Index
Introductory concepts
Recruitment
from soil surface seed populations
Recruitment from buried seed
populations
Chemical factors in recruitment
Germination and Establishment: differentiation of germination and establishment often arbitary, hard to define:
-germination events may be evident ultrastructurally 30 min after
wetting: emergence of radicle happens later, not reliable indicator of germination
-commonly germination determined by appearance of radicle, or emergence from soil
-commonly establishment determined by appearance of photosynthetic tissue from seedling
(cotyledons or leaves) and are capable of independent growth on own
"Safe-site" concept:
-the "environmental sieve" of safe and unsafe sites
-only a small fraction of the seed bank germinates to give seedlings
-presence, absence and density of seedling population depends on availability of seed and
frequency of "safe sites"
A safe site is a place a seed might be that provides:
-stimuli required for relief of seed dormancy
-conditions required for the germination processes to proceed
-the resources (water, oxygen) which are consumed in the course of germination
-place where specific hazards are absent: predators, competitors, toxic soil constituents
(e.g. herbicides, allelochemicals) and pre-emergence pathogens
The micro-scale of seed events:
-the conditions actually permitting establishment must be seen at the
scale of the size of a seed;
-at this very small scale the environment is extremely heterogeneous
-a worm cast is a mountain
-a fallen leaf is a shade from light
-a raindrop is a cataclysm
Periodicity of germination
-major weed germination "flush" of seedlings
-some species have more than one 1 flush per season
-the time of this flush varies by species; by environment
Recruitment from Soil surface (and near surface) Seed Populations
Adequate seed-soil contact necessary for successful seed imbibition of moisture
Moisture
-some species germinate over a large range of water tensions, some very
specific tensions; same for wet-dry cycles
-intensity of water evaporation and loss of water (air humidity, wind, etc.) can not
exceed absorption for imbibition
Soil factors affect contact, water availability to seed for uptake
Compression of soil-seed increases germination in many cases
-example: animals trampling field pressed soil, greater seedling
emergence of Plantago lanceolata (plantain) in compressed soil areas
-example: grazing animals in pasture can press freshly fallen seed into ground
differentially in autumn, germination following spring increased for these compacted seeds
of Ranunculus sp. despite fact hoof marks no longer apparent
Soil surface microtopography: Change in topography does not have an all-or-nothing effect on germination:
-changes in surface roughness do not result in the complete elimination
of a species from an area; it just changes the frequency safe sites encountered, the
frequency that a particular species germinates
-some species thrive in smooth soil surface microtopographies, others in rough; ratio of
species present thusly determined
Microtopography influences on the relative establishment of species by the ways the seed rests on soil:
Example of two bromegrass species illustrates this:
-Bromus rigidus predominates on smooth soil surface: it has a larger seed,
straight, needlelike awn; and will lay flat with good soil contact on smooth soil
-B. madritensis predominates on rougher soil surface: it has a smaller, lighter
seed with curved awn; on a smooth surface the seed rolls over on convex seed side with
embryo pointing ineffectively into air; on a rough surface it can curl around soil clods
for good soil contact
Common lambsquarters example: Chenopodium album seed tuberculate and rough and tend to remain on the side of the soil clod where they first landed
Mustard sp. example: Brassica napus seed are smooth and round and roll down into crevices
Seed shape/structure:
Shape of seed, how it lies relative to water contact points affects
germination potential:
-example: Panicum turgidum: seed has convex and flat sides; germination much better
if they land with flat side down
Seed dispersal structures also play a role in germination and soil
contact. For example the role of the rigid pappus of many species in Compositae family:
-example: Dandelion: achene with pappus lies at angle to soil, evidence this is superior
angle for germination; pappus opens and closes hygroscopically, also altering angle of
seed to soil
-example: Senecio sp. (common groundsel): pappus collapses and becomes
mucilaginous, aiding soil contact
Seeds with micropyle closer to water source contact point often have better germination
Large seeds, rough seeds, tuberculate seeds (small rounded protuberance, hump on seed):
-ratio of seed surface area to surface available for water contact
-more contact results in better germination
Mucilage covered seeds absorb water more efficiently (e.g. groundsel)
Seed polymorphisms: several different soil-substrate contact variants within a species
-tuberculate and non-tuberculate seeds (common in Caryophyllaceae)
-small and large seed (common in Compositae)
Recruitment from Buried Seed
Populations
Emergence
Harder for seed buried to emerge than for seed on surface: extra work load emerging from depth
Greater emergence depths takes longer generally, uses more of the seed reserve; small seeds often cannot emerge from deeper depths (relationship of seed reserves available and depth of seed)
Soil crusting as an impediment to seedling emergence
Buried seeds lack light requirement for germination from depth
Chemical Factors affecting
Recruitment
Most seeds don't need chemicals for germination and establishment: fulfilled by seed food reserves
Toxic chemicals can inhibit germination: evaporation exceeds precipitation and disolved chemicals (salts, etc.) can accumulate in remaining water, inhibit germination
Soil pH: acidic soil conditions inhibit weed species differentially
Soil chemical conditions enhance susceptibility to soil pathogens: many germinate and die right away
Herbicides:
