Forest Ecology & Disturbance Ecology Notes

Definition: A relatively discrete event in time and space that alters the structure of populations, communities, and ecosystems and causes changes in resource availability and/or the physical environment (Pickett and White 1985, Pickett et al. 1999, Chapin et al. 2002).

A substance or object required by a living organism for normal growth, maintenance, and reproduction.
It is consumed by one organism and, as a result, becomes unavailable to another organism.

Must be defined within the normal range of environmental variation that an ecosystem experiences.
- For example, herbivory is normal, but stand-killing insect outbreaks are a disturbance.
It's not an external event that “happens” to an ecosystem.
- Natural disturbances are necessary for ecosystem function; they are a part of the ecosystem.

The impact of a disturbance on an ecosystem depends on several factors:
- Size, pattern, severity, frequency, timing, intensity, and type of disturbance.
These factors together constitute a disturbance regime.

Influences the lateral flow of materials and organisms.
Can shift vegetation density and composition of the ecosystem.
Disturbances are often patchy; patches of undisturbed vegetation can act as seed sources or refugia for new growth influencing the effective size of the disturbance.

Magnitude of change in resource supply or environment.
Typically, is the amount of organic matter (plants/soils) removed by the disturbance.
A major factor determining the rate and trajectory of vegetation development after disturbance.
Increased fire severity can lead to compositional shifts in vegetation, for example a shift from black spruce to aspen following high-severity fires.
Table 1 (Modified from Ryan 2002 and Turner et al. 1994):
- Unburned: Plant parts green and unaltered, no direct effect from heat
- Scorched: Unburned, but plants exhibit leaf loss from radiated heat. Canopy trees with green needles although stems scorched
- Light: Surface litter, mosses, and herbs charred or consumed. Soil organic layer largely intact and charring limited to a few mm depth
- Moderate or severe surface burn: Trees with some canopy cover killed, but needles not consumed. All understorey plants charred or consumed. Fine dead twigs on soil surface consumed and logs charred. Pre-fire soil organic layer largely consumed
- Deep burning or crown fire: Canopy trees killed and needles consumed. Surface litter of all sizes and soil organic layer largely consumed. White ash deposition and charred organic matter to several cm depth

Adventitious roots of black spruce trees lay exposed because the organic soil layer was consumed by wildfire.
Sphagnum moss often holds so much water that fire will pass it by, leaving "sphagnum sheep-sheep sized mounds of light colored, unburned moss peppering the blackened landscape."
Table 1 (Kasischke et al. 2008): Site characteristics used to quantify fire severity in black spruce forests and their possible effects on ecosystems.
- Tree canopy characteristics: Level of consumption of canopy foliage and stems, Percent of tree boles standing after the fire
- Surface organic layer depth characteristics: Depth of burning, Characteristics of exposed substrate (fibric, mesic, humic, or mineral soil), Relative depth of burning (percent reduction compared with pre-burn depth)
- Organic and mineral soil physical and chemical characteristics: Bulk density, Percent carbon, Nutrient content, Ash deposition, Water repellency and infiltration rate
- Ecosystem/environmental effect: Seed dispersal level, Seed dispersal distance, Carbon emissions, loss of nutrients, Seedbed characteristics (seedling recruitment, survival and growth), propagule level for vegetative reproduction, water soil repellency, Changes in soil temperature and moisture, propagule level for vegetative reproduction, Seedbed characteristics, water soil repellency, carbon storage and loss, Carbon storage and loss, Nutrient storage and loss, Nutrient availability, Soil water repellency

Technically, energy release per unit area and time.
For example, Fire intensity = rate of heat production ( $depends$ on mass, consumption rate, and energy content of fuel).
Others call intensity ‘force of disturbance’
Table 1.2: Measurements of disturbance intensity for a variety of natural disturbances.
- Volcano: Volcanic explosivity index (height and volume of tephra plume)
- Earthquake: Horizontal displacement (Richter and moment magnitude scales)
- Dune: Depth of sand added
- Erosion: Depth of soil displaced
- Cyclone: Wind speed (five categories); atmospheric pressure
- Flood: Depth above mean level; rate of rise; flow rate; mm rain per hour
- Drought: Days without rain; departure from mean precipitation and Temperature, duration.
- Fire: Temperature, duration
- Tsunami: Wave height; distance traveled inland

Disturbances can be intense but not severe.
- Very ‘hot’ fire that sweeps through an area but does not consume much organic material.
Key Differences (Keeley 2009):
- Fire Intensity: Energy output from fire.
  - Metrics: time-averaged energy flux in $W/m^2$ , fireline intensity, temperature, residence time, radiant energy.
- Fire Severity: Aboveground and belowground organic matter consumption from fire.
  - Metrics: aboveground measures include tree crown canopy scorch, crown volume kill, bole height scorch, skeleton twig diameter. Belowground and soil measures include ash deposition, surface organic matter, belowground organic matter contributing to soil structure, degree of hydrophobicity, and heat-induced oxidation of minerals. Mortality is a common measure that is best applied to non-sprouting trees in surface fire regimes. In crown fire regimes, aboveground mortality may be useful when fires are patchy.
Ecosystem Response to Intensity and Severity:
- Intensity: Energy released, Erosion
- Severity: Organic matter loss, Vegetation recovery
- Societal Impacts include the loss of life or property and suppression costs

How often the disturbance occurs.
Ranges from daily (e.g., herbivory) to rarely (e.g., volcano eruption).
Ecosystems are most resilient to disturbances they frequently experience.
Affects the ability and amount of time available for recovery.
Severity vs. Frequency:
- Less severe disturbances generally occur more frequently, and vice versa.
- Examples ranging from high severity/low frequency (Siberian meteor strike, glaciations) to low severity/high frequency (annual fires, single tree blowdown).

The time required for an ecosystem to accumulate 50% of its maximum biomass is related to fire frequency (Chapin and Van Cleve 1981).
Ecosystems with frequent disturbance recover more quickly.
Decreased fire return interval (FRI) = less time to recover.
A stand with a "Decreased" FRI will not recover as much as a stand with a "Normal" FRI