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Predicting the onset of net carbon uptake by deciduous forests with soil temperature and climate data: A synthesis of FLUXNET data

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We tested the hypothesis that the date of the onset of net carbon uptake by temperate deciduous forest canopies corresponds with the time when the mean daily soil temperature equals the mean annual air temperature. The hypothesis was tested using over 30 site-years of data from 12 field sites where CO(2) exchange is being measured continuously with the eddy covariance method. The sites spanned the geographic range of Europe, North America and Asia and spanned a climate space of 16 degrees C in mean annual temperature. The tested phenology rule was robust and worked well over a 75 day range of the initiation of carbon uptake, starting as early as day 88 near Ione, California to as late as day 147 near Takayama, Japan. Overall, we observed that 64% of variance in the timing when net carbon uptake started was explained by the date when soil temperature matched the mean annual air temperature. We also observed a strong correlation between mean annual air temperature and the day that a deciduous forest starts to be a carbon sink. Consequently we are able to provide a simple phenological rule that can be implemented in regional carbon balance models and be assessed with soil and temperature outputs produced by climate and weather models.
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Int J Biometeorol (2005) 49: 377–387
DOI 10.1007/s00484-005-0256-4
ORIGINAL ARTICLE
Dennis D. Baldocchi · T. A. Black · P. S. Curtis ·
E. Falge · J. D. Fuentes · A. Granier · L. Gu ·
A. Knohl · K. Pilegaard · H. P. Schmid · R. Valentini ·
K. Wilson · S. Wofsy · L. Xu · S. Yamamoto
Predicting the onset of net carbon uptake by deciduous forests
with soil temperature and climate data: a synthesis of FLUXNET data
Received: 23 June 2004 / Revised: 21 October 2004 / Accepted: 30 December 2004 / Published online: 2 February 2005
C
ISB 2005
Abstract We tested the hypothesis that the date of the
onset of net carbon uptake by temperate deciduous for-
est canopies corresponds with the time when the mean
daily soil temperature equals the mean annual air temper-
ature. The hypothesis was tested using over 30 site-years
of data from 12 field sites where CO
2
exchange is being
measured continuously with the eddy covariance method.
The sites spanned the geographic range of Europe, North
America and Asia and spanned a climate space of 16
Cin
mean annual temperature. The tested phenology rule was
D. D. Baldocchi (
) · L. Xu
Ecosystem Science Division, Department of Environmental
Science, Policy and Management, University of California,
151 Hilgard Hall,
Berkeley, CA 94720, USA
e-mail: baldocchi@nature.berkeley.edu
T. A. Black
Faculty of Agricultural Sciences, University of British
Columbia,
Vancouver, BC, Canada
P. S. Curtis
Department of Evolution, Ecology & Organismal Biology, Ohio
State University,
Columbus, OH, USA
E. Falge
Bayreuth University,
Bayreuth, Germany
J. D. Fuentes
Department of Environmental Sciences, University of Virginia,
Charlottesville, VA, USA
A. Granier
INRA,
Champenoux, France
L. Gu
Environmental Science Division, Oak Ridge National
Laboratory,
Oak Ridge, TN, USA
A. Knohl
Max Planck Institute for Biogeochemistry, Jena Germany,
K. Pilegaard
RISOE,
Roskilde, Denmark
H. P. Schmid
Department of Geography, Indiana University,
Bloomington, IN, USA
R. Valentini
Department of Forest Science and Environment, Universita’ di
Tuscia,
Viterbo, Italy
K. Wilson
Atmospheric Turbulence and Diffusion Division, NOAA,
Oak Ridge, TN, USA
S. Wofsy
Department of Earth and Planetary Science, Harvard University,
Cambridge, MA, USA
S. Yamamoto
National Institute of Advanced Industrial Science and
Technology,
Tsukuba, Ibaraki 305-8569, Japan
Present address:
L. Xu
LICOR,
Lincoln, NE, USA
A. Knohl
ESPM, University of California,
Berkeley, CA, USA
robust and worked well over a 75 day range of the initia-
tion of carbon uptake, starting as early as day 88 near Ione,
California to as late as day 147 near Takayama, Japan. Over-
all, we observed that 64% of variance in the timing when net
carbon uptake started was explained by the date when soil
temperature matched the mean annual air temperature. We
also observed a strong correlation between mean annual air
temperature and the day that a deciduous forest starts to be
a carbon sink. Consequently we are able to provide a sim-
ple phenological rule that can be implemented in regional
378
carbon balance models and be assessed with soil and tem-
perature outputs produced by climate and weather models.
Keywords Phenology
.
Eddy covariance
.
CO
2
exchange
.
Canopy photosynthesis
Introduction
The timing of leaf out, across the temperate deciduous for-
est biome, has major implications on the seasonal variation
of numerous ecosystem/atmosphere interactions. For ex-
ample, the transition between dormant and vegetated states
of deciduous forests causes an abrupt change in surface
albedo, aerodynamic roughness and the surface conduc-
tance to water, carbon dioxide and pollutant transfer. To-
gether, these variables alter the availability of energy and its
partitioning into sensible and latent heat exchange(Blanken
et al. 2001; Moore et al. 1996; Wilson and Baldocchi 2000).
A perturbation in the partitioning of energy is consequen-
tial because it alters the growth rate and ultimate depth
of the planetary boundary layer (McNaughton and Spriggs
1986). The modifications in boundary layer growth, in turn,
affect the diurnal course and amplitude of temperature
and humidity within the surface and planetary boundary
layer (Schwartz and Karl 1990). Furthermore, the absence
or presence of leaves can act as a switch for the forma-
tion of convective clouds through links with the properties
of the planetary boundary layers (Fitzjarrald et al. 2001;
Freedman et al. 2001; Schwartz and Crawford 2001).
The timing of leaf-out marks the beginning of the pho-
tosynthetic season for a deciduous forest and is a major
determinant of its duration (White et al. 1999). With re-
gards to terrestrial carbon cycling, the length of carbon
uptake period has much predictive power about the spatial
variation of the net annual carbon exchange of ecosystems
(NEE) across a latitudinal and continental gradient of de-
ciduous forests (Baldocchi et al. 2001)—the length of the
carbon uptake period explained 80% of the spatial vari-
ance in annual NEE. It has also been reported—on both
experimental (Black et al. 2000; Goulden et al. 1996b;
Schmid et al. 2000; Schmid et al. 2003) and theoretical
bases (White et al. 1999)—that the timing of leaf out pro-
vides partial explanation for the year-to-year variability in
NEE at individual sites; additional controlling factors on
NEE include presence and absence of snow, drought, and
summer cloudiness.
Lately, phenology has received added attention because
of its role as a surrogate in detecting global climate change
(Jackson et al. 2001; Penuelas and Filella 2001; White
et al. 2003). Phenological indices measured across Europe
(Menzel and Fabian 1999) and North America (Cayan et al.
2001) and interannual measurements of biosphere green-
ness, as observed by satellites (Myneni et al. 1997; Tucker
et al. 2001), are detecting a trend towards earlier springtime
leaf-out, portending a potential signal of global warming. In
order to simulate the implications of changing phenology
on biosphere-atmosphere interactions, models that com-
pute the biogeochemical cycling of water, carbon and nu-
trients, atmospheric chemistry, weather and climate need
algorithms that can predict the timing of leaf expansion,
the initiation of photosynthesis and the onset of net carbon
uptake by deciduous forests (White et al. 1997).
