Journal of Vegetation Science && (2012)
Frost as a limiting factor for recruitment and
establishment of early development stages in an alpine
glacier foreland?
Silvia Marcante, Angela Sierra-Almeida, Joachim P. Spindelböck, Brigitta Erschbamer &
Gilbert Neuner
Keywords
adults; alpine plant species; chronosequence;
freezing stress; juveniles; plantlets; primary
succession; seeds and seedlings
Nomenclature
Fischer et al. (2005).
Abstract
Questions: How frost resistant are the early development stages (seeds, seedlings, plantlets and juveniles) of alpine plant species? Do summer frosts impair
establishment of plant species typical of different successional stages on a central
alpine glacier foreland?
Location: Rotmoos glacier foreland, Austrian Central Alps (Obergurgl, Tyrol,
Received 20 December 2010
Accepted 7 February 2012
Co-ordinating Editor: Francesco de Bello
Marcante, S. (corresponding author,
[email protected]), Erschbamer, B.
([email protected]) & Neuner, G.
([email protected]): lnstitute of
Botany, University of Innsbruck,
Sternwartestrasse 15, 6020, Innsbruck,
Austria
Sierra-Almeida A. ([email protected]):
ECOBIOSIS, Departamento de Botânica,
Universidad de Concepcion, Concepcion,
Chile
Spindelböck, J.P. (Joachim.Spindelbock@
hisf.no): Faculty of Science,
Sogn og Fjordane University College,
PO Box 133, N-6851, Sogndal, Norway
Austria).
Methods: Seeds of 12 species typical of different successional stages were collected in the glacier foreland and either sown directly in the field or in a growth
chamber (25/10 °C, 16/8 h) and grown to the investigated development stages.
Frost resistance of the early development and adult stages was determined by
exposing them to a set of freezing temperatures and assessing viability with the
tetrazolium test (LT50, i.e. 50% of samples being lethally frost damaged).
Results: Dry seeds had the highest frost resistance (LT50: 19 °C), followed by
wet seeds after imbibition (LT50: 8 °C). With the onset of germination, frost
resistance decreased rapidly. While germinated seeds tolerated a mean of 3.2 °
C, seedlings and juveniles were less frost resistant (LT50: 2.5 °C). Along the primary succession, seedlings of pioneer species were significantly less frost resistant than early- and late-successional species. However, field grown seedlings,
mainly of pioneer species, showed higher frost resistance (mean: 5 °C) than
the growth chamber seedlings (mean: 3 °C), indicating that frost hardening
(transition from a lower to a higher level of frost resistance) is already possible
during these early stages of development.
Conclusions: The low frost resistance during and after germination may not
suffice to survive summer frosts and may at least in certain years explain the
high seedling mortality rates recognized in the glacier foreland.
Introduction
Establishment of plants in the glacier forelands proceeds
slowly and is often fragmentary. Although a considerable
number of seeds germinate (Niederfriniger Schlag & Erschbamer 2000; Marcante et al. 2009a), species establish
only at low rates due to high seedling mortality (Welling
et al. 2005; Erschbamer et al. 2008; Marcante et al.
2009b). These high seedling mortality rates supposedly
depend on extreme low temperatures and water shortage
caused by rapid drainage of water through the undeveloped soils (Körner 2003). Freezing temperatures during
the growing season could be the dominant factor, particularly if frost resistance in the early development stages of
plants is insufficient.
On recently deglaciated terrain, germination takes place
on the surface of the bare ground. The surface is particularly prone to low temperature extremes during the night,
and additionally, this area is also exposed to cooling winds
coming from the glacier and from the side slopes (Hoinkes
1955; Tackenberg & Stöcklin 2008). At the elevation of glacier forelands in the Austrian Central Alps long-term bioclimatic temperature records indicate that freezing
temperatures down to 5 °C can occur throughout the
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1
Does frost limit recruitment and establishment on a glacier foreland?
whole growing period (Larcher et al. 2010). Field observations indicate that summer frost events damage adult
alpine plants at temperatures between 3.0 and 10.8 °C
in summer (Taschler & Neuner 2004). A substantial fraction of their above-ground tissues can be lost during such
freezing events, although the plants can survive if undamaged below-ground organs have developed (Körner 2003;
Taschler & Neuner 2004; Neuner & Hacker 2010).
