Topic 12- Forest Communities

What is a community?

• Organisms that occupy a common geographic space

• Environment/ ecological conditions past and present that caused the assemblage to evolve and persist.

• Community as a property of ecosystems

Community → Habitat → Ecosystem

Shawnee Hills of Illinois.

1.) Community changes driven by variation in Loess Deposition

2.) More loess on western side than eastern. Causes a productivity gradient in forests where the western forest are more productive than east

3.) More recent time scale, changes in the composition due to fire supression, leading to a mesophycation on once dry sites

4.) Black oaks were displaced by red oaks which were moisture loving

5.) White and black oaks were displaced my sugar maple which is more mesic.

Initial focus in community ecology

Primary vs secondary succession

Clemensonian View

• Plant associations uniform

• Communities as superorganisms

• Climax community was considered stablE

• Predicatbale stages of life

Gleasonian View

• Heterogenity was considered within communities

• Individual attributes considered (Indivisual species responding in their own way)

• Variation due to chance and environmental effects

Continuum Concept

• Species change gradually across the landscape depending on the environment

• Grew out of the Gleason school of community ecology

• Acknowledgment of continuous variation in plant communities across landscapes

• Eventually both continuous change and abrupt change are accepted

• Struggles to explain ecotones

What is a ecotone

• Transition zones between two communities

• Band that defines the ecotone can vary in width, generally not linear

• Abrupt changes in parent material or topography can cause narrow ecotones

Abrupt vs Gradual Ecotones

Abrupt

• Tepuis of Venezuela. Completely different communities on top and bottom, each ecotone has endemic plants on it

Gradual

• Great smoky mountains

Forest- Grassland ecotones

• Abrupt ecotone in many areas

• Fire, grazing, nutrient & water availability, community composition all important factors

• In many deforestation areas that have become grasslands, the ecotone can persist for hundreds of years

Eastern Decidous forest ecotones

• Charcterzied by many gradual changes at both the broad and local scales

• Great smoky mountains are characterized by continuous gradiation

• Greatest complexity in Eastern Ky and Tennessee- Cumberland Plateau

Interactions among organisms

• Competition

• Niche Differentation

• Facilitation

• Verticle structure

• Mutualisms

• Crown Class differentation

• Age of forest

Mutualisms

• Both sides benefits

• Symbioses-Mycorrhizae

• No symbiotic mutualisms- mutual benefit but lack a physical connection

• (Pollination, Seed dispersal, Decomposition, Protection

Competition

• Two species fighting in for the same limiting resource

• Antagonistic relationship between two species attempting to utilize the same limiting resources

• If resources are not limited but one species blocks another from acquiring

• Trade-offs for both species

• Can lead to a decrease in diversity if the fundamental & realized niches of both species overlap

• The weaker species can shrink its realized niche to avoid competition. Diversity can be maintained or even promoted

Progression of Crown Classes

• Even aged stands

• Competition within the stand over time results in differences in the vigor of trees

Progression of trees (Overtopped → Dominant)

Dominant trees

• Trees with crowns extending above the general level of the canopy

• Receiving full light from top and some on the sides

• Well- developed spreading crowns

Co-dominant trees

• Tree crowns developing the average height of the canopy or slightly below

• Receive full light from above but moderate or limited from the sides

• Medium Sized Crowns

Intermediate

• Shorter than the above

• Crowns extend into the canopy but receive little direct light from above and little to none from the sides

• Crowns are small and crowded

Overtopped

• trees with crowns entirely below the canopy Layer

• Not receiving direct light from either the sides or top

Niche Differentation

-Specialization that minimizes completion but maximizes diversity

• different species occupy different habitats of the forest

• Active different times of the day

• Different food sources

• Timing of water use

• Location of water sourcing

• Specialize for a particular light environment (Spatial vs Temporal adaptations)

Facilitation

• One species helps another, not necessarily meaning too and doesn’t receive a benefit

• Hydraulic lift- deep roots pull up deep water, leaks, shallow roots gets it

• Mycorrhizal Networks- fungi on ones roots can move nutrients to another

• Nurse plants- Shurbs can shade/moisture/protect a seedling

• Kin recognition

Vertical Structure

• The dominant species can dictate the structure of the rest of the forest. Can facilitate or inhibit other species from colonizing

Maple Beech Forests

• More dense and let little light through

• Leading fewer shrubs, though some shade tolerant species can be found

Oak dominated forests

• a fair amount of light penetrates the canopy

• Tend to have a well-developed shrub layer

Age of the forest

• The relative importance of different processing in maintains diversity depends on the age of the forest

Fundamental Niche

Where a species could live. Has what the species needs to survive

Realized niche

Actual habitat

Not living somewhere vs can’t live somewhere

Not living somewhere= realized niche limitation. Something is preventing this species from living there. Pest, other dominant species, etc

Can’t live somewhere= fundamental niche limitation. Incapable of survival

Competitive exclusion principle

Two species that use the exact same resources can not coexist, one will win, forcing the other into a reduced realized niche