14
The Prairie Naturalist 29(1): March 1997
Sexual Allocation in Carex stricta, a Monoecious, Tussock-Forming Sedge DENISE M. ESTRELLA and CHARLES E. UMBANHOWAR, Jr.
Department of Biology, St. Olaf College, Northfield,
MN 55057
,
ABSTRACT -- Sexual allocation in plants is a topic of much study. We
examined patterns of sex allocation in Carex stricta Lam., a perennial,
monoecious, anemophilous, tussock-forming sedge. Allocation to male and
female function was measured based on numbers of male, female, and
androgynous inflorescence units (spikes) and estimates of numbers of male
and female flowers. Total number of spikes was not correlated with plant
(tussock) size, and relative number of male, female, or androgynous spikes
increased proportionally in relation to total number of spikes. For individual
inflorescences, mean percent female flowers was 49.7 (:t2.2, SEM), and 77.4
(:t.3.4, SEM) of female flowers contained "mature" seeds. Neither percent
female flowers or seed set were significantly correlated with number of flowers
or height of inflorescence.
Key words: sex allocation, tussock sedge, Carex stricta, sexual reproduction,
sedge.
Sexual reproduction in plants involves dividing limited resources between
rnale and female function (Charnov 1982). And a range of observational and
experimental studies have attempted to elucidate the correlates of plant sex
ratios (Lloyd and Bawa 1984). Resource allocation theory predicts that an
increase in resources will result in increased allocation to female function
(Freeman et al. 1981, Goldman 1991, Herrera 1991, Delesalle 1992, Bickel
and Freeman 1993, Ashman 1994). In contrast, plant height may be a better
predictor of floral sex allocation in cases where pollen and seed dispersal
distances are limited, taller plants being increasingly biased toward male
lunction (Surd and Allen 1988). Changes in floral sex ratios may be most
Jlrectly linked to constraints of plant architecture. adaptive , or otherwise
16
The Prairie Naturalist 29(1): March 1997
(Ganeshaiah and Shaanker 1991, Bickel and Freeman 1993). While simple
counts of male and female flowers may underestimate energy and nutrient
investment in female function (Goldman and Willson 1986), they do provide
a good relative measure .by which to compare different individuals within the
same species (Surd and Allen 1988).
Carex stricta Lam. is a perennial, monoecious, anemophilous tussock­
forming sedge common to shallow-water wetlands; it ranges throughout
northeastern North America and extends well into the northern Great Plains
(Curtis 1959, Great Plains Flora Association 1986, Standley 1989). Plants
commonly grow to a height of 1.25 m and tussocks are composed of living
and dead tillers, roots, rhizomes, and trapped particulates: Tussocks range in
diameter from 2-6 dm, and individual tussocks may live to 40-60 years or more
(Costello 1936). Inflorescences are initiated in the fall and begin to elongate
in March to equal or barely exceed leaf height. Flowering occurs in late May
to early June (Standley 1989). Inflorescences consist of three to seven spikes
(or what Reznicek (1990) terms inflorescence units). The terminal spike
typically bears only male flowers, while the lowest lateral spike is commonly
female; intervening spikes display an androgynous combination of male and
female flowers.
In our study, we examined male:female floral ratios in Carex stricta. We
posed three main questions: First, does reproductive effort' change with plant
size? Second, do male:female floral ratios change as a function of plant size
or height? And third, how do male:female floral ratios change within
inflorescences?
STUDY SITE AND METHODS
Our study site was an approximately 2 ha fen located in the Cannon
River Wilderness Area in Rice County, Minnesota (44°23', 93°12'). Carex
stricta was the dominant species and formed large well-developed tussocks
throughout the site. Each tussock most likely represents an individual genet
composed of many ramets, but it is possible that tussocks contain more than
one genet (Costello 1936).
To investigate the relationship between tussock size and sexual
allocation, two 20 m transects were surveyed in the second and third weeks of
June of 1994. Transects were sampled at 1 m intervals and a total of 22
tussocks with inflorescences sampled. Tussock circumference, measured at
the base of living tillers, number of inflorescences, number of spikes per
tussock, and gender of spike, male, female, or androgynous were recorded in
the field.
Thirty-five reproductive stalks were collected from 20 tussocks in the last
week of June, 1995. Inflorescence height, culm length , and tussock
17
Estrella & Umbanhowar : Sex Allocation in Carex stricta
circumfer~nce
were measured in the field. Inflorescences were returned to the
lab and oven dried at 75°C for 24 hours. For each inflorescence, spike
position, relative to top of inflorescence, length, and gender were recorded.
