Risultati finali
Growth of trees is largely regulated by nitrogen availability. When plants are exposed to elevated atmospheric CO2 concentrations, they often exhibit enhanced growth with increased biomass accumulation which in the long-term can result in an increased nitrogen uptake from the soil. In fact, nitrogen and carbon cycles are strictly related within plants, and nitrogen metabolism is regulated by signals that are derived from carbon metabolism.
Scientific achievements from EuroFACE experiment are as follows:
- At the end of the rotation cycle of a short rotation poplar plantation, we estimated N use by trees of three different poplar species exposed for three years to free air-CO2 enrichment (FACE) and we analysed whether this might influence the future N availability for the plantation.
- Destructive harvest at the end of the three year-rotation cycle was used to determine N concentration and N content of the long-lived (woody) tissues, whereas N uptake of fast-turnover tissues (fine roots and litter) was measured throughout the whole rotation cycle. These observations were related with previously published variations of soil N content during the same period. Moreover, we estimated retranslocation from green canopy and some processes determining N mobilization and N immobilization.
- Elevated CO2 treatment significantly increased Nitrogen Use Efficiency (NUE), i.e. NPP per unit of annual N uptake, in all species; decreased N concentration in all plant tissues, but did not change significantly the cumulative N uptake by trees at the end of the rotation cycle. Total soil N was depleted more under elevated CO2 than ambient CO2, although not significantly for all layers. Despite the differences among species were sometimes relevant, the response to elevated CO2 was similar for all species for most of the parameters analysed.
- The high rate of retranslocation under elevated CO2 in this experiment caused lower N concentration in leaf litter compared to ambient CO2. Once in the litter layer, it appeared that elevated CO2 treatment affected decay rate of such litter in two opposite modes, at least during the first stage of litter decay. While litter produced under elevated CO2 decomposed slower than litter in ambient CO2, leaf litter incubated in elevated CO2 decomposed faster. This mechanism could be also true for fine roots whose C/N was increased under elevated CO2 similarly to leaf litter.
Despite the lower N concentration, litter grown in elevated CO2 did not immobilize N differently from ambient CO2, during the first 250 days of field incubation. This appears to be a result of N limitation in soils under elevated CO2, rather than a decreased N demand by the litter.
- In conclusion, despite the higher productivity, N uptake by poplar trees did not change under elevated CO2 as compared to the ambient. As a consequence, NUE increased under elevated CO2, excluding so far the idea that additional nitrogen would be required to maintain increased yields of carbon uptake under elevated CO2 in such plantations. However, a stronger decrease of soil N was observed under elevated CO2 at the end of the first rotation cycle in comparison with ambient CO2, probably due to a decreased input by leaf litter and decreased decomposition rate.
Moreover we observed some processes indicating a trend, although not significant, of increased N immobilization under elevated CO2 which might lead to limiting conditions of N availability over a longer term period possibly influencing the productivity of such plantations over multiple rotation cycles.
The influence of elevated [CO2] on the physiology and development of plants, including forest trees, have already been intensively studied. However, the effects of CO2 enrichment on the structure and quality of plants and carbon allocation to long- and short-term carbon pools in plants are not clear yet. How does elevated [CO2] affect the wood quality and carbon allocation to long- and short-term pools in the wood of trees?
This question is particularly important because forests contain more than 90% of the carbon of earth�s living organisms. Besides the ecological importance of forests, wood is also economically important for human beings. For instance, wood from forest trees provides structural materials and energy for human beings since a long time. Wood quality is also extremely important for pulp and paper industry because fibre properties, an important aspect of wood quality, affect the quality of final products. With respect to sustainable development, wood quality is also important for forest health because carbon-based secondary compounds, the biochemical aspects of wood quality, such as lignin, phenolics and tannins, affect the ability of forest trees to resist the attack of pathogen and insects. To date only few studies addressed wood quality in response to elevated [CO2], especially to FACE.
To address the above aspects, wood quality and structure of juvenile wood obtained from the EUROFACE site were analyzed by microscopic technique and image analysis. By employment of biochemical methods and allometric relationships, carbon allocation and carbon-based secondary compounds in the wood of poplar responding to FACE and N-fertilization were intensively studied. The implications of the results for forest carbon sequestration were discussed.
