Plant recruitment•Control of recruitment•A recruitment model •Site- and seed-limitation•Historical background•Desert plants

Ben-Gurion University of the Negev

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Vegetation Ecology Course 2018/19 Bertrand Boeken

Plant recruitment

End of a chain of processes (state transitions)1. Seed production, primary and secondary dispersal, arrival, retention

2. Seed dormancy (innate or imposed inability to germinate)

3. Seed imbibition (water uptake) and germination (radicle protrusion)

4. Seedling emergence and establishment

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Seeds on mother plant

Seeds in target location

Germinating seeds

Emerging seedlings

Established seedlings

Photo: Piccolo Namek

Recruitment = addition of individuals to a populationUsually of seedlings from sexual reproduction (seeds),

sometimes ramets from vegetative propagation (clonal growth)

Agur Sands, Negev

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Controllers of recruitment

Seeds on mother plant

Seeds in location

Germinating seeds

Pre-dispersal predation

Secondary dispersal

vector

Primary dispersal

vector

Post-dispersal predation

Conditions

Other plants

Animal predation

Microbial decay

Resources, Conditions

Other plants

Animal predation

Microbial decay

Emerging seedlings

Resources, Conditions

Other plants

Disturbance

Established seedlings

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Predation during dispersal

Seed availabilitySeed availability depends on • Seed production at the source(s) at time t0 and

• Seed arrival (a) at the target until time t1:NSeeds,t1 = a Nseeds in sources,t0

Nseeds in sources,t0 Nseeds,t1a

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For a population of species i with seed sources j:NSeeds,t1 i = Σ aij Ni,j,t0

Arrival rates aij = f (primary and secondary seed dispersal, post-release mortality, arrival at the target)

aij also depends on species traits, dispersal vectors, landscape structure and connectivity, patch properties.

A model of recruitment

Plant recruitment from seeds in a location(from time t1 to t2)

NPlants,t2 = rt1,2 NSeeds,t1

(NPlants,t2 ≤ M,t2)

• Density-dependent recruitment(Micro-)site or establishment limitation

Recruitment depends on the availability

of

suitable microsites M for which r>0

If more seeds present (high NSeeds,t1) than

suitable microsites

e. g. ample seed rain or persistent dormant

seed bank, with specific requirements

• Density-independent recruitmentSeed limitation (Eriksson and Ehrlén 1992)

Recruitment depends on both

seed availability NSeeds,t1

and recruitment rate rt1,2

rt1,2 = f (conditions, resources, interactions,

species traits) from time t1 to t2

where microsite quality is relative to species

requirements and tolerances

in annual plants

NSeeds,t1 NPlants,t2 Plan

t den

sity

N

Plan

ts,t2

Seed availability NSeeds,t1

rt2

Mt2

P(re

crui

tmen

t)

5

rt1,2

Microsite selectivity

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Recruitment is proportional to seed availability (NPlants = r Nseeds) or equal to microsite availability (NPlants = M)

depending on the number of seeds relative to the number of microsites (NSeeds / M):(NSeeds < M) à seed dependence(NSeeds > M) à microsite limitation

NDormant seeds,t1 Microsites

NSeeds,t1 NPlants,t2rt1,2Mt1,2

Low MS: low requirements, efficient acquisition and exploitation, no dormancy and independence of high-quality microsites à M is high in most of the habitat

High MS: specific requirements, often related to good conditions for establishment and growth; seed dormancy à M usually not very high.

This, in turn, depends on• Microsite selectivity – species-specific seed traits

in relation to microsite suitability– seed arrival, microsite acquisition, seed dormancy

and requirements for dormancy-breaking and germination• Local availability of suitable microsites – which decreases with microsite selectivity

Seed or microsite limitation

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Recruitment is seed or microsite-limited, depending on seed and microsite availability (NSeeds/M)

• In species with Low Microsite Selectivity Microsite availability M is usually high, due to high

probability to acquire and/or exploit themà high, seed-dependent recruitment

Herbaceous dominants – large seeds (Rees et al. 2001)

Stipa capensis on biocrust – soil-penetrating seeds and transient seed banks (no dormancy) (Boeken 2018)

à However, in marginal habitats, recruitment may be microsite-limited

• In species with High Microsite SelectivityMicrosite availability is usually low, due to low

probability to acquire high-quality micrositesàlow, microsite-limited recruitment

