Lecture 5: Seed Production

Question 1

Failures in Pollination

Answer 1

Failure to pollination lowers fruit/flower ratios and seed/ovule ratios.
Furthermore, we not only have fewer fruits, but also fruits with fewer seeds.

Failures in pollination is a particularly acute problem for pollinated plants in low-density populations.
This improves with economies of scale:
* high density populations
* mass flowering: years of high pollen and ovule production

Question 2

The average number of seeds/fruits produced by a tree is a function of

Answer 2

the # of fertilized ovules

Question 3

The percentage of fertilized ovules is a function of

Answer 3

the density of pollen in the air.

Question 4

Pollen density

Answer 4

Usually, within ecology and beyond, an increase in density comes with negative effects.
For e.g. with more indiviudals comes more competition for food and mates and increased risk for disease.
Thus, growth and survival rates decrease.
But, in the** case of pollination success, density-dependent effects are positive.
**
But of course, pollen production is correlated with ovule production, Plants don’t over produce pollen, while reducing the number of ovules!

The percentage of seeds filled increases as a function of crop size because of greater pollen density.

Question 5

In short:

Answer 5

1. Pollen density varies from year to year
2. Pollen density varies as a function of tree density
3. Pollen density determines % of fertilized ovules

Question 6

Pollen density at range limits…

Answer 6

At range limit we find fewer indiviudals within a population and fewer populations of that same species at the landscape-level.
Furthermore, these tree are often stunted in growth because of short growing season. Sexual reproduction is therefore often limited because of low investements in ovulate and staminate cones.

Consequently, we have very low pollen density in the air.
At range limits, tree populations resort to asexual reproduction.

Question 7

Seed mass

Answer 7

Flowers, cones and seeds are costly investments. Everything extracts its price. For plants, **sexual reproduction reduces rates of vegetative growth. **

Seed mass affects every aspects of the plant regeneration: dispersal, establishment and seedling survival.

Question 8

Seed mass is the best predictor of the quantity of seed produced.

Answer 8

A species with a large seed (oak acorn) will produce fewer of them while a tree species with small seeds (birch samara) will produce far more.

e.g. in a very good year: 30cm diamter white oak (3g/seed) will produce at best about 500 acorns while a yellow birch (each seed about 1mg) of the same diameter will manufacture 50,000 seeds.

Question 9

Seed mass, we can quantify this relationship for trees:

Answer 9

Seed production is proportional to seed mass raised to the power of** -0.58 **

Same thing for herbaceous plants (the slope is -0.78)

Question 10

Seedlings of large seeds perform better in response to:

Answer 10

• shade
• defoliation
• mineral nutrient deficiency
• drought
• competition

Question 11

Seed mass and environment

Answer 11

Does seed size vary along environmental gradients?
* SHADE: YES, mean seed size increases along a gradient of declining light intensity. Seeds in open habitats tend to be small whereas closed-canopy habitats seeds tend to be large.

• SOIL-FERTILITY: in some studies, seed size is negatively correlated to soil-fertility. As soil fertility declines, seeds get larger to provide a reserve of nutrients to survive for up to 12 weeks. But the relationship between mineral content of soils and seed size is not as strong as shade gradient.

Conclusion: environmnet select, to a degree, an optimum seed size. And bigger seeds are better.

Question 12

So why not have only big seeds?

Answer 12

To attain a given size, requires a long enough growing season. At our latitude seed/fruit size is limited by short summers. Leafing out & flowering too early can be dangerous (late frost). A delay limits time available for fruit/seed development.
The study of the timing of plant events is called phenoloy.

The onset flowering is determined by temperature but also seriously constrained by the size of the fruit.

Question 13

What is the relationship between seed mass and dispersal capacity?

Answer 13

Small seeds are** produced in large quantities and, typically dispersed by wind.**
* wind-dispersed seeds are smaller than animal dispersed seeds.
* of the animal dispersed seeds, size is a function of dispersing agent: ant<bird<mammals

Thus in addition to everything else,** seed size is also influenced by the dispersal agent. **

Question 14

What is the relationship between seed mass and persistence in soil seed bank?

Answer 14

Small, more compact seeds (low variance in seed dimensions) are more persistent in soil seed bank.
So bigger seeds is not always better.

Question 15

Seed production is a function of the size of the individual

Answer 15

The second issue controlling the average crop size is **the size of an individual. **
Within a species bigger trees will produce more seeds than little trees.
The bigger trees have more sugars to play with.

Question 16

Light receipt

Answer 16

Light intensity influences the manufacture of plant food, stem length, leaf color and flowering. Generally speaking, plants grown in low light tend to be spindly with light green leaves. A similar plant grown in very bright light tends to be shorter, better branches, and have larger, dark green leaves.