Deliverables
Models of forest response to future atmospheric and climatic conditions are sensitive to assumptions about the primary and secondary responses of trees to elevated CO2 concentrations, and these assumptions have necessarily been based on the short-term responses of tree seedlings and young saplings.
With the advent of free-air CO2 enrichment (FACE) systems, it is now possible to investigate responses at the scale of the forest stand and over a longer time. The POPFACE experiment in Europe together with three other forest systems in USA using FACE technologies are providing data on longer-term responses at the forest stand level. A common initiative was set up by these four FACE experiments, the first in the world on forest systems, to synthesize information on carbon assimilation, respiration, transpiration, tree growth, soil processes, and other data streams that these experiments have in common. The working group has explored the premise that there is an essential commonality in the primary responses to elevated CO2 across the four sites, and cross-site differences in the outcome of those responses can be attributed to environmental interactions or stand developmental history. The purpose of this initiative is to begin the process of synthesizing the data sets from ongoing forest FACE experiments and make them available for modelling, assessment, and a generalized understanding of the metabolism of forests in the future.
A meeting was already held to review and harmonize experimental protocols for the measuring of the principal ecosystem processes being active in the four forest systems studied under FACE conditions.
Although studied less frequently than photosynthesis, respiration plays an important role in the total carbon budget and, therefore, estimates of respiration rates are essential in productivity models. When plant respiration was assessed at the stage of fully closed canopy and on 6 to 10 m tall trees, no differences were observed on stem respiration between FACE and control treatments except for a small increase of stem CO2 efflux in the FACE treatment for P. nigra.
On the contrary, a highly significant treatment effect on soil respiration was observed during our study; annual soil respiration in the FACE treatment was stimulated by 31% to 48% in the three poplar species. It is possible that new photosynthates are the primary source for root and soil respiration; assuming this is valid for the work presented here, then increased rates of soil respiration are a reflection of increased rates of flux of labile C from the roots (exudates) and increased root and mycorrhizal respiration. From an integrated analysis of the POPFACE results it can be shown that for all species the increase in soil respiration under FACE is largely in excess of the increase in C input via fine root turnover. This would then be in agreement with the low levels of new C found in the bulk soil, in that a large part of the newly fixed photosynthates is used in root, mycorrhizal and soil respiration with only a smaller amount being incorporated as new soil C via root turnover and litterfall. This also suggests that only a low fraction of the C input in root turnover and litterfall is in a recalcitrant form, which accumulates in the soil.
Two poplar species, P. alba and P. nigra, invested a significantly greater proportion of their root biomass into deeper soil horizon under FACE treatment. Average root biomass present in the top 20 cm of soil, expressed as a percentage of total root biomass measured, decreased from about 80 to 60 %, indicating a significantly deeper allocation of root biomass as a result of exposure to elevated CO2, which might have implications on soil C cycling.
Also mycorrhizal colonisation of Populus is affected by FACE treatment; however this effect is genotype specific and differences even among such closely related genotypes occur. The colonisation of fine roots and root tips of P. alba grown under elevated CO2 treatment significantly increased both for vesicular-arbuscular mycorrhizae (VAM) and for ectomycorrhizae (ECM). P. nigra displayed a statistically significant increase only in VAM colonisation while FACE treatment did not affect mycorrhizal colonisation of P. x euramericana. When comparing FACE effect on mycorrhizal colonisation with that on fine root production, it can be hypothesised that P. alba utilised additional assimilated C to explore the soil through higher investment in its mycorrhizal symbionts. On the contrary, P. nigra and P. x euramericana satisfied their need for more nutrients by investing C in growth of fine root systems. Assessment of the impact of elevated CO2 on mycorrhizal community structure did not bring conclusive results and merits further study.
A finite-element Computational Fluid Dynamics model (CFD) was also developed to better understand the mechanisms of air-CO2 mixing in high velocity jets. Such model required the definition of a tri-dimensional domain that was designed using appropriate Computer Aided Design software (Geomesh vs 3.5, Fluent Inc.) and meshed using a tetrahedral grid.
The meshed volume provided a 3D model of the horizontal pipe and of the convergent nozzle. Pressure in the pipe and the airflow in the tri-dimensional space were variable input boundary conditions that could be adjusted prior of every simulation run. Navier-Stokes equations were solved for pressure, velocity and flow using the k-e turbulence model. When converged to an optimal solution, the model provided detailed information on a number of variables in the 3D domain, including velocity fields and vectors, CO2 concentrations, static and dynamic pressures, turbulence and energy. The CFD model of the jet was validated using the experimental data obtained in the laboratory.
The use of choked jets at sonic velocity to release CO2 has a crucial importance for the initial CO2 and air mixing. The so-called ''shock-wave effect'' of sonic jets may favour, in fact, such mixing. The existence of such an effect was clearly detectable by the direct measurement of CO2 concentration near the jet and could be also very well simulated using the CFD model. The model helped to better depict and understand the mechanism of such rapid mixing. Detailed analysis of the air movement revealed in fact that a large depression is created near the choked jet attracting a large amount of ambient air that, due to the highly turbulent regime of the flow, rapidly mixes with CO2. In practice, the model indicates that 10-times dilution of the CO2 flow is already obtained at 2mm downstream of the jet, reaching a 100-times dilution at only 30mm.
