2.26.97
Weedy Life Cycles
seedbank.html
The Soil Seed Bank
Index
Seed Bank Dynamics
Seed bank dynamic model
The weed seed bank: the store of seeds buried in soil; composed of seed
produced on site and seed moved (dispersed) into area
Seed bank flow chart: dynamics of the seed population
1. Dormant seedbank ("deposit seed bank account"): majority are
dormant seed waiting stimuli or conditions before germination
2. Active Seedbank ("current seed bank account"): another part
of seed bank are seed in temporary stage, requiring only favorable temperature
and moisture to germinate
-dispersed seed with simple germination requirements
-dispersed seed whose stimulus requirements have already been met
-seed recruited from the dormant seed bank
3. Two way flow between two accounts
a. seed continually added from seed rain
b. represents an historical record of the past vegetation that grew on or
near the area
Seedbank Size and Quality
Ecosystem and habitat affect seed bank size
-perennial grasses (e.g. prairies) seed bank is often quite small because
seed production very small relative to that with annual species
-annual weed species ecosystems have very large seed banks, especially where
the land has been grazed
Grazed agroecosystem seedbanks: It is unclear why grazing increases seed
bank size:
-dormancy in seeds maintained in the feces
-opening of habitat by tight grazing allows annuals with a high seed production
to thrive and produce a lot of seed
-trampling may create conditions in which seeds retain greater viability
in the soil
Soil disturbance and seed bank size: One local or temporary disturbance
which allows one years annual weed growth and seed production can have profound
impact on seed bank
-seed banks in native prairies of Great Plains: 300-800 seeds/m2
-locally disturbed areas of prairie: up to 20,000 seeds/m2, often composed
of one species
-cultivated soils can have larger seed banks when frequently cultivated:
up to 157,000 seeds/m2
Seed bank quality: crop land seed banks have numerous species present
-often with a group of a few species comprising 70-70% of the total
-often with a smaller subset of species comprising 10-20%
Vertical Movement of Seeds in the
Soil
Vertical distribution of seed banks
-majority of seeds in grasslands (and probably no-till agricultural fields)
is located in upper 1" (2 cm) of soil profile, nearly the entire seed
bank is in the upper 10 cm
-majority of seeds in cultivated soils in upper 15 cm of soil profile, and
can be found as deep as the soil is tilled
-as the intensity of tillage declines, the seed bank moves closer to the
soil surface
Seed Movement Mechanisms Associated with Soil Disturbances:
Cultivation and tillage: continuously stirring and inverting the soil profile
-deeply buried seed probably does not ever germinate until and if it is
brought near the surface; most deeply buried seeds die buried
-bringing up seed previously buried to the upper soil surface layers is
more readily germinated
Vertical seed movement resulting from animal activities, animal vectors
-earthworm cast and mole burial
-burial by caching activities of birds, rodents, ants, etc.
-seed falling down earthworm burrows, cavities left after root decay
-animal action (moles, gophers, earthworms, etc.) can bring seed near surface;
tree falling brings buried seed up
Physical action
-seed burial by falling down cracks caused by drying-wetting cycles in the
soil
-seed burial by surface soil erosion covering seed
Other vertical seed movement mechanisms
-small seed move down soil profile of loose-textured soils with percolating
water
-some species possess self-burial mechanisms: species in the genera of Hordeum,
Triticum, Avena, Erodium; e.g. wild oat hygroscopic
awns
-burial of seeds with the piling up of successive layers of leaf litter
on the soil surface after seed fall
Losses from the seed bank
Seed longevity in the soil
Generalizations about seed longevity
-long-lived seeds are characteristic of disturbed habitats
-most long-lived seeds are annuals or biennials; biennials are especially
prevalent in soil samples taken from dated archaeological sites
-small seeds tend to have much longer soil lives than large ones; very large
ones have very short soil lives (e.g. nuts of trees) (exception: velvetleaf)
-longevity depends on species, depth of seed burial, soil type, tillage,
crop rotations, etc.
-seed longevity increases with depth of burial
Maximum seed viability in the soil:
-Nelumbo nucifera: 150-250 years
-Nelumbo sp.: (1040 +/- 210) years
-Chenopodium album (common lambsquarters), Spergula arvensis:
1600 years
Beal and Duvel experiments
-seeds buried in containers. left for various periods of time, exhumed and
germination/viability assessed
-after first 50 years: curled dock, mullein, black mustard, etc. viable
-native species survived longest compared to crop and non-native weed species
Old seed age in the soil
-in the absence of fungi and seed-destroying pests, seed viability declines
as mutations accumulate during storage
-soil mutagens: soil radon; microflora byproducts, toxins, etc.: other soil
chemical mutagens
Seed decay and predation: Pathogenic seed bank loss processes
-in the presence of soil pathogens, death rates of seeds often are accelerated
-soil microflora may protect or aid seed viability
-ability of soil microflora to cause seed decay constantly changes over
year
-decay conditions best when germination is extremely slow, stressed
Seed losses due to animal, insect, etc. predation
Seed losses due to seed germination
-germination is a source of loss of seeds from the seed bank
-seedling germination increases with decreasing soil burial depth
-seedling germination is greater in cultivated fields than undisturbed soils
-cultivation reduces the soil seed bank more rapidly than in undisturbed
soil -
The soil bud bank
Besides seeds, other dormant meristems, propagules, may accumulate in the
soil in large numbers:
-bulbs
-bulbils (aerial bulb; e.g. lillies)
-buds on rhizomes (e.g. quackgrass)
-corms (enlarged, rounded, underground stem)
-tubers (e.g. jerusalem artichoke)
-buds on rootstocks (e.g. common milkweed, hemp dogbane)
Difference between bud and seed banks:
-buds are clonally reproduced, and as such are of the same genotype successful
in leaving buds in the soil
-seeds buried in the soil are potentially new genotypes, potentially untested;
exception: self-pollinated weed seed species
Here is some additional information about soil seedbanks. This information
is optional, but have a look if you are interested:
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