Effects of carbon dioxide, fertilization, and irrigation on photosynthetic capacity of loblolly pine trees

Branches of nine-year-old loblolly pine trees grown in a 2 x 2 factorial combination of fertilization and irrigation were exposed for 11 months to ambient, ambient + 175, or ambient + 350 micro mol mol(-1) CO(2). Rates of light-saturated net photosynthesis (A(max)), maximum stomatal conductance to water vapor (g(max)), and foliar nitrogen concentration (% dry mass) were assessed monthly from April 1993 until September 1993 on 1992 foliage (one-year-old) and from July 1993 to March 1994 on 1993 foliage (current-year). Rates of A(max) of foliage in the ambient + 175 CO(2) treatment and ambient + 350 were 32-47 and 83-91% greater, respectively, than that of foliage in the ambient CO(2) treatment. There was a statistically significant interaction between CO(2) treatment and fertilization or irrigation treatment on A(max) on only one measurement date for each age class of foliage. Light-saturated stomatal conductance to water vapor (g(max)) was significantly affected by CO(2) treatment on only four measurement dates. Light-saturated g(max) in winter was only 42% of summer g(max) even though soil water during winter was near field capacity and evaporative demand was low. Fertilization increased foliar N concentration by 30% over the study period when averaged across CO(2) treatments. During the study period, the ambient + 350 CO(2) treatment decreased average foliar N concentration of one-year-old foliage in the control, irrigated, fertilized and irrigated + fertilized plots by 5, 6.4, 9.6 and 11%, respectively, compared with one-year-old foliage in the corresponding ambient CO(2) treatments. The percent increase in A(max) due to CO(2) enrichment was similar in all irrigation and fertilization treatments and the effect persisted throughout the 11-month study period for both one-year-old and current-year foliage.     

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

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

Atmospheric carbon dioxide concentration, nitrogen availability, temperature and the photosynthetic capacity of current-year Norway spruce shoots

Branches of field-grown Norway spruce (Picea abies (L.) Karst.) trees were exposed to either long-term ambient or to elevated CO2 concentrations ([CO2]) using the branch bag technique. The light-saturated photosynthetic rates (A(max)) of current-year shoots differing in nitrogen (N) status were measured at various temperatures and at either ambient (360 micromol mol(-1), AMB) or elevated (ambient + 350 micromol mol(-1), EL) [CO2]. The value of A(max) was determined at various intercellular [CO2]s (A/Ci curves) and used to normalize photosynthetic rates to the mean treatment C(i) values, which were 200 micromol mol(-1) (AMB) and 450 micromol mol(-1) (EL), respectively. Needle N status and temperature strongly affected A(max). The response to N increased with temperature, and the photosynthetic temperature optimum increased with N status. This was assumed to be a result of reduced mesophyll CO2 conductance. The relative increase of Amax in the EL treatment compared to the AMB treatment varied from 15 to 90%, and increased with temperature, but decreased with N status. Nevertheless, the absolute photosynthetic response to EL increased with shoot N status. The relative increase in the instantaneous response of A(max) to elevated [CO2] was about 20% higher than the long-term response, i.e., there was downward acclimation in Amax in response to elevated [CO2]. The photosynthetic temperature optimum increased 4 degrees C with either a short- or a long-term increase in [CO2]. The bag treatment itself increased A(max) by approximately 16% and the temperature optimum of A(max) by approximately 3 degrees C.     

Effect of elevated carbon dioxide concentration and root restriction on net photosynthesis, water relations and foliar carbohydrate status of loblolly pine seedlings

To determine the effects of CO(2)-enriched air and root restriction on photosynthetic capacity, we measured net photosynthetic rates of 1-year-old loblolly pine seedlings grown in 0.6-, 3.8- or 18.9-liter pots in ambient (360 micro mol mol(-1)) or 2x ambient CO(2) (720 micro mol mol(-1)) concentration for 23 weeks. We also measured needle carbohydrate concentration and water relations to determine whether feedback inhibition or water stress was responsible for any decreases in net photosynthesis. Across all treatments, carbon dioxide enrichment increased net photosynthesis by approximately 60 to 70%. Net photosynthetic rates of seedlings in the smallest pots decreased over time with the reduction occurring first in the ambient CO(2) treatment and then in the 2x ambient CO(2) treatment. Needle starch concentrations of seedlings grown in the smallest pots were two to three times greater in the 2x ambient CO(2) treatment than in the ambient CO(2) treatment, but decreased net photosynthesis was not associated with increased starch or sugar concentrations. The reduction in net photosynthesis of seedlings in small pots was correlated with decreased needle water potentials, indicating that seedlings in the small pots had restricted root systems and were unable to supply sufficient water to the shoots. We conclude that the decrease in net photosynthesis of seedlings in small pots was not the result of CO(2) enrichment or an accumulation of carbohydrates causing feedback inhibition, but was caused by water stress.     

