Effects of long-term CO2 enrichment and nutrient availability in Norway spruce. I. Phenology and morphology of branches

 Branches of 30-year-old Norway spruce [Picea abies (L.) Karst.] trees were enclosed in ventilated, transparent plastic bags and flushed with air containing ambient (A≈370 μmol CO₂ mol^–1) or ambient plus 340 μmol CO₂ mol^–1 (EL). Light-saturated photosynthesis was on average 56% higher in EL compared to A. Branch phenology and morphology were strongly related to nitrogen concentration (mg g^–1 dry mass) in the foliage and to elevated temperatures in the bags, but no direct effect of EL was found. In 1995, budbreak occurred on average 4 days earlier in the bags compared to the control branches, which was partly explained by the temperature elevation in the bags. No nutrient or EL effect on budbreak was found. Increases in temperature and nitrogen supply increased shoot growth: together they explained 76% of the variation in the extension rate, 63% of the variation in extension duration and 65% of the variation in final length of leading shoots. Shoot morphology was altered both by increased nitrogen availability and by the enclosure induced environmental changes inside the bags, leading to reduced mutual shading between needles. Specific needle area (SNA) was lower in EL, but this was related to lower nitrogen concentrations. Total dry mass of the branches was unaffected by EL. It is concluded that treating individual branches of Norway spruce with elevated CO₂ does not increase branch growth. The nutrient status of the branch and climate determine its growth, i.e. its sink strength for carbon. Increased export of carbohydrates to the rest of the tree is probable in EL treated branches. Received: 20 July 1998 / Accepted 8 October 1998     

The effect of elevated CO2 concentration and nutrient supply on carbon-based plant secondary metabolites in Pinus sylvestris L.

This study investigated changes in carbon-based plant secondary metabolite concentrations in the needles of Pinus sylvestris saplings, in response to long-term elevation of atmospheric CO₂, at two rates of nutrient supply. Experimental trees were grown for 3 years in eight open-top chambers (OTCs), four of which were maintained at ambient (∼350 μmol mol⁻¹) and four at elevated (700 μmol mol⁻¹) CO₂ concentrations, plus four open air control plots. Within each of these treatments, plants received either high (7.0 g N m⁻² year⁻¹ added) or low (no nutrients added) rates of nutrient supply for two years. Needles from lateral branches were analysed chemically for concentrations of condensed tannins and monoterpenes. Biochemical determinations of cellulase digestibility and protein precipitating capacity of their phenolic extracts were made because of their potential of importance in ecological interactions between pine and other organisms including herbivores and decomposers. Elevated CO₂ concentration caused an increase (P<0.05) in dry mass per needle, tree height and the concentration of the monoterpene α-pinene, but there were no direct effects of CO₂ concentration on any of the other chemical measurements made. High nutrient availability increased cellulase digestibility of pine needles. There was a significant negative effect of the OTCs on protein precipitating capacity of the needle extracts in comparison to the open-air controls. Results suggest that predicted changes in atmospheric CO₂ concentration will be insufficient to produce large changes in the concentration of condensed tannins and monoterpenes in Scots pine. Processes which are influenced by these compounds, such as decomposition and herbivore food selection, along with their effects on ecosystem functioning, are therefore unlikely to be directly affected through changes in these secondary metabolites. Received: 20 October 1997 / Accepted: 28 February 1998     

Needle CO2 exchange, structure and defense traits in relation to needle age in Pinus heldreichii Christ – a relict of Tertiary flora

