Belowground competition and the response of developing forest communities to atmospheric CO2 and O3

As human activity continues to increase CO₂ and O₃, broad expanses of north temperate forests will be simultaneously exposed to elevated concentrations of these trace gases. Although both CO₂ and O₃ are potent modifiers of plant growth, we do not understand the extent to which they alter competition for limiting soil nutrients, like nitrogen (N). We quantified the acquisition of soil N in two 8‐year‐old communities composed of trembling aspen genotypes (n= 5) and trembling aspen–paper birch which were exposed to factorial combinations of CO₂ (ambient and 560 μL L⁻¹) and O₃ (ambient = 30–40 vs. 50–60 nL L⁻¹). Tracer amount of ¹⁵NH₄⁺ were applied to soil to determine how these trace gases altered the competitive ability of genotypes and species to acquire soil N. One year after isotope addition, we assessed N acquisition by measuring the amount of ¹⁵N tracer contained in the plant canopy (i.e. recent N acquisition), as well as the total amount of canopy N (i.e. cumulative N acquisition). Exposure to elevated CO₂ differentially altered recent and cumulative N acquisition among aspen genotypes, changing the rank order in which they obtained soil N. Elevated O₃ also altered the rank order in which aspen genotypes obtained soil N by eliciting increases, decreases and no response among genotypes. If aspen genotypes respond similarly under field conditions, then rising concentrations of CO₂ and O₃ could alter the structure of aspen populations. In the aspen–birch community, elevated CO₂ increased recent N (i.e. ¹⁵N) acquisition in birch (68%) to a greater extent than aspen (19%), suggesting that, over the course of this experiment, birch had gained a competitive advantage over aspen. The response of genotypes and species to rising CO₂ and O₃ concentrations, and how these responses are modified by competitive interactions, has the potential to change the future composition and productivity of northern temperate forests.     

Effects of CO2 and light on tree phytochemistry and insect performance

Direct and interactive effects of CO₂ and light on tree phytochemistry and insect fitness parameters were examined through experimental manipulations of plant growth conditions and performance of insect bioassays. Three species of deciduous trees (quaking aspen, Populus tremuloides; paper birch, Betula papyrifera; sugar maple, Acer saccharum) were grown under ambient (387±8 μL/L) and elevated (696±2 μL/L) levels of atmospheric CO₂, with low and high light availability (375 and 855 μmol×m^?2×s^?1 at solar noon). Effects on the population and individual performance of a generalist phytophagous insect, the white‐marked tussock moth (Orgyia leucostigma) were evaluated. Caterpillars were reared on experimental trees for the duration of the larval stage, and complementary short‐term (fourth instar) feeding trials were conducted with insects fed detached leaves.
Phytochemical analyses demonstrated strong effects of both CO₂ and light on all foliar nutritional variables (water, starch and nitrogen). For all species, enriched CO₂ decreased water content and increased starch content, especially under high light conditions. High CO₂ availability reduced levels of foliar nitrogen, but effects were species specific and most pronounced for high light aspen and birch. Analyses of secondary plant compounds revealed that levels of phenolic glycosides (salicortin and tremulacin) in aspen and condensed tannins in birch and maple were positively influenced by levels of both CO₂ and light. In contrast, levels of condensed tannins in aspen were primarily affected by light, whereas levels of ellagitannins and gallotannins in maple responded to light and CO₂, respectively.
The long‐term bioassays showed strong treatment effects on survival, development time, and pupal mass. In general, CO₂ effects were pronounced in high light and decreased along the gradient aspen birch maple. For larvae reared on high light aspen, enriched CO₂ resulted in 62% fewer survivors, with increased development time, and reduced pupal mass. For maple‐fed insects, elevated CO₂ levels had negative effects on survival and pupal mass in low light. For birch, the only negative CO₂ effects were observed in high light, where female larvae showed prolonged development. Fourth instar feeding trials demonstrated that low food conversion efficiency reduced insect performance. Elevated levels of CO₂ significantly reduced total consumption, especially by insects on high light aspen and low light maple.
This research demonstrates that effects of CO₂ on phytochemistry and insect performance can be strongly light‐dependent, and that plant responses to these two environmental variables differ among species. Overall, increased CO₂ availability appeared to increase the defensive capacity of early‐successional species primarily under high light conditions, and of late‐successional species under low light conditions. Due to the interactive effects of tree species, light, CO₂, and herbivory, community composition of forests may change in the future.     