Percent seed set was estimated for all spikes with female flowers, based on
a subsampling of 20 female flowers per spike. Mature achenes filled the
perigynia, while immature fruits were small and the style was much longer than
the ovary. We estimated floral density for each inflorescence by counting
number of female flowers per cm on the lowest spike and number male flower
per cm on the top spike.
Statistical analyses were performed with SYSTAT 5.2 for the Macintosh
(SYSTAT, Inc., 1992). Data were evaluated for assumptions of normality and
homogeneity of variance prior to analysis and transformed where necessary.
RESULTS
Tussock size, number of inflorescences and number of spikes varied
markedly (Table 1). Number of male spikes (12.1) did not differ significantly
1.53, df 21, P
0.13) but
from the number of female spikes (9.3) (t
exceeded the number of androgynous spikes (7.9) (t 3.87, df 21, P <
0.001). Numbers of female and androgynous spikes were statistically the
same (t 1.10, df 21, P 0.28). Number of spikes (total) was not strongly
0.35, n 22, P
correlated with tussock size (data log transformed, rp
0.11), but numbers of female and male spikes (rp 0.82, n 22, P <0.001),
female and androgynous (rp = 0.86, n =22, P <0.001), and androgynous and
male (rp = 0.97, n = 22, P <0.001) spikes were significantly positively
correlated with each other (Fig. 1).
For individual inflorescences, total length of spikes bearing female
flowers did not significantly exceed the length of those bearing male flowers
(Table 2, t = 1.80, df 34, P 0.08), while density of female flowers was
slightly, but not Significantly, below that for male flowers (Table 2, t =-1.74, df
= 34, P = 0.09). Similarly, there was no significant difference between the
llIumber of male and female flowers within an inflorescence (Table 2, t = 0.18,
tlf =34, P = 0.86), and percent female flowers was not significantly correlated
with flower number (Table 2, rp 0.16, n 35, P 0.36). Flowering culms
were pendant so that inflorescence height was always less than culm length
(Table 2), and neither inflorescence height (rp 0.02, n 35, P 0.90) nor
culm length (rp = -0.14, n 35, P 0.42) were significantly correlated with
tussock circumference. Inflorescence height (and also culm length) was not
~ Ignificantly correlated with flower number (rp = 0.12, n
35, P
0.49),
percent female flowers (rp 0.26, P 0.14), nor seed set (rp = 0.17, n 35, P
032) .
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
Table 1. Tussock size and number of inflorescences and spikes (inflorescence
c:.~mnlp.c1 in
units) for 22 Carex stricta tll~~n('lcc::
----- ...... ,,- -_w·-c----­1994.
Mean (+ SEM) .Mi!1.
MM:.
Basal Circumference (m)
0.82(0.08)
0.32
1.70
Number of Inflorescences
7.1 (1.4)
1
32
Number of Male spikes
12.1(3.2)
2
71
Number of Female spikes
9.3(2.4)
1
41
Number of Androgynous spikes
7.9(2.4)
0
50
100
~
"0
c
(I)
Male Spikes
0
Female Spikes
100
6.
0
0
90
0
6. Androgynous Spikes
Q)
~
m
(!)
.c
(J
cd
lJ.J
5
0
!:S 10
(/)
(I)
0
"''5."
(f)
0
ill
'0
Q;
.0
0
ffi
a.
0
80
(f)
o~
~
70
(f)
~o
(rj
0
(j/S)
6.
:::
g
60
LL
50
Q)
ro
{j
E
000 rl.
E
:>
z
Table 2. Summary of reproductive characteristics for 35 Carex stricta
inflorescences sampled in 1995.
Mean (+ SEM)
Min.
~
Inflorescence Height (cm)
62.2 (2 .2)
33
89
67.8 (2.4)
Culm Length (cm)
35
103
4.6(0.1)
3
7
Number of Spikes
2.9
Length of Male
7.9 (0.4)
12.5
Length of Female
2.5
9.0 (0 .6)
18.7
140
61.2 (3 .8)
Density of Male Flowers (fcm)
32
30
146
Density of Female Flowers (fcm)
55.0 (3.7)
Flower Number
9538.2 (577.7) 4116
19716
49.7 (2.2)
71 .5
Percent Female Flowers
23 .5
100
77.4 (3.4)
Percent Seed Set
22.5
.0
~
0
19
Estrella & Umbanhowar: Sex Allocation In Carex stricta
The Prairie Naturalist 29(1): Mar~h 1997
18
Q)
40
LL
~
DO{j M
30
....
Q)
20
(,)
~
l...> l...>'
..or----£r
10
~l .