To characterise wood quality in response to elevated CO2 and N-fertilization, growth and wood anatomy of three poplar clones were investigated. In the three poplar genotypes, most of the anatomical traits showed no uniform response pattern to elevated CO2 or N-fertilization. In P. × euramericana, N-fertilization resulted in significant reductions in fiber lengths. In all three genotypes, N-fertilization caused significant decreases in cell wall thickness. In P. × euramericana and P. alba, elevated CO2 also caused decreases in wall thickness, but less pronounced than nitrogen. In P. nigra and P. × euramericana, elevated CO2 induced increases in vessel diameters. The combination of elevated CO2 and N-fertilization resulted in overall losses in cell wall area of 5 12% in all three clones suggesting that in future climate scenarios, negative effects on wood quality may be anticipated.
Forest soils account for a large part of the stable carbon pool held in terrestrial ecosystems. Future levels of atmospheric CO2 are likely to increase C input into the soils through increased above and below ground production of forest ecosystems. A common response to elevated CO2 is increased allocation of assimilated C below ground. This phenomenon can result in increased belowground C inputs by a shift in C allocation between foliage and roots, increased production and turnover of fine roots, by greater proliferation of mycorrhizal symbionts or by increased root exudation. Assessment of the relative contribution of these processes to the soil C pool is ridden with technical and methodological difficulties. However, it seems that the turnover of ephemeral tissues constitutes the greatest source of C entering the soil C pool.
In EuroFACE experimental facility, elevated CO2 increased both above and belowground biomass production. This effect on root biomass, both coarse and fine, did not diminish even after 6 years of CO2 enrichment and 2 rotation periods. All Populus species have shown similar reaction to elevated CO2, indicating that this increase might be common at least within the genus Populus. From our observations in EuroFACE it appears that the biggest input of C into the soil is from fast turnover belowground biomass, such as fine roots and mycorrhizas attached to them.
Elevated CO2 increased fine root biomass by 85% in P. alba, 86% in P. nigra and 17% P. x euroamericana during second rotation. Moreover, fine root biomass is turning over faster under elevated CO2. This results in a further increased in carbon transferred into the soil through root systems, up to 210% more fine root biomass in P. alba is turned over annually in elevated CO2 compared to ambient control. It is reasonable to assume that mycorrhizal fungi, which have been found colonising fine roots of all three poplar species, will decompose in short time after root death, since fine roots constitute their only source of carbon. This increased input will result in greater sequestration of C only if the additional C enters stable soil C pools.
When assessing whether elevated CO2 increases soil C storage, alongside measuring inputs, equal importance must be given to the estimates of the amount of C leaving the soil. Forest soil respiration has been indicated as a main pathway for C leaving temperate forest ecosystems and therefore plays a major role in determining sequestration potential. Soil CO2 emanating from the soil is a product of both autotrophic and heterotrophic respiration. A likely effect of greater root systems found under elevated CO2 is an increase of autotrophic respiration. Soil CO2 efflux is controlled by diffusion gradient, an increase in CO2 concentration in soil atmosphere resulting from increased respiration will cause increased soil CO2 efflux. The other pathway for C to leave soil leaching of dissolved inorganic carbon to ground water appears to be only of minor significance. Therefore, when considering the effect of elevated CO2 on soil C storage, it is important to consider the response of C inputs and of soil CO2 efflux.
In conclusion, elevated levels of atmospheric CO2 result in increased C allocation below ground. Usually, increased belowground C allocation has been accompanied by greater soil CO2 efflux making predictions of enhanced C sequestration in elevated CO2 difficult. The results from the EuroFACE site suggest that much of the increase in C input is rapidly lost. Whether increased pool of soil C will result in increased C sequestration by temperate forests depends on convolution of various factors such as soil fertility, temperature and moisture. All these factors influence the rate and magnitude of root and microbial respiration and ultimately the fate of extra C allocated to the soil.