Dates of bud break, leaf unfolding, and commencement
of photosynthesis have been used to characterize aspects of
forest phenology (Brugger et al. 2003;Guetal.2003).
Physiological mechanisms for the timing of springtime
phenological events involve a need for dormancy and are
triggered by interactions between growth-promoting and
inhibiting hormones (Schaber and Badeck 2003). The re-
lease of these hormones seems to be triggered by an
accumulation of winter chilling, a critical photoperiod
and springtime warming. Historically, phenological mod-
els have used photoperiod and cumulative heat and chilling
units as independent variables (Chuine et al. 2003; Kramer
et al. 2000; Nizinski and Saugier 1988; Raulier and Bernier
2000; Spano et al. 1999). While this class of models has
many practical applications, it is highly empirical. Conse-
quently, its generality is limited because the threshold sum
of heat and chill units, that determines the date of a partic-
ular phenological event, needs to be calibrated at each site
and for each mix of species (Kramer et al. 2000; Raulier and
Bernier 2000; Taylor 1974). We also note that there can be
considerable imprecision with identifying the specific date
of a phenological event due to natural temporal and spatial
variability and sampling errors. Phenological metrics like
bud break, leaf unfolding and the onset of photosynthesis
are not synchronous and can occur for an extended period
(Brugger et al. 2003; Morecroft et al. 2003). For example, it
takes 11 days for European oak leaves to transcend between
budbreak and a physiological state that maintains a positive
carbon balance (Morecroft et al. 2003). Considerable spa-
tial variation in the timing of phenology will occur within a
woodland, too, due to the presence of multiple species and
because of microclimate variations. Sampling error is yet
another source of variation. Detecting the precise date of a
phenological event depends upon the sampling frequency
and sampling area associated with manual observations or
with the frequency of passage and pixel size associated
with remote sensing instruments mounted on a satellite or
tower.
Our goal is to assess a simple phenological rule that has
a detection criterion that is based on biophysical principles
and can be assessed with information that is commonly
available at weather stations and from weather and clima-
tological forecast models (e.g. air and soil temperature).
We propose and test the hypothesis that: “the date of the
onset of net carbon uptake by a temperate deciduous forest
corresponds with the time when the mean soil tempera-
ture equals its mean annual air temperature”. This working
hypothesis was generated from observing the timing of
leaf-out near Oak Ridge, Tennessee. There, trees tended to
leaf out when the soil temperature was near 13
C (Taylor
1974), a temperature that corresponds with the mean annual
air temperature of the region. While this anecdotal obser-
vation requires further scrutiny with a larger database, we
contend that there may be mechanistic justifications for this
hypothesis. For example, soil temperature acts as a proxy
379
Table 1 List of the field sites used in this analysis, their location, mean annual air temperature and citations describing site characteristics
and meteorological measurements
Site Genera Lat Long Mean annual
temperature
citation
Prince Albert, Saskatchewan Populus 53 N 106 W 0.6 Black et al. (2000)
Douglas Lake, Michigan Populus/Quercus 45 N 84 W 6.2 Schmid et al. (2003)
Borden, Ontario Acer/Populus 44 N 79 W 6.4 Lee et al. (1999)
Collelongo Italy Fagus 41 N 13 E 6.5 Valentini et al. (1996)
Takayama, Japan Betula 36 N 137 E 7.3 Yamamoto et al. (1999)
Harvard Forest, Massachusetts Quercus/Acer 42 N 72 W 7.4 Goulden et al. (1996b)
Hainich, Germany Fagus 51N 10 E 7.5 Knohl et al. (2003)
Soroe, Denmark Fagus 55 N 11 E 7.6 Pilegaard et al. (2001)
Hesse, France Fagus 48 N 7 E 9.2 Granier et al. (2002)
Morgan Monroe, Indiana Quercus 39 N 86 W 11.8 Schmid et al. (2000)
Oak Ridge, Tennessee Quercus/Acer 36 N 84 W 14.9 Wilson and Baldocchi (2001)
Ione, California Quercus 38 N 120 W 16.5 Baldocchi et al. (2004)
for accumulated chill and heat units as it is an integrator
of these measures due to the soil’s thermal inertia and en-
ergy balance. Secondly, deciduous trees have evolved to
be in synchrony with their climate to minimize the expo-
sure of their young leaves to freezing and possibly lethal
temperatures during the spring; the probability of frost be-
comes quite low once mean daily air temperature exceeds
its annual mean temperature.
We test this hypothesis using continuous CO
2
flux mea-
surements, an approach that has been adopted in other
recent phenological studies (Gu et al. 2003; Suni et al.
2003). The analysis is based on over 30 site-years of me-
teorological and carbon flux data from 12 temperate de-
ciduous forest sites that are associated with the FLUXNET
project (Baldocchi et al. 2001). The datasets examined here
were acquired from sites that are distributed across the ge-
ographic domains of Europe, North America and Asia and
they span 16
C in mean annual temperature. An advantage
of quantifying phenology with eddy covariance measure-
ments includes its capacity to make nearly continuous mea-
surements and to sample a large area, as characterized by
its flux footprint (Schmid 2002).
Materials and methods
We restricted this phenological study to FLUXNET study
sites whose trees have broad leaves and deciduous habits.
The key genera at the sites used in this analysis include Pop-
ulus (aspen), Acer (maple), Quercus (oak), Betula (birch)
and Fagus (beech). Forests at the majority of sites inspected
formed closed canopies; their leaf area indices ranged be-
tween 3 and 6 and their tree heights ranged between 8
and 30 m. With regards to age structure, most of the sites
were second-growth forests and were less than 120 years
old. Characteristics of the sites used in this analysis, and
primary references describing additional site details, are
summarized in Table 1.
A common set of meteorological and eddy flux measure-
ments were acquired from each of the sites involved in
the analysis. The datasets scrutinized here included long-
term and simultaneous measurements of air and soil tem-
perature and net ecosystem CO
2
exchange between the
forest and the atmosphere. Air temperature was measured
above the forest stands with aspirated and shielded sensors.
Soil temperatures were measured with either thermistor
or thermocouple sensors; most sites had soil temperature
measurements at 2, 5, 8 or 10 cm depths. Daily means
were computed from the original 30 min datasets using
the mean diurnal course gap-filling method (Falge et al.
2001).
CO
2
flux densities were measured across the forest-
atmosphere interface with the eddy covariance method
(Baldocchi et al. 1988). In Europe, flux measurements
systems were based on closed path CO
2
sensors (Aubinet
et al. 2000), while in North America, both open and closed
path CO
2
sensors were used; independent studies show
that there is no bias by using either an open or closed path
sensor system to measure CO
2
exchange (Billesbach et al.
2004; Suyker and Verma 1993).
Efforts have been made by the AmeriFlux and FLUXNET
communities to inter-compare CO
2
flux measurements and
meteorological measurements with a roving, calibration
system (Baldocchi et al. 2001; Billesbach et al. 2004). In
general, the absolute errors in eddy flux measurements of
CO
2
exchange are less than 15%, with the application of
proper corrections (Goulden et al. 1996a; Hollinger et al.
2004; Massman and Lee 2002).
The date of onset of net carbon uptake was identified as
the day when daily integrated net CO
2
exchange (NEE)
experienced a transition from its winter respiration phase
to its spring/summer assimilation phase (Fig. 1). To deduce
this date with objective and statistical rigor, we regressed
measurements of daily NEE on day of year, using a
subsets of data from this springtime source-sink transition
period. The method has a clean and distinct signal with
a quantifiable error. For the case shown in Fig. 1, the
statistical variation in detecting the precise date of the
zero crossing is ±2 days, based on the 95% confidence
interval.
380
Fig. 1 An example of daily net
ecosystem CO
2
exchange
measurements (NEE)asa
function of time. The solid line
represents the linear regression
and the dashed lines are the
95% confidence interval.
During this transition period a
linear regression was fit through
the data and the ‘leaf-out’ date
was computed by inverting that
regression and solving for when
NEE was zero
On a physiological and fundamental level, it may be
preferable to detect the onset of canopy photosynthesis
rather than rely on transitional observations of NEE. How-
ever, the use of canopy photosynthesis can introduce an
additional source of error and imprecision because canopy
photosynthesis is a derived quantity that is assessed by sub-
tracting an indirect estimate of ecosystem respiration from
direct measurements of NEE (Falge et al. 2002). Ecosystem
respiration (R
eco
), for example, is deduced from night mea-
surements (which are uncertain due to turbulent mixing)
and is calculated during the daytime with a temperature-
dependent function (Falge et al. 2001). During the dynamic
spring growth period, temperature response functions, that
are commonly used to assess ecosystem respiration are apt
to fail because growth respiration is accelerating during
this transitional period and it enhances ecosystem respira-
tion (Xu et al. 2004).
Our working hypothesis presumes that there is a corre-
spondence between the date of the initiation of net carbon
uptake and the day when mean daily-averaged soil temper-
ature crosses a line corresponding with the mean annual
air temperature. Since a tree is unable to sense the mean
annual air temperature a priori, we decided to approximate
mean annual air temperature (
T ) using a low-pass, digital
recursive filter (Hamming 1989). The low-pass character
of this recursive filter provides us with a method that ap-
proximates the temporal mean by weighting the current air
temperature with its history:
T
t
= (1 α)T
t1
+αT
t
(1)
The mean air temperature, at time t, is updated based on
its previous mean value, at t1, and the most recent daily-
averaged air temperature, T
t
. The weighting factor, α,is
defined as:
α = exp
t
τ
(2)
We computed mean air temperature using a 730 day (2
year) time constant, τ , and a 1 day sampling interval, t.
Results and discussion
Before we attempt to test our working hypothesis across
a network of field sites, it is crucial that we demonstrate
whether or not this concept works across a representative
sampling of study sites (Figs. 2, 3). Figure 2 shows a test
of the hypothesis for a beech forest in Denmark; this site
is near the most northerly and coolest end of the deciduous
forest biome range in the FLUXNET network. We observe
that there is very close correspondence between the date
when net CO
2
exchange crossed the zero line and when
mean soil temperature (computed with a 3 day running
mean) crossed the isotherm representing the mean annual
air temperature at that site. A second example is shown for
a contrasting case, a site near Oak Ridge, Tenn. This site
is near the most southern and warmest end of the decidu-
ous forest biome (Fig. 3). Like the Danish forest, there is a
close correspondence between the date of the zero crossing
of canopy CO
2
exchange and when the soil temperature
crosses the line representing the mean annual air temper-
ature at that site. Despite the fact that the commencement
of net CO
2
uptake near Oak Ridge, Tenn. occurs about 30
days earlier and the mean annual air temperature is about
381
Fig. 2 Seasonal course in daily
integrated net ecosystem CO
2
exchange (NEE), mean air
temperature computed with a
digital recursive filter, mean air
temperature computed with a
digital recursive filter and the
mean daily soil temperature at
2 cm (based on a 3 day running
mean). These data were
acquired over a beech forest in
Denmark during 1999. The
double-headed arrow identifies
when soil temperature matched
mean annual air temperature
and when NEE crossedzero(the
lower horizontal line)
6
C warmer than in Denmark, the general relationship ‘that
a critical soil temperature identifies the onset of net carbon
uptake by the ecosystem’ holds.
How well the match between daily mean soil tempera-
ture and mean annual air temperature provides a precise
gauge for predicting the onset of net CO
2
uptake for the en-
tire database is quantified in Fig. 4. We report that a linear
regression between the dependent (the day T
soil
equaled
mean annual air temperature) and independent (the day
NEE equaled zero) variables accounts for 64% of the vari-
ance. Furthermore, the slope of the regression was close to,
but significantly different from one (0.929±0.21) and the
intercept was 17.1±25.3 days. Because the dependent and
independent variables have sampling and measurement er-
rors, we also computed the geometric mean regression. In
this case, the slope was 1.15±0.344 and the intercept was
8.57±41.5.
Another question we can pose and address is: ‘how well
can climatological data describe when a deciduous forest
transcends from being a carbon source to a sink?’ Figure 5
shows that the start-date of net carbon uptake becomes
earlier, in a linear fashion, as the local climate (mean annual
air temperature) becomes warmer. Overall, perturbations in
mean annual air temperature explain 69% of the variance
in the start-date of net carbon uptake.
In sum, both phenology rules seem robust by working
well over a 75 day range of the initiation of carbon uptake,
starting as early as day 88 in near Ione, California to as
late as day 147 near Takayama, Japan. However, the results
shown here may not be universal for all functional plant
types and climate zones. For example, Suni et al. (2003)
reported that soil temperature was not a good indicator
for signaling the onset of photosynthesis across the boreal
forest biome. On the other hand, they found that air temper-
ature was a good indicator of the onset of photosynthesis
for conifers at high latitudes, but they found no unifying re-
lationship that held across the boreal forest biome. Hence,
we advise the reader to apply the functional relationship
between soil temperature and the onset of carbon uptake
only to deciduous broadleaved forests. Close inspection of
Fig. 4 shows that the significant outliers were associated
with measurements at Harvard Forest, in Massachusetts,
where net carbon uptake starts later than one would expect
based on soil temperature measurements. This site is near
the eastern edge of the North American continent and is a
locale subject to much climatic variability during the spring
due to the passing of warm and cold air masses; examin-
ing over 40 years of mean air temperature we found that
the daily mean temperature ranges between 0 and 20
C
around the expected date of leaf out, approximately day
120. So soil temperature may not queue the phenology of
net carbon uptake as well at this site as others. We also
add that our phenology rule does not work well for a de-
ciduous forest in the boreal zone, where the mean annual
temperature is close to zero centigrade and snow cover
keeps soil temperature close to zero during the winter and
early spring (Griffis et al. 2003). Consequently, soil temper-
ature in this region crosses the isotherm representing mean
air temperature much sooner than when leaves emerge
(Fig. 6).
Measurements of net CO
2
exchange have the potential
for assessing the timing of leaf-out if we know the time
382
Fig. 3 Seasonal course in daily
integrated net CO
2
exchange
(NEE) and the mean daily soil
temperature at 8 cm. These data
were acquired over an
oak/maple forest near Oak
Ridge, Tenn. during 1996. The
double-headed arrow identifies
when soil temperature matched
mean annual air temperature
and when NEE crossedzero(the
lower horizontal line)
Fig. 4 The empirical
relationship between the date
when mean daily soil
temperature equals mean annual
air temperature and when daily
net ecosystem carbon exchange
(NEE) crosses zero. The solid
line represents the linear
regression through the data and
the dashed line represents the
95% confidence interval
delay between when leaves unfold and when canopy pho-
tosynthesis matches soil respiration. We detected the date
of leaf-out at a few selected sites using light transmission
measurements through the canopy; leaf-out was identified
when the fraction of beam penetration through the forest
decreased, relative to its winter deciduous state. Our detec-
tion of the date of leaf out, with CO
2
flux measurements,
accounted for over 94% of the variance in the timing of
leaf-out observed with light measurements. The statistics
associated with the linear regression possessed a non-zero
intercept of 22.1±29.1 days and the regression slope that
was less than one (0.744±0.243) (Fig. 7). We also observed
383
Fig. 5 The relationship
between the mean annual air
temperature and the day when
net carbon of deciduous forests
uptake commences. The solid
line represents the linear
regression through the data and
the dashed line represents the
95% confidence interval
Fig. 6 Seasonal course in daily
integrated net ecosystem CO
2
exchange (NEE), the mean daily
soil temperature at 8 cm
(computed with a 3 day running
mean) and mean air
temperature, computed with a
digital recursive filter. These
data were acquired over an
aspen forest in the southern
portion of the boreal zone of
Canada during 2000
that the onset of net carbon uptake, relative to the date of
leaf-out, becomes more delayed as the start of growing
season becomes later.
To compensate for the bias between the date of observed
leaf-out and the onset of net carbon uptake, we produced
a transformed metric using the regression between the ob-
served and inferred dates of leaf-out, discussed in Fig. 7.We
next compared this transformed metric for identifying the
date of leaf out against the date when mean daily soil tem-
perature crosses mean annual air temperature (Fig. 8). For
the dataset in hand, we found that the mean date of leaf out,
detected using soil and air temperature, was ±114.9±14.9
384
Fig. 7 The relationship
between the day net ecosystem
CO
2
exchange crossed zero and
the date that leaf out was
observed. Data were from sites
in Michigan, Tennessee,
Saskatchewan and California
Fig. 8 A test of the timing of
leaf out as detected by the date
when soil temperature crosses
the mean air temperature, as
computed with a recursive
digital filter. Data on the
dependent axis were assessed
by applying the empirical
relation between the observed
date of leaf out and that detected
with CO
2
flux measurements.
Statistical analysis (via a paired
t-test) indicates there is no
significant difference between
the data on the dependent and
independent axes
days and the mean date detected with the transformed CO
2
flux measurements was 116.6±14.3 days. Further analysis
of the data, using Student’s paired t statistic, indicates that
there was no significant difference between the two means
on the 5% probability level (t=0.413; P=0.60; 29 df). A
linear regression between the independent and dependent
variables explained 61% of the variance, had a slope of
0.744 and an intercept of 28.7.
There are several sources of variation associated with the
results in Figs. 5 and 8 that merit further discussion. Soil
temperature was not measured at uniform depths across
the network, so this source of variation may contribute to
some ‘noise’ introduced into the cross-site comparison. In
general, we attempted to minimize this source of variation
by: (1) using soil temperatures measured in the area of
the main root activity, 5 to 16 cm; (2) by relying on daily
385
Fig. 9 Seasonal variation in
mean daily soil at 2 and 32 cm
and mean air temperature
computed with a digital
recursive filter. The data were
collected at an oak woodland
field site in California during
2003. Both measures of soil
temperature crossed the mean
air temperature at day 90
Fig. 10 Seasonal course of
daily-integrated CO
2
flux and
canopy photosynthesis at Oak
Ridge, Tenn. during 1999.
Canopy photosynthesis was
computed by subtracting
understory eddy flux
measurements from the
overstory measurements
mean temperatures, a more conservative metric; and (3) by
applying a 3-day running mean to the soil temperatures.
Overall, site-to-site differences in the depth of soil temper-
ature measurements probably had a minor and secondary
effect on the results shown in Figs. 5 and 8. This claim is
supported with experimental data shown in Fig. 9. We ob-
serve that there was little difference when daily mean soil
temperature, at 2 or 32 cm depths, first crossed the isotherm
representing mean annual air temperature at an oak savanna
field site in California and at other sites where soil temper-
atures was measured at multiple depths as in Oak Ridge,
Tennessee and Soroe, Denmark (data not shown).
386
There may also be imprecision associated with using the
CO
2
flux cross-over date as a measure of leaf out, rather
than canopy photosynthesis. At most temperate deciduous
forest sites there will be some photosynthesis prior to this
date, which offsets soil respiration. But the temporal change
in both NEE and A
c
during spring is rapid and will only
cause a few days lag in the detected leaf out date, as shown
for a case near Oak Ridge, Tenn. (Fig. 10).
A strength of our approach, compared with traditional
phenological models based on heat degree units, is that our
method does not rely on an arbitrary heat unit threshold that
must be calibrated on a site-by-site basis. It is also worth
noting that the simple phenology scheme examined in this
report says nothing about photoperiod, which may also be
a source of variance and a weakness of the method we are
advocating here (Nizinski and Saugier 1988; Raulier and
Bernier 2000).
With regards to further work, we encourage a wider test-
ing of this scheme with remote sensing data at continental
scales. This exercise would involve predicting the seasonal
course of soil temperature at each pixel in the deciduous
forest biome and find the date when it matches the local
mean annual air temperature. Then one would compare
that product with remote sensing data of the green wave of
spring. The phenology algorithm could also use additional
validation against data from independent phenology net-
works (http://www.uwm.edu/mds/markph.html) and new
measurements being produced by the Moderate Resolu-
tion Imaging Spectroradiometer (MODIS) on the TERRA
satellite (Shabanov et al. 2003). Finally, we encourage col-
leagues to install video cameras at all FLUXNET sites and
record the state of the canopy each day.
Acknowledgements We thank the technicians, students and
postdoctoral students who helped collect data at all the field sites
and the funding agencies that supported the numerous team. The
senior author is supported by the NASA FLUXNET project and
DOE Terrestrial Carbon Program (DE-FG0203ER63638).
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... The impact of drought on exchanges at the soil-root and canopy-atmosphere interfaces (e.g., from water and CO 2 flux measurements) resulted in a reduction in transpiration and water uptake. Furthermore, modifications in chlorophyll and leaf pigments and in net carbon assimilation (i.e., decreasing photosynthesis) [9] result in stomatal closure [10]. This combination of declining precipitation and reduced soil water reservoirs coupled with the increasing water consumption will induce severe limitations of water availability for plants (i.e., drought stress). ...
Article
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Forest insects are among the most important factors of disturbance in European forests. The increase in and duration of drought stress events due to climate change not only increase the vulnerability of rural and urban forests but also predispose weakened stands to insect pest calamities. In this context, many German and European forest research institutes and environmental institutions report an increase in the densities and calamity developments of the oak processionary moth (Thaumetopoea processionea L.) not only in oak and mixed-oak forests but also in smaller areas where oak trees regularly occur, e.g., parklands, urban areas, copses, avenues, recreational forests, etc. It is expected that the oak processionary moth (OPM) will benefit from the overall weakened vitality of both individual oaks and oak stands in the future and that mass outbreaks will occur at an increased frequency. This paper reports on the effects that the OPM can have on tree performance for rural forest and urban oak trees by applying the chlorophyll fluorescence non-destructive diagnostic method for the identification and quantification of damage to oak leaves. The aim of the study was to investigate the effects of OPM frass activity on tree leaf health using chlorophyll fluorescence measurements, comparing infested host oaks with non-infested oaks in urban and forest environs. The study assessed: 1. the quantum efficiency of photosystem II (PS II), which counts as an indicator for leaf conditions, 2. the performance index, which indicates the efficiency of the photosynthetic light reaction, 3. the energy loss of the photosynthetic reaction, which is an indicator for cell damage, and 4. the degree of open reaction centers in PS II, which indicates how well light energy can be absorbed for photosynthesis. Infested urban and rural oaks showed a significantly reduced quantum yield of PS II by up to 10% compared to non-infested oak leaves. The leaf performance was significantly reduced by up to 35% for infested urban oaks and by up to 60% for infested forest oaks, respectively. The energy losses were two times higher for infested urban and forest oaks. However, OPM infestation led to a higher reduction in the photosynthetic performance in the leaves of forest oaks compared to that of urban oaks. In order to avoid permanent damage, suitable countermeasures must be taken quickly, as, immediately after pest infestation, the performance decreases significantly. A lower performance means a significant loss in biomass production as well as in tree vitality.
... 2.3.1 Accumulated soil temperature of ≥12 and 14°C at 5-cm depth Soil temperature indexes might offer helpful estimations of plant growth owing to the strong physiological link between underground temperatures and aboveground phenology, therefore, soil temperature data have also been suggested as indicators of seasonal change (Baldocchi et al., 2005;Leeper et al., 2021). Generally, the minimum temperature index required for the suitable sowing date of crops is determined by the temperature 5 cm above the ground surface. ...
Article
Full-text available
Soil temperature change has considerable impact on land surface energy and water balances, and hence on changes in weather/climate, surface/subsurface hydrology, and ecosystems. However, little is known regarding the spatiotemporal variations and influencing factors of changes in hourly soil temperature (depth: 5–320 cm) in low-latitude highland areas. This study analyzed the hourly soil temperature at each hour during 2004–2020 and at 08:00, 14:00, and 20:00 (Beijing Time) during 1961–2020. The results revealed the following. 1) As soil depth increased, average soil temperature increased in autumn and winter, and decreased annually and in spring and summer. It exhibited significant increase during 00:00–23:00 annually, seasonally, and monthly, especially at depths of 40–320 cm during 2004–2020. Average soil temperature increased at 08:00 and decreased at 14:00 and 20:00 with increasing soil depth, but the opposite trend was found annually, seasonally, and monthly at 08:00, 14:00, and 20:00 during 1961–2020. 2) With increasing elevation, average soil temperature decreased at 08:00, 14:00, and 20:00 at depths of 5–20 cm, and showed significant increase trend at 08:00 and 14:00 at depths of 10–20 cm (except at 14:00 at 10-cm depth). 3) At 5-cm depth, the critical accumulated soil temperature of ≥12°C and 14°C extended the potential growing season during 1961–2020. 5) Significant uptrend of hourly soil temperature annually, seasonally, and monthly potentially leads to additional release of carbon to the atmosphere and increased soil respiration, reinforcing climate warming. These findings contribute to better understanding of the variation of shallow soil temperatures and land–atmosphere interactions in low-latitude highland areas.
... Phenological field observations are only made in a few places and over a short period. Using satellite remote sensing data, a different strategy was created that enables explicit spatio-temporal observation on a broad scale [1,13,14], making it an effective tool for the efficient modelling of forest biophysical parameters. NDVI and EVI (enhanced vegetation index), two of the most often used remotely sensed indices, were utilised to track the phenological and seasonal variation in vegetation growth [5,7]. ...
Article
Full-text available
Understanding ecosystem functional behaviour and its response to climate change necessitates a detailed understanding of vegetation phenology. The present study investigates the effect of an elevational gradient, temperature, and precipitation on the start of the season (SOS) and end of the season (EOS), in major forest types of the Kumaon region of the western Himalaya. The analysis made use of the Normalised Difference Vegetation Index (NDVI) time series that was observed by the optical datasets between the years 2001 and 2019. The relationship between vegetation growth stages (phenophases) and climatic variables was investigated as an interannual variation, variation along the elevation, and variation with latitude. The SOS indicates a delayed trend along the elevational gradient (EG) till mid-latitude and shows an advancing pattern thereafter. The highest rate of change for the SOS and EOS is 3.3 and 2.9 days per year in grassland (GL). The lowest rate of temporal change for SOS is 0.9 days per year in mixed forests and for EOS it is 1.2 days per year in evergreen needle-leaf forests (ENF). Similarly, the highest rate of change in SOS along the elevation gradient is 2.4 days/100 m in evergreen broadleaf forest (EBF) and the lowest is −0.7 days/100 m in savanna, and for EOS, the highest rate of change is 2.2 days/100 m in EBF and lowest is −0.9 days/100 m in GL. Winter warming and low winter precipitation push EOS days further. In the present study area, due to winter warming and summer dryness, despite a warming trend in springseason or springtime, onset of the vegetation growth cycle shows a delayed trend across the vegetation types. As vegetation phenology responds differently over heterogeneous mountain landscapes to climate change, a detailed local-level observational insight could improve our understanding of climate change mitigation and adaptation policies.
... Unlike range extension or ecosystem composition change, which can be confounded by other factors such as land use change, temperature is the dominant influence. In addition, changes in the cycle of phenological events and lengthening of the growing season can significantly affect: (i) the global carbon and water cycle (Baldocchi et al. 2005;Churkina et al. 2005;Piao et al. 2007); (ii) shifts in agricultural zoning (Fischer et al. 2005;Fischer et al. 2002); (iii) changing response of vegetation to the atmospheric boundary layer (Schwartz and Crawford, 2001); ...
Thesis
Vegetation phenological stages are important indicators for monitoring vegetation growth, evaluating climate change impacts on vegetation, control atmospheric general circulations and carbon sequestration. Traditional phenology observations rely on fixed-point visual inspection. However, this method is labour-intensive and subjective, and often limited to few species. Remote sensing technology using vegetation indices provides a more objective, long-term, continuous and efficient way to monitor land surface phenology from regional to world wide scale. The European Space Agency (ESA)’s Medium Resolution Imaging Spectrometer (MERIS) data in red/NIR (near infrared) were used to produce the level-2 product of MERIS Terrestrial Chlorophyll Index (MTCI). The MTCI, is strongly linked with the red edge position (REP) in vegetation spectra, and in turn the foliar chlorophyll content, making the MTCI a useful product of vegetation phenology indicator. In this thesis the MTCI data with different resolutions were applied to monitor vegetation phenological variables over mainland China, namely onset of greenness (OG) and end of senescence (ES). Then they were correlated with climatic factors of temperature and precipitation, demonstrated the main drives for major vegetation types in climate zones. Both MTCI and NDVI time-series captured the growth patterns for major vegetation types, the OG estimates were more consistent than the ES, and overall the NDVI gave later ES estimates than the MTCI. 9-year phenology maps showed that the OG was advanced and the ES was delayed in general. The OG was more related with latitude than the ES especially in the north China, while it was the opposite for the ES. And it was found in north China, the temperature was the main driver for the earlier OG, while in the south precipitation played a prominent role in advancing the OG. For the ES, both precipitation and temperature influenced partially. In Qinghai-Tibet Plateau, the precipitation was the main driver for both shifting OG and ES of grass, while less influenced by temperature. Among the vegetations that were examined, the broadleaf forest had the strongest correlation with climatic factors; the needle leaf forest was also greatly influenced by climate in cold temperate zone; the grass was highly affected by climate, while the mixed forest and crops were at moderate level. In the light of the abilities of MTCI in monitoring vegetation phenology, MTCI was applied into specific situations to test the performance on phenology-based applications, including mapping paddy rice in northeast China and predict rice yield. The results were well consistent with the statistical data on the prefectural level and county level in spatial distribution and quantity from 2007 to 2011. The crop yield regression models indicated that the maximum value of MTCI time-series has a better correlation with rice yield. In summary, MTCI has its own advantages than popular index such as normalised difference vegetation index (NDVI). It is more sensitive to high values of chlorophyll content and less sensitive to spatial resolution and atmospheric effects. Although the MERIS was not in operation anymore in May 2012, the successor, the Sentinel satellites were launched, with a wider range of wavelength from blue to shortwave infrared, including red edge bands. Therefore, index for estimating foliar chlorophyll can be produced to combine with other Sentinel products in agricultural, biological, and ecological studies.<br/
... Half-hourly tower measured GPP and SW were average over GS for each site to calculate GS LUE during the period of 2000-2014. We also calculated GS period using tower-measured NEE (NEE > 0) (Baldocchi et al., 2005). The GPP-based GS LUE was strongly correlated with NEE-based GS LUE, with the regression line approaching the 1:1 line (R 2 = 0.94; p < 0.05; Figure S3 in Supporting Information S1). ...
Article
Full-text available
Plain Language Summary Vegetation light‐use efficiency (LUE) is a key factor for reliable estimations of gross primary production, which is an important component of terrestrial carbon cycle. Thorough knowledge of global vegetation LUE and the drivers is required for better understanding the response of terrestrial ecosystems to climate change. Using measurements from 172 flux tower sites and satellite observations, we found large spatial variations in LUE globally. These large variations were primarily driven by environmental factors, plant functional traits and their interactions. However, dynamic global vegetation models strongly underestimated LUE over most sites globally, and could not represent the response of LUE to main drivers, suggesting the potential model improvements through incorporating the trait‐based approaches. Our findings address the urgency of accounting for the synergism of plant traits and their interactions with environmental factors for better diagnosing and predicting the response of vegetation photosynthesis to climate change.
... For example, the major grouping of plants on the basis of shedding of leaves into deciduous and evergreen trees is a well-accepted classification (Fig. 2). Leafing is actually the plant phenological event that outlines the growing season and also controls fundamental ecosystem processes that include nutrient cycling, water storage, regulation of productivity in terrestrial ecosystems and the dynamics of carbon sequestration (Baldocchi et al., 2005;Reich, 1995). Plants are in fact the most appropriate organisms for climatic effect studies on phenology because of their sessile behaviour which makes them endure all weather conditions occurring in their location. ...
Chapter
Greenhouse gases (GHGs) are major contributors to global warming and climate change. These gases modulate the atmospheric radiative forcing and play an important role in Earth's albedo. The emission level, global warming potential and the persistence of a GHG define its accumulation in the atmosphere and relative potential to change radiative forcing. The major anthropogenic GHGs include methane, nitric oxide, ozone, hydrochloroflourocarbons, chloroflourocarbons, sulfur hexaflouride and nitrogen triflouride. Besides these, some gases indirectly act as GHGs like carbon monoxide, non-methane hydrocarbons, and nitrogen oxides. Many scientists have already warned regarding elevated emission trends after the industrial revolution. From last decades the emission of GHGs has tremendously increased in the atmosphere and the natural sinks of GHGs have contracted over time. Generally, fossil fuel burning and change in land use are major sources of GHGs while major sinks include soil, ocean and atmosphere. Interestingly the emission trends of greenhouse gases from different sources as well as the contribution of various countries to global greenhouse gasses budget have changed. Thus previous footprints, trends and projections regarding GHGs are needed to be reevaluated. Specific precautions and strategies are compatible to reduce GHGs emissions while further may help to obtain global temperature to above pre-industrial ambient temperature level by reducing 2°C in current temperature.
... Chemical properties of the soil in the Montiers site. Adapted from (Calvaruso et al., 2017) The deciduous forest has been known to rely on a considerable amount of rainfall (800-1400 mm per year) and can survive freezing winter down to -20°C as well as warm summer up to low 30°C (Baldocchi et al., 2005) on which our site receives on average 1100 mm per year. ...
Thesis
This study is a continuation of our previous geochemical monitoring finding at the injection wells of Rousse 1 ( Total CCS pilot, Lacq- Rousse, France) where it was identified that the soil CO2 mole fraction (χc) evolution in subsoil was negatively correlated with the level of the water table and the CO2 sources were attributed to the CO2-rich aquifers. However, it is still unclear whether this relationship exists in the forest ecosystem, representing a significant proportion of the CO2 atmospheric budget. For this reason, this thesis focuses on monitoring the gas exchange and its main driver of the transport process between soil (-1 m), subsoil (-6 m), and biosphere. We developed and implemented an in-situ geochemical monitoring system for continuous monitoring of CO2 mole fraction in the subsoil coupled with a micrometeorological monitoring system using a pre-established flux tower in the forest Ecosystem (Montiers, Lorraine Region, France). This soil gas measurement infrastructure combining borehole measurement with micrometeorological measurement offers great possibilities for long-term in-situ and continuous gas monitoring to derive the vertical distribution of CO2. Thus, this infrastructure allowed the observation of the temporal dynamics in soil-gas CO2 research. During the study periods, the ecosystem acted as a net carbon sink with a mean annual NEE, GPP, and Reco of -453±122 gC m-2y-1, -1468 ±109 gC m-2y-1, and 1052 ±88 gC m-2y-1 consecutively. The Carbon exchange, climate, and environmental drivers during the drought episodes were compared with long-term reference data recorded from 2014 to 2017. In contrast with some previous research where GPP and Reco parallelly decreased during the drought episodes, our site showed Reco is more sensitive to drought than GPP, resulting in a significant increase in Net Ecosystem exchange. Reco decreased by 20%, and 26% were found in Summer and Autumn (2018-2019) relative to the reference years (2014-2017). This study shows strong empirical shreds of evidence that wind turbulence plays a significant role in driving the deep soil CO2 concentration. We hypothesize that this could be due to pressure pumping effects where it decreases the CO2 molar fraction in the soil during high turbulence and increases the CO2 storage in deep soil during low turbulence. This study also demonstrates that permeability significantly reduced during wet periods diminishing molecular diffusion and advection. This study also revealed a strong biotic influence on CO2 production. The δCCO2 values in our site subsoil can be attributed to the respiration and decomposition of the C3 plants. These biological origins of 13 14 soil CO2 are highly likely to increase air density resulting in gravitational percolation that leads the CO2 stored in a deeper layer of soil. The relationship of subsoil gases also emphasizes that biogenic components dominate the origins and controlling process of subsoil CO2 while the geochemical process plays an insignificant role. Keywords: Geochemical Gas Monitoring, CO2, NEE, Friction velocity, Heterotroph.
... Eddy-covariance (EC), also called turbulent flux method, is a well-established widely used micrometeorological technique and an effective way to quantify large scale net ecosystem carbon exchange (NEE) over long time periods, ranging from hours to years (Baldocchi, 2003(Baldocchi, , 2014. Traditionally, the EC method has been used primarily to measure the canopy-atmosphere gas flux (Law et al., 2003;Baldocchi et al., 2005;Stoy et al., 2005) or soil/land-atmosphere gas flux (Wolf et al., 2011;Xiao et al., 2011;Schwalm et al., 2012), but it can also be used to measure benthic (sediment-water) gas exchange (Attard et al., 2019;Berg et al., 2019;Berger et al., 2020) or fluxes from water and wetland surfaces (Alberto et al., 2014;Guti errez-Loza et al., 2019;Morin, 2019;Benítez-Valenzuela and Sanchez-Mejia, 2020;. Historical information and a practical guide on how to measure and analyze eddy-covariance data can be found in Aubinet et al. (2012), Burba (2013), Baldocchi (2014), and Rebmann et al. (2018) (also see Chapter 4). ...
Chapter
This chapter discusses carbon dioxide (CO2) and methane (CH4) gas exchange at the water-air interface in coastal wetlands. The existing literature was reviewed to report the magnitude of CO2 and CH4 water-air fluxes in mangrove, saltmarsh, and seagrass ecosystems. Based on available data, mangrove waters show a large range of CO2 (13–9726 mg CO2 m− 2 day− 1) and CH4 water-air fluxes (− 1.1–1169 mg CH4 m− 2 day− 1) and are generally a source of CO2 and CH4 to the atmosphere. Similarly, saltmarsh waters are predominantly water-air sources of CO2 (mean: 2823 ± 332 mg CO2 m− 2 day− 1) and CH4 (− 1.5–1510 mg CH4 m− 2 day− 1). In contrast, seagrass waters can act as a source or sink of CO2 (− 3168–3041 mg CO2 m− 2 day− 1) and are likely a source of CH4 to the atmosphere (1.9–4.9 mg CH4 m− 2 day− 1). High spatial and temporal variability and the large range of fluxes are linked to tidal regimes, seasonality, vegetation coverage, and complex biogeochemical processes that occur in coastal wetland sediments and waters. Various direct and indirect drivers are described that can control CO2 and CH4 concentration gradients, transport pathways, and fluxes from sediments to the water column and ultimately to the atmosphere. Finally, the three most commonly used methods to determine water-air gas exchange in coastal waters are reviewed, which are the chamber method, the gradient flux method, and the eddy-covariance technique. Using appropriate methods, more research is needed for a better assessment of long-term and large-scale gas exchange in dynamic coastal wetland waters, to quantify more accurately present and to predict future greenhouse gas trends and potential “blue carbon” offsets in mangrove, saltmarsh, and seagrasses on regional and global scale.
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Photosynthetic carbon assimilation in plant leaves supports biomass accumulation and developmental growth and contributes to the regulation of atmospheric CO2 concentration via the carbon cycle. Photosynthesis and its environmental responses have been the central theme of plant physiological ecology and ecosystem ecology, as photosynthesis is involved in a broad range of natural systems from cells to the biosphere. In particular, the environmental responses of tree leaves and forest ecosystems and their seasonal and interannual changes under ongoing climate change are central research interests in ecology and Earth system science. This chapter reviews the studies of leaf and canopy photosynthesis conducted in a cool-temperate deciduous broadleaf forest site in Japan. Long-term observations and open-field warming experiments were conducted to assess leaf phenology in canopy trees, leaf photosynthesis, and the light environment of understory shrubs, and the application of optical remote sensing on forest canopy photosynthetic productivity helped to clarify single-leaf level ecophysiology and forest ecosystem function. The advancement of integrated ecosystem science coupled with climate monitoring should help us to respond to the urgent need for key data regarding biodiversity and ecosystem conservation and management across landscapes from local to regional scales.KeywordsClimate changeDeciduous broadleaf treesEcosystem master sitePhenologyPhotosynthesis
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Remote sensing of vegetation phenology has long been used to characterize ecosystem functions and responses to climate at spatial and temporal scales unfeasible to field surveys. However, the potential of remote sensing to elucidate mechanistic drivers of phenology and the underlying plant community processes at such scales remains under‐discussed. This review synthesizes possibilities to advance this knowledge using multi‐temporal remote sensing and discusses remaining challenges and progress in instruments and analytical tools. Recent evidence indicates that, besides documenting vegetation seasonality and responses to climate, remote sensing of phenology can help meet emerging needs for indicators of plant diversity, vegetation structure, and ecosystem change. Responses of phenological metrics to stressors over large, heterogeneous regions may provide clues on ecological resilience manifested in asynchronies, recovery of vegetation cycles, and stable microrefugia. At the same time, important barriers persist in relation to choosing among phenological estimation methods and paradigms, characterizing phenological events beyond changes in photosynthetically active biomass, and mechanistic interpretation of phenological patterns. Synthesis. Increasing temporal frequency of products, opportunities for multi‐sensor data fusion, and advances in historically less available hyperspectral, active microwave, and lidar instruments promise to help navigate these barriers and enable more comprehensive assessments of seasonality. Progress in customizable local platforms such as unoccupied aerial vehicles and phenocams may further enrich ground‐level understanding of phenology and validate satellite‐based assessments. However, remote sensing analyses alone are insufficient for mechanistic interpretation of phenology, which can be challenged by artifacts in remote sensing data and sensitivity of estimated metrics to landscape structure and spatial resolution of the inputs. Robust and informative phenological assessments call for rigorous collaborations with field ecological studies, strategic selection of ancillary environmental and geographic data, and wider adoption of causal inference approaches to address these needs and support novel explorations in plant ecology.
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An analysis of boundary layer cumulus clouds and their impact on land surface-atmosphere exchange is presented. Seasonal trends indicate that in response to increasing insolation and sensible heat flux, both the mixed-layer height (zi) and the lifting condensation level (LCL) peak (~ 1250 and 1700 m) just before the growing season commences. With the commencement of transpiration, the Bowen ratio falls abruptly in response to the infusion of additional moisture into the boundary layer, and zi and LCL decrease. By late spring. boundary layer cumulus cloud frequency increases sharply, as the mixed layer approaches a new equilibrium, with zi and LCL remaining relatively constant (~ 1100 and 1500 m) through the summer. Boundary layer cloud time fraction peaks during the growing season, reaching values greater than 40% over most of the eastern United States by June. At an Automated Surface Observing System (ASOS) station in central Massachusetts, a growing season peak is apparent during 1995-98 but reveals large variations in monthly frequency due to periods of drought or excessive wetness. Light-cloud cover regression relationships developed from ASOS ceilometer reports for Orange. Massachusetts, and Harvard Forest insolation data show a good linear fit (r2 = 0.83) for overall cloud cover versus insolation, and a reasonable quadratic fit (r2 = 0.48) for cloud cover versus the standard deviation of insolation, which is an indicator of sky type. Diffuse fraction (the ratio of diffuse to global insolation) shows a very good correlation (r2 = 0.79) with cloud cover. The sky type-insolation relationships are then used to analyze the impact that boundary layer clouds have on the forest ecosystem, specifically net carbon uptake (Fco2), evapotranspiration (ET), and water use efficiency (WUE). During 1995, afternoon Fco2 was 52% greater on days with boundary layer cumulus clouds than on clear days, although ET was the same, indicating greater light use efficiency and WUE on partly cloudy days. For 1996-98, afternoon Fco2 was also enhanced, especially during dry periods. Further analysis indicates that the vapor pressure deficit (VPD) was significantly greater (>8 hPa) during 1995 and parts of 1996-98 on clear days as compared with partly cloudy days. A long-term drought combined with abnormally warm weather likely contributed to the high VPDs, reduced Fco2 ET, and the dearth of clouds observed during 1995. In general, the presence of boundary layer cumulus clouds enhances net carbon uptake, as compared with clear days.
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A boreal deciduous forest in Saskatchewan, Canada, sequestered 144±65, 80±60, 116±35 and 290±50 g C m−2 y−1 in 1994, 1996, 1997 and 1998, respectively. The increased carbon sequestration was the result of a warmer spring and earlier leaf emergence, which significantly increased ecosystem photosynthesis, but had little effect on respiration. The high carbon sequestration in 1998 was coincident with one of the strongest El Niño events of this century, and is considered a significant and unexpected benefit.
Article
The objective of this research is to elucidate the seasonal and inter-annual variations of CO2 exchanges between the atmosphere and a temperate deciduous forest in Japan and to elucidate their relation to meteorological conditions. The uptake rates of CO2 from October 1993 to December 1996 were estimated from field measurements of CO2 concentrations and meteorological conditions using a tower. Net of uptake rate of CO2 was positive (uptake by forest ecosystems) from June to September and negative (release to the air) from October to April. Averages of integrated uptake rates of CO2 were 840, -450 and 390 gCO2/m2/year (2.3, -1.2 and 1.1 tC/ha/year) for daytime, night and whole day (net), but they had notable inter-annual variation due to the differences of averaged insolation and temperature each summer of 1994 to 1996. The errors of CO2 flux due to topographical conditions were investigated through comparison with heat budgets. CO2 uptake rate estimated by tower measurement might be underestimation of 40%, therefore, above net-uptake value, 1.1 tC/ha/year became 1.8. This value of uptake rate was smaller than the results obtained in other temperature deciduous forests. The causes of this are partially in the difference of the height of the site and the short active period of the present forest. According to the CO2 flux measurements in several forests including the present one, the forest ecosystems could be a large sink of CO2 however, more data of the CO2 flux is needed at the various forests and latitudes to reduce the uncertainty of estimation of CO2 uptake on a global scale.
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The nature of energy-flux transitions during the onset of midlatitude spring has not been widely examined, despite their critical implications for global-scale biospheric measures and climate change. Further conventional assessment of these phenomena across multiple locations is restricted by satellite-derived data limitations, and the paucity of surface measurement sites. In this paper, we explore a new phenology-based strategy for evaluating the spring surface energy-balance shift that can address these concerns. Our technique can reflect native-species responses and atmospheric surface layer change at a variety of sites, using synoptic-scale first leaf and first bloom phenology models. We test the approach at three locations with diverse climates, and within a group of stations in the state of Oklahoma, which has moderate climate variations across an east-to-west gradient. The results show that the onset of spring in midlatitudes is a modally abrupt (rather than gradual) seasonal transition in terms of energy balance (sensible and latent heat levels) and carbon-flux change that can be linked directly to vegetation phenology. The consistent temporal pattern and magnitude of flux variations across diverse sites suggest that this technique has potential as a proxy for spring energy-balance change at many locations.
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This chapter presents new and fundamental approaches to understanding the relationships between canopy distribution in time and space, and stand productivity. Among the many issues discussed, the relationship between allocation and productivity is given a special mention. Production is dependent on the proportion of dry matter allocated to a photosynthetic tissue, as well as the proportion of time this tissue was active in the plant (its phenology). Because of the analogy of “compound interest,” the gain in dry matter is higher for a photosynthetic tissue than for a nonphotosynthetic tissue when there are no environmental limitations. The ideas discussed in this chapter encourage the importance of phenology, growth, and allocation for terrestrial primary productivity. Background and examples for a subset of issues relevant to global change are also provided. The chapter highlights trees and forest ecosystems because these systems contain the predominant fraction of global terrestrial carbon stores in plants. The chapter concludes by highlighting several promising areas of research: extension of the present theory of the timing of allocation to include the phenology of belowground organs and reproduction; techniques for improving estimates of belowground productivity and patterns of global allocation above and belowground; and the use of remote sensing combined with modeling for estimating global phenology and productivity. It would be especially useful if one could relate these remotely sensed data of aboveground events to seasonal patterns of belowground production.
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The footprint of a turbulent flux measurement defines its spatial context. With the onset of long-term flux measurement sites over forests and other inherently inhomogeneous areas, and the development of the FLUXNET program, the need for flux footprint estimations has grown dramatically. This paper provides an overview of existing footprint modeling approaches in the critical light of hindsight and discusses their respective strengths and weaknesses. The second main objective of this paper is to establish a formal connection between micrometeorological measurements of scalar fluxes and their mass conservation equation, in a surface-vegetation-atmosphere volume. An important focus is to identify the limitations of the footprint concept and to point out situations where the application of footprint models may lead to erroneous conclusions, as much as to demonstrate its utility and power where warranted. Finally, a perspective on the current state-of-the-art of footprint modeling is offered, with a list of challenges and suggestions for future directions.
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Differences in the seasonal pattern of assimilatory and respiratory processes are responsible for divergences in seasonal net carbon exchange among ecosystems. Using FLUXNET data (http://www-eosdis.ornl.gov/FLUXNET) we have analyzed seasonal patterns of gross primary productivity (GPP), and ecosystem respiration (RE) of boreal and temperate, deciduous and coniferous forests, mediterranean evergreen systems, rainforest, temperate grasslands, and C3 and C4 crops. Based on generalized seasonal patterns classifications of ecosystems into vegetation functional types can be evaluated for use in global productivity and climate change models. The results of this study contribute to our understanding of respiratory costs of assimilated carbon in various ecosystems. Seasonal variability of GPP and RE increased in the order tropical, Mediterranean, temperate coniferous, temperate deciduous, boreal forests. Together with boreal forests, managed grasslands and crops show the largest seasonal variability. In temperate coniferous forests, seasonal patterns of GPP and RE are in phase, in temperate deciduous and boreal coniferous forests RE was delayed compared to GPP, resulting in the greatest imbalance between respiratory and assimilatory fluxes early in the growing season. Gross primary productivity adjusted for the length of the growing season decreased across functional types in the order C4 crops, and temperate and boreal deciduous forests (7.5-8.3 g C m-2 d-1), temperate conifers, C3 grassland and crops (5.7-6.9 g C m-2 d-1), rainforest and boreal conifers (4.6-4.9 g C m-2 d-1). Annual GPP and NEP decreased across climate zones in the order tropical, temperate, boreal. However, the decrease in NEP was greater than the decrease in GPP, indicating a larger contribution of respiratory (especially heterotrophic) processes in boreal systems.