100
1000
10
Total Number of Spikes
0
0-20
21-40
41-60
61-80
81-100
Relative Position within Inflorescence (distal to proximal)
Figure 1. Relationship (log-log sca le) between reproductive output (number
of spikes) and numbers of male, female, and androgynous spikes for 22
tussocks of Carex stricta.
Figure 2. Percent female flowers (mean ± 1 SEM) in spike relative to position
(eflstal to proximal quintiles) within Carex stricta inflorescence. n
total
number of spIkes Within uch aUlnll1
=
20
The Prairie Naturalist 29(1): March 1997
Terminal spikes contained mostly male flowers (Fig. 2) while the lowest
lateral spike was almost exclusively female . Percent femaleness did not
increase linearly with distanCe from the top spike (Fig. 2), and there was often
a large physical gap (as reflected by dearth of samples in 21-40 quintile of Fig.
2) between the lowest and penultimate spike within inflorescences. Percent
seed set was significantly correlated with spike position (rp 0.23, n 99, P
0.02) and declined from lower to higher spikes.
=
=
=
DISCUSSION
We found that sexual reproductive output in Carex stricta, as measured
by number of spikes, was not related to tussock size . Numbers of male and
female spikes, including androgynous, within tussocks, and numbers of male
and female flowers within inflorescences were positively correlated. Relative
allocation to male and female flowers was not correlated with number of
flowers or inflorescence height.
Our results contrast with a variety of studies that have documented a
positive correlation between plant size and reproductive output. Larger plants
are believed to have greater access to energy and nutrients needed to produce
flowers and fruits (Charnov 1982, Goldman and Willson 1986, Devlin 1988,
Oelesalle 1992, Clay 1993). In our study, tussock size may be a good
indicator of age, but not of energy or nutrient status, especially if physiological
integration among tillers is limited or absent (Delph et al. 1993). Studies of
other species of Carex indicate that tillers live for only one or two years
(Bernard 1990), also limiting the total amount of nutrients or energy an
individual tiller could accumulate.
In this species, the absence of specialized mechanisms for seed dispersal
could mean that more female flowers will result in increased competition
among seedlings, while ,efficient wind dispersal of pollen , combined with close
proximity of neighboring plants, may limit the benefrt of additional male flowers
(deJong and Klinkhamer 1994), even though male flowers were more common
on the more distal spikes (Fig. 2) . The absence of any height relationship
could also reflect the fact that, as with many Carex, flowers are initiated the
previous fall (Soukupova 1988, Bernard 1990, Standley pers. comm.) limiting
the ability of individual plants to predict or respond to current growing
conditions (Lloyd and Bawa 1984), or architectural and genetical constraints
or correlations involving floral initiation (McKone and Tonkyn 1986, Solomon
1989, Oiggle 1994, Agren and Schemske 1995). Numbers of spikes per
inflorescence showed little variation , but spike length and flower number did
(Table 2).
Estrella & Umbanhowar: Sex Allocation in Carex str/cta
21
Goldman (1991) suggested that trade-offs between numbers of male and
female flowers need not exist if, as is the case for C. stricta, flowers and fruits
are small relative to the overall size of the plant, or green and so able to
produce some of their own photosynthates. The high percentage of female
flowers containing "mature" achenes suggests that energy and nutrients were
not limiting. The high percentage of flowers containing fruits also suggests
that pollen was not a limiting factor (but see Handel 1976). We do not know
if C. stricta is self-pollinating , or if it is apomictic, both possible alternative
explanations for the high degree of seed set.
Floral sex allocation mayor may not be adaptive , and a wide variety of
floral allocation patterns have been report~ (Lloyd and Bawa 1984, Goldman
and Willson 1986).
Carex stricta tussocks contained relatively few
inflorescences (Table 1) and vegetative reproduction, due to spreading
rhizomes. predominates with the age of some tussocks approaching 50 years
or more (Costelio 1936). This may mean that sexual reproduction is of
relatively less importance and hence under little selection. To the extent that
floral sex allocation in Carex stricta, or other plants. is adaptive, it seems likely
that it may be a product of different and possibly conflicting selective factors
or constraints.
ACKNOWLEDGMENTS
Our work was supported in part by a grant to st. Olaf College by the Howard
Hughes Medical Institution. Thanks to M. McKone for many helpful comments
on an earlier draft of our paper.
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urn, M., and T. F. H. Allen. 1988 Sexual allocation strategy in wind­
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22
The Prairie Naturalist 29(1): March 1997
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Estrella & Um banhowar : Sex Allocation in Carex stricta
23
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~. 6'/ 17/.96'
~/O/7/.97