Photosynthetic responses to understory shade and elevated carbon dioxide concentration in four northern hardwood tree species

Seedling responses to elevated atmospheric CO(2) concentration ([CO(2)]) and solar irradiance were measured over two growing seasons in shade-tolerant Acer saccharum Marsh. and Fagus grandifolia J.F. Ehrh. and shade-intolerant Prunus serotina, a J.F. Ehrh. and Betula papyrifera Marsh. Seedlings were exposed to a factorial combination of [CO2] (ambient and elevated (658 micromol mol-1)) and understory shade (deep and moderate) in open-top chambers placed in a forest understory. The elevated [CO(2)] treatment increased mean light-saturated net photosynthetic rate by 63% in the shade-tolerant species and 67% in the shade-intolerant species. However, when measured at the elevated [CO(2)], long-term enhancement of photosynthesis was 10% lower than the instantaneous enhancement seen in ambient-[CO(2)]-grown plants (P < 0.021). Overall, growth light environment affected long-term photosynthetic enhancement by elevated [CO(2)]: as the growth irradiance increased, proportional enhancement due to elevated [CO(2)] decreased from 97% for plants grown in deep shade to 47% for plants grown in moderate shade. Results suggest that in N-limited northern temperate forests, trees grown in deep shade may display greater photosynthetic gains from a CO(2)-enriched atmosphere than trees growing in more moderate shade, because of greater downregulation in the latter environment. If photosynthetic gains by deep-shade-grown plants in response to elevated [CO(2)] translate into improved growth and survival of shade-intolerant species, it could alter the future composition and dynamics of successional forest communities.     

Effects of elevated carbon dioxide concentration on growth and nitrogen fixation in Alnus glutinosa in a long-term field experiment

Nitrogen-fixing plant species may respond more positively to elevated atmospheric carbon dioxide concentrations ([CO2]) than other species because of their ability to maintain a high internal nutrient supply. A key factor in the growth response of trees to elevated [CO2] is the availability of nitrogen, although how elevated [CO2] influences the rate of N2-fixation of nodulated trees growing under field conditions is unclear. To elucidate this relationship, we measured total biomass, relative growth rate, net assimilation rate (NAR), leaf area and net photosynthetic rate of N2-fixing Alnus glutinosa (L.) Gaertn. (common alder) trees grown for 3 years in open-top chambers in the presence of either ambient or elevated atmospheric [CO2] and two soil N regimes: full nutrient solution or no fertilizer. Nitrogen fixation by Frankia spp. in the root nodules of unfertilized trees was assessed by the acetylene reduction method. We hypothesized that unfertilized trees would show similar positive growth and physiological responses to elevated [CO2] as the fertilized trees. Growth in elevated [CO2] stimulated (relative) net photosynthesis and (absolute) total biomass accumulation. Relative total biomass increased, and leaf nitrogen remained stable, only during the first year of the experiment. Toward the end of the experiment, signs of photosynthetic acclimation occurred, i.e., down-regulation of the photosynthetic apparatus. Relative growth rate was not significantly affected by elevated [CO2] because although NAR was increased, the effect on relative growth rate was negated by a reduction in leaf area ratio. Neither leaf area nor leaf P concentration was affected by growth in elevated [CO2]. Nodule mass increased on roots of unfertilized trees exposed to elevated [CO2] compared with fertilized trees exposed to ambient [CO2]. There was also a biologically significant, although not statistically significant, stimulation of nitrogenase activity in nodules exposed to elevated [CO2]. Root nodules of trees exposed to elevated [CO2] were smaller and more evenly spaced than root nodules of trees exposed to ambient [CO2]. The lack of an interaction between nutrient and [CO2] effects on growth, biomass and photosynthesis indicates that the unfertilized trees maintained similar CO2-induced growth and photosynthetic enhancements as the fertilized trees. This implies that alder trees growing in natural conditions, which are often limited by soil N availability, should nevertheless benefit from increasing atmospheric [CO2].     

