Botanical Journal of the Linnean Society (1996), 122: 301-313. With 1 figure
Reproduction of the rare monocarpic species
Saxifraga mutata L.
ROLF HOLDEREGGER
Institute of Systmatic Botqy, Universi@of Zurich, Zolldmstr. 107, CH-8008 Zurich,
Switzerland
Received May 1996, acceptedfw publication Aupust 1996
The aim of the study is to investigate the impact of reproduction and genetic variation on the persistence
of populations of the prealpine, monocarpic SmJfugu mututu L. The species grows on erosion slopes or
rocks, and its local populations are often small and isolated. Crossing experiments resulted in better seedset than selfing, but both yielded viable seeds. Agamospermy did not occur. In an early-successional
species like S. mututu, successful selfing is important in the colonization of new habitats. Flowers of S.
mututu were visited by Syrphidae and unspecialized Hymenoptera. A germination rate of 40% was
reached in cultivation after 20 weeks but germination continued until the end of the experiment after 92
weeks. Seeds stored dry for 30 months at room temperature mostly lost their germinability. In natural
habitats, seedlings were found almost throughout the year with a peak in spring. Suitable safe sites were
small patches of open soil, bare marl on erosion slopes, and rock crevices. AU individuals investigated
were diploid with 2n = 26. AUozyme electrophoresis showed a lack of segregation within the
populations. Intra- and interpopulation genetic variation was low. These results were in partial
disagreement with theoretical expectations in a mixed mating species. It is concluded that demographic
rather than genetic processes are the main cause of extinction of populations of S. mututu, at least in the
short-term.
Bl996 The Linnean Society of London
ADDITIONAL KEY WORDS: -allozymes - conservation biology - genetic variation - germination
- pollination - population dynamics.
CONTENTS
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .
Material and methods . . . . . . . . . . . . . . . . . . . . . . .
The species . . . . . . . . . . . . . . . . . . . . . . . . .
Study sites . . . . . . . . . . . . . . . . . . . . . . . . .
Pollination experiments . . . . . . . . . . . . . . . . . . . . .
Flower visitors . . . . . . . . . . . . . . . . . . . . . . . .
Seed sizes and germination experiments . . . . . . . . . . . . . . .
Enzyme electrophoresis . . . . . . . . . . . . . . . . . . . . .
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pollination experiments . . . . . . . . . . . . . . . . . . . . .
Flower visitors . . . . . . . . . . . . . . . . . . . . . . . .
Seed sizes and germination experiments . . . . . . . . . . . . . . .
Enzyme electrophoresis . . . . . . . . . . . . . . . . . . . . .
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01996 The Linnean Society of London
302
R. HOLDEREGGER
Discussion . .
. . . . . .
Breeding system and pollination
Genetic variation . .
Germination
Conservation biology .
Acknowledgements
References .
. . . .
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INTRODUCTION
A long standing question in conservation biology is whether genetic or
demographic processes are more important as critical factors affecting the extinction
or persistence of small populations (Lande, 1988). In rare or endangered plant
species, knowledge of demography, in terms of the number of individuals per
population, and knowledge of intrapopulational genetic variation are therefore of
fundamental importance (Harvey, 1985; Gilpin & Soule, 1986). Existing data imply
that demographic processes including environmental stochasticity are more
important as causes of extinction of local populations than are genetic processes
(Lande, 1988; Barrett & Kohn, 1991; Ellstrand & Elan, 1993). Knowledge of the
reproductive biology of a species is a key for the understanding of intrapopulational
processes of both types, demographic and genetic (Loveless & Hamrick, 1984).
