Tree Physiology 22, 725–732
© 2002 Heron Publishing—Victoria, Canada
Coarse and fine root respiration in aspen (Populus tremuloides)
ANNIE DESROCHERS,1–3 SIMON M. LANDHÄUSSER1 and VICTOR J. LIEFFERS1
1
Centre of Enhanced Forest Management, Department of Renewable Resources, University of Alberta, 4-42 Earth Sciences Building, Edmonton, AB
T6G 2E3, Canada
2
Present address: Alberta-Pacific Forest Industries Inc., Box 8000, Boyle, AB T0A 0M0, Canada
3
Author to whom correspondence should be addressed ([email protected])
Received June 7, 2001; accepted November 13, 2001; published online June 3, 2002
Summary Coarse and fine root respiration rates of aspen
(Populus tremuloides Michx.) were measured at 5, 15 and
25 °C. Coarse roots ranged from 0.65 to 4.45 cm in diameter,
whereas fine roots were less than 5 mm in diameter. To discriminate between maintenance and growth respiration, root respiration rates were measured during aboveground growing
periods and dormant periods. An additional measurement of
coarse root respiration was made during spring leaf flush, to
evaluate the effect of mobilization of resources for leaf expansion on root respiration. Fine roots respired at much higher
rates than coarse roots, with a mean rate at 15 °C of 1290 µmol
CO2 m –3 s –1 during the growing period, and 660 µmol CO2 m –3
s –1 during the dormant period. The temperature response of
fine root respiration rate was nonlinear: mean Q10 was 3.90 for
measurements made at 5–15 °C and 2.19 for measurements
made at 15–25 °C. Coarse root respiration rates measured at
15 °C in late fall (dormant season) were higher (370 µmol CO2
m –3 s –1) than rates from roots collected at leaf flush and early
summer (200 µmol CO2 m –3 s –1). The higher respiration rates
in late fall, which were accompanied by decreased total nonstructural carbohydrate (TNC) concentrations, suggest that
respiration rates in late fall included growth expenditures, reflecting recent radial growth. Neither bud flush nor shoot
growth of the trees caused an increase in coarse root respiration
or a decrease in TNC concentrations, suggesting a limited role
of coarse roots as reserve storage organs for spring shoot
growth, and a lack of synchronization between above- and belowground growth. Pooling the data from the coarse and fine
roots showed a positive correlation between nitrogen concentration and respiration rate.
Keywords: carbon, CO2, maintenance respiration, nonstructural carbohydrates, Q10, temperature response.
Introduction
Aspen (Populus tremuloides Michx.) is the most widely distributed tree species in North America, and forms extensive
stands in the boreal forests (Perala 1990). Understanding aspen root respiration is important for several reasons. First, root
carbon economy in aspen is critical for the production and sub-
sequent growth of root suckers (Schier and Johnston 1971, Peterson and Peterson 1992). Second, root respiration, especially
that of the large communal root system of aspen (DesRochers
and Lieffers 2001), can consume large proportions of net primary production (Ericsson et al. 1996) and can therefore affect
the maintenance and survival of aspen stands (Hogg 1999).
Finally, ecosystem models of carbon dynamics depend on accurate estimation of root respiration (Fernandez et al. 1993,
Hogg 1999).
Maintenance respiration, which is the component of respiration that is required to keep existing root cells and tissues
alive, requires considerable amounts of carbohydrates (Kozlowski and Pallardy 1997) and is highly temperature-dependent (Sprugel and Benecke 1991). Root respiration also varies
with season, because during the growing season, it comprises
both maintenance respiration and growth respiration (Lambers
et al. 1983). To isolate the maintenance component of respiration, respiration rates are commonly measured at the end of the
growing season, when it is assumed that growth activity has
ceased (Ryan 1990, Sprugel 1990, Ryan et al. 1995, 1996), although root growth can occur well into the fall (Lyr and
Hoffman 1967). Root respiration rates are also affected by the
proportion of metabolically active meristematic to non-meristematic tissues, leading to higher respiration rates in fine
roots than in coarse roots (Pregitzer et al. 1998). Other endogenous factors, such as total nonstructural carbohydrate (TNC)
and nitrogen concentrations in tissues, are also known to affect
root respiration rates (Kramer and Kozlowski 1979) where the
repair and replacement of proteins is the principal cause for
maintenance respiration (Lexander et al. 1970). Finally, respiration rates may vary depending on the concentration of CO2
at which they are measured (Burton et al. 1997, Clinton and
Vose 1999).
