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Effects of seed size and habitat on
recruitment patterns
in grassland and forest plants
Karin Lönnberg
©Karin Lönnberg, Stockholm University 2012
ISBN 978-91-7447-599-9
Printed in Sweden by US-AB, Stockholm 2012
Distributor: Department of Botany, Stockholm University
Till Ida och Gustav
Doctoral dissertation
Karin Lönnberg
Department of Botany
Stockholm University
SE-106 91 Stockholm
Sweden
Effects of seed size and habitat on recruitment patterns
Abstract – A trade-off between seed size and seed number is central in seed ecology,
and has been suggested to be related to a trade-off between competition and
colonization, as well as to a trade-off between stress tolerance and fecundity. Large
seeds endure hazards during establishment, such as shading, drought, litter coverage and
competition from other plants, better than do small seeds, due to a larger amount of
stored resources in the seed. Small seeds, however, are numerous and small-seeded
species are therefore more fecund. Moreover, a pattern with small-seeded species being
associated with open habitats and large-seeded species being associated with closed
habitats has been reported in the literature. In this thesis I assess effects of seed size on
recruitment, and how relationships between seed size and recruitment may relate to
habitat conditions. Seed sowing experiments were performed in the field to assess interand intra-specific relationships between seed size and recruitment in open and closed
habitats (Paper I and II). Seed removal experiments were performed in the field to
assess what effects seed predation may have on a relationship between seed size and
recruitment (Paper III). A garden experiment was performed based on contests between
larger-seeded and smaller-seeded species, in order to examine different models on coexistence of multiple seed size strategies. The results showed that there was a weak
positive relationship between seed size and recruitment in the field, and that this
relationship was only weakly and inconclusively related to habitat (Paper I and II). Seed
removal was negatively related to seed size in closed habitats and unrelated to seed size
in open habitats (Paper III). This indicates that any positive relationship between seed
size and recruitment may be an effect of higher seed removal in small-seeded species.
However, when grown under controlled conditions in a garden experiment, there was a
clear advantage of larger-seeded species over smaller-seeded species (Paper IV). This
advantage was unaffected by seed density, indicating that there was no competitive
advantage of the larger-seeded species. Instead, indirect evidence suggests that largerseeded species exhibit higher tolerance to stress.
Keywords – Seed size, seedling establishment, seedling survival, recruitment, seed
removal, co-existence, game-theory
List of papers
This thesis is based on the following papers, which are referred to by
their roman numerals:
I.
Lönnberg K, Eriksson O (2012) Seed size and recruitment
patterns in a gradient from grassland to forest. Ecoscience 19:
140-147.
II.
Lönnberg K, Eriksson O. Relationships between intra-specific
variation in seed size and recruitment in four species in two
contrasting habitats. Plant Biology, doi: 10.1111/j.14388677.2012.00676.x
III.
Lönnberg K. Seed removal in relation to seed size in two
contrasting habitats. Manuscript.
IV.
Lönnberg K, Eriksson O. Rules of the seed size game:
contests between large-seeded and small-seeded species.
Accepted for publication in Oikos, doi: 10.1111/j.16000706.2012.00246.x
Contents
Introduction ....................................................................................................... 11
Background .................................................................................................... 11
Co-existence of multiple seed sizes ............................................................... 12
Study system and study area ......................................................................... 14
Aim..................................................................................................................... 15
Methods ............................................................................................................ 15
Results and discussion ....................................................................................... 17
Concluding remarks ........................................................................................... 21
Acknowledgements ........................................................................................... 21
References ......................................................................................................... 21
Svensk sammanfattning .................................................................................... 26
Tack! .................................................................................................................. 31
10
Introduction
Background
Parents invest a lot of resources in their offspring. In plants, the seed is
an important part of the plant life cycle, and seed size has been shown to
affect, for example, seed predation (Gómez 2004; Pérez-Ramos et al.
2008; Gómez et al. 2008), longevity in soil (Thompson et al. 1993; Moles
et al. 2000), germination (Gómez 2004; Moles & Westoby 2004a;
Urbieta et al. 2008) and seedling survival (Paz & Martínez-Ramos 2003;
Moles & Westoby 2004b; Baraloto et al. 2005). Among plants there is a
huge variation in seed size stretching over ten orders of magnitude, from
the dust-like seeds of orchids and some saprophytic and parasitic plants
weighing about 1µg, to the double coconut weighing up to 20 kg (Harper
et al. 1970). Furthermore, among plants within a plant community there
is a variation in seed size typically stretching over five to six orders of
magnitude (Leishman et al. 2000; Westoby et al. 2002).
A central part of seed ecology is the trade-off between seed size and seed
number, which is based on a model of an offspring size – number tradeoff (Smith & Fretwell 1974). This model states that from a given amount
of resources allocated to seed production a plant can either produce a
large number of small seeds, or a small number of large seeds. Large
seeds are well provisioned for and have a high recruitment probability,
whereas producing numerous (small) seeds enhances dispersal and gives
a high fecundity.
Seedlings germinating from large seeds are larger, and can endure
hazards during establishment better than seedling germinating from small
seeds, due to a larger amount of resources stored in the seed. For
instance, large-seeded species have been shown to have a higher survival
at the seedling stage than small-seeded species through events of drought,
herbivory, litter coverage, shade and competition from established
vegetation (Gross & Werner 1982; Gross 1984; Leishman et al. 2000;
Westoby et al. 2002; Moles & Westoby 2004a). This would indicate that
large seeds are advantageous in certain habitats, such as in stressful
environments or in closed-canopy habitats with thick litter layers and
where shading is massive. However, producing a few large seeds may
involve costs for the plant. Producing a few large seeds, rather than
11
numerous small seeds, reduces fecundity and dispersability. Moreover,
based on theory on optimal foraging, large seeds constitute a more
desirable food source for seed predators, such as small rodents (Charnov
1976; Hoffmann et al. 1995; Hulme 1997; Blate et al. 1998; Hulme
1998). Large seeds are also more conspicuous, and take longer time to
become incorporated into the soil, and therefore run a higher risk of
being encountered by seed predators (Moles et al. 2003). Hence,
producing large seeds would only be evolutionary stable under hazardous
conditions, such as stressful or shady habitats, where the costs of having
large seeds are balanced by the advantages (Leishman et al. 2000). In an
open habitat, like a grassland, where there is less shading and
competition from other plants is reduced through disturbance, the costs of
having large seeds may not be balanced by the advantages (Eriksson &
Jakobsson 1999). Producing numerous small seeds and thereby
increasing fecundity and dispersability may instead be more
advantageous, as producing numerous seeds increases the probability that
at least some seeds find unoccupied and suitable microsites. Thus,
species filtering and evolutionary processes favoring large-seeded species
in closed and stressful habitats, and small-seeded species in open habitats
are expected. Patterns with small-seeded species being associated with
open habitats early in the succession sere, and large-seeded species being
associated with closed, late succession habitats have indeed been found ,
and average seed size is generally higher in closed habitats than in open
habitats (Baker 1972; Leishman et al. 2000; Bolmgren & Eriksson 2005).
Co-existence of multiple seed sizes
According to the model by Smith & Fretwell (1974), for each set of
environmental conditions, there is an optimal seed size that is reached
when offspring fitness is exactly balanced by energy invested per seed by
the mother plant. This model states that any increase in seed size would
increase offspring fitness, but this gain would not compensate the loss in
fecundity, resulting in a total decrease in fitness for the mother plant.
Any decrease in seed size would increase the fecundity of the mother
plant, but would at the same time reduce offspring fitness, also resulting
in a total decrease in mother plant fitness. Thus, any increase or decrease
in seed size would be selected against. However, in nature seed sizes
typically vary over five to six orders of magnitude among plants within
12
plant communities (Leishman et al. 2000; Westoby et al. 2002).
Moreover, within plant species or even within plant individuals there is
often a large variation in seed size (Obeso 1993; Vaughton & Ramsey
1998; Leishman et al. 2000), and little support for an optimal seed size
has actually been found. This co-existence of multiple seed sizes has
been puzzling, and density dependent game-theory has attempted to
explain co-existence of multiple seed size strategies within plant
communities. In this kind of models, developed by Geritz (1995) and
Rees & Westoby (1997), asymmetric competition is assumed, such that
larger seeds will always win over smaller seeds in direct competition
over safe sites, leading to a competitive advantage in large seeds, but a
colonization advantage in small seeds. Thus, large-seeded species can
always invade any plant community, due to a competitive advantage.
However, due to their higher abundance, small-seeded species can
always invade any plant community because they will have the highest
probability to be the first to find safe sites unoccupied by any larger
seeds. For this model to work a complete asymmetry of competitive
advantage is essential, such that even a minute difference in seed size
would mean a complete advantage of the larger-seeded species, outcompeting the smaller-seeded species in every single case. Empirical
studies, however, have found that the competitive advantage of largerseeded species is not completely asymmetric, and larger-seeded species
do not always win over smaller-seeded species in direct competition over
safe sites (Eriksson 2005). Moreover, in a study on seed size and species
abundance, no relationship was found between seed size and abundance,
indicating that small seeds are not generally more abundant than large
seeds (Eriksson & Jakobsson 1998). Recently, a model by Muller-Landau
(2010) has attempted to explain the co-existence of multiple seed size
strategies, suggesting a seed-size game where the seed size – number
trade-off is related to a trade-off between stress tolerance and fecundity,
rather than a trade-off between competition and colonization. In this
model large seeds are assumed to tolerate stress during establishment
better than small seeds. Therefore, in stressful environments largerseeded species have an advantage over smaller-seeded species, whereas
smaller-seeded species have an advantage in less stressful environments
due to their higher fecundity.
13
Study system and study area
In this thesis I have assessed seed size and habitat effects on
recruitment patterns, working with a study system consisting of both a
seed size gradient and a habitat gradient. As all experiments were
conducted in south-eastern Sweden, plant species and habitats common
in the area were used. The habitat gradient is a conceptual gradient
stretching from an open disturbance regime to a closed competition
regime. For the field experiments I selected a gradient from grassland to
forest to represent this conceptual gradient, using both a gradient
represented by six habitats from heavily grazed grassland to coniferous
forest (Paper I), and two contrasting habitats – grassland and deciduous
forest (Paper II and Paper III). For the garden experiment (Paper IV)
different treatments of litter and shading were used to mimic conditions
expected to have high influence on recruitment in open and closed
habitats. The seed size gradient consisted of seeds of plant species
selected to represent a variation in seed size among species (Paper I, III
and IV) from small-seeded to large-seeded, as well as within species
(Paper II). All selected plant species are common in the area, which was
essential in order to make collections of large numbers of seeds possible.
Some of the selected species mainly inhabit open habitats, whereas others
are mainly found in closed habitats, although all selected species are
habitat generalists, and occur in both open and closed habitats. Care was
also taken to select species from different plant genera, and as far as
possible also from different plant families, in order to avoid
phylogenetically biased sampling.
The field experiments were conducted in Tullgarn (Paper I
and II) and Angarn (Paper III). The Tullgarn area is a nature reserve
situated by the Baltic Sea, about 45 km southeast of Stockholm (WGS 84
lat, lon: 58.95045, 17.58410). The area has a long history of management
and consists of a variety of habitats, from open and semi-open grasslands
mainly grazed by cattle, to deciduous and coniferous forests. The Angarn
area, too, is a nature reserve, about 25 km north of Stockholm (WGS 84
lat, lon: 59.54270, 18.14809). The reserve is dominated by a bird lake,
surrounded by a mosaic of grassland grazed by cattle and horses, arable
fields and mixed deciduous and coniferous forests.
The garden experiment (Paper III) was conducted in a garden
at the Department of Botany, Stockholm University.
14
Aim
The objectives of this thesis were to assess relationships between seed
size, recruitment and habitat. This was done by addressing the following
questions:
Is there a relationship between seed size and recruitment at the interspecific level (Paper I) and intra-specific level (Paper II)? If so, does this
relationship shift as the habitat shifts from open to closed (Paper I+II)? Is
there evidence for species filtering processes (Paper I) or selection (Paper
II) favoring large-seeded species in closed habitats and small-seeded
species in open habitats? Is there a relationship between seed size and
seed predation, and what implications may that have in recruitment
patterns (Paper III)? Do larger-seeded species generally win over
smaller-seeded species in direct contest over safe sites, and does any seed
size advantage increase in high competition or stressful conditions (Paper
IV)?
Methods
To assess seed size effects on recruitment patterns in relation to different
plant community conditions I have performed seed sowing experiments
in the field (paper I + II) and in a common garden (paper IV), as well as a
seed removal experiment in the field (paper III). Twenty-four plant
species, from fifteen plant families, were initially selected for the two
seed sowing experiments in the field (paper I + II). The plant species
were selected because (i) they all occur in both open and closed habitats
in the study area, (ii) there is a large variation in seed size among, and in
some cases also within, the species, and (iii) all species are common in
the study area which made collection of large numbers of seeds possible.
Fourteen of the species were later also used in the common garden
15
experiment (paper IV) and ten of the species were used in the seed
removal experiment (paper III).
Paper I
To study the effects of seed size and habitat on recruitment, I selected six
vegetation types to represent a gradient from grassland to forest. Seeds of
20 plant species, with a 356-fold inter-specific variation in seed size,
were sown in six sites of each vegetation type. Seed sowings were
performed in two consecutive years, 2005 and 2006. Seedling emergence
and seedling survival was recorded in the beginning and end of each
growing season, for up to three years.
Paper II
To assess intra-specific relationships between seed size and recruitment
in different habitats, I weighed seeds of four plant species (Convallaria
majalis, Frangula alnus, Prunus padus and P. spinosa) individually,
3200 seeds per species. The seeds were sown one by one, while carefully
keeping track of each single individual, in two contrasting habitats,
grassland and deciduous forest. Seed sowings were performed in two
consecutive years, 2005 and 2006. The fate of each individual seed, in
terms of seedling emergence and seedling survival, was followed for up
to three years.
Paper III
To assess whether seed predation may play a role in any relationships
between seed size and recruitment, I conducted seed removal
experiments where seed removal was used as a measure of seed
predation. Seeds of ten plant species varying 1452-fold in seed size were
placed one by one in field sites representing two contrasting habitats,
grassland and deciduous forest. After 48 hours seed removal was
recorded. The set up was repeated seven times in each of ten sites.
16
Paper IV
To test whether larger-seeded species have an advantage over smallerseeded species in direct contests, and if such an advantage increases in
conditions with shade and/or litter, I conducted a garden experiment
where seeds of 14 plants species were sown pair-wise in pots. Each pair
consisted of one larger-seeded and one smaller-seeded species. The seeds
were grown in six different treatments of seed density, shade and litter,
and seedling emergence and seedling survival for both species of all
species pairs were followed for two years.
Results and discussion
In Paper I, I found that generally there was a weak positive relationship
between seed size and recruitment among plant species, and that this
relationship was only weakly affected by habitat. Seed sowings were
performed in two consecutive years, and the recruitment differed
between years. Among the six habitats selected to represent a gradient
from open to closed habitat, there was no relationship between seed size
and recruitment in the two most open grassland habitats, whereas positive
relationships were found in the four most closed habitats, for seeds sown
in 2005. For seeds sown in 2006 there was a positive relationship
between seed size and recruitment only in one habitat – poorly grazed
grassland, the second most open habitat, whereas no relationship was
found for any of the other five habitats. In Paper II, I found that within
species there were positive relationships between seed size and
emergence, and between seed size and recruitment. Here, too, seedling
emergence and final recruitment differed between years, indicating that
seeds sown in 2006 met harsher conditions during establishment than
seeds sown in 2005. Only weak effects of habitat were found; within one
species (Convallaria majalis) final recruitment was positively related to
seed size in the closed habitat, but not in the open habitat, whereas within
another species (Prunus spinosa) final recruitment was positively related
to seed size in the open habitat, but not in the closed habitat. Within the
other two species (Frangula alnus and Prunus padus) any relationships
17
between seed size and recruitment were unrelated to habitat. The results
from these two studies show a coherent pattern between inter-specific
and intra-specific recruitment patterns in the study system. Both at the
inter-specific and at the intra-specific level there was a positive effect of
seed size on recruitment, and habitat effects on this relationship were
weak. This indicates that factors other than those associated with
habitats, such as variation in abiotic factors between years, may have a
large effect on recruitment. A species filtering process, favoring smallseeded species in open habitats and large-seeded species in closed
habitats is suggested, but that such a filter only is active during certain
conditions (Paper I). No clear evidence of a selection pressure favoring
small seeds in open habitats and large seeds in closed habitats was found,
which suggests that opposing selection pressures may instead have a
stabilizing effect on seed size (Paper II).
From these two papers I suggest the following mechanism to explain
recruitment patterns in a gradient from grassland to forest habitats: 1)
When seeds are subjected to minor hazards during establishment, i.e.
during favorable years and in open habitats, all seeds recruit equally well,
resulting in seed size having no effect on recruitment. 2) When seeds are
subjected to major hazards, i.e. during harsh years in closed habitats, all
seeds recruit equally poorly and seed size, again, has no effect on
recruitment. 3) When subjected to intermediate hazards during
establishment, i.e. during favorable years in closed habitats, or during
harsh years in open habitats, seed size has a positive effect on
recruitment. This is consistent with the model by Muller-Landau (2010),
suggesting that a wide range of seed sizes manage to recruit in low stress
environments, whereas only large seeds manage to recruit in high stress
environments. Seeds establishing in a grassland during a favorable year
thus meet low stress conditions, resulting in all seed sizes recruiting
equally well. In Paper I an equal number (50) of seeds were sown per
plot, and therefore no difference in recruitment between seed sizes was
found. At the other extreme, seeds establishing in a forest during a harsh
year meet multiple hazards (e.g. shading, litter, competition and drought).
In such multiple hazard conditions, the environment may be too stressful
for successful recruitment to occur in any seed size, again resulting in no
difference in recruitment between seed sizes. However, seeds
establishing in a grassland during a harsh year, or in a forest during a
favorable year meet intermediate stress conditions. In these conditions
only seeds larger than a certain size manage to recruit, resulting in a
18
positive relationship between seed size and recruitment. To fully
understand this recruitment pattern, further detailed studies to assess
under which conditions seed size has a positive effect on recruitment, and
under which conditions recruitment is unrelated to seed size are required.
The results from Paper III showed that seed removal was much higher in
closed habitats than in open habitats, in most species as much as twice as
high. This was expected as closed habitats probably provide more safe
sites for seed predators, and is consistent with results from previous
studies (Mittlebach & Gross 1984; Reader 1991; Osunkoya 1994; Hau
1997). Based on the theory on optimal foraging I also expected to find a
positive relationship between seed size and seed removal. Quite contrary,
however, I found a negative effect of seed size on seed removal in closed
habitat and no relationship in open habitat. Most previous studies have
typically found a positive relationship between seed size and seed
predation (Reader 1993; Hoffmann et al. 1995; Hulme 1998; Gómez
2004; Pérez-Ramos 2008), or that factors other than seed size control
seed predation rates (Mittlebach & Gross 1984; Hau 1997; Holl & Lulow
1997; Meiners & Stiles 1997; Kollmann et al. 1998). A few studies have,
in coherence with my study, found negative relationships between seed
size and seed predation (Osunkoya 1994; Blate et al. 1998; Moles et al.