 Pinus heldreichii Christ is a long-lived, slow-growing Tertiary relict from the Balkans. In this study we evaluated the physiological characteristics of eight needle-age classes of P. heldreichii grown at the Arboretum of the Institute of Dendrology in Kórnik, Poland. At the end of the growing season, current-year foliage had the highest rates of mass-based light-saturated net photosynthesis (A_sat) of 33.5 nmol CO₂· g^–1· s^–1. A_sat decreased with needle age, but older needle classes retained from approximately 62 to 26% of the current needles’ rate. The relationship between leaf N and chlorophyll a concentration among all needle-age classes was highly significant (r = 0.96, P = 0.0006). The variation in A_sat of 1- to 7-year-old needles was linearly related to needle N concentration (r = 0.98, P = 0.0001). Needle dark respiration rates among these needle age classes ranged from 0.8 to 2.2 nmol · g^–1· s^–1 and decreased with needle age and nitrogen concentration. Total phenols and glucose concentrations increased linearly with needle age. A similar pattern was observed in acid buffering capacity and the pH of tissue homogenates. The water content ranged from 62% for the current needles to 51% for the 6-year-old needles. Greater investment in leaf structural tissue and increased chemical defense is associated with higher structural cost of older needles and may reduce their photosynthetic activity. Significant declines in water and nitrogen content with needle age and an increase in content of phenolics is most likely a defense adaptation of P. heldreichii related to the species’ long-lived leaves. Received: 8 January 1997 / Accepted: 4 March 1997     

Effects of elevated CO<Subscript>2</Subscript> and/or O<Subscript>3</Subscript> on hormone IAA in needles of Chinese pine

This study examined the effects of season-long exposure of Chinese pine (Pinus tabulaeformis) to elevated carbon dioxide (CO₂) and/or ozone (O₃) on indole-3-acetic acid (IAA) content, activities of IAA oxidase (IAAO) and peroxidase (POD) in needles. Trees grown in open-top chambers (OTC) were exposed to control (ambient O₃, 55 nmol mol⁻¹ + ambient CO₂, 350 μmol mol⁻¹, CK), elevated CO₂ (ambient O₃ + high CO₂, 700 μmol mol⁻¹, EC) and elevated O₃ (high O₃, 80 ± 8 nmol mol⁻¹ + ambient CO₂, EO) OTCs from 1 June to 30 September. Plants grown in elevated CO₂ OTC had a growth increase of axial shoot and needle length, compared to control, by 20% and 10% respectively, while the growth in elevated O₃ OTC was 43% and 7% less respectively, than control. An increase in IAA content and POD activity and decrease in IAAO activity were observed in trees exposed to elevated CO₂ concentration compared with control. Elevated O₃ decreased IAA content and had no significant effect on IAAO activity, but significantly increased POD activity. When trees pre-exposed to elevated CO₂ were transferred to elevated O₃ (EC–EO) or trees pre-exposed to elevated O₃ were transferred to elevated CO₂ (EO–EC), IAA content was lower while IAAO activity was higher than that transferred to CK (EC–CK or EO–CK), the change in IAA content was also related to IAAO activity. The results indicated that IAAO and POD activities in Chinese pine needles may be affected by the changes in the atmospheric environment, resulting in the change of IAA metabolism which in turn may cause changes in Chinese pine’s growth. An erratum to this article can be found at     

Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees

The effects of elevated atmospheric CO₂ and increased wet N deposition on leaf quality and insect herbivory were evaluated in nine model ecosystems composed of 7-year-old spruce trees (Picea abies) and three understorey species established on natural forest soil. Each model ecosystem was grown in a simulated montane climate, and was exposed to one of three CO₂ concentrations (280, 420, and 560 μl l⁻¹), and to one of three levels of N deposition (0, 30, and 90 kg ha⁻¹ year⁻¹) for 3 years. In the 3rd year of the experiment second to third instars of the nun moth (Lymantria monacha) were allowed to feed directly on current-year needles of top canopy branches of each tree for 12 days. Specific leaf area (SLA), water content, and N concentration decreased in needles exposed to elevated CO₂, whereas the concentrations of starch, condensed tannins, and total phenolics increased. Increased N deposition had no significant effect on SLA, and water content, but the concentrations of starch, condensed tannins, and total phenolics decreased, and sugar and N concentrations increased. Despite higher relative consumption rates (RCRs) larvae consumed 33% less N per unit larval biomass and per day at the two high CO₂ treatments, compared to those feeding on 280 μl l⁻¹-needles, but they maintained similar N accumulation rates due to increased N utilization efficiencies (NUE). However, over the 12-day experimental period larvae gained less N overall and reached a 35% lower biomass in the two high-CO₂ treatments compared to those at 280 μl l⁻¹. The effects of increased N deposition on needle quality and insect performance were generally opposite to those of CO₂ enrichment, but were lower in magnitude. We conclude that altered needle quality in response to elevated CO₂ will impair the growth and development of L. monacha larvae. Increasing N deposition may mitigate these effects, which could lead to altered insect herbivore distributions depending on regional patterns of N deposition. Received: 8 June 1998 / Accepted: 27 October 1998     