The objectives of this study were to determine seasonal respiration rates of coarse and fine roots in aspen at three soil
temperatures, and at the CO2 concentration of soils of aspen
stands. Because root growth is not observed in aspen at soil
temperatures below 5 °C (Landhäusser and Lieffers 1998,
Wan et al. 1999), we hypothesized that respiration rates measured on roots collected in late fall, when soil temperatures
are below 5 °C, will be lowest and represent maintenance res-
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DESROCHERS, LANDHÄUSSER AND LIEFFERS
piration. Respiration rates measured in the spring during leaf
flush, when soil temperatures are still below 5 °C (no root
growth), should reflect the additional energy required for the
mobilization and translocation of reserve carbohydrates for
developing leaves (Sprugel 1990). In early summer, when
trees are actively growing and soil temperatures are above
5 °C, respiration rates should be highest and represent total
respiration (maintenance + growth respiration).
Materials and methods
Coarse roots sampling
Coarse roots were collected from a 55-year-old pure aspen
stand near Devon (53°23′ N, 113°45′ W), Alberta, Canada,
within a 50 × 50 m area. The stand, categorized as an aspendominated low-bush cranberry ecosite (Beckingham and
Archibald 1996), is on a moderately well-drained mesic site
with a medium to rich nutrient supply. Mean elevation is
682 m, with undulating terrain and podzolic sandy loam soil
(Bowser et al. 1962). Mean summer and winter air temperatures are 14.4 and –8.7 °C, respectively, and mean annual precipitation is 424 mm (Strong and Leggat 1992).
Roots were collected in late fall (early December 1998) and
during leaf flush in spring (May 1999). Soil temperatures at
20-cm depth were below 5 °C at both collection times. Because fall 1998 was unusually warm, it can be expected that
soil temperatures drop below 5 °C earlier in the season during
more typical years. A third root collection was made 1 month
after leaf flush, in early June 1999, when soils were warming
(range 6–8 °C), to estimate total respiration. On each collection date, a large pit was excavated and 50-cm-long sections of
roots ranging from 0.5 to 5 cm in diameter were carefully
hand-excavated at soil depths ranging from 10 to 30 cm,
mostly in the B soil horizon. Ninety roots were collected per
sampling date. The roots were gently cleaned with diluted
bleach solution (1 ml of 5% hypochlorate per liter of distilled
water) and stored in moistened sphagnum moss at 2 °C for a
maximum of 4 days until respiration was measured. New collections were made after 4 days, in the same aspen stand but
from different trees. Four collections were made per sampling
date, over a period of 2 weeks.
A relationship between root volume (V; cm3) and dry mass
(Md; g) was developed based on a sample of 20 roots from
0.5 to 4.7 cm in diameter: Md = 0.41V (r 2 = 0.99, P < 0.001).
Fine roots sampling
For fine roots, dormant, 1-year-old, container-grown (6 cm diameter, 15 cm deep) aspen seedlings from a local seed source
were obtained from a commercial nursery. Before planting in
pots (15 cm diameter, 18 cm deep) filled with sand, the root
systems of 18 seedlings were carefully washed under running
distilled water to remove all soil. After planting, pots were
placed in a heating system to provide a constant soil temperature of 15 °C (Landhäusser and Lieffers 1998). The seedlings
were grown for 6 weeks in a growth chamber in an 18-h photoperiod at a day/night temperature of 18/16 °C. Photosynthetic
photon flux density was 350–400 µmol m –2 s –1 at pot level.
The pots were watered and fertilized as needed. After each watering, a hose was inserted into the drainage area (false bottom) of each pot to suction out excess water. Respiration rates
were measured on half of the seedlings after 6 weeks, when
they were still actively growing. Dormancy was induced in the
rest of the seedlings by shortening the day length to 6 h, lowering the day/night temperature to 11/8 °C and the soil temperature to 5 °C, and withholding fertilization. After 2 weeks, the
plants were placed in a dark refrigerator at 2 °C for another
2 weeks. Before the respiration measurements, the roots were
gently washed free of sand by rinsing the root mass with distilled water. Excess water on the roots was blotted with a paper
towel.
