2.26.97
Weedy Life Cycles
The Seed
disperse.html
Weed Seed Dispersal
Index
Introductory Concepts
There are two demographic ways of looking at dispersal (two parts of the
same whole):
-the expanding range and increasing population size of of an invading weed
species into a new area
-the part of the process by which an established and stabilized weed species
in an area maintains itself within that area
Separate processes:
-dispersal (leaving mother plant)
-post-dispersal events (subsequent movement)
Seed rain as a process in time
Dispersal of seed occurs in 4 dimensions (not just 2):
-length
-width
-height (soil depth, in the air)
-time
Space: the first 3 dimensions of dispersal
-2-D: Land/habitat/soil surface area phenomena: distance, width
-3-D: soil depth
Time: 4th Dimension
Differences in time:
-seed ripens on parent plant
-seed retained on parent before its dispersed (seed shattering)
-example: weeds with long flowering period and corresponding long period
of ripening and release: seed shatters immediately after ripening on parent
-example: weeds flowering and seed production in narrower time period, often
require harvesting activity to release seed: evolved with crop, maximize
chance of dispersal with crop
Selection for time of seed release: adaptation for quick or slow release
from parent plant?
-quick release: oversaturate predator demands of feeding and leave enough
after that for new colonizers
-slow release: maximize chance some seeds land on ground during favorable
time for quick germination or escape from predators
Seed Dispersal: Non-animal
Gravity dispersal: most of our common, unspecialized weed seeds don't move
very far from mother plant without animals, humans
-preventative weed control is very important
-invasion from the outside overemphasized
Poverty of seed fall in the immediate neighborhood of the parent characteristic
of of plants in isolation; e.g. mullein, musk thistle is pasture
Wind dispersal of weed seed is a function of:
-how fast seed falls: weight, density, ability to float in air
-height of release: may be most important factor in distance spread; mullein
seed
-speed, direction and turbulence of wind between release point and ground
-very light, dust-like seed (poppies; fungal spores)
-species with specialized wind dispersal mechanisms generally don't colonize
as a horizon but as isolated individuals over a greater distance
Wind-blown seed, or seeds on the soil surface moved by water:
-accumulate near obstacles (fences, furrows, etc.), or fall in soil crevices,
dried soil cracks
-Composite family weeds with pappus (or other similar attached seed structure)
on the seed are held at a constant distance in the crack (soil) by the pappus
and do not enter the crack.
"Tumble Weeds: movement of parent plant (with seeds) on ground after
dispersal, -blow with wind;
-seeds dispersed in rolling action;
-examples: kochia on mother plant, forms rolling ball;
-tumbleweed;
-velvetleaf capsules
Seed movement by water:
-surface water, irrigation, falling in rivers, lakes, etc.
-movement with water on soil surface, runoff
-specialized structures:
a. low specific gravity seed float easily (milkweed)
b. flattened seed shape for floating on water surface
c. "corky" seed wings: curled dock seed
Specialized seed structures
-plumed seeds or fruits: e.g. dandelion, pappus; milkweed, plume; thisles,
pappus; sowthistles; pappus is characteristic of Compositae family seeds
-winged seeds: maple seeds: lift provided by wings; asymetrical wing can
be an advantage
Specialized mechanisms of some weed species to bury themselves: wild oat
hygroscopic awns that twist themselves into the soil (self-seeding)
Seed ejaculation
-example: mistletoe seed propelled by parent plant
-sticky seed surface to adhere to tree host, bird foot vector
The Role of Animals in Dispersal
Animal Dispersal: affected by animal feeding patterns: specific weeds fed
on by specific animal feeders
Animal behavior affects where the moved seed ends up, and its success thereafter:
-the territorial and migratory behavior of animals dispersing seed
-animal feeding (eating, digestion) affects seed viability
-animal storage of seed: distance, concentration, location;
Birds:
-ones with hard gizzards that destroy seeds;
-soft gizzards that pass on viable seed
Ants play important role, especially in storage concentrations
Specialized seed structures affect whether, and how, animals spread seed:
-burrs: cocklebur, sandbur seed
-appendages: beggarticks seed
Human dispersal
-new continent, region invasions
-local dispersal: machinery, crop seed contamination
Crop mimicry dispersal
-weed seed adaptations to look like crop seed: plant body or seed same size,
shape, morphology as crop
-example: barnyardgrass biotype looking like rice escapes handweeding and
is dispersed with rice
-example: nightshade fruit ("berries") same size, shape as dry
beans, harvested and dispersed with beans
This information is optional, not required, but if you are interested:
BackTo:
