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Consequences of clear-cutting on the production of fine roots, CO2, CH4 and N2O down to the water table in Eucalyptus grandis stands conducted in coppice in a throughfall-exclusion experiment

Germon A.. 2019. Montpellier : Montpellier SupAgro, 273 p.. Thèse de doctorat -- Ecologie fonctionnelle.

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 spatio-temporal dynamics of fine roots down to the root front is an important issue to identify more sustainable silvicultural practices for planted forests installed in areas with low soil fertility and prolonged drought periods. The Eucalyptus gender is the most planted in Brazil covering about 5.1 million hectares in 2012. Coppice management can be an advantage against water stress in eucalyptus plantations because the trees are likely to benefit of a root system exploring deep soil layers where the availability of water can be higher than in the topsoil. The objective of this study is to assess the production of fine roots, CO2, CH4 and N2O down to the water table after cutting the trees in Eucalyptus grandis plantation conducted in coppice, under two contrasting water supply regimes,. Two pits were excavated down to a depth of 17 m in a throughfall exclusion experiment: one in a plot with 37% of throughfall excluded by plastic sheets, and one without rain exclusion. Another pit in an adjacent stand of the same age not harvested will make it possible to assess the effect of cutting the trees. Fine roots dynamics will be studied using the Minirhizotron technique: twenty four transparent polyvinyl chloride tubes were installed in the pits in 2014 (12 per pit) down to 17 m deep and 7 tubes down to 4 m deep in the stand not harvested. Images will be obtained by a circular scanner every 14 days over 1 year before cutting the trees and over 1.5 years in coppice after harvesting. The winrhizotron software will be used to estimate root growth in length and area. Gas sampling will be performed every two weeks for 24 months (6 months before and 1.5 years after cutting the trees) throughout the soil profiles down to the groundwater. Gas samples will be analysed by gas chromatography and a modelling approach will be used to estimate CO2, CH4 and N2O production rates. Dynamics of gas concentrations can provide additional insights into the heterogeneous gas exchange processes through soil depth and how the control of the fluxes at the soil surface are correlated to root dynamics in the whole soil profile. Improving our understanding of the factors controlling the fluxes of greenhouse gases throughout very deep soil profiles is needed to develop reliable process-based models likely to predict wood production at a large scale in forest companies.

Mots-clés : eucalyptus grandis; coupe rase; racine; croissance; gaz à effet de serre; sol tropical; lessivage du sol; régime du taillis; brésil

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