Plant Communities: Diversity, Structure and Organization Notes

Plant Communities: Diversity, Structure and Organization

The Plant Community as a Unit of Study

• Plant communities are defined as groups of plants that co-occur in a particular place and time.
• Community Ecology, also known as Synecology, studies these plant communities.
• Criteria for naming communities include:
  • Characteristic species
  • Habitat or physical features
  • Functional groups

Are Communities Natural Units? The Community Unit Perspective

• This perspective views communities as discrete, definable entities.
• Species co-occur and form tight, interdependent relationships.
• Frederic Clements (early 1900s) proposed the "Superorganism" concept, suggesting communities function as a single organism.

Individualistic View of Communities

• H.A. Gleason and R.H. Whittaker supported this view.
• Communities are assemblages of species with similar environmental requirements.
• There is no "super-organism;" species distributions are independent.

Continuity and Discontinuity in Communities: Predictions

• Ecotones (transition zones between communities) would support the community "unit" view.
• An absence of ecotones would support the "individualistic" nature of communities.

Field Studies: Robert Whittaker, Great Smoky Mountains

• Whittaker's gradient analysis of tree species in the Great Smoky Mountains.
• Species distributions along moisture gradients:
  • Xeric (dry) to Mesic (moist) conditions.
  • Examples: White pine, Pitch pine, Hemlock, Beech, Red maple, White oak, Dogwood, Virginia pine, American basswood, Tulip poplar, Chestnut oak, Scarlet oak.

Contemporary View of Communities

• Stresses the individualistic nature of communities.
• Acknowledges interdependencies and coevolution among species.
• The community unit view persists for practical reasons:
  • Conservation Biology
  • Utilitarian value in naming and classifying communities
  • Phytosociology = the study of naming and classifying plant communities.

Community Properties

• Species Richness = the number of species in a community.
• Species Dominance/Evenness = the distribution of abundance among species.
• Species Diversity = Species richness, plus evenness or sharing of abundance.
• Shannon-Weiner Diversity Index (H’) measures species diversity.

Species Diversity and Dominance

• Community A: 5 species; high dominance, low diversity.
• Community B: 5 species; low dominance, high diversity.

Plant Community Description: Sampling Techniques

• Quadrats:
  • Best for herbaceous vegetation.
  • Dimensions vary, but 1 m X 1 m is common.
• Plotless techniques:
  • Best for woodland/forest vegetation.
  • Several methods, point-quarter is common.
  • Measured distances at 90°.

Community Data

• Cover = spatial coverage of species within the plot.
• Frequency = number of plots in which the species occurs.
• Density = number of plants in each plot.
• Importance Value (IV) = sum of cover, frequency, and density.

Summary data from four woodland plant communities in central Texas

• Species data from four woodland plant communities in central Texas, including Parkland, Woodland, North Slopes, and South Slopes.
• Plant species and their growth forms (shrub/tree) and family are listed with Importance Values (IV) for each habitat.
• Sum of IV is calculated per habitat.

Quantitative Descriptions of Plant Communities

• Community data is multivariate data.
• Classification & Ordination:
  • Mathematical techniques for identifying and depicting communities based on similarity in species composition.
  • Often correlate with environmental variables.

Ordination of Lodgepole Pine Forests

• Graphical representation of 63 stands of Lodgepole Pine Forests.
• Elevation (m) as one variable.
• Moisture index (drier to wetter) as another variable.
• Stands are plotted based on these variables.

Numerical techniques to define communities

• Classification complements ordination.
• Cluster diagram.
• Polythetic-agglomerative cluster analysis.
• Pinus contorta forests of Banff and Jasper.

Identification and ordination of major plant communities

• Elevation (m) and moisture index are used to identify major plant communities.
• Communities include Ledum/Vaccinium, Menziesia/Vaccinium, Shepherdia/Elymus, Alnus/Linnaea, and Shepherdia/Arctostaphylos.

Other Descriptive Tools: Mapping Community Distributions Using Aerial Photos or Spectral Quality

• Aerial photos are used for mapping community distributions.
• Example: Cloud forest, Oak forest, Pine-oak forest, Pine forest, Developed area, Agriculture/pastureland.

Community Organization and Species Diversity

• Theoretical Expectations
• Predictions from Competitive Exclusion Principle (One species-One Niche)
  • Low diversity
  • Why the opposite?
• Questions of Community Organization:
  • What determines how many species can coexist in communities?
  • Why do habitats differ in species diversity?

Species diversity varies over multiple spatial scales

• Species diversity varies across different geographic locations, such as the Pacific and Atlantic Oceans.

Local Patterns in Species Diversity

• Examples from Nebraska Sandhills (Barnes et al. 1984):
  • #spp (number of species) = 72, H’ (Shannon-Weiner Diversity Index) = 3.01
  • #spp = 60, H’ = 2.56
  • #spp = 67, H’ = 2.49

Equilibrium Perspective on Community Organization

• Traditional view of communities:
  • Populations near carrying capacities (i.e., equilibrium conditions).
  • Biotic interactions intense.
  • Emphasis on competition & niche relationships.
  • Resource partitioning reduces competitive interactions and allows for species coexistence.

Vertical Stratification in a Temperate Forest

• Overstory, Understory, Shrub Layer, Herb Layer.
• Examples:
  • Temperate deciduous forest, North Carolina

Belowground Resource Partitioning in Grasslands

• Root systems of sand-dune forbs and grasses:
  • Lathyrus siipulaceus (LS), Asclepias arenaria (AS), Psoralea lanceolata (PS), Lithospermum gmelini (LG), Tradescantia occidentalis (TO), and Calamovilfa longifolia (CL).

Temporal (Phenological) Resource Partitioning

• Calendar for Blooming of Wild Flowers on Nine Mile Prairie, Nebraska.

Non-equilibrium Perspective on Community Organization

• A more recent perspective:
  • Populations rarely reach K (carrying capacity).
  • Competitive exclusion doesn’t operate.
  • Emphasis on disturbance and other factors that keep populations from reaching equilibrium.

Disturbance and Diversity in a Tallgrass Prairie

• Fire frequency:
  • High: Dominance by \text{C}4 grasses, Competitive exclusion of \text{C}3 species, Low numbers of species.
  • Low: Decreased \text{C}4 dominance, Presence of \text{C}3 species, High numbers of species.
• Fire and grazing on the Konza Prairie, Kansas.

Intermediate Disturbance Hypothesis

• J.H. Connell (1979)
• Diversity is highest at intermediate levels of disturbance.

Relationships between species diversity and disturbance are complicated

• Unimodal, U-shaped, Positive, Negative, No correlation disturbance-diversity patterns