vegetation ecology course 2018/19 bertrand boeken plant
TRANSCRIPT
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)
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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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