Plant Life Strategies

Resource Allocation and Life History Strategies

Plant Life Histories

• Life History: Patterns of reproduction and mortality exhibited by organisms.
• Life History Traits:
  • Number of offspring produced
  • Size of adults and offspring
  • Age of first reproduction
  • Frequency of reproduction
  • Extent of parental care
  • Life-span and patterns of survival (mortality)

"Typical" Plant Life History

• An idealized plant life history involves stages such as seed pool, environmental sieve, seedling cohort, vegetative daughter, seed production, and seed dispersal.

Variation in Life History: Life Span

• Annuals: Life span of less than one year (seed to seed).
• Biennials: Life span of approximately two years.
  • First year: vegetative growth (basal rosette).
  • Second year: bolting, flowering, and death.
• Perennials: Life span of more than two years.

Extreme Variation in Maximum Life Span

• Bristlecone Pine (Pinus longaeva): Can live for approximately 5,000 years.
• Arabidopsis thaliana: Completes its life cycle in about 6 weeks, from seed to seed.

Theoretical Survivorship Curves

• Type I: High survival rate throughout most of the life span, with mortality concentrated at old age.
• Type II: Constant mortality rate throughout the life span.
• Type III: High mortality rate early in life, with a higher survival rate for those that survive.

Plant Survivorship Curves

• Examples of actual plant survivorship curves include:
  • The tropical palm Euterpe globosa.
  • Phlox drummondii.

Plant Species Vary Greatly in Size

• Examples include:
  • Wolffia borealis
  • Giant Sequoia (Sequoiadendron giganteum).

Plant Species Vary in Reproduction Frequency

• Iteroparous (polycarpic): Reproduce multiple times during their life.
  • Masting (e.g., in oaks) involves synchronized, highly variable seed production from year to year.
• Semelparous (monocarpic): Reproduce only once in their lifetime.
  • Annuals complete their life cycle in one year.
  • Perennials may live for many years before a single, large reproductive event ("Big bang reproducers"), such as the Mauna Kea silversword (Argyroxiphium sandwicense).

Plant Species Vary in Energy Devoted to Reproduction

• Examples include:
  • Rafflesia arnoldii: The world’s largest flower.
  • Titan Arum (Amorphophallus titanum).

Variation in Size of Offspring: Seed Size

• Examples include:
  • Lodoicea maldivica (Double coconut): Seeds can weigh up to 40 lbs (approximately 18 kg).
  • Epiphytic orchids: Seeds can be as small as 0.000035 oz, resembling "dust particles."

Making Sense of Variation in Life History Patterns: Evolutionary “Strategies”

• Ecological use of the term "strategy" refers to evolved solutions to environmental challenges that enhance survival and reproduction.
• Evolution of Life History Strategies:
  • Selective forces in the environment shape these strategies.
  • Assumptions about resource allocation and trade-offs are made.

Plant Economics and Allocation

• Plants allocate resources (energy and structural building blocks) to various processes, including:
  • Growth
  • Reproduction
  • Maintenance
  • Tolerance/Resistance
• These allocations lead to products, such as:
  • Biomass
  • Seeds
• The ultimate goals are increased competitive ability, increased numbers, and increased survival.

Principle of Allocation

• Energy is limited, so allocation to one function reduces allocation to other functions.

Tradeoffs: Reproductive Effort and Life Span

• Century plants (Agave spp.) may live in a vegetative state for over 70 years and then reproduce massively before dying.

Tradeoffs: Seed Size & Number

• Seed Size & Number: There is often a tradeoff between seed size and seed number.
  • Large Seeds = little, few seeds.
  • Small Seeds = a lot, more seeds.
• Plants that produce larger seeds produce fewer seeds, and vice versa.
• Examples of plants with small seeds:
  • Asteraceae (daisies)
  • Poaceae (grasses)
  • Brassicaceae (mustards)
  • Fabaceae (beans)
• This relationship is shown in a graph plotting number of seeds per plant versus average seed mass (data from Stevens 1932).

