Biometric and eddy-covariance based estimates of annual carbon storage in five eastern North American deciduous forests

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Abstract

Quantifying net carbon (C) storage by forests is a necessary step in the validation of carbon sequestration estimates and in assessing the possible role of these ecosystems in offsetting fossil fuel emissions. In eastern North America, five sites were established in deciduous forests to provide measurements of net ecosystem CO2 exchange (NEE) using micro-meteorological methods, and measures of major carbon pools and fluxes, using a combination of forest mensurational, eco-physiological, and other biometric methods. The five study sites, part of the AmeriFlux network, ranged across 10° of latitude and 18° of longitude, but were all of similar age, canopy height, and stand basal area. Here we present a cross-site synthesis of annual carbon storage estimates, comparing meteorological and biometric approaches, and also comparing biometric estimates based on analyses of autotrophic carbon pools and heterotrophic carbon fluxes (net ecosystem production, NEP) versus those based on measurements of change in two major carbon pools (ΔC). Annual above-ground net primary production (ANPP) varied nearly two-fold among sites and was strongly correlated with average annual temperature and with annual soil nitrogen mineralization (Nmin). Estimates of NEP ranged from 0.7 Mg C per hectare per year in northern lower Michigan to 3.5 Mg C per hectare per year in central Indiana, and were also well correlated with Nmin. There was less variation among sites in estimates of ΔC (range, 1.8–3.2 Mg C per hectare per year). In general, ΔC more closely matched NEE than did NEP, but there was no systematic pattern among sites in over- versus under-estimation of the biometric compared to the meteorologically based measures. Root and soil carbon dynamics were significant sources of uncertainty in our biometric measures and represent a prerequisite area of study needed for accurate estimates of forest carbon storage.

Introduction

Temperate deciduous forests are an important and much altered component of the Northern Hemisphere. Absent human disturbance, deciduous forests are the dominant vegetation type over most of temperate eastern North America. Much of the region was converted to agricultural land use, effectively removing more than 95% of the above-ground carbon from these systems. Tillage also resulted in carbon losses from soils (Lal et al., 1995, Compton and Boone, 2000). Much of these agricultural lands have since been abandoned, and a substantial portion of the eastern deciduous forest region contains trees less than a century old. However, these forests represent a substantial and growing carbon pool. Birdsey (1992) estimated from forest inventory data an average net accrual in the eastern deciduous forest region of from 1 to 2.4 Mg C per hectare per year, 45% of US forest carbon storage. Carbon is accruing in these forests not only in standing live wood, but also in other above- and below-ground carbon pools, including standing dead wood, forest floor litter, coarse woody debris, roots, and soil organic matter.

Because forests have substantially higher growth rates and hence carbon storage than most other native vegetation types, their importance in continental and global carbon cycling and storage is magnified. Temperate forests average from twice to 20 times higher productivities than desert, grassland, or shrubland vegetation (Whittaker and Likens, 1975) and store up to several orders of magnitude more carbon above-ground than do these other vegetation types. Consequently, much recent attention has focused on the status of temperate deciduous forests as sources or sinks in the global carbon cycle (Fan et al., 1998, Canadell et al., 2000). The net gain, or loss, of carbon from an ecosystem is defined as net ecosystem production (NEP, or PNE) and results from the gain of carbon from autotrophic organisms (gross primary productivity, GPP, or PGP) minus its loss from autotrophic (Ra) and heterotrophic (Rh) respiration:PNE=PGP−Ra−Rh

The difference between GPP and Ra, or net primary production (NPP, or PNP), has been of interest to ecologists for several decades (e.g. Reichle, 1970, Lieth and Whittaker, 1975, US National Committee for the International Biological Program, 1975). Forest NPP estimates have been based primarily upon measurements of stem, branch, woody root, and foliage mass gain using allometric relationships for large trees (Baskerville, 1972) and harvest methods for other ecosystem components. That isPNP=PGP−Ra=L+D+Hwhere for some unit of time (typically a year), L is the increment in live plant mass, D the increment in dead plant mass (detritus), and H is the increment lost to herbivory (Waring and Schlesinger, 1985). In principle, each variable includes above- and below-ground processes. For simplicity we define these and other plant- or soil-based approaches as ‘biometric’ measurements and note that all terms can be expressed on either a tissue dry mass or carbon mass basis.