The effect of nutrition on the seasonal course of needle respiration in Norway spruce stands

 Respiration of 1-year-old needles of 30-year-old Norway spruce trees [Picea abies (L.) Karst.] was studied in a nutrient optimisation experiment in northern Sweden. Respiration rates of detached needles, from ten control (C) and ten irrigated-fertilised (IL) trees, were measured on 16 occasions from June 1992 to June 1993. The aim of the study was to determine the influence of temperature on the seasonal course of needle maintenance respiration, and the effect of nitrogen concentration [N] and carbohydrate content on needle respiration in young Norway spruce trees subjected to long-term fertilisation. The IL treatment significantly affected needle size, in terms of dry mass and length, but not specific needle length (SNL). There was, however, a strong tree-specific effect on SNL (P<10^–9, R ² = 0.75). Needle starch content varied markedly with season (0–25% of total dry mass). This, unless accounted for, would cause erroneous estimates of nutrient concentrations, and of rates of needle respiration, within and between treatments. There was considerable seasonal variation in needle respiration, both in terms of maintenance respiration and temperature dependence (Q₁₀). Q₁₀ had its highest value (2.8) during winter and its lowest (2.0) in the middle of summer. In early autumn (August, September), respiration rate and needle [N] were significantly related (C: P = 0.001, IL: P<0.0005). There was no significant difference in the slope between the two regression lines, but a difference in intercept. At the same needle [N], needles from IL-plots always had a lower respiration rate than needles from control plots. No obvious explanation for the observed difference in intercept was found, but some plausible assumptions are put forward and discussed. Received: 24 January 1997 / Accepted: 1 July 1997     

Elevated [CO2], temperature increase and N supply effects on the turnover of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO₂] (700 μl l^-1 CO₂) and temperature increase (+3 °C) on carbon turnover in grassland soils were studied during 2.5 years at two N fertiliser supplies (160 and 530 kg N ha^-1 y^-1) in an experiment with well-established ryegrass swards (Lolium perenne) supplied with the same amounts of irrigation water. During the growing season, swards from the control climate (350 μl l^-1 [CO₂] at outdoor air temperature) were pulse labelled by the addition of ¹³CO₂. The elevated [CO₂] treatments were continuously labelled by the addition of fossil-fuel derived CO₂ (^{13 C} of -40 to -50 ‰). Prior to the start of the experimental treatments, the carbon accumulated in the plant parts and in the soil macro-organic matter (‘old’ C) was at −32‰. During the experiment, the carbon fixed in the plant material (‘new’ C) was at −14 and −54‰ in the ambient and elevated [CO₂] treatments, respectively. During the experiment, the ¹³C isotopic mass balance method was used to calculate, for the top soil (0–15 cm), the carbon turnover in the stubble and roots and in the soil macro-organic matter above 200 μ (MOM). Elevated [CO₂] stimulated the turnover of organic carbon in the roots and stubble and in the MOM at N+, but not at N−. At the high N supply, the mean replacement time of ‘old’ C by ‘new’ C declined in elevated, compared to ambient [CO₂], from 18 to 7 months for the roots and stubble and from 25 to 17 months for the MOM. This resulted from increased rates of ‘new’ C accumulation and of ‘old’ C decay. By contrast, at the low N supply, despite an increase in the rate of accumulation of ‘new’ C, the soil C pools did not turnover faster in elevated [CO₂], as the rate of ‘old’ C decomposition was reduced. A 3 °C temperature increase in elevated [CO₂] decreased the input of fresh C to the roots and stubble and enhanced significantly the exponential rate for the ‘old’ C decomposition in the roots and stubble. An increased fertiliser N supply reduced the carbon turnover in the roots and stubble and in the MOM, in ambient but not in elevated [CO₂]. The respective roles for carbon turnover in the coarse soil OM fractions, of the C:N ratio of the litter, of the inorganic N availability and of a possible priming effect between C-substrates are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Multi-objective environment chamber system for studying plant responses to climate change

