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11 the Functional Role of Soil Seed Banks in Natural Communities

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11 the Functional Role of Soil Seed Banks in Natural Communities 11 The Functional Role of Soil Seed Banks in Natural Communities Arne Saatkamp,1* Peter Poschlod2 and D. Lawrence Venable3 1Institut Méditerranéen de Biodiversité et d’Ecologie (IMBE UMR CNRS 7263), Université d’Aix-Marseille, Marseille, France; 2LS Biologie VIII, Universität Regensburg, Regensburg, Germany; 3Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, USA Introduction some plants make such prominent seed banks and others none at all? When I was a child, playing in the meadows Some of us would be satisfied with and woods, I (A.S.) was fascinated by all the answers like ‘the large size of Calendula seedlings coming out of seemingly lifeless and Carduus seeds limits the number that soil where the ponds dried out, a new river can be produced by the plant and which will bank was exposed or a mole built its hill. get buried’ or ‘decades ago heathland grew Beggarticks (Bidens tripartita) quickly cov- where the pinewood used to be’. But others ered the former pond; the river bank turned of us, inspired by Darwin’s ‘three table- blue with forget-me-nots (Myosotis praten- spoonsful of mud’ from which he grew sis); and molehills were crowned with 537 plants, also want to understand the stitchwort (Stellaria media). It was a diffi- evolution of soil seed banks, pursuing the cult experience when my parents had me deeper sense to the ‘why’ question in biol- weed out our overgrown vegetable garden ogy that Darwin (1859) gave us. The goal of where lambsquarters (Chenopodium album) this chapter is to help to answer the ques- from the seed bank grew faster than the rad- tions on: (i) types and definitions of soil ishes we had sown. I learned, however, to seed banks; (ii) how soil seed reservoirs can distinguish the few Calendula seedlings evolve; (iii) what functional role seed banks and to keep some flowers for my mother. play in the dynamics of natural communi- Later, my fascination persisted as I asked ties; and (iv) what are adaptive traits myself why there were so many heather to build up soil seed banks. seedlings in the place where the pinewoods By the ‘functional role’ of soil seed burned, but so few thistles? Why did many banks we mean their role in population seedlings sometimes emerge in a footprint, dynamics, their adaptive role, the effect but not just beside it? Why did the annual seed banks have on communities and coex- grass Bromus rubens show up every year, istence, and the role of soil seed banks in but on the same site Glaucium cornicula- the evolution of other plant traits through tum only every other year? And why did interactive selection. These aspects will * E-mail: [email protected] © CAB International 2014. Seeds: The Ecology of Regeneration in Plant Communities, 3rd Edition (ed. R.S. Gallagher) 263 264 A. Saatkamp et al. help us to understand the build-up and Leubner-Metzger, 2006). Seeds in the soil existence of soil seed banks. We use ‘natural seed bank may occur in or on the soil, but in communities’ in a pragmatic sense to mean many situations, there is a continuity between any spontaneous plant assemblage. The seeds at the surface, partly buried and com- functional role of seed banks in agroecosys- pletely buried seeds (Thompson, 2000; tems is treated in detail in Chapter 10 of this Benvenuti, 2007). In practice, it is rarely volume. possible to properly separate buried seeds from the seeds in the litter. Seeds of several plant species hardly ever enter the soil but persist at its surface or in the litter for many Types and Definitions years, prominent examples are the large and of Soil Seed Banks hard fruits of Medicago and Neurada, which contain dozens of seeds and can give rise to Soil seed banks include all living seeds in a several plants over several years. soil profile, including those on the soil sur- Plants differ in the duration their seeds face. Here we simply speak of seeds, remain in the soil and even within a species although in the beginning, soil seed banks and among seeds of the same cohort there is are also composed of dispersal units, which variability in the time they spend in the soil are seeds or fruits surrounded by structures seed bank. Thompson and Grime (1979) serving for dispersal and sometimes contain proposed a system of soil seed bank types, other plant parts such as bracts or stems. based on the study of the seasonal dynamics Over time, the dispersal structures, as well and the duration of soil seed banks for the flora as seed coats, can decompose, leaving only of Central England (Fig. 11.1). According to germination units. For example, Ranunculus their data, they distinguished between tran- arvensis has a thick seed coat and spikes sient seed banks for species that have viable which both decompose after burial in soil seeds present for less