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Seasonal dynamics and age of stemwood nonstructural Seasonal dynamics and age of stemwood nonstructural
carbohydrates in temperate forest trees carbohydrates in temperate forest trees
Andrew D. Richardson
Harvard University
, arichardson@oeb.harvard.edu
Mariah S. Carbone
National Center for Ecological Analysis and Synthesis
, mcarbone@nceas.ucsb.edu
Trevor F. Keenan
Harvard University
Claudia I. Czimczik
University of California - Irvine
, czimczik@uci.edu
David Y. Hollinger
USDA Forest Service
, davidh@hypatia.unh.edu
See next page for additional authors
Follow this and additional works at: https://digitalcommons.unl.edu/usdafsfacpub
Richardson, Andrew D.; Carbone, Mariah S.; Keenan, Trevor F.; Czimczik, Claudia I.; Hollinger, David Y.;
Murakami, Paula; Schaberg, Paul G.; and Xu, Xiaomei, "Seasonal dynamics and age of stemwood
nonstructural carbohydrates in temperate forest trees" (2012).
USDA Forest Service / UNL Faculty
Publications
. 230.
https://digitalcommons.unl.edu/usdafsfacpub/230
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Seasonal dynamics and age of stemwood nonstructural
carbohydrates in temperate forest trees
Andrew D. Richardson
1
, Mariah S. Carbone
2
, Trevor F. Keenan
1
, Claudia I. Czimczik
3
, David Y. Hollinger
4
,
Paula Murakami
5
, Paul G. Schaberg
5
and Xiaomei Xu
3
1
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, 02138, USA;
2
National Center for Ecological Analysis and Synthesis, Santa Barbara, CA, 93101,
USA;
3
Department of Earth System Science, University of California, Irvine, CA, 92697, USA;
4
USDA Forest Service, Northern Research Station, Durham, NH, 03824, USA;
5
USDA Forest
Service, Northern Research Station, Burlington, VT, 05403, USA
Author for correspondence:
Andrew D. Richardson
Tel: +1 617 496 1277
Email: arichardson@oeb.harvard.edu
Received: 27 June 2012
Accepted: 7 October 2012
New Phytologist (2012)
doi: 10.1111/nph.12042
Key words: carbon allocation, carbon cycle
model, mean residence time, nonstructural
carbohydrate (NSC) reserves, radiocarbon
(
14
C), starch.
Summary
Nonstructural carbohydrate reserves support tree metabolism and growth when current
photosynthates are insufficient, offering resilience in times of stress.
We monitored stemwood nonstructural carbohydrate (starch and sugars) concentrations of
the dominant tree species at three sites in the northeastern United States. We estimated the
mean age of the starch and sugars in a subset of trees using the radiocarbon (
14
C) bomb spike.
With these data, we then tested different carbon (C) allocation schemes in a process-based
model of forest C cycling.
We found that the nonstructural carbohydrates are both highly dynamic and about a
decade old. Seasonal dynamics in starch (two to four times higher in the growing season,
lower in the dormant season) mirrored those of sugars. Radiocarbon-based estimates indi-
cated that the mean age of the starch and sugars in red maple (Acer rubrum) was 7–14 yr.
A two-pool (fast and slow cycling reserves) model structure gave reasonable estimates of
the size and mean residence time of the total NSC pool, and greatly improved model predic-
tions of interannual variability in woody biomass increment, compared with zero- or one-pool
structures used in the majority of existing models. This highlights the importance of nonstruc-
tural carbohydrates in the context of forest ecosystem carbon cycling.
Introduction
Forest trees, like all plants, accumulate and store nonstructural
carbohydrates (NSC) as resources to be used to support future
growth and metabolism (Chapin et al., 1990). The NSC pool is
the sum of soluble sugars, mainly sucrose, plus starch. The
dynamics of NSCs are considered indicators of carbon (C) source–
sink relationships. K€orner (2003) made the analogy that the size
of a tree’s NSC pool reflects its C ‘fueling status’. Recent analyses
(Barbaroux & Breda, 2002; Gough et al., 2009) suggest that a
large fraction of a tree’s annual C budget is allocated to the NSC
pool, and W€urth et al. (2005) estimated the total NSC pool in a
tropical forest ecosystem to be 8% of living biomass, which scales
to 25 Pg C if this proportion holds globally.
Processes and pathways related to NSC allocation and storage
could influence the impact of climate change on forest ecosystem
C balance. However, C alloc ation processes remain poorly und er-
stood (Le Roux et al., 2001; Trumbore, 2006; Keel et al., 2007;
Wiley & Helliker, 2012), and many models treat allocation in an
overly simplistic manner (Friedlingstein et al., 1999). Progress is
hampered by the scarcity of field data necessary for model testing,
with additional studies in mature natural forests in particular
being needed (Barbaroux & Breda, 2002; Hoch et al., 2003;
Gough et al., 2009).
The standard conceptual model for NSCs is that the pool is:
depleted when demand exceeds supply, for example, when
metabolism and growth requirements are high, or when produc-
tion of photoassimilates is limited by environmental conditions;
and recharged when the supply exceeds demand, for example,
when environmental conditions permit high rates of photosyn-
thesis, or when metabolism and growth requirements are low
(Chapin et al., 1990; Grulke et al., 2001; Gleason & Ares, 2004)
(cf. K€orner’s (2003) argument that the size of the NSC pool is
primarily driven by demand-side factors). In this framework,
storing NSCs for future use is viewed as a ‘bet hedging’ strategy,
providing reserves that the tree can draw on in times of stress
(Dunn et al., 1990; Kozlowski, 1992; Bond & Midgley, 2001;
Gleason & Ares, 2004). Carbon isotope labeling studies have
shown conclusively that stored NSCs are used to fuel growth and
respiration when the supply of current photoassimilates is inade-
quate (Kagawa et al., 2006a; Keel et al., 2006, 2007; Carbone &
Trumbore, 2007; Kuptz et al., 2011). In addition, there is
mounting evidenc e that stored NSCs, particularly in below-
ground organs, are still accessible a decade after assimilation
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(Schuur & Trumbore, 2006; Vargas et al., 2009; Carbone et al.,
2011).
The NSC pool may also play an important role in ecosystem
C cycling . For example, at the Howland Forest AmeriFlux site
the mean ( 1 SD, 1996–2005) annual rate of net ecosystem
exchange (NEE) of CO
2
, 185 47 g C m
2
yr
1
,isin
substantial agreement with the mean annual rate of live tree bio-
mass increment, 163 15 g C m
2
yr
1
(Fig. 1). While ther e is
no correlation between year-to-year fluctuations of the two time-
series (r = 0.07, P = 0.85), there is a strong correlation between
NEE in one year and biomass increment in the following year
(r
lag
= 0.80, P < 0.01). As hypothesized previously (Arneth et al.,
1998; Kagawa et al., 2006b; Rocha et al., 2006; Navarro et al.,
2008; Gough et al., 2009; Rocha & Goulden, 2009), time lags
between C uptake and biomass growth could be explained by a
dynamic NSC pool that functions as temporary storage, with
accumulated NSC not being drawn upon for allocation to
growth until the following year.
We present the results of a multi-year (2007–2010) study of
NSC dynamics in temperate forest trees. At quarterly intervals,
we measured stemwood NSC concentra tions in the dominant
tree species at three sites in the northeastern USA. Our objectives
were to:
assess the seasonal dynamics and interannual variability in
stemwood NSC (starch and sugars) concentrations, and evaluate
whether these vary among species or across sites;
quantify the mean age of stemwood starch and sugars, using
the radiocarbon (
14
C) bomb spike method;
compare different approaches (no-, one- and two-pool NSC
representations) to modeling C allocation and NSC dynamics in
a parsimonious forest ecosystem C cycling model (Keenan et al.,
2012). We use Howland Forest as a case study, in order to inves-
tigate our hypothesis for the lag observed in Fig. 1.
Materials and Methods
Study sites
Field research was conducted at three sites in the northeastern
USA: Howland Forest, Bartlett Experimental Forest, and
Harvard Forest (Table 1). Monthly and annual departures from
the longer-term (2001–2010) climatological means indicate
substantial regional coherence for air temperature, but not solar
radiation or precipitation, anomalies (see Supporting Informa-
tion Fig. S1).
Field sampling for NSC analyses
In May 2007, transects were established in the AmeriFlux tower
footprint at each site, and 60 trees (20 trees for each of three
dominant species at each site; Table 2) were tagged and mea-
sured. We sought out healthy, dominant or codominant individ-
uals of 25 cm DBH (diameter at breast height, 1.3 m),
although in a small fraction of cases (17/180 trees), it was neces-
sary to relax the minimum diameter requirement to 20 cm.
At quarterly intervals (June, August, November and March),
one-half of the trees of each species at each site were cored at
breast height, on the south side of the trunk, to a depth of 3 cm
with a standard 4.3 mm increment borer (H€aglof Company
Group, Langsele, Sweden). Each tree was cored six times over the
3-yr course of measurements. Deciduous trees were in leaf during
the June and August sampling dates, but were leafless in Novem-
ber and March. Cores were placed in clean plastic drinking straws
(McDonald’s Corporation, Oak Brook, IL, USA), labeled, and
frozen in the field on dry ice.
Three additional sets of cores were collected:
To investigate factors associated with the high rates of mortal-
ity observed for paper birch at Bartlett Experimental Forest, a
subset of trees (20 paper birch, and 10 each of red maple and
American beech) were cored to the pith for ring width measure-
ments and age determination in Summer 2010. At the same time,
we rated tree vigor according to Millers et al. (1991). These
results are presented in the Supporting Information, Notes S2,
Fig. S4;
To investigate the mean ages of stemwood starch and sugars,
two cores were collected from nine red maple at each site, and
nine eastern hemlock at Howland Forest and Harvard Forest, in
November 2010. The first of these cores was used for determina-
tion of NSC concentrations, while the second was used for radio-
carbon (which we denote ‘
14
C-NSC’) analyses.
To investigate relationships between tree age, the number of
rings in the outer 2 cm of stemwood, and the age of starch and
sugars, each of the nine
14
C-NSC red maples at each site was
cored to the pith for ring width measurements and age determi-
nation in Summer 2011.
Concentrations of starch and sugars
Analytical procedures for NSC determination followed Wong
et al. (2003). Analysis was conducted only on the outer 2 cm of
1980 1985 1990 1995 2000 2005
100
150
200
250
300
Year
C sequestration (g C m
–2
yr
–1
)
Woody biomass
increment
Tower C uptake
(r = 0.06, ns)
Lagged tower
(r = 0.80, P < 0.01)
100 200 300
140
160
180
200
Lagged tower
Wood increment
Fig. 1 Comparison of carbon (C) sequestration estimates for the Howland
Forest AmeriFlux site, based on (1) measurements of woody biomass
increment estimated from tree rings (see Richardson et al. 2010) and (2)
tower measurements of net ecosystem exchange (NEE) of CO
2
measured
by eddy covariance (‘tower C uptake’; see Hollinger et al. 2004). The
correlation between woody biomass increment and current-year tower C
uptake is not statistically significant (r = 0.06, P = 0.85), but woody
biomass increment is well correlated with tower C uptake in the previous
year (r = 0.80, P < 0.01), as shown in the inset plot.
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each increment core. Cores were vacuum-infiltrated with 80%
ethanol at 52 KPa for 15 min and then boiled. Samples were
finely diced with a razor blade and macerated in existing ethanol
solutions using a polytron (Brinkman Instruments, Westbury,
MA, USA). Macerated samples were extracted twice with 5 ml
fresh 80% ethanol at 80°C for 15 min and centrifuged at
3000 rpm. Supernatants for each sample were combined, filtered
through a 0.45 lm syringe filter and used for soluble sugar analy-
sis. The ethanol-insoluble pellets were used to determine starch
content.
Ethanol-soluble fractions were analysed for sucrose, glucose,
fructose, raffinose, and stachyose using a Waters (Milford, MA,
USA) Alliance high-pressure liquid chromatography (HPLC) sys-
tem with a Waters Sugar-pak column and solvent (0.1 mmol
1
Ca EDTA) at 90°C (Wong et al., 2003). Sugars were detected
with a Waters 2414 refractive index detector and Waters PC-
based Empower software. The separated soluble sugars were iden-
tified and quantified with known standards and converted to mg
sugar per g oven-dry wood.
The branched form of starch was determined after gelatiniza-
tion with 0.1 M KOH in a boiling water bath for 30 min fol-
lowed by neutralization with acetic acid by hydrolysis with
amyloglucosidase for 30 min at 55°C (Wargo et al., 2002). Enzy-
matic digestions were terminate d by placing the digests in a boil-
ing water bath for 4 mi n. Glucose formed by hydrolysis was
determined with a glucose hexokinase kit (Pointe Scientific,
Canton, MI, USA) at 340 nm with a microplate reader (ELx800
UV; Bio-Tek Instruments, Winooski, VT, USA). Starch concen-
trations were calculated from standard curves and are expressed
here as mg starch per g oven-dry wood.
The standard deviation (across trees of the same species on a
given sampling date) of the total (starch + sugars) stemwood
NSC was 1r = 25% of the measured value. With n = 10 repli-
cate trees per species at each samp le date, the standard error on
the species-level mean is thus c. 9%.
Determination of
14
C-based ages
We took advantage of the
14
C bomb spike to directly estimate
the age of extracted NSC. This approach uses the
14
C label that
was produced during the period of atmospheric thermonuclear
weapons testing, which approximately doubled the
14
C content
of CO
2
in the north ern hemisphere atmosphere by 1963
(Fig. 2a). Since then, the
14
C content of atmospheric CO
2
has
decreased owing to dilution through mixing with ocean and bio-
sphere C reservoirs, and by the addition of
14
C-free CO
2
from
fossil fuel burning (Levin et al., 2010). We estimated the ages of
stemwood starch, sugars and ring cellulose by direct comparison
with the northern hemisphere atmospheric record (Levin et al.,
2008; I. Levin, pers. comm.) following Gaudinski et al. (2001).
This is possible because the C in photosynthate reflects the
14
C
content of the atmosphere in the year assimilation occurred and
contributes to the NSC pool
14
C content. For pools with a mean
residence tim e (MRT) of < 20 yr, the
14
C age of the pool is
Table 1 Study sites where field sampling was conducted
Site Lat./Long. Elev.
a
MAT/MAP
b
Vegetation Reference
Howland Forest
(Maine)
45.25°N, 68.73°W 60 m 6.7°C, 850 mm Spruce-fir boreal transition Hollinger et al. (2004)
Bartlett
Experimental
Forest (New
Hampshire)
44.06°N, 71.29°W 270 m 7.3°C, 1270 mm Maple-beech-birch northern
hardwoods
Jenkins et al. (2007)
Harvard Forest
(Massachusetts)
42.53°N, 72.17°W 340 m 8.2°C, 1270 mm Oak-dominated transition
hardwoods
Urbanski et al. (2007)
a
Elevation in m ASL (above sea level).
b
MAT, mean annual temperature; MAP, mean annual precipitation, based on 2007–2010.
Table 2 Dimensions of trees sampled for stemwood nonstructural carbohydrate measurements
Site Species
Diameter at breast height (cm)
Mean 1 SD Minimum Maximum
Howland Forest Red maple (Acer rubrum L.) 28 720 41
Red spruce (Picea rubens Sarg.) 31 723 46
Eastern hemlock (Tsuga canadensis (L.) Carri
ere) 38 430 46
Bartlett Experimental Forest Red maple (Acer rubrum L.) 35 825 48
Paper birch (Betula papyrifera Marsh.) 33 525 43
American beech (Fagus grandifolia Ehrh.) 31 525 46
Harvard Forest Red maple (Acer rubrum L.) 28 523 43
Red oak (Quercus rubra L.) 42 925 69
Eastern hemlock (Tsuga canadensis (L.) Carri
ere) 40 823 56
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