Determining photosynthetic responses of forest species to elevated [CO2]: Alternatives to FACE

Free air CO₂ enrichment (FACE) experiments are considered the most reliable approach for quantifying our expectations of forest ecosystem responses to changing atmospheric CO₂ concentrations [CO₂]. Because very few Australian tree species have been studied in this way, or are likely to be studied in the near future because of the high installation and maintenance costs of FACE, there are no clear answers to questions such as: (1) which species will be the winners in Australia's natural forests and what are the implications for biodiversity and carbon (C) sequestration; and (2) which will be the most appropriate species or genotypes to ensure the sustainability of Australia's plantation forests.     

Scots pine responses to elevated temperature and carbon dioxide concentration: growth and wood properties

Growth and wood properties of 20-year-old Scots pine (Pinus sylvestris L.) trees were studied for 6 years in 16 closed chambers providing a factorial combination of two temperature regimes (ambient and elevated) and two carbon dioxide concentrations ([CO2]) (ambient and twice ambient). The elevation of temperature corresponded to the predicted effect at the site of a doubling in atmospheric [CO2]. Annual height and radial growth and wood properties were analyzed during 1997-2002. Physical wood properties analyzed included early- and latewood widths and their proportions, intra-ring wood densities, early- and latewood density and mean fiber length. Chemical wood properties analyzed included concentrations of acetone-soluble extractives, lignin, cellulose and hemicellulose. There were no significant treatment effects on height growth during the 6-year study. Elevated [CO2] increased ring width by 66 and 47% at ambient and elevated temperatures, respectively. At ambient [CO2], elevated temperature increased ring width by 19%. Increased ring width in response to elevated [CO2] resulted from increases in both early- and latewood width; however, there was no effect of the treatments on early- and latewood proportions. Mean wood density, earlywood density and fiber length increased in response to elevated temperature. The chemical composition of wood was affected by elevated [CO2], which reduced the cellulose concentration, and by elevated temperature, which reduced the concentration of acetone-soluble extractives. Thus, over the 6-year period, radial growth was significantly increased by elevated [CO2], and some wood properties were significantly affected by elevated temperature or elevated [CO2], or both, indicating that climate change may affect the material properties of wood.     

Response of photosynthesis in second-generation Pinus radiata trees to long-term exposure to elevated carbon dioxide partial pressure

Second-generation Pinus radiata D. Don trees, propagated from cuttings of 4-year-old trees previously grown at ambient (36 Pa) and elevated (65 Pa) CO2 partial pressure (Ca) were grown under the same conditions in open-top chambers for a further year. As cuttings of the original trees, these second-generation trees were physiologically the same age as the first-generation trees with the only difference between the two being size. This allowed us to test the effects of tree size independently of age or duration of exposure. Total non-structural carbohydrate concentration, area-based nitrogen concentration, leaf mass per unit area and chlorophyll concentration measured in three foliage age cohorts were unaffected by either age or Ca. There were no signs of photosynthetic down-regulation in trees grown at elevated Ca. When measured at the growth Ca, photosynthetic rate in young needles during summer, autumn and spring was 34, 43 and 38% higher, respectively, in trees grown at elevated Ca than in trees grown at ambient Ca. In older needles, the corresponding photosythetic rate increases were 26, 47 and 49%. Water-use efficiency, determined by stable carbon isotope analysis, was 49% higher in foliage in the elevated Ca treatment than in foliage in the ambient Ca treatment. This increase was entirely due to photosynthetic enhancement, because stomatal conductance did not differ between treatments. We conclude that down-regulation of photosynthesis at elevated Ca is related to tree size rather than tree age or duration of exposure, and that enhanced photosynthetic rates can be maintained while sink strength is high enough to use the excess photosynthates.elevated CO2, needle age, photosynthetic down-regulation, photosynthetic enhancement, sink strength, water-use efficiency.     

