Modelling carbon dynamics and response to environmental change along a boreal fen nutrient gradient
Highlights
► We model historical carbon gas exchange over 31 years in minerotrophic fens. ► Variability in greenhouse gas dynamics was large among fens and among years. ► Climate change and local flooding disturbance impact fen gas production. ► Predicting future response must account for differences among fens.
Introduction
Peatlands store approximately 25% of the global soil carbon (C) pool (IPCC, 2000). Boreal and subarctic peatlands account for 15–30% of C stored, with Russia and Canada containing the majority of peatlands (Gorham, 1991, Tarnocai and Stolbovoy, 2006). These peatlands occupy a large and integral part of the boreal landscape and are valued for a variety of ecosystem services, including water storage, stream flow regulation, and water quality (Hanson et al., 2008). Peatlands also perform an important climate regulation ecosystem service by steadily accumulating C. This service has been ongoing since peatland inception following deglaciation (Zoltai et al., 1988, Tarnocai and Stolbovoy, 2006, Gorham et al., 2007) because more carbon dioxide (CO2) was fixed through photosynthesis than was released through organic matter decomposition in these cold, wet soils.
Peatlands are largely categorized into ombrotrophic bogs and minerotrophic fens, both with differing C cycles (Rydin et al., 2006). In Canada minerotrophic fens account for approximately one-third of the peatland area (Tarnocai, 2001). Fens are of particular interest for boreal C dynamics as they span a wide minerotrophic spectrum from rich (pH values greater than 6.5), where mineral and groundwater input from neighbouring uplands and peat substratum is high, to poor (pH values between 4.5 and 5.5), where mineral and groundwater inputs contributing circum-neutral water are low (National Wetlands Working Group, 1997). This variability in minerotrophy creates heterogeneity in C exchange (Martikainen et al., 1995, Bubier et al., 2003a, Riutta et al., 2007b).
Abiotic and biotic properties change along rich to poor fen gradients (Aerts and Ludwig, 1997, Moore et al., 2002, Jungkunst and Fiedler, 2007). Water table level and external nutrient input are two important abiotic controls on C cycling. As water tables deepen along the rich to poor fen gradient, nutrient inputs decrease, area of aerobic decomposition increases and nutrient retention times in peat lengthen (Webster and McLaughlin, 2010). Changes in the biota are related to changes in the abiotic environment. High water table levels generally lead to decreasing aboveground biomass (sedges, brown mosses) and increasing anaerobic microbes (Godin et al., 2012, Meyers et al., 2012), whereas lower water table levels generally lead to increasing aboveground biomass composed of Sphagnum mosses, shrubs and trees and aerobic microbes. The differences in plant phenology and photosynthetic efficiency, as well as plant and microbe respiration rates, may be the most important factors controlling differences in net C balance among fen types (Laiho et al., 2003, Limpens et al., 2008, Leppälä et al., 2011).
Carbon exchange with the atmosphere also varies due to seasonal and annual variability in fen hydrology driven by short-term meteorological conditions (Clair et al., 2002, Nykänen et al., 2003, Trettin et al., 2006). Meteorological conditions differentially affect CO2 uptake and release, and the amount of methane produced and emitted to the atmosphere across fen types (Weltzin et al., 2000, Bubier et al., 2003a, Pelletier et al., 2011). Thus, different types of fens are expected to differ in their response to changes in temperature and precipitation. However, changing meteorological conditions may have less effect on fens where local disturbances, such as the impacts of beaver impoundments, control their hydrology.
Over the longer term, as the global climate changes so will the balance between CO2 sequestration and release within peatlands. Biophysical differences among fen peatlands may result in complex C cycling responses to climate change. For example, increasing temperature will enhance decomposition, resulting in elevated CO2 effluxes to the atmosphere (Lafleur et al., 2005). However, because peatland C dynamics are strongly linked to hydrology, temperature changes may be concomitant with (1) abiotic changes, including decreased precipitation and groundwater inputs or increased evapotranspiration rates, all of which contribute to lower water table levels (Gorham, 1991), and (2) biotic changes, such as shifts in plant community composition (Waddington and Roulet, 2000, Riutta et al., 2007a). Water table level and vegetation composition interactively regulate peatland C cycling (Trettin et al., 2006). In addition to short-term meteorological and long-term climate changes, local disturbances, such as beaver impoundments also affect peatland C dynamics. Thus abiotic and biotic changes occurring within peatlands are complex, making their future C responses difficult to predict. If production continues to exceed releases through decomposition and methane (CH4) effluxes, peatlands could remain a C sink, but if releases of C exceed production they may shift to a source (Frolking and Roulet, 2007).
Our ability to predict ecosystem responses to changes in climate depends on a more complete understanding of the factors that control C dynamics across a range of peatland plant communities (Bubier et al., 2003b) over long periods. We applied the process-based model Wetland-DNDC (Zhang et al., 2002) to examine the effects of fen type (rich, intermediate and poor), historical and projected climate, and local disturbance on C dynamics by asking:
- 1.
How does the timing and magnitude of gas effluxes within the year affect annual C exchange among fens?
- 2.
How does C exchange vary from year to year and what environmental conditions correlate with this variability?
- 3.
How do prolonged droughts, as might be expected under climate change, or prolonged flooding, as might be expected by increased beaver activity, affect C exchange?
Section snippets
Study area
The study fens are part of the White River Experimental Watershed Study and are within first-order catchments in the White River basin in northern Ontario, Canada (centred at 48°21′ N, 85°21′ W) that drains into Lake Superior. The climate is continental and strongly influenced by the proximity of the lake, with mean annual precipitation and temperature of 970 mm and 2.2 °C, respectively, during a 31-year record (1981–2011) from a nearby weather station in Wawa, Ontario, Canada (47°58′ N, 84°47′
Results and discussion
Modelling C dynamics from interpolated climate and hydrological data provides insight into how fens respond to changing environmental conditions. While these modelling studies do not replace the need for long-term monitoring studies, they help to interpret results from short-term studies in a broader context.
Conclusions
DNDC, combined with the climate record and reconstructed water tables allowed us to examine recent changes in carbon dynamics in a rich, intermediate and poor fen and predict how these dynamics may change under different local and global scenarios. We determined that variability in modelled C dynamics across fen peatland types was due to (1) spatial heterogeneity in plant productivity and decomposition rates of the different fen plant communities and (2) spatial and temporal differences in
Acknowledgements
Support for this project was provided by the Ontario Ministry of Natural Resources’ Ontario Forest Research Institute (OFRI) under the auspices of project CC-167 to JWM and Natural Resources Canada-Canadian Forest Service A-base to KLW. The authors also thank Lisa Buse, John Ralston, Adam Kinnunen, Mark Crofts, Aaron Godin, Sheri-Ann Kuiper, Sandra Wawryszyn, and Ravi Kanipayor for their assistance with various aspects of the project.
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