Forest Succession and Disturbance

ENVS 31 - Forest Ecology

What is a Disturbance?

Any discrete event in time that disrupts an ecosystem, community, or population structure, composition, or function and changes resource availability and physical structure.

Gap-Scale Disturbances

Disturbances that are small-scale and create patches, rather than being large-scale/regional.

How is disturbance severity classified?

Based on the amount of vegetation killed.

Disturbances result in…..

Succession: The process of how a community or ecosystem shifts over time. Driven by differential species availability and performance OR site availability/disturbance severity.

How do management strategies for the same disturbance types vary?

Vary based on the goals are in the particular location, context-dependent.

What are the two models of succession?

The Facilitation Model of Succession and The Tolerance Model of Succession

The Facilitation Model of Succession

Earliest species to arrive following a disturbance (pioneer species) modify the environment in positive ways to benefit later species (e.g. increase soil moisture and nutrients) but hinder their own dominance (b/c this allows new species to thrive and outcompete the pioneers). Each level of succession facilitates the next level by altering the physical environment and allowing the next stage to outcompete in it.

The Tolerance Model of Succession

Earliest species (pioneer species) modify the environment in neutral ways to neither benefit nor hurt other species. Later successional species replace pioneer species b/c they are more successful at tolerating lower resource levels, rather than being due to modification or active inhibition.

Tree Species Silvics

The qualities of a tree and how it interacts with its environment. You can often tell if species will do well in an environment based on these qualities.

Silvics

Biological characteristics of individual trees and communities of trees. This includes growth and reproduction, as well as response to the physical environment (soils, moisture, light availability).

What is the key Silvical Characteristic?

Shade tolerance: The capacity of trees to grow and persist in low light levels.

Which species of tree are extremely tolerant of low light conditions?

Balsam Fir, Eastern Hemlock, American Beech, Sugar Maple.

Which species of tree is extremely intolerant of low light conditions?

Aspen

What does the severity of disturbance change about the succession that follows it?

Changes the forms of reproduction that are favored, and what types of trees are favored.

High Severity Disturbances Favor…

Shade intolerant pioneer species.

• Wind-dispersed seeds (aspen, paper birch).

• On-site sources of seeds

Moderate Severity Disturbances Favor…

Predominantly intolerant to mid-tolerant species.

• Wind-dispersed seeds.

• Thick-barked species (red, white, ponderosa pine).

• Stump Sprouting Species (oaks, maples) can regenerate post-moderate disturbances.

Low Severity Disturbances Favor…

Predominantly shade tolerant species with slow juvenile growth.

• Advanced Regeneration: Seedlings or saplings that develop or are present in the understory.

What is an example of a large, infrequent disturbance in New England Forests.

Hurricanes.

What did hurricane modeling in New England show?

• Increasing wind 8% increased the affected area by a magnitude of 10, and 16% a magnitude of 27x.

• Each hurricane wipes out 10+ years of carbon sequestration: A single storm was shown to down 121-250MMTCO2e, which is equal to 4.6% to 9.4% of total aboveground forest carbon, while only 15.8MMTCO2e is sequestered by New England forests annually. 

• 66% of the downed carbon storage will be emitted after 30 years.

• 79% emitted after 60 years.