EuroFACE Infrastructure:
The EuroFACE infrastructure, located in central Italy (Tuscania, VT) has been completely updated and reorganised before the beginning of the fumigation operations. The poules located at the vertices of the fumigated plots have been re-built with metal frames extending up to 12 meters above the ground. Each poule has been erected over a concrete platform on ground. Signal and power cables have been carefully checked prior the beginning of the FACE operations and some previously damaged infrastructure has been replaced. In March 2003, the control units located at the centre of each ring have been re-installed and refurbished. As originally planned, the six IRGAs that are used within EuroFACE have been replaced with a new model having superior performance (PPSystem SBA-4). A new Panel-PC will be installed at the site in early September 2003.
EuroFACE operations:
FACE management was started as planned in April 2003 and maintained until present. One-minute CO2 concentration data have been acquired and stored in the local PC throughout the season with only minor interruptions due to power failures and the replacement of the data acquisition system. Spatial performance measurements are underway at present. For this, a new sonic anemometer and a fast response IRGA are being used.
CFD Modelling:
A new Computational Fluid Dynamic model has been developed and implemented to simulate gas dispersion in the EuroFACE system. The model now uses a Large Eddy Simulation approach to simulate turbulence and gas dispersion from the horizontal venting pipe. Such a model allows dynamic time-dependent simulations with time steps of 0.1 seconds which provide a very powerful tool to better understand the interaction between the weather conditions and the overall performance of the FACE system.
Understanding how forest soils will respond to elevated [CO2], in terms of C stability and sequestration potential is a must for corrected prediction of future C cycling. The expected shift in C allocation from leaves to fine roots and the increased substrate availability for microorganisms, under elevated CO2, may lead to increased microbial activity and increased soil aggregation. Aggregates physically protect organic matter against microbial decomposition, and an increase in soil aggregation under elevated CO2 is expected to enhance carbon sequestration in soil. A growing number of studies are addressing the effects of elevated CO2 on soil organic C and SOM aggregation, however, for forest ecosystems results are still controversial. Despite the increased C input to the organic layer, no changes in stabilised SOM fraction in the mineral soil were observed at an aggrading Loblolly pine plantation after 6 years of FACE exposure. In contrast, in a ten-year old Liquidambar styraciflua plantation, a significant increase of C in the microaggregate fraction of SOM, after 5 years of FACE exposure, has been reported.
At the EUROFLUX site, we performed a study aimed to determine how SOM pools and dynamics are affected: 1) by changing land use from a wheat crop to a poplar plantation; 2) comparing poplar plantations, by exposure to FACE.
In particular, the study was conducted to test the hypotheses that: 1) poplar plantation increases the aggregation of a soil previously managed to grow wheat; 2) exposure to elevated [CO2] further stimulates soil aggregation, through enhancing C input belowground; 3) in both cases, the increased C input belowground and aggregate formation, promote C sequestration into the afforested soil and does it to a larger extent under elevated [CO2]. The additional C was mainly expected to be in the physically protected microaggregate fraction. SOM was fractionated by size and density, following recently established procedures.
Soil organic C dynamics were not significantly modified by afforestation: SOM aggregation, contrary to our hypotheses, was not enhanced in the afforested soil, as compared to the agricultural soil, and only an initial trend toward a stabilization of C was observed in physically protected fractions.
When afforested soils exposed to ambient and FACE atmospheres were compared, elevated CO2 appeared to promote soil aggregate formation, with an increase of 8.8% and 18.0% being observed in the macroaggregates fraction (> 250m) of soil, under P. nigra and P. x euramericana, respectively. In the afforested FACE soils, however, a significant decrease, on average of 60%, was observed in the physically protected C of microaggregates (53-250m) and silt and clay (< 53?m) fractions, as compared to afforested control soils.
This effect was likely induced by a priming of these pools.
Our results suggest that, in order to detect how SOM dynamics might be altered in the short term by environmental changes, it is indispensable to consider different SOM pools and not the soil as a unique entity. Only by applying this study approach it was possible to underline SOM dynamics determined by this type of fast growing woody plantation.
