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The effects of 11 yr of CO 2 enrichment on roots in a Florida scrub-oak ecosystem

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The effects of 11 yr of CO 2 enrichment on roots in a Florida scrub-oak ecosystem

Abstract and Figures

Uncertainty surrounds belowground plant responses to rising atmospheric CO2 because roots are difficult to measure, requiring frequent monitoring as a result of fine root dynamics and long-term monitoring as a result of sensitivity to resource availability. We report belowground plant responses of a scrub-oak ecosystem in Florida exposed to 11 yr of elevated atmospheric CO2 using open-top chambers. We measured fine root production, turnover and biomass using minirhizotrons, coarse root biomass using ground-penetrating radar and total root biomass using soil cores. Total root biomass was greater in elevated than in ambient plots, and the absolute difference was larger than the difference aboveground. Fine root biomass fluctuated by more than a factor of two, with no unidirectional temporal trend, whereas leaf biomass accumulated monotonically. Strong increases in fine root biomass with elevated CO2 occurred after fire and hurricane disturbance. Leaf biomass also exhibited stronger responses following hurricanes. Responses after fire and hurricanes suggest that disturbance promotes the growth responses of plants to elevated CO2 . Increased resource availability associated with disturbance (nutrients, water, space) may facilitate greater responses of roots to elevated CO2 . The disappearance of responses in fine roots suggests limits on the capacity of root systems to respond to CO2 enrichment.
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The effects of 11 yr of CO
2
enrichment on roots in a Florida
scrub-oak ecosystem
Frank P. Day
1
, Rachel E. Schroeder
1
, Daniel B. Stover
2
, Alisha L. P. Brown
1
, John R. Butnor
3
, John Dilustro
4
,
Bruce A. Hungate
5
, Paul Dijkstra
5
, Benjamin D. Duval
6
, Troy J. Seiler
7
, Bert G. Drake
8
and C. Ross Hinkle
9
1
Department of Biological Sciences, Old Dominion University, Norfolk, VA 23529, USA;
2
Office of Biological and Environmental Research, US Department of Energy, Washington, DC
20585, USA;
3
Southern Research Station, USDA Forest Service, Burlington, VT 05405, USA;
4
Department of Biology, Chowan University, Murfreesboro, NC 27855, USA;
5
Department of
Biological Sciences and the Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ 86011, USA;
6
Global Change Solutions, Institute for Genomic
Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA;
7
ENSCO Inc., Melbourne, FL 32940, USA;
8
Smithsonian Environmental Research Center, Edgewater, MD
21037, USA;
9
Department of Biology, University of Central Florida, Orlando, FL 32816, USA
Author for correspondence:
Frank P. Day
Tel: +1 757 683 4198
Email: fday@odu.edu
Received: 3 December 2012
Accepted: 19 February 2013
New Phytologist (2013)
doi: 10.1111/nph.12246
Key words: CO
2
enrichment, disturbance,
ground-penetrating radar, minirhizotrons,
root biomass, root closure, scrub-oak.
Summary
Uncertainty surrounds belowground plant responses to rising atmospheric CO
2
because
roots are difficult to measure, requiring frequent monitoring as a result of fine root dynamics
and long-term monitoring as a result of sensitivity to resource availability.
We report belowground plant responses of a scrub-oak ecosystem in Florida exposed to
11 yr of elevated atmospheric CO
2
using open-top chambers. We measured fine root
production, turnover and biomass using minirhizotrons, coarse root biomass using ground-
penetrating radar and total root biomass using soil cores.
Total root biomass was greater in elevated than in ambient plots, and the absolute differ-
ence was larger than the difference aboveground. Fine root biomass fluctuated by more than
a factor of two, with no unidirectional temporal trend, whereas leaf biomass accumulated
monotonically. Strong increases in fine root biomass with elevated CO
2
occurred after fire
and hurricane disturbance. Leaf biomass also exhibited stronger responses following
hurricanes.
Responses after fire and hurricanes suggest that disturbance promotes the growth
responses of plants to elevated CO
2
. Increased resource availability associated with distur-
bance (nutrients, water, space) may facilitate greater responses of roots to elevated CO
2
. The
disappearance of responses in fine roots suggests limits on the capacity of root systems to
respond to CO
2
enrichment.
Introduction
Increased interest in the global carbon cycle and carbon storage
by forest ecosystems demands accurate methods of quantification
of belowground biomass (Watson et al., 2000). Vegetation
accounts for nearly 80% of carbon stored in forest ecosystems
(Richter et al., 1999; Barton & Montagu, 2004). Coarse roots
(>5 mm in diameter) constitute a major belowground perennial
carbon sink and, compared with their aboveground counterparts,
often persist for long periods after tree harvest or disturbances
such as fire (Johnsen et al., 2001, 2005; Ludovici et al., 2002;
Miller et al., 2006). More carbon may be stored in roots in tropi-
cal and temperate forests, shrublands and savannas than previ-
ously thought, with recent estimates of at least 268 Pg of carbon
stored in roots in these ecosystems (Robinson, 2007). Thus, there
is a critical need to reassess belowground carbon storage by roots.
In shrub ecosystems that are fire prone and subject to drought,
the unique belowground morphology of the dominant plants
includes large rhizomes, underground stems and lignotubers
(Canadell & Zedler, 1995). Estimates of belowground biomass
in our study in a Florida scrub-oak ecosystem suggest that bio-
mass and carbon pools are much greater (c. 8000 g m
2
of root
mass to 60 cm depth) than previously reported for forested sys-
tems (c. 5000 g m
2
for sclerophyllous shrublands and tropical
evergreen forests; Jackson et al., 1996; Robinson et al., 2003). We
attribute this primarily to large structures that do not typically
exist in many forests (lignotubers and large rhizomes). These
massive structures potentially serve as one of the largest sinks of
carbon in the system (Stover et al., 2007). They are the primary
means of plant regeneration in fire-dominated ecosystems
(Schmalzer & Hinkle, 1992a,b).
Root abundance and biomass production are often stimulated
by elevated CO
2
(Rogers et al., 1994; Jongen et al., 1995;
Bernston & Bazzaz, 1996; Pritchard et al., 1999, 2008; Matamala
& Schlesinger, 2000; Norby et al., 2004; Iversen et al., 2008;
Pregitzer et al., 2008; Jackson et al., 2009). In a 25-yr-old stand
of Pinus taeda,CO
2
enrichment increased fine root biomass by
24% in the top 15 cm of soil, with no sign of this response
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diminishing after more than a decade (Jackson et al., 2009). This
response may be driven by the need to acquire additional below-
ground resources to support more rapid biomass accumulation,
such that plants exposed to elevated CO
2
invest more carbon in
root systems for expanded exploitation of the soil (Matamala &
Schlesinger, 2000).
Plant responses to elevated CO
2
are often greater under higher
levels of other resources, such as space, light and nutrients
(Bazzaz, 1990; Field et al., 1992; Oren et al., 2001; Korner,
2006; Reich et al., 2006; Garten et al., 2011). Therefore, changes
in resource availability could shape responses of plants to CO
2
over time. For example, progressive nutrient limitation has been
postulated to reduce long-term plant response to elevated CO
2
driven by changes in nutrient cycling (Luo et al., 2004; Johnson,
2006). The influences of exogenous phenomena that increase
resource availability, such as disturbance, are not as well charac-
terized, but would be expected to have the opposite effect,
enhancing CO
2
responses by causing resource pulses (Korner,
2006; Li et al., 2007). The investigation of both above- and
belowground responses to elevated CO
2
is especially important
because biomass allocation may shift in response to relative
resource limitation (Poorter & Nagel, 2000).
Complementary and comprehensive sampling
The substantial pool of carbon in roots has been difficult to
quantify because of shortcomings in methodologies for the mea-
surement of belowground biomass and root dynamics in general
(Fitter & Stickland, 1992; Butnor et al., 2003). Most studies on
elevated CO
2
have sampled only fine roots near the soil surface,
providing a limited picture of root responses to increased CO
2
.
Roots of different diameters can vary in functions, and so lump-
ing roots together may miss key responses (Pregitzer et al., 2002;
Keel et al., 2012).