However, to the best of our knowledge, there is a lack of
information on the frost resistance of early development
stages of alpine plants, such as germinating seeds, seedlings, juveniles and of vegetative dispersal units such as
plantlets. As far as we know, only one preliminary study
has addressed this point. This study, investigating four
alpine plants, suggests that frost resistance of early development stages (Maria Wildner-Eccher and Walter Larcher,
unpublished data) is lower than for adults, leading to a
higher risk of being damaged by frosts. Additionally, restoration after frost damage of above-ground plant parts is less
likely to occur during early development stages as such
plants lack a sufficiently large below-ground biomass.
Growing plant tissues are known to be particularly frost
susceptible, and their frost hardening potential, i.e. the
reversible transition from a lower to a higher level of frost
resistance upon exposure to triggering environmental conditions, is also limited (Taschler et al. 2004; Neuner & Beikircher 2010; Hacker et al. 2011). Tissue growth processes
are dominant during early plant development and might
affect frost hardening.
Moreover, we hypothesized that field-grown seedlings
may possess a higher frost resistance than seedlings cultivated in a growth chamber, as the natural environmental
conditions may force them to become frost hardened. As
far as we know, little if anything, is known about the frost
hardening potential during early stages of plant development.
As for seedlings, plantlets and bulbils from viviparous
and pseudo-viviparous species such as Poa alpina and Persicaria vivipara are released from the mother plants at a very
sensitive stage. It is recognized that the production of
P. alpina plantlets is governed by temperature (Heide
1989): they are produced under harsh conditions, whereas
propagation by seed seems to prevail under more benign
conditions. As plantlets are largely responsible for maintaining plant populations in the pioneer stages (Marcante
et al. 2009b; Winkler et al. 2010), we hypothesized that
they may also have a higher frost resistance than seedlings.
Our general hypothesis is that frost could be a limiting
factor for the colonization of open areas in alpine glacier
forelands due to frost damage that can affect imbibed
seeds, seedlings, juveniles or plantlets. Along the successional gradient of a glacier foreland, a temperature gradient
exists, with soil surface temperatures during the growing
2
S. Marcante et al.
season of 7.5–9.0 °C at the 35-yr ice-free sites to 10.5–
12.0 °C at the oldest ice-free sites (Schwienbacher et al.
2012). Freezing may occur at any time during the growing
season on the recently ice-free sites. Thus, early colonizing
species should be adapted to lower temperatures and
should have higher frost resistance at all development
stages compared to species occurring at older successional
stages. Hence, we investigated frost resistance of glacier
foreland species, from pioneer to late-successional species,
to test to what extent frost affects plant population dynamics and plant survival along the primary succession.
Our experimental approach was designed to answer the
following questions:
1. Do early development stages of glacier foreland species
differ in their frost resistance?
2. Does frost resistance of pioneer species differ to earlyand late-successional species?
3. Are adult alpine plants more frost resistant than their
seedlings?
4. Do plantlets of P. alpina acquire a higher frost resistance
than seedlings?
5. Are there differences in frost resistance between seedlings cultivated in the field and in the growth chamber, i.e.
do seedlings possess a frost hardening capacity?
Methods
Research area
The research area is situated in the Austrian Central Alps
on the pioneer stage (1971 moraine) of the glacier foreland
of the Rotmoos Valley (Obergurgl, Ötztal, Tyrol, Austria,
46°52′N, 11°02′E) at 2400 m a.s.l. The valley is almost flat
and ascends only slightly near the glacier tongue. Over the
last 150 yr the glacier has retreated by more than 2 km.
The largely well-preserved chronosequence exhibits a series of glacier moraines (e.g. 1971, 1923, 1870), delimitated
by a terminal moraine ridge dated to 1858 (G. Patzelt, University of Innsbruck, unpublished data). The chronosequence proceeds from a pioneer stage (1971) via an early(1923) to a late-successional stage (1858; Raffl & Erschbamer 2004; Raffl et al. 2006a). Soil development shows
slow progressive development from Syrozems on the
youngest moraines to Pararendzinas on the oldest sites
(Erschbamer et al. 1999).