Response of quaking aspen genotypes to enriched CO2: foliar chemistry and tussock moth performance

Abstract

• 1 Genetic variation in the phytochemical responses of plants to CO₂ enrichment is likely to alter trophic dynamics, and to shift intraspecific selection pressures on plant populations. We evaluated the independent and interactive effects of atmospheric CO₂ and quaking aspen (Populus tremuloides Michx.) genotype on chemical composition of foliage and performance of the whitemarked tussock moth (Orgyia leucostigma J. E. Sm.).
• 2 This research was conducted at the Aspen FACE (Free Air CO₂ Enrichment) site in northern Wisconsin, U.S.A. Leaf samples were collected periodically from each of three genetically variable aspen genotypes growing under ambient and elevated CO₂, and analysed for levels of primary and secondary metabolites. Tussock moth larvae were reared in situ on experimental trees, and development times and pupal masses were recorded.
• 3 Foliar chemical composition varied among aspen genotypes and in response to CO₂ enrichment. However, chemical responses of trees to elevated CO₂ were generally consistent across genotypes.
• 4 Larval development times varied among host genotypes and increased slightly for insects on high‐CO₂ plants. Enriched CO₂ tended to reduce insect pupal masses, particularly for females on one of the three aspen genotypes.
• 5 CO₂ × genotype interactions observed for plant chemistry and insect performance in this study with a small number of genotypes are probably too few, and too weak, to shift selection pressures in aspen populations. These results differ, however, from earlier work in which more substantial CO₂ × genotype interactions were observed for plant chemistry.

     

Interacting elevated CO2 and tropospheric O3 predisposes aspen (Populus tremuloides Michx.) to infection by rust (Melampsora medusae f. sp. tremuloidae)

We investigated the interaction of elevated CO₂ and/or (Ozone) O₃ on the occurrence and severity of aspen leaf rust (Melampsora medusae Thuem. f. sp. tremuloidae) on trembling aspen (Populus tremuloides Michx.). Furthermore, we examined the role of changes in leaf surface properties induced by elevated CO₂ and/or O₃ in this host–pathogen interaction. Three‐ to five‐fold increases in levels of rust infection index were found in 2 consecutive years following growing‐season‐long exposures with either O₃ alone or CO₂ + O₃ depending on aspen clone. Examination of leaf surface properties (wax appearance, wax amount, wax chemical composition, leaf surface and wettability) suggested significant effects by O₃ and CO₂ + O₃. We conclude that elevated O₃ is altering aspen leaf surfaces in such a way that it is likely predisposing the plants to increased infection by aspen leaf rust.     

Atmospheric change alters performance of an invasive forest insect

Atmospheric change and species invasions are arguably two of the most important factors affecting the long‐term sustainability of natural ecosystems. We examined the independent and interactive effects of atmospheric carbon dioxide (CO₂) and tropospheric ozone (O₃) on the foliar quality of two host species and performance of an invasive folivorous insect. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) were grown at the Aspen FACE research site in northern Wisconsin, USA, under all combinations of ambient and elevated CO₂ and O₃. We measured the effects of elevated CO₂ and O₃ on aspen and birch phytochemistry and on the survivorship, development time, growth, and fecundity of the gypsy moth (Lymantria dispar). Elevated CO₂ had little effect on, whereas elevated O₃ altered, the composite phytochemical profiles of aspen and birch. Nutritional quality in aspen and birch leaves was marginally affected by elevated CO₂ and reduced by elevated O₃. Both gases increased concentrations of phenolic and structural compounds in aspen and birch. Elevated CO₂ offset reduced foliar quality under elevated O₃, but only in aspen, and to a greater extent later than earlier in spring. Elevated CO₂ generally had beneficial effects on, while elevated O₃ detrimentally affected, gypsy moth performance. Elevated CO₂ ameliorated most of the reductions in gypsy moth performance under elevated O₃. Our findings suggest that atmospheric change can alter foliar quality in gypsy moth hosts sufficiently to influence gypsy moth performance, but that these responses will depend on interactions among CO₂, O₃, and tree species. Our findings also contrast with those of earlier studies at Aspen FACE, indicating that foliar quality responses to environmental change are likely influenced by tree stand age and longevity of exposure to pollutants to the extent that they affect plant‐herbivore interactions differently over decadal time spans.     

