Where do we expect to find deep plant roots?

Plant roots have been observed up to 70 m in depth – what would compel a plant to root so deeply? Earlier work shows that the climate, soil and drainage all affect rooting depth, but with conflicting results. For example, both the deepest and shallowest roots are found in arid regions. Here, we compiled > 2400 globally distributed rooting‐depth observations of individual plants and applied simple correlation analysis to assess the impact of global climate, local topography and substrate, and individual plant size, and their combinations controlling where and why plants root deeply. At the global scale, deep roots are driven by climate. Both concentrated wet periods and prolonged droughts are required to drive deep roots, and we find the deepest roots in semi‐arid climates with strong precipitation seasonality or interannual variability. At the landscape scale, drainage modulates rooting depth. An accessible water table facilitates deep roots at midslopes, but it is too deep to impact roots further upslope. Instead, the deep vadose zone moisture reserve is the primary driver for deep rooting. Thus, the deepest roots are observed on well‐drained uplands with deep vadose zones under climates with distinct wet and dry periods. At the plot scale, substrate structure and hydraulic properties modulate deep rooting – B‐horizons limit deep roots, while woody plants often root below the bedrock surface, provided it is fractured. At the individual plant scale, deep roots are limited to high‐biomass woody plants. Together, these findings sharpen our understanding of where and why plants root deeply, highlighting intersections of climate, drainage, terrain and biomass and identifying conditions where deep roots may serve as a lifeline during prolonged drought, meanwhile weathering rock, sequestering carbon, and bringing the living world far deeper than the conventional ‘root zone'.