Claire Marsden

Stand-level patterns of carbon fluxes and partitioning in a Eucalyptus grandis plantation across a gradient of productivity, in São Paulo State, Brazil

Wood production represents a large but variable fraction of gross primary production (GPP) in highly productive Eucalyptus plantations. Assessing patterns of carbon (C) partitioning (C flux as a fraction of GPP) between above- and belowground components is essential to understand mechanisms driving the C budget of these plantations. Better knowledge of fluxes and partitioning to woody and non-woody tissues in response to site characteristics and resource availability could provide opportunities to increase forest productivity. Our study aimed at investigating how C allocation varied within one apparently homogeneous 90 ha stand of Eucalyptus grandis (W. Hill ex Maiden) in Southeastern Brazil. We assessed annual above-ground net primary production (ANPP: stem, leaf, and branch production) and total belowground C flux (TBCF: the sum of root production and respiration and mycorrhizal production and respiration), GPP (computed as the sum of ANPP, TBCF and estimated aboveground respiration) on 12 plots representing the gradient of productivity found within the stand. The spatial heterogeneity of topography and associated soil attributes across the stand likely explained this fertility gradient. Component fluxes of GPP and C partitioning were found to vary among plots. Stem NPP ranged from 554 g C m(-2) year(-1) on the plot with lowest GPP to 923 g C m(-2) year(-1) on the plot with highest GPP. Total belowground carbon flux ranged from 497 to 1235 g C m(-2) year(-1) and showed no relationship with ANPP or GPP. Carbon partitioning to stem NPP increased from 0.19 to 0.23, showing a positive trend of increase with GPP (R(2) = 0.29, P = 0.07). Variations in stem wood production across the gradient of productivity observed at our experimental site were a result of the variability in C partitioning to different forest system components.

Soil microbial diversity drives the priming effect along climate gradients: a case study in Madagascar

The priming effect in soil is proposed to be generated by two distinct mechanisms: ‘stoichiometric decomposition’ and/or ‘nutrient mining’ theories. Each mechanism has its own dynamics, involves its own microbial actors, and targets different soil organic matter (SOM) pools. The present study aims to evaluate how climatic parameters drive the intensity of each priming effect generation mechanism via the modification of soil microbial and physicochemical properties. Soils were sampled in the center of Madagascar, along climatic gradients designed to distinguish temperature from rainfall effects. Abiotic and biotic soil descriptors were characterized including bacterial and fungal phylogenetic composition. Potential organic matter mineralization and PE were assessed 7 and 42 days after the beginning of incubation with 13C-enriched wheat straw. Both priming mechanisms were mainly driven by the mean annual temperature but in opposite directions. The priming effect generated by stoichiometric decomposition was fostered under colder climates, because of soil enrichment in less developed organic matter, as well as in fast-growing populations. Conversely, the priming effect generated by nutrient mining was enhanced under warmer climates, probably because of the lack of competition between slow-growing populations mining SOM and fast-growing populations for the energy-rich residue entering the soil. Our study leads to hypotheses about the consequences of climate change on both PE generation mechanisms and associated consequences on soil carbon sequestration.

Les avancées de la recherche dans le domaine de la modélisation des interactions sol-arbre

Les outils de modelisation au service de la politique, de la planification et de la gestion forestiere ne permettent pas encore une evaluation simultanee de l’impact des changements globaux et de la gestion sur les differentes fonctions ecologiques et productives assurees par les ecosystemes forestiers. Nous proposons un etat des lieux des modeles disponibles dans les differentes disciplines que sont la dendrometrie, l’ecophysiologie et les sciences du sol et comment l’integration des concepts de ces trois disciplines peut permettre d’elaborer les outils necessaires au gestionnaire. In fine, ces nouveaux outils devront etre suffisamment complets pour repondre a des questions a l’echelle de l’ecosysteme ; distinguer ce qui releve du processus generique de ce qui reste site-dependant ; permettre de definir puis simuler des scenarios de gestion varies et innovants en environnement incertain, avec des contraintes de plus en plus fortes ; et etre documentes et references pour garantir leur perennite et leur utilisation.

Advances in modeling interactions between soils and trees

Forest ecosystems are, by their very nature, highly variable. — Vertical variation: the interaction with the environment extends from the parent rock to the atmosphere. — Horizontal variation: the spatial variability of the topography, the soil properties, the type of forest management and the previous soil use and management means that the structure and functioning of an ecosystem can vary significantly within a limited area. — Temporal variation: rotations can vary from under ten years (typically eucalyptus or acacia forests in tropical environments), through several decades (beech and conifers in temperate and tropical environments) to several hundreds of years (oak and sequoia). These rotations may be affected by abrupt or gradual changes or by long-term natural or human forcing, resulting in dynamics, the scale and progression of which are not immediately evident.

VARIABILIDADE DAS MEDIDAS DE FLUXOS DE CO2 DO SOLO OBTIDAS PELO MÉTODO DA CORRELAÇÃO DE VÓRTICES

Variabilidade das medidas de fluxos de CO2 do solo obtidaspelo metodo da correlacao de vortices