Microclimate
To assess the severity and frequency of low temperature
extremes at the pioneer stage in the glacier foreland, a
microclimate station was installed at the research area and
temperature data were collected throughout three successive summer periods (2008–2010). Air (at 15 cm above
the soil surface) and soil surface temperatures were
Journal of Vegetation Science
Doi: 10.1111/j.1654-1103.2012.01411.x © 2012 International Association for Vegetation Science
Does frost limit recruitment and establishment on a glacier foreland?
S. Marcante et al.
determined with thermocouple sensors (Type T; solder
junction diameter: 0.2 mm; Thermo-Est, Vienna, Austria).
Soil temperatures (2-, 3-, 5- and 10-cm depth) were measured with thermistors (107; Campbell Scientific, Logan,
UT, USA). All sensor types were connected to a data logger
(CR10X; Campbell Scientific), collecting data every
10 min from each sensor and recording mean values at
30-min intervals.
have sufficient field-grown seedlings for the laboratory
experiments in 2009. Bulbils of P. vivipara were regarded
as seeds, given that they also occur in the seed banks of
the Rotmoos glacier foreland (Marcante et al. 2009a).
The experimental field was regularly watered during the
2009 growing season to prevent drought stress to seedlings. Adults and plantlets were collected in the glacier
foreland during the 2009 growing season.
Plant material
Determination of frost resistance
Frost resistance of seeds and early development stages
was studied in 12 alpine plant species, all of which occur
along the primary succession of the Rotmoos glacier foreland (Raffl & Erschbamer 2004; Table 1). The nomenclature follows Fischer et al. (2005). Almost 500 seeds per
species were collected at the research site. For the laboratory experiments, seeds of all species were sown into
Petri dishes and exposed in a growth chamber (Sanyo,
E&E Europe BV, Leicestershire, UK; MLR-350H; 25/10 °
C, 16/8 h light/dark, 400 lmol photons m 2·s 1) and
further cultivated to the respective development stages.
The following development stages were investigated: dry
seeds (G0, after field collection), imbibed seeds (G1, wet
seeds), germinated seeds (G2, emerged radicle at least 2mm long), seedlings (G3S, cotyledons completely
extended), plantlets (G3P, two leaves completely
extended, no roots), juveniles (G4, first pair of true leaves
completely extended), and adults (A, more than two
leaves). To test frost-hardening capacity, seedlings were
cultivated at the research area on a flat site at the 1971
moraine (pioneer stage) and then the frost resistance
experiments were performed in the laboratory. The seeds
were sown into the substrate in autumn 2008 in order to
In the laboratory, frost resistance of seeds, seedlings,
plantlets, juveniles and adult plants was determined by
exposing samples in a temperature-controlled, convective freezing chamber. In the freezing chamber, temperatures were lowered at a moderate cooling rate of
2 K·h 1. Target temperatures identical for all the species
and ranging from 0 to 10 °C in 2 °C steps, were maintained for 4 h, i.e. a frost treatment protocol generally
used in frost resistance studies (Sakai & Larcher 1987).
The target temperatures were identical for all the species
and were selected after preparatory experiments on the
experimental species. The highest temperature was chosen to cause no frost damage and the lowest to cause
100% frost damage to the sample. Thawing took place
at a rate of 4 K·h 1. Sample temperature was recorded
with fine-wire thermocouples connected to a data logger
(CR10X; Campbell Scientific). Thermocouple sensors
were placed at the surface of the analysed organs and
attached with a special adhesive (Transpore; BM-Austria
GmbH, Vienna, Austria). After a latency period of 5 d,
viability of the samples as a whole and of single organs
was tested with the topographic tetrazolium (TTC) test
(Ruf & Brunner 2003; Larcher et al. 2010). Dehydrogenase activity reduces the colourless tetrazolium salt to
red-coloured triphenyl formazan; so that red-coloured
cells and organs can be rated as viable (Larcher 1969).
The percentage of frost damage to the samples was calculated with image analysis software (OPTIMAS) and
then plotted against the treatment temperature. A classic
logistic function was fitted to the data with P-Fit software (Biosoft, Durham, NC, USA): Y = Min +
(Max Min)/(1 + e k(X X50)), where X is the input
variable (target temperature), Y is the output variable
(injury as a percentage), Min and Max are asymptotic
upper and lower limits of the curve (0%, 100%), X50 is
the input variable at the inflection point (Min Max)/2
and k is a slope factor. Values of X50 were read directly
from the fitted curve and used as a measure of frost
resistance [i.e. taken as LT50, the lethal temperature (°C)
for half of the samples]. For each development stage
and each species values of at least five measurements
were included.
