The relationship between seasonal changes in rhizome carbohydrate reserves and
recovery following disturbance in Calamagrostis canadensis
EDWARD
H. HOGGA N D VICTORJ. LIEFFERS'
Department of Forest Science, 855 General Services Building, Universily of Alberta. Edmonton, Alta., Canada T6G 2 H I
Received March 1 , 1990
HOGG,E. H., and LIEFFERS,
V. J. 1991. The relationship between seasonal changes in rhizome carbohydrate reserves and
recovery following disturbance in Calamagrostis canadensis. Can. J. Bot. 69: 641-646.
The grass Calarnagrostis canadensis (Michx.) Nutt. often forms a dense growth after logging and interferes with replanting
on moist sites in northern Alberta. The objective was to determine if seasonal changes in rhizome carbohydrate reserves can
provide useful predictions of the time of year when this species is most vulnerable to disturbance. Small (3 x 3 m) plots
were mowed to ground level in May, June, July, or August 1988, or on all four of these dates. The lowest seasonal levels
of rhizome carbohydrate reserves (10-12%) occurred early in the season (mid-May to early July). Contrary to expectations,
in September 1988 the belowground carbohydrate reserves in plots mowed once during this period had increased to levels
that were significantly greater (17-18%) than in unmowed control plots (15%). In plots mowed on all four dates, rhizome
carbohydrate levels (1 1%) and biomass were significantly lower than in unmowed plots in September 1988. Despite these
differences, regrowth of shoot biomass in 1989 was similar among all mowed and unmowed treatment plots. Thus rhizome
carbohydrate levels were a poor predictor of shoot biomass regrowth following disturbance treatments. It is suggested that
improvements in microclimate following the removal of shoot biomass and litter may have compensated for the negative
effects of disturbance treatments on the plant's carbon balance.
Key words: defoliation, biomass, regrowth, TNC, Gramineae.
HOGG,E. H., et LIEFFERS,V. J. 1991. The relationship between seasonal changes in rhizome carbohydrate reserves and
recovery following disturbance in Calnmagrostis canadensis. Can. J. Bot. 69 : 641-646.
Le Calarnagrostis canadensis (Michx.) Nutt., une plante herbacie, forme souvent une croissance dense aprks la coupe
des arbres et interfkre avec la replantation des sites humides du nord de 1'Alberta. L'objectif Ctait de diterminer si les
changements saisonniers des reserves en glucides dans les rhizomes peuvent permettre de predire le moment de I'annCe oh
cette espkce est la plus vulnCrable aux dkrangements. De petites places Cchantillons (3 X 3 m) ont CtC tondues au niveau du
sol en mai, en juin, en juillet ou en aoct 1988 aussi bien qu'i chacune de ces dates. Les teneurs les plus faibles (10-12%)
en glucides dans les rhizomes au cours de la saison se retrouvent t8t au debut de la saison (mi-mai au debut de juillet).
Contrairement a ce qu'on attendait, les reserves en glucide des parties souterraines en septembre 1988 Ctaient significativement
supCrieures (17-18%) dans les parcelles tondues une fois au cours de la saison que dans les parcelles tCmoins (15%) non
tondues. Dans les parcelles tondues a chacune des quatre dates les teneurs en glucide des rhizomes ainsi que la biomasse
etaient, en septembre 1988, significativement plus faibles (1 1%) que dans les parcelles non tondues. En depit de ces diffirences, la repousse en biomasse des parties aCriennes en 1989 s'est avtrie la mime pour tous les traitements, avec ou sans
tonte. Les auteurs concluent que la teneur en glucide des rhizomes n'est pas un parametre utile pour prCdire la repousse a
la suite de traitements perturbateurs. 11s suggkrent que les amCliorations microclimatiques apportCes par 1'Climination de la
biomasse aCrienne et de la litikre pourraient avoir compensC les effets nCgatifs des perturbations sur le bilan carbon6 de la
plante.
Mots clPs : defoliation, biomasse, repousse, TNC, graminees.
[Traduit par la rCdaction]
Introduction
Belowground carbohydrate reserves of herbaceous clonal
perennials have received considerable attention because these
stores of energy are available for new growth following disturbance or unfavourable growth periods (White 1973;
Deregibus et al. 1982). It has often been assumed that plants
are most vulnerable to disturbance when their levels of
belowground total nonstructural carbohydrate (TNC) are at a
seasonal minimum. This concept has been widely applied in
range management, where the aim is to minimize the impact
of grazing on vegetation while maintaining high productivity
of forage (Coyne and Cook 1970; Steele et al. 1984; Krueger
and Bedunah 1988). In contrast, seasonal TNC cycles in rhizomes have been used to plan the timing of disturbance treatments to maximize the control of weedy clonal perennials in
wetlands (Linde et al. 1976; Thompson and Shay 1985; Krusi
and Wein 1988). However, the usefulness of TNC reserves in
'Author to whom correspondence should be addressed.
Pr~ntedIn Crnddd i lrnprlrnC au Canada
predicting vegetation regrowth after disturbance has been a
controversial topic over the last few decades (May 1960; White
1973; Deregibus et al. 1982; Richards and Caldwell 1985).
In the present study we examined the relationship between
rhizome TNC and recovery following disturbance in the native,
perennial grass Calamagrostis canadensis (Michx.) Nutt. This
species dominates clearcuts after logging in moist areas of
boreal mixed-wood forest in western Canada. The resulting
buildup of shoot biomass and litter strongly reduces the success of replanted conifers (Eis 1981; Haeussler and Coates
1986). Calamagrostis carzadensis is used as forage for livestock in Alaska (Laughlin et al. 1984) and is also an important
component in the diet of bison herds in the Slave River lowland, N.W.T., Canada (Reynolds et al. 1978).
A few studies have examined the effects of mowing or clipping regimes on C . canadensis (Corns and Schraa 1962) or
C . rubescerzs Buck. (Stout et al. 1980, 1981; Krueger and
Bedunah 1988), but the treatments used resulted in shoot stubble heights of'5-15 cm above the soil surface. Disturbances
that cause total elimination of leaf area should produce more
CAN. 1. BOT. VOL. 69, 1991
642
severe effects (Ward and Blaser 196 1 ; Stout et al. 198 l), particularly if they occur when below ground TNC levels are low.
The experimental disturbance used in this study was a mowing
of the vegetation to ground level that removed all aboveground, photosynthetic tissues and accumulated litter. This
treatment is similar to intense fire because it causes initial shoot
regrowth to be independent on mobilization of belowground
organic reserves. Mowing was used instead of burning because
it can be applied consistently and is less dependent on weather
conditions at the time of disturbance.
We postulated that disturbance applied when rhizome TNC
reserves were at a seasonal minimum would cause a decrease
in rhizome biomass at the end of the growing season. This
would be expected if the plant shifts its carbon allocation away
from belowground growth in favour of shoot regrowth
(Caldwell et al. 1981; Archer and Tieszen 1983; Richards
1984), or if the depletion of belowground TNC causes
increased mortality of belowground organs (Marshall and
Waring 1985). These effects could then lead to a decrease in
shoot biomass regrowth during the following growing season.
Study area
The study area was located in northern Alberta (55"04'N,
113"05'W) about 15 km southeast of Calling Lake, at an elevation
of 650 m. Two sites (A and B). situated 1 km apart, were chosen in
cutblocks 3-4 years old that had been previously dominated by Picea
glauca (Moench) Voss, Picea mariana (Mill.) BSP, and trembling
aspen Populus rremuloi(1es Michx. When the present study was initiated in 1988, both sites had become dominated by the grass Calarnagrosris canaderlsis. Other locally common species inclu.ded
Epiblobium angrrsrifolium L., Rosa acicularis Lindl., Merrerzsia paniculara (Ait.) G.Don, and some regrowth of P. rrernuloides. The substrate consisted of a surface organic layer 5-15 cm over a clay mineral soil.
The region has a boreal continental climate, with a mean annual
temperature near 0°C at Calling Lake (Environment Canada 1982).
Mean monthly temperatures range from -20°C in January to 16°C
in July. More than half (248 mm) of the mean annual precipitation
(424 mm) occurs during the summer months of June, July, and
August. During the growing season (May-September) mean temperatures were slightly warmer than average during both years of the
study (12.9"C in 1988 and 13.1°C in 1989, compared with the longterm normal mean of 12.3"C). Growing season precipitation was 13%
below normal in 1988 and within 1% of normal in 1989.
