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.