Table 1. Occurrence of the 12 investigated species at three successional
stages (pioneer, early and late) deglaciated since 1971, 1923 and 1858,
respectively, and their seed appendages.
Species
Abbreviations
Successional
stage
Seed
appendage
Saxifraga aizoides
Artemisia genipi
Oxyria digyna
Trifolium pallescens
Persicaria vivipara
Poa alpina
Leontodon hispidus
Anthyllis vulneraria
subsp. alpicola
Achillea moschata
Erigeron uniflorus
Silene acaulis subsp.
exscapa
Trifolium badium
SAXAIZ
ARTGEN
OXYDIG
TRIPAL
PERVIV
POAALP
LEOHIS
ANTALP
Pioneer
Pioneer
Pioneer
Early
Early
Early
Late
Late
No
Pappus
Wings
No
No
No
Pappus
No
ACHMOS
ERIUNI
SILACA
Late
Late
Late
No
Pappus
No
TRIBAD
Late
No
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3
Does frost limit recruitment and establishment on a glacier foreland?
S. Marcante et al.
occurred at a low frequency of up to 1.3% of days during
the snow-free growing season (Table 2, Fig. 1). The 2008
growing season was particularly prone to freezing with
severe temperatures down to 5.6 °C at the soil surface,
mainly at the beginning of autumn. At a depth of 5 cm,
the mean minimum temperatures were much less
extreme ( 0.8 °C). During the 2008 growing season,
freezing temperatures occurred only at a frequency of 4%
of snow-free days. Air temperature records at 15 cm
above the ground showed a higher frequency of significant freezing temperatures than soil temperatures. During
the 2008 growing season almost 4% of the daily temperatures were below 2.5 °C.
Statistical analysis
The LT50 was used to compare frost resistance of the investigated development stages, species and successional
stages. Differences between species and development
stages, our first hypotheses, were tested with a generalized
linear mixed model after log transformation of the data.
Development stages were used as a fixed factor and species
as a random factor. To test our second hypothesis on the
effect of successional stages to the development stages;
another model was run with development stages as fixed
factor and the species categorized by successional stages as
random factor. The frost resistance of adult individuals
compared to the frost resistance of their seedlings and the
frost hardening capacity of seedlings were tested with the
Mann-Whitney U-test. Median, 25th and 75th percentile
and minimum and maximum values are shown in box
plots. Statistics were performed using STATISTICA 6.1
(Stat. Soft Inc., Tulsa, OK, USA).
Frost resistance along the primary succession
Significant differences (P < 0.001) in frost resistance (LT50)
between different development stages were observed
(Fig. 2a). Frost resistance (median) decreased from dry
seeds ( 19.8 °C) to imbibed seeds ( 8.0 °C). There was a
considerable variability in frost resistance within both dry
seeds (range: 22 to 8 °C, median: 19.8 °C) and
imbibed seeds (range: 25 to 2 °C, median: 7.8 °C).
During germination, frost resistance decreased further to
3.2 °C for G2 and to 2.4 °C for G3. Interspecific variability of frost resistance was still high in germinated seeds
Results
Microclimate
During the 2008–2010 growing seasons about 10% of daily
soil surface temperature minima were lower than 0 °C.
However, significant freezing temperatures below 2.5 °C
Table 2. Mean minimum temperatures (MIN) and standard deviations (SD) recorded during three growing seasons (1 June–1 Oct; 2008–2010) and in
autumn winter–spring (1 Oct–1 June; 2008–2010) at 15 cm above the soil surface (air; two sensors), on the soil surface (28 sensors) and in the soil at a depth
of 5 cm, and frequency (%) of the records lower than 0, 2.5 and 5 °C.
