Thongo M'Bou et al., 2010. Growth and maintenance respiration of roots of clonal Eucalyptus cuttings: scaling to stand-level

Thongo M'Bou, A., Saint-Andre, L., Grandcourt, A.s., Nouvellon, Y., Jourdan, C., Mialoundama, F.l., Epron, D., 2010. Growth and maintenance respiration of roots of clonal Eucalyptus cuttings: scaling to stand-level. Plant Soil 332, 41-53.

Abstract

Root respiration consumes an important part of the daily assimilated carbon but the magnitude of this component of forest net ecosystem exchange and its partitioning among the different energy demanding processes in roots are still poorly documented. 5-month old Eucalyptus cuttings were grown in a greenhouse in pot filled with coarse sand. They were fertilized with three different amounts of a slow-release fertilizer with the doses of 8, 24 and 48 g of nitrogen per plant. Root respiration was measured using an infrared gas analyser by perfusing air through the pot on 9 plants per treatment on three dates 14 days apart. Measure of root respiration of the three treatments over time was made in order to obtain a large range of growth and nutrient uptake. Root respiration normalized at 22°C ranged from 0.09 to 0.23 g_C d⁻¹ for the three treatments during all the experiment. It was well predicted with a model that includes root growth rate and root nitrogen content.The nitrogen related maintenance coefficient was negatively correlated to the root nitrogen concentration suggesting a decrease in protein turnover with increasing fertility. Growth rate of fine root in a virtual stand was simulated using age-related allometric equations and further used to estimate root respiration in the field. Simulated root respiration increased over time from 0.39 to 3.14 g_C m⁻² d⁻¹ between 6 and 126 months assuming a turnover of 2 yr⁻¹ for fine roots. The major fraction of simulated root respiration in the field (78–92%) was used for the maintenance of the existing biomass.

Epron et al., 2009. Soil carbon dynamics following afforestation of a tropical savannah with Eucalyptus in Congo.

Epron, D. et al., 2009. Soil carbon dynamics following afforestation of a tropical savannah with Eucalyptus in Congo. Plant Soil 323, 309-322.

Abstract

Soil organic matter is a key factor in the global carbon cycle, but the magnitude and the direction of the change in soil carbon after afforestation with Eucalyptus in the tropics is still a matter of controversy. The objective of this work was to understand the dynamics of soil carbon in intensively managed Eucalyptus plantations after the afforestation of a native savannah. The isotopic composition (δ) of soil carbon (C) and soil CO₂ efflux (F) were measured on a four-age chronosequence of Eucalyptus and on an adjacent savannah. δ _F was used to partition F between a C3 component and a C4 component, the latter corresponding to the decomposition of a labile pool of savannah-derived soil carbon (C _SL). The mean residence time of C_SL was 4.6 years. This further allowed us to partition the savannah-derived soil carbon into a labile and a stable (C _SS) carbon pool. C _SL accounted for 30% of soil carbon in the top soil of the savannah (0–5 cm), and only 12% when the entire 0–45 cm soil layer was considered. The decrease in C _SL with time after plantation was more than compensated by an increase in Eucalyptus-derived carbon, and half of the newly incorporated Eucalyptus-derived carbon in the top soil was associated with the clay and fine silt fractions in the 14-year-old. stand. Increment in soil carbon after afforestation of tropical savannah with Eucalyptus is therefore expected despite a rapid disappearance of the labile savannah-derived carbon because a large fraction of savannah-derived carbon is stable.

Marsden et al., 2008. Relating coarse root respiration to root diameter in clonal Eucalyptus stands in the Republic of the Congo.

Marsden, C., Nouvellon, Y., Epron, D., 2008. Relating coarse root respiration to root diameter in clonal Eucalyptus stands in the Republic of the Congo. Tree Physiology 28, 1245-1254.