Estimates of root carbon stocks may be substantially low (40%
of actual root mass on average) because of incomplete sampling
(e.g. small cores missing large roots and inefficient recovery of
the smallest roots; Robinson, 2007). Long-term experiments that
involve belowground measurements pose a particular challenge,
because these studies can tolerate only minimal disturbance to
the soil (Bledsoe et al., 1999; Fahey et al., 1999). Thus, destruc-
tive sampling in the form of pit excavations and soil core extrac-
tions, although more direct, is severely limited. Large
belowground structures are frequently not sampled. For this
reason, most inferences regarding belowground responses to ele-
vated CO
2
are based on measurements of fine roots (Hendrick &
Pregitzer, 1992; Norby, 1994; Day et al., 2006). Ground-
penetrating radar (GPR; Stover et al., 2007; Butnor et al., 2008)
and minirhizotrons offer viable options for the quantification of
both coarse and fine root biomass throughout the course of a
long-term experiment without the major disturbances that are
associated with pits and cores and without the inherent sampling
problems suggested by Robinson (2007). In this study, we
employed minirhizotrons to quantify fine roots and GPR to
quantify coarse roots. Soil cores were also periodically taken to
compare with the other two methods.
Florida study
A fire-maintained scrub-oak ecosystem on the east coast of Flor-
ida was exposed to 11 yr of elevated atmospheric CO
2
using
open-top chambers (OTCs) following a controlled burn in 1996.
Earlier findings on the effects of CO
2
enrichment on roots at this
study site included reports on fine root abundance and turnover
for 19961997 (Day et al., 1996; Dilustro et al., 2002) and
20022004 (Stover et al., 2010), fine root abundance for 1996
2004 and distribution by depth for 1997 (Day et al., 2006), fine
root biomass for 2002 based on core data (Brown et al., 2007)
and for 19962006 based on minirhizotron data (Brown et al.,
2009), and coarse root biomass from GPR measurements for
2005 (Stover et al., 2007).
Here, we bring together these past results, together with new
data from the final harvest in 2007 and previously unpublished
data from soil cores taken throughout the duration of the experi-
ment. We also compare fine root and leaf responses to CO
2
enrichment over time to assess responses in allocation to above-
and belowground resource acquisition. Our synthesis tests the
following hypotheses: (1) elevated CO
2
stimulates fine and coarse
root biomass; and (2) elevated CO
2
causes larger responses in fine
root biomass relative to leaf biomass. We are particularly inter-
ested in the influence of disturbance on CO
2
responses. Unlike
typical mature forested systems that experience infrequent distur-
bances, the Florida scrub-oak woodland is frequently disturbed
by fire and coastal storms (hurricanes), similar to Mediterranean
shrublands (e.g. chaparral) and savannahs.
Materials and Methods
Study site
The study site was located at Kennedy Space Center on Merritt
Island National Wildlife Refuge (28°38N, 80°42W) on the east
coast of Florida, USA. The sandy soils are acidic, well drained
and nutrient poor (Schmalzer & Hinkle, 1992a,b). The climate is
subtropical with a wet season between late June and October and
a dry season between April and early June. Lightning associated
with thunderstorms is responsible for igniting wildfires. Although
fire is the dominant ecosystem disturbance at the study site
(715-yr fire cycle), other natural disturbances include periodic
drought and severe weather from tropical storms and hurricanes.
Scrub-oak shrublands occupy over 15 000 acres of Merritt
Island, and fire is essential for maintaining the community. The
scrub-oak vegetation is dominated by woody evergreen species
with extensive belowground storage organs, such as lignotubers
and rhizomes, that allow the plants to re-sprout after fire
(Schmalzer & Hinkle, 1992a; Menges & Kohfeldt, 1995). The
two co-dominant species are Quercus myrtifolia Willd. (myrtle
oak) and Quercus geminata Small (sand live oak).
Experimental design
The study site was burned in early 1996 before the installation of
the chambers. Sixteen plots were grouped on the basis of
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pre-burn vegetation into eight blocks and randomly assigned to a
treatment of ambient CO
2
or elevated CO
2
. Chamber frames
were made with 4-in-diameter PVC pipe in an octagonal design
with panels covered with clear Mylar film. The chambers enclosed
9.4 m
2
of ground area and were 2.5 m tall and 3.5 m wide. CO
2
addition began in May 1996 and was controlled at 350 ppm above
ambient throughout the experiment, except for brief periods in
1999 and 2004 during repairs to the chambers after damaging
storms. Ambient CO
2
was c. 350 ppm in 1996 and had increased
to c. 380 ppm in 2007. CO
2
addition was stopped in May 2007.
Fine root measurements
Minirhizotrons were installed in the study plots in 1996 after fire,
but before chamber construction. Details of the minirhizotron
design, installation and sampling protocol are described in Day
et al. (2006). Minirhizotron recordings were obtained approxi-
mately every 3 months throughout the study, but, because of
time constraints, some dates were not digitized. Images were con-
verted to jpeg files and digitized following the protocol of Day
et al. (2006). The primary metric was root length per frame area
(mm cm
2
; root length density). Root length density was used to
calculate fine root biomass following the methods detailed by
Brown et al. (2009). The principles behind this method are
described by Johnsen et al. (2001), Hendrick & Pregitzer (1996)
and Taylor et al. (1970).
Previously reported minirhizotron estimates of fine root bio-
mass for this system (Brown et al., 2009) included roots >2mm
in diameter. Here, we restricted the analyses to roots <2mm in
diameter because an extremely small number of roots >2mmin
diameter indicated inadequate sampling by minirhizotrons (dis-
cussed in Brown et al., 2009). For example, in the minirhizotron
images, there were no >2-mm-diameter roots in ambient CO
2
plots in March 2007 and there were only two roots in this size class
in elevated plot images. Thus, we recalculated fine root biomass
(roots <2 mm in diameter) for all minirhizotron sampling dates,
and these revised minirhizotron fine root biomass data presented
here are different from those reported by Brown et al. (2009). Fur-
ther details of the methods can be found in Dilustro et al. (2002),
Day et al. (2006), Brown et al. (2009) and Stover et al. (2010).
Coarse root measurements
After CO
2
addition had ended and the chambers had been
removed, all aboveground vegetation was clipped to the soil sur-
face, dried and weighed to obtain total aboveground biomass
(Seiler et al., 2009). In June 2007, <1 wk after the aboveground
vegetation in experimental plots was harvested, GPR was used to
image roots in all plots with a Subsurface Interface Radar
(SIR-3000) and 1500-MHz antenna (Geophysical Survey Sys-
tems Inc., North Salem, NH, USA). The high-frequency antenna
provides high resolution to a depth of 60 cm. Soil cores taken to a
depth of 200 cm indicate that GPR captured at least 80% of total
coarse roots in the top 100 cm of soil. GPR signal reflection was
correlated with coarse root biomass estimates from soil cores,
resulting in a linear relationship, which was then used to estimate
root biomass nondestructively from the study plots. In order to
obtain a more complete picture of total root biomass by combina-
tion with the minirhizotron data (to a depth of 100 cm), we
extrapolated the GPR data to 100 cm (see Soil cores methods sec-
tion below).
A292-m
2
fiberglass frame was positioned within the footprint
of each experimental plot. A 2-m-long beam with a freely moving
shuttle was positioned on the frame. The radar antenna (with cali-
brated survey wheel) was attached to the shuttle arm, and the shut-
tle guided the antenna along 2-m transects. Plots were scanned
every 16 cm in an xand ydirection (resulting in a total of 26 scans
per plot). Individual 2-m-long GPR scans were processed using
Radan version 6.5 (Geophysical Survey Systems Inc.). The pro-
cessing protocol was similar to that used in Stover et al. (2007).
For each experimental plot, 25 random intersections from the grid
of GPR scans were selected. After digital processing of the corre-
sponding scan, a 15-cm-wide section was cropped at each intersec-
tion. This was equivalent to the size of the cores used to establish
the relationship between GPR signal intensity and root biomass
(Stover et al., 2007). Cropped GPR images were converted to
bitmaps using Radan to Bitmap Conversion Utility 2.1 (Geophys-
ical Survey Systems Inc.) and converted to 24-bit grayscale with
SigmaScan Pro Image Analysis software version 5.0 (SPSS Inc.,
Chicago, IL, USA). Pixels within an intensity range of 70227,
representing live roots, were counted for each image. Pixel counts
were applied to a regression equation relating pixel number and
root biomass, where coarse root biomass per 15-cm-diameter
area =0.1262 9pixel count (R
2
=0.47).
To validate the GPR biomass estimation method, a 2 92-m
2
plot separate from the experimental plots was cleared of above-
ground vegetation and scanned with the 1500-MHz GPR
antenna using 13 scans in the xand ydirections. A 1 92-m
2
pit
in half of the scanned area was excavated to a depth of 60 cm.
Roots from the pit were extracted on site using a 6-mm mesh
sieve. Root samples were washed, oven dried at 70°C until the
weight loss was stable and weighed.
Soil cores
Fine roots in the uppermost soil layer (010 cm) were sampled
using cores throughout the study. On several dates in 1998, 1999
and 2001, three cores were removed from each plot using a 1.9-cm-
diameter punch auger. The cores were composited and hand
picked to remove fine roots, which were subsequently washed,
dried and weighed. Core samples were collected in May 2002
using a 7-cm-diameter soil corer, as described in Brown et al.
