Burnt paddocks have an increased risk of erosion
Fire intensity matters
Fire intensity – how hot the fire is for how long – has a major effect on recovery of pasture.
The 3 categories of intensity are:
- Cool–moderate burns: most dead plant material is burnt with some seed and most subterranean clover surviving. There will usually be a small residue of unburnt pasture or stubble. There is an increased risk of wind erosion, but there is little effect on a following crop. Soil organic carbon is not reduced, and soil organisms are largely unaffected.
- Hot burns: all dead plant material and many seeds are burnt. The topsoil usually appears charred and bare. Weed control in a following crop is improved; soil pH under burnt windrows will be increased. Soil organic carbon might be reduced in the top few millimetres, and soil organisms will be reduced or changed in that zone.
- Very hot burns: the surface layer of soil is virtually sterilised. All surface plant material and seed is destroyed, and some sub-clover seed is bunt or sterilised. Soil organic carbon might be reduced in the top 10–15 mm, and soil organisms will be killed or reduced in that zone.
Temperatures at the soil surface reach about 600°C in a very hot burn, about 100–250°C in a hot burn, and 50–150°C in a cool–moderate burn. The soil 15 mm below the surface is usually not heated by more than 10°C and returns to its original temperature within 5 minutes (CSIRO). Plants that bury their seed or that have growing points below the surface have better survival after fire.
What you can do to help pasture recovery
For different fire intensity:
- For a cool–moderate burn, the pasture should recover to its original density during the following year, given adequate moisture and the absence of soil nutrient deficiencies.
- For a hot burn, manage the recovering pasture to maximise flowering and seedset for subsequent years.
- For a very hot burn, almost all plant material will be dead so the area should be cropped or resown to pasture following the fire.
Options to improve recovery of pasture after fire are:
- Sow new pasture seed: if weeds are dense, chemical weed control may be necessary before sowing the seed.
- Heavy harrow the paddock when the soil is moist: heavy harrowing can unearth buried seed and improve germination, especially of subterranean clover. Wind erosion risk will be increased with this option, and this requires special management.
- Defer grazing: do not graze the burnt pastures for 6 or more weeks after germination.
- Add fertiliser: where there is an adequate density of regenerating plants, or pasture seed has been sown, adding fertiliser may speed early growth and recovery of the pasture. Use soil test results to decide on fertiliser rates.
- Control broadleaf weeds: where broadleaf weeds could dominate the recovering pasture, control them with selective herbicides or spray-grazing.
- Avoid cutting for hay or heavy grazing pressure during flowering and seedset. Aim to increase seedset and seed reserves for the following season.
Effects of fire on annual species
Most annual grasses produce very little dormant seed. Usually 80–90% of the seed in one season will germinate in the following autumn. This means that fire which destroys annual grass seed will cause a drastic reduction in the annual grass component in the following pasture.
All seed of the major volunteer grass species in south-west Western Australian pastures – such as barley grass, brome grass and silver grass – remain on or very close to the soil surface. This makes the seed vulnerable to being destroyed or sterilised as the fire passes.
Subterranean clover buries its seed, substantially reducing the damage to seed caused by fire. Unlike annual grasses, subterranean clover also has large reserves of dormant seed in the soil.
Survival and growth of subterranean clover and the annual grasses after a fire is highly dependent on rains after the break of season. Soil dries out faster where fire has removed surface litter. After a reasonable germination, many plants may be stunted or die following short dry spells. However, hot fires will probably remove most of the grass seed and competition for moisture at the start of the season will be reduced. It is very important to defer grazing on these pastures.
Effect on perennial species
Recently resown perennial grass pastures can be seriously damaged by fire. The young perennial plants without well-established root systems and reserves are more vulnerable to fire damage, especially if the pasture was sown with a cover crop that fuels the fire. Almost all well-established perennial grasses survive a cool–moderate burn well. The ability to survive a hot burn varies between species: grasses with growing points below the soil surface survive best.
Established lucerne survives even a very hot burn because of the large underground root reserves and growing points. Newly sown lucerne (less than 6 months old) can be killed by a moderate burn, particularly if there are a lot of weeds mixed through the stand to fuel the fire.
Effect on weeds
The seed of annual weeds can be destroyed by fire in the same way as annual pasture plant seed. The massive amount of seedset by weeds, such as capeweed, often means that some still survive in the pasture in reduced densities. Also, burnt pastures are more susceptible to weed invasion.
Effect on soil fertility
Nutrients present in standing plant material are returned to the soil in the ash. In the long term, a pasture does not gain any additional nutrients as a result of a fire.
Fire generally reduces the amount of nitrogen available to the recovering pasture because organic matter near the soil surface is burnt, releasing nitrogen to the atmosphere. As a result, some burnt pastures show symptoms of nitrogen deficiency (pale to yellow leaves) during the following winter.
Pastures with a good fertiliser history recover much faster and to a greater degree than those with a poor history. After a cool–moderate burn on pastures on low to moderately fertile soil, we recommend soil testing and adding nutrients based on the results.
Effect on carrying capacity
Fire reduces standing feed and regrowth, which reduces the carrying capacity of a pasture in the year after a fire. In subclover-based pastures, most of the carrying capacity loss is from annual grass seed being killed.
The loss of carrying capacity is less with:
- a cooler burn
- an early and reliable autumn break
- a higher proportion of improved perennials in the pasture before the fire
- higher soil fertility.
Most south-west pastures will recover their carrying capacity by the second growing season after a late summer fire.