Fine roots are seasonal, respond relatively rapidly to environmental changes and are key regulators of nutrient and carbon cycling. While a significant fine-root response to warming and elevated CO2 (e[CO2]) has been observed in upland ecosystems, fine-root dynamics are unclear in nutrient-poor and water-logged peatlands. Peatlands are long-term reservoirs of carbon and belowground production is an important input to this sink. Climate warming could lead to drying or to faster nutrient cycling rates and increase production of fine roots in peatlands. However, increased nutrients could also result in lower root biomass allocation relative to aboveground allocation. We investigated fine-root responses at SPRUCE (Spruce and Peatland Responses Under Climatic and Environmental change), a whole-ecosystem warming and e[CO2] experiment at a peatland. Using ingrowth cores, we measured woody fine-root growth of shrub and tree plant functional types (PFT) along a temperature treatment gradient (0, +2.25, +4.5, +6.75 and +9 °C above ambient) crossed with ambient or e[CO2] treatment. To investigate response mechanisms, we also evaluated fine-root traits and aboveground biomass at each treatment.
Results/Conclusions
We found that warming increased total fine-root production through increased fine-root length and tissue density. Fine-root diameter and specific root length (length per mass) remained consistent across treatments. Elevated CO2 did not have a significant effect on ecosystem-scale fine-root dynamics after one growing season of treatment, but there were trait responses in some PFTs. For example, shrub fine-root tissue density decreased in e[CO2] plots, highlighting different responses on PFT- or ecosystem-scales. In addition to being PFT-dependent, fine-root response was also microtopography-dependent. Increased fine-root growth in response to warming and drying was driven more by hollows than by hummocks. Lastly, relative to aboveground, belowground C allocation increased after warming.
Our study is the first to provide detailed insight into peatland fine-root warming and e[CO2] response using an ecosystem-scale manipulative experiment. In this decade-long experiment, we expect more pronounced effects of e[CO2] and drying in the future. Plant community shifts from prolonged drying may also alter belowground responses. Our results highlight plasticity of peatland fine-root traits and allocation strategies; the extent and mechanisms of which vary by PFT and microhabitat. Our trait-based approach is a first step toward modeling fine-root contribution to peatland carbon stability against global change.