Seasonal patterns of light-saturated photosynthesis and leaf conductance for mature and seedling Quercus rubra L. foliage: differential sensitivity to ozone exposure

Extrapolation of the effects of ozone on seedlings to large trees and forest stands is a common objective of current assessment activities, but few studies have examined whether seedlings are useful surrogates for understanding how mature trees respond to ozone. This two-year study utilized a replicated open-top chamber facility to test the effects of subambient, ambient and twice ambient ozone concentrations on light-saturated net photosynthesis (P(max)) and leaf conductance (g(l)) of leaves from mature trees and genetically related seedlings of northern red oak (Quercus rubra L.). Gas exchange measurements were collected four times during the 1992 and 1993 growing seasons. Both P(max) and g(l) of all foliage followed normal seasonal patterns of ontogeny, but mature tree foliage had greater P(max) and g(l) than seedling foliage at physiological maturity. At the end of the growing season, P(max) and g(l) of the mature tree foliage exposed to ambient ( approximately 80-100 ppm-h) and twice ambient ( approximately 150-190 ppm-h) exposures of ozone were reduced 25 and 50%, respectively, compared with the values for foliage in the subambient ozone treatment ( approximately 35 ppm-h). In seedling leaves, P(max) and g(l) were less affected by ozone exposure than in mature leaves. Extrapolations of the results of seedling exposure studies to foliar responses of mature forests without considering differences in foliar anatomy and stomatal response between juvenile and mature foliage may introduce large errors into projections of the response of mature trees to ozone.     

Effects of elevated CO(2) concentration and nutrition on net photosynthesis,stomatal conductance and needle respiration of field-grown Norway spruce trees

To study the effects of elevated CO(2) on gas exchange, nonstructural carbohydrate and nutrient concentrations in current-year foliage of 30-year-old Norway spruce (Picea abies (L.) Karst.) trees, branches were enclosed in ventilated, transparent plastic bags and flushed with ambient air (mean 370 &mgr;mol CO(2) mol(-1); control) or ambient air + 340 &mgr;mol CO(2) mol(-1) (elevated CO(2)) during two growing seasons. One branch bag was installed on each of 24 selected trees from control and fertilized plots. To reduce the effect of variation among trees, results from each treated branch were compared with those from a control branch on the same whorl of the same tree. Elevated CO(2) increased rates of light-saturated photosynthesis on average by 55% when measured at the treatment CO(2) concentration. The increase was larger in shoots with high needle nitrogen concentrations than in shoots with low needle nitrogen concentrations. However, shoots grown in elevated CO(2) showed a decrease in photosynthetic capacity compared with shoots grown in ambient CO(2). When measured at the internal CO(2) concentration of 200 &mgr;mol CO(2) mol(-1), photosynthetic rates of branches in the elevated CO(2) treatments were reduced by 8 to 32%. The elevated CO(2) treatment caused a 9 to 20% reduction in carboxylation efficiency and an 18% increase in respiration rates. In response to elevated CO(2), starch, fructose and glucose concentrations in the needles increased on average 33%, whereas concentrations of potassium, nitrogen, phosphorus, magnesium and boron decreased. Needle nitrogen concentrations explained 50-60% of the variation in photosynthesis and CO(2) acclimation was greater at low nitrogen concentrations than at high nitrogen concentrations. We conclude that the enhanced photosynthetic rates found in shoots exposed to elevated CO(2) increased carbohydrate concentrations, which may have a negative feedback on the photosynthetic apparatus and stimulate cyanide-resistant respiration. We also infer that the decrease in nutrient concentrations of needles exposed to elevated CO(2) was the result of retranslocation of nutrients to other parts of the branch or tree.     