FACE stands for Free Air CO2 Enrichment. FACE technology is capable of providing a means by which the environment around growing plants may be modified to realistically simulate future concentrations of atmospheric carbon dioxide (CO2). Unlike growth chambers and greenhouses, no containment is required with FACE designs. Previously difficult-to-study natural conditions such as temperature, precipitation, pollination, wind, humidity, and sunlight are now possible. Therefore, long-term studies may be conducted. FACE field data represent plant and ecosystem responses to concentrations of atmospheric CO2 in a natural setting possible during the next century.
A new type of FACE facility, based on a different concept has been developed, tested and used within the POPFACE project. In the POPFACE system, pure carbon dioxide is released to the atmosphere through a very large number of small gas jets, at high velocity. Such innovation was introduced because the theory of fluid mechanics says that when a gas jet reaches sonic velocity, a shock wave is created at the jet outlet and the air-CO2 mixing is supposed to be greatly enhanced.
The operational system was built in three replicates and each FACE octagon enclosed an area of approximately 350 m2 with a diagonal of 22.2 meters. The size of the octagon was such to delimit a ''geometrical sweet spot'' having a circular shape and a diameter of 16m. The pipes located on the perimeter of the octagon were suspended at the height of the poplar canopy using eight telescopic poles located at the vertices of the octagon. The poles allowed easy and rapid vertical movements of the pipes to account for the rapid changes in the height of the poplars.
Long-term performances of the POPFACE system were overall satisfactory. The system was operational for about 90% of the time, with major interruptions of fumigation due to maintenance and CO2 refilling. The mean overall long-term CO2 concentrations in the centres of the three FACE plots were between 540 and 545-mmol mol-1.
FACE research technology creates 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 would encourage 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 provides 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 poplar plantation was 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 3-year long rotation cycle; height of trees at the end of first year was less than 2m; in November 2000 tree height reached maximum values of 6.0 to 7.0 m for the three poplar species producing a completely closed tree canopy whereas at the end of the third growing season the height of the canopy was almost 10 m, reaching out the maximum height allowed by the FACE infrastructure that was raised during the 3-year experiment to match the height of the experimental trees. Tree harvesting was conducted in winter 2001/02; all trees were cut at the base of the stem and a large sample was utilized for biomass analyses.
A smaller group was also excavated to measure the biomass of the various belowground tree components. This material produced under modified atmospheric conditions is valuable also for other European projects and for wood technological plants.
Wood density was not influenced by FACE treatment in all genotypes; on the contrary a significant difference was observed between genotypes. Mean values for whole stem wood ranged from 0.35 g/cm3 in P. x euramericana and 0.41 g/cm3 in P. nigra.
No significant differences were observed also in the lignin concentrations of leaf litter for any of the three Populus genotypes in response to the CO2 treatment. However, the elevated CO2 treatment significantly decreased N concentrations in both fresh leaves and leaf litter, on average by 30%. Litter decomposition was indeed affected by the elevated CO2 treatment and, independently of the species, litter generated under elevated CO2 decomposed at a lower rate than litter generated under ambient CO2, whereas litter incubated under elevated CO2 showed a higher decay rate than litter incubated under ambient CO2. P. nigra leaf litter was the most affected by the treatment, with a reduction in decay rate of 12% for the litter generated under elevated CO2 as compared to ambient, and an increase of 22% for the litter incubated under elevated CO2 as compared to the litter incubated under ambient CO2.
The results of the POPFACE experiment first suggest that the additional carbon assimilated under FACE conditions is invested in growth and not in the formation of secondary compounds like lignin; secondly, the atmospheric CO2 enrichment decreased litter decay rates by altering litter quality. Therefore, the conclusions of the present study appear to re-open the debate on the litter quality hypothesis.
Limitations to plant productivity across large areas of Europe as a consequence of altered rainfall patterns will be exacerbated in future; even in temperate northern Europe, many predictions shown that future summer rainfall will be inadequate for the maintenance of crop growth and production. Yet, virtually no data exist on the impact of future predicted changes in atmospheric CO2 on forest and tree plantation hydrological balance. Much of this research has been conducted at a very limited scale, on individual pot- or chamber -grown trees, where stomata may exert a large control over transpiration, a situation that may be far removed from reality for field-grown plantation trees across large tracts of land. Plantation forest canopies may very well be largely 'uncoupled' from the atmosphere suggesting that small reductions in stomatal conductance, under elevated CO2, may be inadequate to alter seasonal canopy water use.
To clarify these uncertainties a novel programme was initiated on IR image collection combined with a residual energy balance approach to determine canopy-scale remote estimates of transpiration waster loss. At the stand level a new remote sensing technique using IR images was developed to quantify canopy-level transpiration water fluxes from the three poplar species and in relation to canopy development, the FACE treatment and with soil moisture. Remote images were obtained with airborne flights in combination with a scanning IR camera and images digitised. The research activity was integrating data from canopy level (IR remote images) with leaf-level (stomatal conductance) and tree level data (sap flow gauges).
