Résumé
In tree ecosystems, tree fine roots (defined as those of diameter ≤ 2 mm) comprise only a small proportion of total biomass, but play the most active and fundamental role in both tree functioning (via water and nutrition uptakes) and carbon flux dynamics (via respiration, exudation and high turn-over rate). These processes have a beneficial effect on the promotion of underground biodiversity, as fine roots with their rhizospheres can provide “cradle effect” for sheltering and nourishing soil micro- and macro-organisms in both direct (e.g. mycorrhizal fungi growth on roots) and indirect (e.g. creating soil organic matter) manners. As a result, in order to better explore underground biodiversity repartition and evaluate its service, a reliable characterisation of the spatial distribution of fine roots is indispensable. The spatial distribution of fine root can be served either as a proxy of repartition of certain fauna or as an environmental factor to be correlated with biodiversity indicators. However, our knowledge concerning fine roots distribution is still limited, partially due to the lack of effective root modelling and data analysis methods. This proposal aims to introduce and highlight a set of methodologies suitable to quantitatively describe root spatial distribution. The methods are statistically based and easy to use in conjunction with conventional root sampling techniques (e.g. root trench, coring). Fine root density from trees at a given point can be modelled by a modified logistic function parametrised by tree dimension and position, presence of obstacles between a tree and the given point, and root competition level with roots from understory species. For fine root vertical distribution, by introducing a recently published piecewise linear model’, namely “hockey stick model,” an ecological threshold served to distinguish shallow rooting zone and deep rooting zone, might be thus detected with statistical diagnostic. Applying a log-normal function and Gompertz function allows us to better fit the shape of root diameter spectrum (cumulative root density with increasing diameter). Regarding the phenomenon that fine root distribution exhibits a pattern of patchiness (or namely root nuggets), application of geo-statistical approaches allows us to better quantify aggregation and dispersion rate of roots within/amongst nuggets.In the ongoing projects ANR ECOSFIX (Ecosystem Services of Roots – Hydraulic Redistribution, Carbon Sequestration and Soil Fixation) and ARANGE FP7 (Advanced multifunctional management of European mountain forests), these above methods have been successfully applied to several French tree ecosystems, varying from heterogeneous mixed mountain forests (with low intervention intensity), to homogenous agroforests and plantations (with high intervention intensity). In the framework of Cost Action Biolink, it might be therefore particularly interesting to have these novel root analysis methods applied toward new root data, in order to better evaluate the state and roles of belowground soil biodiversity.