Impacts of afforestation and silviculture on the soil C balance of tropical tree plantations: belowground C allocation, soil CO2 efflux and C accretion

Tropical forest plantations will provide a large part of the global wood supply which is anticipated to increase sharply in the next decades, becoming a valuable source of income in many countries, where they also contribute to land use changes that impact the global carbon (C) cycle. Tropical forest plantations established on previous grasslands are potential C sinks offsetting anthropogenic CO2 emissions. When they are managed on short rotations, the aboveground biomass is frequently removed and transformed into wood products with short lifetimes. The soil is thus the only compartment for durable C sequestration. The soil C budget results from the inputs of C from litterfall, root turnover and residues left at logging stage, balanced by C losses through heterotrophic respiration and leaching of organic C with water flow. Intensive researches have been conducted these last ten years in eucalypt plantations in the Congo on the effects of management options on soil fertility improvement and C sequestration. Our aim is to review important results regarding belowground C allocation, soil CO2 efflux and C accretion in relation to management options. We will specifically address (i) the soil C dynamics after afforestation of a tropical savannah, (ii) the impact of post-harvest residue management, and (iii) the beneficial effect of introducing nitrogen fixing species for C sequestration. Our results on afforestation of previous savannah showed that mechanical soil disturbance for site preparation had no effect on soil CO2 efflux and soil C balance. Soil C increased after afforestation despite a rapid disappearance of the labile savannah-derived C because a large fraction of savannah-derived C is stable and the aboveground litter layer is as the major source of CO2 contributing to soil CO2 efflux. We further demonstrated that the C stock in and on the soil slightly increased after each rotation when large amounts of residues are left at logging stage and that most of eucalypt-derived C is recovered in the fine particulate organic matter fraction (0.25-0.05 mm) and the organo-mineral fraction (< 0.05 mm). While the early tree growth is related to the heterotrophic component of soil CO2 efflux, thus largely dependent on the nutrients released by the decomposition of organic residues left at harvest, the stabilization of the old soil organic C derived from the savannah may depends on the amount of organic residues left at harvest. A greater C accumulation was observed in the soil when eucalypts were grown in mixture with a nitrogen fixing tree despite similar aboveground litter fall and lower fine root biomass. A slowdown of C turnover related to N enrichment might thus be postulated in nitrogen-poor tropical soils, and mixed-species plantation with nitrogen fixing trees might be an important strategy of reforestation or afforestation to offset C emissions

Etude de la végétation naturelle de la zone cotonnière ouest du Burkina Faso à partir des données spot

Au Burkina Faso, diverses pressions entraînent une dégradation importante des milieux naturels et du patrimoine foncier. La télédétection, qui permet d'avoir une vue globale et répétitive, peut fournir des informations interessantes pour suivre l'état de l'environnement, comprendre et analyser les mécanismes des divers changements et orienter les décideurs dans leurs actions de développement, de gestion et de planification. Notre objectif a été d'étudier les potentialités des images SPOT prises à différentes périodes pour l'identification et la cartographie de la végétation naturelle de la zone cotonnière Ouest du Burkina Faso. Les types de végétation à délimiter ont été définis selon les classifications de Y agambi et de l'UNESCO. Puis des segments aériens photo-interprètés ont servi de base pour l'analyse visuelle et numérique d'images SPOT prises en février, juin et septembre. Ces analyses ont permis d'identifier les thèmes distingables pour chaque date et de mettre en évidence les sources de confusion entre thèmes. Elles ont montré que les images de février et juin présentaient peu d'intérêt. En revanche, l'image de septembre s'avère intéressante pour la discrimination des différents thèmes et leur cartographie. De meilleurs résultats sont encore obtenus en combinant les images de septembre de deux années consécutives. La classification dirigée effectuée à partir de cette combinaison montre que la végétation naturelle de cette région est principalement constituée de savanes arbustives ou arborées. Les savanes boisées sont également bien représentées et le taux d'occupation par les sols nus est loin d'être négligeable. (Résumé d'auteur

Light use efficiency and productivity of 16 genotypes of Eucalyptus along a 6-year rotation in Brazil

