Fine‐root responses to warming and elevated CO ₂ vary among plant functional types

The viability of peatlands as terrestrial carbon sinks is unknown as peatland carbon losses could be offset, or exacerbated, by increased plant growth. The response of fine‐roots to changing conditions in these ecosystems will be critical to understanding plant access to water, acquisition of nutrients, and ecosystem structure and function.

We examined the plant functional type‐specific responses of fine‐root production to warming and elevated CO ₂ treatments, and to warming treatment‐induced deepening of water‐tables in a forested boreal bog. We hypothesized that shrubs would show the most dynamic responses of fine‐root production. We used minirhizotron cameras to estimate fine‐root production, depth and peak annual standing crop from images of individually tracked roots across 7 years (2015–2021) in the SPRUCE (Spruce and Peatland Responses Under Changing Environments) experiment.

Fine‐root production increased with warming for shrubs 10.3, trees 2.2 and herbs 4.6%°C ⁻¹ . When water‐tables dropped to their lows of 78 cm deep, shrubs deepened their fine‐root production from 7 to 29 cm deep, and trees from 9 to 17 cm, while herbs produced fine‐roots around 31 cm deep independent of shifts in water‐tables. Fine‐root standing crop increased with warming most strongly for shrubs, even more so under elevated CO ₂ .

Synthesis. We found that the fine‐roots of shrubs responded the most strongly to warming, elevated CO ₂ and lowered water‐tables. The greater responsiveness of shrub fine‐root production may partially explain increasing ‘shrubification’ at this site and in other peatlands with long‐term drainage.