(2009). The 2002 sampling included exhaustive treatment of the
depth distribution of fine and coarse root biomass. We used these
data to develop plot-specific models of root depth distribution,
using the model of Gale & Grigal (1987), where the proportion
of total root biomass (Y) at depth (d, cm) is equal to
Y¼1bd;Eqn 1
where bis a fitted parameter. To fit the model, we calculated the
cumulative proportion of fine or coarse root biomass at each
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depth interval to 1 m based on the 2002 sampling, and compared
this with modeled estimates using Eqn 1. We minimized the sum
of squared differences between measured and modeled values of
Yusing the Solver function in Microsoft Excel. Values of bbased
on coarse roots from 2002 for each plot were used to estimate
coarse root biomass for the 60100-cm soil depth in 2007, the
fraction of the top meter which the GPR readings could not
assess. Additional coarse root biomass estimated using this
method amounted to, on average, only 7% of the total coarse
root biomass in each plot, and so biomass estimates using this
approach do not substantially alter the calculations of total root
mass. Nevertheless, the inclusion of this fraction of coarse root
biomass in our estimates provides a more complete picture of the
total root biomass in the top meter of soil in this ecosystem.
At the end of the experiment (JuneJuly 2007), five soil cores
were collected in each chamber plot with a 7-cm-diameter soil
corer. The cores were collected in 10-cm increments for the top
meter of soil, and then in 30-cm increments until the water table
was reached (180330 cm). Core samples were sieved with a
2-mm mesh sieve, followed by a 1-mm sieve to separate roots
from mineral soil. Coarse roots were hand picked from the frac-
tion retained on the sieve, and fine roots were picked from a
subsample, scaled to the entire mass of retentate by weight. Roots
were washed, oven dried and weighed to determine biomass.
Total biomass per square meter was calculated by summing the
root mass for all cores per plot and dividing by the total core
area.
Statistical analyses
CO
2
treatments were replicated (n=8 for each treatment). For
measurements that included subsampling within experimental
plots, the model residuals were tested for normality (using the
ShapiroWilk test) and homogeneity of variances (using the
Levene statistic) with PASW Statistics 17.0 (SPSS Inc.). Model
residuals for the data failed tests for normal distribution and vari-
ance homogeneity, and so the data were transformed. Data with
subsampling included minirhizotron biomass data (log trans-
formed) and GPR data (square root transformed). Fine root bio-
mass estimated using minirhizotrons was tested with a repeated
measures ANOVA (SAS version 9.1; SAS Institute Inc., Cary,
NC, USA). GPR data were tested with SAS Proc GLM using a
two-factor nested ANOVA with 25 subsamples per chamber;
chamber was assigned as the random effect and CO
2
treatment as
the fixed effect.
Data that did not have subsampling included total root bio-
mass from the combination of GPR and minirhizotron estimates,
root biomass from cores, aboveground biomass and root-to-shoot
ratios. Nonparametric tests were run on these data using SAS
Proc NPAR1WAY to test for differences between CO
2
treat-
ments. All results were considered to be significant at a<0.05,
but trends were recognized at 0.05 <a<0.15 (following Runion
et al., 2006).
We compared responses to CO
2
by fine roots and leaves in
order to assess the relative responses to elevated CO
2
of resource-
acquiring organs above- and belowground. Total leaf biomass
was estimated from measurements of stem diameter and allomet-
ric relationships developed for the three co-dominant oak species,
as described in Dijkstra et al. (2002) and Seiler et al. (2009).
Total fine root biomass was estimated on the basis of the minirhi-
zotron measurements and, where available, the soil cores. For
each year of the study, 19962007, we used all available data esti-
mating total fine root biomass and total leaf biomass in each plot.
We used bootstrapping to estimate confidence intervals for the
difference in log response ratio below- vs aboveground:
LogeðEr=ArÞlogeðEl=AlÞ;
where Eis the mean for the elevated CO
2
treatment, Ais the
mean for the ambient CO
2
treatment, the subscript ‘r’ indicates
fine roots and the subscript ‘l’ indicates leaves. In this way,
positive values indicate larger relative responses belowground,
and negative values show larger relative responses aboveground.
The use of log response ratios facilitates comparison of relative
responses with symmetrical distributions regardless of sign.
Results
Fine roots
Fine root biomass varied over time from <2000 g m
2
at the
beginning and end of the study to nearly 5000 g m
2
during year 3
(Fig. 1). There were significant (P<0.001) increases during the
first 2 yr, followed by a general decline until a significant increase
(P<0.001) from 2003 to 2005. Increases in fine root biomass
during the first year or two probably represent regrowth after
tube installation (Strand et al., 2008). Changes over time did not
show the sustained increase after fire disturbance that we postu-
lated, but rather showed strong temporal variation apparently
associated with multiple disturbances. Fine root biomass showed
a repeating pattern of CO
2
stimulation after or coincident with
disturbance, followed by declining biomass and diminution of
the CO
2
effect (Fig. 1). The fine root biomass increase during the
0
1000
2000
3000
4000
5000
6000
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008
Fine root biomass (g m
–2
)
HurricaneFire
Fig. 1 Fine (<2 mm in diameter) root biomass to a depth of 100 cm
estimated using minirhizotrons over the 11-yr study period in a scrub-oak
shrubland community in Florida, USA. Values are means SE. Results
presented are revised from those published previously by Brown et al.
(2009). Disturbance events are noted. Ambient, white circles. Elevated,
black circles.
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first 3 yr of the study was greater under elevated CO
2
, but subse-
quently peaked and then declined to levels found in the ambient
chambers. Surface soil core data, although obtained at different
intervals from the minirhizotron observations, nonetheless
showed the high in fine root biomass under elevated CO
2
in early
1998, followed by a decline (Table 1). Declines in root mass were
generally associated with periods of below average rainfall (1999
2002 and 2006). In 2005, following a hurricane, which occurred
in September 2004 and severely reduced leaf area (Li et al.,
2007), fine root biomass peaked again and was significantly
greater in elevated CO
2
plots. This was again followed by a
decline that persisted until the end of the study. At the end of the
study in March 2007, fine root biomass was not significantly
greater under elevated CO
2
than under ambient CO
2
(Table 2,
Fig. 2). These values were lower than at any time since the begin-
ning of the study in 1996. Estimates of fine root biomass
obtained through soil cores also showed low biomass at the end
of the study compared with other time periods (Tables 1, 2).
Fine root production, mortality and turnover were higher in
the elevated chambers during the first 2 yr of the study (Dilustro
et al., 2002). However, our findings 5 yr later revealed that treat-
ment differences in productivity, mortality and turnover were no
longer present (Stover et al., 2010). The results indicated that
CO
2
enrichment was no longer driving changes in fine root
dynamics.
After fire disturbance at the beginning of the study, responses
of fine roots and leaves to elevated CO
2
were roughly comparable
(Fig. 2). Later, elevated CO
2
usually stimulated leaf growth more
than fine root growth (negative response values indicate greater
CO
2
effects on leaves). This was the trend for 1999, 2000 and
2006, and was significant for 20012004 and 2007. In 2005, the
year after the hurricane, responses in fine roots and leaves were
comparable, as found in the early years associated with fire distur-
bance.
Coarse roots
Coarse root biomass estimated using GPR averaged
5476 g m
2
for the top 060 cm (Table 2, Fig. 3). Scaled to
the top meter of soil (after Gale & Grigal, 1987; see Materials
and Methods), this average increased to 5949 g m
2
(Table 2).
Table 1 Surface fine root biomass (top 10 cm) from soil cores in a scrub-oak shrubland community in Florida, USA
June 1998 July 1998 September 1998 December 1998 September 1999 May 2002 July 2007
Ambient 1400 162 1273 118 1268 183 1406 276 1249 186 1700 244 675 147
Elevated 2096 194 1973 309 1182 158 948 120 1377 191 1178 134 472 61
Pvalue 0.011 0.040 0.709 0.126 0.614 0.222 0.036
Values are biomass (g m
2
) plus or minus one standard error of the mean.