Forest stemwood productivity depends on the amount of absorbed light and on the Light Use Efficiency (LUE), i.e. the amount of stemwood produced per amount of absorbed light. Other growth limiting factors than absorbed light are for instance water and nutrients, carbohydrates allocation processes, and management practices. In fertilized eucalyptus plantations in Brazil, a shift in the main factor limiting growth is expected, from light at the beginning of the rotation (not limited by water because soil has been recharged during the interval between rotation) to other factors such as water in the subsequent years. Changes of allocation patterns and foliar traits also occur along the rotation. These trends may differ between genotypes. These hypotheses were tested along a 6-year rotation, with 16 contrasted genotypes planted in 10 randomized repetitions in São Paulo Region, Brazil. Absorbed light was estimated using the MAESTRA 3D model precisely parameterized at tree scale for each plot. Stand growth was computed based on allometric relationships calibrated on regular destructive biomass measurements. Results at stand scale showed that 1) LUE increased with stand age for all genotypes, from 0.15 to 0.70 gdry_matter/MJ on average; 2) light was the main limiting factor during the first year of growth (R2 between 0.5 and 0.95). Subsequently, the variability of wood production explained by light was variable among genotypes (R2 of 0.25 on average; 3) The effect of genotype on stemwood production remains high and significant along the rotation. These results and their implications for plantation management are discussed

Vegetation - atmosphere interactions – the crucial role of tropospheric ozone

Vegetation is the dominant source of biogenic volatile organic compounds (bVOCs). On a global scale, the source strengths of bVOCs exceed those of anthropogenic VOCs (aVOCs) by an order of magnitude. Due to their high reactivity, VOCs play important roles in determining atmospheric processes such as secondary organic aerosol (SOA), or when VOCs are in the presence of anthropogenic nitrogen oxides (NOx), they increase ozone formation and alter the concentrations of hydroxyl radicals, the main atmospheric oxidant. Thus, in changing the oxidative capacity of the troposphere, VOCs influence the local and regional air composition through altering the chemical lifetime of reactive gases with substantial impacts on vegetation and climate. Biogenic emissions from vegetation are species-specific and the terpenoids isoprene and monoterpenes normally dominate the overall bVOC profile of woody plants. Isoprene and monoterpenes are predominantly emitted in a 'constitutive' manner as a function of light, temperature, and seasonality. In addition to 'constitutive' emissions, significant quantities of 'stress-induced' BVOCs can be emitted into the atmosphere following abiotic (e.g., ozone) and/or biotic (e.g. herbivores) stresses. For instance, some monoterpenoids, the classes of sesquiterpenoids, benzenoids, and volatile lipoxygenase products (so called 'green leaf volatiles') are typically induced and emitted from green foliage after exposure to ozone or herbivores. However, despite the potential of terpenoids and benzoids to influence ozone and SOA formation, stress-induced bVOC fluxes are rarely considered in the context of atmospheric chemistry. In particular the net effect of multiple stress factors, which frequently co-occur in nature, on stressinduced BVOC emission remains still poorly understood. Besides it's release of VOCs, vegetation also represents a major sink of atmospheric VOC oxidation products, i.e. carbonyls and ketones (i.e. methyl vinyl ketone) challenging the plants' defense system. Thus in anthropogenically polluted urban and suburban areas, the vegetation can suffer twice - from the deposition of ozone and from VOC breakdown products generated during ozone formation processes. The presentation will give an overview on the complexity of biosphere-atmosphere interactions and will highlight future research goals and possible strategies to mitigate harmful atmospheric feedbacks on vegetation. (Résumé d'auteur

Competition for light and light use efficiency for Acacia mangium and Eucalyptus grandis trees in mono-specific and mixed-species plantations in Brazil

Mixed plantations with N-fixing species might be an attractive option for limiting the use of fertilizer in highly productive Eucalyptus plantations. A randomized block design was set up in southern Brazil, including a replacement series and an additive series design, as well as a nitrogen fertilization treatment, and conducted during a full 6 years rotation. The gradient of competition between Eucalyptus and Acacia in this design resulted in very different conditions of growth of Acacia, from totally dominated up to dominant canopies. We used the MAESTRA model to estimate the amount of absorbed photosynthetically active radiation (APAR) at tree level. This model requires the description of the scene and distinct structural variables of the two species, and their evolution with time. The competition for light is analysed by comparing the inter-specific values of APAR during a period of 2 years at the end of the rotation. APAR is further compared to the measured increment in stem wood biomass of the tree, and their ratio is an estimation of the light use efficiency for stemwood production at tree-scale. Variability of these LUE are analysed in respect to the species, the size of the tree, and at plot scale (competition level). Stemwood production was 3400, 3900 and 2400 gDM/m2 while APAR was 1640, 2280 and 2900 MJ/y for the pure Eucalyptus, pure Acacia and 50/50 mixed plantation, respectively, for an average LAI of 3.7, 3.3 and 4.5, respectively. Individual LUE for stemwood was estimated at an average value of 1.72 and 1.41 gDM/MJ/tree for Eucalyptus and Acacia, respectively, and at 0.92 and 0.40 gDM/MJ/tree when they were planted in mixed 50/50 plantations. LUE was highly dependant on tree size for both species. At the plot scale, LUE for stemwood were 2.1 gDM/MJ and 1.75 for Eucalyptus and Acacias, respectively, and 0.85 for the mixed 50/50 plantation. These results suggest that the mixed 50/50 plantation, which absorbed a higher amount of light, produce less stemwood since half of the canopy (Acacias) are dominated, and the other half does not benefit much in terms of tree growth compared to absorbed light. The eventual benefit of the nitrogen-fixing species is not visible in the mixture with 50% of each species. More attention has to be paid to introducing acacias in an additive series with the same density of eucalyptus trees as in the monospecific stands