In species with formerly large but recently small and isolated populations like
Salvia pratensis L., Scabiosa columbari.a L. and Gentiana pneumonanthe L., genetic erosion
may cause extinction due to inbreeding effects resulting in low genetic variation (van
Treuren et al., 1993; Ouborg & van Treuren, 1994; Oostermeijer et al., 1995). In
species that are naturally rare and have always occupied isolated habitat patches
deleterious genetic processes due to inbreeding may be of minor importance or
totally absent. This was shown in several rock-inhabiting fern species (Holderegger
& Schneller, 1994; Schneller & Holderegger, 1996). Nevertheless, the importance of
genetic variation within populations of rare perennial plant species that exhibit
marked changes in population size and structure due to naturally occurring
environmental stochasticity is still insufficiently known. A study of the demography
of the rare, prealpine Saxi.ftaga mutata L. has indicated that size structure (in terms of
rosette diameters), recruitment, and timing of reproduction are connected with the
course of succession (Holderegger, in press), and that stochastic erosion events
strongly influence the persistence of local populations.
The present paper deals with the reproductive biology and the intrapopulational
genetic variation of S. mutata in order to assess their potential impact on its
conservation biology. Pollination experiments and germination experiments were
undertaken, field observations on flower visitors were made, and allozyme diversity
was determined to evaluate the importance of genetic factors for the survival of the
species' local populations.
MATERIAL AND METHODS
The species
Saxifraga mutata L. (Saxifragaceae) is characterized by widely scattered and often
transient populations in the European prealps and Carpathian mountains (Webb &
REPRODUCTION OF SAXZFRAGA MUIATA
303
Gornall, 1989). Most of its sometimes very small populations are found at altitudes
between 800 and 1200 m a.s.1. (Kaplan, 1995). In river valleys or ravines it inhabits
calcareous rocks and debris or gravel on erosion slopes. Its dark green, basal rosettes
with lime-secretinghydathodes can reach diameters of 20 cm. The flowering panicles
usually bear about 60 flowers with linear, yellow to faint-red petals. The flowering
season is from July to late August. Single rosettes of S. mututu are normally strictly
monocarpic, requiring several years from seed to flowering, but genets may
sometimes behave as true perennials by means of vegetative offsets (Webb & Gornall,
1989).
In Switzerland, S. mutata is a rare plant species (Landolt, 1991). Populations are
usually isolated from each other, but in a river valley or ravine the species may
occupy several small habitat patches.
study sites
The study was made in the Canton of Zurich in a mountainous region of
northeastern Switzerland. Three populations of S. mututu were investigated:
(T) Tosstal; coordinates of the Swiss national grid 715.100/241.250. A population
of about ten thousand individuals of S. mututu inhabits steep erosion slopes and rocks
of calcareous conglomerate in a deep river valley at altitudes of about 800 m a.s.1.
(FJ Falatsche near Zurich; coordinates: 680.500/243.250. A population of S. mututu
with about three thousand individuals is found at altitudes between 700 m and 780 m
a d . on a steep slope of marl or calcareous sandstone in the shape of a semicircular
funnel. Erosion is frequent at this study site.
(K) Kusnacht; coordinates: 687.800/241.500. A small population of S. mututu of
about 200 rosettes occupies slopes of marl in a small ravine at an altitude of 500m
a.s.1. in the vicinity of Zurich (Holderegger, 1994).
Pollination expmimts
Experiments were performed in summer 1990 in all three populations in order to
investigate the breeding system of S. mutab. Flower buds were bagged in synthetic,
water-resistant silk with a mesh-width of 12 pm (Nybolt Gaze, Schweizer Seidenfabrik AG) one week before the onset of flowering. Only flowers of second order within
the panicle were taken because flower size depends on the position within the
panicle. One flower bud from each of 70 plants was used per treatment, 140 flowers
were marked as controls. Substantial losses of marked and experimentally treated
flowers occurred because they were damaged by deer and chamois, land slides, or
hail (see sample sizes).
Five different treatments were carried out. (1) Artificial crossings: stamens of the
pollen-receiving flowers removed before bagging; pollen for crossing was collected
from individuals within a distance of 5 m from the experimental plant and mixed;
n = 33. (2) Spontaneous selfing: flower buds were bagged without hrther treatment;
n = 65. (3) Artificial selfing: stamens of the experimental flowers were removed
before bagging; selfing was performed with pollen from another flower of the same
individual ( = geitonogamy); n = 67. (4) Agamospermy: flower buds were bagged
304
R. HOLDEREGGER
after stamen-removal; no further treatment; n = 42. (5) Open pollination: control; no
treatment; n = 139.
