Lecture 10 - Establishment Success: Seedbeds and Thinning Stage Flashcards
Seeds and germination
Most seeds contain less than 20% water. This water is bound to macromolecules, and therefore unavailable for metabolic reactions.
Given that the water potential of seeds is so low, water is easily imbibed to initiate germination. As the seed imbibe water, metabolism begins. If dormancy has been broken, germination will occur.
Seed anatomy: Cotyledon
Embyonic leaves rich in energy-yielding nutrients.
* Monocots have a single cotyledon.
* Basal angiosperms and eudicots have two cotyledons
* Gymnosperms have many cotyledons
For all plants, cotyledon are storage organs that seldom look anything like true leaves.
Seed anatomy: Embryo
The immature, developing plant inside the seed.
Seed anatomy: Endosperm
Nutrients are diverted into the cotyledons as the embryo matures, Nutrients include a combination of strach, oil, or protein.
Two types of germinants
Epigeal and Hypogeal
Epigeal vs. Hypogeal germinants
Epigeal germinants = cotyledons are above the ground, and are generally photosynthetic.
Hypogeal germinants = cotyledons at ground level or tucked within the seed coat. They translocate nutrients to the emerging germinant.
The emerging germinant consist of:
- The epicotyl extends above the cotyledons. It is also called the embryonic stem. This become the trunk and branches of the tree.
- The hypocotyl is the root/shoot junction and extends between the cotyledons and the radicles.
- The radicle is the embryonic root and becomes the taproot.
On the relationsip of seed mass and germinant size.
In seeds, the radius is about the length of the emerging germinants.
The hypocotyl makes up almost the entire length of an epigeal germinants
- Now, if volume is proportional to radius ^3 then,
- Seed volume is proportional to the germinant^3
- Since seed volume is proportional to seed mass, then,
- Seed mass is proportion to the germinants^3
- Flipping this arguments around:
- Germinant length is proportion to seed mass^1/3
Germinant length is crucial:
It determines wether the hypocotyl can push dried leaves out of the way. Also determines if the radicle can reach the moist soil beneath the litter layer.
Seedbed-related mortality
A seedbed is simply the surface that the seed finds itself atop after deposition by wind, defecation or caching.
- Fresh/dead litter layer: cones, twigs, leaves. Dries out very quickly.
- Fermentation layer: organic matter in various stages of decomposition, but still recognizable. Dries out quickly.
- Humification layer: organic material decomposed, beyond recognition. Dries out slowly.
- Mineral soil: do not contain organic matter. Dries out very slowly
Leaf litter
Leaf litter seedbeds of about 5-7 leaves are the main seedbed.
What is the max thickness that the germinants of a species can tolerate.
Exposed humus and mineral soil seedbeds
Humus = organic matter (leaves moss) decayed beyond recognition.
Humus is so well decomposed that the pores among the organic material are **sufficiently small that capillary action can allow water to move upward.
Mineral soil is the best possible medium for the initial root of the germinant because it loses water slowly. **
While mineral soil and humus are excellent seedbeds for germination, together account for less than 1% of the seedbeds found in typical forests.
Litter layer: hardwood forest
In a forest dominated by hardwoods the great majority of the area conisist of fallen leaves; in a conifer forest, moss cover may be greater than leaf cover.
Litter layer: hardwood forest
In a forest dominated by hardwoods the great majority of the area conisist of fallen leaves; in a conifer forest, moss cover may be greater than leaf cover.
What is a good seedbeds?
Present no mechanical obstructions; do not dry out quickly during drought; are easy for radicles to penetrate.
Good seedbeds include:
- rotted wood
- exposed mineral soil
- thin humus or thin moss
- thick moss (if water nearby): Sphagnum
Terrible seedbeds:
- thick leaf litter and thick Of
- thick moss (non-sphagnum)
Lethal seedbeds:
- rocks, puddles, fresh logs
Big-seeded species of course do not care about seedbeds.
Thinning
For any given density there is a maximum average biomass that the plants can attain.
Any further increase in biomass can only be achieved by having a lower density
Given that plants must grow, it follows that density must keep going down with time. Therefore, plants must die. This on-going mortality is called thinning.
Who are the losers?
A fully stocked understory spruce stand. Almost all trees in smallest quartile die. Trees in third quartile greatest individual response.
Large plants supress small plants. In this hierarchy of dominance and suppression, **the smaller plants are at an accumulating disadvantage and finally die. **
The place an individual occupies in the hierarchy of a plant population seems to be determines very early on.
The weight of an individual is largely dependent on its:
* starting capital - embryonic investment
* time of emergence
* the relative growth rate (RGR) of the genotype of the individual in the environment provided
* length of time for which this growth rate is determined
* restrictions on RGR imposed by neigbors
* herbivory
* in short, it is genetic and luck
Thus, plants start off at difference sizes, competition for light accentuates these diferences, and then the shorter ones start to die from lack of light.
Natural thinning
When a stand is this dense, no light reaches the lower portion of the stand.
What you cannot see:
* most of the stems are dead
* lower branches on the survivors are also dead
