Carbon dioxide enrichment accelerates the decline in nutrient status and relative growth rate of Populus tremuloides Michx. seedlings

A meta-analysis of responses of C3 plants to atmospheric CO2 : dose-response curves for 85 traits ranging from the molecular to the whole-plant level

Generalised dose-response curves are essential to understand how plants acclimate to atmospheric CO₂ . We carried out a meta-analysis of 630 experiments in which C₃ plants were experimentally grown at different [CO₂ ] under relatively benign conditions, and derived dose-response curves for 85 phenotypic traits. These curves were characterised by form, plasticity, consistency and reliability. Considered over a range of 200-1200 µmol mol^-1 CO₂ , some traits more than doubled (e.g. area-based photosynthesis; intrinsic water-use efficiency), whereas others more than halved (area-based transpiration). At current atmospheric [CO₂ ], 64% of the total stimulation in biomass over the 200-1200 µmol mol^-1 range has already been realised. We also mapped the trait responses of plants to [CO₂ ] against those we have quantified before for light intensity. For most traits, CO₂ and light responses were of similar direction. However, some traits (such as reproductive effort) only responded to light, others (such as plant height) only to [CO₂ ], and some traits (such as area-based transpiration) responded in opposite directions. This synthesis provides a comprehensive picture of plant responses to [CO₂ ] at different integration levels and offers the quantitative dose-response curves that can be used to improve global change simulation models.

Do above-ground growth dynamics of poplar change with time under CO2 enrichment?

•  In a free-air CO₂ enrichment (FACE) study, above-ground growth of Populus alba, Populus nigra and Populus×euramericana was followed continuously during the first rotation cycle of a short rotation culture (SRC) plantation to test possible changes in the response to elevated CO₂ occurring from planting until canopy closure. •  Height, stem basal area, stem volume index, branch production, and bud phenology were monitored for 3 yr. Moreover the coefficient of variation and a competition index were calculated to analyse the onset and the typology of competition. •  Volume index was higher under elevated CO₂ by 77%, 24% and 22%, as mean value for the three species, in the first, second and third years, respectively. The stimulating response, although univocal, differed in extent among species. Branch production was stimulated only in the first year, whereas bud phenology was unaffected. •  The analysis of these results show that growth was stimulated by elevated CO₂ only in the first year, although differences in volume index remained significant even in the second and third years. In the third year, under canopy closure, only competitively advantaged individuals profited by the FACE treatment.

The impact of elevated ozone and carbon dioxide on young Acer saccharum seedlings

The effects of high O3 (200 nl l-1 during the light period) and high CO2 (650 &mgr;l l-1 CO2, 24 h a day) alone and in combination were studied on 45-day-old sugar maple (Acer saccharum Marsh.) seedlings for 61 days in growth chambers. After 2 months of treatment under the environmental conditions of the experiment, sugar maple seedlings did not show a marked response to the elevated CO2 treatment: the effect of high CO2 on biomass was only detected in the leaves which developed during the treatment, and assimilation rate was not increased. Under high O3 at ambient CO2, assimilation rate at days 41 and 55 and Rubisco content at day 61 decreased in the first pair of leaves; total biomass was reduced by 43%. In these seedlings large increases (more than 2-fold) in glucose 6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) activity and in anaplerotic CO2 fixation by phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were observed, suggesting that an enhanced reducing power and carbon skeleton production was needed for detoxification and repair of oxidative damage. Under high O3 at elevated CO2, a stimulation of net CO2 assimilation was observed after 41 days but was no longer observed at day 55. However, at day 61, the total biomass was only reduced by 21% and stimulation of G6PDH and PEPC was less pronounced than under high O3 at ambient CO2. This suggests that high CO2 concentration protects, to some extent, against O3 by providing additional carbon and energy through increased net assimilation.

Water relations and gas exchange in poplar and willow under water stress and elevated atmospheric CO2

Predictions of shifts in rainfall patterns as atmospheric [CO2] increases could impact the growth of fast growing trees such as Populus spp. and Salix spp. and the interaction between elevated CO2 and water stress in these species is unknown. The objectives of this study were to characterize the responses to elevated CO2 and water stress in these two species, and to determine if elevated CO2 mitigated drought stress effects. Gas exchange, water potential components, whole plant transpiration and growth response to soil drying and recovery were assessed in hybrid poplar (clone 53-246) and willow (Salix sagitta) rooted cuttings growing in either ambient (350 &mgr;mol mol-1) or elevated (700 &mgr;mol mol-1) atmospheric CO2 concentration ([CO2]). Predawn water potential decreased with increasing water stress while midday water potentials remained unchanged (isohydric response). Turgor potentials at both predawn and midday increased in elevated [CO2], indicative of osmotic adjustment. Gas exchange was reduced by water stress while elevated [CO2] increased photosynthetic rates, reduced leaf conductance and nearly doubled instantaneous transpiration efficiency in both species. Dark respiration decreased in elevated [CO2] and water stress reduced Rd in the trees growing in ambient [CO2]. Willow had 56% lower whole plant hydraulic conductivity than poplar, and showed a 14% increase in elevated [CO2] while poplar was unresponsive. The physiological responses exhibited by poplar and willow to elevated [CO2] and water stress, singly, suggest that these species respond like other tree species. The interaction of [CO2] and water stress suggests that elevated [CO2] did mitigate the effects of water stress in willow, but not in poplar.