Effects of predicted future and current atmospheric temperature and [CO2] and high and low soil moisture on gas exchange and growth of Pinus taeda seedlings at cool and warm sites in the species range

Predicted future changes in air temperature and atmospheric CO(2) concentration ([CO(2)]), coupled with altered precipitation, are expected to substantially affect tree growth. Effects on growth may vary considerably across a species range, as temperatures vary from sub-optimal to supra-optimal for growth. We performed an experiment simultaneously at two locations in the current range of loblolly pine, a cool site and a warm site, to examine the effect of future climate conditions on growth of loblolly pine seedlings in contrasting regions of the species range. At both sites 1-year-old loblolly pine seedlings were grown in current (local ambient temperature and [CO(2)]) and predicted future atmospheric conditions (ambient +2 °C temperature and 700 μmol mol(-1) [CO(2)]). Additionally, high and low soil moisture treatments were applied within each atmospheric treatment at each site by altering the amount of water provided to the seedlings. Averaged across water treatments, photosynthesis (A(net)) was 31% greater at the cool site and 34% greater at the warm site in elevated temperature and [CO(2)] compared with ambient temperature. Biomass accumulation was also stimulated by 38% at the cool site and by 24% at the warm site in that treatment. These results suggest that a temperature increase of 2 °C coupled with an increase in [CO(2)] (predicted future climate) will create conditions favorable for growth of this species. Reduced soil moisture decreased growth in both current and predicted atmospheric conditions. Biomass accumulation and A(net) were reduced by ～39 and 17%, respectively, in the low water treatment. These results suggest that any benefit of future atmospheric conditions may be negated if soil moisture is reduced by altered precipitation patterns.     

Photosynthetic capacity of red spruce during winter

We measured the photosynthetic capacity (P(max)) of plantation-grown red spruce (Picea rubens Sarg.) during two winter seasons (1993-94 and 1994-95) and monitored field photosynthesis of these trees during one winter (1993-94). We also measured P(max) for mature montane trees from January through May 1995. Changes in P(max) and field photosynthesis closely paralleled seasonal changes in outdoor air temperature. However, during thaw periods, field photosynthesis was closely correlated with multiple-day temperature regimes, whereas P(max) was closely correlated with single-day fluctuations in temperature. There was a strong association between short-term changes in ambient temperature and P(max) during the extended thaw of January 1995. Significant increases in P(max) occurred within two days of the start of this thaw. Repeated measurements of cut shoots kept indoors indicated that temperature-induced increases in P(max) can occur within 3 h. Although significant correlations between P(max) and stomatal conductance (g(s)) or intracellular CO(2) concentration (C(i)) raised the possibility that increases in P(max) resulted from increases in stomatal aperture, fluctuations in g(s) or C(i) explained little of the overall variation in P(max). Following both natural and simulated thaws, P(max) increased considerably but plateaued at only 37% of the mean photosynthetic rate reported for red spruce during the growing season. Thus, even though shoots were provided with near-optimal environmental conditions, and despite thaw-induced changes in physiology, significant limitations to winter photosynthesis remained.     

Effects of season,needle age and elevated atmospheric CO(2) on photosynthesis in Scots pine (Pinus sylvestris)

Five-year-old Scots pine (Pinus sylvestris L.) seedlings were grown in open-top chambers at ambient and elevated (ambient + 400 &mgr;mol mol(-1)) CO(2) concentrations. Net photosynthesis (A), specific leaf area (SLA) and concentrations of nitrogen (N), carbon (C), soluble sugars, starch and chlorophyll were measured in current-year and 1-year-old needles during the second year of CO(2) enrichment. The elevated CO(2) treatment stimulated photosynthetic rates when measured at the growth CO(2) concentration, but decreased photosynthetic capacity compared with the ambient CO(2) treatment. Acclimation to elevated CO(2) involved decreases in carboxylation efficiency and RuBP regeneration capacity. Compared with the ambient CO(2) treatment, elevated CO(2) reduced light-saturated photosynthesis (when measured at 350 &mgr;mol mol(-1) in both treatments) by 18 and 23% (averaged over the growing season) in current-year and 1-year-old needles, respectively. We observed significant interactive effects of CO(2) treatment, needle age and time during the growing season on photosynthesis. Large seasonal variations in photosynthetic parameters were attributed to changes in needle chemistry, needle structure and feedbacks governed by whole-plant growth dynamics. Down-regulation of photosynthesis was probably a result of reduced N concentration on an area basis, although a downward shift in the relationship between photosynthetic parameters and N was also observed.     

