Many pastures will recover well following bushfires. Recovery will depend on many factors such as pre-fire residue levels, soil type, pasture type and previous pasture management. In many cases cropping programs have a good chance of being relatively unaffected, although burnt stubble can mean poorer capture of fallow rain.
Past CSIRO research categorises fire impact into four levels; 'cool burn', 'moderate burn', 'hot burn' and 'very hot burn'. Because of drought, many pastures were well grazed prior to fire with impact likely to be in the "moderate burn" category. Where denser dead plant material, such as lightly grazed good pastures or un-grazed crop stubble, moderate to hot burn would likely be the category. Un-grazed pastures with high dry matter loads can result in hot to very hot burns. Zero till farming with stubble retention commonly has fire pass through them rapidly.
Research suggests temperature below the soil surface, especially for cool to moderate burns, does not change dramatically. Below 15 millimetres it is normally not raised more than 10 degrees, and returns to its original temperature within five minutes. This suggests plants that bury their seed, such as subterranean clover, or that have growing points below the surface, such as many perennial grasses, will survive well.
One of the biggest impacts of post-fire recovery is lack of ground cover. Heavy rain events on bare sloping ground, unfortunately commonly result in much of it not entering the soil with it running off the paddock. That is much the same situation as heavily drought grazed paddocks with little to no retained ground cover.
In time, these paddocks are capable of good recovery, speed of which will largely depend on rain amount and intensity, and allowing new growth to provide ground cover for more efficient rain capture. I have seen frightfully burnt country recover to full productivity, provided carefully managed as ground cover post-fire occurs.
Especially good at recovery have been tropical grasses with recent experience in a large damaging bushfire at Dunedoo Coolah Cassilis (over three years ago) and seven years ago in the Coonabarabran area. Premier digit, Consol lovegrass and Bambatsi panic especially recovered well. Where good rain followed, in some cases within a couple of weeks, these were back in full production within a couple of months.
Slowest recovery is common on badly burnt heavier soil areas. Smaller rain events can quickly start pasture recovery on lighter soils but soaking rains are more needed for heavy soil ones. However with reasonable rains most perennial pastures, provided they haven't thinned for other reasons, such as drought related, will recover well.
Native grasses are generally slower at recovery but are well adapted to fires. Temperate perennials like phalaris are fairly fire immune but shallower rooting species like cocksfoot and perennial ryegrass are more fire susceptible, especially if the fire was rated above moderate. Generally, lucerne recovery is successful, provided not over damaged via prior drought conditions. Its lack of ground cover, especially on harder setting soils on sloping country often means it can be slow coming back.
Annual winter legumes with high hard seed content generally have good levels of viable seed for following autumn regeneration. Sub clover buries a lot of its seed (most but not all varieties). Seed of aerial seeding species, commonly have a fair bit of it mixed into the top few centimetres of soil, providing it with useful soil reserves.
Weeds are commonly a post-fire issue. It is important to assess carefully herbicide treatment of weeds as some herbicides can adversely affect legumes like various clovers. If the following autumn/winter is below par, that legume herbicide retardation can add to winter feed shortage.
Sound grazing management is an important part of post-fire recovery. Where possible allow pastures to develop good growth before grazing.
More details can be found in the NSW DPI updated publication "Regaining pasture productivity after fire", available on their web site of from LLS offices'.
Next week: Cropping with low to moderate soil water levels.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