Effects of genotype,elevated CO2 and elevated O3 on aspen phytochemistry and aspen leaf beetle Chrysomela crotchi performance

• 1 Trembling aspen Populus tremuloides Michaux is an important forest species in the Great Lakes region and displays tremendous genetic variation in foliar chemistry. Elevated carbon dioxide (CO₂) and ozone (O₃) may also influence phytochemistry and thereby alter the performance of insect herbivores such as the aspen leaf beetle Chrysomela crotchi Brown.
• 2 The present study aimed to relate genetic‐ and atmospheric‐based variation in aspen phytochemistry to C. crotchi performance (larval development time, adult mass, survivorship). The experiment was conducted at the Aspen Free‐Air CO₂ Enrichment (FACE) site in northern Wisconsin. Beetles were reared on three aspen genotypes under elevated CO₂ and/or O₃. Leaves were collected to determine chemical characteristics.
• 3 The foliage exhibited significant variation in nitrogen, condensed tannins and phenolic glycosides among genotypes. CO₂ and O₃, however, had little effect on phytochemistry. Nonetheless, elevated CO₂ decreased beetle performance on one aspen genotype and had inconsistent effects on beetles reared on two other genotypes. Elevated O₃ decreased beetle performance, especially for beetles reared on an O₃‐sensitive genotype. Regression analyses indicated that phenolic glycosides and nitrogen explain a substantial amount (27–45%) of the variation in herbivore performance.
• 4 By contrast to the negative effects that are typically observed with generalist herbivores, aspen leaf beetles appear to benefit from phenolic glycosides, chemical components that are largely genetically‐determined in aspen. The results obtained in the present study indicate that host genetic variation and atmospheric concentrations of greenhouse gases will be important factors in the performance of specialist herbivores, such as C. crotchi, in future climates.

     

Foliar quality influences tree-herbivore-parasitoid interactions: effects of elevated CO<Subscript>2</Subscript>, O<Subscript>3</Subscript>, and plant genotype

This study examined the effects of carbon dioxide (CO₂)-, ozone (O₃)-, and genotype-mediated changes in quaking aspen (Populus tremuloides) chemistry on performance of the forest tent caterpillar (Malacosoma disstria) and its dipteran parasitoid (Compsilura concinnata) at the Aspen Free-Air CO₂ Enrichment (FACE) site. Parasitized and non-parasitized forest tent caterpillars were reared on two aspen genotypes under elevated levels of CO₂ and O₃, alone and in combination. Foliage was collected for determination of the chemical composition of leaves fed upon by forest tent caterpillars during the period of endoparasitoid larval development. Elevated CO₂ decreased nitrogen levels but had no effect on concentrations of carbon-based compounds. In contrast, elevated O₃ decreased nitrogen and phenolic glycoside levels, but increased concentrations of starch and condensed tannins. Foliar chemistry also differed between aspen genotypes. CO₂, O₃, genotype, and their interactions altered forest tent caterpillar performance, and differentially so between sexes. In general, enriched CO₂ had little effect on forest tent caterpillar performance under ambient O₃, but reduced performance (for insects on one aspen genotype) under elevated O₃. Conversely, elevated O₃ improved forest tent caterpillar performance under ambient, but not elevated, CO₂. Parasitoid larval survivorship decreased under elevated O₃, depending upon levels of CO₂ and aspen genotype. Additionally, larval performance and masses of mature female parasitoids differed between aspen genotypes. These results suggest that host-parasitoid interactions in forest systems may be altered by atmospheric conditions anticipated for the future, and that the degree of change may be influenced by plant genotype.     

Elevated CO2 levels and herbivore damage alter host plant preferences

Interactions between the moth Spodoptera littoralis and two of its host plants, alfalfa (Medicago sativa) and cotton (Gossypium hirsutum) were examined, using plants grown under ambient (350 ppm) and elevated (700 ppm) CO₂ conditions. To determine strength and effects of herbivore‐induced responses assays were performed with both undamaged (control) and herbivore damaged plants. CO₂ and damage effects on larval host plant preferences were determined through dual‐choice bioassays. In addition, larvae were reared from hatching to pupation on experimental foliage to examine effects on larval growth and development. When undamaged plants were used S. littoralis larvae in consumed more cotton than alfalfa, and CO₂ enrichment caused a reduction in the preference for cotton. With damaged plants larvae consumed equal amounts of the two plant species (ambient CO₂ conditions), but CO₂ enrichment strongly shifted preferences towards cotton, which was then consumed three times more than alfalfa. Complementary assays showed that elevated CO₂ levels had no effect on the herbivore‐induced responses of cotton, whereas those of alfalfa were significantly increased. Larval growth was highest for larvae fed undamaged cotton irrespectively of CO₂ level, and lowest for larvae on damaged alfalfa from the high CO₂ treatment. Development time increased on damaged cotton irrespectively of CO₂ treatment, and on damaged alfalfa in the elevated CO₂ treatment. These results demonstrate that elevated CO₂ levels can cause insect herbivores to alter host plant preferences, and that effects on herbivore‐induced responses may be a key mechanism behind these processes. Furthermore, since the insects were shown to avoid foliage that reduced their physiological performance, our data suggest that behavioural host plant shifts result in partial escape from negative consequences of feeding on high CO₂ foliage. Thus, CO₂ enrichment can alter both physiology and behaviour of important insect herbivores, which in turn may to impact plant biodiversity.     

Responses of trembling aspen (Populus tremuloides) phytochemistry and aspen blotch leafminer (Phyllonorycter tremuloidiella) performance to elevated levels of atmospheric CO2 and O3

1 This research was conducted at the Aspen FACE (Free Air CO₂ Enrichment) site located in northern Wisconsin, U.S.A. where trembling aspen (Populus tremuloides Michaux) trees were exposed to one of four atmospheric treatments: elevated carbon dioxide (CO₂; 560 µL/L), elevated ozone (O₃; ambient × 1.5), elevated CO₂ and O₃, or ambient air. We evaluated the effects of these fumigants on aspen foliar quality and the performance of aspen blotch leafminer (Phyllonorycter tremuloidiella Braun). 2 CO₂ and O₃ each affected foliar quality, with the major changes consisting of an 11% reduction in nitrogen under elevated CO₂ and a 20% reduction in tremulacin under elevated O₃. In the CO₂ + O₃ treatment, nitrogen levels were reduced by 15% and CO₂ ameliorated the O₃‐mediated reduction in tremulacin levels. 3 Phyllonorycter tremuloidiella were allowed to colonize trees naturally. Elevated CO₂ and O₃ reduced colonization rates by 42 and 49% relative to ambient CO₂ and O₃, respectively. The only effect of fumigation treatments on larval performance occurred under elevated O₃, where male development time and larval consumption increased by 8 and 28%, respectively, over insects reared under ambient O₃. 4 These data demonstrate that the individual and combined effects of CO₂ and O₃ can alter aspen foliar chemistry and that these alterations in foliar chemistry produce little to no change in larval performance. However, both CO₂ and O₃ greatly reduced oviposition. In order to ascertain the full effects of CO₂ and O₃ on insect performance, future studies should address both population‐ and individual‐level characteristics.     

Soil CO2 efflux of two silver birch clones exposed to elevated CO2 and O3 levels during three growing seasons

In the present open‐top chamber experiment, two silver birch clones (Betula pendula Roth, clone 4 and clone 80) were exposed to elevated levels of carbon dioxide (CO₂) and ozone (O₃), singly and in combination, and soil CO₂ efflux was measured 14 times during three consecutive growing seasons (1999–2001). In the beginning of the experiment, all experimental trees were 7 years old and during the experiment the trees were growing in sandy field soil and fertilized regularly. In general, elevated O₃ caused soil CO₂ efflux stimulation during most measurement days and this stimulation enhanced towards the end of the experiment. The overall soil respiration response to CO₂ was dependent on the genotype, as the soil CO₂ efflux below clone 80 trees was enhanced and below clone 4 trees was decreased under elevated CO₂ treatments. Like the O₃ impact, this clonal difference in soil respiration response to CO₂ increased as the experiment progressed. Although the O₃ impact did not differ significantly between clones, a significant time × clone × CO₂× O₃ interaction revealed that the O₃‐induced stimulation of soil respiration was counteracted by elevated CO₂ in clone 4 on most measurement days, whereas in clone 80, the effect of elevated CO₂ and O₃ in combination was almost constantly additive during the 3‐year experiment. Altogether, the root or above‐ground biomass results were only partly parallel with the observed soil CO₂ efflux responses. In conclusion, our data show that O₃ impacts may appear first in the below‐ground processes and that relatively long‐term O₃ exposure had a cumulative effect on soil CO₂ efflux. Although the soil respiration response to elevated CO₂ depended on the tree genotype as a result of which the O₃ stress response might vary considerably within a single tree species under elevated CO₂, the present experiment nonetheless indicates that O₃ stress is a significant factor affecting the carbon cycling in northern forest ecosystems.     