Methods
In April 1988, 75 plots measuring 3 .X 3 m were selected at each
of the two sites within areas uniformly dominated by Calarnagrosris
canadensis. On each of four dates during the 1988 growing season
(May 11, June 15, July 6, and August 9), five experimental plots at
each site were randomly selected and mowed to ground level using
a gas-powered rotary trimmer. At each site, an additional five plots
were randomly selected and mowed on all four of these dates. After
mowing treatments, plots were raked free of aboveground plant material. The remaining 5 0 plots in each site were left untreated and served
as control plots.
Sampling was restricted to the central 1.5 x 1.5 m area in each
experimental plot, thus allowing a 75 cm buffer strip around areas
actually sampled. Following mowing treatments, shoot regrowth was
monitored in experimental plots at 2- to 3-week intervals from May
11 to September 17, 1988, and on September 10, 1989. The number
of shoots and the height of the three tallest shoots were recorded in
each of three 15-cm diameter circular quadrats placed randomly within
each experimental plot. Shoot growth was sampled using the same
method in 10 randomly chosen control plots on each sampling date,
but shoots were also harvested and oven dried at 70°C for biomass
determinations. Shoot biomass was similarly determined for all
experimental plots on September 17, 1988, and on September 10,
1989; the northern portion of each 1.5 X 1.5 m plot was sampled in
1988 and the southern portion was sampled in 1989. A larger quadrat
size of 45-cm diameter was used to sample shoot litter (i.e., dead
material from growth in previous years) in control plots at the end of
each growing season.
In conjunction with the shoot biomass sampling procedure
described above, two soil cores 15 cm in diameter and 15 cm deep
were obtained from each plot and all living C . carzaderlsis rhizomes
and shoot bases were extracted. The most recent growth of rhizomes
(new growth) was white or lightly pigmented and was separated from
the older rhizome portion prior to oven drying (70°C) for biomass
estimates. Each of the dried components was then ground through a
40-mesh screen in a Wiley mill and stored in sealed bottles at - 12°C
prior to analysis for nonstructural carbohydrates. TNC was measured
using the Shaeffer-Somogyi copper iodometric titration method
(Smith 1981) following TNC extraction with boiling distilled water
(Hogg and Lieffers 19910). Total pools of rhizome TNC were calculated for each combination of mowing treatment and site, based on
mean rhizome dry mass (g m-2) and %TNC.
Model 111 two-factor analysis of variance (ANOVA)was used (Zar
1984) to examine the effects of mowing treatment (fixed factor) and
site (random factor) on rhizome biomass and TNC in September 1988,
and on shoot regrowth (height, density, and biomass) in 1989. The
results from the unmowed control plots were compared with results
from each mowing treatment using the two-tailed Dunnett's test (Zar
1984), as recommended by Day and Quinn (1989). Results from the
two sites were pooled for presentation when no significant differences
between sites, and no significant interaction term between site and
the primary variable of interest were detected (P > 0.05). We also
used multiple regression analysis to examine the relationship between
total shoot biomass regrowth in 1989 (dependent variable) and pools
of rhizome TNC in September 1988 (independent variable), with site
included as a second independent variable.
Results
Phenological developmetzt in unmowed plots
Heavy accumulations of Calarnagrostis canadensis shoot
litter insulated the soil and delayed ground thawing until early
May. When the first mowing treatment was conducted (1 1 May
1988), about 60% of shoots had emerged and were in the Ito 2-leaf stage of development, and shoot biomass was low
(mean ? SE, n = 10 plots: 15 +- 1 g mP2). By the second
mowing date (15 June 1988), significant vegetative growth of
shoot biomass (103 t 9 g m-2) had occurred in unmowed
plots and most shoots had reached the 4- to 5-leaf stage. Flowering was initiated between the second and third (6 July 1988)
mowing dates, while shoot biomass increased to 233 t
19 g m-'. On the fourth mowing date (9 August 1988), the
C. canadensis shoots had reached their seasonal maximum
height and density, and shoot biomass (437 t 37 g m-2) was
near its seasonal maximum. The first severe frost (-6°C) at
the study site occurred on 9 September. On the major autumn
sampling date of 17 September 1988, most stems were green
but senescence of leaves was nearly complete.