2003). In some cases a negative relationship is explained by that larger
seeds have harder seeds coats, and therefore smaller seeds are preferred
by seed predators (Osunkoya 1994). This explanation is unlikely in my
study, as the largest seeds within this study were cherry kernels, known
to be a desirable food source to small rodents (personal observation). My
study assessed a rather wide range of seed sizes, from 0.14 mg – 203 mg.
In a previous study where a similarly wide range of seed sizes were
assessed, a negative relationship between seed size and seed removal was
also found (Moles et al. 2003), and explained by different guilds of seed
predators feeding on different seed sizes. This explanation seems likely
in my study, given that different guilds of seed predators have different
habits and abundance. Another explanation may be that smaller seeds are
removed through secondary dispersal, rather than by seed predators.
The results from Paper III may offer some explanation to the positive
relationship between seed size and recruitment found in Paper I and II. If
smaller seeds are removed at a higher rate than larger seeds, this may
explain why recruitment is lower in smaller seeds. The result that there
was no relationship between seed size and seed removal in open habitats
19
is coherent with results from Paper I, showing no relationship between
seed size and recruitment in open habitats. If all seed sizes are removed
at the same rate, all seed would be expected to have the same probability
of recruitment, given that seed size is unrelated to recruitment. The result
that there was a negative relationship between seed size and seed removal
in closed habitat is also coherent with results from Paper I, showing a
positive relationship between seed size and recruitment in closed
habitats. If small seeds are removed at a higher rate, a lower probability
of recruitment in small seeds would be expected, resulting in a positive
relationship between seed size and recruitment, even if seed size was
unrelated to recruitment.
The results from Paper IV showed that larger-seeded species generally
had an advantage over smaller-seeded species in direct contests for safe
sites. This advantage increased in strength in treatments with litter,
compared to when not subjected to litter, whereas shading and seed
density had no effect on the strength of the advantage. My study provides
no direct evidence as to whether this advantage of larger-seeded species
is a result of higher competitive ability or tolerance to stress. However,
indirect support for a model based on a trade-off between tolerance to
stress and fecundity is given, as seed density had no effect on the strength
of the advantage of the larger-seeded species, which would be expected
in a competition – colonization driven mechanism.
In Paper I and II only weak effects of seed size on recruitment was found,
whereas in Paper IV a clear advantage of larger-seeded species was
found. This incoherence may be resulted from confounding factors in the
field obscuring any relationship between seed size and recruitment. In the
garden experiment these confounding factors were eliminated, with a
clearer signal as a result. In the field it is often difficult to isolate
different factors from each other, and what effects they may have. The
results from Paper IV indicate that negative effects on recruitment in
closed habitats may be an effect of litter (among others), but that the
effect of shading is minor.
20
Concluding remarks
The general conclusions of this thesis are that seed size has a positive
effect on recruitment. This positive effect is weak in field conditions, but
stronger when confounding effects in the field are controlled for in a
garden experiment. Habitat effects on the positive relationship between
seed size and recruitment are weak and inconclusive, and only occur
under certain conditions. Other factors, such as differences in abiotic
factors between years, may have a higher influence on any relationship
between seed size and recruitment. However, when isolated from
confounding factors in the field, there is a clear effect of litter, increasing
the advantage of larger-seeded species. Any positive relationship
between seed size and recruitment may, however, be an effect of higher
seed removal in small-seeded species within the study area. There is no
evidence of a competitive advantage in larger-seeded species, but rather
indirect evidence is found for a better ability to tolerate stress in largerseeded species.
Acknowledgments
I am grateful to Ove Eriksson, Kjell Bolmgren and Ellen Schagerström
for valuable comments on this thesis.
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ecological consequences of seed size. Plant cover and seeddrop timing. Oikos 117: 1386-1396.
Reader R J (1991). Control of seedling emergence by ground cover: a
potential mechanism involving seed predation. Canadian
Journal of Botany 69: 2084-2087.
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Reader R J (1993). Control of seedling emergence by ground cover and
seed predation in relation to seed size for some old-field
species. Journal of Ecology 81: 169-175.
Rees M, Westoby M (1997). Game-theoretical evolution of seed mass in
multi-species ecological models. Oikos 78: 116-126.
Smith C C, Fretwell S D (1974). The optimal balance between size and
number of offspring. The American Naturalist 108: 499506.
Thompson K, Band S R, Hodgson J G (1993). Seed size and shape
predict persistence in soil. Functional Ecology 7: 236-241.
