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Effects of seed size and habitat on recruitment patterns in grassland and forest plants
Stockholm University, Faculty of Science, Department of Botany.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 inter- and 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 co-existence 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 larger-seeded species exhibit higher tolerance to stress.

Place, publisher, year, edition, pages
Stockholm: Department of Botany, Stockholm University , 2012. , 32 p.
Keyword [en]
Seed size, seedling establishment, seedling survival, recruitment, seed removal, co-existence, game-theory
National Category
Ecology
Research subject
Plant Ecology
Identifiers
URN: urn:nbn:se:su:diva-82384ISBN: 978-91-7447-599-9 (print)OAI: oai:DiVA.org:su-82384DiVA: diva2:567510
Public defence
2012-12-14, Föreläsningssalen, Botaniska institutionen, Lilla Frescativägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Accepted.

 

Available from: 2012-11-22 Created: 2012-11-13 Last updated: 2012-11-15Bibliographically approved
List of papers
1. Seed size and recruitment patterns in a gradient from grassland to forest
Open this publication in new window or tab >>Seed size and recruitment patterns in a gradient from grassland to forest
2012 (English)In: Ecoscience, ISSN 1195-6860, Vol. 19, no 2, 140-147 p.Article in journal (Refereed) Published
Abstract [en]

Seedlings germinating from large seeds are known to endure hazards such as shading, competition, and litter coverage better than seedlings germinating from small seeds. However, few studies have assessed the relationships between seed size and recruitment comparing plant communities with different structures in order to establish the conditions under which a seed-size advantage prevails. Here, seeds from 20 species varying in seed size from 0.05 to 17.8 mg were sown in 6 different vegetation types, representing a gradient from open grassland to closed canopy coniferous forest. We hypothesized that the effect of seed size on recruitment is generally positive, but that there is a stronger positive effect of seed size in closed than in open communities. Our results provided only limited support for this hypothesis. Firstly, the results varied between years, suggesting that any seed size advantage may depend on factors varying on an annual basis. Secondly, although there were trends of significantly positive relationships between seed size and seedling emergence, seedling survival, and recruitment success, particularly in relatively more closed vegetation types, the strongest positive effects of seed size were found in intermediate (semi-open) habitats along the gradient. We conclude that the filtering of species into the investigated communities is only weakly related to seed size, and that several factors other than canopy probably influence the link between seed size and recruitment.

Keyword
habitat, recruitment, seed size, seedling establishment, seedling survival
National Category
Environmental Sciences Ecology
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-81308 (URN)10.2980/19-2-3521 (DOI)000307358200006 ()
Note

AuthorCount:2;

Available from: 2012-10-15 Created: 2012-10-15 Last updated: 2017-12-07Bibliographically approved
2. Relationships between intra-specific variation in seed size and recruitment in four species in two contrasting habitats
Open this publication in new window or tab >>Relationships between intra-specific variation in seed size and recruitment in four species in two contrasting habitats
2013 (English)In: Plant Biology, ISSN 1435-8603, E-ISSN 1438-8677, Vol. 15, no 3, 601-606 p.Article in journal (Refereed) Published
Abstract [en]

Large seeds contain more stored resources, and seedlings germinating from large seeds generally cope better with environmental stresses such as shading, competition and thick litter layers, than seedlings germinating from small seeds. A pattern with small-seeded species being associated with open habitats and large-seeded species being associated with closed (shaded) habitats has been suggested and supported by comparative studies. However, few studies have assessed the intra-specific relationship between seed size and recruitment, comparing plant communities differing in canopy cover. Here, seeds from four plant species commonly occurring in ecotones between open and closed habitats (Convallaria majalis, Frangula alnus, Prunus padus and Prunus spinosa) were weighed and sown individually (3200 seeds per species) in open and closed-canopy sites, and seedling emergence and survival recorded over 3 years. Our results show a generally positive, albeit weak, relationship between seed size and recruitment. In only one of the species, C. majalis, was there an association between closed canopy habitat and a positive seed size effect on recruitment. We conclude that there is a weak selection gradient favouring larger seeds, but that this selection gradient is not clearly related to habitat.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2013
Keyword
Convallaria, Frangula, Prunus, seed size, seedling recruitment
National Category
Ecology
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-82371 (URN)10.1111/j.1438-8677.2012.00676.x (DOI)000317602900021 ()
Funder
Swedish Research Council
Available from: 2012-11-13 Created: 2012-11-13 Last updated: 2017-12-07Bibliographically approved
3. Seed removal in relation to seed size in two contrasting habitats
Open this publication in new window or tab >>Seed removal in relation to seed size in two contrasting habitats
(English)Manuscript (preprint) (Other academic)
National Category
Ecology
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-82379 (URN)
Available from: 2012-11-13 Created: 2012-11-13 Last updated: 2012-11-15Bibliographically approved
4. Rules of the seed size game: contests between large-seeded and small-seeded species
Open this publication in new window or tab >>Rules of the seed size game: contests between large-seeded and small-seeded species
2013 (English)In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 122, no 7, 1080-1084 p.Article in journal (Refereed) Published
Abstract [en]

The coexistence of multiple seed size strategies within plant communities have been considered puzzling, based on a theoretical expectation of the existence of an optimal seed size under each set of specific environmental conditions. A model aimed at explaining the coexistence of different seed sizes has been suggested, where a seed size – seed number trade-off is connected to a trade-off between competition and colonization, leading to a competitive advantage in larger-seeded species and a colonization advantage in smaller-seeded species. Recently an alternative model has been suggested, based on a trade-off between stress tolerance and fecundity, associated with the variation from large to small seeds. Here, we examine the role of seed size for recruitment in two-species contests subjected to various treatments. In a garden experiment seeds of 14 plant species were combined pair-wise into seven pairs, each with one larger-seeded species and one smaller-seeded species. Each species-pair was sown with sparse and dense seed densities and subjected to different treatments of shading and litter. Recruitment was recorded during two years. Our results showed a general advantage of larger-seeded species over smaller-seeded species. This seed size advantage increased in treatments with litter, whereas there were minor effects of shade, and no effect of seed density was found. We thus found little support for a density dependent seed size game as assumed in models of a competition-colonization trade-off, whereas our results fit well with a model based on a trade-off between stress tolerance and fecundity. Our experiment provides novel empirical data to theoretical models on co-existence between multiple seed size strategies.

National Category
Ecology Environmental Sciences
Research subject
Plant Ecology
Identifiers
urn:nbn:se:su:diva-82376 (URN)10.1111/j.1600-0706.2012.00249.x (DOI)000320618900014 ()
Funder
Swedish Research Council
Available from: 2012-11-13 Created: 2012-11-13 Last updated: 2017-12-07Bibliographically approved

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