This paper describes the technical information and performance of a new multi-objective chamber system enabling the control of environmental variables (e.g., temperature, CO₂, air humidity, wind speed, and UV-B radiation) for understanding plant responses to climate change. Over a whole growing season, four different climate scenarios were evenly programmed into the system’s 16 chambers as ambient environment (AMB), elevated temperature (ET), elevated CO₂ concentration (EC) and elevated temperature and CO₂ concentration (ETC). Simultaneously, the chamber effects were assessed regarding the physiological responses and growth of a boreal perennial grass (reed canary grass, Phalaris arundinacea L.). During the growing season, the chamber system provided a wide variety of climatic conditions for air temperature (T _a), relative humidity (RH) and CO₂ concentration (C _a) in the AMB chambers following outside conditions. The target temperature (+3.5°C) was achieved to a good degree in the ET and ETC chambers, being on average 3.3°C and 3.7°C higher than ambient conditions, respectively. The target concentration of CO₂ (700 ppm) was also well achieved in the EC and ETC chambers, being on average 704 ppm and 703 ppm, respectively. The stable airflow condition inside all of the chambers provided a homogeneous distribution of gases and temperature. The decreases in RH and increases in vapour pressure deficit (VPD) in the elevated temperature chambers were also maintained at a low level. Chamber effects were observed, with some physiological and growth parameters of plants being significantly lower in the AMB chambers, compared to outside conditions. The plant growth was negatively affected by the reduced radiation inside the chambers.     

Species-specific and needle age-related responses of photosynthesis in two Pinus species to long-term exposure to elevated CO2 concentration

There is, so far, no common conclusion about photosynthetic responses of trees to long-term exposure to elevated CO₂. Photosynthesis and specific leaf area (SLA) of 1-year-old and current-year needles in Pinus koraiensis and P. sylvestriformis grown in open-top chambers were measured monthly for consecutive two growing seasons (2006, 2007) after 8–9 years of CO₂ enrichment in northeastern China, to better understand species-specific and needle age-related responses to elevated CO₂ (500 μmol mol⁻¹ CO₂). The light-saturated photosynthetic rates (P _Nsat) increased in both species at elevated CO₂, but the stimulation magnitude varied with species and needle age. Photosynthetic acclimation to elevated CO₂, in terms of reduced V _cmax (maximum carboxylation rate) and J _max (maximum electron transport rate), was found in P. koraiensis but not in P. sylvestriformis. The photosynthetic parameters (V _cmax, J _max, P _Nsat) measured in different-aged needles within each species responded to elevated CO₂ similarly, but elevated CO₂ resulted in much pronounced variations of those parameters in current-year needles than in 1-year-old needles within each species. This result indicated that needle age affects the magnitude but not the patterns of photosynthetic responses to long-term CO₂ enrichment. The present study indicated that different species associated with different physioecological properties responded to elevated CO₂ differently. As global change and CO₂ enrichment is more or less a gradual rather than an abrupt process, long-term global change experiments with different plant species are still needed to character and better predict the global change effects on terrestrial ecosystems.     

Effects of elevated CO2 and temperature on secondary compounds in the needles of Scots pine (Pinus sylvestris L.)

The aim of this study was to evaluate the long-term effects of elevated CO₂ concentration (doubling of ambient CO₂ concentration) and temperature (2–6°C elevation) on the concentration and content of secondary compounds in the needles of Scots pine (Pinus sylvestris L.) saplings grown in closed-top environmental chambers. The chamber treatments included (1) ambient temperature and CO₂, (2) ambient temperature and elevated CO₂, (3) elevated temperature and ambient CO₂, and (4) elevated temperature and elevated CO₂. The needle sampling and analyses of monoterpenes, HPLC-phenolics and condensed tannins in current- and 1-year-old needles were made in two consecutive years. The results showed that the effects of elevation of CO₂ and temperature were greatest on the monoterpene concentration in the needles while the concentration of HPLC-phenolics remained almost unaffected by the changed growing conditions. Most of the observed decrease in monoterpene concentration was caused by the CO₂ enrichment while the effect of elevated temperature alone was not as significant. The accumulation of condensed tannins tended to increase due to the elevation of CO₂ alone compensating the reduced carbon allocation to monoterpenes. Overall, the responses of the concentrations of secondary compounds to the elevation of CO₂ and temperature are variable and depend strongly on the properties and characteristics of each compound as well as on the interrelation between the production of these compounds and the primary production of trees.     

