Sensitivity of Northern Peatland Carbon Dynamics to Holocene Climate Change Zicheng Yu,1 David W. Beilman,2,3 and Miriam C. Jones1 In this paper, we evaluate the long-term climate sensitivity and global carbon (C) cycle implications of northern peatland C dynamics by synthesizing available data and providing a conceptual framework for understanding the dominant controls, processes, and interactions of peatland initiation and C accumulation. Northern peatlands are distributed throughout the climate domain of the boreal forest/taiga biome, but important differences between peatland regions are evident in annual temperature vs. precipitation (T-P) space, suggesting complex hydroclimatic con trols through various seasonal thermal-moisture associations. Of 2380 available basal peat dates from northern peatlands, nearly half show initiation before 8000 calendar years (cal years) B.P. Peat-core data from sites spanning peatland T-P space show large variations in apparent C accumulation rates during the Holocene, ranging from 8.4 in the Arctic to 38.0 g C m-2 a-1 in west Siberia, with an overall time-weighted average rate of 18.6 g C m-2 a-1. Sites with multiple age determina tions show millennial-scale variations, with the highest C accumulation generally at 11,000–8000 cal years B.P. The early Holocene was likely a period of rapid peat land expansion and C accumulation. For example, maximum peat expansion and accumulation in Alaska occurred at this time when climate was warmest and possibly driest, suggesting the dominant role of productivity over decomposition processes or a difference in precipitation seasonality. Northern peatland C dynamics contributed to the peak in atmospheric CH4 and the decrease in CO2 concentrations in the early Holocene. This synthesis of data, processes, and ideas provides baselines for understanding the sensitivity of these C-rich ecosystems in a changing climate.
1
1. INTRODUCTION
Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania, USA. 2 CHRONO Centre for Climate, the Environment and Chronology, School of Geography, Archaeology and Palaeoecology, Queen’s University Belfast, Belfast, UK. 3 Now at Department of Geography, University of Hawai’i at Manoa, Honolulu, Hawaii, USA.
Northern peatland ecosystems have cycled and stored substantial amounts of global land carbon (C) over the Holocene (the last 11,700 years). Today, peatlands are one of the largest terrestrial biosphere C pools and are the largest natural source of methane (CH4) in the northern hemisphere. Owing to their large accumulated C mass and dynamic greenhouse gas fluxes, these ecosystems have been an important component of the high-latitude C cycle for thousands of years. Peatland ecosystems and their C-rich peat archives have been studied for several decades, mostly for reconstructing past climate [Charman, 2002], and have been central
Carbon Cycling in Northern Peatlands Geophysical Monograph Series 184 Copyright 2009 by the American Geophysical Union. 10.1029/2008GM000822 55
56 SENSITIVITY OF PEATLAND DYNAMICS TO HOLOCENE CLIMATE CHANGE
to early ideas about recurrent climate changes [Sernander, 1908]. Over recent decades, attention has also turned to carbon cycling and the implications of long-term peatland ecosystem dynamics and climate sensitivity [Clymo, 1984; Gorham, 1991; Rydin and Jeglum, 2006]. Peat C accumulation is determined by the balance of biological inputs (plant growth and litter production) and outputs (organic matter decomposition); both of these processes are sensitive to climate change and climate variability or are indirectly affected by climate through related processes. In this chapter, we provide a conceptual framework for understanding the dominant controls, processes, and interactions of northern peatland initiation and long-term peat C accumulation and dynamics using climate data and peat-core data. We use modern instrumental climate data to explore the climate envelope of today’s northern peatland distribution. We synthesize spatial and temporal patterns of peat C accumulation rates during the Holocene in different regions and discuss climatic and autogenic influences. We also discuss the implications of peatland dynamics for the global carbon cycle. Understanding the causal connection between peat C dynamics and past climate would provide insight into the possible future response of these C-rich ecosystems to climate change in different regions and over different timescales. 2. CLIMATE CONTROLS OVER DISTRIBUTION, INITIATION, AND EXPANSION OF NORTHERN PEATLANDS Northern peatlands occur mostly in boreal and subarctic regions in the northern hemisphere. A cool climate, low evaporation rates, and high effective moisture (precipitation minus evaporation) are essential for the formation and development of northern peatlands on suitable substrates and in suitable topographic settings. Despite the generally short summer seasons at high latitudes and the moderate net primary production (NPP) of peatland vegetation, peatlands accumulate excess organic matter as peat owing to depressed decomposition in waterlogged and anoxic conditions and the chemical recalcitrance of some peatland plant tissues. Extensive development of northern peatland ecosystems has occurred in west Siberia, central Canada, northwest Europe, and Alaska (Plate 1). Due to different regional climate and deglaciation histories, the timing of peatland initiation varies greatly from region to region [Kuhry and Turunen, 2006], but the majority of today’s peatlands first expanded in the early and mid-Holocene [MacDonald et al., 2006; Gorham et al., 2007] (Plate 1). To explore the climate domain of northern peatlands, particularly in relation to the boreal forest/taiga ecoregion [Olson et al., 2001], we compared the distribution of peatlands
