Weed Seed Dispersal

Index

Introductory concepts
Seed rain as a process in time
Seed dispersal: Non-animal
The role of animals in dispersal

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
Self-burial.  Some weed species are able 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:

Advanced Topics in Seed Dispersal

©jdekker-1999