Biomass allocation and canopy development in spruce model ecosystems under elevated CO2 and increased N deposition

Ecosystem-level experiments on the effects of atmospheric CO₂ enrichment and N deposition on forest trees are urgently needed. Here we present data for nine model ecosystems of spruce (Picea abies) on natural nutrient-poor montane forest soil (0.7 m² of ground and 350 kg weight). Each system was composed of six 7-year-old (at harvest) trees each representing a different genotype, and a herbaceous understory layer (three species). The model ecosystems were exposed to three different CO₂ concentrations (280, 420, 560 μl l⁻¹) and three different rates of wet N deposition (0, 30, 90 kg ha⁻¹ year⁻¹) in a simulated annual course of Swiss montane climate for 3 years. The total ecosystem biomass was not affected by CO₂ concentration, but increased with increasing N deposition. However, biomass allocation to roots increased with increasing CO₂ leading to significantly lower leaf mass ratios (LMRs) and leaf area ratios (LARs) in trees grown at elevated CO₂. In contrast to CO₂ enrichment, N deposition increased biomass allocation to the aboveground plant parts, and thus LMR and LAR were higher with increasing N deposition. We observed no CO₂ ×  N interactions on growth, biomass production, or allocation, and there were also no genotype × treatment interactions. The final leaf area index (LAI) of the spruce canopies was 19% smaller at 420 and 27% smaller at 560 than that measured at 280 μl CO₂ l⁻¹, but was not significantly altered by increasing N deposition. Lower LAIs at elevated CO₂ largely resulted from shorter branches (less needles per individual tree) and partially from increased needle litterfall. Independently of N deposition, total aboveground N content in the spruce communities declined with increasing CO₂ (−18% at 420 and −31% at 560 compared to 280 μl CO₂ l⁻¹). N deposition had the opposite effect on total above ground N content (+18% at 30 and +52% at 90 compared to 0 kg N ha⁻¹ year⁻¹). Our results suggest that under competitive conditions on natural forest soil, atmospheric CO₂ enrichment may not lead to higher ecosystem biomass production, but N deposition is likely to do so. The reduction in LAI under elevated CO₂ suggests allometric down-regulation of photosynthetic carbon uptake at the canopy level. The strong decline in the tree nitrogen mass per unit ground area in response to elevated CO₂ may indicate CO₂-induced reductions of soil N availability. Received: 11 May 1997 / Accepted: 4 August 1997     

Assessing the impact of elevated O3 and CO2 on gas exchange characteristics of differently K supplied clonal Norway spruce trees during exposure and the following season

 Well-supplied and K-deficient 4-year-old clonal Norway spruce trees were exposed to combinations of two levels of ozone (20 and 80 nl l^{–  1} O₃) and carbon dioxide (350 and 750 μl l^{–  1} CO₂) to study the effects of possible future climate factors on gas exchange characteristics. The fumigation was performed in environmental chambers for a complete growing season. After the exposure, plants were cultivated outdoors to investigate possible recovery and delayed effects. During the exposure 1-year-old needles responded to the 80 nl l^{–  1} O₃ treatment by a sharp but transient decrease of both apparent carboxylation efficiency (CE) and maximum photosynthetic capacity (A₂₅₀₀). Elevated CO₂ also reduced CE and A₂₅₀₀. The effect became stronger in the course of the exposure and was accompanied by decreases of N and P as well as chlorophyll contents. In case of K deficiency, the acclimation response of current-year needles was even more pronounced reflecting lower sink capacities for carbon metabolites. The joint application of elevated O₃ and CO₂ resulted in the lowest values of gas exchange parameters and chlorophyll contents. At the beginning of the growing season after the exposure and under outdoor conditions, all these treatment effects disappeared in the needles which had developed during the fumigation. In the course of the development of the new flush, however, the well-supplied 1-year-old needles which had been treated with 80 nl l^{–  1} O₃ and 350 μl l^{–  1} CO₂ in the year before, exhibited a sharp decline of CE and A₂₅₀₀. Simultaneously, chlorotic mottle and bands developed. These delayed symptoms are discussed in the context of the previously published “memory” effect for O₃ (Sandermann et al. 1989). Additionally, evidence is presented that shoot development is altered in plants which had been exposed to elevated O₃. Accepted: 5 August 1996     