[MacDonald et al., 2006] to gridded instrumental climate data (0.5° ´ 0.5° grids) for land north of 45°N (1960–1990 [Rawlins and Willmott, 1999]). Northern peatlands typically occur where mean annual temperatures are between -12° and 5°C and mean annual precipitation is between 200 and 1000 mm. This distribution spans most of the climate domain of the boreal ecoregion (Plate 2). Peatlands are often abundant in regions that receive 1500 mm, owing to coastal and orographic influences. The distinct character of the peatland regions in modern T-P space suggests that the maintenance of peatland hydrology suitable for long-term peat C accumulation is the result of various thermal-moisture associations and precipitation seasonality. In the same way, climate histories and temperature and precipitation associations in the past were likely also very different between regions. As a result, a regional perspective would be most informative in understanding and projecting C cycling responses to climate change. In particular, peatlands located in regions near the limits of peatland climate space may be the first to experience expansion and shrinkage under changing regional climates. Preliminary results from similar analysis of relative humidity (RH) show that northern peatlands have high annual RH values ranging from 65 to 95%. Peatlands with the highest RH occur in regions with a mean annual temperature around -10°C. A surprising pattern is that peatlands with the highest RH (and also a wide range of RH) tend to oc-
YU et al. 57
P1
Plate 1. The distribution of northern peatland regions (light blue [MacDonald et al., 2006]), the boreal/taiga biome (green [Olson et al., 2001]), basal peat ages north of 45°N latitude (circles with ages in legends [MacDonald et al., 2006; Gorham et al., 2007]; n = 2380), and 33 sites with detailed peat C accumulation data (yellow triangles; site numbers as in Table 1). Terrain is from the ETOPO2 data set. The extent of the Laurentide Ice Sheet at 8000 calendar years B.P. [Dyke et al., 2004] is shown by the crosshatched pattern.
cur at low annual precipitation of 2000 basal peat dates from the northern hemisphere. Also, peat C accumulation appears to have been highest in the first few millennia of the Holocene, especially in regions that experienced Holocene thermal maximum conditions at that time. We observe that peatlands having high C accumulation rates tend to occur in regions with intermediate temperature and precipitation. These regions have the largest peatland areas in the world, including west Siberia and western Canada. On the other hand, high precipitation may not necessarily result in high C accumulation, e.g., in eastern Canada and British Columbia, suggesting that water budgets and carbon balance between production and decomposition are key to net C accumulation. The early Holocene peatland expansion and C accumulation contributed to the peak in global CH4 concentration and the decline in CO2 concentration at this time. Also, the estimates on the basis of our synthesis of the largest available data sets show that, in the early Holocene before 8000 cal years B.P., northern peatlands alone may have sequestered about 100 Pg of atmospheric carbon. This synthesis of data and ideas has identified some major outstanding issues and key future research directions. 1. Our data compilation and synthesis show major data gaps for peatland initiation and carbon accumulation histories in the Russian Far East, East Siberia, and the Hudson Bay Lowland (Plate 1). These regions represent geographic locations of intermediate temperatures and high precipitation in modern climate space (Plate 2), where peat-core data are lacking. Therefore, filling these gaps will further inform our understanding of the climate sensitivity of peatland C dynamics. 2. Further refined analysis of the large data sets of available basal peat ages could provide useful information for understanding climate control and sensitivity of peatland expansion, including separation of paludified and terrestrial-
YU et al. 67
ized peatlands since these peatland formation pathways have very different climate controls. 3. There is a need to develop and integrate process-based peatland dynamic models that take into account interactions and feedbacks of local and regional factors as well as fast and slow processes affecting production and decomposition to determine net peat C accumulation. 4. Developing novel peat-based proxies will facilitate our understanding of climate sensitivity of specific ecosystem processes, including independent proxies for productivity and decomposition. Also, new proxies to indicate hydrological and permafrost dynamics would improve our understanding of these important processes. Acknowledgments. We thank Andy Baird and an anonymous reviewer for their constructive comments, and Daniel Brosseau and Julie Loisel for assistance with data analysis. Yu and Jones were supported by the United States National Science Foundation (NSF)—Biocomplexity in the Environment: Carbon and Water in the Earth System Program (ATM 0628455) for their peatland research in Alaska. Beilman was supported by the Marie Curie Incoming International Fellow Program of the European Commission (MC IIF 40974). We acknowledge the stimulating discussions and ideas presented at the NSF-supported PeatNet workshop in March 2008, which were part of the impetus for this paper.
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D. W. Beilman, Department of Geography, University of Hawai’i at Manoa, 2424 Maile Way, Honolulu, HI 96822, USA. (beilman@ hawaii.edu) M. C. Jones and Z. Yu, Department of Earth and Environmental Sciences, Lehigh University, 31 Williams Drive, Bethlehem, PA 18015, USA. (
[email protected];
[email protected])
