Plant-Plant Interactions: Competition

Plant-Plant Interactions: Competition

Classification of Species Interactions

• Species interactions are classified based on their effects on each species involved:

  • Mutualism (including symbiosis): Both species benefit (+, +).

  • Predation (including Herbivory, Parasitism, Plant-Pathogen Interactions): One species benefits, and the other is harmed (+, -).

  • Competition: Both species are negatively affected (-, -).

Ecological and Evolutionary Consequences of Competition

• Definition of Competition:

  • Competition occurs when species have similar resource use (niche overlap).

  • Resources are limiting (in short supply).

• Ecological Effects of Competition:

  • Impacts individuals and populations:

  • Plays a role in shaping communities.

• Evolutionary Consequences of Competition:

  • Creates selection pressures.

  • Leads to niche separation (resource partitioning).

Plant-Plant Interactions

• Plant-plant interactions are generally indirect, mediated through the environment.

• Intermediaries: Resources (light, water, nutrients) and chemicals.

Types of Competition

• Classification Based on Plants Involved:

  • Intraspecific Competition: Occurs between individuals of the same species.

  • Interspecific Competition: Occurs between individuals of different species.

  • Diffuse Competition: Involves many species.

• Classification Based on Mechanisms of Operation:

  • Interference Competition:

    • Involves chemical intermediaries.

    • Allelopathy: A form of interference competition where plants release chemicals that harm other plants.

  • Exploitative Competition:

    • Involves resource intermediaries (light, water, nutrients).

    • Also known as resource competition.

Intraspecific Competition

• Intraspecific competition occurs among individuals of the same plant species.

Law of Constant Final Yield

• The law of constant final yield demonstrates that at high densities, yield becomes insensitive to density as dry weight increases when mac seeds are added to a pot.

• Yield is insensitive to density beyond some point.

• Graph shape started the same (cats).

• More biomass is produced until some point.

Agronomic Application of the Law of Constant Final Yield

• There's a minimum number of seeds required to achieve optimal yield.

• The optimum seed rate differs based on whether seeds are drilled or broadcast.

Density Effects on Average Plant Weight

• As plant density increases, the size of individual plants decreases, but the yield remains constant.

• Average individual plant weight decreases as more seeds are added.

Density Effects on Size Distributions

• Density affects the size distribution of plants over time.

• Initially, with N=400, an even population.

• Two weeks: N=395, still mostly even.

• Four weeks: N=250, mortality occurs and a few big ones emerge, with most still being smaller plants.

• Six weeks: N=103, few big plants and more small ones.

• Eight weeks: Few big plants and mostly small ones, leading to mortality and skewed size distributions.

• The main effect is competition happening.

Asymmetric Competition

• Asymmetric competition leads to skewed size distributions.

• Light competition is one-dimensional; larger plants intercept more light, creating a snowball effect where they get bigger, and smaller plants get less light.

Priority of Emergence

• Priority of emergence significantly affects seedling dry weight.

Self-Thinning

• Self-thinning describes competitive effects on plant mortality.

• -3/2 thinning law: $w = cN^{-3/2}$, where:

  • $w$ = mean plant dry weight

  • $c$ = constant

  • $N$ = density

Applications of the Self-Thinning Law

• The self-thinning law is applied in forest-thinning projects to reduce plant size and decrease mortality.

• Thinning increases fire resistance.

• Differing levels of thinning (heavy, moderate, light, and unthinned stands) are compared.

Interspecific Competition

• Interspecific competition occurs among individuals of different plant species.

Approaches to Studying Interspecific Competition

• Laboratory Studies:

  • Additive experiments: Add neighbors to a target species.

  • Substitutive experiments: Replace individuals of one species with individuals of another, maintaining constant density.

• Field Studies:

  • Pattern analyses (inferential studies): Infer competition from observed spatial patterns.

  • Removal experiments: Remove one species to observe the response of another.

  • Transplant experiments: Move individuals to new locations to assess performance.

Additive Experiments

• Additive experiments involve adding neighbors to a target species to assess the impact of competition.

Additive Experiments to Examine Crop-Weed Competition

• Additive experiments assess crop-weed competition by measuring the yield of cotton in the presence of different densities of weeds such as redroot pigweed and sicklepod.

Substitutive Experiments

• Substitutive experiments involve monocultures and mixtures to determine if I or J can produce well.

Interpretation of Substitutive Experiments

• Substitutive experiments determine which species is the better competitor; Model IIa allows a visual representation to determine if I or J is a better competitor.

Substitutive Experiment: An Example

• Experiments with wild oat species (Avena fatua and Avena barbata) in monoculture and mixtures.

Competition In Nature

• Illustration example

Approaches to Study Competition

• Indirect Approaches:

  • Study patterns in nature.

  • Infer competition from observed patterns.

  • Patterns reflect competition in the past.

• Direct Approaches:

  • Involve experimental manipulation.

  • Either add or remove species.

Inferential Studies: Spacing Patterns in Desert Shrubs

• Spacing patterns in desert shrubs, such as creosote bush (Larrea tridentata), suggest competition for resources.

Competition for Water Alters Shrub Spacing

• Competition for water alters shrub spacing. Small shrubs establish in high densities and produce a clumped distribution.

Removal Experiments: Direct Tests of Competition

• Removal experiments directly test competition by removing one species (e.g., Larrea or Ambrosia) and observing the response of the other.

Results of Removal Experiments Example

• Removal experiments demonstrate differential effects on plant xylem pressure potentials.

• Interspecific competition > Intraspecific competition for both species.

Transplant Experiment

• Transplant experiments assess the distribution of Hesperostipa neomexicana (C3 grass) and C4 grasses along a topographic gradient.

Results of Transplant Experiments

• Interspecific competition appears to limit the distribution of Hesperostipa.

Interspecific Competition Along Productivity Gradients

• The intensity of interspecific competition varies along productivity gradients.

• Lower borders are set by physical stress (soil salinity and anoxia), while upper borders are set by competition.

Allelopathy

• Allelopathy is chemical interference among plants.

Allelopathy: Chemical Interference Among Plants

• Various chemicals leach from leaves and roots that act as allelochemicals.

Various Chemicals Leach from Leaves

• Rainwater that falls through a canopy (throughfall) contains various nutrients.

Roots Exude Various Chemicals

• Roots exude various chemicals, including sugars, amino acids/amides, organic acids, cations, and anions.

Ecological Significance of Allelopathy

• Do plants have a toxic influence on other species in the field that is distinct from resource competition?

• Requirements for demonstration of allelopathy:

  • Source plant contains toxins.

  • Toxin can move from source to target (mobile).

  • Toxin is present in sufficient concentration.

  • No other simpler explanations exist.

Alternative Explanations

• Other factors besides allelopathy may influence plant interactions.

Allelopathy Observed in Succession

• Allelopathy plays a role in plant succession in abandoned cropland, with species containing gallotannins and chlorogenic acid either promoting or inhibiting the next stage.

• Indirect effects of allelopathy may involve inhibiting nitrogen-fixing bacteria.

Plants May Use Allelochemicals to Communicate

• Plants may use allelochemicals to communicate with one another belowground, influencing factors such as antimicrobial defenses, mycorrhizal fungi, and root nodulating rhizobia.