Digging deeper - soil water availability reshapes fine root distribution in hybrid Poplar (Populus Nigra × P. maximowiczii) short-rotation coppice plantation

Fine roots play a critical role in water and nutrient uptake, contributing significantly to the ecosystem carbon cycle. This study investigates the effects of soil water availability on the vertical distribution of fine root annual production and turnover rates in hybrid poplar (Populus nigra × P. maximowiczii, clone J-105) short-rotation coppice plantation. We hypothesized that reduced throughfall would lead to a deeper vertical distribution of fine roots and affect their annual production and turnover rates. The study was conducted in a drought experiment established in a coppiced poplar plantation in the Czech Republic. Root biomass and in-growth cores were used to quantify root vertical distribution, production, and turnover rates under control and drought (throughfall reduction) treatments. Root distributions were modeled using the asymptotic equation and the logistic dose-response curve. Although total fine root biomass (255 g DM m− 2; 0–60 cm) and production (298 g DM m− 2 y− 1; 0–60 cm) were not significantly affected by drought, the vertical distribution of fine roots was influenced by soil water availability. Model results show that at lower soil water content in May (20%) corresponded to a greater proportion of roots (50% roots) at deeper soil layers as compared with higher soil water content (35%), where only 20% of roots were allocated to deeper layers. Our findings highlight the importance of accounting for the plasticity in root distributions when modeling drought impacts on ecosystem processes. Shifts in the vertical distribution of fine roots can profoundly impact soil carbon inputs and nutrient uptake efficiencies, which are not fully captured by changes in total root biomass alone. Ecosystem models should incorporate root distribution dynamics to improve predictions of terrestrial carbon cycling under drought conditions. Soil moisture shapes the vertical distribution of fine roots in poplar, highlighting root plasticity as a key trait for modeling ecosystem responses to changing water availability.