Respiration measurements
Because studies have shown that respiration may be artificially increased by low CO2 concentrations (Qi et al. 1994,
Burton et al. 1997, Clinton and Vose 1999), we measured soil
CO2 concentrations at a depth of 20 cm at 16 random locations
in the aspen stand. Samples were collected by slowly drawing
20 ml of soil air with a syringe connected to an aluminum
probe. The air was injected in Vacutainers (Fisher Scientific) tubes and the CO2 concentrations measured by gas chromatography. Soil concentrations at 5 °C were 3233 ± 609 ppm.
We therefore used a CO2 concentration of 3000 ppm in the air
circulated through the respiration chamber during measurements.
A clamp-on respiration chamber was constructed from ABS
pipe to accommodate both the coarse and fine roots. It included a small fan for air circulation and a set of fine-gauge
copper-constantan thermocouple wires inserted in the xylem
(coarse roots) or root mass (fine roots) to monitor root temperature inside the respiration chamber. The respiration chamber
was sealed from outside air with non-toxic, non-drying putty
and placed inside a dark cooling chamber (Landhäusser et al.
1996) while respiration was measured at constant root temperatures of 5, 15 and 25 °C. Respiration was measured with an
open-system infrared gas analyzer (IRGA, CIRAS I, PP Systems, Haverhill, MA). Bottled gas containing CO2 at a concentration of 3000 ppm was supplied to a 9.6-l mixing chamber to
maintain a stable CO2 concentration in the supplied air. A
slightly positive pressure was maintained inside the respiration chamber to avoid outside air leaking into the chamber.
This positive pressure was monitored after each sample was
installed inside the respiration chamber, and again before each
measurement, with a water bubble system linked to the outflow pipe going to the IRGA. Inflow rate to the chamber was
maintained at 200–350 ml min –1. Because the respiration calculations were based on the flow rate into the chamber, and the
IRGA sampled CO2 concentrations in the chamber, assuming
complete mixing, minor leaks should not have caused substantial errors. This system is similar to open systems used for
measurements of photosynthesis.
Roots were allowed to acclimate to the measurement temperature and CO2 concentration for 45 to 90 min, and respira-
TREE PHYSIOLOGY VOLUME 22, 2002
RESPIRATION OF COARSE AND FINE ROOTS
tion rate was allowed to stabilize for about 20 min before it
was recorded. Over the range of measurement temperatures,
relative humidity was maintained at near saturation. For
coarse root respiration, a range of root sizes was randomly assigned to a measurement temperature of 5, 15 or 25 °C. To
avoid wound respiration caused by cutting the roots (Müller
1924), the respiration chamber was clamped around the midsection (10-cm long) of the 50-cm-long coarse root segment,
thereby excluding the cut parts of the root. Root volume enclosed in the respiration chamber was calculated assuming
that roots were cylindrical.
For the fine roots, the whole seedling root system was enclosed in the respiration chamber, with the aboveground portions left intact but excluded from the chamber. For each
seedling, respiration was measured at 5, 15 and 25 °C (repeated measures). Root volume was calculated by water displacement immediately after respiration was measured.
Values of Q10 (respiration rate at temperature T over respiration rate at temperature T – 10) were calculated for coarse and
fine roots.
727
completed in roots collected in the fall and had not yet started
in roots collected in spring and summer. Even in the largest
coarse roots, no heartwood (central portion of xylem without
live parenchyma cells) was observed. Occasionally, small
dead areas were observed in the xylem; however, these areas
constituted less than 5% of root volume. Mean coarse root diameter was 2.06 cm (range 0.65–4.45 cm), and mean root age
was 30 years (range 4–56 years). Nitrogen concentrations did
not vary with season (P = 0.07), averaging 0.4% of root dry
mass (Figure 1a). Mean TNC concentration was 11.7, 15.4 and
17.1% of root dry mass for coarse roots collected in the fall,
spring and summer, respectively (Figure 1b). Coarse roots collected in the fall had lower TNC concentrations than coarse
roots collected in the spring and summer (P < 0.05), whereas
there was no difference in TNC concentrations between roots
collected in the spring and summer (P > 0.05, Figure 1b).
