The real-time measurement of net photosynthesis and dark respiration of plants has been possible since the development of the infra-red analyzer and its application in the 1940's. This technique has allowed intensive investigations of the mechanisms and dynamics of carbon dioxide assimilation by green plants (Sestak, Catsky, and Jarvis, 1971). As a result, physiologic and ecologic aspects of net carbon dioxide exchange are the subject of much research (eg, Osmond, Bjorkman and Anderson, 1980).

The effects of CO2 enrichment and water stress on gas exchange of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) seedlings were examined for individuals grown from seed under high (1000 μmol·m-2·s-1) and low (250 μmol·m-2·s-1) photosynthetic photon flux density at 350, 675 and 1000 μl·l-1 CO2. At 8 weeks of age, half the seedlings in each CO2-irradiance treatment were subjected to a drying cycle which reduced plant water potential to about -2.5 MPa in the most stressed plants, while control plants remained well-watered (water potentials of -0.3 and -0.7 MPa for sweetgum and loblolly pine, respectively). During this stress cycle, whole seedling net photosynthesis, transpiration and stomatal conductance of plants from each CO2-irradiance-water treatment were measured under respective growth conditions.

Root growth and physiological uptake capacity for NH 4 + and NO 3 − were examined for seedlings of loblolly and ponderosa pine grown for 160 days under two CO2 levels, ambient (35 Pa) and ambient plus 35 Pa (70 Pa). Fraction of biomass allocated to active fine roots as well as total N (NH 4 + + NO 3 − ) absorption per unit root dry mass were unaffected by CO2. On a whole-plant basis, elevated CO2 led to a significant increase in N acquisition in loblolly but not in ponderosa pine. However, even in loblolly pine where CO2 significantly increased plant N acquisition, the relative increase, in biomass far exceeded the gain in N, i.e. a 60% increase in total dry weight was accompanied by only a 30% increase in N gain in response to high CO2. We suggest that the commonly reported decline in tissue N concentration of these and other species at high CO2 is largely caused by inability of the root systems to sufficiently compensate for increased N demand. Elevated CO2 significantly altered root uptake capacity of the different N forms, i.e., high CO2 significantly increased NO 3 − absorption rates, but decreased NH 4 + absorption rates in both species though the decrease in loblolly was insignificant. However, elevated CO2 increased root respiration rate in loblolly pine while significantly decreasing it in ponderosa pine. This indicates that CO2-induced changes in plant preference for inorganic N forms is not simply regulated by root energy status. If changes in plant preference for inorganic N forms represent typical responses to elevated CO2, the results could have important implications for N dynamics in managed and natural plant communities.

•  Availability of growth limiting resources may alter root dynamics in forest ecosystems, possibly affecting the land-atmosphere exchange of carbon. This was evaluated for a commercially important southern timber species by installing a factorial experiment of fertilization and irrigation treatments in an 8-yr-old loblolly pine (Pinus taeda) plantation. •  After 3 yr of growth, production and turnover of fine, coarse and mycorrhizal root length was observed using minirhizotrons, and compared with stem growth and foliage development. •  Fertilization increased net production of fine roots and mycorrhizal roots, but did not affect coarse roots. Fine roots had average lifespans of 166 d, coarse roots 294 d and mycorrhizal roots 507 d. Foliage growth rate peaked in late spring and declined over the remainder of the growing season, whereas fine roots experienced multiple growth flushes in the spring, summer and fall. •  We conclude that increased nutrient availability might increase carbon input to soils through enhanced fine root turnover. However, this will depend on the extent to which mycorrhizal root formation is affected, as these mycorrhizal roots have much longer average lifespans than fine and coarse roots.

Atmospheric CO 2 partial pressure ( p CO 2 ) was as low as 18 Pa during the Pleistocene and is projected to increase from 36 to 70 Pa CO 2 before the end of the 21st century. High p CO 2 often increases the growth and reproduction of C 3 annuals, whereas low p CO 2 decreases growth and may reduce or prevent reproduction. Previous predictions regarding the effects of high and low p CO 2 on C 3 plants have rarely considered the effects of evolution. Knowledge of the potential for evolution of C 3 plants in response to CO 2 is important for predicting the degree to which plants may sequester atmospheric CO 2 in the future, and for understanding how plants may have functioned in response to low p CO 2 during the Pleistocene. Therefore, three studies using Arabidopsis thaliana as a model system for C 3 annuals were conducted: (1) a selection experiment to measure responses to selection for high seed number (a major component of fitness) at Pleistocene (20 Pa) and future (70 Pa) p CO 2 and to determine changes in development rate and biomass production during selection, (2) a growth experiment to determine if the effects of selection on final biomass were evident prior to reproduction, and (3) a reciprocal transplant experiment to test if p CO 2 was a selective agent on Arabidopsis . Arabidopsis showed significant positive responses to selection for high seed number at both 20 and 70 Pa CO 2 during the selection process. Furthermore, plants selected at 20 Pa CO 2 performed better than plants selected at 70 Pa CO 2 under low CO 2 conditions, indicating that low CO 2 acted as a selective agent on these annuals. However, plants selected at 70 Pa CO 2 did not have significantly higher seed production than plants selected at 20 Pa CO 2 when grown at high p CO 2 . Nevertheless, there was some evidence that high CO 2 may also be a selective agent because changes in development rate and biomass production during selection occurred in opposite directions at low and high p CO 2 . Plants selected at high p CO 2 showed no change or reductions in biomass relative to control plants due to a decrease in the length of the life cycle, as indicated by earlier initiation of flowering and senescence. In contrast, selection at low CO 2 resulted in an average 35% increase in biomass production, due to an increase in the length of the life cycle that resulted in a longer period for biomass accumulation before senescence. From the Arabidopsis model system we conclude that some C 3 annuals may have produced greater biomass in response to low p CO 2 during the Pleistocene relative to what has been predicted from studies exposing a single generation of C 3 plants to low p CO 2 . Furthermore, C 3 annuals may exhibit evolutionary responses to high p CO 2 in the future that may result in developmental changes, but these are unlikely to increase biomass production. This series of studies shows that CO 2 may potentially act as a selective agent on C 3 annuals, producing changes in development rate and carbon accumulation that could not have been predicted from single-generation studies.

Loblolly pine (Pinus taedaL.) were grown in the field, under non‐limiting nutrient conditions, in open‐top chambers for 4 years at ambient CO2partial pressures (pCO2) and with a CO2‐enriched atmosphere (+ 30 PapCO2compared to ambient concentration). A third replicate of trees were grown without chambers at ambientpCO2. Wood anatomy, wood density and tree ring width were analysed using stem wood samples. No significant differences were observed in the cell wall to cell lumen ratio within the latewood of the third growth ring formed in 1994. No significant differences were observed in the density of resin canals or in the ratio of resin canal cross‐sectional area to xylem area within the same growth ring. Ring widths were significantly wider in the CO2‐enrichment treatment for 3 of 4 years compared to the ambient chamber control treatment. Latewood in the 1995 growth ring was significantly wider than that in the ambient control and represented a larger percentage of the total growth‐ring width. Carbon dioxide enrichment also significantly increased the total wood specific gravity (determined by displacement). However, when determined as total sample wood density by X‐ray densitometry, the density of enriched samples was not significantly higher than that of the ambient chamber controls. Only the 1993 growth ring of enriched trees had a significantly higher maximum latewood density than that of trees grown on non‐chambered plots or ambient chambered controls. No significant differences were observed in the minimum earlywood density of individual growth rings between chambered treatments. These results show that the most significant effect of CO2enrichment on wood production in loblolly pine is its influence on radial growth, measured as annual tree ring widths. This influence is most pronounced in the first year of growth and decreases with age.

Increasing concentrations of atmospheric CO(2) are predicted to impact both current and future ecosystems. Elevated CO(2) is also predicted to affect biological processes at many levels of organization. In this overview, we summarize the responses of plants to elevated CO(2) including primary physiological and molecular responses, growth and reproductive responses, effects on plant-plant competition and interactions with other organisms, evolutionary responses, and effects at the ecosystem level. The objectives of this paper are to: (a) overview studies in this issue that were presented at a 1997 meeting entitled "Critical Assessment of the Response of Forest Ecosystems to Elevated Atmospheric Carbon Dioxide," which was sponsored by the Global Change and Terrestrial Ecosystems (GCTE) group of the International Geosphere Biosphere Program (IGBP), (b) review areas of recent progress in CO(2) research, (c) generalize patterns arising from past research, and (d) list critical areas of research for the future.

SummaryInteractive effects of CO2 and water availability have been predicted to alter the competitive relationships between C3 and C4 species over geological and contemporary time scales. We tested the effects of drought and CO2 partial pressures (pCO2) ranging from values of the Pleistocene to those predicted for the future on the physiology and growth of model C3 and C4 species. We grew co‐occurring Abutilon theophrasti (C3) and Amaranthus retroflexus (C4) in monoculture at 18 (Pleistocene), 27 (preindustrial), 35 (current), and 70 (future) Pa CO2 under conditions of high light and nutrient availability. After 27 days of growth, water was withheld from randomly chosen plants of each species until visible wilting occurred. Under well‐watered conditions, low pCO2 that occurred during the Pleistocene was highly limiting to C3 photosynthesis and growth, and C3 plants showed increased photosynthesis and growth with increasing pCO2 between the Pleistocene and future CO2 values. Well‐watered C4 plants exhibited increased photosynthesis in response to increasing pCO2, but total mass and leaf area were unaffected by pCO2. In response to drought, C3 plants dropped a large amount of leaf area and maintained relatively high leaf water potential in remaining leaves, whereas C4 plants retained greater leaf area, but at a lower leaf water potential. Furthermore, drought‐treated C3 plants grown at 18 Pa CO2 retained relatively greater leaf area than C3 plants grown at higher pCO2 and exhibited a delay in the reduction of stomatal conductance that may have occurred in response to severe carbon limitations. The C4 plants grown at 70 Pa CO2 showed lower relative reductions in net photosynthesis by the end of the drought compared to plants at lower pCO2, indicating that CO2 enrichment may alleviate drought effects in C4 plants. At the Pleistocene pCO2, C3 and C4 plants showed similar relative recovery from drought for leaf area and biomass production, whereas C4 plants showed higher recovery than C3 plants at current and elevated pCO2. Based on these model systems, we conclude that C3 species may not have been at a disadvantage relative to C4 species in response to low CO2 and severe drought during the Pleistocene. Furthermore, C4 species may have an advantage over C3 species in response to increasing atmospheric CO2 and more frequent and severe droughts.

The purpose of this paper is to summarize the results of a series of greenhouse and open-top chamber studies on the effects of N and elevated atmospheric CO2 on ponderosa and loblolly pine (Pinus ponderosa Laws, and P. taeda L.) to evaluate common patterns of response. Growth response to elevated CO2 ranged from zero to more than 1000%, depending largely upon N status, In both species, growth response to CO2 was greater under moderate N deficiency than under extreme N deficiency or N sufficiency/excess. Elevated CO2 generally caused lowered tissue N concentrations in many (but not all) cases, which in turn resulted in smaller increases in N uptake than in biomass. Growth response to N ranged from -50 (in ponderosa pine) to more than 1000%, depending upon the N status of the control medium. Growth response to N was enhanced by elevated CO2 when N was in the extreme deficiency range but not when N was in the moderate deficiency range. In two separate studies, ponderosa pine responded negatively to high N inputs, and in each case this response was mitigated by elevated CO2. Collectively, these results show that (i) N deficiency is a continuum rather than a step function, (ii) responses to elevated CO2 vary across this continuum of N deficiency, and (iii) elevated CO2 greatly enhances growth response to N additions when N is initially in the extremely deficient range.

