Weedy Life Cycles

The Soil Seed Bank


Seed bank dynamics
Seed bank size and quality
Vertical movement of seeds in the soil
Losses from the seed bank
The soil bud bank

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:
-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:

Advanced Topics about the Soil Seedbank


Weed Life Cycles | The Seed