Short-term effects of fertilization on photosynthesis and leaf morphology of field-grown loblolly pine following long-term exposure to elevated CO(2) concentration

We examined effects of a first nitrogen (N) fertilizer application on upper-canopy needle morphology and gas exchange in approximately 20-m-tall loblolly pine (Pinus taeda L.) exposed to elevated carbon dioxide concentration ([CO(2)]) for 9 years. Duke Forest free-air CO(2) enrichment (FACE) plots were split and half of each ring fertilized with 112 kg ha(-1) elemental N applied in two applications in March and April 2005. Measurements of needle length (L), mass per unit area (LMA), N concentration (N(l)) on a mass and an area basis, light-saturated net photosynthesis per unit leaf area (A(a)) and per unit mass (A(m)), and leaf conductance (g(L)) began after the second fertilizer application in existing 1-year-old foliage (F(O)) and later in developing current-year first-flush (F(C1)) and current-year second-flush (F(C2)) foliage. Elevated [CO(2)] increased A(a) by 43 and 52% in F(O) and F(C1) foliage, respectively, but generally had no significant effect on any other parameter. Fertilization had little or no significant effect on L, LMA, A or g(L) in F(O) foliage; although N(l) was significantly higher in fertilized trees by midsummer. In contrast, fertilization resulted in large increases in L, N(l), and A in F(C1) and F(C2) foliage, increasing A(a) by about 20%. These results suggest that, although both needle age classes accumulate N following fertilization, they use it differently-current-year foliage incorporates N into photosynthetic machinery, whereas 1-year-old foliage serves as an N store. There were no significant interaction effects of elevated [CO(2)] and fertilization on A. Elevated [CO(2)] increased the intercept of the A:N(l) relationship but did not significantly affect the slope of the relationship in either foliage age class.     

Atmospheric carbon dioxide, irrigation, and fertilization effects on phenolic and nitrogen concentrations in loblolly pine (Pinus taeda) needles

Concentrations of total soluble phenolics, catechin, proanthocyanidins (PA), lignin and nitrogen (N) were measured in loblolly pine (Pinus taeda L.) needles exposed to either ambient CO(2) concentration ([CO(2)]), ambient plus 175 or ambient plus 350 micromol CO(2) mol(-1) in branch chambers for 2 years. The CO(2) treatments were superimposed on a 2 x 2 factorial combination of irrigation and fertilization treatments. In addition, we compared the effects of branch chambers and open-top chambers on needle chemistry. Proanthocyanidin and N concentrations were measured in needles from branch chambers and from trees in open-top chambers exposed concurrently for two years to either ambient [CO(2)] or ambient plus 200 micromol CO(2) mol(-1) in combination with a fertilization treatment. In the branch chambers, concentrations of total soluble phenolics in needles generally increased with needle age. Concentrations of total soluble phenolics, catechin and PA in needle extracts increased about 11% in response to the elevated [CO(2)] treatments. There were no significant treatment effects on foliar lignin concentrations. Nitrogen concentrations were about 10% lower in needles from the elevated [CO(2)] treatments than in needles from the ambient [CO(2)] treatments. Soluble phenolic and PA concentrations were higher in the control and irrigated soil treatments in about half of the comparisons; otherwise, differences were not statistically significant. Needle N concentrations increased 23% in response to fertilization. Treatment effects on PA and N concentrations were similar between branch and open-top chambers, although in this part of the study N concentrations were not significantly affected by the CO(2) treatments in either the branch or open-top chambers. We conclude that elevated [CO(2)] and low N availability affected foliar chemical composition, which could in turn affect plant-pathogen interactions, decomposition rates and mineral nutrient cycling.     