Table 2 Root biomass (g m
2
; means standard errors) measured using minirhizotron image analysis, ground-penetrating radar (GPR) and 7-cm-diameter
soil cores in a scrub-oak shrubland community in Florida, USA
Minirhizotron (MR) GPR MR +GPR Cores
<2mm >5 mm Total <2mm >2 mm Total
0100 cm 060 cm 60100 cm 0100 cm 0100 cm 0100 cm 0100 cm 0100 cm
Ambient CO
2
1644 173 5105 418 345 84 5451 371 7094 381 2800 189 2690 246 5490 307
Elevated CO
2
1942 168 5830 487 617 166 6448 615 8390 552 2932 612 3014 467 5947 942
Pvalue 0.236 0.277 0.174 0.191 0.076 0.842 0.552 0.657
Root diameter categories are indicated for each method. GPR estimates are reported for a depth of 060 cm, imaged directly, and for a depth of 60
100 cm, estimated using the model of Gale & Grigal (1987). Totals are shown for the sum of the two indirect measures (Minirhizotron+GPR) and the sum
of diameter categories for the cores. Note that the core sampling includes the roots between 2 and 5 mm in diameter, a range that may not be adequately
detected by the indirect measurements. All of these data were collected during the final harvest in 2007. Pvalues are reported for two-tailed t-tests assum-
ing unequal variance.
–1.0
–0.5
0
0.5
1.0
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Fire Hurricanes
Difference between root and leaf response
to CO
2
Log
e
(E
r
/A
r
) – log
e
(E
l
/A
l
)
Fig. 2 Relative effect of CO
2
on fine roots vs leaves for the 11-yr study
period in a scrub-oak shrubland community in Florida, USA. Values are
means 90% confidence intervals estimated by bootstrapping. The yaxis
is the difference in log response ratios to elevated CO
2
,belowground
aboveground. Thus, negative values indicate greater relative responses
to elevated CO
2
aboveground relative to belowground. Disturbance
events are noted.
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These estimates were 22% lower than those from 2005, where
the average coarse root biomass for 060 cm was estimated at
7070 g m
2
(calculated from Stover et al., 2007). The reliabil-
ity of GPR biomass estimates was confirmed at the end of the
study by the validation plot (1 9290.6-m
3
pit (length 9
width 9depth) in July 2007), with an estimate of 7770 g m
2
roots to a depth of 60 cm using GPR, compared with an
actual biomass of 8222 g m
2
.
Coarse root biomass was not significantly different between
treatments after more than a decade of exposure to experimen-
tally increased atmospheric CO
2
concentration (Table 2),
although coarse root biomass was generally higher in the elevated
treatment plots. Average coarse root biomass was 726 g m
2
higher in the elevated CO
2
treatment for the top 060 cm and
997 g m
2
for the top meter of soil (Table 2). The previous esti-
mate of coarse root biomass (Stover et al., 2007) suggested a
greater treatment difference of 1881 g m
2
(P=0.12) 2 yr earlier
(8010 796 g m
2
elevated and 6129 1010 g m
2
ambient
for the top 060 cm).
Total root biomass
Combining the minirhizotron estimate of fine root biomass mea-
sured in March 2007 and the GPR estimate of coarse root bio-
mass from June 2007, total root biomass for plots exposed to
elevated CO
2
was 8390 552 g m
2
, compared with
7094 381 g m
2
for ambient plots (P=0.076; Table 2). The
combined total root biomass from these two methods is probably
an underestimate given the methodological gap in detecting roots
between 2 and 5 mm in diameter. Minirhizotrons are limited to
roots <2 mm in diameter and the GPR detection limit is
assumed to be roots >5 mm in diameter in this study. Although
the biomass of roots >2 mm in diameter could not be accurately
determined using minirhizotrons, GPR may have detected a
portion of the roots <5 mm in diameter. Dense mats of near-
surface fine roots and clusters of fine roots may be captured by
GPR (R. Schroeder, pers. obs.). The CO
2
fertilization effect
aboveground was significant and proportionally greater than that
belowground (Figs 2, 3). However, belowground biomass, partic-
ularly coarse roots, was the major contributor to total plant bio-
mass (84% in ambient and 78% in elevated CO
2
), and the
absolute difference between elevated and ambient biomass was
greater belowground: 846 g m
2
aboveground (Seiler et al.,
2009) and 1296 g m
2
belowground (Table 2, Fig. 3). The ratio
of total belowground to total aboveground biomass averaged
3.9 0.4 for elevated CO
2
plots, significantly less than the aver-
age of 5.5 0.5 for ambient CO
2
plots (P=0.02).
Biomass estimated from cores
Total root biomass estimated using 7-cm-diameter soil cores was
26% lower than the combined total from the minirhizotron and
GPR sampling methods, an average across treatments of
5719 g m
2
for the cores compared with 7742 g m
2
for the
imaging methods (Table 2). The CO
2
effect on total root bio-
mass estimated from cores was not significant (Table 2), and the
456 g m
2
difference in means between treatments was smaller
than that observed for minirhizotrons and GPR, probably in part
because the cores underestimated coarse root biomass by c. 50%.
Discussion
Fine root response over time in a frequently disturbed
woodland ecosystem
The stimulation of fine root biomass as a result of CO
2
enrich-
ment was transient, and appeared to be associated with distur-
bance: responses were strongest at the beginning of the experiment
after fire, and again during year 8 after hurricanes, periods during
which aboveground biomass was either completely removed (fire)
or severely reduced (hurricane). This phenomenon could be a
result of increased resource use efficiency, or limits on soil
resources over time. A decrease in fine root response after long-
term CO
2
enrichment was observed by Bader et al. (2009), a find-
ing that contrasts with the majority of empirical studies. The loss
of a treatment effect after 7 yr of enrichment was attributed to
increased soil moisture (through reduced transpiration) under ele-
vated CO
2
which may have caused reduced biomass allocation to
fine roots. The lack of CO
2
stimulation of root growth reported
by Handa et al. (2008) was thought to be evidence that mature
ecosystems may not show a belowground treatment effect as much
as an expanding or early successional community. The idea that
vegetation responses to elevated CO
2
are strongly controlled by
ecosystem successional state and plant demography was explored
by Korner (2006). Korner (2006) identified several ecosytem types
based on nutrient cycling properties, but he indicated that some
systems have unique combinations of these properties. We suggest
that the Florida scrub-oak woodland may represent such a system.
Unlike most forests, the scrub-oak system is frequently
impacted by disturbances (fire and hurricanes). Key drivers in a
disturbance-prone system are likely to be different from those in
10 000
8000
6000
4000
2000
0
2000
4000
Biomass (g m–2)
Ambient Elevated
Fig. 3 Live aboveground biomass (above the horizontal line) and root
biomass to a depth of 60 cm (below the horizontal line) in g m
2
SE for
the CO
2
treatments at the time of the final harvest in 2007 in a scrub-oak
shrubland community in Florida, USA. Below the horizontal line, the white
bar represents coarse root biomass and gray represents fine root biomass.
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6
systems that are disturbed infrequently. Fire and hurricanes pro-
duce resource (nutrient and water) pulses that interact with CO
2
to produce unexpected responses. An elevated CO
2
experiment
in a desert ecosystem revealed sporadic CO
2
treatment effects
over time (Ferguson & Nowak, 2011). Periodic rain events in
this system apparently act as an environmental signal eliciting
CO
2
responses. Elevated CO
2
had greater effects on fine root
dynamics during certain phenological events influenced by soil
moisture (Sonderegger et al., 2013). Similar pulse drivers occur
in savanna ecosystems (Chen et al., 2003; February & Higgins,
2010; Tomlinson et al., 2012). We suggest that nutrient avail-
ability is the primary pulse driver in the scrub-oak system. Fires
mobilize nutrients and can increase nutrient availability to plants
(Ojima et al., 1994; Turner et al., 1997), which could enhance
the CO
2
response. The hurricanes defoliate the trees, resulting in
increased nutrient input to the soil via increased leaf fall, and
reduce water demand by the canopy. Several months following
the hurricane, concentrations of soil extractable micronutrients
increased, and then declined over the next 2 yr (B. A. Hungate
et al., unpublished). Thus, both nutrient and water pulses may
have enhanced the CO
2
response after the hurricanes.
Day et al. (2006) proposed that fine roots reached a dynamic
equilibrium (‘root closure’) in the study system 3 yr after the
experiment began, and that this root closure was reached shortly
before canopy closure. However, the analogy to closure above-
ground is limited, because resource availability belowground is
far more dynamic. In other words, light and CO
2
, the major
aboveground resources, do not vary over space and time as much
as belowground resources (water and nutrients). This might
explain the large fluctuations in fine root biomass. The concept
of root ‘closure’ may be resource (other than just space) depen-
dent. Disturbances, such as fire or drought, appear to reduce fine
root biomass to levels below the soil’s carrying capacity for fine
roots. During the recovery phase, after disturbance and root die-
back, fine roots respond to CO
2
fertilization (a resource pulse).
One important implication of this finding is that elevated CO
2
may result in greater carbon inputs to soils following disturbance.
After root closure, limited resources (space, water and nutrients)
result in a loss of the CO
2
fertilization effect.