Elevated CO2 and ozone reduce nitrogen acquisition by Pinus halepensis from its mycorrhizal symbiont

The effects of 700 µmol mol-1 CO2 and 200 nmol mol-1 ozone on photosynthesis in Pinus halepensis seedlings and on N translocation from its mycorrhizal symbiont, Paxillus involutus, were studied under nutrient-poor conditions. After 79 days of exposure, ozone reduced and elevated CO2 increased net assimilation rate. However, the effect was dependent on daily accumulated exposure. No statistically significant differences in total plant mass accumulation were observed, although ozone-treated plants tended to be smaller. Changes in atmospheric gas concentrations induced changes in allocation of resources: under elevated ozone, shoots showed high priority over roots and had significantly elevated N concentrations. As a result of different shoot N concentration and net carbon assimilation rates, photosynthetic N use efficiency was significantly increased under elevated CO2 and decreased under ozone. The differences in photosynthesis were mirrored in the growth of the fungus in symbiosis with the pine seedlings. However, exposure to CO2 and ozone both reduced the symbiosis-mediated N uptake. The results suggest an increased carbon cost of symbiosis-mediated N uptake under elevated CO2, while under ozone, plant N acquisition is preferentially shifted towards increased root uptake.

The likely impact of rising atmospheric CO2 on natural and managed Populus: a literature review

Because of their prominent role in global biomass productivity, as well as their complex structure and function, forests and tree species deserve particular attention in studies on the likely impact of elevated atmospheric CO2 on terrestrial vegetation. Poplar (Populus) has proven to be an interesting study object due to its fast response to a changing environment, and the growing importance of managed forests in the carbon balance. Results of both chamber and field experiments with different poplar species and hybrids are reviewed in this contribution. Despite the variability between experiments and species, and the remaining uncertainty over the long term, poplar is likely to profit from a rising atmospheric CO2 concentration with a mean biomass stimulation of 33%. Environmental conditions and pollutants (e.g. O3) may counteract this stimulation but with managed plantations, environmental constraints might not occur. The predicted responses of poplar to rising atmospheric CO2 have implications for future forest management and the expected forest carbon sequestration.

The influence of CO2 and O3 , singly and in combination, on gas exchange, growth and nutrient status of radish (Raphanus sativus L.)

Five days after emergence radish (Raphanus sativus L. ev. Cherry Belle) plants were transferred to a phytotron at the GSF München, where they were exposed in four large controlled climate chambers to two atmospheric concentrations of CO₂ , ('ambient', daily means of ∼ 385 μmol^-1 ; elevated, daily means of ∼ 765 μmol mol^-1 ) and two O₃ regimes ('non-polluted' air, 24 h mean of 20 nmol mol^-1 ; polluted air, 24 h mean of 73 nmol mol^-1 ). Leaf gas-exchange measurements were made at intervals, and visible O₃ damage, effects on growth, dry matter partitioning and mineral composition were assessed at a final whole-plant harvest after 27 d. In 'non-polluted air' CO₂ enrichment resulted in a progressive stimulation in A_sat , whilst there was a decline in g which decreased E (i.e. improved WUE_i ). The extra carbon fixed in elevated CO₂ stimulated growth of the root (+ hypocotyl) by 43 %, but there was no significant effect on shoot growth or leaf area. Moreover, a decline in SLA and LAR in CO₂ -enriched plants suggested that less dry matter was invested in leaf area expansion. Tissue concentrations of N, S, P, Mg and Ca were lower (particularly in the root + hypocotyl) in elevated CO₂ , indicating that total uptake of these nutrients was not affected by CO₂ , and there was an increase in the C:N ratio in root (+ hypocotyl) tissue. In contrast, O₃ depressed A_sat , (∼ 26%) and induced slight stomatal closure, with the result that WUE, declined. All plants exposed to 'polluted' air developed typical visible symptoms of O₃ injury, and effects on carbon assimilation were reflected in reduced growth, with shoot growth maintained at the expense of the root. In addition, O₃ increased the P and K concentration in shoot and root (+ hypocotyl) tissue, indicating enhanced uptake of these nutrients from the growth medium. However, there was no affect of O₃ on tissue concentrations of N, S, Mg and Ca. Interactions between the gases were complex, and often subtle. In general, elevated CO₂ counteracted (at least in part) the detrimental effects of phytotoxic concentrations of O₃ , whilst conversely, O₃ reduced the impact of elevated CO₂ . Moreover, there were indications that cumulative changes in source: sink relations in O₃ -exposed plants may limit plant response to CO₂ -enrichment to an even greater extent in the long-term. The future ecological significance of interactions between CO₂ and O₃ are discussed.