The influence of elevated O3 and CO2 concentrations on secondary metabolites of Scots pine (Pinus sylvestris L. ) seedlings

Terpene, resin acid and total phenolic concentrations in five‐year‐old Scots pine (Pinus sylvestris L.) seedlings were analysed after exposure to ambient and realistically elevated (2 × ambient) O₃ and CO₂ concentrations and their combination in open‐top chambers during two growing seasons. Under O₃ exposure, limonene concentration in needles and isopimaric concentration in stems decreased significantly. As a response to elevated CO₂, α‐pinene and total phenolic concentrations in needles increased significantly, while bornyl acetate concentration in needles and palustric + levopimaric and neoabietic acid concentrations in stems decreased significantly. Some terpenes and resin acids were found at lower concentrations in the combined O₃ and CO₂ treatment than in O₃ exposure or elevated CO₂. A negative chamber effect was found: seedlings growing inside the chambers with ambient air had significantly lower concentrations of some terpenes and resin acids than seedlings growing outside the chambers. There was a lot of between‐tree variation in terpene and resin acid concentrations, which is typical of open‐pollinated populations. The results of this study suggest that, at least in short‐term experiments, Scots pine secondary metabolites are relatively insensitive to climate change factors. Total phenolics in the needles were the most responsive group showing about 25% increase in elevated CO₂, and O₃ exposure did not mitigate this CO₂ effect. Terpenes and resin acids were less responsive, although some individual compounds showed notable responses, e.g. α‐pinene in needles, which increased about 50% in response to elevated CO₂. As a consequence, although there were only slight effects on total pools of needle secondary metabolites, considerable O₃ and CO₂ effects on certain individual compounds might have ecological significance via trophic amplification, e.g. in decomposing processes of needle litter.     

Long-term response of Sitka spruce (Picea sitchensis (Bong.) Carr.) to CO2 enrichment and nitrogen supply. I. Growth,biomass allocation and physiology

ABSTRACT

After a 3-year exposure to elevated CO₂, young trees of Sitka spruce (Picea sitchensis (Bong.) Carr.) were planted in native, nutrient-deficient forest soil and grown for two more years with three CO₂ treatments in open-top chambers, and with two nutrient treatments (with and without supplied N). Elevated CO₂ resulted in larger fresh mass, dry mass, leaf area and leaf thickness in two-year old needles, but had no effect on one-year old and current needles. Tree height, basal diameter and biomass production also increased, regardless of N supply. In trees without added N, elevated CO₂ resulted in higher root-to-shoot and absorbing roots-to-stump ratios. Regardless of N supply, trees grown in elevated CO₂ had lower photosynthetic rates on a leaf area basis. Photosynthesis reduction was accompanied by a decline in Rubisco activity and leaf N concentration. Under elevated CO₂, added N elevated photosynthesis and Rubisco activity, suggesting a dependence on N availability of the photosynthetic response to elevated CO₂. Stomatal conductance of trees grown with added N decreased in response to elevated CO₂. This may account for the larger reduction in intercellular CO₂ concentration, and hence photosynthesis, in the trees supplied with N than in those without N supply.     