Fine roots are temporally dynamic. Root biomass, production
and mortality vary seasonally (McClaugherty et al., 1982;
Hendrick & Pregitzer, 1992) and interannually (Espeleta &
Clark, 2007). In CO
2
enrichment studies, the CO
2
treatment
effect on fine root biomass varied over the course of a year
(Norby et al., 2004; Jackson et al., 2009) and between years
(Norby et al., 2004). Natural variations in root biomass over time
may complicate the evaluation of CO
2
treatment effects in long-
term studies. At the end of the Florida study, fine root biomass
was lower than it had been at any time since the beginning of the
study; thus, the biomass values and differences between treat-
ments from 2007 represent a point in time when fine root
biomass was low and CO
2
effects were not apparent. Thus, the
end-of-study results may have underestimated the CO
2
response
by roots. Conclusions based on fine root ‘lows’ may differ from
those based on the ‘highs’, as the CO
2
effects were strongest dur-
ing periods of recovery. The value of long-term studies is that
they provide a time series that encompasses such variability and
provides the opportunity to assess relationships among ecosystem
response variables with environmental drivers or pulses that may
not be apparent from short-term studies or snapshot or end-of-
study results. This may explain the differences in 2005 and 2007
estimates of root mass by GPR. The biomass validation plot data
and the interannual fine root biomass data both support the idea
that a substantial portion of roots of <2 mm in diameter are
detected by GPR. They may exist as clumps or as part of the
dense surface mat. The decrease in coarse root estimates from
2005 to 2007 measured using GPR is not likely to be the result
of an actual decrease in coarse root biomass, but may just reflect
the decrease in fine root biomass over that time period. Addi-
tional testing of GPR is needed to evaluate the sensitivity to fine
root clusters.
By contrast, some long-term CO
2
enrichment studies have
shown sustained root biomass stimulation under elevated CO
2
over more than a decade of CO
2
enrichment (Jackson et al.,
2009). Fine root peak standing crop was approximately doubled
across all years in a 9-yr free air carbon dioxide enrichment
(FACE) study in a sweetgum plantation (Norby et al., 2004;
Iversen et al., 2008). Averaged over 6 yr of FACE treatment in a
loblolly pine plantation, root standing crop was increased by
23% (Pritchard et al., 2008). Resource availability, including
space, may explain these differences.
Coarse roots in a frequently disturbed woodland ecosystem
Although fine root biomass was monitored throughout the study,
coarse root biomass was only measured during the last few years of
the experiment. Limits on destructive sampling in long-term
experiments using small-diameter cores may limit observations of
CO
2
stimulation of root biomass. The doubling of coarse roots
under elevated CO
2
, reported by Jackson et al. (2009), was not
observed until pits were dug in 2008. This poses a significant prob-
lem for biomass estimates, because belowground biomass, particu-
larly coarse roots, constituted the majority of total plant biomass at
the Florida site (84% in ambient and 79% in elevated CO
2
plots).
The data presented here and previously collected from the site
indicate that coring techniques, especially those using small-
diameter corers, probably underestimate coarse root biomass. In
a study comparing actual (whole-tree harvest extraction) and esti-
mated (5-cm-diameter soil cores) lateral root density, the soil
cores underestimated root density by as much as half the value
determined by whole-tree harvest (Retzlaff et al., 2001). Our
GPR-based estimates of coarse root biomass were greater than
those obtained from coring, but comparable with coarse root bio-
mass sampled directly in the 1 92-m
2
validation soil pit. Thus,
the GPR data accurately reflected destructive sampling on a larger
spatial scale (2 m
2
), and suggested that the smaller area of the
cores (c. 0.02 m
2
) underestimated coarse root biomass. The small
number of cores allowed in this long-term study is not sufficient
to cover the spatial heterogeneity of coarse root distribution, and
results in bias towards values of zero, which may lead to an
underestimation of coarse root biomass (B. A. Hungate, unpub-
lished).
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The validation pit dug near the study plots yielded over
8000 g m
2
of root biomass to a depth of 60 cm. This is consid-
erably higher than the upper range of 5000 g m
2
of root bio-
mass in global biomes analyzed by Jackson et al. (1996). Studies
in systems with large belowground structures, such as rhizomes
and lignotubers, have found high root biomass similar to the
Florida site, for example the scrub-oak of the garrigue in southern
France, in which large belowground structures (>5 mm in dia-
meter) constituted 85% of the 7200 g m
2
total root biomass
(Kummerow et al., 1990).
Root-to-shoot ratios
Elevated CO
2
stimulated the production of aboveground bio-
mass; however, this response was species specific with
Q. myrtifolia increasing in aboveground biomass by 128% and
Q. geminata displaying no significant treatment effect after 11 yr
of enrichment (Seiler et al., 2009). Live aboveground biomass
values were 1257 107 and 2103 184 g m
2
for ambient and
elevated CO
2
plots, respectively (Seiler et al., 2009; Fig. 3). At
the end of the study, the average root-to-shoot ratio was higher in
plots exposed to ambient CO
2
. The contrasting responses of the
dominant oaks to CO
2
enrichment may have affected the pat-
terns of biomass allocation above- and belowground. Because
roots were not quantified by species, we cannot determine the
direct contribution of each species to root biomass, but it is possi-
ble that differences in biomass partitioning among species could
have affected total root biomass at the end of the study.
Biomass partitioning under CO
2
enrichment does not seem to
follow a predictable pattern. Although a meta-analysis by Luo
et al. (2006) showed slightly higher root-to-shoot ratios in plants
grown under elevated CO
2
, there are many studies in which this
is not the case. Stulen & den Hertog (1993) believed that the
determination of root-to-shoot ratios was highly susceptible to
experimental error, such as the subjectivity surrounding the point
at which shoots end and roots begin. The lack of a consistent
pattern in biomass partitioning found in a literature review by
BassiriRad et al. (2001) was attributed to variations in experimen-
tal protocol and/or interspecific differences. Wang & Taub
(2010) found that abiotic stresses (i.e. drought or exposure to
ozone) had a more pronounced effect on the fraction of root to
total biomass than did exposure to elevated CO
2
.
The relatively greater CO
2
stimulation of leaf biomass relative
to fine root biomass contrasts with earlier speculation that
increasing CO
2
should promote relatively stronger responses in
roots (Norby et al., 2004). Two phenomena could account for
this response: (1) elevated CO
2
increases the functional efficiency
of fine roots; or (2) elevated CO
2
stimulates nutrient availability.
We have no direct evidence of increased physiological efficiency
of fine roots (e.g. nutrient uptake kinetics), and responses in
other studies have been mixed (BassiriRad, 2000). As an exten-
sion of the nutrient scavenging surface, mycorrhizae can effec-
tively increase the functional efficiency of fine roots; in the scrub-
oak system, elevated CO
2
stimulated ecto-mycorrhizal biomass
and infection (Langley et al., 2003), soil fungal markers consis-
tent with mycorrhizae (Klamer et al., 2002) and the ratio of fungi
to bacteria in soil (Carney et al., 2007), suggesting that increased
mycorrhizal infection, a general response to elevated CO
2
(Treseder, 2004), may have contributed to the relatively smaller
response to CO
2
of fine roots. Second, increased nutrient avail-
ability may also partly explain the smaller response of fine roots
to elevated CO
2
: we found that elevated CO
2
stimulates the turn-
over of soil organic matter in this system (Carney et al., 2007;
Langley et al., 2009; B. A. Hungate et al., unpublished), and
increases plant nitrogen uptake (B. A. Hungate et al., unpub-
lished), a pattern also found in a pine forest (Drake et al., 2011).
In the first several years of CO
2
exposure in the Florida scrub-oak
system, fine root turnover increased with elevated CO
2
, but this
response was no longer observed later in the study (Dilustro
et al., 2002; Stover et al., 2010). Additional observations are
required to fully address this question.
Conclusions
Data from this 11-yr CO
2
enrichment study in a fire-maintained,
shrub-dominated woodland ecosystem marginally supported the
hypothesis that elevated CO
2
would stimulate root biomass; how-
ever, fine root biomass was at a low point at the end of the study and,
although statistically insignificant (P=0.07), there was a substantial
absolute difference in belowground biomass in elevated vs ambient
CO
2
plots. Strong CO
2
effects on fine root biomass were seen after
disturbance by fire and hurricane during periods of recovery, fol-
lowed by periods in which CO
2
effects diminished. Possibly, distur-
bance increased resource availability and altered plant sink strength
above- and belowground, modulating belowground responses to
CO
2
in this ecosystem. Total root biomass was as much as five times
greater than aboveground biomass in this system, reflecting the
importance of belowground structures as a carbon reservoir. This
study suggests that both minirhizotrons and GPR are effective and
compatible in covering the range of root size classes, but there are
unknown areas of overlap that need resolution. Analysis and inter-
pretation of the entire 11-yr dataset was necessary to fully elucidate
the root response to long-term CO
2
enrichment at this site.