Afforestation of cropped soils induced only a trend of increasing C that was stabilized in physical protected fractions: this suggests that, in the long term, C sequestration can be promoted in these soils. Contrarily to our hypothesis, aggregation was not enhanced by afforestation, while it was by elevated CO2, possibly as a consequence of altered rhyzosphere processes and microbial biomass. FACE soils, however, showed a reduction in total C induced primarily by a priming effect of microaggregates and mineral-associated C, thus indicating those fractions as particularly important in determining SOM response to environmental changes.
It must be remarked, however that the higher C input is promoting the stabilization of C in microaggregates protected by macroaggregates (mM) especially in soils under the most reactive Populus species (P. nigra). Even if initial conditions promotes the decomposition of stable and protected pools, it is not to be excluded that, if increased C input is sustained in FACE soils, C accrual in protected microaggregate fractions may be seen, in the long term, in afforested soils exposed to elevated CO2.
EuroFACE research infrestructure, one of the few on forest ecosystems in Europe, has created a platform for multidisciplinary, ecosystem-scale research on the effects of elevated atmospheric CO2 concentrations over extended periods of time. In doing so, a large amount of high-CO2-grown plant material can be produced, enough to support the research of many cooperating scientists. This has encouraged research by teams of investigators, who can study different aspects of an ecosystem's response to CO2 enrichment.
This concurrent use by numerous independent scientists has provided economies of scale and the potential to gain new insights into ecosystem responses that are difficult or impossible to obtain with smaller scale studies. The managed forest ecosystem, a poplar plantation, was firstly established in late spring 1999 using uniform hardwood cuttings (length 25 cm).
The entire 9-ha field was planted with Populus x euramericana genotype I-214 at a planting density of 5000 trees per ha (spacing 2 m x 1 m). The six experimental plots were planted with three different poplar genotypes at a planting density of 10000 trees per ha (spacing 1 m x 1 m) in order to have a sufficient number of experimental trees and a closed canopy after a short time. Growth of trees was rapid over the first, 3-year long, rotation cycle; at the end of the third growing season the height of the canopy was almost 10 m. Silvicultural management consisted of harvesting the poplar trees and naturally regenerating the plantation through coppicing.
A second rotation cycle was then initiated with the production of a multi-stem forest plantation. At the end of the second rotation a tree stand, 12 m high, was obtained; a second complete harvesting was implemented and the derived biomass feeded to a bio-energy plant to produce renewable, C-neutral, electrical power. The biomass produced under modified atmospheric conditions is valuable also for other European projects and for wood technological tests.
The NPP of the three-genotype stands was increased in e[CO2] as compared to control by 20 to 36%, in both successive rotations. Therefore, rising atmospheric [CO2] might further expand the potential of forest plantations for C-sequestration, in the above- and belowground biological components.
Forests account for more than 75% of the carbon stored in terrestrial ecosystems and approximately 40% of the carbon exchange between the atmosphere and the terrestrial biosphere each year. Elevated CO2 is reported to stimulate tree growth and net primary production (NPP) in intact forest ecosystems. In the EUROFACE project, the different carbon pools of above and belowground ecosystem processes were determined by the different partners during the period 2002-2004. We integrated the different pools to calculate the NPP and net ecosystem productivity (NEP). As elevated CO2 increased both above- and belowground carbon pools, and soil C content increased after an initial decline (i.e. priming effect), we hypothesize that the net carbon storage capacity of this poplar plantation may increase in a future CO2 enriched atmosphere.
From the different teams of the EUROFACE consortium, data were gathered and structured into lists of input parameters for the ANAFORE model describing the climate, the EUROFACE stand, species characteristics, and the soil under control conditions.
As the model simulates ecosystem processes on a daily bases, and integrates the pools and fluxes to a yearly scale, data collected in the field were structured on a yearly base, showing the differences between CO2 and fertilisation treatments. Preliminary runs show that the ANAFORE model is able to simulate the effects of future atmospheric CO2 levels on the above and belowground carbon partitioning in a poplar coppice plantation. However, as the data from the three growing seasons are only becoming available now after finalisation of the project, the final parameterisation is under way.
