Agricultural and Forest Meteorology 101 2000 1–14
CO
2
exchange at the floor of a boreal forest
Ann-Sofie Morén
a,∗
, Anders Lindroth
b,1
a
Department for Production Ecology, Swedish University of Agricultural Sciences, P.O. Box 7042, SE-750 07 Uppsala, Sweden
b
Department of Physical Geography, Lund University, Sölvegatan 13, SE-223 62 Lund, Sweden Received 26 February 1999; received in revised form 3 November 1999; accepted 16 November 1999
Abstract
Net CO
2
exchange at the forest floor in a mixed boreal spruce and pine forest in central Sweden was studied during 1996. Forest floor CO
2
efflux was measured continuously by means of a ventilated open soil chamber, covering a surface area of 0.6 m
2
. The chamber was transparent and thus measured soil respiration by night, and soil respiration reduced by photosynthetic uptake by forest floor vegetation by day. Maximum nocturnal efflux rates were 0.2–0.7 and daytime rates
were 0.05–0.2 mg m
− 2
s
− 1
. Measured efflux rates were higher than reported in other studies, but have earlier been found to agree with nocturnal CO
2
exchange of the forest ecosystem measured by eddy-covariance technique. Soil temperature at 5 cm explained 49 of the variation in nocturnal soil respiration, while moss and air temperature explained 29 and 17 of
the variation, respectively. For the relationship with soil temperature at 5 cm, base respiration rate and effective Q
10
, derived from data over the seasonal course, were 0.04 mg m
− 2
s
− 1
and 4.75, respectively. Corresponding figures for the relationship with air temperature were 0.11 mg m
− 2
s
− 1
and 1.89. Over the 6 months May–October covered by measurements, August had the largest CO
2
efflux, 0.89 kg m
− 2
and May the smallest efflux, 0.20 kg m
− 2
. During daytime photosynthetic uptake by forest floor vegetation reduced potential soil efflux through respiration by ca. 20. On an annual basis total forest floor
respiration was estimated to be 4.5 kg CO
2
m
− 2
and gross photosynthesis to be 0.7 kg CO
2
m
− 2
, resulting in a net efflux of 3.8 kg CO
2
m
− 2
. ©2000 Elsevier Science B.V. All rights reserved.
Keywords: Soil respiration; Net assimilation; Chamber system; Scots pine; Norway spruce
1. Introduction
Soils throughout the world, and boreal forest soils in particular, are currently attracting the attention of
the scientific community. One reason for this is that global circulation models GCM, indicate for a range
of CO
2
emission scenarios, a continuous increase in temperatures, the largest increases being expected
∗
Corresponding author. Tel.: +46-018-67-2559; fax: +46-018-67-3376.
E-mail addresses: ann-sofie.morenspek.slu.se A.-S. Mor´en,
anders.lindrothnatgeo.lu.se A. Lindroth.
1
Fax: +46-046-2224011.
at high latitudes. Recent simulations for the period 1990−2050 predict a global warming of 2–3
◦
C in winter and 1–2
◦
C in summer for most of the boreal region Greco et al., 1994. Soil respiration and soil
organic matter SOM decomposition are most sen- sitive to a temperature increase in areas where soil
temperatures are low, as is the case in tundra and boreal forests Lloyd and Taylor, 1994; Kirschbaum,
1995. Therefore, the combination of a large carbon pool in boreal soils Dixon et al., 1994 and increas-
ing temperature, will inevitably increase soil respi- ration rates and SOM decomposition, which might
transform many boreal forests into carbon sources Kirschbaum, 1995. Both Goulden et al. 1998 and
0168-192300 – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 9 9 0 0 1 6 0 - 4
2 A.-S. Mor´en, A. Lindroth Agricultural and Forest Meteorology 101 2000 1–14
Lindroth et al. 1998 found that boreal forests can already be a source of carbon for substantial periods,
which contrasts sharply with the common belief that forests are always carbon sinks Wofsy et al., 1993;
Grace et al., 1995; Goulden et al., 1996a.
Technical developments during the 1990s made it possible to measure the instantaneous exchange of
CO
2
between the biosphere and the atmosphere di- rectly, by means of the eddy-covariance technique e.g.
Baldocchi and Meyers, 1991; Grelle and Lindroth, 1996, and at present the technique is widely applied
in ecosystem studies Kaiser, 1998. Data from such studies give, for example, information on the response
of ecosystems to climate cf. Goulden et al., 1997, while the governing processes, such as photosynthe-
sis, and soil and woody biomass respiration, can only be studied at smaller spatial scales. In particular, pre-
dictions of the exchange of CO
2
at ecosystem level require a mechanistic understanding of the governing
processes Steffen et al., 1998. To increase under- standing of diurnal, seasonal and between-year vari-
ation encountered at ecosystem level, measurements must be supplemented by measurements at lower lev-
els of scale, in combination with models which scale compartment fluxes to ecosystem level e.g. Sellers
et al., 1997; Steffen et al., 1998. In particular, com- paratively little attention has been paid to roots and
below-ground organisms and their function Jones et al., 1998. Since photosynthesis and respiration, in
boreal ecosystems, are almost equal in absolute terms on an annual basis e.g. Lindroth et al., 1998 our
knowledge of the direct and indirect long-term effects of increasing CO
2
concentration and temperature on boreal forest ecosystems therefore is very uncertain.