Tissue analyses
The age of each coarse root was determined by counting the
number of annual growth rings in a cross section. A 2-cm section of root was soaked for 24 h in a solution of 1% triphenyltetrazolium chloride, to identify sapwood area (Ryan 1990).
Another section was oven-dried at 68 °C to constant mass and
ground in a Wiley mill to 40 mesh. For the seedlings, the entire
fine root system was dried and ground. For total nitrogen analysis, ground samples were digested by the Kjeldhal method
(Kalra and Maynard 1991), and total nitrogen was quantified
with a Technicon AutoAnalyzer II (Tarrytown, NY). For TNC
(sugars and starch), samples were digested for 1 h in 0.2 N
H2SO4 at 115 °C, mixed with phenolsulfuric acid and quantified spectrophotometrically (Smith et al. 1964).
Experimental design and statistical analyses
Coarse root respiration rates were analyzed as a randomized
3 × 3 factorial design with three seasons (fall, spring and summer) and three temperatures (5, 15 and 25 °C) as fixed main effects. The fine root respiration data were analyzed as a univariate repeated measures design with two seasons (growing
and dormant) and a repeated temperature factor (5, 15 and
25 °C). Respiration data were log-transformed for the coarse
and fine roots, to correct for unequal variances. Relationships
between respiration rate and total nitrogen and TNC were
tested with linear regression, and as covariates in analyses of
covariance. Least significant difference (LSD) procedure was
used for comparison of treatment means. We used SAS statistical software (SAS Institute, Cary, NC) to perform the data
analysis. A significance level of P < 0.05 was chosen.
Results
Coarse roots
Cross sections of coarse roots showed that radial growth was
Figure 1. Coarse root (a) nitrogen concentration (N), (b) total nonstructural carbohydrate concentration (TNC) and (c) respiration rate
measured at 5, 15 and 25 °C (logarithmic scale). Bars with the same
letter are not significantly different at P < 0.05.
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DESROCHERS, LANDHÄUSSER AND LIEFFERS
Coarse root respiration rate was significantly affected by
season (P = 0.002). Respiration rates were significantly higher
in roots collected in the fall (370 µmol CO2 m –3 s –1) than in
roots collected in spring and summer (200 µmol CO2 m –3 s –1)
(P < 0.05), whereas there was no difference between respiration rates of roots collected in spring and summer (P > 0.05,
Figure 1c). As expected, respiration rates increased exponentially with measurement temperature (P < 0.001). The respiration response to changing temperature was similar in roots
collected in fall, spring and summer, i.e., the slopes of the regression lines between respiration rates and temperature were
not significantly different between seasons (P > 0.05). Mean
Q10 over all three seasons was 2.15; however, mean Q10 was
1.72 for the 5–15 °C temperature range and 2.57 for the 15–
25 °C temperature range (statistically non-testable).
Storage of roots collected in fall and spring for more than
1 day increased respiration rates measured at 15 °C (P = 0.01)
(Table 1). However, storage time had no effect on respiration
rates of roots collected in the summer or on respiration rates
measured at 5 and 25 °C. Respiration rate was negatively correlated with root diameter (r 2 = –0.28; P < 0.001) and weakly
positively correlated with nitrogen concentration (r 2 = 0.08;
P < 0.001). Analysis of covariance showed that root diameter
(P < 0.001) and nitrogen (P < 0.001) were significant covariates for coarse root respiration rates; however, season and
measurement temperature still significantly affected root respiration rates after adjusting for root diameter, nitrogen concentration and days of storage. There was no relationship
between TNC concentration and respiration rate across seasons and measurement temperature combinations (P > 0.05),
and TNC concentration did not contribute significantly to the
model as a covariate (P = 0.76). The TNC concentration was
unrelated to root diameter (P = 0.85).
mass). Nitrogen concentration of fine roots did not vary between the growing and dormant seasons (P = 0.36), with a
mean of 1.6% of dry mass (Figure 2a). Fine roots had significantly higher mean TNC concentrations during the growing
period than during the dormant period (13.2 versus 9.2% of
root dry mass; P = 0.03) (Figure 2b). The decrease in TNC
during the dormant period was accompanied by an increase in
root dry mass (P = 0.02, Table 2), whereas root volumes did
not differ between the start and end of dormancy (P = 0.56).