2010
Air
MIN all year, °C
MIN growing season, °C
MIN autumn–winter–spring, °C
Frequency < 0 °C growing season, %
Frequency < 2.5 °C growing season, %
Frequency < 5 °C growing season, %
Soil Surface
MIN all year, °C
MIN growing season, °C
MIN autumn–winter–spring, °C
Frequency < 0 °C growing season, %
Frequency < 2.5 °C growing season, %
Frequency < 5 °C growing season, %
Soil (5 cm)
MIN all year, °C
MIN growing season, °C
MIN autumn–winter–spring, °C
Frequency < 0 °C growing season, %
Frequency < 2.5 °C growing season, %
Frequency < 5 °C growing season, %
4
2009
2008
Mean
SD
Mean
SD
Mean
SD
2.8
2.8
1.2
17.3
0.1
0.0
0.1
0.1
0.02
9.3
2.9
4.9
11.8
0.1
0.0
2.4
0.1
1.3
7.6
7.6
3.6
12.6
3.7
0.5
1.1
1.1
0.6
2.9
2.9
0.9
10.7
0.1
0.03
2.1
2.1
0.7
5.4
1.2
2.9
5.6
5.3
2.8
5.9
1.3
0.2
2.5
2.9
1.2
0.8
0.2
0.8
9.9
0.0
0.0
7.2
2.3
3.6
8.3
0.05
0.0
2.8
0.2
1.6
10.3
0.0
0.0
2.8
0.8
2.04
3.8
0.1
0.0
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Does frost limit recruitment and establishment on a glacier foreland?
S. Marcante et al.
Fig. 1. Daily minimum temperatures over 2009 (Jan–Dec) collected at the pioneer stage of the Rotmoos glacier foreland (2400 m a.s.l.). Air temperatures
were collected at 15 cm above the soil surface. Soil temperatures were collected at the surface, and at depths of 5 and 10 cm.
Imbibed
seeds
Germinated
seeds
a
b
Seedlings
c
Juveniles
Adults
c
bc
LT50 (°C)
(a)
(b)
Germinated
seeds
Imbibed
seeds
a
b
a
b
a
Juveniles
Seedlings
a
b
b
a
b
Adults
c
a
b
a
LT50 (°C)
a
Developmental stages
Fig. 2. Frost resistance (LT50) of all investigated alpine species (a) and
categorized by successional stage (b). Determined are the following
development stages: imbibed seeds (n = 96), germinated seeds (n = 76),
seedlings (n = 97), juveniles (n = 72) and adult individuals (n = 55).
Different letters (a, b, c) represent multiple comparisons of P -values
performed with the GLMM (n = 396).
( 14.7 to 0.7 °C) but was low in seedlings and juveniles.
From G3 to G4 ( 2.5 °C) it remained unchanged. Multiple
comparisons showed a significant difference (P < 0.001)
between the frost resistance of dry and imbibed seeds (G0
and G1), germinated seeds (G2) and seedlings (G3). Adults
differed (P < 0.001) in their frost resistance from the dry
and imbibed seeds but not from germinated seeds (G2),
seedlings (G3) and juveniles (G4).
Along the primary succession (Fig. 2b), germinated
seeds and seedlings of pioneer species possessed a lower
frost resistance ( 3.3, 1.8 °C, respectively), than germinated seeds and seedlings of early-successional
species ( 7.3, 3.3 °C, respectively) and of late-successional species, even if not always significant ( 3.1,
2.6 °C, respectively). The juveniles of pioneer and
early-successional
species
differed
significantly
(P < 0.001). Frost resistance of adult individuals differed
significantly (P < 0.01) between pioneer, early- and
late-successional species ( 2.2,
4.8 and
3.6 °C,
respectively).
Significant differences in frost resistance during development among the investigated species were observed
(P < 0.001; Fig. 3a–e). Imbibed seeds (G1) were characterized by a high variability in frost resistance (Fig. 3a).
Among the pioneer species, Oxyria digyna seeds showed a
higher frost resistance than Saxifraga aizoides and Artemisia
genipi seeds. Imbibed seeds of the early-successional species
Trifolium pallescens showed an extremely high frost resistance (LT50: 25 °C). Among the late-successional species
Leontodon hispidus and Achillea moschata exhibited a
frost resistance lower than 20 °C, while other species
could only tolerate freezing temperatures between 4 and
8 °C.
With the onset of germination frost resistance decreased
rapidly (Fig. 3b). The highest frost resistance was found for
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Does frost limit recruitment and establishment on a glacier foreland?
