Despite enormous devastation resulting from massive bushfires, commonly pastures recover and with care can return to full potential. However recovery rate is dependent on many factors, including follow-up rain, previous grazing management and strength of the pasture, groundcover level and related fire heat and duration on a given area.
Heat plays a factor in pasture recovery. Most paddocks are affected between “cool” and “hot” burns. “Very hot” burns are defined as “all plant material destroyed, the soil surface virtually sterilised”. This mainly happens with high levels of herbage at the time of fire (a very good pasture closed since spring).
Where plant material is moderate, such as lightly grazed pasture or crop stubble, burns are mainly rated “moderate to hot”. Research indicates that during a bushfire soil surface temperature usually varies from 50 to 250 degrees Celsius. Temperature at soil depth below 15 millimetres is normally not raised more than 10C, and returns to original temperature within five minutes. This indicates that seed of annual plants buried, such as sub clover, or with growing points below the surface, such as many perennial grasses, will survive and regenerate.
Four years ago one of the most ferocious bushfires ever recorded in our district burned out large areas of nearly all pasture types, from native to introduced tropical grasses, such as bambatsi panic and Premier digit grass. Depending on follow-up rain and seasonal conditions, most of these returned to productivity levels similar to their normal expectation. Elliot and Kate Shannon, “Tiona”, Bugaldie, for example, had tropical pastures based on Premier digit grass return to full production once good rains occurred. Yet the bushfire across parts of their property was ferocious, coming out of adjoining Warrumbungle national park.
Lucerne generally recovers well after fire, especially if stands are strong. However older stands or those with low root reserves (a consequence of previous grazing management and/or age) may be more severely damaged and slower to recover.
Temperate perennials like phalaris are generally dormant or semi dormant in summer, especially if a dry period preceded the fire. Often they recover well, especially if stands have been well established and managed. However recovery is often not until autumn.
Viable seed reserves of sub clover are particularly adept at surviving bushfires, as varieties grown on many properties have a large percentage of their seed buried into the ground at spring seed down. Also a reasonable percentage of seed is “hard”, providing extra protection against short-term hot soil temperatures.
Annual legumes such as serradella, biserrula, medics, various clovers, and woolly pod vetch, while aerial seeders, tend to have good levels of seed survive bushfires. Commonly by midsummer much of their seed has fallen to the ground and mixed into topsoil. A good percentage of seed is also “hard”, bolstering fire protection.
Levels of annual legume soil seed reserves before a bushfire are an important consideration. Fortunately for many pastures, 2016 was a good spring with good seed set. Previous management including soil fertility impacts on pre-fire seed set and soil seed reserves.
Soil type must be considered. Lighter soils respond to small rainfall events, but heavy soil pastures may require far bigger rainfall events to recover. Managing recovering pastures is also important. The challenge is to maximise post-fire pasture (and crop) growth and this generally means holding off grazing to allow plants to develop adequate root systems (annuals) and enough leaf area to best utilise light.
Next week. Early sowing and more nitrogen big canola issues.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