Altered genotypic and phenotypic frequencies of aphid populations under enriched CO2 and O3 atmospheres

Environmental change is anticipated to negatively affect both plant and animal populations. As abiotic factors rapidly change habitat suitability, projections range from altered genetic diversity to wide-spread species loss. Here, we assess the degree to which changes in atmospheric composition associated with environmental change will influence not only the abundance, but also the genotypic/phenotypic diversity, of herbivore populations. Using free-air CO₂ and O₃ enrichment (FACE) technology, we assess numerical responses of pea aphids (Acyrthosiphon pisum) exhibiting a pink–green genetic polymorphism and an environmentally determined wing polyphenism on broad bean plants (Vicia faba) under enriched CO₂ and/or O₃ atmospheres, over multiple generations. We show that these two greenhouse gases alter not only aphid population sizes, but also genotypic and phenotypic frequencies. As the green genotype was positively influenced by elevated CO₂ levels, but the pink genotype was not, genotypic frequencies (pink morph : green morph) ranged from 1 : 1 to 9 : 1. These two genotypes also displayed marked differences in phenotypic frequencies. The pink genotype exhibited higher levels of wing induction under all atmospheric treatments, however, this polyphenism was negatively influenced by elevated O₃ levels. Resultantly, frequencies of winged phenotypes (pink morph : green morph) varied from 10 : 1 to 332 : 1. Thus, atmospheric conditions associated with environmental change may alter not just overall population sizes, but also genotypic and phenotypic frequencies of herbivore populations, thereby influencing community and ecosystem functioning.     

Atmospheric change alters foliar quality of host trees and performance of two outbreak insect species

This study examined the independent and interactive effects of elevated carbon dioxide (CO₂) and ozone (O₃) on the foliar quality of two deciduous trees species and the performance of two outbreak herbivore species. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) were grown at the Aspen FACE research site in northern Wisconsin, USA, under four combinations of ambient and elevated CO₂ and O₃. We measured the effects of elevated CO₂ and O₃ on aspen and birch phytochemistry and on gypsy moth (Lymantria dispar) and forest tent caterpillar (Malacosoma disstria) performance. Elevated CO₂ nominally affected foliar quality for both tree species. Elevated O₃ negatively affected aspen foliar quality, but only marginally influenced birch foliar quality. Elevated CO₂ slightly improved herbivore performance, while elevated O₃ decreased herbivore performance, and both responses were stronger on aspen than birch. Interestingly, elevated CO₂ largely offset decreased herbivore performance under elevated O₃. Nitrogen, lignin, and C:N were identified as having strong influences on herbivore performance when larvae were fed aspen, but no significant relationships were observed for insects fed birch. Our results support the notion that herbivore performance can be affected by atmospheric change through altered foliar quality, but how herbivores will respond will depend on interactions among CO₂, O₃, and tree species. An emergent finding from this study is that tree age and longevity of exposure to pollutants may influence the effects of elevated CO₂ and O₃ on plant–herbivore interactions, highlighting the need to continue long-term atmospheric change research.     

Aphid individual performance may not predict population responses to elevated CO2 or O3