Root growth and physiological uptake capacity for NH 4 + and NO 3 − were examined for seedlings of loblolly and ponderosa pine grown for 160 days under two CO 2 levels, ambient (35 Pa) and ambient plus 35 Pa (70 Pa). Fraction of biomass allocated to active fine roots as well as total N (NH 4 + + NO 3 − ) absorption per unit root dry mass were unaffected by CO 2 . On a whole-plant basis, elevated CO 2 led to a significant increase in N acquisition in loblolly but not in ponderosa pine. However, even in loblolly pine where CO 2 significantly increased plant N acquisition, the relative increase, in biomass far exceeded the gain in N, i.e. a 60% increase in total dry weight was accompanied by only a 30% increase in N gain in response to high CO 2 . We suggest that the commonly reported decline in tissue N concentration of these and other species at high CO 2 is largely caused by inability of the root systems to sufficiently compensate for increased N demand. Elevated CO 2 significantly altered root uptake capacity of the different N forms, i.e., high CO 2 significantly increased NO 3 − absorption rates, but decreased NH 4 + absorption rates in both species though the decrease in loblolly was insignificant. However, elevated CO 2 increased root respiration rate in loblolly pine while significantly decreasing it in ponderosa pine. This indicates that CO 2 -induced changes in plant preference for inorganic N forms is not simply regulated by root energy status. If changes in plant preference for inorganic N forms represent typical responses to elevated CO 2 , the results could have important implications for N dynamics in managed and natural plant communities.

We investigated how changes in loblolly Dine needle phytochemistry caused by elevated CO2, leaf age, and soil nutrient levels affected the performance of 2 individual generations of the multivoltine folivorous insect pest Neodiprion lecontei (Fitch). In 2 feeding trials, mature needles produced in the previous (spring) and current (fall) year from seedlings grown in open-topped chambers under 4 CO2 and 2 soil nutrient levels were fed to 2 separate generations of redheaded pine sawfly larvae. Strong seasonal differences (i.e., spring versus fall) in leaf nutritional and defensive constituents resulted in significant between-generation differences in the growth, consumption, and growth efficiency of sawfly larvae, Enriched CO2-grown needles had higher levels of starch and starch/nitrogen ratios in older, overwintering spring needles, which were lower in leaf nitrogen and monoterpenes than younger, current year needles (fall). Overall, larval growth was higher and consumption lower on the fall needles, presumably because of higher levels of leaf nitrogen compared with the spring needles. The plant CO2 concentration significantly contributed to the larval consumption responses between seasons (significant CO2 X season interaction), demonstrating that the 2 sawfly generations were affected differently by CO2-induced phytochemical alterations in spring versus fall needles. The data presented here suggests that when investigating multivoltine folivorous insect responses to elevated CO2-grown tree seedlings in which multiple leaf flushes within a growing season expose insects to an array of leaf phytochemical changes, >1 insect generation should be investigated.

The decomposition of plant-derived organic matter exerts strong control over the cycling of carbon and nutrients in terrestrial ecosystems and may be significantly altered by increased precipitation and nitrogen deposition associated with global change. It was the goal of this study to quantify the rate of belowground decomposition in an intact loblolly pine forest, and determine how this was affected by increased availability of water and nitrogen. A randomized complete-block factorial of irrigation and fertilization treatments was installed in an 8 yr old loblolly pine plantation in Scotland county, North Carolina. Fresh root samples of three size classes were buried in fiberglass mesh bags in January, 1994 and recovered at two-month intervals for two years. Samples were analyzed for percent mass remaining and contents of macro-nutrients. Roots decomposed in a two stage process: early in the incubation mass loss was correlated to size class and nutrient concentrations, but this correlation disappeared later in the incubation when rates of mass loss converged for all size classes. Decomposition was seldom affected by the irrigation and fertilization treatments, due to the buffering capacity of soil moisture and complex ecosystem-level responses to fertilization. Net mineralization of N, P, K, Ca, and Mg occurred in the smaller size classes of roots providing a source of these nutrients to the aggrading plantation for an estimated 2 to 15 years. The largest size class of roots was a sink for N, Ca, and Mg for the duration of this study, and was a source of P and K for an estimated 20 and 4 years, respectively. It is concluded that in moist temperate ecosystems belowground decomposition will be less affected by the projected increases in moisture and nutrient availability than will decomposition of the forest floor due to the buffering capacity of the soil. Further, small roots provide important sources of macro-nutrients for several decades to aggrading forests after large-scale disturbances such as harvesting of aboveground biomass.