Subordinate and sparse species – small seeds(Rees et al. 2001)

Most desert plants, with dormant seed banksàSometimes seed-limited, due to restricted

dispersal from distant seed sources

Recruitment limitation in vegetation science• Migration (arrival of propagules) and ecesis (establishment and persistence)

(Clements 1904, 1905, 1916)

– Seed dispersal (migration) recognized, but arrival assumed to be unimpeded(esp. in climax communities)

– �Eventually, all sites will be occupied by suitable species�– Focus on the match between species traits and the biophysical environment,

and reaction (facilitation by environmental modification)– ‘Supra-organismal’ view of plant associations

• Assumption of ever-present, randomly mixed seed rain(Beijerinck 1913, Baas Becking 1934 in Dutch)

– "Everything is everywhere, and the environment selects" – Based on cosmopolitan bacteria and ruderal plants– Communities are determined by climate and soil (Braun-Blanquet 1932, Oosting 1953)

– Classification, syntaxonomy, mapping

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Recruitment limitation in population ecology

• Stochasticity of migration (Gleason 1926)

– Communities are �too noisy� to study because of random migration

– Associations are coincidental (individualistic species)

– �Only population-based focus will reveal rules�

• Recruitment site concept: safe sites (Harper 1965), regeneration niche (Grubb 1977)

– Plants establish in suitable sites • Interactions (predation, competition), abiotic conditions, resource availability

– �Plant populations are governed by local processes�

– Individual, not area-based

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Seed limitation in ecological theory

• Seed limitation of abundance – Seed shadows (radial gradients from mother plant) (Janzen 1971)

– Implied in metapopulation theory (rescue effect) (Hanski and Gilpin 1991)

– First mentioned in plant ecology textbook by Crawley (1990) (ca. 100 words)

– New empirical and theoretical studies (Eriksson and Ehrlén 1992, Turnbull 2000, Muller-

Landau et al. 2002)

– Experimental test by seed addition (Turnbull 2000) and removal (Boeken 2018)10

• Vegetation dynamics (Pickett and McDonnell 1989)

– Species availability important, besides site availability and species

performance

– Multiple factors, responses, scales, and levels of complexity

– No equilibrium assumptions (saturation, DD-density dependence)

– Includes disturbance, landscape heterogeneity and human impacts

– Species availability implies a switch in observation

• from the source (individual mother plant)

• to the target (a community in a heterogeneous landscape)

Seed dormancy in deserts• Seed banks (Went 1947, 1948)

– Desert annuals have large dormant seed banks– Respond strongly to seasonal rainfall, thresholds– Therefore microsite-limited

• Predictive and delayed germination(e.g. Koller 1956, Negbi and Evenari 1962, Evenari and Gutterman 1966, Venable 1989)

– Highly selective: germination if conditions very good– Prolonged dormancy of a large fraction of the seed bank– Maximization of reproductive value (Cohen 1966, 1967)

– Germination upon rain dispersal (Gutterman 1993)

• High germination rate with no secondary dormancy– Low Microsite Selectivity (and seed-dependent recruitment)

is also a Evolutionary Stable Strategy – Stipa capensis (Boeken 2018)

– No trade-off with colonization ability (�alternative deserts�, MacArthur 1972)?

Seed dispersal in deserts still debated: strongly scale-dependent, probably short distance (1-100 cm) during flow events

Many seeds without appendages – ground dispersal (Boeken and Shachak 1994)

– But non-selective germination may have advantages– During drought, after ample seed crop in previous year (Boeken 2018)

Antelope Valley, CA, www.desertusa.com

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Stipa capensis in Park Shaked, 20/03/2016

Importance of seed limitationWhen is seed limitation of recruitment important?

– During or after drought

• Annuals without dormant seeds should be

seed-limited after poor years (Boeken, in prep.)

– Herbivory and granivory– Landscape fragmentation– Regional extinctionSpatial context and scale

When is it unimportant?– Seed saturation (seed rain, seed bank)– Accessible locations– Highly selective species (predictive germination)– For species with high DD juvenile mortality

e.g. self-thinning perennials Local conditions

(i.e., site availability and limitation)Erucaria pinnata, Park Shaked 2004

Messor ebeninus, Park Shaked 2004

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