The role of plant phenology in regulating the peatland carbon cycle

Abstract

Boreal peatlands are a major global carbon (C) store and a significant natural source of methane (CH4), thereby regulating the climate system. However, future peatland C cycle feedbacks to climatic changes remain highly uncertain. Accurate predictions of peatland carbon-climate interactions require an in-depth understanding of how vegetation dynamics and environmental conditions jointly govern the net exchange of carbon dioxide (CO2), plant carbon use efficiency (CUE) and CH4 emissions. To date, detailed knowledge of how key plant functional groups (PFGs), particularly vascular plants and Sphagnum mosses, regulate C dynamics in response to environmental conditions on seasonal scales remains limited. This thesis used high-resolution CO2 and CH4 flux data from an automated chamber system established over vegetation removal plots to explore the separate roles of key PFGs in regulating net CO2 exchange component fluxes (Paper I), CUE (Paper II) and CH4 emissions (Paper III) in a boreal peatland over three growing seasons. Flux measurements and detailed vegetation monitoring were combined with in vivo measurements of plant cell respiratory pathways (Paper II) and porewater substrate sampling (i.e., sugars, organic acids) (Paper III). Results revealed distinct roles of vascular plants and Sphagnum mosses in regulating CO2, CUE and CH4 dynamics and their sensitivity to environmental drivers on diel to seasonal timescales. Specifically, vascular plants dominated gross primary production (GPP) during the peak season and autotrophic respiration throughout the growing season. In addition, their presence mediated magnitude and temperature sensitivity of CH4 emissions. However, variations in porewater concentrations of plant-derived substrates showed no clear relationship with seasonal CH4 dynamics. In contrast, Sphagnum mosses contributed most to GPP during green-up and senescence and promoted CH4 oxidation through Sphagnum-associated methanotrophs. Sphagnum exhibited persistently higher CUE than vascular plants. However, respiratory pathway partitioning only partially explained seasonal CUE dynamics. Overall, the findings highlight the central role of PFG-specific phenology in regulating peatland C cycling, with important implications for predictions of carbon-climate feedbacks.