Changes in atmospheric composition affect plant quality and herbivore performance. We used the Aspen Free Air CO₂ Enrichment (FACE) facility to investigate the impacts of elevated carbon dioxide (CO₂) and ozone (O₃) on the performance of the aphid Cepegillettea betulaefoliae Granovsky feeding on paper birch (Betula papyrifera Marsh.). In Year 1, we simultaneously measured individual performance and population growth rates, and in Year 2 we surveyed natural aphid, predator and parasitoid populations throughout the growing season. Aphid growth and development (relative growth rate (RGR), development time, adult weight, embryo number and the birth weight of newborn nymphs) were unaffected by CO₂ and O₃. Aphid fecundity decreased on trees grown at elevated CO₂, O₃ and CO₂+O₃. Neither nymphal performance nor adult size were reliable indicators of future fecundity at elevated CO₂ and/or O₃. Aphid populations protected from natural enemies were unaffected by elevated CO₂, but increased significantly at elevated O₃. Individual fecundity in elevated CO₂ and O₃ atmospheres did not predict population growth rates, probably because of changes in the strength of intraspecific competition or the ability of the aphids to induce nutrient sinks. Natural aphid, predator and parasitoids populations (Year 2) showed few significant responses to CO₂ and O₃, although CO₂ and O₃ did affect the timing of aphid and natural enemy peak abundance. Elevated CO₂ and O₃ affected aphid and natural enemy populations independently: no CO₂× O₃ interactions were observed. We conclude that: (1) aphid individual performance did not predict population responses to CO₂ and O₃ and (2) elevated CO₂ and O₃ atmospheres are unlikely to affect C. betulaefoliae populations in the presence of natural enemy communities.     

Below-ground responses of silver birch trees exposed to elevated CO2 and O3 levels during three growing seasons

Field‐growing silver birch (Betula pendula Roth) clones (clone 4 and 80) were exposed to elevated CO₂ and O₃ in open‐top chambers for three consecutive growing seasons (1999–2001). At the beginning of the OTC experiment, all trees were 7 years old. We studied the single and interaction effects of CO₂ and O₃ on silver birch below‐ground carbon pools (i.e. effects on fine roots and mycorrhizas, soil microbial communities and sporocarp production) and also assessed whether there are any clonal differences in these below‐ground CO₂ and O₃ responses. The total mycorrhizal infection level of both clones was stimulated by elevated CO₂ alone and elevated O₃ alone, but not when elevated CO₂ was used in fumigation in combination with elevated O₃. In both clones, elevated CO₂ affected negatively light brown/orange mycorrhizas, while its effect on other mycorrhizal morphotypes was negligible. Elevated O₃, instead, clearly decreased the proportions of black and liver‐brown mycorrhizas and increased that of light brown/orange mycorrhizas. Elevated O₃ had a tendency to decrease standing fine root mass and sporocarp production as well, both of these O₃ effects mainly manifesting in clone 4 trees. CO₂ and O₃ treatment effects on soil microbial community composition (PLFA, 2‐ and 3‐OH‐FA profiles) were negligible, but quantitative PLFA data showed that in 2001 the PLFA fungi : bacteria‐ratio of clone 80 trees was marginally increased because of elevated CO₂ treatments. This study shows that O₃ effects were most clearly visible at the mycorrhizal root level and that some clonal differences in CO₂ and O₃ responses were observable in the below‐ground carbon pools. In conclusion, the present data suggests that CO₂ effects were minor, whereas increasing tropospheric O₃ levels can be an important stress factor in northern birch forests, as they might alter mycorrhizal morphotype assemblages, mycorrhizal infection rates and sporocarp production.     

Wood properties of two silver birch clones exposed to elevated CO2 and O3

Effects of elevated carbon dioxide (CO₂) and ozone (O₃) on wood properties of two initially 7‐year‐old silver birch (Betula pendula Roth) clones were studied after a fumigation during three growing seasons. Forty trees, representing two fast‐growing clones (4 and 80), were exposed in open‐top chambers to the following treatments: outside control, chamber control, 2 × ambient [CO₂], 2 × ambient [O₃] and 2 × ambient [CO₂]+2 × ambient [O₃]. After the 3‐year exposure, the trees were felled and wood properties were analyzed. The treatments affected both stem wood structure and chemistry. Elevated [CO₂] increased annual ring width, and concentrations of extractives and starch, and decreased concentrations of cellulose and gravimetric lignin. Elevated O₃ decreased vessel percentage and increased cell wall percentage in clone 80. In vessel percentage, elevated CO₂ ameliorated the O₃‐induced decrease. In clone 4, elevated O₃ decreased nitrogen concentration of wood. The two clones had different wood properties. In clone 4, the concentrations of extractives, starch, soluble sugars and nitrogen were greater than in clone 80, while in clone 80 the concentrations of cellulose and acid‐soluble lignin were higher. Clone 4 also had slightly longer fibres, greater vessel lumen diameter and vessel percentage than clone 80, while in clone 80 cell wall percentage was greater. Our results show that wood properties of young silver birch trees were altered under elevated CO₂ in both clones, whereas the effects of O₃ depended on clone.     

