Lecture 14 - Disturbances Fire

Question 1

Fire-as a driving force for biodiversity and guide for silvicultural managment

Answer 1

Fire has been present since the origin of terrestrial plants. Undoubtedly, it has acted as en evolutionnary force shaping biodiversity. But the negative socioeconomics impact of many wildfires has indered our understanding of fire ecology, and only are we starting to build a body of information supporting the evolutionnary role of fire.

Gaining insight into this topic is not only important from an evolutionary point of view, but also affects how we view and manage our ecosystems. In places where fire is the main disturbance, silvicultural managment is based on emulating this type of disturbance. But this close-to-nature management approach does have its critics.

Question 2

Fire and insects epidemics account for the dominant distrubances in CAD.

Answer 2

It is in our best interest to understand as much as possible if we hope to emulate these disturbances as part of sustainable forestry.

When considering forest fires we can ask;
* WHAT IS THE FIRE INTERVAL?
* WHAT IS THE FIRE INTENSITY?
* WHAT IS THE FIRE SEVERITY?

Question 3

Grasslands

Answer 3

Grasses are the great success story of evolution. They occur in nearly every habitat-from Antarctic to lush tropics. When drought is periodically severe, grasslands take over the landscape. Grasses are experts as seizing control of the -2m to +2m zone and holding it against the competition from other life forms.

Grasslands fires with short return intervals, prevent encroachment from trees. As the inerval gets longer, we get mixtures of grasses with shrubs and trees: shrublands, steppes and savannas.

Grasses are ideally suited for repeated disturbances: by placing meristematic tissue near, at or below the surface of the soil, they recover quickly from fire and herbivory.

Question 4

Grasses are also designed to burn

Answer 4

Grasses have high surface to volume ratios. A dead grass leaf can be thoroughly soaked by rain at night, yet dry to a point of ignition by mid-day (given moderate humidity levels, some sun and wind).

Before a fuel element can flame, the water it contains must first evaporate so that the fuel element temperature can be raised to 200C.

200C is the temp at which volatilization of organic components begins. **In other words we have combustion. **

Question 5

Rates of spread in grassland fires

Answer 5

The fine and abundant fuel elements require very small inputs of heat energy to reach the point of ignition, allowing fire to propagate readily through the fuel bed.

Rates of spread for a set of Australian wildfires:
* 20/24 wildfires spread at a rate of 100m/min with the fastest 383m/min

Question 6

Forest fires, TWO types:

Answer 6

1. Ground Fires - common in savannas and woodlands (mix of grass and trees)
2. Crown Fires

Question 7

Fire Intensity

Answer 7

Fire intensity is the rate of heat released at the linear flaming front.

Unlike “fire severity” it has a specific meaning along with units:
Fire intensity is measured in kilowatts per m (kW m-1)
The rate of heat release at the flaming front is measured as:
I = 259L^2.2 where, I is intensity (kW/m) and L is length of the flame (m).

Question 8

Ground Fires

Answer 8

For ground fires, flame height is only a few m high at best, thus are relatively of low intensity (e.g. 100 to 15,000kW/m).

Unlike grassland fires, they do not usually spread quickly. Unlike crown fires, they burn only through surface (ground) fuels.

Depending on their intensity: kill the aboveground portion of herbaceous and woody plants… also may partially kill canopy trees.

Question 9

Ground Fires are common in open ecosystems

Answer 9

Ground fires can become passive crown fires. Flame rises up the tree briefly turning into a crown fire, but then drops back down again.

In order to maintain a crown fire, you need high rates of fire spread which can be achieved if trees are densely packed (measured as: crown bulk densities (kg/m3)), or really high wind speeds.

Question 10

Crown Fires

Answer 10

Crown fires form a wall of flames extending from the ground to the tree canopies.

Relatively of high intensity (e.g. 8,000 to 100,000kW/m).
Burn through ground fuels and crown fuels.
Usually spread rapidly.

Crown fires are also high severity: all strata burn in Canada, only conifer forests will support crowning combustion.

Question 11

The occurence of active crown fires is limited by three factors:

Answer 11

1. A critical fire intensity that ignites the crown fuels.
2. A critical rate of spread below which the crown fire cannot be sustained-hence dependent on bulk density.
3. A critical horizontal heat flux that results primarily from burining tree crowns (intensity).

Question 12

Surface fuels organic litter (falen leaves) live herbaceous plants.

Answer 12

Canopy fuels include small diameter branches, leaves, buds. The high surface-to-volume ratio allows greater thermal contact with the air allowing it to reach temperature of volatilization.

Fuels that ignite at the lowest temperatures and release high-energy gases are called extractives. These includes waxes, oils, fats. The combustion of extractives is necessary to raise the temperature of fuels like cellulose allowing them to volatilize.