- Variable length of time of dispersal - maybe years.
Lodgepole
is an example of a serotinous coned species. Click on the photo
to learn more.Vegetational succession depends on the fugitive properties of plants. All pioneer species are doomed in their present habitats, and their continued survival depends on escape and establishment elsewhere.
Pioneer species: The first species to colonize or recolonize a barren or disturbed area.
A. Dispersal - Points to Ponder
1. Significance of dispersal in the life cycle depends on the spatial
and temporal heterogeneity of the environment.
2. Dispersal habit:
- reflects natural selection through time
- determines present ecological range and population size.
3. Many plants are widely dispersed:
- helps prevent competition among seedlings
- enables colonization of new habitats
- promotes genetic variation
4. Some species not widely dispersed:
- some species have very specific ecological requirements
- niche may be surrounded by unfavorable habitats (islands are an easy
example to visualize).
B. Dispersal may directly affect population size over the potential
range of a species.
The population size of a species may be low because:
1. The areas suitable for its occupancy (e.g. availability of sites for
germination ) are few.
2. The habitable areas are separated by distances that are great relative
to the dispersibility of the species.
3. The carrying capacity of the habitable sites is low.
4. The time for which these sites are suitable is short relative to the
rate of dispersal of propagules.
5. The habitability of a site is of short duration (e.g. successional displacement
is rapid).
6. Colonization and full exploitation of the carrying capacity of a habitable
site is slow.
C. How to classify dispersal mechanisms.
1. Agent of dispersal
- animals
- wind
- water
2. Structure of Propagules
- fleshy vs. dry
- dehiscent vs. indehiscent
- wings
- plumes
3. Size
4. Odor
A. Seed Fall a Function of:
1. height and distance of seed source
2. concentration of seed at source
3. dispersibility of seed
- weight
- wings of plumes
B. The Dispersal of Seeds by Wind Depends on:
1. How fast the seed falls through air - its terminal velocity.
2. The height at which the seed is released.
3. Speed and turbulence of wind.
a. Turbulence is important in keeping seed airborne and seeds tend to
fall in zones of locally reduced turbulence.
b. Densest seed rain just within the margin of a forest.
C. Dispersal by Wind - Different Strategies:
1. Dispersal of dust seeds depends on air flow and movement.
2. Dispersal of plumed seeds and fruits very complicated
-depends on structure of plume and wind velocity.
3. Dispersal of winged seeds which rotate
- Seeds actually develop lift, very difficult to forecast distance.
- Animals often have very-species specific feeding habits
- Territorial and migratory behavior is very important.
- May eat and digest seeds (lost).
- Eat and pass fruits undamaged in feces.
A. Mistletoe
-Individual sticky seeds rubbed of bird's bill into crevices in
bark of tree where they will establish and germinate.
B. Heavy Seeded Species
- Walnuts, Oaks, Hickories
- Seed distributed near parents (animals - squirrels)
- Dispersal usually less than 30m.
- Black spruce, lodgepole pine, jackpine
- Variable length of time of dispersal - maybe years.
Lodgepole
is an example of a serotinous coned species. Click on the photo
to learn more.
Dormancy: A state in which viable seeds or buds fail to germinate under conditions favorable for germination or begetative growth.
The developmental processes in the seeds or buds that release dormancy and trigger the onset of growth are often regulated by temperature. However, some seeds must be scarified (e.g. pass through the gut of a bird), while others must be heated (an obvious advantage for colonization just after a fire).
- Seeds disseminated to the forest floor are typically stored there.
A. Diagram - Points
1. Compared to seeds of herbaceous plants and shrubs, tree seeds are relatively
uncommon in the dormant seed bank.
2. Tree seeds subject to heavy predation- large and yummy.
3. Some species germinate immediately
- pioneer species (cottonwoods, aspen, maple,lodgepole pine, ponderosa
pine, - fire species)
4. Pioneer- shrub and herb species are common in dormant seed bank.
Germination: The commencement of growth of a seed.
A. Seeds must:
- Imbibe water
- Activate metabolic processes
- Initiate growth of embryo
1. Dormant if one of these functions is blocked.
2. Over two thirds of North American trees have dormancy requirements.
B. Patterns of Germination
1. Epigeous - Cotyledons elevated above surface and ps active
- seeds store little food.
2. Hypogenus - Cotyledons remain below surface
C. Establishment
Establishment is the most critical stage in the life history of an
individual.
Most woody plants have rather specific seedbed requirements. Some species (for example, many pines) require a mineral seedbed. Mineral seedbeds sometimes occur after wildfire. The successful establishment of individuals can be constrained by the type and frequency of natural disturbances like wildfire.
Mineral
seedbed established on a recently burned clear-cut. Click on
photo to learn more.
Other species find partially decomposed logs or moss-covered rocks suitable sites for establishment. Examples include: eastern and western hemlock, and yellow birch.
Example
of old growth & seedling establishment of western hemlock. Click
on photos to learn more.
Tip-up mounds caused by wind-thrown trees are yet another example of a specific site for establishment that depends on natural disturbance.
Some species (for example, sugar maple) have the ability to germinate in the litter. The cotyledons grow up through (penetrate) through the decomposing leaf litter that covered the seeds the previous fall.
It is very important to understand the specific dispersal, dormancy, and seedbed requirements of each individual species.
These life-history attributes play a pivotal role in the successful establishment of individuals. Forest managers have devised numerous silvicultural techniques to mimic natural disturbances. This is done to encourage suitable seed dispersal and seedbed conditions.
Silviculture: The application of the knowledge of the life-history
attributes of species in the management of a forest. The theory and practice
of controlling forest establishment, composition, and growth.
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