These findings can be applied to future work in several ways.
First, belowground carbon budgets and predictions regarding the
effect of increasing atmospheric CO
2
on root biomass will need
to take into account root closure as a possible limit on carbon soil
dynamics in mature ecosystems. Second, belowground biomass is
temporally dynamic and undergoes natural cycles that are
affected by ecosystem disturbances in systems with strong distur-
bance regimes. Similar responses have been observed in desert
plants, with CO
2
effects dependent on soil moisture (Ferguson
& Nowak, 2011; Sonderegger et al., 2013). The change in root
biomass over time means that one-time sampling will not give an
accurate representation of root parameters over the long term.
Third, elevated CO
2
may enhance root growth following distur-
bance and potentially speed up the recovery.
Acknowledgements
This research was funded by US Department of Energy grant
(DE-FG-02-95ER61993) to the Smithsonian Institution with
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Phytologist
8
subcontract (95-59-MPOOO02) to F.P.D. at Old Dominion
University, and by grants from the National Science Foundation
(DEB 9873715, 0092642 and 0445324) to B.A.H. at Northern
Arizona University. We thank the US Fish and Wildlife Service
at Merritt Island National Wildlife Refuge and the National
Aeronautics and Space Administration at Kennedy Space Center
for their cooperation. Soil coring was assisted by J. Brown,
J. Blankinship, J. Coyle, C. LaViolete, Z. Wu, Tom Powell and
Pat Megonigal. Kadrin Getman provided invaluable field assis-
tance. Dan Welch of Geophysical Survey Systems Inc. provided
help with data processing questions, and Dayanand Naik assisted
with statistical analyses.
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... A number of studies have shown promising results estimating root biomass using GPR [7,[19][20][21][25][26][27][28][29][30][41][42][43][44][45]. GPR has been shown to routinely identify roots as small as 5 mm in diameter in field soils using a 1500 MHz antenna [19]. ...
... Lower frequency antennas are not as useful for this application. However, these studies have only utilized either single direction scanning transects [7,[19][20][21][25][26][27]29,30,42,44,45] or perpendicular, two-direction scanning transects [28,41,43]. It has been shown that the angle at which the GPR antenna crosses over a root affects the amplitude of the reflected waveform, thus affecting the estimated size of the root [46][47][48]. ...
... Strong regressions were obtained using total root biomass from sample cores to 60 cm depth, and using pixels within a threshold range for biomass estimation previously used by Stover et al. [20], Butnor et al. [2], Samuelson et al. [27,44,45], and Day et al. [43]. The number of transects in each direction was not paramount, but the inclusion of four scanning directions for each core and plot was important. ...
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Accurate quantification of coarse roots without disturbance represents a gap in our understanding of belowground ecology. Ground penetrating radar (GPR) has shown significant promise for coarse root detection and measurement, however root orientation relative to scanning transect direction, the difficulty identifying dead root mass, and the effects of root shadowing are all key factors affecting biomass estimation that require additional research. Specifically, many aspects of GPR applicability for coarse root measurement have not been tested with a full range of antenna frequencies. We tested the effects of multiple scanning directions, root crossover, and root versus soil moisture content in a sand-hill mixed oak community using a 1500 MHz antenna, which provides higher resolution than the oft used 900 MHz antenna. Combining four scanning directions produced a significant relationship between GPR signal reflectance and coarse root biomass (R2 = 0.75) (p < 0.01) and reduced variability encountered when fewer scanning directions were used. Additionally, significantly fewer roots were correctly identified when their moisture content was allowed to equalize with the surrounding soil (p < 0.01), providing evidence to support assertions that GPR cannot reliably identify dead root mass. The 1500 MHz antenna was able to identify roots in close proximity of each other as well as roots shadowed beneath shallower roots, providing higher precision than a 900 MHz antenna. As expected, using a 1500 MHz antenna eliminates some of the deficiency in precision observed in studies that utilized lower frequency antennas.
... The inter-annual fluctuation and seasonal pattern of fine root demographics are the two main sources of the variability (Krasowski et al. 2010;McCormack et al. 2014). Few previous studies have reported that fine root production, mortality and turnover rate showed widely inter-annual variation (Joslin et al. 2000;Iversen et al. 2008;Strand et al. 2008;Day et al. 2013;Krasowski et al. 2018). For instance, fine root production and mortality varied more than 4-folds in subtropical forests (Joslin et al. 2000;Kou et al. 2018), and Communicated by Louis Stephen Santiago. up to 30-folds in a temperate forest (Krasowski et al. 2010). ...
... For instance, fine root production and mortality varied more than 4-folds in subtropical forests (Joslin et al. 2000;Kou et al. 2018), and Communicated by Louis Stephen Santiago. up to 30-folds in a temperate forest (Krasowski et al. 2010). The variability in fine root annual demographics may depend on climatic factors (Day et al. 2013;Kou et al. 2018), or closely linked to study sites and tree species (Krasowski et al. 2010(Krasowski et al. , 2018. Therefore, more studies on fine root interannual variation are needed in other sites and forest types. ...
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Fine roots play a key role in carbon, nutrient, and water biogeochemical cycles in forest ecosystems. However, inter-annual dynamics of fine root production, mortality, and turnover on the basis of long-term measurement have been less studied. Here, field scanning rhizotrons were employed for tracking fine root by branch order over a 6 years period in a larch plantation. For total fine roots, from the first- to the fifth-order roots, annual root length production, length mortality, standing crops, and turnover rate varied up to 3.4, 2.3, 1.5, and 2.3-folds during the study period, respectively. The inter-annual variability of those roots indices in the first-order and the second-order roots were greater than that of the higher order (third- to fifth-order) roots. The turnover rate was markedly larger for the first-order roots than for the higher order roots, showing the greatest variability up to 20 times. Seasonal dynamics of root length production followed a general concentrated pattern with peak typically occurring in June or July, whereas root length mortality followed a general bimodal mortality pattern with the dominant peak in May and the secondary peak in August or October. Furthermore, the seasonal patterns of root length production and mortality were similar across years, especially for the first-order and the second-order roots. These results from long-term observation were beneficial for reducing uncertainty of characterizing fine root demography in consideration of large variation among years. Our findings highlight it is important for better understanding of fine root dynamics and determining root demography through distinguishing observation years and root branch orders.
... Scrub oaks have extensive belowground roots and rhizomes (Guerin 1993, Day et al. 2013, as do ericads. Belowground biomass of scrub at KSC/ MINWR makes up approximately 85% of total biomass (Stover et al. 2007). ...
... Sprouting is rapid and scrub vegetation reestablishes cover quickly (Abrahamson 1984a, 1984b, Schmalzer and Hinkle 1992, Schmalzer 2003. Fine roots also recover rapidly after fire (Day et al. 2013). Oak cover returns to preburn values usually within 5 yr (Schmalzer 2003). ...
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Sandy gaps in the shrub matrix of oak (Quercus L.)-saw palmetto (Serenoa repens (W. Bartram) Small) scrub are created by fire but typically close quickly because of rapid regrowth. Such gaps are important habitat features for rare scrub flora and fauna and appear to have been more common in the historical landscape. We followed, from 1993 to 2016, the dynamics of 12 gaps (32.2-98.1 m²) created by burning slash piles as part of restoration of long-unburned scrub. Gaps closed slowly, primarily by canopy spread of oaks around the gaps. In the absence of subsequent fire, gaps closed within approximately 12 yr. When burned a second or third time, gap area increased to near the initial after-burn size but then declined in area more rapidly than after the initial fire. Vegetation that reestablished in gaps differed from that of the scrub matrix in having less cover of scrub oaks, less cover of S. repens > 0.5 m, greater cover of native shrubs and forbs > 0.5 m, and more bare ground. Soil heating from slash-pile burning killed the roots and rhizomes from which scrub oaks, Serenoa, and ericaceous shrubs sprout; this altered and slowed the after-fire recovery.
... Collecting root samples in a core sampler-in triplicate-once in the period of June to November, may not take into account the spatial and temporal diversity of fine roots found across the studied plots [18]. On the other hand, the findings in [94] emphasized the advantages of soil-core methods (cylinders) for the validation of other FRB estimation methods; therefore, it may be assumed that the most accurate method was chosen for our research work. In the described shallow organic soil horizons (Table 6: 2 to 4 cm), roots were not detected during field description. ...