Elevated CO2 alters birch resistance to Lagomorpha herbivores

We studied the three‐way interaction of elevated CO₂, nitrogen (N), and temperature (T), and the two‐way interaction of elevated CO₂ and early‐season defoliation on the secondary chemistry and resistance of Eurasian silver birch (Betula pendula) and North American paper birch (B. papyrifera) against the Eurasian hare (Lepus timidus) and the North American eastern cottontail rabbit (Sylvilagus floridanus), respectively. Elevated CO₂ decreased the palatability of winter‐dormant silver and paper birch stems to both hares and rabbits, respectively. But the effect on hares was only apparent at intermediate levels of N fertilization. Elevated T had no effect on palatability. The effects of elevated CO₂, N, and T on levels of silver birch bark phenolics and terpenoids were dominated by two‐way interactions between N and CO₂, and N and T. Generally, however, N amendments elicited a parabolic response in carbon partitioning to most biosynthetic classes of silver birch phenolics (i.e. highest concentrations occurring at intermediate N). CO₂ elevation was most enhancing at highest levels of N. On the other hand, T increases, more often than not, elicited reductions in phenolics, but especially so at the highest N level. In the case of B. papyrifera, elevated CO₂ increased carbon partitioning to Folin‐Denis stem and branch phenolics and condensed tannins. Early‐season defoliation, on the other hand, had no effect on phenolics and tannins but lowered both N and energy levels of branches. Elevated CO₂ substantially ameliorated the negative effects of severe defoliation on tree growth. These results support the hypothesis that continuing anthropogenic alterations of the atmosphere may trigger significant changes in plant phenotypic resistance to mammalian herbivores owing to an increasing net carbon balance between the highly vagile supply and demand capacities of plant carbon sources and sinks.     

Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and/or O3

Leaf gas exchange parameters and the content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2‐year‐old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO₂ and/or O₃. The plants were exposed either to ambient air (control), elevated CO₂ (560 p.p.m.) elevated O₃ (55 p.p.b.) or a mixture of elevated CO₂ and O₃ in a free air CO₂ enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light‐saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age‐related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO₂ and decreased by O₃ at both control and elevated CO₂. The relative stomatal limitation to photosynthesis (l_s) was in both clones about 10% under control and elevated O₃. Exposure to elevated CO₂ + O₃ in both clones and to elevated CO₂ in clone 259, decreased l_s even further – to about 5%. The corresponding changes in Rubisco content and the stability of C_i/C_a ratio suggest that the changes in photosynthesis in response to elevated CO₂ and O₃ were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O₃ tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable C_i under the given treatment, that indicates close coupling between stomatal and mesophyll processes.     

Fungal community composition and function after long‐term exposure of northern forests to elevated atmospheric CO2 and tropospheric O3

The long‐term effects of rising atmospheric carbon dioxide (CO₂) and tropospheric O₃ concentrations on fungal communities in soil are not well understood. Here, we examine fungal community composition and the activities of cellobiohydrolase and N‐acetylglucosaminidase (NAG) after 10 years of exposure to 1.5 times ambient levels of CO₂ and O₃ in aspen and aspen–birch forest ecosystems, and compare these results to earlier studies in the same long‐term experiment. The forest floor community was dominated by saprotrophic fungi, and differed slightly between plant community types, as did NAG activity. Elevated CO₂ and O₃ had small but significant effects on the distribution of fungal genotypes in this horizon, and elevated CO₂ also lead to an increase in the proportion of Sistotrema spp. within the community. Yet, although cellobiohydrolase activity was lower in the forest floor under elevated O₃, it was not affected by elevated CO₂. NAG was also unaffected. The soil community was dominated by ectomycorrhizal species. Both CO₂ and O₃ had a minor effect on the distribution of genotypes; however, phylogenetic analysis indicated that under elevated O₃Cortinarius and Inocybe spp. increased in abundance and Laccaria and Tomentella spp. declined. Although cellobiohydrolase activity in soil was unaffected by either CO₂ or O₃, NAG was higher (～29%) under CO₂ in aspen–birch, but lower (～18%) under aspen. Time series analysis indicated that CO₂ increased cellulolytic enzyme activity during the first 5 years of the experiment, but that the magnitude of this effect diminished over time. NAG activity also showed strong early stimulation by elevated CO₂, but after 10 years this effect is no longer evident. Elevated O₃ appears to have variable stimulatory and repressive effects depending on the soil horizon and time point examined.     