Ripe fruits were harvested before dehiscence, and the number of ripe seeds per
fruit was determined. Viability of these seeds was tested in a germination experiment
(for experimental design see below).
Flower Uzritors
Insects visiting flowers of S. mutata were caught in July and August 1990 during
four hours in the afternoon of two days with changing weather conditions in
population T, three days in population F, and one day in population K. Each species
was collected and recorded only once, and no attempt was made to count flower
visitors. Flower visitors were identified at least to the level of genera.
Seed sizes and germination expm'ments
Differences in seed sizes among populations could have an effect on germination
rates. Therefore, ripe seeds were sampled in all three populations. Length and width
of five seeds per plant were measured under a dissecting microscope. Seeds of 30
individuals were analysed in population T, 29 in F, and 28 in K.
Seeds used in the germination experiments A 4 were collected in late August and
September 1990. Fifty ripe seeds from a single individual of S. mutata were scattered
on wet filter paper in a petri dish. Since no fungicide was applied, some seeds were
infected, but their germination was usually not inhibited or delayed when compared
with the germination of uninfected seeds. Nevertheless, some losses occurred. The
petri dishes were exposed to a constant temperature of 2OoC and to a 16h light/8 h
darkness regime. Tap water was added once a week. Germination rate was checked
every two weeks.
Experiment A: this experiment was started immediately after sampling and lasted
for 20 weeks. Seeds from 26 individuals were harvested from population T, 30 from
F, and 23 from K. A total of 3907 seeds was used for analysis.
Experiment B: seeds were stored dry for 10 months at room temperature before
the experiment started (T: n = 40 individuals; F n = 40; K: n = 6). Additionally,
capsules of five individuals of S. mutata of the previous season were sampled from
population F in spring 1991. These seeds were also incorporated in this experiment.
Germination was checked for 92 weeks starting in June 1991. A total of 450 1 seeds
was taken into account.
Experiment C: seeds were tested after a dry storage of 30 months at room
temperature (T: n = 10 individuals; F: n = 10; K: n = 5). The experiment was
carried on for 96 weeks; 1250 seeds were used.
Enzyme electrophoresis
The chromosome number of S. mututu has been debated several times (Kupfer &
Rais, 1983). Therefore, in populations T , F, and K the meiosis of 13 individuals in
REPRODUCTION OF SAXIFRAGA M U A T A
305
total was examined using the classical acetocarmine squash method (Darlington &
LaCour, 1976).
Genetic variation within populations was investigated with allozyme electrophoresis. Both starch-gel electrophoresis and agarose-gel electrophoresis were used
(Wendel & Weeden, 1989; Schneller & Scheffrahn, 1989). Modifications of the
loading technique enabled the analysis of 52 individuals per agarose gel. Best results
were obtained with agarose-gel electrophoresis (Schneller & Scheffrahn, 1989),but a
second run was usually done using starch-gel electrophoresis in order to confirm the
banding patterns. The grinding buffer of Schneller & Scheffrahn (1989: 199) with
20% w/v PVP 40000 was used. PGM and GPI were best scored with buffer system
No 5 of Soltis et al. (1983)adjusted to pH 7.0 with citric acid and gel buffer No 5 of
Schneller & Scheffrahn (1989). For IDH the electrode buffer No 5 had a pH 7.2.
MDH and SKD were analysed with electrode and gel buffers No 1 of Schneller &
Scheffrahn (1989).
Problems with the material during grinding and loading the gels (withering)
resulted in bad resolution of bands after staining. This is the reason for the different
sample sizes per enzyme system and per population (Table 3). Only fresh plant
material could be used. Five enzyme systems with a total of seven loci were
interpreted at the allelic level, namely Qi-2, Idh, Mdh-1, Mdh-2, Mdh-3, Pp-2, and
Skd (Weeden & Wendel, 1989). Results are presented as genotypes per loci.
