The Distribution and Dynamics of Fine Roots in a Forested Bog in Northern Minnesota, USA.

Our objective was to determine the distribution and dynamics of fine roots in an ombrotrophic bog that is the location of the Spruce and Peatland Responses Under Climatic and Environmental change (SPRUCE) experiment (http://mnspruce.ornl.gov/). We used several methods to characterize fine roots, including minirhizotrons installed across gradients of tree density and microtopography to quantify root production and mortality throughout the peat profile, species-specific root voucher specimens to investigate the relationships among root order, root morphology, and root chemistry, peat cores to determine rooting depth distribution, in-growth cores to capture newly-produced fine roots, and novel, automated minirhizotron technology to track the dynamics of ephemeral roots and mycorrhizal hyphae at high spatial and temporal resolution.

We found that: (1) Minirhizotrons, a technology that is rarely used in wetlands, provided an opportunity to examine fine roots in an important and understudied ecosystem. (2) Root standing crop increased quickly in the spring, well before wood growth was initiated, and standing crop was lowest in areas of low tree density. (3) Root standing crop and production were much greater in raised hummocks when compared with saturated hollow depressions. (4) Common vascular plant species in the bog encompassed a range of root morphology and diameter distributions, as well as mycorrhizal colonization. Across a range of root orders, root diameter was strongly related with root mass per length and root nitrogen concentration. (5) Intact shrub roots removed from peat samples as deep as 2-m were well-preserved, dead roots with a calibrated ¹⁴C age of ~5000 years; living roots were confined to the aerobic zone above the water table. (6) Fungal hyphae occurred only in shallow peat. Root or hyphal growth during winter months was limited. These measurements, taken prior to the SPRUCE experimental manipulation, will be used to parameterize ecosystem and land surface models to refine hypotheses regarding the expected effects of warming and elevated CO₂ on root distribution, dynamics, and carbon and nitrogen cycling in globally important peatland ecosystems.