Physiological adjustment of two full-sib families of ponderosa pine to elevated CO(2)

Seeds from two full-sib families of ponderosa pine (Pinus ponderosa) with known differences in growth rates were germinated and grown in an ambient (350 micro l l(-1)) or elevated (700 micro l l(-1)) CO(2) concentration. Gas exchange at both ambient and elevated CO(2) concentrations was measured 1, 6, 39, and 112 days after the seed coat was shed. Initial stimulation of CO(2) exchange rate (CER) by elevated CO(2) was large (> 100%). On Day 1, CER of seedlings grown in elevated CO(2) and measured at ambient CO(2) was significantly lower than the CER of seedlings grown and measured at ambient CO(2), indicating physiological adjustment of the seedlings exposed to elevated CO(2). Physiological acclimation to elevated CO(2) was complete by Day 39 when there was no significant difference in CER between seedlings grown and measured at ambient CO(2) and seedlings grown and measured at elevated CO(2). After 4 months, the light response of seedlings in the two treatments was determined at both ambient and elevated CO(2). Light compensation point, CER at light saturation, and apparent quantum efficiency of seedlings grown and measured at ambient CO(2) were not significantly different from those of seedlings grown and measured at elevated CO(2). With a short-term increase in CO(2), CER at light saturation (5.16 +/- 0.52 versus 3.13 +/- 0.30 micro mol CO(2) m(-2) s(-1)) and apparent quantum efficiency (0.082 +/- 0.011 versus 0.045 +/- 0.003 micro mol CO(2) micro mol(-1) quanta) were significantly increased. Leaf C/N ratio was significantly increased in the elevated CO(2) treatment. There were few significant differences between families for any response to elevated CO(2). Under the experimental conditions, high growth rate was not correlated with a greater response to elevated CO(2).     

Elevated CO(2) concentration affects leaf photosynthesis-nitrogen relationships in Pinus taeda over nine years in FACE

To investigate whether long-term elevated carbon dioxide concentration ([CO(2)]) causes declines in photosynthetic enhancement and leaf nitrogen (N) owing to limited soil fertility, we measured photosynthesis, carboxylation capacity and area-based leaf nitrogen concentration (N(a)) in Pinus taeda L. growing in a long-term free-air CO(2) enrichment (FACE) facility at an N-limited site. We also determined how maximum rates of carboxylation (V(cmax)) and electron transport (J(max)) varied with N(a) under elevated [CO(2)]. In trees exposed to elevated [CO(2)] for 5 to 9 years, the slope of the relationship between leaf photosynthetic capacity (A(net-Ca)) and N(a) was significantly reduced by 37% in 1-year-old needles, whereas it was unaffected in current-year needles. The slope of the relationships of both V(cmax) and J(max) with N(a) decreased in 1-year-old needles after up to 9 years of growth in elevated [CO(2)], which was accompanied by a 15% reduction in N allocation to the carboxylating enzyme. Nitrogen fertilization (110 kg N ha(-1)) in the ninth year of exposure to elevated [CO(2)] restored the slopes of the relationships of V(cmax) and J(max) with N(a) to those of control trees (i.e., in ambient [CO(2)]). The J(max):V(cmax) ratio was unaffected by either [CO(2)] or N fertilization. Changes in the apparent allocation of N to photosynthetic components may be an important adjustment in pines exposed to elevated [CO(2)] on low-fertility sites. We conclude that fundamental relationships between photosynthesis or its component processes with N(a) may be altered in aging pine needles after more than 5 years of exposure to elevated atmospheric [CO(2)].     

Photosynthesis and light-use efficiency by plants in a Canadian boreal forest ecosystem

Measurements of the photosynthetic response to midsummer irradiance were made for 11 species representing the dominant trees, understory shrubs, herbaceous plants and moss species in an old black spruce (Picea mariana (Mill.) B.S.P.) boreal forest ecosystem. Maximum rates of photosynthesis per unit foliage area at saturating irradiance, A(max), were highest for aspen (Populus tremuloides Michx.), reaching 16 micromol m(-2) s(-1). For tamarack (Larix laricina (Du Roi) K. Kock) and P. mariana, Amax was only 2.6 and 1.8 micromol m(-2) s(-1), respectively. Values of A(max) for understory shrubs and herbaceous plants were clustered between 9 and 11 micromol m(-2) s(-1), whereas A(max) of feather moss (Pleurozium schreberi (Brid.) Mitt.) reached only 1.9 micromol m(-2) s(-1). No corrections were made for differences in shoot structure, but values of photosynthetic light-use efficiency were similar for most species (70-80 mmol CO2 mol(-1)); however, they were much lower for L. laricina and P. mariana (15 mmol CO2 mol(-1)) and much higher for P. schreberi (102 m;mol CO2 mol(-1)). There was a linear relationship between Amax and foliage nitrogen concentration on an area basis for the broad-leaved species in the canopy and understory, but the data for P. mariana, L. laricina and P. schreberi fell well below this line. We conclude that it is not possible to scale photosynthesis from leaves to the canopy in this ecosystem based on a single relationship between photosynthetic rate and foliage nitrogen concentration.     