Fine root respiration rates were on average 49% higher during the growing season compared with the dormant season
Fine roots
Root systems of aspen seedlings consisted mostly of fine roots
less than 2 mm in diameter, but a small portion of the fine roots
were larger, ranging up to 5 mm in diameter (~5% of total root
Table 1. Mean respiration rate at 15 °C of aspen coarse roots collected
in the fall, spring and summer and stored at 2 °C for 1–4 days before
measurement. Values followed by the same letter are not significantly
different at P < 0.05.
Storage days
Respiration rate (µmol CO2 m –3 s –1)
Fall
Spring
Summer
1
190 a
n=5
180 a
n = 16
174 a
n=7
2
400 b
n = 10
340 b
n=9
200 a
n=8
3
430 b
n=7
290 b
n=4
180 a
n = 10
4
330 b
n=7
–
180 a
n=4
Figure 2. Fine root (a) nitrogen concentration (N), (b) total nonstructural carbohydrate concentration (TNC) and (c) respiration rates (logarithm scale) measured at 5, 15 and 25 °C. Bars with the same letter
are not significantly different at P < 0.05.
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RESPIRATION OF COARSE AND FINE ROOTS
Table 2. Mean volume and dry mass and relationship between volume
(V ) and dry mass (Md ) of fine roots of growing and dormant aspen
seedlings. Standard deviations are given in parentheses. Values followed by the same letter are not significantly different at P < 0.05.
Growing seedlings
Dormant seedlings
Volume (cm3)
17.6 a
(4.9)
19.9 a
(10.9)
Dry mass (g)
2.60 b
(0.77)
4.51 c
(2.69)
Relationship
Md = 0.14V + 0.41
(R2 = 0.41, P < 0.001)
Md = 0.23V
(R2 = 0.97, P < 0.001)
(P = 0.001, Table 3, Figure 2c). Mean respiration rate at 15 °C
was 1290 µmol CO2 m –3 s –1 during the growing period compared with 660 µmol CO2 m –3 s –1 during the dormant period.
Respiration rates increased exponentially with increasing temperature, with a mean Q10 of 3.06 over the 5–25 °C temperature range. However, the increase in respiration rate was
significantly higher between 5 and 15 °C (Q10 = 3.90) than between 15 and 25 °C (Q10 = 2.19) (P = 0.001). The temperature
response of respiration rate did not differ between dormant
and growing seedlings (P = 0.31).
Analysis of covariance showed that nitrogen and TNC were
not significant covariates for fine root respiration rate (nitrogen: P = 0.06; TNC: P = 0.40). However, TNC concentration
was positively correlated with respiration rate measured at
15 °C (r 2 = 0.49; P = 0.03) and 25 °C (r 2 = 61; P = 0.01),
whereas there was no correlation between TNC concentration
and respiration rate measured at 5 °C (r 2 = 0.27; P = 0.27).
There was no correlation between nitrogen concentration and
respiration rate when respiration rates were examined individually at each measurement temperature (P > 0.05).
Discussion
Fine root respiration rates of growing seedlings were nearly
double the rates measured in dormant seedlings. Fine root respiration of dormant seedlings was assumed to represent main-
729
tenance respiration alone (Figure 2c). However, although the
seedlings did not grow new roots or increase in root diameter
during the dormant period, root mass increased (Table 2),
which could account for respiration expenditures unrelated to
maintenance. Despite the possible overestimation of maintenance respiration, root respiration rates of dormant seedlings
were 15–30% lower than other published estimates for aspen.
On a dry mass basis, we found a respiration rate of 3.0 nmol
CO2 g –1 s –1 at 15 °C in dormant seedlings (Table 3), compared
with rates of 4.4 nmol CO2 g –1 s –1 at 15 °C (Lawrence and
Oechel 1983) and 3.5–4.2 nmol CO2 g –1 s –1 at 10 °C in fieldexcavated fine roots (Ryan et al. 1997). Assuming that the respiration chamber remained leak-proof during measurements,
lower respiration rates in our study could be attributed to
higher concentrations of CO2 during measurement (3000 ppm
compared with 500–1400 ppm in the study by Ryan et al.
