Ecosystem respiration and its components in an old-growth forest in the Great Lakes region of the United States
Introduction
Net ecosystem exchange (NEE) between the atmosphere and forests has become a focus of climate change research due to the potential of forests to reduce enhanced atmospheric CO2 concentration (IPCC, 2001, Tans et al., 1990, Fan et al., 1998). Because NEE is the small difference between two large fluxes of photosynthesis and respiration and is typically an order of magnitude smaller than respiration or photosynthesis (Goulden et al., 1996a, Law et al., 1999), NEE is sensitive to both respiration and photosynthesis and often changes sign within and among sites (e.g., Euskirchen et al., 2006). Despite the possibly higher importance of respiration than photosynthesis in determining the variability of NEE across latitudinal gradients (Valentini et al., 2000), respiration and its components have been the focus of fewer studies (Law et al., 1999).
Ecosystem respiration is composed of autotrophic and heterotrophic components whose contributions to total respiration vary in space and time. There is no consensus on methods for measuring each component of respiration and estimating the annual sum. Components of respiration include soil respiration, stem respiration, leaf respiration, and woody and surface litter respiration. Soil respiration may be further partitioned into root respiration with associated rhizosphere respiration, and microbial respiration. These components all respond primarily to temperature, but many components are affected by additional factors. For example, soil respiration may be controlled by photosynthesis in additional to environmental variables (Hogberg et al., 2001, Tang et al., 2005a). Soil moisture is an important control on soil respiration in arid and semi-arid ecosystems (Xu and Qi, 2001, Tang and Baldocchi, 2005). Precipitation frequency and duration may affect soil respiration during and after the drought (Xu et al., 2004). Leaf respiration may be driven by temperature and related to species and leaf nitrogen content (Bolstad et al., 1999). It is therefore difficult to measure and model respiration components.
The eddy covariance technique has provided a useful tool to continuously measure NEE from hourly to daily, annual and interannual periods (Aubinet et al., 2000, Baldocchi, 2003). However, eddy covariance measurements do not provide direct information on component fluxes and are difficult in assessing over spatially heterogeneous areas due to inherent variation in the measurement footprint with time. Eddy covariance methods need to be complemented and compared to component fluxes in order to interpret and understand the variability of fluxes. As a complementary method, chamber measurements have been used to sample component fluxes and upscale to the annual carbon budget (Law et al., 1999, Xu et al., 2001, Bolstad et al., 2004).
Studying carbon fluxes from old-growth forests helps us to understand the successional change of carbon fluxes and the future trend of current second-growth forests. While there are a large and growing number of eddy flux sites (Baldocchi, 2003), there are relatively few in old-growth forests (Paw et al., 2004, Chen et al., 2004, Desai et al., 2005), which have received negligible human disturbance and are dominated by trees greater than two centuries old. Whole system and component measurements in older forests allow us to test the conceptual model that forest net primary production (NPP) and NEE declines with forest age (Kira and Shidei, 1967, Odum, 1969, Ryan et al., 1997, Gower et al., 1996), and to test the assumption that old-growth forests reach NEE equilibrium compared with young and recovering forests (Carey et al., 2001). NEE is typically positive (i.e., a carbon source) during stand establishment periods due to large heterotrophic losses to the atmosphere from the soil and surface litter. NEE becomes negative (i.e., a carbon sink) as standing biomass increases and net photosynthesis balances and then surpasses ecosystem respiration. Respiration is expected to continuously increase as stems and detritus accumulate in older forests, and eventually balances deceasing photosynthesis when forests age (Odum, 1969, Kira and Shidei, 1967). However, there are no empirical data to support this conceptual model that old forests reduce photosynthesis but enhance respiration (Ryan et al., 2004). A few empirical studies directly measuring NEE using the eddy covariance method have revealed that old-growth forests are carbon sinks (Grace et al., 1995, Carswell et al., 2002, Roser et al., 2002, Paw et al., 2004, Desai et al., 2005), possibly due to the fertilization effect from increasing atmospheric CO2 concentration and nitrogen deposition rates (Grace et al., 1995). Despite the importance of old-growth forests in studying respiration over the course of succession, we have seen few publications separately measuring component respiration in old-growth forests except for Law et al. (2001) and Harmon et al. (2004).
Our objectives were to (1) measure respiration components from two stands with different dominant species in an old-growth northern forests, (2) estimate the annual sum of respiration and percentage of each component, and (3) compare respiration from the old-growth forest with a young and a mature second-growth forest under similar climate, and derive the successional pattern of respiration.
Section snippets
Site description
The study area is located on the boundary of the Sylvania Wilderness and Recreation Area of the Ottawa National Forest in the upper peninsula of Michigan, USA (46°14′31″N, 89°20′52″W). Average elevation is 542 m. The climate is northern continental, characterized by short growing seasons and long, cold winters. Annual average precipitation and air temperature measured in a nearby weather station over 1961–1990 is 896 mm and 3.9 °C, respectively. Precipitation is evenly distributed in all seasons.
Soil respiration
Measurements of soil respiration indicated that the seasonal pattern of soil respiration ranged from 1.3 to 4.5 μmol m−2 s−1 in the hardwood stand and from 1.1 to 4.0 μmol m−2 s−1 in the hemlock stand (Fig. 1). Soil respiration in the hardwood stand was systematically higher than that in the hemlock stand, except on days 270 and 283 in 2003. Soil respiration peaked in July (day 199) in 2002 but peaked in late August (day 238) in 2003 in the hardwood stand, and peaked at the end of June (day 179) in
Controls on respiration
Temperature was the primary control on respiration in this northern forest site, and an exponential response function appears to explain most of the observed temporal variation. While temperature sensitivity (Q10) may be temperature-dependent (Lloyd and Taylor, 1994, Kirschbaum, 1995), and Q10 may change with soil moisture (Xu and Qi, 2001, Tang et al., 2005b), fixed Q10 values over the season provide useful estimates for component and summed total ecosystem respiration at our site.
In contrast
Conclusions
Chamber-based flux measurements combined with spatial and temporal upscaling allow us to estimate component respiration and total ecosystem respiration. Temperature was the primary control on respiration in the old-growth forest in the Great Lake region. Exponential functions explained most of the observed temporal variations in respiration in response to temperature.
Cumulative ecosystem respiration was estimated to be 1013 and 922 g C m−2 y−1 in the hardwood and hemlock stands, respectively.
Acknowledgments
We thank D. Hudleston, L. Kreller, and Jing Zhang from University of Minnesota and staff from University of Wisconsin Kemp Natural Resources Station for their fieldwork assistance. We also thank the guest editor, Dr. Jiquan Chen, and three anonymous reviewers for many valuable comments and suggestions in improving the early version of this manuscript. This work was primarily funded by the Office of Science/BER, U.S. Department of Energy Terrestrial Carbon Processes program (DE-FG02-00ER63023).
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