Carbon budget of Pinus sylvestris saplings after four years of exposure to elevated atmospheric carbon dioxide concentration

To study the responses of Scots pine (Pinus sylvestris L.), a commercially important tree species in Europe, to future increases in atmospheric CO2 concentration ([CO2]), we grew saplings for 4 years in the ground in open-top chambers in ambient or ambient + 400 micromol mol(-1) CO2, without supplemental addition of nutrients and water. Carbon (C) budgets were developed for trees in both CO2 treatments based on productivity and biomass data obtained from destructive harvests at the end of the third and fourth years of treatment, and simulations of annual gross photosynthesis (P(tot)) and maintenance respiration by the model MAESTRA. Simulated P(tot) was enhanced by elevated [CO2], despite significant down-regulation of photosynthetic capacity. The subsequent increase in C uptake was allocated primarily to tissues with limited longevity (needles and fine roots), which explains why the measured annual increment in woody biomass did not differ between CO2 treatments. Thus, our results suggest that accelerated stem growth only occurs in the first 2 years in the presence of elevated [CO2] and that forest rotations will not be shortened significantly in response to increasing [CO2]. In elevated [CO2], a higher proportion of available C was allocated below ground, resulting in altered biomass distribution patterns. In trees of equal size, measured ratios of fine root/needle biomass and belowground/aboveground biomass were almost twice as large in the elevated [CO2] treatment. Although there are uncertainties in scaling from saplings to mature canopies, the data indicate that, in nutrient-limited Scots pine forests, elevated [CO2] is unlikely to accelerate tree growth significantly, but is likely to increase C inputs to soil.     

湖南省现有森林植被主要树种的碳含量

探讨了湖南省现有森林植被17种树种(杉木、马尾松、湿地松、柏木、乐昌含笑、红花木莲、樟树、桢楠、甜槠、青冈栎、木荷、杜英、山矾、枫香、拟赤杨、杨树、毛竹)各器官的碳含量及其各器官碳含量的算术平均值。结果表明:同一树种不同器官碳含量差异不显著,不同树种各器官碳含量由高至低的排序不完全一致,不同树种同一器官或同一树种不同器官碳含量算术平均值存在一定的差异;针叶树、常绿阔叶树、落叶阔叶树和毛竹各器官碳含量(g·g-1)的变化范围分别为0.491~0.566,0.421~0.549,0.449~0.550,0.470~0.496,各树种种内各器官碳含量算术平均值在0.486~0.551 g·g-1之间变化,柏木最高,毛竹最低;针叶树碳含量高于阔叶树、毛竹,各树种树干碳含量普遍较高于其它各器官,变化范围在0.493~0.556 g·g-1之间,地上部分碳含量普遍高于相应树种的地下部分;17种树种各器官碳含量的算术平均值为0.504 g·g-1。     

Ecophysiological responses and carbon distribution of Pinus koraiensis seedlings to elevated carbon dioxide

The net CO2 assimilation rate, stomatal conductance, RuBPcase (ribulose 1,5-biphosphate carboxylose) activity, dry weight of aboveground and belowground part, plant height, the length and diameter of taproot of Pinus koraiensis seedlings were measured and analyzed after six-week exposure to elevated CO2 in an open-top chamber in Changbai Mountain of China from May to Oct. 1999. Seedlings were planted in four different conditions: on an open site, control chamber, 500 μ L.L-1 and 700 μL.L-1 CO2 chambers. The results showed that the total biomass of the seedlings increased whereas stomatal conductance decreased. The physiological responses and growth to 500 μL.L-1 and 700 μ L.L-1 CO2 varied greatly. The acclimation of photosynthesis was downward to 700 μL.L-1 CO2 but upward to 500 μ L.L-1 CO2. The RuBPcase activity, chlorophyll and soluble sugar contents of the seedlings grown at 500 μL.L-1 CO2 were higher than that at 700 μ L.L-1 CO2. The concentration 500 μ L.L-1 CO2 enhanced the growth of aboveground part whereas 700 μL.L-1 CO2 allocated more carbon to belowground part. Elevated CO2 changed the carbon distribution pattern. The ecophysiological responses were significantly different between plants grown under 500 μL.L-1 CO2 and 700 μL.L-1 CO2.     

