Understanding Ecosystem Disturbances: Wind and Fire Impacts
Ecosystem Disturbances: Wind and Fire Impacts
A disturbance is a relatively discrete event in time that causes a disruption in ecosystems, changing the substrate and availability of resources, and creating opportunities for the establishment of new individuals. Allogenic disturbances are partly allogenic and autogenic because their impact is the result of both the magnitude of the disturbance and the susceptibility of the community to the particular type of alteration.
Key Temporal and Spatial Attributes of Disturbances
- Type: Agents of disturbance (natural or anthropic)
- Area (scale): Size of the area altered
- Frequency: Average number of events per unit of time
- Return Interval: The average time between disturbances (1/Frequency)
- Intensity: The physical strength of the event per area and time
- Severity: Impact on organisms or community
- Timing and Seasonality: When the disturbance occurs
- Interaction: How disturbances interact with each other
Wind Disturbance and Increased Susceptibility
Mature trees dominant in the canopy, those exposed in the stand or at gap edges, and those that grow in shallow soils with shallow rooting are more susceptible to wind damage.
The impact of the wind is accentuated when accompanied by snow (due to added mass) and rain (as it weakens the roots’ hold on the soil). Trees weakened by insect and fungal attacks, damaged by lightning, or with abundant epiphyte biomass (> 0.5 tons) are also more susceptible.
Types of Winds
- Hurricane (200-250 km/hr): Can cause great damage to the landscape (e.g., 50 km-wide with Hurricane Andrew in 1992).
- Tornadoes (> 430 km/hr): Damage is more localized.
- Wind gusts associated with thunderstorms (90-200 km/hr): Affect a variable surface area, uprooting trees or causing breakage.
- Downburst (heavy rain, rough and windy): Intense, localized downdrafts.
Stands most affected by the wind are those within 600m of the summit with a north-east exposure.
Conclusions on Wind Disturbance
- Topographic position and fire history influence windthrow patterns.
- Younger forests were less affected than mature forests (especially with severity < 80%).
- Stands at higher altitudes, near the summits, and with specific exposures were most affected.
- Legacies of earlier unrest, several centuries ago, influence the effects of subsequent disturbances.
- Interactions between disturbances and the physical environment determine the dynamics of the ecosystem.
Fire Disturbance
Conditions for Fire: Accumulation of organic matter, dry weather, landscape conducive to propagation, and an ignition source (e.g., lightning and low humidity).
Types of Fires
- Underground:
- Fires without flames that consume the organic matter in the soil.
- Spreads slowly, burning the roots and killing all species.
- Produce little smoke and are therefore difficult to detect.
- In fens or bogs, they can be very dangerous to people because they are not easily seen.
- Surface:
- Common in open woods with a long dry season during the year.
- Kills most of the herbaceous and shrub layers, young trees, and consumes the litter layer, humus, and dead woody material.
- Usually does not damage the roots, bulbs, rhizomes of plants, or the trunks of trees.
- Spreads rapidly, and depending on fire intensity and thickness of bark, can lead to death or scars on trees.
- Crown Fires:
- Common in dense forests where fire travels from treetop to treetop.
- High fire severity and speed of propagation, resulting in high mortality.