Responses of loblolly pine seedlings to elevated CO(2) and fluctuating water supply

Osmotic adjustment of loblolly pine (Pinus taeda L.) seedlings to fluctuating water supply in elevated CO(2) was investigated. Seedlings were grown in controlled-environment chambers in either 350 or 700 micro l l(-1) CO(2) with weekly watering for four months, after which they were either watered weekly (well-watered treatment) or every two weeks (water-stress treatment) for 59 days. Osmotic adjustment was assessed by pressure-volume analysis of shoots and by analysis of soluble carbohydrates and free amino acids in roots during the last drying cycle. In well-watered seedlings, elevated CO(2) increased the concentration of soluble sugars in roots by 68%. Water stress reduced the soluble sugar concentration in roots of seedling growing in ambient CO(2) to 26% of that in roots of well-watered seedlings. Elevated CO(2) mitigated the water stress-induced decrease in the concentration of soluble sugars in roots. However, this was probably due, in part, to carbohydrate loading during the first four months when all seedlings were grown in the presence of a high water supply, rather than to osmotic adjustment to water stress. Water stress caused a doubling in the concentration of free primary amino acids in roots, whereas elevated CO(2) reduced primary amino acid and nitrogen concentrations to 32 and 74%, respectively, of those in roots of seedlings grown in ambient CO(2). There was no indication of large-scale osmotic adjustment to water stress or that elevated CO(2) enhanced osmotic adjustment in loblolly pine.     

Phosphorus supply affects the photosynthetic capacity of loblolly pine grown in elevated carbon dioxide

Effects of phosphorus supply and mycorrhizal status on the response of photosynthetic capacity to elevated CO(2) were investigated in loblolly pine (Pinus taeda L.) seedlings. Seedlings were grown in greenhouses maintained at either 35.5 or 71.0 Pa CO(2) in a full factorial experiment with or without mycorrhizal inoculum (Pisolithus tinctorius (Pers.) Coker & Couch) and with an adequate or a limiting supply of phosphorus. Assimilation versus internal CO(2) partial pressure (C(i)) curves were used to estimate maximum Rubisco activity (V(c,max)), electron transport mediated ribulose 1,5-bisphosphate regeneration capacity (J(max)), phosphate regeneration capacity (PiRC) and daytime respiration rates (R(d)). Nonmycorrhizal seedlings grown with limiting phosphorus had significantly reduced V(c,max) and PiRC compared to seedlings in other treatments. Elevated CO(2) increased photosynthetic capacity in nonmycorrhizal seedlings in the low phosphorus treatment by increasing PiRC, whereas it induced phosphorus limitation in mycorrhizal seedlings in the low phosphorus treatment and did not affect the photosynthetic capacity of seedlings in the high phosphorus treatment. Despite the variety of effects on photosynthetic capacity, seedlings in the elevated CO(2) treatments had higher net assimilation rates than seedlings in the ambient CO(2) treatments. We conclude that phosphorus supply affects photosynthetic capacity during long-term exposure to elevated CO(2) through effects on Rubisco activity and ribulose 1,5-bisphosphate regeneration rates.     

Vertical profiles reveal impact of ozone and temperature on carbon assimilation of Betula pendula and Populus tremula

Rising temperature and tropospheric ozone (O(3)) concentrations are likely to affect carbon assimilation processes and thus the carbon sink strength of trees. In this study, we investigated the joint action of elevated ozone and temperature on silver birch (Betula pendula) and European aspen (Populus tremula) saplings in field conditions by combining free-air ozone exposure (1.2?×?ambient) and infrared heaters (ambient +1.2 °C). At leaf level measurements, elevated ozone decreased leaf net photosynthesis (P(n)), while the response to elevated temperature was dependent on leaf position within the foliage. This indicates that leaf position has to be taken into account when leaf level data are collected and applied. The ozone effect on P(n) was partly compensated for at elevated temperature, showing an interactive effect of the treatments. In addition, the ratio of photosynthesis to stomatal conductance (P(n)/g(s) ratio) was decreased by ozone, which suggests decreasing water use efficiency. At the plant level, the increasing leaf area at elevated temperature resulted in a considerable increase in photosynthesis and growth in both species.     

