Root respiration at the level of a forest stand, an important component of ecosystem carbon balan... more 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 micromol m(-2) s(-1), c. 50% higher than the mean estimation obtained with the indirect technique (1.05 micromol 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.
Eucalyptus plantations have been introduced since 1978 on savannah soils of the coastal plains of... more Eucalyptus plantations have been introduced since 1978 on savannah soils of the coastal plains of Congo, but there is still little information on the effect of silvicultural practices on soil organic carbon dynamics after afforestation on these savannahs. The objectives of this study were to assess the effects of two experimental site preparation treatments on soil CO 2 efflux, tree growth and soil carbon balance during the first year following plantation establishment. One treatment involved mechanical soil disturbance with disk harrowing (D), whereas in the second treatment (H), savannah grasses were killed by herbicide application before planting, without mechanical soil disturbance. Soil respiration and soil water content were monitored for 1 year following treatment application, at 2-week intervals. We hypothesized that mechanical soil disturbance would increase soil CO 2 efflux, but the results did not support this hypothesis. The cumulated soil CO 2 efflux over 1 year was not significantly different in the two treatments and averaged 658 g C m À2. In contrast, tree growth was significantly increased by disk harrowing, maybe as a result of decreased soil penetration resistance. Carbon inputs to the soil from savannah residues (428 g C m À2) were outweighed by the annual carbon outputs through heterotrophic respiration (505 and 456 g C m À2 in the H and D treatments, respectively) leading to a slightly negative soil carbon budget in both treatments 1 year after afforestation.
Specific leaf area (SLA; m 2 leaf kg À1 leaf) is a key ecophysiological parameter influencing lea... more Specific leaf area (SLA; m 2 leaf kg À1 leaf) is a key ecophysiological parameter influencing leaf physiology, photosynthesis, and whole plant carbon gain. Both individual tree-based models and other forest process-based models are generally highly sensitive to this parameter, but information on its temporal or within-stand variability is still scarce. In a 2-4-year-old Eucalyptus plantation in Congo, prone to seasonal drought, the within-stand and seasonal variability in SLA were investigated by means of destructive sampling carried out at 2-month intervals, over a 2-year period. Within-crown vertical gradients of SLA were small. Highly significant relationships were found between tree-average SLA (SLA t) and tree size (tree height, H t , or diameter at breast height, DBH): SLA t ranged from about 9 m 2 kg À1 for dominant trees to about 14-15 m 2 kg À1 for the smallest trees. The decrease in SLA t with increasing tree size was accurately predicted from DBH using power functions. Stand-average SLA varied by about 20% during the year, with lowest values at the end of the 5-month dry season, and highest values about 2-3 months after the onset of the wet season. Variability in leaf water status according to tree size and season is discussed as a possible determinant of both the within-stand and seasonal variations in SLA.
Land use changes such as savannah afforestation with eucalypts impact the soil carbon (C) balance... more Land use changes such as savannah afforestation with eucalypts impact the soil carbon (C) balance, therefore affecting soil CO 2 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 CO 2 , 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. Keywords Afforestation Á Eucalyptus Á Litter Á Respiration Á Soil carbon Á Dissolved organic carbon Á Land use changes Á Congo
In savannah and tropical grasslands, which account for 60% of grasslands worldwide, a large share... more In savannah and tropical grasslands, which account for 60% of grasslands worldwide, a large share of ecosystem carbon is located below ground due to high root:shoot ratios. Temporal variations in soil CO 2 efflux (R S) were investigated in a grassland of coastal Congo over two years. The objectives were (1) to identify the main factors controlling seasonal variations in R S and (2) to develop a semi-empirical model describing R S and including a heterotrophic component (R H) and an autotrophic component (R A). Plant above-ground activity was found to exert strong control over soil respiration since 71% of seasonal R S variability was explained by the quantity of photosynthetically active radiation absorbed (APAR) by the grass canopy. We tested an additive model including a parameter enabling R S partitioning into R A and R H. Assumptions underlying this model were that R A mainly depended on the amount of photosynthates allocated below ground and that microbial and root activity was mostly controlled by soil temperature and soil moisture. The model provided a reasonably good prediction of seasonal variations in R S (R 2 = 0.85) which varied between 5.4 mol m −2 s −1 in the wet season and 0.9 mol m −2 s −1 at the end of the dry season. The model was subsequently used to obtain annual estimates of R S , R A and R H. In accordance with results reported for other tropical grasslands, we estimated that R H accounted for 44% of R S , which represented a flux similar to the amount of carbon brought annually to the soil from below-ground litter production. Overall, this study opens up prospects for simulating the carbon budget of tropical grasslands on a large scale using remotely sensed data.
