Thèse d'Amandine Germon le vendredi 21 juin à Montpellier
Amandine Germon soutiendra sa thèse "Consequences of clear-cutting on the production of fine roots, CO2, N2O and CH4 down to the water table in Eucalyptus grandis stands conducted in coppice in a throughfall-exclusion experiment"
le vendredi 21 juin 2019, 13h00, à l'amphithéâtre Jacques Alliot, CIRAD, Batiment 4, avenue Agropolis, Montpellier
Soutenance publique en anglais
Tree growth is highly dependent on the absorptive function of fine roots for water and nutrients. Fine roots also play a major role in the global carbon (C) cycle, mainly through production, respiration, exudation and decomposition processes. Improving our understanding of the spatiotemporal dynamics of fine roots and greenhouse gases in deep soil layers is a key component to identify more sustainable silvicultural practices for planted forests in a context of climate change and to improve the current biogeochemical models.
Our study aimed to assess the effect of clear-cutting and drought on fine-root production, soil CO2, CH4 and N2O effluxes and production throughout deep soil profiles down to the water table in Brazilian coppice-managed Eucalyptus plantations. Fine roots (i.e. diameter < 2 mm) were sampled down to a depth of 17 m in a throughfall exclusion experiment comparing stands with 37% of throughfall excluded by plastic sheets (-W) and stands without rain exclusion (+W). Root dynamics were studied using minirhizotron in two permanent pits down to a depth of 17 m in treatments -W and +W, over 1 year before clear-cutting, then over 2 years in coppice, as well as down to a depth of 4 m in a non-harvested plot (NH) serving as a control. CO2, CH4 and N2O surface effluxes were measured over three years using the closed-chamber method in treatments -W, +W and NH. CO2, CH4 and N2O concentrations in the soil were measured from the pits down to a depth of 15.5 m in treatments -W, +W and NH over 3 months before the clear-cut and 1.5 years after in coppice.
After harvesting, spectacular fine root growth of trees conducted in coppice occurred in very deep soil layers (> 13 m) and, surprisingly, root mortality remained extremely low whatever the depth and the treatment. Total fine-root biomass in coppice down to a depth of 17 m was 1266 and 1017 g m-2 in treatments -W and +W, respectively, at 1.5 years after the clear-cut and was 1078 g m-2 in NH 7.5 years after planting. Specific root length and specific root area were about 15% higher in -W than in +W. CO2, CH4, and N2O effluxes were not significantly different between treatments -W and +W and did not change after clear-cutting in the coppice-managed stands compared to non-harvested stand. CO2 and CH4 concentrations greatly increased with depth and N2O concentrations remained roughly constant from the soil surface down to a depth of 15.5 m. Mean CO2 and N2O concentrations in -W were 20.7% and 7.6% lower than in +W, respectively, and CH4 concentrations in -W were 44.4% higher than in +W throughout the soil profiles. A diffusivity model showed that CO2, N2O and CH4 production and consumption occurred at great depths and were similar in treatments +W, -W and NH. Clear-cutting did not increase CO2, CH4 and N2O effluxes and productions, whatever the water supply regime.
Establishing deep root systems in tropical planted forests could help trees withstand the long drought periods expected under climate change in many tropical regions. Our study carried out in coppice-managed eucalypt stands representative of large areas in tropical regions suggests that greenhouse gas emissions could be little influenced by changes in rainfall patterns due to climate change.
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