Urbieta I R, Pérez-Ramos I M, Zavala M A, Marañón T, Kobe R K
(2008). Soil mater content and emergence time control
seedling establishment on three co-occurring Mediterranean
oak species. Canadian Journal of Forest Research 38: 23822393.
Vaughton G, Ramsey M (1998). Sources and consequences of seed mass
variation in Banksia marginata (Proteaceae). Journal of
Ecology 86: 563-573.
Westoby M, Falster D S, Moles A T, Vesk P A, Wright I J (2002). Plant
ecological strategies: some leading dimensions of variation
between species. Annual Review of Ecology and
Systematics 33: 125-159.
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Svensk sammanfattning
Växter investerar stora resurser i reproduktion och frösättning, och
storleken på frön har effekter på, exempelvis, groning och etablering.
Från en given mängd resurser avsatta för fröproduktion, gör växter en
avvägning mellan att producera ett stort antal små frön eller att producera
ett litet antal stora frön. Stora frön innehåller en stor mängd lagrad
energi, vilket gör att groddplantor som gror från stora frön är stora och
kan klara av påfrestningar under etableringen väl. Exempelvis har man
funnit att storfröiga arter har högre överlevnad än småfröiga arter om de
utsätts för skuggning eller konkurrens från andra arter under
etableringsfasen. Därmed har stora frön hög sannolikhet att etableras.
Små frön, däremot, kan produceras i stort antal, och har därmed hög
sannolikhet att kunna spridas till platser som är fria från konkurrens från
andra växter.
Eftersom stora frön har högre sannolikhet för etablering än små frön,
särskilt under påfrestande förhållanden, kan man förvänta sig att stora
frön skulle ha en fördel över små frön i slutna habitat, t ex skog, där det
råder skuggiga förhållanden och ofta hård konkurrens mellan växter. Här
skulle alltså sorteringsprocesser eller selektionsprocesser kunna göra att
stora frön gynnas. Att producera stora frön innebär dock kostnader för
växter, eftersom då endast ett relativt litet antal frön kan produceras.
Dessutom löper stora frön ofta högre risk att bli uppätna av fröpredatorer,
som smågnagare. I öppna habitat, t ex gräsmarker, råder oftast inte
skuggiga förhållanden, och konkurrensen mellan växter är oftast låg, till
följd av att störning, t ex betning, hindrar annars konkurrenskraftiga arter
att ta över. Därmed skulle små frön kunna gynnas i öppna habitat,
eftersom fördelen med att producera många lättspridda frön här väger
tyngst.
I växtvärlden varierar fröstorleken från ca. en miljondels gram till ca 10
kilo. Även inom växtsamhällen förekommer en betydande
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fröstorleksvariation. Inom arter, och till och med inom individer,
förekommer dessutom ofta en variation i fröstorlek. Hur denna stora
variation i fröstorlek kan förekomma, och hur arter med olika stora frön
kan samexistera kan anses förbryllande. Hur kan flera olika
fröstorleksstrategier vara lika väl anpassade? Borde inte en viss fröstorlek
vara den optimala? Olika modeller har försökt förklara samexistensen av
arter med olika stora frön. Å ena sidan kan avvägningen mellan att
producera ett litet antal stora frön eller ett stort antal små frön kopplas
ihop med en avvägning mellan att vara en god konkurrent eller att vara
en god kolonisatör. I denna modell kan stora frön etableras till följd av att
de är goda konkurrenter och kan konkurrera ut alla mindre frön på en
groningsplats. Små frön kan däremot etableras till följd av att de är goda
kolonisatörer och kan finna groningsplatser som är obesatta av större
frön. Å andra sidan kan avvägningen mellan att producera ett litet antal
stora frön eller ett stort antal små frön även kopplas ihop med en
avvägning mellan att vara stresstålig eller att ha hög fekunditet, dvs att
frön produceras i stort antal. I denna modell gynnas stora frön i
påfrestande miljöer, eftersom stora frön har mer lagrad energi och bättre
kan klara av påfrestande förhållanden. Små frön, däremot, gynnas i
miljöer med låg påfrestning till följd av att de förekommer i störst antal.
