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Plant recruitment • Control of recruitment • Models of recruitment
• Seed flow • Site-‐ and seed-‐limita7on
• Historical background • Desert plants
Ben-Gurion University of the Negev"
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Vegeta7on Ecology Course 2015/16 Bertrand Boeken
Plant recruitment
End of a chain of processes
1. Seed produc4on, primary and secondary dispersal, arrival, reten4on
2. Seed imbibi4on (water uptake) and germina4on (radicle protrusion)
3. Seedling emergence and establishment
www.tutorvista.com
Seeds on mother plant
Seeds in target location
Germinating seeds
Emerging seedlings
Established seedlings
Photo: Piccolo Namek
Recruitment = addi4on of individuals to a popula4on
Usually of seedlings from sexual reproduc4on (seeds),
some4mes ramets from vegeta4ve propaga4on (clonal
growth)
Agur Sands, Negev
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Controllers of recruitment
Seeds on mother plant
Seeds in location
Germinating seeds
Pre-‐dispersal preda4on
Secondary dispersal vector
Primary dispersal vector
Post-‐dispersal preda4on
Condi4ons
Other plants
Animal preda4on
Microbial decay
Resources, Condi4ons
Other plants
Animal preda4on
Microbial decay
Emerging seedlings
Resources, Condi4ons
Other plants Disturbance
Established seedlings
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Preda4on during dispersal
Models of recruitment
Plant recruitment from seeds in a loca4on (from 4me t1 to t2)
NPlants,t2 = rt1,2 NSeeds,t1 (NPlants,t2 ≤ M = NSites,t2)
• Density dependence
“Site limita4on” Recruitment depends on availability of microsites M
for which r>0
Condi4ons: sufficient seeds present (large NSeeds,t1) e. g. ample seed rain or long-‐term dormant seed bank
• Density independence “Seed limita4on” (Eriksson and Ehrlén 1992) Recruitment depends on both
seed availability NSeeds,t1
and recruitment rate rt1,2 rt1,2 = f (condi4ons, resources, interac4ons,
species traits) from 4me t1 to t2 (site quality rel. to species requirements) Assump4on: r includes low juvenile mortality
in annual plants
NSeeds,t1 NPlants,t2 rt1,2 Pla
nt d
ensi
ty
NP
lant
s,t2
Seed availability NSeeds,t1
rt2
Mt2
P(r
ecru
itmen
t)
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Seed availability • Seed availability Depends on seed produc4on at the source(s) at
4me t0 and
arrival (a) at the target un4l 4me t1:
NSeeds,t1 = a Nseeds in sources,t0
For species i, seed sources j:
NSeeds,t1 i = Σ aij Ni,j,t0
Arrival rates aij = f (dispersal I & II, post-‐release
preda4on, accessibility, pathogens,
disrup4on);
aij also depends on species traits, vectors,
landscape structure, patch proper4es.
• Seed limita4on if seed availability is restricted (< M) due to
– No [input from] dormant seed bank
– Reduced produc4on at the sources
– Low probability to arrive
• Improbable dispersal from source
• High post-‐release preda4on • Low dispersal into target (import)
Seed import Local seed production
aj
Seeds
Sites
r M
Dormant seeds
Plants
Nseeds in sources,t0 Nseeds,t1 a Seed flow model
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Site and seed limita7on • Is recruitment either seed-‐ or site-‐limited?
– At similar intermediate values, at low density (N/area)? – Even if seed-‐limited, sites mager.
• If recruitment is seed-‐limited, is this due to – Limited produc4on – Dispersal limita4on, – Seed removal/decay, or
• Is this fixed per species, or does it vary? – Is recruitment limita4on a specific trait under selec4on? – Or is recruitment limita4on an emergent demographic property?
(due to popula4on responses to environmental varia4on)
• Are sites either suitable (or safe) for recruitment or not, or somewhere along a site quality con7nuum?
– Few high-‐quality sites and many low-‐quality sites undis4nguishable – Many quan4ta4ve and qualita4ve factors determine suitability
• Is recruitment an on/off process, or gradual, depending on variable site quality? – Variable germina4on, seedling emergence, growth and establishment rates – Each responding differently to combina4ons of factors, at different 4mes
• Do only a few seeds find a suitable site by chance, or do seeds keep moving un7l they do?
– Repeated dispersal (e.g. wind, runoff) – Agractors – diggings, shrub patches
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Site limita7on in vegeta7on science • Migra7on (arrival of propagules) and ecesis (establishment and persistence)
(Clements 1904, 1905, 1916) – Seed dispersal (migra4on) recognized, but arrival assumed to be unimpeded
(esp. in climax communi4es)
– “Eventually, all sites will be occupied by suitable species” – Focus on the match between species traits and the biophysical environment,
and reac4on (facilita4on by environmental modifica4on)
– ‘Supra-‐organismal’ view of plant associa4ons
• Assump7on 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
– Communi4es are determined by climate and soil (Braun-‐Blanquet 1932, Oos4ng 1953)
– Classifica4on, syntaxonomy, mapping 7
Site limita7on in popula7on ecology
• Stochas7city of migra7on (Gleason 1926)
– Communi4es are “too noisy” to study because of random migra4on
– Associa4ons are coincidental – “Only popula4on-‐based focus will reveal rules”
• Recruitment site concept: safe sites (Harper 1965), regenera7on niche
(Grubb 1977)
– Plants establish in suitable sites • Interac4ons (preda4on, compe44on), abio4c condi4ons, resource availability
– “Plant popula4ons are governed by local processes”
– Individual, not area-‐based
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Seed limita7on in ecological theory
• Seed limita7on of abundance – Seed shadows (radial gradients from mother plant) (Janzen 1971)
– Implied in metapopula4on theory (Hanski and Gilpin 1991)
– First men4oned in plant ecology textbook by Crawley (1990) (ca. 100 words)
– New empirical and theore4cal studies (Eriksson and Ehrlén 1992, Turnbull 2000, Muller-‐Landau et al. 2002)
– Experimental test by seed addi4on (Turnbull 2000) and removal (Boeken, in prep.) 9
• Vegeta7on dynamics (Pickeg and McDonnell 1989) – Site availability, species availability, and species performance
– Mul4ple factors, responses, scales, and levels of complexity
– No equilibrium assump4ons (satura4on, DD-‐density dependence)
– Includes disturbance, landscape heterogeneity and human impacts
– Species availability implies a switch in observa4on
• 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 – Rainfall threshold – Therefore site-‐limited
• Predic7ve and delayed germina7on (e.g. Koller 1956, Negbi and Evenari 1962, Evenari and Gugerman 1966, Venable 1989) – Highly selec4ve: germina4on if condi4ons very good – Prolonged dormancy of a large frac4on of the seed bank – Maximiza4on of reproduc4ve value (Cohen 1966, 1967) – Germina4on upon rain dispersal (Gugerman 1993)
• High germina7on rate with no secondary dormancy – If dispersal is good (MacArthur 1972, Venable and Lawlor 1980)? – Maximiza4on of fitness by coloniza4on ability in spa4ally heterogeneous
and temporally variable landscapes (“alterna4ve deserts”)? – Such species may experience seed-‐limita4on
Seed dispersal in deserts s4ll 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-‐selec4ve germina4on may have advantages – During drought, ayer ample seed crop in previous year (B. Boeken in prep.)
www.desertusa.com
Aizoon hispanicum
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Importance of seed limita7on When is seed limita7on of recruitment important?
– During or ayer drought
• Annuals without dormant seeds should be
seed-‐limited ayer poor years (Boeken, in prep.)
– Herbivory and granivory – Landscape fragmenta4on – Regional ex4nc4on
Spa4al context and scale
When is it unimportant? – Seed satura4on (seed rain, seed bank) – Accessible loca4ons – Highly selec4ve species (predic4ve germina4on) – For species with high DD juvenile mortality
e.g. self-‐thinning perennials
Local condi4ons (i.e., site availability and limita4on)
Erucaria pinnata, Park Shaked 2004
Messor ebeninus, Park Shaked 2004
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