Stem wood properties of Populus tremuloides, Betula papyrifera and Acer saccharum saplings after 3 years of treatments to elevated carbon dioxide and ozone

The aim of this study was to examine the effects of elevated carbon dioxide [CO₂] and ozone [O₃] and their interaction on wood chemistry and anatomy of five clones of 3‐year‐old trembling aspen (Populus tremuloides Michx.). Wood chemistry was studied also on paper birch (Betula papyrifera Marsh.) and sugar maple (Acer saccharum Marsh.) seedling‐origin saplings of the same age. Material for the study was collected from the Aspen Free‐Air CO₂ Enrichment (FACE) experiment in Rhinelander, WI, USA, where the saplings had been exposed to four treatments: control (C; ambient CO₂, ambient O₃), elevated CO₂ (560 ppm during daylight hours), elevated O₃ (1.5 × ambient during daylight hours) and their combination (CO₂+O₃) for three growing seasons (1998–2000). Wood chemistry responses to the elevated CO₂ and O₃ treatments differed between species. Aspen was most responsive, while maple was the least responsive of the three tree species. Aspen genotype affected the responses of wood chemistry and, to some extent, wood structure to the treatments. The lignin concentration increased under elevated O₃ in four clones of aspen and in birch. However, elevated CO₂ ameliorated the effect. In two aspen clones, nitrogen in wood samples decreased under combined exposure to CO₂ and O₃. Soluble sugar concentration in one aspen clone and starch concentration in two clones were increased by elevated CO₂. In aspen wood, α‐cellulose concentration changed under elevated CO₂, decreasing under ambient O₃ and slightly increasing under elevated O₃. Hemicellulose concentration in birch was decreased by elevated CO₂ and increased by elevated O₃. In aspen, elevated O₃ induced statistically significant reductions in distance from the pith to the bark and vessel lumen diameter, as well as increased wall thickness and wall percentage, and in one clone, decreased fibre lumen diameter. Our results show that juvenile wood properties of broadleaves, depending on species and genotype, were altered by atmospheric gas concentrations predicted for the year 2050 and that CO₂ ameliorates some adverse effects of elevated O₃ on wood chemistry.     

Interannual climatic variation mediates elevated CO2 and O3 effects on forest growth

We analyzed growth data from model aspen (Populus tremuloides Michx.) forest ecosystems grown in elevated atmospheric carbon dioxide ([CO₂]; 518 μL L^?1) and ozone concentrations ([O₃]; 1.5 × background of 30–40 nL L^?1 during daylight hours) for 7 years using free‐air CO₂ enrichment technology to determine how interannual variability in present‐day climate might affect growth responses to either gas. We also tested whether growth effects of those gasses were sustained over time. Elevated [CO₂] increased tree heights, diameters, and main stem volumes by 11%, 16%, and 20%, respectively, whereas elevated ozone [O₃] decreased them by 11%, 8%, and 29%, respectively. Responses similar to these were found for stand volume and basal area. There were no growth responses to the combination of elevated [CO₂+O₃]. The elevated [CO₂] growth stimulation was found to be decreasing, but relative growth rates varied considerably from year to year. Neither the variation in annual relative growth rates nor the apparent decline in CO₂ growth response could be explained in terms of nitrogen or water limitations. Instead, growth responses to elevated [CO₂] and [O₃] interacted strongly with present‐day interannual variability in climatic conditions. The amount of photosynthetically active radiation and temperature during specific times of the year coinciding with growth phenology explained 20–63% of the annual variation in growth response to elevated [CO₂] and [O₃]. Years with higher photosynthetic photon flux (PPF) during the month of July resulted in more positive growth responses to elevated [CO₂] and more negative growth responses to elevated [O₃]. Mean daily temperatures during the month of October affected growth in a similar fashion the following year. These results indicate that a several‐year trend of increasingly cloudy summers and cool autumns were responsible for the decrease in CO₂ growth response.