The efflux of CO
2
from the soil is in principle the re- sult of two processes: the production and the transport
of CO
2
. In forest soil, plant roots and soil microbes are the dominant CO
2
producers. Their CO
2
produc- tion depends on a number of external factors, such as
soil organic content, soil moisture, temperature, oxy- gen supply, CO
2
concentration, nutrient availability, etc., as well as a number of internal factors, such as
root biomass, and the size and composition of the mi- crobial population see e.g. Glinski and Stepniewski,
1985. Transport takes place both under the influence of pressure gradients — mass flow, and concentra-
tion gradients — diffusion flow Glinski and Step- niewski, 1985. Normally, diffusion is believed to be of
far greater significance than mass flow Simunek and Suarez, 1993. When the water content of a soil is close
to saturation, however, the importance of mass flow increases, and at saturation it is more important than
the contribution from gas-phase diffusion cf. Suarez and Simunek, 1993; Freijer and Leffelaar, 1996.
Despite a long history of soil CO
2
efflux measure- ments, this process remains one of the most difficult
to measure in an accurate and appropriate manner see e.g. Norman et al., 1997; Rayment and Jarvis,
1997. The methods available are usually based on one of the four main principles: closed-static and
closed-dynamic chamber systems, open-chamber sys- tems and eddy-covariance systems e.g. Nakayama,
1990; Norman et al., 1997. Closed-static chambers with chemical traps such as soda lime, have been
widely used, but the technique has been shown to be hampered by systematic errors Janssens and Ceule-
mans, 1998. Today, the most common technique employed is probably the closed-dynamic chamber,
in which air circulates between the chamber and an external IRGA, and where the increase in CO
2
concentration as a function of time is proportional to the CO
2
flux. Open-chamber systems and the eddy-covariance technique are, however, increasingly
common, because they can be left unattended for an extended period to observe time courses, temperature
responses, etc. e.g. Iritz et al., 1997; Rayment and Jarvis, 1997. Furthermore, the eddy-covariance tech-
nique has the advantage of not affecting the emission of CO
2
from the soil, but requires large, uniform areas with no other sources and sinks between the
surface and the measuring height e.g. Verma, 1990; Baldocchi and Vogel, 1996; Baldocchi et al., 1997.
Many sources of error are associated with chamber design, which may seriously affect the CO
2
efflux rate see Rayment and Jarvis, 1997. Today there
exists no independent method to measure soil CO
2
efflux. An alternative approach is to compare scaled compartment fluxes to nocturnal ecosystem exchange
measured by eddy-covariance technique above forest ecosystems. Lavigne et al. 1997 estimated forest
ecosystem respiration, by scaling chamber measure- ments at six boreal coniferous forest sites and com-
pared to nocturnal ecosystem respiration. On average for the six sites, ecosystem respiration was consis-
tently 27 lower than scaled chamber measurements. Similarly, Goulden et al. 1996b, for a temperate
A.-S. Mor´en, A. Lindroth Agricultural and Forest Meteorology 101 2000 1–14 3
deciduous forest, found that nocturnal ecosystem res- piration was about 65 of scaled chamber measure-
ments. In contrast to these two studies, Lindroth et al. 1998, found good agreement, though for a limited
period, between nocturnal ecosystem respiration and scaled chamber measurements. The surface area of the
chamber is also important, because of the great spatial heterogeneity encountered in forest soils Rayment
and Jarvis, 1997; Norman et al., 1997. To investigate differences between soil chamber systems, Norman
et al. 1997 compared six different systems, each of the basic types of system mentioned above being rep-
resented. Adjustment factors, to bring all the systems into agreement, varied from 0.93 to 1.45, with an un-
certainty of ca. 10–15. The commercially available LI-6200 LI-COR, Inc., Lincoln, NE, with a chamber
covering a surface area of about 80 cm
2
, was used as a reference system. The primary source of variability
between chamber systems was associated with spatial heterogeneity. Thus many questions related to mea-
surements of soil CO
2
remain to be answered. One of the most crucial questions is which soil-chamber
system gives the most accurate measurements; an- other is how to account for the spatial variation of
soil CO
2
efflux encountered in forest ecosystems. Furthermore, as addressed by Lavigne et al. 1997,
the question also remains whether scaled chamber or eddy-covariance measurements provide more accurate
estimates of ecosystem respiration rates.
As indicated above the exchange between soil and atmosphere is a vital field of study. Not the least mea-
surement techniques constitute an ongoing challenge to the scientific community. In order to quantify forest
floor contribution to total ecosystem CO
2
exchange on daily as well as on a seasonal basis, a recently
developed chamber monitoring system was used. The system, which before only was applied over a clay
soil, continuously measures CO
2
and water exchange at surface areas of 0.6 m
2
, which is 4–75 times larger than that of the soil chambers commonly used in soil
respiration studies e.g. Mathes and Schriefer, 1985; Goulden and Crill, 1997; Norman et al., 1997. The
main aim of the present study was to quantify and model the CO
2
exchange at the forest floor of a bo- real forest in central Sweden. This paper i presents
the site and measurements made with the chamber, ii quantifies the CO
2
efflux of the forest floor at the site and discusses its seasonal variation, iii
partitions the net CO
2
efflux of the forest floor into gross photosynthesis and respiration, iv quantifies
the effect of abiotic environment on the gross photo- synthesis of the forest floor, and v discusses advan-
tages and disadvantages of the system as applied in a boreal forest.
2. Materials and methods