RESULTS
Pollination experimts
Mean seed-set per capsule decreased in the following order: open pollination
(161.92) > artificial crossing (45.39) > spontaneous selfing (24.75) > artificial selfing
(5.03) > agamospermy (0.00). The large standard errors in Table 1 show that there
was high variation in seed-set within each treatment. According to a Kruskal-Wallis
analysis (d.f. = 3, P < 0.001) with subsequent Nemenyi pairwise comparisons
(2' = 0.05; Sachs, 1992) open pollination had significantly higher seed-set than all
other treatments, and artificial crossing resulted in a significantlyhigher seed number
compared with the agamospermy experiment. Neither of the other comparisons
exhibited significant differences.
Mean germination rates after 92 weeks were 90.4% in open pollinated flowers
(number of sampled plants n = 86, total number of seeds n = 4300), 73.3% in
artificially crossed flowers (n = 8, seeds n = 400), 96.0% in spontaneously selfed
TABLE1. Mean number of ripe seeds per capsule (K), standard error (SE), and number of
replicates ( n ) , i.e. the number of flowers per treatment in the pollination experiments of
Saxapaga mutata
Open
pollination
X
SE
n
161.92
10.47
139
Artificial
crossing
45.39
13.31
33
Spontaneous
selfing
ArtifiCal
selfing
Agamospermy
24.75
5.72
65
5.03
1.88
67
0.00
42
R. HOLDEREGGER
306
flowers (n = 2, seeds n = loo), and 65.8% in artificially selfed flowers (n = 10, seeds
n = 500).
Flower uisitors
Only Hymenoptera and syrphids were caught in all three populations;
Hymenoptera were: Andrm bicolor (Fabricius, 1775), Apis mellfera (Linnaeus, 1758),
Bombus patorum (hnaeus, 176l), B. pascuorum (Scopoli, 1763), hioglossum calceatum
(Scopoli, 1763), L.JiLlviCom (IGrby, 1802), L. morio (Fabricius, 1793), and L. ?.lsfitarse
(Zetterstedt, 1838); Syrphidae were: EpZrtrophe sp., Plapckrus sp., Scaeva sp.,
Sphwophoria sp., and Sy-hus sp.
Seed sizes and gemination experiments
Seeds of S. mututa are spindle-shaped with a mean length of about 0.8 mm and a
mean width of about 0.3 mm (Table 2). Seed sizes were only significantly different
between populations T and F (ANOVA, d.f. = 83, length: P = 0.003, width:
P < 0.001; 95% LSD pairwise comparison). Nevertheless, the differences in seed
sizes were $0small among populations that they were assumed not to have significant
effects on germination.
All populations showed essentially the same course of germination in experiment
A (Fig. 1A-C). Germination started slowly, and germination rate of all three
populations averaged about 4o0/o after 20 weeks.
In experiment B, populations reached germination rates between 40% and 80%
after 20 weeks (Fig. 1D-F).Substantial differences occurred between the populations.
In populations T and K germination was higher after stratification (experiment B)
than without stratification (experiment A). Only a few seeds germinated after 24
weeks in populations T and K. In population F (Fig. 1E) germination after 24 weeks
took an almost linear course. At the end of the experiment after 92 weeks,
germination rates were between 83% and 96%. Seeds that had overwintered in
capsules in the field germinated very quickly reaching 60% after four weeks. After 92
weeks, their germination rate was 88% (Fig. 1G).
After dry storage at room temperature for 30 months, the germination of seeds
was drastically decreased (experimentC; Fig. 1H-K). In populations T and K almost
no germination occurred, while a germination rate of 22% was still observed in
population F.
TABLE
2. Seed length and seed width (pm) of Smj-ugu mututa in the populations TGsstal (T),
FalHtsche (F), and KGsnacht (K). Mean (3,standard error (SE), and the number of replicates
(mean of five seeds per individual; n)
T
Seed length
X
SE
Seed width
X
SE
n
854.4
11.2
327.4
4.1
30
F
793.0
9.9
302.6
4.1
29
K
827.2
12.6
322.3
4.4
28
REPRODUCTION OF U Z F R A G A MUTATA
307
Enzyme elecpophoresti
AU cytologically investigated individuals of S. mutata from the three populations
showed regular chromosome pairing in meiosis. The chromosome number was
2n = 26.