Biometric NPP estimates commonly ignore volatile organic compounds which may be lost from ecosystems (Hanson and Hoffman, 1994, Isebrands et al., 1999) as well as the loss of dissolved organic carbon with water flow. Carbon transport by vertebrates (i.e. seed movements by birds and squirrels) is assumed to be random and losses from a particular area are balanced by gains from other locations. There are additional NPP components (e.g. nectar and pollen production) that commonly are not accounted for and are assumed to be small (but see Clark et al., 2001).

It follows that NEP can be expressed as the difference between NPP and Rh:PNE=(L+D+H)−Rhor the summed increment of all autotrophic carbon pools less heterotrophic respiratory losses. Soil microbial respiration, usually measured as the difference between total soil respiration and root respiration (Hanson et al., 2000), is the main contributor to Rh.

If mass losses from H, volatile organic compounds, or other processes are small, and the annual increment in foliar and fine root mass is approximately zero, forest annual carbon storage can also be expressed as the annual increment in wood (W) plus soil (S) carbon pools:ΔC=W+Ssince S is the difference between D and Rh. While NEP and ΔC are not independent, ΔC does not require measurement of below-ground D or Rh, variables which cannot be easily measured directly.

With improvements in micro-meteorological instrument technology, the eddy-covariance technique has been applied to the measurement of energy and mass exchange over forest canopies, enhancing our understanding of soil–vegetation–atmosphere carbon exchanges (Wofsy et al., 1993, Baldocchi et al., 1996, Goulden et al., 1996, Goulden et al., 1998, Baldocchi and Meyers, 1998). Net ecosystem CO2 exchange (NEE), measured using eddy-covariance methods, integrates vegetation and soil CO2 fluxes, and can provide a measure of net carbon exchange on a sub-hourly basis. However, eddy-covariance measurements become unreliable or unavailable during precipitation, icing, or very weak turbulence, as sometimes occurs at night (Lee, 1998, Baldocchi et al., 2000). A variety of modeling techniques (Schmid et al., 2000; and references in this volume) are used to make corrections for the estimation of annual NEE. Conceptually, NEE and NEP are equivalent in that both constitute the difference between GPP and total ecosystem respiration (Rtotal=Ra+Rh). Methodogically, however, they are independent, having unrelated measurement errors. As such, their comparison helps constrain estimates of forest carbon storage and may provide insight into the causes of within (Barford et al., 2001) and among site differences in annual carbon sequestration (Wofsy and Hollinger, 1998).

In this paper we describe the estimation of annual carbon storage using biometric methods at five sites within the eastern deciduous forest biome of North America and compare them with eddy-covariance based estimates of NEE from the same sites. These sites are part of the AmeriFlux carbon cycle research program (Wofsy and Hollinger, 1998). Our objective was to conduct a cross-site comparison of biometric estimates of annual carbon storage and to evaluate whether such a synthesis could provide an independent constraint on meteorologically based estimates of the same variable. We also hoped to gain insight into the major sources of uncertainty in our ecosystem carbon budgets.

Section snippets

Methods

At each of the five sites, efforts have been made to derive biometric estimates of annual carbon storage by measurement of major carbon pools and fluxes as presented in Table 1, and to measure annual NEE using eddy-covariance methods. Given their ecological similarities and their use of standard methods for quantifying many carbon cycle components, there was considerable overlap among sites in the details of their biometric and meteorological measurements. However, differences among sites in

Results

Annual soil net Nmin varied more than 10-fold among sites but with two distinct groups evident: two southern sites on fine textured soils with high Nmin, and three northern sites on sandy soils with much lower Nmin (Table 2). Climatic and edaphic differences notwithstanding, the five sites are quite similar in stand age and structure. All are late secondary successional forests of comparable height and stand basal area, having regrown following widespread disturbance in the late 19th or early

Discussion

Above-ground net primary productivity at these five AmeriFlux sites fell within the range expected for eastern North American deciduous forests. We found in the more productive, southern sites annual carbon accumulation of ∼5 Mg C per hectare per year, comparable to that reported for similar aged stands in New Hampshire (Whittaker et al., 1974), Wisconsin (Pastor and Bockheim, 1981), and Kentucky (Liu and Muller, 1993). Our northern sites, growing on poorer soils and experiencing colder

Acknowledgements

This work began during discussions at a workshop, ‘Biometry based estimates of forest NEP: an evaluation of methods and results across AmeriFlux sites’ held 4–7 May 2000 at the University of Michigan Biological Station, Pellston, Michigan, USA. Workshop funding was provided by the Midwestern Center of the National Institute for Global Environmental Change through the US Department of Energy (Cooperative Agreement No. DE-FC03-90ER61010). Any opinions, findings, and conclusions or recommendations

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