Boreal plant roots do not grow deeper to take advantage of depressed water tables

Boreal bogs host large stores of terrestrial carbon as peat. Long-term stability of peat is threatened by global change via altered rhizosphere conditions, temperatures, and hydrology. High water tables, cold temperatures and low pH in bogs favors Sphagnum moss production over vascular plant functional types (PFTs): trees, shrubs and sedges. We hypothesize that warming will increase the growth of vascular plant roots, particularly in deeper peat as water tables sink. Additionally, we hypothesize elevated [CO2] to amplify root productivity responses to warming, by inducing greater plant nutrient demand via CO2 fertilization. We tested these hypotheses in the SPRUCE experiment, a whole-ecosystem warming gradient (+0 to +9 °C in 2.5°C steps) ✕ ambient and elevated (500 ppm) [CO2] manipulation. Manual minirhizotron images from 2015-2021 were used to estimate root production and depth for each PFT. Root production by ericaceous shrubs increased with warming under elevated, but not ambient [CO2]. Rather than being more deeply distributed under warming elevated [CO2], this root production was either more shallowly distributed or remained at the same depth. Altogether these results suggest that vascular plants in boreal bogs will respond to global change through shallow root production, possibly to capture nutrients from recent litter inputs.