ABSTRACTAtmospheric CO2 partial pressure may have been as low as 18 Pa during the Pleistocene and is expected to increase from 35 to 70 Pa before the end of the next century. Low CO2 reduces the growth and reproduction of C3 plants, whereas elevated CO2 often increases growth and reproduction. Plants at high elevation are exposed to reduced CO2 partial pressure and may be better adapted to the low CO2 of the Pleistocene. We examined genotypes of Arabidopsis thaliana from different elevations for variation in growth and reproduction at the CO2 levels of the Pleistocene, the present and the future. Genotypes exhibited limited genetic variation in the response of the production of biomass to changes in CO2, but showed significant variation in reproductive characters. We found evidence that plants from high elevations may be better adapted to low CO2 when considering seed number, which is an important component of fitness. Genotypes showed greater variation in the response of seed number between 35 and 20 Pa CO2 compared to 35 and 70 Pa CO2. We conclude that present‐day C3 annuals may have greater potential for evolution in response to the low CO2 of the Pleistocene relative to the elevated CO2 predicted for the future.

We measured CO2 efflux from intact root/rhizosphere systems of 155 day old loblolly (Pinus taeda L.) and ponderosa (Pinus ponderosa Dougl. ex Laws.) pine seedlings in order to study the effects of elevated atmospheric CO2 on the below-ground carbon balance of coniferous tree seedlings. Seedlings were grown in sterilized sand culture, watered daily with either 1, 3.5 or 7 mt M NH 4 + , and maintained in an atmosphere of either 35 or 70 Pa CO2. Carbon dioxide efflux (μmol CO2 plant−1 s−1) from the root/rhizosphere system of both species significantly increased when seedlings were grown in elevated CO2, primarily due to large increases in root mass. Specific CO2 efflux (μmol CO2 g root−1 s−1) responded to CO2 only under conditions of adequate soil nitrogen availability (3.5 mt M). Under these conditions, CO2 efflux rates from loblolly pine increased 70% from 0.0089 to 0.0151 μmol g−1 s−1 with elevated CO2 while ponderosa pine responded with a 59% decrease, from 0.0187 to 0.0077 μmol g−1 s−1. Although below ground CO2 efflux from seedlings grown in either sub-optimal (1 mt M) or supra-optimal (7 mt M) nitrogen availability did not respond to CO2, there was a significant nitrogen treatment effect. Seedlings grown in supra-optimal soil nitrogen had significantly increased specific CO2 efflux rates, and significantly lower total biomass compared to either of the other two nitrogen treatments. These results indicate that carbon losses from the root/rhizosphere systems are responsive to environmental resource availability, that the magnitude and direction of these responses are species dependent, and may lead to significantly different effects on whole plant carbon balance of these two forest tree species.

We examined the extent to which carbon investment into secondary compounds in loblolly pine (Pinus taeda L.) is changed by the interactive effect of elevated CO2 and N availability and whether differences among treatments are the result of size-dependent changes. Seedlings were grown for 138 days at two CO2 partial pressures (35 and 70 Pa CO2) and four N solution concentrations (0.5, 1.5, 3.5, and 6.5 mmol l−1 NO3NH4) and concentrations of total phenolics and condensed tannins were determined four times during plant development in primary and fascicular needles, stems and lateral and tap roots. Concentrations of total phenolics in lateral roots and condensed tannins in tap roots were relatively high regardless of treatment. In the smallest seedlings secondary compound concentrations were relatively high and decreased in the initial growth phase. Thereafter condensed tannins accumulated strongly during plant maturation in all plant parts except in lateral roots, where concentrations did not change. Concentrations of total phenolics continued to decrease in lateral roots while they remained constant in all other plant parts. At the final harvest plants grown at elevated CO2 or low N availability showed increased concentrations of condensed tannins in aboveground parts. The CO2 effect, however, disappeared when size differences were adjusted for, indicating that CO2 only indirectly affected concentrations of condensed tannins through accelerating growth. Concentrations of total phenolics increased directly in response to low N availability and elevated CO2 in primary and fascicular needles and in lateral roots, which is consistent with predictions of the carbon-nutrient balance (CNB) hypothesis. The CNB hypothesis is also supported by the strong positive correlations between soluble sugar and total phenolics and between starch and condensed tannins. The results suggest that predictions of the CNB hypothesis could be improved if developmentally induced changes of secondary compounds were included.