(1997)). Low CO2 concentrations artificially increase root respiration rates (Qi et al. 1994, Burton et al. 1997, Clinton and
Vose 1999). For example, Coleman et al. (1996) reported a
fine root respiration rate of 15.3 nmol CO2 g –1 s –1 at 20 °C and
350 ppm of CO2 for aspen, which is almost twice the respiration rates we measured, at 15 °C and 3000 ppm of CO2, in
growing seedlings (Table 3). However, it was recently found
that even minor leaks in respiration chambers when making
measurements at high CO2 concentrations can result in underestimation of root respiration rates (Burton and Pregitzer
2002).
Our experimental design was based on the assumption that,
in coarse roots collected in the fall, growth respiration had
ceased because soil temperatures were below 5 °C and the annual growth ring was fully developed. However, we found that
roots collected in the fall had 31% higher respiration rates
(when measured at 15 and 25 °C) than roots collected in the
spring and summer (Figure 1c). This suggests that respiration
rates of roots collected in the fall included growth expenditures, in which case they did not solely represent maintenance
respiration. Root growth can occur even if shoots are in deep
dormancy (Lyr and Hoffmann 1967, Vogt et al. 1980). Lavigne
(1988) also noted high respiration rates in balsam fir (Abies
balsamea (L.) Mill.) stems for over a month after radial
growth had stopped. It appears that roots collected in the fall
Table 3. Summary of coarse root and fine root respiration rates of aspen seedlings measured at 5, 15 and 25 °C and expressed on a dry mass basis.
Standard errors are given in parentheses.
Measurement temperature (°C)
Respiration rate (nmol CO2 g –1 s –1)
Coarse roots
Fine roots
Fall
Spring
Summer
Dormant
Growing
5
0.36 (0.07)
n = 23
0.26 (0.03)
n = 31
0.31 (0.02)
n = 31
0.72 (0.12)
n=9
2.50 (0.47)
n=9
15
0.75 (0.09)
n = 28
0.51 (0.04)
n = 29
0.40 (0.03)
n = 29
3.00 (0.22)
n=9
7.63 (0.72)
n=9
25
1.63 (0.16)
n = 24
1.15 (0.11)
n = 30
1.35 (0.09)
n = 30
7.42 (0.63)
n=9
15.60 (1.38)
n=8
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DESROCHERS, LANDHÄUSSER AND LIEFFERS
retained high overall cell activity after a recent period of radial
growth, possibly because of cell wall thickening. We found
that fine roots increased in dry mass during the dormancy period, even though fine root volumes did not change. High respiration rates could also be a result of increased cell activity
associated with the transformation of reserve starch into sugars at the onset of low temperatures (Marvin et al. 1971).
The spring and early summer growth flush was unaccompanied by an increase in coarse root respiration rates. Coarse
roots collected in the spring and early summer had increased
TNC concentrations compared with coarse roots collected in
the fall (Figure 1b). This seasonal pattern of TNC concentrations contrasts with the generally acknowledged pattern for
deciduous species, where root TNC concentrations are maximal in the fall and minimal at and just after bud flush (Siminovitch 1981, Larcher 1995). The low TNC concentrations that
we observed in coarse roots collected in the fall suggests that
root TNC reserves were depleted by fall root growth and high
root respiration rates. Translocation of TNC to the coarse roots
in the spring and summer resulted in high concentrations in
preparation for summer radial root growth. A similar seasonal
pattern of TNC accumulation in coarse roots has been observed in Sitka spruce (Picea sitchensis (Bong.) Carr.) (Deans
and Ford 1986) and sugar maple (Acer saccharum Marsh.)
(Wargo 1979), i.e., substantial carbohydrate storage in roots in
spring preceding radial root growth. Although leaf and shoot
growth of trees had started by the summer collection date, radial growth of coarse roots had not begun. These results suggest that the role of coarse roots as carbohydrate storage
organs to sustain bud flush and early growth of mature trees
may be limited. In mature aspen trees, aboveground biomass
represents about 60–80% of total tree biomass (Peterson and
Peterson 1992), hence carbohydrates stored in the branches,
twigs and stem may play a more important role than coarse
roots as TNC storage organs for bud flush and early growth. A
potential role of branches and twigs as major TNC storage
organs for spring growth flush has also been suggested for
Pacific silver fir (Abies amabilis Dougl. (Forbes)) (Sprugel
1990).