Abstract

Root respiration is an important component of the carbon balance of a forest ecosystem. We measured CO₂ efflux of excised fine roots and intact coarse roots in 3-, 4- and 13-year-old Eucalyptus stands in the region of Pointe-Noire, Republic of the Congo. A transportable and adaptable closed chamber gas exchange system directly measured CO₂ efflux of roots from 0.5 to 32 mm in diameter. Fluxes were corrected for measurement system leaks and normalized to a reference temperature of 30 °C. Mean fine root respiration rates at the reference temperature varied between 8.5 and 10.8 μmol CO₂ kg⁻¹ s⁻¹ depending on the stand. Coarse root respiration was strongly negatively correlated to root diameter. We propose a model based on a radial gradient of respiratory activity within the root to simulate the exponential decrease in respiration with diameter. Although many sources of uncertainty in the measurements remain, as discussed in this paper, these results provide a basis for scaling up organ-level root respiration measurements to the tree and stand levels.

Marsden et al., 2008. Two independent estimations of stand-level root respiration on clonal Eucalyptus stands in Congo: up scaling of direct measurements on roots versus the trenched-plot technique.

Marsden, C., Nouvellon, Y., M’Bou, A.T., Saint-Andre, L., Jourdan, C., Kinana, A., Epron, D., 2008. Two independent estimations of stand-level root respiration on clonal Eucalyptus stands in Congo: up scaling of direct measurements on roots versus the trenched-plot technique. New Phytol. 177, 676-687.

Summary

• • 
  Root respiration at the level of a forest stand, an important component of ecosystem carbon balance, has been estimated in the past using various methods, most of them being indirect and relying on soil respiration measurements.
• •
  On a 3-yr-old Eucalyptus stand in Congo-Brazzaville, a method involving the upscaling of direct measurements made on roots in situ, was compared with an independent approach using soil respiration measurements conducted on control and trenched plots (i.e. without living roots). The first estimation was based on the knowledge of root-diameter distribution and on a relationship between root diameter and specific respiration rates.
• •
  The direct technique involving the upscaling of direct measurements on roots resulted in an estimation of 1.53 µmol m⁻² s⁻¹, c. 50% higher than the mean estimation obtained with the indirect technique (1.05 µmol m⁻² s⁻¹).
• • 
  Monte-Carlo simulations showed that the results carried high uncertainty, but this uncertainty was no higher for the direct method than for the trenched-plot method. The reduction of the uncertainties on upscaled results requires more extensive knowledge of temperature sensitivity and more confidence and precision on the respiration rates and biomasses of fine roots.

Epron et al., 2006. Soil carbon balance in a clonal Eucalyptus plantation in Congo: effects of logging on carbon inputs and soil CO2 efflux.

Epron, D., Nouvellon, Y., Deleporte, P., Ifo, S., Kazotti, G.U.Y., Thongo M'Bou, A., Mouvondy, W., Andre, L.S., Roupsard, O., Jourdan, C., Hamel, O., 2006. Soil carbon balance in a clonal Eucalyptus plantation in Congo: effects of logging on carbon inputs and soil CO2 efflux. Global Change Biol. 12, 1021-1031.

Abstract

Soil CO₂ efflux was measured in clear-cut and intact plots in order to quantify the impact of harvest on soil respiration in an intensively managed Eucalyptus plantation, and to evaluate the increase in heterotrophic component of soil respiration because of the decomposition of harvest residues. Soil CO₂ effluxes showed a pronounced seasonal trend, which was well related to the pattern of precipitation and soil water content and were always significantly lower in the clear-cut plots than in the intact plots. On an annual basis, soil respiration represented 1.57 and 0.91 kg_C m⁻² yr⁻¹ in intact and clear-cut plots, respectively. During the first year following harvest, residues have lost 0.79 kg_C m⁻² yr⁻¹. Our estimate of heterotrophic respiration was calculated assuming that it was similar to soil respiration in the clear-cut area except that the decomposition of residues did not occur, and it was further corrected for differences in soil water content between intact and clear-cut plots and for the cessation of leaf and fine root turnover in clear cut. Heterotrophic respiration in clear-cut plots was estimated at 1.18 kg_C m⁻² yr⁻¹ whereas it was only 0.65 kg_C m⁻² yr⁻¹ in intact plots (41% of soil respiration). Assumptions and uncertainties with these calculations are discussed.

Nouvellon et al. 2012. Age-related changes in litter inputs explain annual trends in soil CO2 effluxes over a full Eucalyptus rotation after afforestation of a tropical savannah

Nouvellon, Yann, et al. 2012. Age-related changes in litter inputs explain annual trends in soil CO2 effluxes over a full Eucalyptus rotation after afforestation of a tropical savannah. Biogeochemistry 111.1-3 (2012): 515-533.