The different carbon pools in the high density stand of three poplar species were calculated using data from the different teams collected during the second rotation cycle of the EUROFACE plantation. Elevated CO2 increased the standing biomass pools, both above (stem + branch biomass) and below (coarse and fine root biomass)-ground. Fertilisation did not significantly increase biomass production indicating nutrients were sufficient. Remarkable was the strong leap elevated CO2 gave during the first year after coppice: trees showed an accelerated growth under high atmospheric CO2, but during the third growing season (2004), although the standing biomass under FACE was stimulated and significantly larger, control trees gained more biomass during that year. We postulate that the initial stimulation declined when the canopy reached a climax. This pattern was confirmed by the leaf litter production: during the first and second year, poplar trees produced more leaves under elevated CO2, resulting in a larger yearly litter production. Fertilisation did not affect the litter pool. Litter production increased after the first growing season after coppice, and stabilized by the end of the second cycle.
The fact that our climate is changing is no longer an issue of debate, and predictions of the impact of rising atmospheric CO2 levels on the stocking of carbon on both regional and global scale are crucial to take the necessary precautions and actions in future. After all, an increased stocking of carbon in terrestrial biomes (forests), above and/or belowground, may slow down the present atmospheric CO2 increase and the greenhouse effect. Since decades, the effects of elevated CO2 have been studied, known by a gradual broadening of the eco-physiological processes and species investigated, and amelioration of the technique to simulate a future high CO2 atmosphere. We are now able to study, thanks to the FACE technique (Free air Carbon dioxide Enrichment), whole ecosystems under quasi natural circumstances. Apart from the focus of many scientists to understand the CO2 effects on processes at the cellular, leaf, or tree level, it is mainly important to integrate the data to the scale of the canopy or the ecosystem, to be able to validate current ecosystem models, and extrapolate predictions to a regional or even global scale. The full carbon budget of the EUROFACE plantation, the result of 2 rotation cycles of growth under elevated CO2, is the finalization of the EUROFACE project, and will gain insight in the long term response of the carbon sequestration in a bio-energy poplar short rotation coppice.
Litter decomposition is one of the major processes determining C fluxes between the terrestrial biosphere and the atmosphere and contributing to soil organic matter (SOM) formation. Nonetheless, factors influencing the amount of C lost by decomposing litter and its partitioning into C emission to the atmosphere and to C input belowground are still not well known. Positive mutual feedbacks were found between leaf litter and rhizosphere respiration. On the other side, generally, mass loss rates for easily degradable litter tend to increase with N availability, particularly in the early stages of decomposition, while rates for lignified litter and humified organic matter tend to decline. With the aims to quantify the contribution of aboveground litter to C sequestration in soil and to test the hypothesis that elevated atmospheric CO2 and soil N availability can influence the amount and partitioning of C lost by decomposing litter, a 13C-labelled litter decomposition experiment was performed at the experimental EUROFACE site. In this site the rhizosphere activity was found to be stimulated by the enrichment in atmospheric CO2 concentration, as well as at the early stage, litter decomposition proceeded faster in CO2 enriched plots where, however, it showed sign of N limitation in the soil.
A standard P. nigra leaf litter, strongly enriched in 13C (?13C -160 0), was incubated from September 2004 to August 2005 under P. nigra trees, exposed to FACE and ambient atmosphere, and in natural and N fertilized conditions, in 3 replicates for each of the four treatment combinations. During the ten months of field incubation soil CO2 efflux and its isotopic composition was measured monthly. At harvest, litter was carefully collected from within collars, dried and weighted for determination of mass loss. Leaf litter mass loss ranged between, 62 and 97 % of the initial mass. Soil profiles under litter and control collars were sampled and divided into 4 depth intervals: 0-2, 2-5, 5-10, 10-20 cm. Soil profiles under litter show a typical trend for all treatments: going from more 13C enriched towards more 13C depleted values with depth (? in figure 8.2.1 a-d). Compared to control soil (? in figures 8.2.1 a-d), only the first two (0-2 and 2-5 cm) upper soil layers under litter were significantly enriched in 13C.
A two source mixing model was applied to quantify the litter contribution to the litter-derived C input to SOM, by depth. A significant fraction of litter derived C was only found for the first two upper soil layers. For the upper soil layer (0-2 cm) incubated with litter, the fraction of litter derived C was, on average, around 5 % of the total C in this soil layer. No significant difference was observed in the fraction of litter derived C between the four different treatments.