No distinct trends in phenology of the belowground organs
were observed. Total rhizome biomass was higher in site A
(mean t SE, n = 8 sampling dates: 143 t 13 g m-2) than
in site B (109 t 10 g mP2), but seasonal changes in total
rhizome biomass were not detected (two-factor ANOVA;df =
7,7; F = 0.49; ns). Biomass of new rhizomes showed a gradual seasonal increase between May and September but were
always a minor component (0-7 g m-2) of total rhizome bio-
HOGG AND LIEFFERS
Sept 1988
Sept 1989
gXXB Before mowing
U Sept 1988
Sept 1989
600
Unmowed
May
June
July
Aug
11
15
5
9
All f o u r
dates
Mowing date
1988
FIG. 1. Total regrowth of C. canadensis shoot biomass in mowed
and unmowed plots in 1988 and 1989 (mean 5 SE of 10 plots per
treatment). Cross-hatching includes shoot regrowth between mowing
treatments for plots mowed on all four dates.
TABLE1. Seasonal changes in total nonstructural carbohydrate (%)
in components of Calamagrostis canadensis from unmowed
plots in 1988
Date
May 11
May 27
June 15
July 6
Julv 26
A U ~9 .
Sept. 17
Oct. 20
Total
rhizomes
New
rhizomes
10.450.7
11.650.7
10.350.6
12.1 5 0 . 7
14.350.7
16.850.6
15.250.5
18.550.7
-
6.3
5.7
5.9
9.1
12.5
15.6
19.4
Belowground
shoots
7.5
2.6
3.8
5.9
7.7
11.O
-
10.1
Roots
5.5
-
Aboveground
shoots
6.7
7.2
6.8
6.0
7.8
-
-
10.1
10.3
1.3
8.6
9.5
NOTE:For total rhizomes, mean 2 SE for 10 plots per sampling date are shown. For
all other components, samples from the 10 plots were pooled prior to analysis, and mean
values are based on two pooled samples from each sampling date.
mass. This may indicate that the new rhizome category
included only a small fraction of the current year's rhizome
growth. The belowground portions of shoots were also a minor
component of belowground biomass, reaching a seasonal maximum of only 15 g m P 2 by September.
Carbohydrate reserves in unmowed plots
In 1988, the lowest levels of rhizome TNC (10-12%) were
recorded during the period between May and early July when
the first three mowing treatments were conducted (Table 1).
Rhizome TNC levels had increased to 16.8% when the fourth
mowing treatment was conducted in early August and reached
a seasonal maximum of 18.5% in October. No differences in
rhizome TNC between sites were detected. Rhizomes collected
the following April (when the ground was still frozen) and
15% TNC, and levels decreased rapidly during the subsequent
spring growth in 1989 (Hogg and Lieffers 1 9 9 1 ~ ) .
Belowground shoots and new rhizome growth showed seasonal changes in %TNC (Table l) that were similar to those
observed in old rhizomes. However, these components contributed little to TNC reserves because of their low biomass,
as previously noted. Root biomass could not be accurately
determined but was estimated to be <20% or less of total
belowground biomass. Root TNC levels also tended to increase
Unmowed
May
June
July
Aug
11
15
5
9
All f o u r
dates
Mowing date
1988
FIG.2. Density (A) and height (B) of C. canaderlsis shoots in
mowed and unmowed plots in 1988 and 1989 (mean ? SE of 10
plots per treatment).
during the growing season (Table I), but we assume that these
reserves are of little importance to shoot regrowth (White 1973;
Richards and Caldwell 1985).
Aboveground shoots contained substantial concentrations of
TNC during the growing season (6-10%). By late summer, a
high ratio (>3:1) of aboveground to belowground biomass
(excluding roots) had developed. Thus, we estimate that up to
70% of TNC was situated in the aboveground portions of the
plant at this time of year. Between 17 September and 20 October 1988, shoot TNC levels decreased sharply while rhizome
TNC levels showed a substantial increase (Table 1). This suggests that part of the seasonal downward translocation of TNC
occurred during this late autumn period.
Recovery following mowing treatments
In plots mowed in May, June, or July, there was rapid
regrowth of shoot biomass: while little regrowth occurred following the August mowing treatment (Fig. 1). During the 1988
growing season, total growth of shoot biomass (i.e., including
growth prior to mowing treatments) was lowest in plots mowed
on all four dates (35% of biomass in unmowed plots). In plots
mowed in June or July, total growth of shoot biomass was
depressed to a lesser extent (62% and 72%, respectively, of
biomass in unmowed plots) (Fig. 1).
