperatures ambiantes etaient moins dommageables. La resistance du bouleau blanc au rechauffement, probablement due a ses exigences thermiques plus grandes pour le debourrement et a son desendurcissement naturellement plus lent, indique que cette espece est mieux adaptee pour resister aux variations de temperature hivernale qui pourraient survenir a cause des changements climatiques. On devrait tenir compte des effets du gel hivernal sur le debourrement pour ameliorer la precision de la modelisation phenologique. [Traduit par la Redaction] Mots-cles: resistance au froid, rechauffement hivernal, desendurcissement, especes feuillues boreales., Introduction In temperate and boreal regions, tree growth cessation in the fall and initiation in the spring is synchronized with annual cycles in environmental conditions (Saxe et al. 2001). In [...]

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. Key words: frost damage, refoliation, leaf morphology. Les auteurs ont compare la resistance au stress chez des plantules agees d'une annee, de bouleau a papier (Betula payrifera Michx.), de peuplier baumier (Populus balsamifera L.) et de peuplier faux-tremble (Populus tremuloides Michx.) apres exposition au gel et a la defoliation. Chez les feuilles ayant survecu au gel (-5 [degre]C), la photosynthese a immediatement decline apres le traitement, mais s'est retablie completement en moins de 3 semaines. La defoliation n'augmenta pas significativement la photosynthese dans les autres feuilles. La refoliation survient apres un gel ayant tue les tiges terminales et supprime l'effet de la dominance apicale sur les bourgeons actifs, alors que de nouvelles feuilles plus grandes se forment par croissance continue des tiges terminales, apres 50 % et 100 % de defoliation. Le gel et la defoliation totale reduisent la croissance en diametre et en hauteurs chez toutes les especes, alors qu'une defoliation a 50 % n'affecte pas la croissance. Ces resultats illustrent certaines reactions physiologiques et morphologiques en reaction de la perte de feuillage chez des especes boreales a larges feuilles pouvant aider a maintenir la croissance et la productivite sous un climat en rechauffement, lequel pourra conduire a des gels printaniers et des defoliations par les insectes plus frequents. [Traduit par la Redaction] Mots-cles : dommage par le froid, refoliation, morphologie foliaire., Introduction Although most studies of North American boreal forests have focused on the more economically important conifer species, broadleaved trees are nevertheless important ecologically and as potential sources of fibre. [...]

C[O.sub.2]--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 C[O.sub.2] concentrations ([C[O.sub.2]]) of 360 and 720 [micro]mol x [mol.sup.-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 [C[O.sub.2]] increased RCD by 33% and enhanced stem volume by 67%, 98%, and 84% under 100%, 50%, and 30% light, respectively. The C[O.sub.2] enhancement of total biomass was relatively higher under lower light, and greater for Sb than Sw. Elevated [C[O.sub.2]] decreased specific leaf area under 50% light only. Root mass was generally higher under 100% light than when shaded. Elevated [C[O.sub.2]] increased the root mass of Sb under 100% light, but decreased it under 30% light. Elevated [C[O.sub.2]] decreased the shoot/root ratio under 100% light, but increased it under 30% light. Our data suggest that raising [C[O.sub.2]] will likely increase species competitiveness under low light conditions, and that the increase will be greater in species that are relatively shade tolerant. Key words: boreal forests, elevated [C[O.sub.2]], [C[O.sub.2]]--light species interactions, black spruce, white spruce seedlings, climate change. L'interaction lumiere--C[O.sub.2] peut influencer la competition entre plantes boreales, mais on les comprend mal. Les auteurs ont examine de telles interactions sur la croissance et la biomasse de plants d'un an d'epinettes noires (Sb) (Picea mariana (Mill.) B.S.P.) et d'epinettes blanches (Sw) (Picea glauca (Moench.) Voss), en presence de 360 et 720 mol-1 [C[O.sub.2]] et 30, 50 et 100% de lumiere, en serres. On retrouve des interactions dans deux directions et dans trois directions. Le diametre au collet (RDC) de Sw diminue avec une diminution de la lumiere alors qu'il n'y a pas de difference significative chez Sb entre 50 et 30% de lumiere. La hauteur est plus grande avec 100% de lumiere, comparativement a l'ombre. Le [C[O.sub.2]] eleve augmente le RDC de 33%. Il augmente la biomasse totale de 67, 98 et 84%, respectivement a 100, 50 et 30% de luminosite. L'augmentation de la biomasse totale sous l'effet du [C[O.sub.2]] est relativement plus elevee a faible intensite lumineuse et plus elevee chez Sb que chez Sw. L'augmentation du [C[O.sub.2]] diminue la surface foliaire specifique seulement sous 50% de luminosite. On observe une biomasse generalement plus importante a 100% de luminosite qu'a l'ombre. L'augmentation de [C[O.sub.2]] augmenta la biomasse racinaire chez Sb a 100% de lumiere, mais la diminue a 30%. L'augmentation de [C[O.sub.2]] diminue le rapport tige/racine a 100% de lumiere, mais la diminue a 30%. Les donnees suggerent que les augmentations de [C[O.sub.2]] favorisent vraisemblablement la competitivite des especes sous des conditions de faible luminosite et l'augmentation sera plus forte chez les especes qui tolerent relativement mieux l'ombre. Mots-cles : forets boreales, [C[O.sub.2]] eleve, interactions especes lumiere-[C[O.sub.2]], epinette noire, epinette blanches, plantule, changement climatique., Introduction The global atmospheric carbon dioxide concentration ([C[O.sub.2]]) has increased from the pre-industrial value of 280 [micro]mol x [mol.sup.-1], to approximately 379 [micro]mol x [mol.sup.-1] in recent years, and the [...]

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Abstract: To investigate the genetic variation of trembling aspen (Populus tremuloides Michx.) in ecophysiological responses to [C[O.sub.2]] elevation, 1-year-old seedlings of four provenances (three families per provenance) from northwestern Ontario [...]