Photosynthetic induction responses to variable light under field conditions in three species grown in the gap and understory of a Fagus crenata forest

Photosynthetic induction responses to abrupt increases in photon flux density (PFD) to 800 and 1500 &mgr;mol m(-2) s(-1) from either darkness or 100 &mgr;mol m(-2) s(-1) were examined in situ in leaves of Fagus crenata Blume, Daphniphyllum humile Maxim., and Acer rufinerve Siebold & Zucc. growing in a gap and the understory of an F. crenata forest. Among the species studied, F. crenata exhibited the highest assimilation rate (A(100)), stomatal conductance (g(s100)) at the background PFD of 100 &mgr;mol m(-2) s(-1), and A(100)/A(max) (A(max) = maximum assimilation rate), in both the gap and the understory. Time required for full induction depended on both background PFD and maximum PFD. The induction period was 2-4-fold shorter at a background PFD of 100 &mgr;mol m(-2) s(-1) than in darkness. For the three understory species, time required to full induction was 2-3-fold longer when irradiance was increased from darkness to 800 &mgr;mol m(-2) s(-1) than when irradiance was increased from darkness to 1500 &mgr;mol m(-2) s(-1). Acer rufinerve showed higher initial stomatal conductance (g(s0)) and a shorter induction period in the understory than in the gap. Fagus crenata exhibited a similar g(s0) and induction period in both habitats. Daphniphyllum humile demonstrated lower g(s0) and a longer induction period in the understory than in the gap. These findings indicate that initial stomatal conductance is closely correlated with the photosynthetic induction response. We conclude that the photosynthetic induction response is affected by the light conditions experienced by plants before the sudden increase in irradiance and by the extent of the increase in irradiance.     

Photosynthetic response of Pinus sylvestriformis to elevated carbon dioxide and its influential factor analysis

IntroductionOur world is changing in the way and at the speedthat are describable, but we are unable to predictthese changes with any degree of accuracy. Radioactive and chemical properties of the atmosphere,global climate, and global ecology are dynamic andmeasurable, but they also linked to each other incomplex and poorly understood ways (Rayal andRamanathan 1989). While many of the physical andbiological sub-processes are understood and modeled in detail, predictive capabilities are poor i…     

Radiation-use efficiency of a forest exposed to elevated concentrations of atmospheric carbon dioxide

We compared radiation-use efficiency of growth (epsilon;), defined as rate of biomass accumulation per unit of absorbed photosynthetically active radiation, of forest plots exposed to ambient (approximately 360 micro l l-1) or elevated (approximately 560 micro l l-1) atmospheric CO2 concentration ([CO2]). Large plots (30-m diameter) in a loblolly pine (Pinus taeda L.) plantation, which contained several hardwood species in the understory, were fumigated with a free-air CO2 enrichment system. Biomass accumulation of the dominant loblolly pines was calculated from monthly measurements of tree growth and site-specific allometric equations. Depending on the species, leaf area index (L*) was estimated by three methods: optical, allometric and litterfall. Based on the relationship between tree height and diameter during the first 3 years of exposure, we conclude that elevated [CO2] did not alter the pattern of aboveground biomass allocation in loblolly pine. There was considerable variation in L* estimates by the different methods; total L* was 18-42% lower when estimated by the optical method compared with estimates from allometric calculations, and this discrepancy was reduced when optical measurements were corrected for the non-random distribution of loblolly pine foliage. The allometric + litterfall approach revealed a seasonal maximum total L* of 6.2-7.1 with about 1/3 of the total from hardwood foliage. Elevated [CO2] had only a slight effect on L* in the first 3 years of this study. Mean epsilon; (+/- SD), calculated for loblolly pine only, was 0.49 +/- 0.05 and 0.62 +/- 0.04 g MJ-1 for trees in the ambient and elevated [CO2] plots, respectively. The 27% increase in epsilon; in response to CO2 enrichment was caused primarily by the stimulation of biomass increment, as there was only a small effect of elevated [CO2] on L* during the initial years of fumigation. Long-term increases in atmospheric [CO2] can increase epsilon; in closed-canopy forests but the absolute magnitude and duration of this increase remain uncertain.     