ABSTRACT Fast-growing plantations are increasingly being established on tropical soils, where fer... more ABSTRACT Fast-growing plantations are increasingly being established on tropical soils, where fertility is largely supported by soil organic matter (SOM) and where different management options of harvest organic residues is thought to impact the long-term sustainability of these plantations. The objectives of this study were: (1) to quantify the effect of contrasting methods of organic residue management on tree growth and soil CO2 effluxes in the first 2 years after planting and (2) to evaluate the impact of organic residue manipulations on the mineralization of soil organic matter over the length of the experiment. Three treatments were setup in 0.125 ha plots and replicated in three blocks at the harvesting of a Congolese Eucalyptus stand, resulting in an aboveground organic residue mass ranging from 0 to 6.3 kg m−2. The mineralization of SOM was deduced in each treatment by partitioning sources of soil CO2 effluxes using decomposition experiments and by upscaling specific root respiration. Soil CO2 effluxes were greatly affected by seasons and organic residue manipulation, although there were no significant changes in topsoil water content and topsoil temperature over most of the study period. Aboveground organic residue was the first contributor to soil CO2 efflux in the two treatments with a litter layer. Organic residue management did not significantly influence the mineralization of SOM in our study, probably due to the low quality of Eucalyptus litter, or to the hypothetical lack of dissolved organic carbon transfers from litter to soil. A strong relationship was found between cumulative heterotrophic CO2 efflux and tree growth, supporting the hypothesis that the early growth of Eucalyptus trees in a sandy tropical soil is largely dependent on the nutrients released by the decomposition of organic residues.
Root respiration at the level of a forest stand, an important component of ecosystem carbon balan... more 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 micromol m(-2) s(-1), c. 50% higher than the mean estimation obtained with the indirect technique (1.05 micromol 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.
Eucalyptus plantations have been introduced since 1978 on savannah soils of the coastal plains of... more Eucalyptus plantations have been introduced since 1978 on savannah soils of the coastal plains of Congo, but there is still little information on the effect of silvicultural practices on soil organic carbon dynamics after afforestation on these savannahs. The objectives of this study were to assess the effects of two experimental site preparation treatments on soil CO 2 efflux, tree growth and soil carbon balance during the first year following plantation establishment. One treatment involved mechanical soil disturbance with disk harrowing (D), whereas in the second treatment (H), savannah grasses were killed by herbicide application before planting, without mechanical soil disturbance. Soil respiration and soil water content were monitored for 1 year following treatment application, at 2-week intervals. We hypothesized that mechanical soil disturbance would increase soil CO 2 efflux, but the results did not support this hypothesis. The cumulated soil CO 2 efflux over 1 year was not significantly different in the two treatments and averaged 658 g C m À2. In contrast, tree growth was significantly increased by disk harrowing, maybe as a result of decreased soil penetration resistance. Carbon inputs to the soil from savannah residues (428 g C m À2) were outweighed by the annual carbon outputs through heterotrophic respiration (505 and 456 g C m À2 in the H and D treatments, respectively) leading to a slightly negative soil carbon budget in both treatments 1 year after afforestation.