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Forest ecosystems significantly contribute to the global organic carbon (OC) pool, exhibiting high spatial heterogeneity in this respect. Some of the components of the OC pool in a forest (woody aboveground biomass (wAGB), coarse root biomass (CRB)) can be relatively easily estimated using readily available data from land observation and forest inventories, while some of the components of the OC pool are very difficult to determine (fine root biomass (FRB) and soil organic matter (SOM) stock). The main objectives of our study were to: (1) estimate the SOM stock; (2) estimate FRB; and (3) assess the relationship between both biotic (wAGB, forest age, foliage, stand density) and abiotic factors (climatic conditions, relief, soil properties) and SOM stocks and FRB in temperate forests in the Western Carpathians consisting of European beech, Norway spruce, and silver fir (32 forest inventory plots in total). We uncovered the highest wAGB in beech forests and highest SOM stocks under beech forest. FRB was the highest under fir forest. We noted a considerable impact of stand density on SOM stocks, particularly in beech and spruce forests. FRB content was mostly impacted by stand density only in beech forests without any discernible effects on other forest characteristics. We discovered significant impacts of relief-dependent factors and SOM stocks at all the studied sites. Our biomass and carbon models informed by more detailed environmental data led to reduce the uncertainty in over- and underestimation in Cambisols under beech, spruce, and fir forests for mountain temperate forest carbon pools.
... We further checked and confirmed our data in the datasets of root biomass and litter mass which contained some anomalously high values than that reported in two previous databases 35,37 . We found that the northern hardwood forests in Michigan, USA and a Florida scrub-oak forest on the east coast of Florida, USA had great root biomass densities of 5,318 and 6,522 g m −2 , respectively 38,39 . In addition, two alpine meadows showed greater root biomass densities (9,524 and 13,696 g m −2 ) 40,41 than those in forests. ...
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Numerous ecosystem manipulative experiments have been conducted since 1970/80 s to elucidate responses of terrestrial carbon cycling to the changing atmospheric composition (CO 2 enrichment and nitrogen deposition) and climate (warming and changing precipitation regimes), which is crucial for model projection and mitigation of future global change effects. Here, we extract data from 2,242 publications that report global change manipulative experiments and build a comprehensive global database with 5,213 pairs of samples for plant production (productivity, biomass, and litter mass) and ecosystem carbon exchange (gross and net ecosystem productivity as well as ecosystem and soil respiration). Information on climate characteristics and vegetation types of experimental sites as well as experimental facilities and manipulation magnitudes subjected to manipulative experiments are also included in this database. This global database can facilitate the estimation of response and sensitivity of key terrestrial carbon-cycling variables under future global change scenarios, and improve the robust projection of global change-terrestrial carbon feedbacks imposed by Earth System Models.
Article
Some experiments and observations of free-living plants have found that increasing atmospheric concentration of CO2 (pCO2) is directly correlated with increasing discrimination against ¹³C during photosynthesis (Δ¹³C) in C3 plants. The inverted form of this correlation has been used to estimate pCO2 in the geological past (i.e. the C3 plant proxy), but there has been little experimental work to establish the relative importance of pCO2 as a driver of discrimination in more natural settings and over a range of pCO2 relevant to the deep-time geologic record. Here we report on an experiment exploring the relationship between pCO2 and Δ¹³C in Ginkgo biloba, a plant long used to infer past CO2 levels because of the strong similarity of extant to fossil Ginkgo and the abundance of Ginkgo fossils with preserved cuticle from late Mesozoic and Cenozoic periods of warm global climate. We grew Ginkgo biloba plants for three years under ambient pCO2 (∼425 ppm) and elevated levels (∼600, ∼800, and ∼1000 ppm) while measuring the carbon isotope composition of air (δ¹³Cair) and leaves (δ¹³Cleaf) as well as the ratio of internal to external CO2 concentration (ci/ca), maximum photosynthetic assimilation rate (Amax), C:N ratio, and leaf mass per area (LMA). We found no significant relationship between pCO2 and Δ¹³Cleaf or ci/ca. We did find a direct correlation of pCO2 with Amax, LMA, and C:N ratio. The lack of increase in Δ¹³Cleaf with rising pCO2 may result from the lack of change in ci/ca, thicker leaves that slow the rate of diffusion of CO2 through the leaf to mesophyll cells, higher Amax that drives more rapid consumption of intracellular CO2 and/or changes in the relative proportions of starches, lipids or other compounds that have distinct isotopic compositions. Our results, along with a compilation of data from the literature on Δ¹³Cleaf in many different types of C3 plants, suggest that Δ¹³Cleaf does not consistently increase with increasing pCO2. Rather, there is a diversity of responses, both positive and negative, that are not clearly related to taxonomic group or growth form but may reflect changes in leaf structure, stomatal response and Amax under higher pCO2. Given the complex relationship between Δ¹³Cleaf and pCO2 in living plants we consider Δ¹³Cleaf of fossil plants to be an unreliable proxy for paleo-atmospheric pCO2.
Article
Elevated atmospheric CO2 (eCO2) typically increases aboveground growth in both growth chamber and free‐air carbon enrichment (FACE) studies. Here we report on the impacts of eCO2 and nitrogen amendment on coarse root biomass and net primary productivity (NPP) at the Duke FACE study, where half of the eight plots in a 30‐year‐old loblolly pine (Pinus taeda, L.) plantation, including competing naturally regenerated broadleaved species, were subjected to eCO2 (ambient, aCO2 plus 200 ppm) for 15‐17 years, combined with annual nitrogen amendments (11.2 g N m‐2) for 6 years. Allometric equations were developed following harvest to estimate coarse root (> 2 mm diameter) biomass. Pine root biomass under eCO2 increased 32%, 1.80 kg m‐2 above the 5.66 kg m‐2 observed in aCO2, largely accumulating in the top 30 cm of soil. In contrast, eCO2 increased broadleaved root biomass more than two‐fold (aCO2: 0.81, eCO2: 2.07 kg m‐2), primarily accumulating in the 30‐60 cm soil depth. Combined, pine and broadleaved root biomass increased 3.08 kg m‐2 over aCO2 of 6.46 kg m‐2, a 48% increase. Elevated CO2 did not increase pine root:shoot ratio (average 0.24) but increased the ratio from 0.57 to 1.12 in broadleaved species. Averaged over the study (1997‐2010), eCO2 increased pine, broadleaved, and total coarse root NPP by 49, 373, and 86%, respectively. Nitrogen amendment had smaller effects on any component, singly or interacting with eCO2. A sustained increase in root NPP under eCO2 over the study period indicates that soil nutrients were sufficient to maintain root growth response to eCO2. These responses must be considered in computing coarse root carbon sequestration of the extensive southern pine and similar forests, and in modelling the responses of coarse root biomass of pine‐broadleaved forests to CO2 concentration over a range of soil N availability.
Article
Fine roots are a key component of carbon and nutrient dynamics in forest ecosystems. Rising atmospheric [CO2] (eCO2) is likely to alter the production and activity of fine roots, with important consequences for forest carbon storage. Yet empirical evidence of the role of eCO2 in driving root dynamics is limited, particularly for grassy woodlands, an ecosystem type of global importance. We sampled fine roots across seasons over a two‐year period to examine the effects of eCO2 on their biomass, production, turnover and functional traits in a native mature grassy Eucalyptus woodland in eastern Australia (EucFACE). Fine root biomass, production and turnover varied greatly through time, increasing as soil water content declined. Despite a lack of consistent effects of eCO2 on fine root biomass, production or turnover across the two‐year sampling period, we found enhanced production pulses under eCO2 between 10‐30 cm soil depth. In addition, eCO2 led to greater carbon and phosphorus concentrations in fine roots and increased root diameter, but no detectable effects on other morphological traits. Synthesis. We found minor quantitative effects of eCO2 on fine root biomass dynamics that were largely driven by temporal variations in soil water availability. Our results suggest that in this mature grassy woodland, and perhaps also in other similar forested ecosystem types, eCO2 effects are small and transient. This also implies a limited ability of these systems to mitigate climate change through belowground mechanisms.
Chapter
Roots make up 5 to 45 percent of tree biomass in upland forests worldwide (Cairns et al., 1997), but estimates from urban settings are limited. Roots of eld-grown trees are difcult to assess and delineate without laborious and destructive excavations. For this reason, relative to studies on aboveground biomass, root systems have rarely been studied; often more is assumed about their nature than is really known. Foresters and ecologists are interested to learn how different management techniques (site preparation, cultural practices, and harvesting methods) and natural disturbances affect tree productivity and carbon sequestration. Tree roots play a dynamic role in sustainable forest productivity and serve as a conduit for atmospheric carbon to enter the rhizosphere. After a harvest, re, or other disturbance, the majority of the recalcitrant carbon is retained in roots for a time and slowly released by oxidative processes, and a small fraction becomes a stable constituent of the soil. Research interests aside, trees play an important role in urban environments, providing shade, reducing temperatures, and providing aesthetics. Municipalities, arborists, and property owners are interested in mapping roots to establish protection zones during construction (Jim, 2003), assessing tree health and taking proactive steps to help urban trees thrive.