Carbon isotope discrimination and stomatal responses of mature Pinus sylvestris L. trees exposed in situ for three years to elevated CO2 and temperature

The Climate Change Experiment (CLIMEX) is a unique large scale facility in which an entire undisturbed catchment of boreal vegetation has been exposed to elevated CO₂ (560 ppm) and temperature (+3°C summer, +5°C winter) for the past three years with all the soil-plant-atmosphere linkages intact. Here, carbon isotope composition and stomatal density have been analysed from sequential year classes of needles of mature Scots pine trees (Pinus sylvestris L.) to investigate the response of time-integrated water-use efficiency (UWE) and stomatal density to CO₂ enrichment and climate change. Carbon isotope discrimination decreased and WUE increased in cohorts of needles developing under increased CO₂ and temperature, compared to needles on the same trees developing in pretreatment years. Mid-season instantaneous gas exchange, measured on the same trees for the past four years, indicated that these responses resulted from higher needle photosynthetic rates and reduced stomatal conductance. Needles of P. sylvestris developing under increased CO₂ and temperature had consistently lower stomatal densities than their ambient grown counterparts on the same trees. The stomatal density of P. sylvestris needles was inversely correlated with δ¹³C-derived WUE, implying some effect of this morphological response on leaf gas exchange. Future atmospheric CO₂ and temperature increases are therefore likely to improve the water economy of P. sylvestris, at least at the scale of individual needles, by affecting stomatal density and gas exchange processes.     

Effects of carbon dioxide enrichment on response of mycorrhizal pitch pine (Pinus rigida) to aluminum: growth and mineral nutrition

 Carbon dioxide enrichment may increase the Al tolerance of trees by increasing root growth, root exudation and/or mycorrhizal colonization. The effect of elevated CO₂ on the response of mycorrhizal pitch pine (Pinus rigida Mill.) seedlings to Al was determined in two experiments with different levels of nutrients, 0.1- or 0.2-strength Clark solution. During each experiment, seedlings inoculated with the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch were grown 13 weeks in sand irrigated with nutrient solution (pH 3.8) containing 0, 6.25, 12.5, or 25 mg/l Al (0, 232, 463, or 927 μM Al) in growth chambers fumigated with 350 (ambient) or 700 (elevated) μl/l CO₂. At ambient CO₂, in the absence of Al, mean total dry weights (DW) of seedlings at the high nutrient level were 164% higher than those at the low level. Total DW at elevated CO₂, in the absence of Al, was significantly greater than that in ambient CO₂ at the low (+34%) and high (+16%) nutrient levels. Root and shoot DW at both nutrient levels decreased with increasing Al concentrations with Al reducing root growth more than shoot growth. Although visible symptoms of Al toxicity in roots and needles were reduced by CO₂ enrichment, there were no significant CO₂ × Al interactions for shoot or root DW. The percentage of seedling roots that became mycorrhizal was negatively related to nutrient level and was greater at elevated than at ambient CO₂ levels. Generally, elevated CO₂ had little effect on concentration of mineral nutrients in roots and needles. Aluminum reduced concentrations of most nutrients by inhibiting uptake. Received: 18 June 1997 / Accepted: 8 December 1997     

Monoterpene emission from coniferous trees in response to elevated CO2 concentration and climate warming

It was hypothesized that high CO₂ availability would increase monoterpene emission to the atmosphere. This hypothesis was based on resource allocation theory which predicts increased production of plant secondary compounds when carbon is in excess of that required for growth. Monoterpene emission rates were measured from needles of (a) Ponderosa pine grown at different CO₂ concentrations and soil nitrogen levels, and (b) Douglas fir grown at different CO₂ concentrations. Ponderosa pine grown at 700 μmol mol^–1 CO₂ exhibited increased photosynthetic rates and needle starch to nitrogen (N) ratios when compared to trees grown at 350 μmol mol^–1 CO₂. Nitrogen availability had no consistent effect on photosynthesis. Douglas fir grown at 550 μmol mol^–1 CO₂ exhibited increased photosynthetic rates as compared to growth at 350 μmol mol^–1 CO₂ in old, but not young needles, and there was no influence on the starch/N ratio. In neither species was there a significant effect of elevated growth CO₂ on needle monoterpene concentration or emission rate. The influence of climate warming and leaf area index (LAI) on monoterpene emission were also investigated. Douglas fir grown at elevated CO₂ plus a 4 °C increase in growth temperature exhibited no change in needle monoterpene concentration, despite a predicted 50% increase in emission rate. At elevated CO₂ concentration the LAI increased in Ponderosa pine, but not Douglas fir. The combination of increased LAI and climate warming are predicted to cause an 80% increase in monoterpene emissions from Ponderosa pine forests and a 50% increase in emissions from Douglas fir forests. This study demonstrates that although growth at elevated CO₂ may not affect the rate of monoterpene emission per unit biomass, the effect of elevated CO₂ on LAI, and the effect of climate warming on monoterpene biosynthesis and volatilization, could increase canopy monoterpene emission rate.     

Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 2. Net assimilation and stomatal conductance of leaves

Atmospheric CO₂ concentration continues to rise. It is important, therefore, to determine what acclimatory changes will occur within the photosynthetic apparatus of wheat (Triticum aestivum L. cv. Yecora Rojo) grown in a future high-CO₂ world at ample and limited soil N contents. Wheat was grown in an open field exposed to the CO₂ concentration of ambient air [370 μmol (CO₂) mol⁻¹; Control] and air enriched to ∼200 μmol (CO₂) mol⁻¹ above ambient using a Free-Air CO₂ Enrichment (FACE) apparatus (main plot). A High (35 g m⁻²) or Low (7 and 1.5 g m⁻² for 1996 and 1997, respectfully) level of N was applied to each half of the main CO₂ treatment plots (split-plot). Under High-N, FACE reduced stomatal conductance (g _s) by 30% at mid-morning (2 h prior to solar noon), 36% at midday (solar noon) and 27% at mid-afternoon (2.5 h after solar noon), whereas under Low-N, g _s was reduced by as much as 31% at mid-morning, 44% at midday and 28% at mid-afternoon compared with Control. But, no significant CO₂ × N interaction effects occurred. Across seasons and growth stages, daily accumulation of carbon (A′) was 27% greater in FACE than Control. High-N increased A′ by 18% compared with Low-N. In contrast to results for g _s, however, significant CO₂ × N interaction effects occurred because FACE increased A′ by 30% at High-N, but by only 23% at Low-N. FACE enhanced the seasonal accumulation of carbon (A′′) by 29% during 1996 (moderate N-stress), but by only 21% during 1997 (severe N-stress). These results support the premise that in a future high-CO₂ world an acclimatory (down-regulation) response in the photosynthetic apparatus of field-grown wheat is anticipated. They also demonstrate, however, that the stimulatory effect of a rise in atmospheric CO₂ on carbon gain in wheat can be maintained if nutrients such as nitrogen are in ample supply. This revised version was published online in June 2006 with corrections to the Cover Date.     

Internal Nitrogen Dynamics in the Graminoid Molinia caerulea Under Higher N Supply and Elevated CO2 Concentrations

Nutrient resorption from senescing leaves is an important aspect of internal plant nutrient cycling. Global environmental change very likely affects this process. In an 8-month experiment, we investigated the effect of increased nitrogen (N) availability and CO₂ concentration on the contribution of leaf N resorption to the internal nitrogen dynamics of the perennial deciduous graminoid Molinia caerulea (L.) Moench. Plants were grown in a factorial combination of two levels of N (65 and 265 N ha⁻¹ year⁻¹) and CO₂ (380 and 700 μL L⁻¹) in a greenhouse. Both N and CO₂ addition increased the total biomass and the total N pools of mature Molinia plants considerably, without a significant interaction. Nitrogen-resorption efficiency from senescing leaves (% of the mature leaf N pool that is resorbed) was neither affected by the N- nor by the CO₂ treatments. When averaged over the treatments, the N-resorption efficiency was 85% ± 1 (SE). The final N concentration in the litter (N-resorption proficiency) was also not affected by the treatments and was on average 3.6 mg N g⁻¹ ± 0.25 (SE). The contribution of resorbed N from senescing leaves to the late seasonal N requirements (seed and stem production and storage of N for next year’s growth) of M. caerulea plants was (negatively) affected by the N treatment only, and no interaction effects with CO₂ were found. Resorption from stems and/or direct reserve and seed formation during growth became relatively more important. Thus, internal N cycling processes in Molinia caerulea are only affected when N availability is increased, but not under elevated CO₂ concentrations. Under high N conditions, this species shifts from a N recycling strategy to reserve formation during growth.