Theory of r- and K-selection

• MacArthur and Wilson (1967) proposed that the environment is a continuum of habitats varying from:
  • Highly disturbed or ephemeral habitats to
  • Very stable, persistent, undisturbed habitats.
• Different selective forces in each environment shape the life history traits of organisms.

Highly Disturbed or Ephemeral Habitats for Plants

• Examples include:
  • Gopher mounds
  • Sand dunes
  • Riparian areas that frequently flood
  • Habitats that frequently burn

Selective Forces in Disturbed, Ephemeral Habitats

• r-selected populations.

Stable or Persistent Habitats for Plants

• Examples include:
  • Unplowed virgin prairie
  • Undisturbed deserts
  • Old-growth forests

Selective Forces in Undisturbed, Stable Habitats

• K-selected population.

Characteristics of r-selected and K-selected Species

• r-selected Species:
  • Population size: Highly variable, often in log phase; good colonizers.
  • Mortality: Unpredictable, sometimes high.
  • Survivorship curve: Generally Type II or III.
  • Competition: Weak.
  • Reproduction and development: Low age at maturity, rapid growth, often semelparity.
  • Emphasis: Quantity of offspring, little or no parental care.
  • Seed Size: Small
• K-selected Species:
  • Population size: Constant, high, near carrying capacity (K).
  • Mortality: Constant, usually due to gradual abiotic factors.
  • Survivorship curve: Generally Type I.
  • Competition: Intense (especially intraspecific).
  • Reproduction and development: High age at maturity, slower growth, often iteroparity.
  • Emphasis: Quality of offspring, much parental care.
  • Offspring Size: Larger

Philip Grime’s CSR Life History Theory

• Limitations of r- and K-selection theory.
• Basis of Grime’s Classification:
  • Disturbance and Resource Availability (Stress).
  • Potential Habitats (4):
    • High disturbance, high resource availability.
    • High disturbance, low resource availability.
    • Low disturbance, high resource availability.
    • Low disturbance, low resource availability.
  • Actual Habitats (3) - triangular representation.

Grime's CSR Model

• Grime’s CSR model classifies plant strategies based on disturbance and stress (resource availability).
• The three primary strategies are:
  • Competitors (C): Thrive in low disturbance and high resource availability environments.
  • Stress-tolerators (S): Dominate in low disturbance and low resource availability environments.
  • Ruderals (R): Adapted to high disturbance and high resource availability environments.

CSR Strategies and Environmental Conditions

• Competitive species: Predominate under conditions of low disturbance and low stress.
• Ruderals: Are dominant under conditions of high disturbance and low stress.
• Stress-tolerant species: Predominate under conditions of low disturbance and high stress.

CSR Examples

• Annual herbs
• Biennial herbs
• Trees and shrubs
• Perennial herbs and ferns
• Bryophytes
• Lichens

Life History Characteristics of Competitors, Ruderals, and Stress Tolerators

• Competitors:
  • Herbs, shrubs, or trees
  • Large size with a fast potential growth rate
  • Reproduction at a relatively early age
  • Small proportion of production allocated to seeds
  • Seed bank sometimes, vegetative spread often important
• Ruderals:
  • Herbs, usually annuals
  • Small size, die early
  • Reproduction at an early age
  • Large proportion of production to seeds
  • Seed bank and/or highly vagile seeds, highly dispersed
  • Vegetative spread important
• Stress Tolerators:
  • Lichens, herbs, shrubs, or trees; usually evergreen
  • Potential growth rate: slow
  • Reproduction at a relatively late age
  • Small proportion of production to seeds
  • slow growing woody plants
  • Most effort goes to tolerate maintence function

Reconciliation of CSR and r- and K- Selection Theories

• The frequency of ruderal, competitive, and stress-tolerant strategies can be mapped along the r-K continuum.