Unveiling the forest subsurface and its invisible water, what can geophysics bring to forest ecohydrology

Forests cover almost one third of the Earth's land area and are central in the carbon and water cycles. Soil water availability is one of the most important factors regulating transpiration, biomass production and plant species distribution in ecosystems. The carbon and water cycles are closely linked and so understanding the functioning and evolution of forest environments and their relation to subsurface structure and water availability is essential to improve understanding of the water cycle under a changing climate. Studying the forest subsurface is a challenge because of its heterogeneous nature and difficult accessibility. Traditional approaches used by ecologists are also often point measurements that have a low spatial representativity. Near-surface geophysics offers a wide range of methods to characterize the spatial and temporal variability of subsurface properties and associated processes in a non-destructive and integrative way. Geophysical methods allow us to obtain new information that complements ecophysiological methods to better understand ecosystem functioning, and in particular processes linked to ecohydrology. The use of geophysical methods in forests is growing, both by geophysicists seeking to apply their tools to more complex environments, and by ecologists seeking to better characterize their experimental sites. One of the major applications and assets of geophysics in forests is to quantify and monitor water stocks and dynamics. For example, geoelectrical monitoring can be used to assess the distribution and spatial variations of water content in the subsoil. In this work, we show the example of a recently developed ensemble approach to quantitatively relate electrical conductivity monitoring and the distribution and dynamic of water in forest soils. We believe that such interdisciplinary advances can help us improving the quantitative assessment of forest responses to the environment and their adaptation to climate change.