Photosynthetic responses to lightflecks ofFagus crenata seedlings grown in a gap and understory of a deciduous forest

Photosynthetic responses to a series of 1-min lightflecks (1,000μmol m⁻² s⁻¹) superimposed on a background with different duration (1, 5, and 10 min) and intensity (25 and 50μmol m⁻² s⁻¹) of low background photosynthetic photon flux density (PPFD) were measured in the leaves ofFagus crenata grown in a gap and understory of aFagus crenata forest in the Naeba Mountains. The two background PPFD intensities most frequently occurred in understory and gap sites respectively. The maximum net photosynthetic rate (P _Nmax) and maximum stomatal conductance (g _smax) were higher in the gap seedlings than in the understory seedlings. However, when the background PPFD was 25μmol m⁻²s⁻¹, the net photosynthetic rate (P ₂₅) and stomatal conductance (g _s25) were almost the same between the gap and understory. When the background PPFD duration was 1-min, the net photosynthetic rate (P _N ) at the end of each lightfleck increased progressively. When the background PPFD duration was 5- and 10-min, the increase inP _N at the end of each lightfleck was less. This indicates that background PPFD duration is important to photosynthetic responses to lightflecks. The higher ratios ofP ₂₅/P _Nmax andg _s25/g _smax in the understory seedlings indicate that the understory seedlings can maintain relatively lower levels of biochemical and stomatal limitations than the gap seedlings under low light conditions. The ratios ofP _N /P _Nmax at the end of each lightfleck (IS) and light utilization efficiency of single lightflecks (LUE _s) that showed the influence of lightflecks on carbon gain were higher in the understory seedlings than in the gap seedlings when the background PPFD was 25μmol m⁻² s⁻¹. This means that understory seedling are capable of utilizing fluctuating light more efficiently under low light conditions than the gap seedlings although the net carbon gain of single lightflecks (CG _s) in the understory seedlings was not higher than that in the gap seedlings. There were no significant differences inIS andLUE _s between understory seedlings at a background PPFD of 25μmol m⁻² s⁻¹ and gap seedlings at a background PPFD of 50μmol m⁻² s⁻¹. However,CG _s in gap seedlings was higher than in understory seedlings. These results provide more evidence thatF. crenata acclimate to a natural light environment in respect to relative induction state at low background PPFD and can capture the fluctuating light at the same efficiency in both the gap and understory seedlings under natural light environments. This study was funded by the research project, Evaluation of Total CO₂ Budget in Forest Ecosystems, coordinated by the Ministry of Agriculture, Forestry and Fisheries of Japan.     

Canopy position affects photosynthetic adjustments to long-term elevated CO2 concentration (FACE) in aging needles in a mature Pinus taeda forest

Few studies have examined the effects of elevated CO2 concentration ([CO2]) on the physiology of intact forest canopies, despite the need to understand how leaf-level responses can be aggregated to assess effects on whole-canopy functioning. We examined the long-term effects of elevated [CO2] (ambient + 200 ppm CO2) on two age classes of needles in the upper and lower canopy of Pinus taeda L. during the second through sixth year of exposure to elevated [CO2] in free-air (free-air CO2 enrichment (FACE)) in North Carolina, USA. Strong photosynthetic enhancement in response to elevated [CO2] (e.g., +60% across age classes and canopy locations) was observed across the years. This stimulation was 33% greater for current-year needles than for 1-year-old needles in the fifth and sixth years of treatment. Although photosynthetic stimulation in response to elevated [CO2] was maintained through the sixth year of exposure, we found evidence of concurrent down-regulation of Rubisco and electron transport capacity in the upper-canopy sunlit leaves. The lower canopy showed no evidence of down-regulation. The upper canopy down-regulated carboxylation capacity (Vcmax) and electron transport capacity (Jmax) by about 17-20% in 1-year-old needles; however, this response was significant across sampling years only for Jmax in 1-year-old needles (P < 0.02). A reduction in leaf photosynthetic capacity in aging conifer needles at the canopy top could have important consequences for canopy carbon balance and global carbon sinks because 1-year-old sunlit needles contribute a major proportion of the annual carbon balance of these conifers. Our finding of a significant interaction between canopy position and CO2 treatment on the biochemical capacity for CO2 assimilation suggests that it is important to take canopy position and needle aging into account because morphologically and physiologically distinct leaves could respond differently to elevated [CO2].     