B
0
4
8
12
16
20
0 12 24 36 48 60 72 84
-
0
tc
4
8
12
16 20
0 12 24 36 48 60 72 84
0
4
8
12
16 20
0 12 24 36 48 60 72 84
0
0 12 24 36 48 60 72 84
100
s
'1
0
'5
H
K
I
80
60
40
20
0
0 12 24 36 48 60 72 84 96
0 12 24 36 4g 60 72 84 96
0 12 24 36 48 60 72 84 96
week
week
week
Figure 1. Germination rates of seeds of Smzuga mutata (mean f SD). A-C, germination experiment A (no
stratification). D-G, germination experiment B (I-F, seeds stored dry at room temperature for 10
months. G, seeds averwinteredin capsules in the field). H-K,germinationexperiment C (seedsstored dry
at room temperature for 30 months). A,D,H, Population Tosstal, B,E,G,I, population Falltrche; C,F,K,
population Kiisnacht. Sample sizes are given in the text.
R. HOLDEREGGER
308
Enzyme loci Gpi-2,Mdh-1, Mdh-2, Mdh-3 were completely uniform. All individuals
were heterozygous at the three Mdh loci and homozygous for Gpi-2.In population F
all investigated individuals exhibited exactly the same multilocus genotype (Table 3).
Only low variation was detected at the loci Idh and Skd in population K. The greatest
variation, though still low, was detected in population T in Idh, P p - 2 and Skd. A
strange Idh-enzyme phenotype, which occurred in two individuals, was interpreted as
a combination of two heterozygote loci ‘ac’ and ‘bc’ (Table 3). Heterozygotes at the
polymorphic loci Pgm-2 and Skd were rare in the investigated populations.
DISCUSSION
Breeding system and pollination
Outbreeding is the dominant mating system in several Saxzjagu species, but selfing
usually also yields seed (Stenstrom & Molau, 1992; Molau & Prentice, 1992;
Lindgaard Hansen & Molau, 1994). In S. mutata, open pollinated individuals
(assumed to receive mainly foreign pollen), and artificially crossed flowers had
greater seed-set than spontaneously selfed or artificially selfed flowers (Table 1). In all
pollination experiments viable seeds were obtained. Smajiaga mutata is thus expected
to be mainly outbred but selfing is possible. The dominant mating system in its
populations may be mixed. Artificially treated flowers yielded fewer seeds than
flowers in spontaneous pollination treatments. This indicates that artificial pollination was not very successful (for possible reasons see Young & Young, 1992).
Experiments showed no agamospermy in the studied populations.
TABLE3. Genotype frequencies at 7 loci in the populations T6sstal (T), Falgtsche (F), and
Kiisnacht (K) of Saxzpugu mututa (enzyme abbreviations according to Weeden & Wendel, 1989;
number of individuals in parenthesis);a,b,c denote different alleles (e.g. ‘ab’ is a heterozygote
and ‘aa’a homozygote)
Locus
Genotype
T
F
K
(3-2
aa
1.oo
Idh
aa
ac
(80)
0.80
1.oo
(53)
1.00
(39)
0.14
0.86
~~
bc
0.12
=/bc
0.08
(25)
1 .oo
(73)
Mdh-1
ab
Mdh-2
ab
Mdh-3
ab
Pgm-2
aa
ab
bb
Skd
aa
ab
bb
1.oo
(73)
1 .oo
(73)
0.02
0.03
0.95
(66)
0.12
0.88
(25)
-
1 .oo
-
-
(11)
1 .oo
(37)
1 .oo
(37)
1 .oo
(37)
(14)
1 .oo
(41)
1.00
(41)
1.oo
(41)
1.00
1.00
(36)
-
-
(22)
-
1.00
(6)
-
-
0.03
0.97
(30)
~
REPRODUCTION OF SAXIFRAGA MULATA
309
Selfing may be supported by the kind of dichogamy in the protandrous flowers of
S. mutata. At dehiscence, stamens bend towards the gynoecium one after the other.
After this movement, they return to their original position. Stigmas reach receptivity
at this time. After this female phase, just before the petals begin to wither, all the
stamens again bend inwards and the remaining pollen may be deposited on the
stigma surfaces (Holderegger, pers. observ.).