Abstract

Land use changes such as savannah afforestation with eucalypts impact the soil carbon (C) balance, therefore affecting soil CO₂ efflux (F _s ), a major flux in the global C cycle. We tested the hypothesis that F _s increases with stand age after afforestation, due to an increasing input of fresh organic matter to the forest floor. In a Eucalyptus plantation established on coastal savannahs in Congo, bimonthly measurements of F _s were carried out for 1 year on three adjacent stands aged 0.9, 4.4 and 13.7 years and presenting similar growth patterns. Litterfall and litter accumulation on the forest floor were quantified over a chronosequence. Equations were derived to estimate the contribution of litter decomposition to F _s throughout the rotation. Litterfall increased with stand age after savannah afforestation. F _s , that was strongly correlated on a seasonal basis with soil water content (SWC) in all stands, decreased between ages 0.9 year and 4.4 years due to savannah residue depletion, and increased between ages 4.4 years and 13.7 years, mainly because of an increasing amount of decomposing eucalypt litter. The aboveground litter layer therefore appeared as a major source of CO₂, whose contribution to F _s in old stands was estimated to be about four times higher than that of the eucalypt-derived soil organic C pool. The high litter contribution to F _s in older stands might explain why 13.7 years-old stand F _s was limited by moisture all year round whereas SWC did not limit F _s for large parts of the year in the youngest stands.

Epron et al., 2013. Partitioning of net primary production in Eucalyptus and Acacia stands and in mixed-species plantations: Two case-studies in contrasting tropical environments.

Epron, D., Nouvellon, Y., Mareschal, L., Moreira, R.M.e., Koutika, L.-S., Geneste, B., Delgado-Rojas, J.S., Laclau, J.-P., Sola, G., Gonçalves, J.L.d.M., Bouillet, J.-P., 2013. Partitioning of net primary production in Eucalyptus and Acacia stands and in mixed-species plantations: Two case-studies in contrasting tropical environments. For. Ecol. Manage. 301, 102-111.

Abstract

The introduction of nitrogen fixing species (NFS) in fast-growing tree plantations is an alternative option to reduce fertilizer inputs. However, the success of mixed-species plantations depends on the balance between positive interactions among species (resulting from facilitation and/or complementarity) and the negative effects of interspecific competition.

Using a carbon budget approach and coupling measurements of standing biomass, aboveground litterfall and soil CO₂ efflux, we assessed the influence of replacing half of eucalypt trees by Acacia mangium on total belowground carbon flux (TBCF), net primary production (NPP) and its partitioning between above- and belowground growth at two tropical sites in Brazil (Itatinga) and in Congo (Kissoko) exhibiting contrasting climates, edaphic conditions and wood productions.

Annual soil CO₂ efflux (F_S) was significantly lower in the acacia monocultures than in eucalypt monocultures and mixed-species stands at both sites. Annual F_S was significantly lower at Itatinga compared to Kissoko for all stands while TBCF was significantly lower in the eucalypt stands only. In the eucalypt monocultures we found a significantly lower aboveground NPP (ANPP) and wood production (wood NPP) at Kissoko compared to Itatinga that was almost fully balanced by a significantly higher belowground NPP (BNPP), leading to similar NPP. Similarly, acacia monocultures exhibited significantly higher ANPP and wood NPP at Itatinga than at Kissoko. The mixed-species stands exhibited a significantly lower wood NPP and ANPP than the eucalypt monocultures at the Brazilian site while NPP of the mixture was not significantly different than the average NPP of the two monocultures. At the Congolese site, NPP of the mixture was significantly higher than the average NPP of the two monocultures. NPP was similar in the mixed-species stand and the eucalypt monoculture with a significantly lower partitioning of NPP to belowground production, leading to a one third higher wood biomass at harvest in the mixed-species stand.

A positive effect of growing eucalypts with the nitrogen fixing acacia trees on stand wood production occurred at Kissoko but not at Itatinga. Mixed-species plantations with NFS can be advocated at sites where the productive gains resulting from nitrogen fixation are not compromised by other resource limitations.