S. Marcante et al.
(a)
(b)
(c)
(d)
(e)
Fig. 3. Frost resistance (LT50) of the investigated species sorted by development stages (a–e). Boxes indicate the range of LT50 lethal temperatures ( °C) of
half of the samples. Within development stages, differences between species were tested with GLMM (P < 0.001). Different letters (a, b, abc and c)
represent multiple comparisons of P-values. Species abbreviations follow Table 1. Insufficient replicates caused the missing values of species within the
juveniles and adult individuals.
T. pallescens (LT50: 13 °C), the lowest for O. digyna (LT50:
0.7 ° C). With the elongation of hypocotyls and cotyledons, the frost resistance further decreased (Fig. 3c,d).
Seedlings (G3) and juveniles (G4) showed a similar trend:
the frost resistance ranged from 5.6 to 0.7 °C for G3
and from 3.7 to 1.5 °C for G4. The most frost-resistant
species in G3 and G4 was P. vivipara.
6
Frost resistance of different organs (root, hypocotyl
and cotyledons) was assessed at the seedling stage. Roots
were generally the most frost susceptible parts, followed
by cotyledons, first leaves and the hypocotyl (data not
shown).
Frost resistance of adult plants was compared with that
of seedlings grown in the field (Fig. 4). Taking all species
Journal of Vegetation Science
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S. Marcante et al.
Fig. 4. Frost resistance (LT50) comparison between adult individuals and
seedlings grown in the field (G3 field). Differences are among species and
between development stages (n.s., not significant; *P < 0.05; **P < 0.01;
***P < 0.001). Species abbreviations follow Table 1.
together, no significant difference was detected [P > 0.05,
KW-H (1, 101) = 0.4]. However, some species-specific differences between adult individuals and seedlings were
observed (Fig 4). Adult individuals of A. genipi and T. pallescens showed a significantly higher frost resistance than
seedlings. In contrast, adult individuals of the pioneer species S. aizoides and O. digyna had a lower frost resistance
than their seedlings. There was little difference in frost
resistance between adults and seedlings of the late-successional species L. hispidus and Anthyllis alpicola. The frost
resistance of plantlets of P. alpina was similar to that of
adult individuals ( 10.3 ± 0.2 and 9.8 ± 2.7 °C, respectively), whereas seedlings showed a significantly
(P < 0.001) lower frost resistance ( 2.4 ± 0.5 °C).
Frost resistance between seedlings grown in the field
and growth chamber varied markedly across species
(Fig. 5). In some pioneer species the frost resistance was
higher when grown in the field, even if not always significant (S. aizoides P < 0.05; O. digyna P < 0.05; A. genipi
P > 0.05). In contrast, the frost resistance of the earlysuccessional species T. pallescens and of the latesuccessional species L. hispidus (P < 0.01) and A. alpicola
(P = 0.05) was higher when grown in the growth chamber. The seedlings of P. alpina and P. vivipara were not
affected by the environmental conditions during cultivation (P > 0.05).
Discussion
Establishment and growth of alpine plants along the primary successions in glacier forelands have frequently been
shown to be under abiotic control (Bliss 1971; Körner
2003; Dolezal et al. 2008). Our results demonstrate that
Does frost limit recruitment and establishment on a glacier foreland?
Fig. 5. Comparison between frost resistance (LT50) of seedlings grown in
the growth chamber (G3) and seedlings grown in the field (G3 field).
Differences are among species and between development stages. (n.s.,
not significant; *P < 0.05; **P < 0.01; ***P < 0.001). Species
abbreviations follow Table 1.
freezing temperatures during the growing season are only
sporadic events in the alpine glacier foreland, but they are
potentially severe enough to kill early development stages.
This can significantly affect the recruitment and speed of
establishment of species, as well as their ability to maintain
populations.
Each of the early development stages investigated was
affected by freezing temperatures in a different manner.
Dehydrated seeds were not endangered during the growing season because their frost resistance (LT50 – 20 °C) is
sufficient to survive the lowest freezing temperatures that
occurred at the research site. Although air temperatures
can drop below 20 °C, soil is usually covered by snow,
which moderates temperatures significantly (Körner et al.
2003; Fig. 1). Absolute soil temperature minima at the
research area were between 4.9 and 1.0 °C during the
study years 2008–2010 (data shown only for 2009).