In September 1988, plots that had been mowed in May,
June, or July had shoot densities about twice as high as in
unmowed plots (Fig. 2A). Tillering was most rapid in plots
mowed in ~ u n e where
,
shoot densities reached their seasonal
maximum of about 2000 m P 2 within 20 days after mowing.
In the remaining treatment plots (mowed in August or mowed
644
CAN. J. BOT.
A
200]
E
,
EEi Total rhizomes
I
New rhizomes
T
Unmowed
Unrnowed
May
June
July
Aug
11
15
5
9
All four
dates
May
June
July
Aug
11
15
5
9
All f o u r
dates
Mowing date
1988
Mowing date
FIG.4. Total nonstructural carbohydrate (TNC) concentration in
FIG.3. Biomass of C. canadetzsis rhizomes in mowed and
unmowed plots on 17 September 1988, shown separately for sites A
and B (mean I+_ SE of five plots per site and treatment combination).
C. canadensis rhizomes in mowed and unmowed plots on 17 Sep-
on all four dates) autumn shoot densities were similar to those
in unmowed plots. Differences in shoot height (Fig. 2B) in
September 1988 largely reflect differences in the period of time
available for regrowth following each mowing treatment.
Comparisons between mowed and unmowed treatments in
September 1988 (Fig. 3) indicated that rhizome biomass was
significantly lower within plots mowed four times during the
1988 growing season (Dunnett's test following two-factor
ANOVA;df = 7,7; F = 5.70; P < 0.05). New rhizome biomass was greater in unmowed plots than in mowed plots, but
the values obtained are probably strong underestimates of rhizome growth during the 1988 growing season.
Rhizome carbohydrate reserves in September 1988 showed
significant differences among treatments (two-factor ANOVA;
df = 5 3 ; F = 10.7; p < 0.025) but not between sites. Surprisingly, plots mowed either in May, June, or July had significantly greater carbohydrate reserves (TNC = 17.1-17.6%)
than in unrnowed plots (15.2%) (Fig. 4). Plots mowed on all
four dates showed a significant depletion of rhizome carbohydrate reserves (TNC = 11.4%). Rhizome carbohydrate
pools (%TNC x rhizome biomass) in the plots mowed four
times were estimated to be 9.5 g m-2, compared with
19 g m-2 in unmowed plots, whereas in plots mowed in May,
June, or July, TNC pools ranged from 21 to 23 g mP2.
On 17 September 1988, concentrations of TNC in aboveground shoots were similar among treatments (9-1 1%). Therefore we assume that the quantity of TNC translocated from
shoots to rhizomes after this date was approximately proportional to autumn shoot biomass in each treatment. Based on
this assumption, differences in rhizome TNC levels between
plots mowed in the spring (high shoot biomass, high rhizome
TNC) and the plots mowed four times (low shoot biomass,
low rhizome TNC) were probably even greater by October.
Regrowth of shoot biomass in 1989 indicated that C. canadensis had fully recovered from the mowing regimes applied
the previous year. In September 1989, no differences were
observed in shoot biomass among mowed and unmowed plots
(Fig. 1; two-factor ANOVA;df = 5 3 ; F = 0.59; ns). Multiple
regression analysis also indicated no significant relationship
between shoot biomass regrowth in 1989 and rhizome TNC
pools in September 1988, based on the mean values for each
of the six treatments within the two sites (n = 12; df = 9;
2 = 0.19; ns). However, mowed plots had shoot densities
that were nearly twice as high as in unmowed plots (Fig. 2A,
Dunnett's test; p < 0.05). Differences in shoot heights were
not detected statistically but may have been slightly lower in
mowed plots than in unmowed plots (Fig. 2B). A decrease in
shoot biomass was noted in unmowed plots between 1988 (498
t 53 g m-') and 1989 (241 t 23 g rnp2), suggesting that
conditions were generally less favourable for growth in 1989
(Fig. 1).
tember 1988 (mean
* SE of 10 plots per treatment).
Discussion
The mowing treatment used in this study was more severe
than that used in comparable experimental studies on Calamagrostis species (Corns and Schraa 1962; Stout et al. 1980,
1981; Kmeger and Bedunah 1988) because it resulted in the
complete removal of aboveground photosynthetic tissue.