The acid digestion method used for determination of TNC
concentrations has been used previously for aspen roots
(Shepperd and Smith 1993). However, in roots of grasses and
legumes, it has been observed that this method is likely to
overestimate TNC concentrations when little starch is present
and to underestimate TNC concentrations when large amounts
of starch are present (Grotelueschen and Smith 1967, Greub
and Wedin 1969). If the same holds true for aspen coarse roots,
the seasonal differences in TNC may be larger than reported,
because starch concentrations are usually lower in the winter
than in the summer (Kramer and Kozlowski 1979).
On a wet volume basis, fine root respiration rates at 15 °C
were 2.5–3.5 times higher than coarse root respiration rates
(Figures 1 and 2), probably reflecting the higher physiological
activity and higher proportions of meristematic and phloem
tissues in fine roots compared with coarse roots (Kramer and
Kozlowski 1979). No heartwood was observed in the coarse
roots; however, sapwood xylem contains proportionally fewer
living cells than cambium and phloem tissues (Ryan 1990).
Pregitzer et al. (1998) showed that root nitrogen concentration
is a better indicator of root activity and respiration than root
size. A large part of maintenance respiration supports repair
and replacement of proteins (Amthor 1984), and because most
of the nitrogen in plant tissue is associated with proteins (Lexander et al. 1970), nitrogen is usually a good indicator of root
activity. There was no clear relationship between maintenance
respiration and nitrogen concentration when coarse root or
fine root data were analyzed independently, because there was
little variation in nitrogen concentration in the samples of a
treatment combination (Figures 1a and 2a). In addition, maintenance respiration may match poorly with nitrogen concentration in systems where nitrogen is in excess (Ryan 1995),
which could explain why it was not a significant covariate for
fine roots in our well-fertilized seedlings. However, after pooling the data obtained from both coarse and fine roots to increase the range in nitrogen concentrations, as suggested
by Pregitzer et al. (1998) for sugar maple, a relationship was
evident and explained 65% of the variation in respiration rates
(Figure 3). Inferences from the data presented in Figure 3 must
be made carefully, however, because the coarse and fine roots
came from two different populations of roots. The absence of a
significant relationship between nitrogen concentrations and
respiration rates of coarse and fine roots may result from a failure to measure maintenance respiration independently of
growth respiration.
In fine roots, the increasing positive correlation between
TNC concentration and respiration rate with increasing
measurement temperature suggests that TNC became more
limiting as temperature increased. This was not observed in
coarse roots, probably because they respire at much lower
rates than fine roots, even at 25 °C (Table 3). The changing
temperature dependency of fine root TNC may be a result of
Figure 3. Relationship between root respiration rate at 15 °C and nitrogen concentration (N). Pooled data from coarse (䊏䊏) and fine roots
(䊉).
TREE PHYSIOLOGY VOLUME 22, 2002
RESPIRATION OF COARSE AND FINE ROOTS
the affinity between enzymes and their substrate, which usually declines with temperature and is modified by substrate
concentration (Hunt and Loomis 1979). Limited access to
TNC at high respiration rates could explain the lower Q10 observed between 15 and 25 °C than between 5 and 15 °C for fine
roots (Wassink 1972, cited by Hunt and Loomis 1979). Lawrence and Oechel (1983) also found a lower Q10 for the 15–
25 °C temperature range than for the 5–15 °C temperature
range. Thus, Q10 values are affected by temperature (Thierron
and Laudelout 1996), and should therefore be applied with
care.
In summary, respiration rates and TNC concentrations of
coarse roots indicated that roots collected in the fall still comprised growth expenditures, whereas roots collected in the
spring and summer were still dormant even though the aboveground portions of the trees had started to grow. Similarly, in
fine roots, the increase in dry mass and low TNC concentrations during the dormancy period suggest growth respiration
expenditures in fine roots during the aboveground dormant
season. We conclude that respiration rates measured on seemingly dormant seedlings or trees may not represent only the
maintenance component of respiration.
Acknowledgments
We thank Ben Seaman, Goeff Eerkes, Sarah Lieffers, Alain Plante and
Line Blackburn for field and laboratory assistance. The study was financially supported by Ainsworth Lumber, Alberta-Pacific Forest Industries, Daishowa Marubeni International, Millar Western Forest
Products, Slave Lake Pulp Corporation, Weyerhaeuser Canada, the
University of Alberta and the Natural Sciences and Engineering Research Council of Canada.
References
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