Long-term biophysical conditions and carbon dynamics of a temperate swamp in Southern Ontario, Canada

Abstract

In Canada, wetlands cover a land area of 1.5  106 km2 and store ~129 Pg C. However, the carbon (C) cycling of swamps has been understudied even though they store substantial quantity of C in their biomass and can also accumulate peat. In particular, southern Ontario swamps are estimated to hold ~1.1 Pg C under distinct hydroclimatic conditions. Previous studies on temperate swamp C fluxes were mostly based on short-term (<5 years) field measurements that limit our understanding of the multi-decadal dynamics that exist between this ecosystem’s C flux and biophysical conditions. To elucidate the long-term interactions and feedbacks that are important to temperate swamp C dynamics, a process-based model (CoupModel) was used to simulate plant processes, energy, water and C fluxes in one of the most well-preserved swamps in southern Ontario over 78-year period (1983–2060). CoupModel reasonably simulated the C flux and controlling variables when validated with compiled historic field measurements (1983–2023) with coefficient of determination (R2) values of 0.60, 0.95 & 0.61 for soil respiration, surface soil temperature (0–5 cm) and water table level (WTL). Systematic calibration of the initialized model for Beverly Swamp with the Generalized Likelihood Uncertainty Estimation (GLUE) approach moderately reduced the uncertainty associated with modelling processes and assisted in identifying the important parameters that greatly influence temperate swamp C flux simulations. Plant-related processes and hydrological variables exerted the strongest control on the simulation of carbon dioxide (CO2) efflux through soil respiration. The forcing of the GLUE calibrated CoupModel with an ensemble of climate projections downscaled from earth system models (ESMs) under shared socio-economic pathway (SSP5) by mid-century (2060) produced a decline in the swamp’s C uptake capacity as net ecosystem exchange (NEE) of CO2. Relative to the reference period of 1983–2002, the projected increase in mean air temperature (4.3 ± 0.8 oC) and precipitation (0.2 ± 0.1 mm) by 2050s triggered increase in 5 cm deep soil temperature, vapor pressure deficit, and evapotranspiration at Beverly Swamp. These changes to the swamp’s thermal and hydrological conditions dropped its WTL and VMC. Consequently, drier and warmer conditions raised the swamp’s CO2 efflux through ecosystem respiration, while its GPP moderately increased. These bidirectional feedbacks contributed to a reduction in the swamp’s net C uptake (NEE) by the 2050s but it mostly still maintained its net C sink role. While uncertainty in future climate projections and model fit limit our confidence in the precise estimate of future carbon exchange, it was clear that seasonal timing of warming and precipitation played an important role in the swamp response, with coincident declines in precipitation and warming temperatures in summer that caused water stress for plants. Results from this long-term study will help improve our understanding of the important ecohydrological interactions and feedbacks that drive the C cycle of temperate swamps, and their contributions to regional terrestrial C and water cycles. This will help inform decision making on the role of swamp peatlands as nature-based climate solutions through improved understanding of their net C exchange with the atmosphere.