Plant Community Ecology

Plant Life Histories

Life History- A plant’s schedule of birth, mortality, and growth

Life Cycles: Annuals, Biennials, Perennials

Idea of Trade-offs—limiting resources is central to thinking about natural selection on life histories

What would an ideal plant species look like (life history traits) if there were no trade-offs?

e.g. root-shoot ratios, fast growing vs wood density

What are the trade-offs between large, few seeds vs. small, numerous seeds?

r strategistUnstable environment,density independent

K strategistStable environment,density dependent

Small size of organism Large size of organism

Much energy for reproduction Little energy for reproduction

Many offspring produced Few offspring produced

Early maturity Late maturity (often after parental care)

Short life expectancy Long life expectancy

Semelparous Iteroparous

Type III survivorship curve Type I or II survivorship curve

MacArthur’s Life History Selection-r vs K species

Productivity-idea at low population densities, selection is strongest on traits that increase population growth (r), high densities, selection strongest on traits that increase population size on carrying capacity K

Grime’s Triangular ModelGrime’s Model-extended r-k selection theory to plants; plant populations face consistent selective pressures mainly from physiological stress (all external factors to a plant that limit growth)

1) stress-tolerators—slow-growing species that inhabit low-fertility, low-disturbance sites (conservation of captured resources)

2) competitors—fast-growing species that inhabit high fertility, low disturbance sites (maximize captured resources)

3) ruderals-fast-growing species that inhabit high-fertility, high-disturbance sites (high seed production)

Grime’s Triangular Model

Grime’s Triangular Model

Dealing with Variation—seed germination

Try to maximize fitness (how an organism apportions resources among competing demographic functions (survival, growth, reproduction) in a varying environmentSpread reproduction more evenly among yearsIncrease seed dispersal areaIncrease dispersal time via seed banksWhen should seeds germinate?Controlled by temperature, light, moisture, othersCan plants evolve predictable germination? - depends on predictability of environment

Dealing with VariationMasting - large and erratic variation among years in the size of the seed crop producedSynchronized among plants within the population

Often explained as a response to avoid seed predatorsOverwhelm abilities of predators to eat all the seed - some survive to germinate

Dealing with Variation

Synchrony achieved by low reproduction one year (bad weather, low pollen), low resource expenditures, greater likelihood of more reproduction next year

Plant Phenology

Timing of growth and reproduction within a yearMuch variation among species, populations, individualsConstrained mostly by temperature, moisturehttp://www.youtube.com/watch?v=8Ks5IG6Mu9g

What influences leaf expansion in deciduous trees?

canopy is elevated

late spring frosts

individual metabolic enzymes have defined temp ranges over which they can operate and are most efficient at particular temps

Reproductive Phenology: abiotic factors

Reproductive Phenology: biotic factors

Pollinator interactions; pollen-limited, animal-pollinated plants may be strongly controlledHerbivory; flowering time delayed

Reproductive Phenology: biotic factors

Fruit or seed eaters more likely to be affected by plant phenology than to affect itCrowding by plant neighbors delays flowering, may select for faster growth or earlier flowering