Percentages of C and N, as well as C/N ratios, decreased along the profile for both soil under litter and control soil. Concentrations of C in the first layer of soil incubated with litter, for all combination of treatments, are higher than the one in the respective control soil, whereas a decrease of N concentration is measured in comparison to control soil. However, differences between depths and treatments were not significant.
The fraction of litter C lost as an input into the soil were higher than the fraction released as CO2 for all the different treatments. This result, together with a very high litter mass loss, was interpreted hypothesising that litter derived C entered the soil matrix mainly as fragmented litter pieces, going to enrich the coarse particulate organic matter fraction in soils. Moreover, comparison with a previous decomposition experiment at the same site, where litter samples lost between 15 and 18 % of their initial mass, suggests how litter bags can limit decomposition by avoiding litter fragments to enter the soil matrix.
The ability of plants to regulate gas exchange through stomata allows them to control water relations and carbon assimilation and the aperture of the stomatal pore reflects a compromise between the photosynthetic requirement for carbon dioxide (CO2) and the availability of water. Consequently, the study of stomatal response to change is important. As intercellular [CO2] is a key variable sensed by guard cells and used to co-ordinate stomatal opening, many studies have been designed to quantify stomatal responses to rising atmospheric [CO2]. Since stomatal responses are involved in determining CO2-induced enhancements of photosynthesis and leaf-level water use efficiency, differential stomatal responses under elevated atmospheric [CO2] have been of interest in the study of terrestrial carbon dynamics and the focus of multi-scale investigations of transpiration and water use efficiency. Few studies, however, have been completed where longterm (years) responses of ecosystems are considered; to our knowledge, however, no-one has previously reported the progression of stomatal frequency or development in response to elevated [CO2] during canopy closure in a FACE exposure, the focus of this study.
Scientific achievements from EuroFACE experiment are as follows:
After five growing seasons, the stomatal conductance of P. · euramericana still responded to FACE whereas the frequency of stomata, measured as stomatal density and index, did not. Consequently, stomatal aperture and not stomatal number was more important in determining leaf water loss in elevated [CO2]. Reductions in stomatal index and density may be a consequence of changes in the number of stomatal complexes, changes in epidermal cell density, or a combination of both, each of which may represent a separate response (Lake et al. 2002). In addition, stomatal density may be influenced by differential epidermal cell expansion. We have previously shown that the sensitivity of epidermal cell density and expansion to elevated [CO2] was altered with both leaf and tree age in this experiment and that stomatal density and index varied significantly with leaf age (defined as position) in the Populus clones Beaupre´ and Robusta. The data here suggest a similar alteration in sensitivity with age and possibly the length of exposure to elevated [CO2]. In younger trees stomatal density and index were reduced, whereas in older trees, exposed for a longer time, there was no significant reduction or stimulation. The mechanism for stomatal response to [CO2] is not well understood but aperture, at least, responds differentially to [CO2] in epidermal peels indicating that sensitivity is not due to a mesophyll signal but is controlled at the guard cell or cuticle.
To quantify carbon allocation between short- and long-term pools in wood in response to elevated CO2 and N-fertilization, in P. nigra, carbon concentrations and stocks were quantified. Although elevated CO2, N-fertilization and season had significant tissue-specific effects on carbon partitioning to the fractions of structural carbon, soluble sugars and starch as well as to residual soluble carbon, the overall magnitude of these shifts was small. The major effect of elevated CO2 and N-fertilization was on biomass production, resulting in about 30% increases in above ground stocks of cell wall mass. Relative C-partitioning between mobile and immobile C-pools was not significantly affected by elevated CO2 or N-fertilization. These data demonstrate high metabolic flexibility of P. nigra to maintain C-homeostasis under changing environmental conditions.
