Warming and elevated CO ₂ cause greater and deeper root growth by shrubs in a boreal bog

Boreal bogs sequester large stores of terrestrial carbon in waterlogged peat occupied by extensive networks of vascular plant roots. Historically, cold temperatures and shallow water tables in bogs have favored the growth of Sphagnum over vascular plants. Seasonally variable water tables in bogs constrain woody shrub and tree root production to shallow, oxic acrotelm peat horizons while herbs with aerenchymous root tissue can grow below the water table. Warming and elevated CO ₂ are altering ecosystem functioning in nutrient-limited, rain-fed (ombrotrophic) bogs, via direct impacts on physiology and indirectly via altering water table levels. The environmental changes are shifting interactions between vascular plant fine-roots and the surrounding peat horizons and increasingly favoring vascular plant growth relative to Sphagnum sp. Altered functioning of fine-roots may dramatically affect the role of plants in these ecosystems because of the importance of these organs for plant resource acquisition. We examined how fine-roots across vascular PFTs differentially respond to warming & elevated CO ₂ manipulations and the consequent water table depression in a forested, boreal bog. We used minirhizotrons (cameras inserted belowground) to measure fine-root production, depth distribution, and standing crop from 2015-2021 for each PFT. We found that daily rates of fine-root production accelerated more for shrubs than for trees or herbs. Shrubs and trees grew their roots more deeply with depressed water table levels and shrub fine-roots became narrower. On an annual basis, fine-root production increased with warming, though these rates varied among PFTs and exhibited interactions with elevated CO ₂ . Standing crop of fine-roots increased with warming temperatures, most strongly for shrubs under elevated CO ₂ , because of these productivity responses. From these results, we expect boreal bogs will become increasingly dominated by shrubs under future warmer temperatures, higher atmospheric CO ₂ concentrations, and lower water table levels.