Water consumption and energy balance in a future world with a modified atmospheric composition is a critical issue that was examined in the POPFACE experiment.
Leaf, tree and stand-scale measurements of water loss were made over the study period utilizing a set of independent techniques, as stomatal conductance by leaf porometry, sap flow gauges and infra-red thermometry. For this mature poplar forest canopy, stomatal conductance was consistently reduced by the FACE treatment up to 25%. The consequences of this response for whole tree and canopy water loss, may, however be limited, depending on compensatory effects of FACE on leaf area development, the coupling of the canopy to the atmosphere and the importance of feedbacks.
Elevated CO2 had also a significant effect on cell size for young expanding leaves of P. x euramericana and this was correlated with stimulation in lamina area expansion rate.
Although the photosynthetic response of tree species to growth at an elevated CO2 concentration (CO2) has been widely investigated, very few studies have grown trees through to canopy closure. Critically, canopy closure increases competition for resources such as light, water and nutrients, all factors likely to modify the photosynthetic response to elevated [CO2]. This study took advantage of FACE technology to investigate the effect of elevated [CO2] on photosynthesis in three Populus clones, P. alba, P. x euramericana and P. nigra in an intensively managed agro-forestry system. Throughout the study, all three clones showed large increases in photosynthesis.
The stimulation of diurnal carbon sequestration ranged from 40% to 60%, while light saturated photosynthesis increased between 17% and 25%. To determine whether photosynthetic acclimation had occurred, the photosynthetic capacity was quantified by measuring the maximum velocity of carboxylation (Vc,max) and the maximum rate of RuBP regeneration (Jmax).
Throughout this study significant photosynthetic acclimation was infrequent, and did not increase in magnitude or occurrence during canopy closure in any of the three clones. Interestingly, some reduction in Rubisco protein appeared to occur in all three clones of poplar, however this was not important in decreasing photosynthesis at elevated [CO2]. A loss of photosynthetic capacity has been frequently correlated with an accumulation of carbohydrates. In keeping with this, no consistent increase in carbohydrate accumulation was evident.
In conclusion elevated [CO2] failed to decrease the photosynthetic capacity of any of the three Populus clones, which sustained a significant stimulation of photosynthesis throughout canopy closure.
The decomposition of plant litter is a key process in the formation of soil organic matter. The rate of accumulation of organic C in soils is a function of primary production and decomposition. Soil organic matter is a complex system, given by different fractions, each characterised by different physical and chemical properties, microbial degradability and turnover time. SOM fraction techniques do exist and are well validated, but since the soil organic C pool is large, small shifts, even if significant on the long term, might not be appreciated by conventional methodologies within the time frame of this project.
The natural 13C-labelling technique offers therefore an interesting alternative. Briefly, in plant/soil systems where the 13C signal of the C input is different from that of the native SOM, for example, where C3 plants (d13C ca. -28 permil) grow on soils derived from a C4 crop (with d13C ca. -17 permil), the relative contribution of 'new C3' and 'native C4' soil organic C can be quantified using the mass balance of stable isotope content. This project has implemented a modified isotopic technique that takes into account also C and 13C losses from the soil and accordingly allows the exact determination of even small changes in SOC stocks over relatively short times.
For this purpose large quantities of 'C4 soil' (a soil where maize was continuously cultivated for about 40 years) were used to fill numerous soil bags that were then included in the soil of the experimental FACE plots. The change of isotopic signature of these bags was then measured over the research period.
Because productivity depends primarily on leaf area and photosynthetic rates, and forest-atmosphere exchanges of energy and water occur at the leaf level, a key question in climate change research on trees is whether total leaf area per unit land area will be enhanced in a future CO2-enriched atmosphere. LAI was significantly increased by the FACE treatment in the first two years of growth because of an increased production of sylleptic branches. In the 3rd year, after canopy closure, LAI was not significantly enhanced by FACE vs. Control; it is postulated that LAI was not affected by FACE after canopy closure because FACE caused increased shading and competition resulting in enhanced leaf fall or leaf turnover.
The surplus of assimilated carbon for the tree genotypes growing in a CO2 enriched atmosphere determined a 24% mean stimulation of total biomass production, at the end of the stand rotation cycle. The increase of aboveground biomass production ranged from 15% to 27% while the effect of elevated CO2 on belowground biomass was even greater, from 22% to 38%, depending on the genotype. Biomass accumulation over the 3-year study period, under FACE conditions, ranged from 58 to 72 t ha-1 of dry matter for P. x euramericana and P. nigra respectively, while P. alba had intermediate results. This is a relevant output of the experiment that allows us to quantify the future productivity and carbon sequestration capacity of these agro-forestry plantations in an elevated CO2 world. It is remarkable that this increased biomass production was obtained because of an increased foliage efficiency.
Searching for OpenAIRE data...
There was an error trying to search data from OpenAIRE
No results available