Liquid water content of snowpack in the Central European Alps is mostly highest at the soil surface throughout
winter and steadily increases until snowmelt (Kuhn 2012).
In the glacier foreland, the water from snowmelt imbibes
seeds when they are still covered by snow at the beginning
of June (Niederfriniger Schlag & Erschbamer 2000), thus
seed imbibition and seed germination start when freezing
temperatures are still a possibility. The dehydrated seeds
are stored either in transient seed banks (0–5-cm deep;
Marcante et al. 2009a), which could still be affected by
freezing temperatures at the beginning of the growing season, or in permanent seed banks (5–10-cm deep) where
freezing events should be prevented unless unpredictable
frost events occur before a snow cover develops (Körner
et al. 2003). Our results demonstrate that imbibed seeds
(LT50: 8 °C) may still be relatively unaffected by the
Journal of Vegetation Science
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7
Does frost limit recruitment and establishment on a glacier foreland?
freezing temperatures occurring in the field. In contrast,
germinated seeds are highly frost susceptible and frost
damage may still occur in transient seed banks. However,
at our research area, numerous seedlings can be counted
in the field after snowmelt but, in most cases, up to 90% of
these seedlings subsequently die (Niederfriniger Schlag &
Erschbamer 2000; Marcante et al. 2009b). Our results
indicate that this high seedling mortality, also found elsewhere in alpine plant communities (Chambers 1995; Kiviniemi & Eriksson 1999; Forbis 2003), could be caused by
frost damage and especially by night freezing with temperature minima lower than frost resistance of these development stages. The high frost susceptibility of these stages
may be connected to the onset of active metabolism that
results in tissue growth processes leading to the appearance
of the radicle. Growing tissues as also found in sprouting
shoots (Taschler et al. 2004), in roots (Sakai & Larcher
1987) or during flower development (Hacker et al. 2011)
and are generally known to be the most frost susceptible
plant parts, with only limited frost hardening potential.
The primary succession along the glacier foreland is
characterized by a shift in species composition, where pioneers are replaced by early-successional colonizers, which
are followed by species of late-successional stages (Matthews & Whittaker 1987). As pioneers have to cope with
stronger constraints during establishment than late-successional species and that colonization on bare ground areas is
also limited by lack of safe sites, we expected pioneer
species to be more frost resistant than early- and latesuccessional species. The results only partially supported
this hypothesis, e.g. dry and imbibed seeds of O. digyna
were among the most frost resistant, whereas germinated
seeds and seedlings were highly susceptible to freezing
temperatures. Even if differences between the species
groups remained relatively weak, pioneer species more
often proved to have a lower frost resistance than earlyand late-successional species. As Caccianiga et al. (2006)
demonstrated, several pioneer glacier foreland species
share ruderal traits, although Grime (2001) suggested that
primary successions should be dominated by stress-tolerant
species. Our results regarding the lower frost resistance of
the pioneer species seem to confirm that these species are
adapted to disturbance and less so to abiotic stress. Along
the primary succession, some differences in frost resistance
among species could be explained with seed morphological
traits. Seed mass and seed shape, tested against seed frost
resistance in previous experiments (unpublished data), did
not clarify our results; however, some outcomes could be
explained by the presence or absence of seed appendages.
O. digyna has flattened and winged seeds. These appendages ensure that the seeds remain at the soil surface and
have to survive freezing temperatures in autumn and winter. Within the early-successional species, the dry and
8
S. Marcante et al.
imbibed seeds of T. pallescens exhibit high frost resistance.
Seeds of this species can frequently be found unburied on
the soil surface after snowmelt, making high frost resistance essential for survival. Among the late-successional
species, dry and imbibed seeds of L. hispidus were also
among the most frost resistant. The seeds of L. hispidus
carry an elongated pappus that also prevents burial of seeds
and thus they remain exposed to temperature extremes
(Peart 1984).
The intraspecific differences in frost resistance between
adults and seedlings were very divergent in the Rotmoos
glacier foreland. The seedlings of the most important pioneer species in the study site, S. aizoides, exhibited a higher
significant frost resistance in the field than their adult individuals. In contrast, adult individuals of the pioneer species
A. genipi and of the early-successional species T. pallescens
and P. alpina showed a significantly higher frost resistance
than their seedlings, which supports earlier results on tree
seedlings (Larcher 1969; Meza-Basso et al. 1986; Rios
et al. 1988; Neuner et al. 1997). In the late-successional
species group, seedlings in the field were less frost resistant
than adults.
Our results illustrated that early development stages of
P. alpina, growing and establishing all across the glacier
foreland (Raffl & Erschbamer 2004), do not possess a significant frost hardening capacity, even if the adult individuals are highly frost resistant (Taschler & Neuner 2004):
the leaves of this species freeze independently of each
other due to anatomical ice barriers in their shoots, some
leaves can even supercool down to 11.6 °C (Hacker et al.
2008). As an example, we compared the response to frost
events of the sexual and asexual offspring of P. alpina. At
the pioneer stage, this species propagates mainly through
plantlets, successfully enhancing its establishment (Marcante et al. 2009b; Winkler et al. 2010). This may be due to
the significantly higher frost resistance of the plantlets than
the seedlings. The lower frost resistance of seedlings could
hinder successful establishment in the pioneer stage and
lead to enforced asexual reproduction, as found in other
alpine species (Lee & Harmer 1980; Stöcklin 1992; Pluess &
Stöcklin 2005; Weppler et al. 2006). Despite the apparent
reliance on clonal regeneration of some alpine plants,
there is evidence that many alpine species produce hundreds of viable seeds (Sayers & Ward 1966; Chambers et al.
1987; S. Marcante unpublished data) that contribute to
extensive soil seed banks (Freedman et al. 1982; Cooper
et al. 2004; Marcante et al. 2009a). Most of them are able
to germinate easily (Kibe & Masuzava 1994; Forbis 2003;
Welling et al. 2005), ensuring an increase in plant population size (Marcante et al. 2009b) and maintaining a high
genetic diversity (Raffl et al. 2006b, 2008). The high frost
susceptibility of the early development stages implies that
establishment will depend on microsites protected from
Journal of Vegetation Science
Doi: 10.1111/j.1654-1103.2012.01411.x © 2012 International Association for Vegetation Science
S. Marcante et al.
low temperature extremes (Harper et al. 1961; Matthews
& Whittaker 1987; Svoboda & Henry 1987; Whittaker
1991; Frenot et al. 1998; Jumpponen et al. 1999). Concave soil surfaces, coarse substrate and small stones provide
favourable microclimate conditions, avoiding temperature
extremes and desiccation. Temperature extremes moderated by only 1–2 °C would already be sufficient for seeds
and seedlings to survive in our research area. In this initial
stage of primary succession seedlings with insufficient frost
hardening capacity rely on suitable microsites to resist climate extremes.
Conclusions
In alpine areas freezing temperatures have always been
considered to be one of the main abiotic factors, besides
drought and heat, that contribute to high seedling mortality rates (Billings & Mooney 1968; Bell & Bliss 1980; Chapin & Bliss 1989; Niederfriniger Schlag & Erschbamer
2000; Forbis 2003; Körner 2003). Our results show that at
the glacier foreland of the Rotmoos Valley, due to its relatively low elevation, frost events critical for survival are
relatively rare in summer and may not fully explain the
high seedling mortality (Marcante et al. 2009b) in all
years. A better understanding of the susceptibility of seeds
and seedlings to critical soil surface heating and frequent
drought in the undeveloped soils will further help to
explain mortality rates at the pioneer stage. Freezing temperatures during the growing season can cause frost damage in early development stages of alpine plants. They are,
in contrast to adult plants, unable to recuperate via
undamaged below-ground organs. Nevertheless, adult
plants showed some frost hardening capacity. Nevertheless, the risk of frost damage to early development stages
remains a potential threat that can retard plant colonization in the glacier foreland.
Acknowledgements
We thank Othmar Buchner for constructing the microclimate station in the field and for assistance in data collection, Jürgen Hacker for the laboratory assistance during
the experiments and Erich Schwienbacher for field assistance. We are particularly grateful to four anonymous
reviewers and the Co-ordinating Editor Francesco de Bello
for many constructive comments on previous versions of
the manuscript. This project was financed by the Austrian
Science Fund (FWF 19090-B16).
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