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

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

Partititioning concurrent influences of nitrogen and phosphorus supply on photosynthetic model parameters of Pinus radiata

Responses of photosynthesis (A) to intercellular CO(2) concentration (C(i)) were measured in a fast- and a slow-growing clone of Pinus radiata D. Don cultivated in a greenhouse with a factorial combination of nitrogen and phosphorus supply. Stomatal limitations scaled with nitrogen and phosphorus supply as a fixed proportion of the light-saturated photosynthetic rate (18.5%) independent of clone. Photosynthetic rates at ambient CO(2) concentration were mainly in the V(cmax)-limited portion of the CO(2) response curve at low-nitrogen supply and at the transition between V(cmax) and J(max) at high-nitrogen supply. Nutrient limitations to photosynthesis were partitioned based on the ratio of foliage nitrogen to phosphorus expressed on a leaf area basis (N(a)/P(a)), by minimizing the mean square error of segmented linear models relating photosynthetic parameters (V(cmax), J(max), T(p)) to foliar nitrogen and phosphorus concentrations. A value of N(a)/P(a) equal to 23 (mole basis) was identified as the threshold separating nitrogen (N(a)/P(a) < or = 23) from phosphorus (N(a)/P(a) > 23) limitations independent of clones. On an area basis, there were significant positive linear relationships between the parameters, V(cmax), J(max), T(p) and N(a) and P(a), but only the relationships between T(p) and N(a) and P(a) differed significantly between clones. These findings suggest that, in genotypes with contrasting growth, the responses of V(cmax) and J(max) to nutrient limitation are equivalent. The relationships between the parameters V(cmax), J(max), T(p) and foliage nutrient concentration on a mass basis were unaffected by clone, because the slow-growing clone had a significantly greater leaf area to mass ratio than the fast-growing clone. These results may be useful in discriminating nitrogen-limited photosynthesis from phosphorus-limited photosynthesis.     

Elevated atmospheric CO2 concentration alters the effect of phosphate supply on growth of Japanese red pine (Pinus densiflora) seedlings

We demonstrated that the inorganic phosphate (P(i)) requirement for growth of Japanese red pine (Pinus densiflora Sieb. & Zucc.) seedlings is increased by elevated CO(2) concentration ([CO(2)]) and that responses of the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch to P(i) supply are also altered. To investigate the growth response of non-mycorrhizal seedlings to P(i) supply in elevated [CO(2)], non-mycorrhizal seedlings were grown for 73 days in ambient or elevated [CO(2)] (350 or 700 micromol mol(-1)) with nutrient solutions containing one of seven phosphate concentrations (0, 0.02, 0.04, 0.06, 0.08, 0.10 and 0.20 mM). In ambient [CO(2)], the growth response to P(i) was saturated at about 0.1 mM P(i), whereas in elevated [CO(2)], the growth response to P(i) supply did not saturate, even at the highest P(i) supply (0.2 mM), indicating that the P(i) requirement is higher in elevated [CO(2)] than in ambient [CO(2)]. The increased requirement was due mainly to an altered shoot growth response to P(i) supply. The enhanced P(i) requirement in elevated [CO(2)] was not associated with a change in photosynthetic response to P(i) or a change in leaf phosphorus (P) status. We investigated the effect of P(i) supply (0.04, 0.08 and 0.20 mM) on the ectomycorrhizal fungus P. tinctorius in mycorrhizal seedlings grown in ambient or elevated [CO(2)]. Root ergosterol concentration (an indicator of fungal biomass) decreased with increasing P(i) supply in ambient [CO(2)], but the decrease was far less in elevated [CO(2)]. In ambient [CO(2)] the ratio of extramatrical mycelium to root biomass decreased with increasing P(i) supply but did not change in elevated [CO(2)]. We conclude that, because elevated [CO(2)] increased the P(i) requirement for shoot growth, the significance of the ectomycorrhizal association was also increased in elevated [CO(2)].