Combining ecosystem modelling and remote-sensing to establish the spatial and temporal dynamics of the carbon budget of fast-growing tropical Eucalyptus plantations

In the aim of estimating regional-scale carbon budgets of Eucalyptus plantations, the G'Day ecosystem model was combined with remotely-sensed estimates of the daily fraction of absorbed photosynthetically active radiation (fAPAR). (Résumé d'auteur

Productivity and carbon allocation in monospecific and mixed-species plantations of Eucalyptus grandis and Acacia mangium in Brazil

Nitrogen fertilizer inputs are required in fast growing eucalypt plantations to meet tree requirements, and to compensate for the large nitrogen outputs associated with wood exportation at the end of the short rotations. Due to the economic and potential environmental cost of fertilizers, mixed-species plantations (MSP) with N-fixing species (NFS) such as Acacia sp. might be an attractive option to improve the long-term soil N (and possibly soil carbon) status. In such MSP, increases in N availability may influence the productivity and C partitioning of the non-N fixing species. To investigate the effects of NFS on nutrient cycling, wood production, C sequestration, and soil fertility, a randomized block design including monocultures of Eucalyptus grandis (100%E) and Acacia mangium (100%A), and mixtures of these species (50%E:50%A) was set up in southern Brazil. Our specific goals in the present study were to compare the production and C allocation patterns of these plantations, during the two last years of the 6-yr rotation. We hypothesized that 1) a large part of the differences in wood production between monospecific stands would be explained by differences in C allocation; and 2) the C allocation patterns of each species would be strongly modified in mixed- species plantations compared to mono-specific plantations due to inter-specific interactions and shifts in soil N status. Biomass increase (growth, G) in the different plant compartments was assessed by means of inventories and allometric relationships. Total aboveground net primary productivity (ANPP), and the productivity of each aboveground plant compartment were estimated from measurements of G and litterfall (L) (ANPP=G L). Total belowground C allocations (TBCA) were estimated using a mass-balance approach as soil CO2 efflux C minus the C input from aboveground litter plus changes in the C stored in roots, in the forest floor litter layer, and in soil. Over this first rotation, mixing NFS with eucalypt did not increase wood production: at the end of the 6 yr-old rotation, total aboveground biomass was the highest in the 100%E stands (68.2 tC/ha), lowest in the MSP 50%E:50%A (62.0 tC/ha), and intermediary in the 100%A (66.0 tC/ha). Although 100%E stands had a stronger growth than 100%A during the first 4 yrs of the rotation, the reverse was observed at the end of the rotation: during the two last yrs, total growth was 15.9 tC/ha/yr for 100%A, and 12.7 and 10.4 tC/ha/yr for 100%E and 50�0%A, respectively. These differences in growth were explained by differences in ANPP (19.2, 17.8 and 15.2 tC/ha/yr, for 100%A, 100%E, and 50�0%A, respectively), and differences in the ratio litter production/ANPP (0.17, 0.29, and 0.31 for 100%A, 100%E, and 50�0%E, respectively). Furthermore, the ratio TBCA/ANPP was the lowest in 100%A, and the highest in the MSPs (0.44, 0.62, and 0.78, for 100%A, 100%E, and 50,�0%E, respectively). These results suggest that inter-specific interactions have a strong effect on the C allocation pattern observed at the stand level in MSPs

Calibration of home-made heat dissipation probes for a full rotation of Eucalyptus grandis trees in Brazil

With the aim of proposing an appropriate calibration equation that could be used for Eucalyptus grandis of any size over a rotation of seven years, we carried out direct measurements of water consumption for 3 trees at 19, 45, 54 and 72 months after planting, and measured values of tree transpiration were compared with estimations from HDPs installed on the trunks. The trees used for direct measurements were cut and kept in situ, standing with the bottom inserted in a water tank. The accuracy of the calibration was checked by two independent methods: 1) directs measurements for other individual trees, and 2) through Eddy covariance measurements. We compared direct measurements for 3 trees of different size at the ages of 29 and 65 months with HDP. At the stand level, 15 trees selected to cover the range of sizes of one commercial plantation at the end of the rotation were monitored by HDP over 8 months. The comparison with eddy-covariance measurements was carried out during dry periods. Resulted showed that the calibration developed for these home-made probes can be used with great reliability at the tree and stand scales