Specific leaf area (SLA; m 2 leaf kg À1 leaf) is a key ecophysiological parameter influencing lea... more Specific leaf area (SLA; m 2 leaf kg À1 leaf) is a key ecophysiological parameter influencing leaf physiology, photosynthesis, and whole plant carbon gain. Both individual tree-based models and other forest process-based models are generally highly sensitive to this parameter, but information on its temporal or within-stand variability is still scarce. In a 2-4-year-old Eucalyptus plantation in Congo, prone to seasonal drought, the within-stand and seasonal variability in SLA were investigated by means of destructive sampling carried out at 2-month intervals, over a 2-year period. Within-crown vertical gradients of SLA were small. Highly significant relationships were found between tree-average SLA (SLA t) and tree size (tree height, H t , or diameter at breast height, DBH): SLA t ranged from about 9 m 2 kg À1 for dominant trees to about 14-15 m 2 kg À1 for the smallest trees. The decrease in SLA t with increasing tree size was accurately predicted from DBH using power functions. Stand-average SLA varied by about 20% during the year, with lowest values at the end of the 5-month dry season, and highest values about 2-3 months after the onset of the wet season. Variability in leaf water status according to tree size and season is discussed as a possible determinant of both the within-stand and seasonal variations in SLA.
Land use changes such as savannah afforestation with eucalypts impact the soil carbon (C) balance... more Land use changes such as savannah afforestation with eucalypts impact the soil carbon (C) balance, therefore affecting soil CO 2 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 CO 2 , 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. Keywords Afforestation Á Eucalyptus Á Litter Á Respiration Á Soil carbon Á Dissolved organic carbon Á Land use changes Á Congo
In savannah and tropical grasslands, which account for 60% of grasslands worldwide, a large share... more In savannah and tropical grasslands, which account for 60% of grasslands worldwide, a large share of ecosystem carbon is located below ground due to high root:shoot ratios. Temporal variations in soil CO 2 efflux (R S) were investigated in a grassland of coastal Congo over two years. The objectives were (1) to identify the main factors controlling seasonal variations in R S and (2) to develop a semi-empirical model describing R S and including a heterotrophic component (R H) and an autotrophic component (R A). Plant above-ground activity was found to exert strong control over soil respiration since 71% of seasonal R S variability was explained by the quantity of photosynthetically active radiation absorbed (APAR) by the grass canopy. We tested an additive model including a parameter enabling R S partitioning into R A and R H. Assumptions underlying this model were that R A mainly depended on the amount of photosynthates allocated below ground and that microbial and root activity was mostly controlled by soil temperature and soil moisture. The model provided a reasonably good prediction of seasonal variations in R S (R 2 = 0.85) which varied between 5.4 mol m −2 s −1 in the wet season and 0.9 mol m −2 s −1 at the end of the dry season. The model was subsequently used to obtain annual estimates of R S , R A and R H. In accordance with results reported for other tropical grasslands, we estimated that R H accounted for 44% of R S , which represented a flux similar to the amount of carbon brought annually to the soil from below-ground litter production. Overall, this study opens up prospects for simulating the carbon budget of tropical grasslands on a large scale using remotely sensed data.
ABSTRACT Fast-growing plantations are increasingly being established on tropical soils, where fer... more ABSTRACT Fast-growing plantations are increasingly being established on tropical soils, where fertility is largely supported by soil organic matter (SOM) and where different management options of harvest organic residues is thought to impact the long-term sustainability of these plantations. The objectives of this study were: (1) to quantify the effect of contrasting methods of organic residue management on tree growth and soil CO2 effluxes in the first 2 years after planting and (2) to evaluate the impact of organic residue manipulations on the mineralization of soil organic matter over the length of the experiment. Three treatments were setup in 0.125 ha plots and replicated in three blocks at the harvesting of a Congolese Eucalyptus stand, resulting in an aboveground organic residue mass ranging from 0 to 6.3 kg m−2. The mineralization of SOM was deduced in each treatment by partitioning sources of soil CO2 effluxes using decomposition experiments and by upscaling specific root respiration. Soil CO2 effluxes were greatly affected by seasons and organic residue manipulation, although there were no significant changes in topsoil water content and topsoil temperature over most of the study period. Aboveground organic residue was the first contributor to soil CO2 efflux in the two treatments with a litter layer. Organic residue management did not significantly influence the mineralization of SOM in our study, probably due to the low quality of Eucalyptus litter, or to the hypothetical lack of dissolved organic carbon transfers from litter to soil. A strong relationship was found between cumulative heterotrophic CO2 efflux and tree growth, supporting the hypothesis that the early growth of Eucalyptus trees in a sandy tropical soil is largely dependent on the nutrients released by the decomposition of organic residues.
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