Sax$iaga mutata has actinomorphic flowers, which are visited by Syrphidae and
polylectic Hymenoptera, which unspecifically collect pollen (Kugler, 1970; Westrich,
1989). Both insect groups are frequently found in substantial numbers on flowering
panicles of S. mutata. This spectrum of flower visitors is in accordance with other
studies on SUX&~Uspecies (McGuire & Armbruster, 1991;Warncke & Olesen, 1991;
Warncke et al., 1993; Lindgaard Hansen & Molau, 1994). Bees seldom fly on cloudy
or cold days (Westrich, 1989), while Syrphidae are not so strongly influenced by
adverse weather (Faegri & van der Pijl, 1971). On relatively cold, cloudy days in
summer 1990 only Syrphidae were caught. They may ensure pollination of S. mutata
flowers even in weather unfavourable for bees. The high seed-set in open pollinated
flowers combined with self-compatibility and autodeposition indicate no pollen
limitation in S. mutata. A pollen limited reproductive success was found in a subarctic
tundra population of the almost obligate outcrosser S. opposit@liu L. (Stenstrom &
Molau, 1992).
Genetic vanation
Hamel (1950, 1953) reported a chromosome number of 2n = 28 for S. mututa, but
Kupfer & Rais (1983) tested some individuals near the sites where Hamel had
sampled and found only plants with 26 chromosomes. From the Carpathians there
is a report indicating a different chromosome number of 2n = 32 (Tarnavschi, 1981).
In the present study all individuals had a chromosome number of 2n = 26. My
counts thus agree with those of Kiipfer & Rais (1983), and it seems most probable
that S. mutata is a diploid with a basic number of x = 13, which is found in many
other Smtj?aga species (Webb & Gornall, 1989).
Genetic variation within and among the three populations of S. mututa was very
low. It was impossible to calculate meaningful genetic diversities using the methods
of Nei (1972). In view of the small number of investigated loci and the differing
sample sizes, the results on allozymes should be interpreted with caution.
Low genetic variation is somewhat unexpected in an at least partially outbreeding
species. They are often characterized by high levels of variation within populations
(Schoen & Brown, 1991; Hamrick et aL, 1991). In addition, monocarpic species have
been found to exhibit high genetic variation among populations (Hamrick, 1989).
The presented allozyme data on S. mutata, therefore, are not in agreement with those
that might have been predicted from the results of the breeding experiments.
At the Mdh-loci, all plants showed the same heterozygous genotypes. At these Mdhloci, the diploid S. mutata had banding patterns similar to tetraploid species with fixed
heterozygosity. Most plants were heterozygous at the Idh-locus; only very few
homozygous plants were found. There were two individuals in population T with an
Idh-banding pattern that looked like a combination of two heterozygous patterns
(Table 3). Whether this was the result of a single gene duplication cannot be analysed
without analysis of progeny. In P p - 2 and Skd most plants were homozygous.
310
R. HOLDEREGGER
It seems unlikely that inbreeding could be the sole cause of the low genetic
variation found in S. mutata, especially in view of the lack of differentiation between
widely separated populations. Apomixis or vegetative reproduction could in principle
lead to genetic uniformity in populations of Saxzzaga species (Bauert, 1994), but the
former did not occur (Table l), and the latter is too rare to influence strongly the
intrapopulational genetic variation; less than 8% of flowering individuals produced
offsets (Holderegger, in press). A reason for the apparent genetic uniformity may be
that the three populations were founded by diaspores with similar genotypes despite
the great distances between the populations. Another possible explanation is that
allozyme electrophoresis may have provided an inadequate representation of the
extent of genetic variation in S. mutata.
Germination
Germination rates of Saxtj?aga species vary substantially. Reports show quick and
high germination in S. hirculus L. (Ohlson, 1989; Dahlgaard & Warncke, 1995) or
slow and low germination in S. paniculata Miller (Sapzhenkova, Syenchyna &
Druchas, 1986). Germination rates of S. mutata reached about 40% after 20 weeks.
After dry storage for more than 30 months, germination rates drastically
decreased.
Germination experiments are usually carried on for 2 or 3 months. Such
experiments would not have detected the unusual course of germination in S. mutata.
After 3 months only a moderate germination rate would have been detected (Fig. 1).
Nevertheless, rates between 83% and 96% were reached after 92 weeks in
experiment B.
Successful establishment of many monocarpic species is only possible in
vegetation-free gaps (Silvertown, 1983). Field observations showed that on sparsely
colonized erosion-slopes seedling of S. mutata often grew in the shade of other plants,
especially mature plants of S. mutata, Campanula cochleari$olia Lam., or Leontodon hzipidus
ssp. hyoseroides (Welwitsch)J. Murr. Inflorescences of S. mutata are bent downwards
during winter. If there are still viable seeds in the capsules, they will be released near
to the mother plant. Many seedlings were found just below dead rosettes of S. mutata.
In more closed vegetation, seeding occurred only on small patches of open soil. On
rocks and cliffs, seedlings grew in crevices or on small ledges. Seedlings also grew in
cushions of mosses like Barbula crocea (Brid.) Web. & Mohr, but not in stands of taller
bryophytes like DitrichumJIexicaule (Schwaegr.) Hampe (Holderegger, pers. observ.).
Suitable safe sites for S. mutata exist at any time during the year. A life history
strategy, including seeds that either germinate immediately or else germinate in low
numbers throughout the year, would increase the chance of successful establishment
in an unpredictable habitat. In the field, seedling abundance of S. mutata was highest
in spring (April, May, and earlyJune), but germination occurred almost throughout
the year (Holderegger,pers. observ.). These field observations are in accordance with
the results of the germination experiments.
Conservation biology
Populations of S. mutata are often small and isolated from each other (Webb &
Gornall, 1989). In a river-valley or along a steep mountain slope, the species usually
REPRODUCTION OF SAXZFMGA M W A T A
311
occupies several suitable sites, forming a set of local populations or a metapopulation
(Hanski, 1991). Unfortunately, nothing is known about the potential dispersal
abilities of S. mutata. Earlier authors (e.g. Daniker, 1939) have stated that S. mutata
colonizes open erosion slopes but that ongoing succession excludes it from these sites.
At the local scale S. mutatu will only survive if it is able to disperse and re-locate to
another, perhaps recently eroded slope. Extinction and colonization events are
inherent in the dynamics of local populations or metapopulations of S. mututu.
Selfing may be of great importance for the successful colonization of hitherto
unoccupied patches in the above mentioned population dynamics (Baker, 1955). In
respect to the allozyme data and the large set of viable seeds of selfed flowers, one
may conclude that selfing has had no deleterious effects in populations of S.
mutata.
Lande (1988) concludes that demographic processes are more important for the
maintenance of local plant populations than are genetic processes. Frankham (1 995)
stresses that the responses of populations to demographic and environmental
stochasticity are affected by inbreeding or loss of genetic variation as well. In this
case, extinctions can be incorrectly attributed to nongenetic factors rather than to
interactions between genetic and nongenetic factors. Lande & Schannon (1996)show
that in the short-term genetic variation is theoretically less critical than other
determinants of population persistence, but that in the long-run it plays a decisive
role in population persistence and adaptation to a changing environment. In S.
mutata, succession and erosion are most important in determining the size of local
populations (Holderegger, in press). Size structure, timing of reproduction, and
recruitment within the populations are strongly related to the course of succession.
Loss of landscape erosive processes due to protective measures like river
embankments is thus the major threat to populations of S. mutata.
ACKNOWLEDGEMENTS
Elena Conti, C.D.K. Cook, F. Gugerli, and JJ. Schneller made valuable
comments on earlier versions of the manuscript and improved my English. A. Muller
helped in the determination of Hymenoptera. Special thanks go to Q.O.N. Kay for
very carefully reviewing the paper, and greatly enhancing its contents and
strength.
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