than 1 year, and per- after a few years, leaving coatless seeds sistent seed banks for species with viable (A. Saatkamp, 2009, unpublished data). Soil seeds that remain for more than 1 year. seed banks resemble other biological reser- Persistent soil seed banks can be subdi- voirs, such as invertebrate eggs, tubercles vided further into short-term persistent for and bulb banks, spores of non-spermato- seeds that are detectable for more than 1 but phyte plants and fungi, or seeds retained on less than 5 years, and long-term persistent mother plants (serotiny). Many of these rest- seed banks that are present for more than ing stages share similar evolutionary con- 5 years (Maas, 1987; Bakker, 1989; Thompson straints and physiological functioning, in and Fenner, 1992). A classification key for such a way that hatching of invertebrate the three basic types can be found in Grime eggs and seed germination can be modelled (1989), which is based on the abundance in the same way (Trudgill et al., 2005). and depth distribution of seeds in the soil Soil seed banks vary much according to seed bank, their seed size, their seasonality seed proximity, seed persistence and physio- and the presence/absence of a plant in the logical state. Living seeds have been found established vegetation around the seed bank in or on the soil for different durations (Duvel, sample. More detailed classifications have 1902; Priestley, 1986; Roberts, 1986; Poschlod been proposed but they did not gain wider et al., 1998), different seasons (Roberts, usage, mostly because necessary data are 1986; Poschlod and Jackel, 1993; Milberg rarely available (reviewed in Csontos and and Andersson, 1997), at different depths Tamás, 2003; e.g. Poschlod and Jackel, 1993). (Duvel, 1902; Grundy et al., 2003; Benvenuti, For temperate regions, Thompson and 2007), in different quantities (Thompson Grime (1979) also used seasonality to sepa- and Grime, 1979; Thompson et al., 1997) rate winter and summer seed banks for and in different states of dormancy or pro- plants with autumn and spring germination cession to germination (Baskin and Baskin, (Fig 11.1). Since timing of seed dispersal 1998; Walck et al., 2005; Finch-Savage and and germination vary greatly among species Role of Soil Seed Banks in Natural Communities 265 (a) Transient seed bank, autumn germination Year 1 Year 2 Year 2 Year 3 Dispersal Germination ************ ooooo ********** ooooo Seed survival Transient Persistent D D ND ND Spring Summer AutumnWinter SpringSummer Autumn Winter Spring (b) Transient seed bank, spring germination Dispersal Germination ooooo ********** ooooo ********** Seed survival Transient Persistent DD ND ND Spring SummerAutumnWinter Spring Summer Autumn Winter Spring (c) Persistent seed bank, autumn germination Dispersal Germination *************** ooooo********* ooooo Seed survival Transient Persistent ND ND D D Spring SummerAutumnWinter Spring Summer Autumn Winter Spring Fig. 11.1. Seed bank types according to timing of dispersal, germination and survival of seeds. (a) Transient seed bank with autumn germination; (b) transient seed bank with spring germination; (c) persistent seed bank with autumn germination. Note that the limit between transient or persistent seed banks as defined by Walck et al. (2005) does not coincide with a 1 year distance from the dispersal of seeds, the limit is indicated by a line (redrawn from Thompson and Grime, 1979 and Walck et al., 2005). D, dormant; ND, non-dormant. and among climates (Baskin and Baskin, plants can build up seed banks when their 1998; Dalling et al., 1998; Boedeltje et al., seeds are buried during disturbance and 2004), Walck et al. (2005) suggested that the stay ungerminated due to a light require- time between dispersal and the first germina- ment but germinate nearly completely when tion season should be used to distinguish tran- they remain at the surface (see Chapters 5 sient from persistent seed banks (Fig. 11.1). and 6 of this volume). Soil seed banks are a Some plants produce both transient dynamic part of plant populations with a and persistent seeds, in varying ratios set of factors that quantitatively influence (Clauss and Venable, 2000; Cavieres and their entry, persistence and exit, all of Arroyo, 2001; Tielbörger et al., 2011) and which vary according to plant biology, time variation in the environment leads to vari- and their environment. Such an approach able seed exit by germination from the seed will improve our ability to predict ecologi- bank (Meyer and Allen, 2009). Whereas cal outcomes in response to community dis- simple seed bank types are useful for multi- turbance and/or community invasion. species comparisons, we need also to con- Research on soil seed banks differs in sider dynamic and quantitative aspects of the type of data collected, sometimes con- seed banks if we want to predict more pre- sisting of (i) studying soil samples by iden- cisely the role of seed banks.
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