Levels of TNC in above ground shoots were substantial
(Table l), thus this mowing treatment should have resulted in
residual TNC pools considerably lower than those remaining
when shoots were mowed to heights of 5-15 cm above the
soil surface. However, in the present study, Calamagrostis
canadensis showed rapid recovery from these severe mowing
treatments, regardless of when they were applied during the
growing season.
In contrast to our expectations, mowing treatments applied
when rhizome TNC levels were near their seasonal minimum
in late spring and early summer (Table 1) appeared to have
improved overall carbon balance in the plant by the end of the
growing season. This is indicated by the greater autumn TNC
levels in plots mowed in May, June, or July relative to
unmowed plots (Fig. 4), and the absence of any significant
reductions in September rhizome biomass under these treatments (Fig. 3). Unmowed plots had heavy accumulations of
shoot litter (>500 g mp') that caused a decrease of 3.8"C in
mean daily soil temperatures at the 10-cm depth between May
and August, relative to frequently mowed plots (Hogg and
Lieffers 1991b). This, coupled with reduced shading, may
explain the improvement in rhizome carbohydrate status after
mowing treatments.
There were major differences in carbohydrate pools among
treatments in September 1988. Most notably, TNC pools in
plots mowed four times were reduced to levels only about half
as high as in control plots, which reflects decreases in both
HOGG AND LIEFFERS
rhizome biomass (Fig. 3) and T N C concentrations (Fig. 4).
However, these differences did not appear to affect regrowth
of shoot biomass the following season (Fig. 1). The absence
of a relationship between rhizome T N C pools and shoot
regrowth may have been affected by changes in microclimate
plots mowed four
mowing treatments. For
times had the lowest T N C levels and rhizome biomass in 1988,
but also had a negligible buildup of shoot litter and thus warmer
soils during the 1989 growing season (Hogg and Lieffers
19916). Laboratory experiments have shown a positive relationship between soil temperature and growth of shoot biomass
in grasses and sedges even when air temperature is held constant (Smoliak and Johnston 1968; Younkin 1974; Kummerow
and Ellis 1984). Other effects of litter include shading of young
growing shoots and thermally induced decreases in soil nutrient
cycling (Daubenmire 1968; Rice and Parenti 1978).
Each of the mowing treatments in this study promoted tillering and resulted in shoot densities the following season that
were about twice as high as in unmowed plots. Similar
increases in shoot density are commonly observed in grasses
and sedges after disturbances such as fire, grazing, or mowing
(Deregibus et al. 1982; Diemer and Pfadenhauer 1987; Olson
and Richards 1988; Svejcar and Browning 1988). Release of
dominance can be an
a x i l l a r ~buds from
mechanism governing this response, particularly if stem apices
are damaged by the disturbance. In the present study, this
mechanism may have been important because most cf the
mowing treatments (June-August) occurred after culm elongation had elevated apical
above the soil surface
(Hogg and Lieffers 199 la). However, litter removal alone and
the associated changes in microclimate can also promote tillering in grasses, often at the expense of shoot height growth
(Hurlbert 1969; Willms et al. 1986; Willms 1988). Thus the
observed increase in shoot density after mowing probably
reflects the combined effects of reduced apical dominance and
shading, coupled with warmer soil thermal regimes.
The main conclusion of this study is that shoot regeneration
following disturbance was unrelated to belowground levels of
TNC in C , canadensis. Changes in microclimate caused by
mowing treatments may have influenced the results of this field
experiment. However, the same conclusion was reached in
another study of seasonal change in shoot regrowth potential
in this species (Hogg and Lieffers 1991a), even though the
experiment was conducted under constant laboratory conditions, 'rhus w e suggest that seasonal change in belowground
T N C levels is unlikely to be useful in predicting the response
of C. canadensis to defoliation caused by disturbances such as
fire and grazing.
Acknowledgements
W e wish to thank Stephen Davis and Dale Stelter for field
and laboratory assistance and Russ Horton for useful advice
on the carbohydrate analysis. Accommodation near the field
site was provided
the MeanOOk
Station,
ment of Zoology. This work was funded by a Forestry Transition grant f r o m the Natural Sciences and Engineering
Research Council of Canada (NSERC) to V.J.L. and an
NSERC postdoctoral fellowship to E.H.H.
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