Woody Plants BI237 Week 4
1. Sexual reproduction
2. Seed dispersal
3. Seed bank (active or dormant)
4. Seed germination
Think about different "strategies" or niches. Why is there not one all-purpose "best" way to disperse, germinate, etc.?
1. ECOLOGY OF SEXUAL REPRODUCTION
A. When and how does pollination occur?
(In temperate forest. Obviously different for rainforest, savannah, etc.)
i. Wind pollinated species. Early spring, before leaves.
Populus, Betula, Ulmus, Quercus
To see a picture, search on google images for quercus flowers, etc.
ii. Insect-pollinated species. Later; when bees active.
B. How long until seeds develop?
i. Most temperate trees, spring --> fall of one year.
ii. A full year following pollination for fertilization in Pinus and the red oaks.
iii. Wetland species, seeds ripen very fast! A few weeks.
Salix, Acer saccharinum, Acer rubrum, Ulmus.
2. SEED DISPERSAL: gravity, wind, water, animals
A. Heavy seeds. Many near parent. Quercus.
Variation within genera: Acer sachharum has heavy seeds; Acer rubrum has light seeds.
Heavy-seeded species can usually establish in the shade. Seeds contain lots of energy.
B. Light seeds. Travel far in wind. Populus, Betula, Salix.
Light-seeded species usually need to establish in sun. Seeds contain little stored energy.
(For pictures of small birch seeds, search google images for betula catkins.)
3. SEED BANK (in temperate forests)
A. Short-term dormancy.
Most seeds are dormant when they are dispersed in the fall.
Will not germinate until exposed to cold.
Cold temperatures degrade hormone (Abscisic acid) that inhibits germination.
Cold/wet can also help rupture seed coat. Let water in.
B. Long-term dormancy
Stay in seed bank several years.
Characteristic of "gap-phase" species (Prunus serotina, Fraxinus americana)
Gap-phase species need light gaps in forest in order to succeed.
Always a few seedlings around in case gap opens up; not dependent on
current year's production.
Note: Many herbaceous plants, especially weeds, may have very long residence times in seed banks. Sometimes require disturbance to germinate.
C. No dormancy
Many tropical trees (too much rot/predation to stay in bank!)
Many wetland species. Disperse in spring, grow same year.
(Acer rubrum, Salix, Ulmus)
D. Dormancy broken by fire
Pinus banksiana, Pinus rigida, Pinus contorta
“Seed bank" in closed cones on trees, fire causes cone scales to open and release seeds.
Two major categories of germination:
A. Epigeous (on top of ground)
Cotyledons come above ground, act to collect photosynthate.
Conifers, Acer, Fraxinus, Betula, Fagus, etc.
To see pictures, try Fraxinus cotyledon in google image
B. Hypogeous (below ground)
Fleshy cotyledons remain below ground.
Stored food gives energy available to develop root system.
Characteristic of Quercus (also hickories).
Gives advantage on dry sites.
To see pictures, try google image on oak cotyledon.
Single most critical stage in life cycle.
Most die within one year; many more within five years.
Different regeneration "strategies".
A. Pioneer species
Seed into open areas following major disturbances such as fire, flooding, windstorm, landstorm (or more recently -- human disturbance)
Environmental conditions harsh (hot, cold, dry, wet, or windy)
Germination and growth rapid
Examples: Populus, Betula populifolia, Pinus banksiana, Pinus rigida
B. Gap-phase species
Establish under forest canopy and persist until local disturbance causes gap
When gap forms, can grow very fast to fill space
Examples: Fraxinus americana, Prunus serotina, Quercus rubra, Quercus alba, Acer rubrum, Betula alleghaniensis, Tilia americana
C. Shade/understory-tolerant species
Establish in shaded understory in moist (mesic) sites
Grow slowly for long periods in understory
Eventually grow to canopy
Can't tolerate water stress
Examples: Acer saccharum, Fagus grandifolia, Tsuga, Abies
VEGETATIVE REPRODUCTION (as opposed to sexual reproduction)
Sexual reproduction makes a new genotype due to recombination of genetic material. Vegetative reproduction propagates the same genotype.
Vegetative reproduction = asexual reproduction = cloning.
Vegetative reproduction common in broad-leaf plants but uncommon in conifers.
Picea mariana reproduces when branch tips pressed into ground by snow.
Methods of vegetative reproduction:
1. Sprouting from bud in the root-collar where stem joins root -- makes multiple stem:
Quercus rubra, Tilia americana, Betula papyrifera, Acer rubrum.
This is especially useful if something destroys or damages the original stem (cutting, fire, etc.).
2. Sprouting from roots: Populus, Fagus americana, Rhus, Sassafras.
This can allow individuals to occupy more space.
It is advantageous because they already have a root system in place.
Long-distance dispersal (by seed) is still necessary to ensure long-term survival.
3. Rhizomes: Rubus, Acer pensylvanicum, Cornus.
Rhizomes are underground, have stem anatomy, with nodes.
Ecologically, similar to sprouting from roots.
4. Fragmentation. bits break off : Salix, cottonwood, many tropical plants.
River-flood plain species can regenerate this way.
Woody plants in the tropical rainforest are always having stuff fall on them (branches from bigger trees, etc.)
Moist air and soils help prevent drying out while a new root system is being established.