I denna avhandling har jag försökt ta reda på hur rekryteringsmönster
styrs av fröstorlek och habitat. Detta har jag gjort genom att studera om
etablering påverkas av fröstorlek, både på mellanartsnivå och
inomartsnivå, samt om det finns någon skillnad i hur denna påverkan ser
ut i olika habitat. Jag har även studerat om det finns något samband
mellan fröstorlek och fröpredation, och hur detta skiljer sig mellan
habitat. Dessa studier har utförts i fält, i Tullgarns naturreservat och i
Angarns naturreservat. För att studera hur etablering påverkas av
fröstorlek mellan arter sådde jag ut frön av 20 arter i sex olika naturtyper
som representerar en gradient från öppet habitat till slutet habitat. För att
studera hur etablering påverkas av fröstorlek inom arter sådde jag ut frön
från fyra olika arter i två olika naturtyper – gräsmark och lövskog. Dessa
frön hade först vägts ett och ett, och såddes sedan också ett och ett, så att
jag kunde hålla reda på varje enskilt frö. Groning och etablering för
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samtliga utsådda frön följdes sedan upp till tre år. För att studera hur
fröpredation påverkas av fröstorlek lade jag ut frön från tio olika arter i
två olika habitat – gräsmark och lövskog. Frön placerades ut ett och ett,
och efter 48 timmar läste jag av vilka frön som försvunnit. Detta
upprepades sju gånger på sammanlagt tio lokaler. Jag har även studerat
om stora frön har en fördel över små frön vid samexistens med varandra.
Denna studie utfördes i en bänkgård på Stockholms universitets campus,
dvs under kontrollerade förhållanden, med frön sådda i krukor. I studien
ingick 14 arter som delades in i sju par, där varje par bestod av en
storfröig och en småfröig art. Frön från båda arterna inom ett par såddes
tillsammans i krukor med 25 frön av varje art eller 50 frön av varje art,
och utsattes för sex olika behandlingar med förna och skuggning.
Groning och etablering förljdes under två år.
Resultaten från de båda utsåningsförsöken i fält visade att groning och
rekrytering påverkas positivt av fröstorlek, men att effekten av fröstorlek
är ganska svag. I vissa fall kunde en effekt av habitat även ses, så att den
positiva effekten av stora frön blev starkare i slutna habitat (skog) än i
öppna habitat (gräsmark), men denna effekt uteblev i andra fall. Detta
kan bero på att variationer i abiotiska fröhållanden (t ex torka) mellan
olika år kan ha en större effekt på groning och rekrytering än de
skillnader som förekommer mellan olika habitat. Utifrån dessa resultat
föreslår jag följande förklaringsmodell: 1) Under gynnsamma
förhållanden (t ex i gräsmark under gynnsamma år) gror och etableras all
frön lika bra oberoende av fröstorlek, och därmed syns ingen effekt av
fröstorlek på groning och etablering. 2) Under mycket påfrestande
förhållanden (t ex i sluten skog under torra år) gror och etableras alla frön
lika dåligt, och därmed syns återigen ingen effekt av fröstorlek på
groning och etablering. 3) Under intermediärt påfrestande förhållanden (t
ex i gräsmark under torra år eller i skog under gynnsamma år) gynnas
stora frön framför små frön, till följd av stora fröns bättre förmåga att
klara av påfrestningar under etableringsfasen. Därmed uppkommer en
positiv effekt av fröstorlek på groning och rekrytering.
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Resultaten från fröpredationsexperimentet i fält visade att långt fler frön
försvinner i slutna habitat (lövskog) än i öppna habitat (gräsmark). Detta
var förväntat, eftersom slutna habitat antagligen erbjuder fler skyddade
platser för fröpredatorer än öppna habitat. Resultaten visade även ett
negativt samband mellan fröstorlek och andelen försvunna frön, dvs ju
mindre frön desto större sannolikhet att försvinna. Detta resultat var
oväntat, eftersom stora frön förväntas löpa större risk att bli uppätna.
Detta oväntade resultat kan bero på att små frön lättare försvinner till
följd av sekundär spridning (t ex blåser bort eller sprids av djur), snarare
än faktisk förpredation. En annan förklaring kan vara att olika grupper av
fröpredatorer föredrar olika fröstorlekar, så att exempelvis myror och
sniglar föredrar små frön, medan smågnagare föredrar stora frön,
förutsatt att de olika grupperna av fröpredatorer har olika förekomst.
Om små frön har större risk att försvinna kan detta ha effekter på de
rekryteringsmönster funna i de båda utsåningsförsöken i fält. Det positiva
sambandet mellan fröstorlek och rekrytering, dvs att stora frön har större
sannolikhet att etableras, skulle alltså kunna vara en effekt av att små
frön i högre grad försvinner.
Resultaten från bänkgårdsexperimentet visade att stora frön generellt har
en fördel över små frön i direkt tävling om groningsplatser. Denna fördel
ökade i behandlingar med förna. Skuggning hade endast svag effekt på
fördelen av stora frön, och då snarast så att stora frön gynnades mer i
behandlingar utan skugga jämfört med behandlingar med skugga.
Däremot fann jag ingen effekt av frötäthet på fördelen av stora frön, dvs
fördelen som stora frön hade över små frön var densamma oavsett om
frön hade såtts glest (25 frön av varje art) eller tätt (50 frön av varje art).
Dessa resultat ger stöd för modeller för samexistens av arter med olika
fröstorlek, så tillvida att stora frön har en fördel över små frön. I en
modell baserad på en avvägning mellan konkurrens och kolonisation
skulle man förvänta sig att stora frön skulle gynnas mer i hög frötäthet än
i låg frötäthet, till följd av att många frön då skulle konkurrera om ett
begränsat antal groningsplatser – en tävling som de stora fröna skulle
vinna. Jag fann dock lite stöd för en sådan modell, eftersom frötäthet inte
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hade någon effekt på storfröiga arters fördel. Däremot fann jag visst stöd
för en modell baserad på en avvägning mellan stresstolerans och
fekunditet, eftersom stora frön gynnades mer i behanlingar med förna än
i behandlingar utan förna. Att ta sig igenom ett förnalager innebär en
påfrestning för groddplantor som gror i habitat med tjockt förnalager, och
en sådan påfrestning skulle alltså kunna vara avgörande för vilka arter
som lyckas etableras. Jag fann även en svag effekt av att stora frön
gynnades mer i behandlingar utan skugga än i behandlingar med skugga.
Detta skulle kunna bero på att skuggväven som användes i fösöken för att
skapa skugga, även hade en isolerande effekt så att frön som skuggades
fick ett lite mer gynnsamt mikroklimat och inte blev uttorkade lika lätt
som oskuggade frön. Även detta ger visst stöd för en modell baserad på
en avvägning mellan tolerans och fekunditet, där stora frön förväntas ha
fördel över små frön i påfrestande förhållanden.
Sammanfattningsvis finns alltså ett rekryteringsmönster där rekrytering
påverkas positivt av fröstorlek, och där denna effekt är starkare i slutna
habitat än i öppna habitat. Effekterna av fröstorlek och, särskilt, habitat är
dock svaga, och faktorer som exempelvis variationer i väderlek mellan år
kan ha större effekter på rekrytering. Ett rekryteringsmönster med ett
positivt samband mellan fröstorlek och etablering kan vara en effekt av
att små frön i högre grad försvinner till följd av fröpredation eller
sekundär spridning. Under kontrollerade förhållanden finns dock en
tydlig fördel av stora frön över små frön – en fördel som ökar vid
påfrestande förhållanden.
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Tack!
Under sju och ett halvt år har botan varit min arbetsplats, och min
doktorandtid här har varit fantastisk, mycket tack vare alla sköna
människor. Jag trivs otroligt bra på botan, och det har känts roligt att åka
till jobbet varje dag.
Först och främst vill jag tacka min handledare Ove Eriksson. Tack för att
du alltid haft tid, för att du alltid läst och kommenterat manus så snabbt
och för din otroliga förmåga att se de enkla sambanden. Tack för att jag
med din handledning fått möjlighet att växa! Jag vill också tacka min
biträdande handledare Kjell Bolmgen. Tack för inspirerande diskussioner
om allt från de stora ekologiska frågorna till film och litteratur.
Tack alla room-mates i rum 539 som förgyllt tillvaron. Tack Didrik och
Hugo för alla roliga diskussioner om allt som rör ekologi, och lite till.
Tack Mathias, du var också med. Tack myrorna. Tack Malin, för kul
hästsnack. Tack Ellen. För allt! Tack Rammstein. Rum 540 – förlåt.
Tack Lenn, din gulkämpar-föreläsning fick mig att inse att det var
växtekolog jag ville bli. Tack Jocke, för delad förvirring i början. Tack
Tove och Helena, för delad ångest på slutet. Tack Petter, för ditt lugn och
för det bästa (och självklaraste) rådet under hela doktorandtiden: ”det är
bara att skriva tills det är klart”. Tack Lisa och Lina för sköna pubar.
Tack Tove, för mysigt ressällskap. Tack Gundula, för din cynism. Tack
Peter och Ingela för råd och hjälp i växthuset och bänkgården, samt för
trevliga pratstunder. Tack alla vuxna, post-docs, doktorander, assistenter
och ex-jobbare genom åren, för att ni gör växtekologiska avdelningen till
en sådan stimulerande miljö. Jag kommer sakna er alla!
Tack alla assistenter som hjälpt till med de olika projekten. Utan er hade
detta inte varit möjligt. Tack Irja för insamling och hjälp med utsåning av
300.000 frön. Tack Lena, för avläsning och utsåning, men också för
svamp, mögelost och trevligt sällskap på Tullbotorp. Tack Maria, för
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utsåning och vägning av frön, trots att du var lika gravid som jag. Tack
Samira och Malin S, för avläsningar i fält och i bänkgården. Tove, du var
också med på ett hörn.
Tack mamma och pappa för stöd och uppmuntran. Tack Jessica. Tack
stalltjejerna och hästarna på Örsta. Tack Patrik. Du höll mig under
armarna när det var jobbigt. Utan dig hade det inte blivit någon
avhandling. pok. Tack Ida och Gustav. Ni är det finaste och viktigaste av
allt ♥ Nu kommer mamma ut ur bubblan.
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