Carbon exchange rates, chlorophyll content, and carbohydrate status of two forest tree species exposed to carbon dioxide enrichment

Seedlings of yellow-poplar (Liriodendron tulipifera L.) and white oak (Quercus alba L.) were exposed continuously to one of three CO(2) concentrations in open-top chambers under field conditions and evaluated after 24 weeks with respect to carbon exchange rates (CER), chlorophyll (Chl) content, and diurnal carbohydrate status. Increasing the CO(2) concentration from ambient to +150 or +300 microl l(-1) stimulated CER of yellow-poplar and white oak seedlings by 60 and over 35%, respectively, compared to ambient-grown seedlings. The increases in CER were not associated with a significant change in stomatal conductance and occurred despite a reduction in the amounts of Chl and accessory pigments in the leaves of plants grown in CO(2)-enriched air. Total Chl contents of yellow-poplar and white oak seedlings grown at +300 microl l(-1) were reduced by 27 and over 55%, respectively, compared with ambient-grown seedlings. Yellow-poplar and white oak seedlings grown at +300 microl l(-1) contained 72 and 67% more morning starch, respectively, than did ambient-grown plants. In contrast, yellow-poplar and white oak seedlings grown at +300 microl l(-1) contained 17 and 27% less evening sucrose, respectively, than did plants grown at ambient CO(2) concentration. Diurnal starch accumulation and the subsequent depletion of sucrose contributed to a pronounced increase in the starch/sucrose ratio of plants grown in CO(2)-enriched air. All seedlings exhibited a substantial reduction in dark respiration as CO(2) concentration increased, but the significance of this increase to the carbohydrate status and carbon economy of plants grown in CO(2)-enriched air remains unclear.     

Exposure to elevated carbon dioxide concentration in the dark lowers the respiration quotient of Vitis cane wood

Cane cuttings of the grapevine rootstock Vitis rupestris Scheele x V. riparia Michx. cv. 3309 Couderc were brought out of endodormancy by warming at 30 degrees C. Cane pieces (12 to 13 cm long) with nodes containing a primary bud were placed in a gas exchange system and monitored for net respiratory fluxes of CO2 and O2. Grapevine respiration rates expressed on a wood volume basis were 1.4 to 3.4 mmol CO2 or O2 m-3s-1, which is higher than stem respiration rates reported for many other woody taxa but similar to rates measured for ecodormant buds of other Vitis species. Passive water loss from canes was 0.7 to 1.2 mmol H2O m-3s-1. During a 7-day period, nonstructural carbohydrate concentrations in cane wood declined only slightly, whereas sucrose was nearly completely consumed. When ambient CO2 concentration ([CO2]) was raised from 300 to 750 micro molmol-1 and then 2000 micromol mol-1, net CO2 exchange rates declined by 5.9 +/- 0.6 and then 11.0 +/- 0.6%, whereas net O2 consumption rates remained about constant. The mean respiration quotient (net CO2/O2 flux) for canes with intact ecodormant buds was 0.99 +/- 0.03 when the [CO2] was 300 micromol mol-1, and decreased to 0.87 +/- 0.03 and 0.088 +/- 0.02 when the [CO2] was increased to 750 and 2000 micromol mol-1, respectively. The results support the hypothesis that, in Vitis canes, inhibition of respiratory CO2 efflux in response to high [CO2] is an indirect consequence of non-photosynthetic carboxylation reactions, and not a result of inhibition of respiratory metabolism.     