To characterise secondary metabolites and internal N-pools responding to elevated CO2 and N-fertilization, carbon-based secondary compounds, concentrations of total N and Klason lignin-bound N were measured in P. nigra. Elevated CO2 had no influence on lignin, cell wall-bound phenolics and soluble condensed tannins. Higher N-supply slightly but markedly stimulated formation of carbon-based secondary compounds. Elevated CO2 decreased internal N-pools in wood, but external N-supply increased the internal N-pools. In wood, 17-26% of N was bound to Klason lignin probably forming a recalcitrant N-fraction. Neither elevated CO2 nor higher N-supply altered N-partitioning between lignin-bound N and other N-containing compounds. Positive correlations existed between the biosynthesis of proteins and secondary compounds in P. nigra. These data imply that the growth and defence of forest trees are well orchestrated.
Based on the above results, it is concluded that in future climate scenario negative effects on wood quality may be anticipated. However, non-structural carbon compounds can be utilized more rapidly for structural growth under elevated atmospheric CO2 in fertilized agro-forestry systems. The growth and defence of forest trees were homeostatically balanced even if increases in atmospheric [CO2] were accompanied by increased N availability.
A recent study of old-growth forest, suggests that there is no increase in carbon retention by the trees under elevated [CO2]. If mature natural forests are unable to accumulate more C, then this leaves plantation forests as the only option for partial mitigation of rising [CO2] by woody systems. Fast growing coppice poplar systems producing repeated crops of timber could be one system that would remove CO2 from the atmosphere at least over a period of decades, potentially slowing the rate of rise in atmospheric [CO2] and buying time for longer-term solutions. We demonstrated that in the three poplar clones grown under Free Air CO2 Enrichment (EuroFACE) revealed, over 3 seasons, a significant and large increase in light saturated photosynthesis (Asat) was sustained.
However, some transient acclimatory down-regulation of photosynthetic potential at elevated [CO2] occurred in the season following coppice, when net photosynthesis was initially no higher with growth at elevated [CO2] compared to growth at current ambient [CO2]. The lack of sustained and significant acclimation of photosynthetic potential, increased gross primary production and increased growth rates, at both leaf and canopy level, suggest that poplar trees may be able to escape long term acclimatory changes that lead to more severe down regulation of photosynthesis.
Furthemore, we examined the hypothesis whether poplars, grown throughout a complete growth cycle in the field under elevated [CO2], show evidence of biochemical acclimation in terms of accumulation of leaf carbohydrates and decreased expression of key enzymes of photosynthetic carbon metabolism.
We demonstrated that:
- Poplar trees sustain close to the predicted increase in leaf photosynthesis when grown under long term elevated CO2 concentration ([CO2]).
- Young expanding leaves of P.x euramericana become photosynthetically competent more rapidly in elevated CO2. This aspect of has not been considered in previous studies and may contribute significantly to CO2 uptake.
- No increase in the levels of soluble carbohydrates was observed, in young expanding or mature sun leaves.
- Substantial increases in starch levels were observed in the mature leaves of all three poplar genotypes
- No changes in the expression of photosynthetic Calvin cycle proteins, or in the starch biosynthetic enzyme AGPase, were observed.
- No long-term photosynthetic acclimation to CO2 occurred in these plants.
- Poplar trees are escaper from long term, acclimatory down regulation of photosynthesis through a high capacity for starch synthesis and carbon export.
These results are very significant with respect to managing carbon sequestration. They show that acclimatory loss of the initial increase in photosynthetic rate under elevated [CO2] is not inevitable at least when trees have a high genetic productivity potential and are grown with adequate nutrients and water. Plantations may therefore be developed that could show a continued stimulation of CO2 uptake in proportion to the rise in atmospheric [CO2]. Given that canopy photosynthetic carbon gain in these species is proportional to wood increment, this implies that the increased photosynthesis will result in more carbon in wood. The residence of the additional carbon in the biosphere will depend on subsequent use of the harvested wood. Importantly, and in sharp contrast to old-growth forest, these results suggest that there are management options for creating forest plantations which can achieve increased sequestration of carbon into wood as atmospheric [CO2] rises. Specifically, these results indicate that poplar species, selected for rapid growth, may be well suited to a future elevated [CO2] environment and particularly suited to afforestation projects aimed to increase carbon uptake into wood in the near-term.
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