Forest trees are major components of the terrestrial biome and their response to rising atmospheric CO2 plays a prominent role in the global carbon cycle. In this study, loblolly pine seedlings were planted in the field in recently disturbed soil of high fertility, and CO2 partial pressures were maintained at ambient CO2 (Amb) and elevated CO2 (Amb + 30 Pa) for 4 years. The objective of the study was to measure seasonal and long-term responses in growth and photosynthesis of loblolly pine exposed to elevated CO2 under ambient field conditions of precipitation, light, temperature and nutrient availability. Loblolly pine trees grown in elevated CO2 produced 90% more biomass after four growing seasons than did trees grown in ambient CO2. This large increase in final biomass was primarily due to a 217% increase in leaf area in the first growing season which resulted in much higher relative growth rates for trees grown in elevated CO2. Although there was not a sustained effect of elevated CO2 on relative growth rate after the first growing season, absolute production of biomass continued to increase each year in trees grown in elevated CO2 as a consequence of the compound interest effect of increased leaf area on the production of more new leaf area and more biomass. Allometric analyses of biomass allocation patterns demonstrated size-dependent shifts in allocation, but no direct effects of elevated CO2 on partitioning of biomass. Leaf photosynthetic rates were always higher in trees grown in elevated CO2, but these differences were greater in the summer (60-130% increase) than in the winter (14-44% increase), reflecting strong seasonal effects of temperature on photosynthesis. Our results suggest that seasonal variation in the relative photosynthetic response to elevated CO2 will occur in natural ecosystems, but total non-structural carbohydrate (TNC) levels in leaves indicate that this variation may not always be related to sink activity. Despite indications of canopy-level adjustments in carbon assimilation, enhanced levels of leaf photosynthesis coupled with increased total leaf area indicate that net carbon assimilation for the whole tree was greater for trees grown under elevated CO2 compared with ambient CO2. If the large growth enhancement observed in loblolly pine were maintained after canopy closure, then these trees could be a large sink for fossil carbon emitted to the atmosphere and produce a negative feedback on atmospheric CO2.

This report covers the results of a long-term project with the primary objective of developing and testing hypotheses on the environmental and physiological controls of loblolly pine response to atmospheric CO{sub 2} enrichment. Earlier research under DOE funding had provided information from loblolly pine and other plant species which allowed the development of specific hypotheses. Phase 1 of this research was a two year pot study of loblolly seedlings to determine the interaction of CO{sub 2} enrichment with soil nutrition. Phase 2 began with the enrichment of loblolly seedlings being grown in the ground, rather than pots, and continued through December 1995. Phase 3 began in April 1994 with the enrichment of undisturbed Piedmont North Carolina old field undergoing succession, including herbaceous annual plants, perennial grasses, and loblolly pine tree seedlings. Phase 3 was designed to gather preliminary information on a regenerating loblolly forest to be used for the development of hypotheses and measurement techniques for a long-term Free Air CO{sub 2} Enrichment (FACE) study of regenerating forest in Duke Forest.

The use of short-lived positron emitter isotopes in studying the dynamics of biological systems provides an indepth understanding of the regulating functions of the system, that is otherwise unattainable. When we coupled such studies with tracer kinetics models, and a system approach of data analysis, in vivo simultaneous processes and their interactions are understood. The techniques applied, results of their applications and system analysis of data are reported.