Wood properties of Scots pines (Pinus sylvestris) grown at elevated temperature and carbon dioxide concentration

Impacts of elevated temperature and carbon dioxide concentration ([CO2]) on wood properties of 15-year-old Scots pines (Pinus sylvestris L.) grown under conditions of low nitrogen supply were investigated in open-top chambers. The treatments consisted of (i) ambient temperature and ambient [CO2] (AT+AC), (ii) ambient temperature and elevated [CO2] (AT+EC), (iii) elevated temperature and ambient [CO2] (ET+AC) and (iv) elevated temperature and elevated [CO2] (ET+EC). Wood properties analyzed for the years 1992-1994 included ring width, early- and latewood width and their proportions, intra-ring wood density (minimum, maximum and mean, as well as early- and latewood densities), mean fiber length and chemical composition of the wood (cellulose, hemicellulose, lignin and acetone extractive concentration). Absolute radial growth over the 3-year period was 54% greater in AT+EC trees and 30 and 25% greater in ET+AC and ET+EC trees, respectively, than in AT+AC trees. Neither elevated temperature nor elevated [CO2] had a statistically significant effect on ring width, early- and latewood widths or their proportions. Both latewood density and maximum intra-ring density were increased by elevated [CO2], whereas fiber length was increased by elevated temperature. Hemicellulose concentration decreased and lignin concentration increased significantly in response to elevated temperature. There were no statistically significant interaction effects of elevated temperature and elevated [CO2] on the wood properties, except on earlywood density.     

Photosynthetic acclimation of overstory Populus tremuloides and understory Acer saccharum to elevated atmospheric CO2 concentration: interactions with shade and soil nitrogen

We exposed Populus tremuloides Michx. and Acer saccharum Marsh. to a factorial combination of ambient and elevated atmospheric CO2 concentrations ([CO2]) and high-nitrogen (N) and low-N soil treatments in open-top chambers for 3 years. Our objective was to compare photosynthetic acclimation to elevated [CO2] between species of contrasting shade tolerance, and to determine if soil N or shading modify the acclimation response. Sun and shade leaf responses to elevated [CO2] and soil N were compared between upper and lower canopy leaves of P. tremuloides and between A. saccharum seedlings grown with and without shading by P. tremuloides. Both species had higher leaf N concentrations and photosynthetic rates in high-N soil than in low-N soil, and these characteristics were higher for P. tremuloides than for A. saccharum. Electron transport capacity (Jmax) and carboxylation capacity (Vcmax) generally decreased with atmospheric CO2 enrichment in all 3 years of the experiment, but there was no evidence that elevated [CO2] altered the relationship between them. On a leaf area basis, both Jmax and Vcmax acclimated to elevated [CO2] more strongly in shade leaves than in sun leaves of P. tremuloides. However, the apparent [CO2] x shade interaction was largely driven by differences in specific leaf area (m2 g-1) between sun and shade leaves. In A. saccharum, photosynthesis acclimated more strongly to elevated [CO2] in sun leaves than in shade leaves on both leaf area and mass bases. We conclude that trees rooted freely in the ground can exhibit photosynthetic acclimation to elevated [CO2], and the response may be modified by light environment. The hypothesis that photosynthesis acclimates more completely to elevated [CO2] in shade-tolerant species than in shade-intolerant species was not supported.     

Chlorophyll and carotenoid concentrations in two varieties of Pinus ponderosa seedlings subjected to long-term elevated carbon dioxide

Two varieties of ponderosa pine (Pinus ponderosa Dougl. var. scopulorum (Rocky Mountain variety) and P. ponderosa var. ponderosa (Sierran variety)) seedlings were subjected to elevated atmospheric CO(2) for two and a half years. The CO(2) concentrations were ambient, ambient + 75 microl l(-1), ambient + 150 microl l(-1) and ambient + 300 microl l(-1), or approximately 350, 425, 500 and 650 microl l(-1) CO(2). After one and a half years of exposure to elevated CO(2) and until the end of the study, seedlings of both varieties showed symptoms of stress including mottling, mid-needle abscission and early senescence. In both varieties, exposure to CO(2) concentrations greater than ambient + 75 microl l(-1) resulted in lower chlorophyll a, chlorophyll b and carotenoid concentrations. At elevated CO(2) concentrations, the concentrations of pigments in needles of the Sierran variety were lower than those in the Rocky Mountain variety. Also, at elevated CO(2) concentrations, the pigment concentrations in the 1-year-old needles of both P. ponderosa varieties were lower than those in current-season needles.