Article
Total root production (∑P), total root loss (∑L), net root production. (NP), and biomass production were determined for seedlings of Betula papyrifera and Acer rubrum in ambient and elevated CO2 environments. ∑P, ∑L, and NP were calculated from sequential, independent observations of root length production through plexiglass windows. Elevated CO2 increased ∑P, ∑L, and NP in seedlings of Betula papyrifera but not Acer rubrum. Root production and loss were qualitatively similar to whole-plant growth responses to elevated CO2. Betula showed enhanced ∑P, ∑L, and biomass with elevated CO2 but Acer did not. However, the observed effects of CO2 on root production and loss did not alter the allometric relationship between root production and root loss for either Acer or Betula. Thus, in this experiment, elevated CO2 did not affect the relationship between root production and root loss. The results of this study have important implications for the potential effects of elevated CO2 on root dynamics. Elevated CO2 may lead to increases in root production and in root loss (turnover) where the changes in root turnover are largely a function of the magnitude of root production increases.
Article
A tree's root system accounts for between 10 and 65% of its total biomass, yet our understanding of the factors that cause this proportion to vary is limited because of the difficulty encountered when studying tree root systems. There is a need to develop new sampling and measuring techniques for tree root systems. Ground penetrating radar (GPR) offers the potential for direct nondestructive measurements of tree root biomass and root distributions to be made. We tested the ability of GPR, with 500 MHz, 800 MHz and 1 GHz antennas, to detect tree roots and determine root size by burying roots in a 32 m(3) pit containing damp sand. Within this test bed, tree roots were buried in two configurations: (1) roots of various diameters (1-10 cm) were buried at a single depth (50 cm); and (2) roots of similar diameter (about 5 cm) were buried at various depths (15-155 cm). Radar antennas were drawn along transects perpendicular to the buried roots. Radar profile normalization, filtration and migration were undertaken based on standard algorithms. All antennas produced characteristic reflection hyperbolas on the radar profiles allowing visual identification of most root locations. The 800 MHz antenna resulted in the clearest radar profiles. An unsupervised, maximum-convexity migration algorithm was used to focus information contained in the hyperbolas back to a point. This resulted in a significant gain in clarity with roots appearing as discrete shapes, thereby reducing confusion due to overlapping of hyperbolas when many roots are detected. More importantly, parameters extracted from the resultant waveform through the center of a root correlated well with root diameter for the 500 MHz antenna, but not for the other two antennas. A multiple regression model based on the extracted parameters was calibrated on half of the data (R-2 = 0.89) and produced good predictions when tested on the remaining data. Root diameters were predicted with a root mean squared error of 0.6 cm, allowing detection and quantification of roots as small as 1 cm in diameter. An advantage of this processing technique is that it produces results independently of signal strength. These waveform parameters represent a major advance in the processing of GPR profiles for estimating root diameters. We conclude that enhanced data analysis routines combined with improvements in GPR hardware design could make GPR a valuable tool for studying tree root systems.
Chapter
This chapter sets the scene for many of the topics covered in detail later in this volume. We discuss first the basic problems that plants face when growing on land. These problems reflect the many physical, chemical and biological constraints that soil imposes on the functioning of roots in terms of growth and resource capture.
Article
The allocation of biomass to different plant organs depends on species, ontogeny and on the environment experienced by the plant. In this paper we first discuss some methodological tools to describe and analyse the allocation of biomass. Rather than the use of shoot:root ratios, we plead strongly for a subdivision of biomass into at least three compartments: leaves, stems and roots. Attention is drawn to some of the disadvantages of allometry as a tool to correct for size differences between plants. Second, we tested the extent to which biomass allocation of plants follows the model of a 'functional equilibrium'. According to this model, plants respond to a decrease in above-ground resources with increased allocation to shoots (leaves), whereas they respond to a decrease in below-ground resources with increased allocation to roots. We can-led out a meta-analysis of the literature, analysing the effect of various environmental variables on the fraction of total plant biomass allocated to leaves (leaf mass fraction), stem (stem mass fraction) and roots (root mass fraction). The responses to light, nutrients and water agreed with the (qualitative) prediction of the 'functional equilibrium' theory. The notable exception was atmospheric CO2, which did not affect allocation when the concentration was doubled. Third, we analysed the quantitative importance of the changes in allocation compared to changes in other growth parameters, such as unit leaf rate (the net difference between carbon gain and carbon losses per unit time and leaf area), and specific leaf area (leaf area: leaf biomass). The effects of light, CO2 and water on leaf mass fractions were small compared to their effects on relative growth rate. The effects of nutrients, however, were large, suggesting that only in the case of nutrients, biomass allocation is a major factor in the response of plants to limiting resource supply.
Article
Global Change Biology (1999) 5, 807-837 Review Elevated CO2 and plant structure: a review SETH G . PRITCHARD,* HUGO H . ROGERS,* STEPHEN A . PRIOR* and CURT M . PETERSON 1 *USDA-ARS National Soil Dynamics Laboratory, 411 South Donahue Drive, Auburn, AL 36832, USA, 1 Department of Biological Sciences, University of Northern Colorado, Greeley, CO 80639, USA. Abstract Consequences of increasing atmospheric CO2 concentration on plant structure, an important determinant of physiological and competitive success, have not received sufficient attention in the literature. Understanding how increasing carbon input will influence plant developmental processes, and resultant form, will help bridge the gap between physiological response and ecosystem level phenomena. Growth in elevated CO2 alters plant structure through its effects on both primary and secondary meristems of shoots and roots. Although not well established, a review of the literature suggests that cell division, cell expansion, and cell patterning may be affected, driven mainly by increased substrate (sucrose) availability and perhaps also by differential expression of genes involved in cell cycling (e.g. cyclins) or cell expansion (e.g. xyloglucan endotransglycosylase). Few studies, however, have attempted to elucidate the mechanistic basis for increased growth at the cellular level. Regardless of specifc mechanisms involved, plant leaf size and anatomy are often altered by growth in elevated CO2, but the magnitude of these changes, which often decreases as leaves mature, hinges upon plant genetic plasticity, nutrient availability, temperature, and phenology. Increased leaf growth results more often from increased cell expansion rather than increased division. Leaves of crop species exhibit greater increases in leaf thickness than do leaves of wild species. Increased mesophyll and vascular tissue cross­-sectional areas, important determinates of photosynthetic rates and assimilate transport capacity, are often reported. Few studies, however, have quantified characteristics more reflective of leaf function such as spatial relationships among chlorenchyma cells (size, orientation, and surface area), intercellular spaces, and conductive tissue. Greater leaf size and/or more leaves per plant are often noted; plants grown in elevated CO2 exhibited increased leaf area per plant in 66% of studies, compared to 28% of observations reporting no change, and 6% reported a decrease in whole plant leaf area. This resulted in an average net increase in leaf area per plant of 24%. Crop species showed the greatest average increase in whole plant leaf area (+ 37%) compared to tree species (+ 14%) and wild, nonwoody species (+15%). Conversely, tree species and wild, nontrees showed the greatest reduction in specific leaf area (- 14% and - 20%) compared to crop plants (- 6%). Alterations in developmental processes at the shoot apex and within the vascular cambium contributed to increased plant height, altered branching characteristics, and increased stem diameters. The ratio of internode length to node number often increased, but the length and sometimes the number of branches per node was greater, suggesting reduced apical dominance. Data concerning effects of elevated CO2 on stem/branch anatomy, vital for understanding potential shifts in functional relationships of leaves with stems, roots with stems, and leaves with roots, are too few to make generalizations. Growth in elevated CO2 typically leads to increased root length, diameter, and altered branching patterns. Altered branching characteristics in both shoots and roots may impact competitive relationships above and below the ground. Understanding how increased carbon assimilation affects growth processes (cell division, cell expansion, and cell patterning) will facilitate a better understanding of how plant form will change as atmospheric CO2 increases. Knowing how basic growth processes respond to increased carbon inputs may also provide a mechanistic basis for the differential phenotypic plasticity exhibited by different plant species/functional types to elevated CO2. Keywords: anatomy, development, elevated carbon dioxide, morphogenesis, morphology, ultrastructure Received 9 October 1998; resubmitted and accepted 21 December 1998 Correspondence: Dr Seth G. Pritchard, tel +1/334-844-4741 ext.142, fax +1/334-887-8597, e-mail pritcsg@mail.auburn.edu © 1999 Blackwell Science Ltd.