RESEARCH LETTER No. 11

Between LIFE and DEATH: the GROWTH and CURING of GRASSES

The proportion of dead grass in a pasture has a major effect on the rate of spread of grass fires (McArthur 1966; Cheney et al. 1998) and is important to the calculation of ‘grassland fire danger’ (McArthur 1966), the basis of Canberra fire-danger rating. This proportion is called ‘curing’.

If the pasture is all green, fires won’t travel. If it is all dead and dry there is no dampening of spread. Somewhere in between, then, there is a boundary line, a threshold, delimiting ‘spread’ from ‘no-spread’ conditions - all else being the same. McArthur’s meter (McArthur 1966) starts at a curing of 70% thereby implying a threshold at this level. Based on experiment and expert knowledge, however, Cheney et al. (1998) suggested that 50% is the threshold although they indicated that the effect of curing between 50 and 60% on head fire rate of spread is relatively small.

The effect of curing on fires is through its moisture content. The more green grass there is the higher is the moisture content. Dead grass might reach 30% moisture content but green grass may be 100% (i.e. equal weights of water and solids). In using curing the assumption is that the moisture content of green grass is constant. This assumption hasn’t been systematically tested. The moisture content of dry grass will vary with temperature humidity, solar radiation and wind (after Byram and Jemison 1943).

How do you determine curing? Mostly this is done by direct estimation – by eye. In the ACT aerial assessment is used to create maps of the variation along with careful observation at ground stations. The latter observations have been found to agree well with a computer model for the growth of grasslands (Moore et al.1997) used by the author in association with Peter Moore (CSIRO Plant Industry) and Rick McRae (Emergency Services Bureau). Satellite-sensing can also be used.

The main problems in determining curing at any point on the ground are:

Curing, at any time, varies according to the species’ of grass involved. The curing of Phalaris may differ from that of Themeda (Kangaroo Grass) for long periods of the year. Broad-leafed weeds can have a major effect on green-ness too.

The difficulties are not just with the determination of curing at a point. When a figure is given for an entire grassland or landscape questions of patchiness emerge. Should there be one value given for the whole of the lower elevation ACT or should several areas be targeted when grassland fire danger is calculated and broadcast? If all the ridges were cured but narrow strips along the valleys were green then do you say there is 99% curing? Grazing affects curing too so different paddocks will vary according to stocking rate.

Determining curing, and fire spread, either locally or across landscapes, when curing is incomplete is a difficult task. When everything is dead, or everything green and alive, it’s much easier.

Literature cited

Byram, G.M. and Jemison, G.M. Solar radiation and forest fuel moisture. J. Agric. Res. 67, 149-176.

Cheney, N.P., Gould, J.S. and Catchpole, W.R. (1998). Prediction of fire spread in grasslands. International Journal of Wildland Fire 8, 1-13.

McArthur, A.G. (1966). Weather and grassland fire behaviour. Comm. of Australia Forestry and Timber Bureau Leaflet 100, 23p.

Moore, A. D., Donnelly, J.R. and Freer, M. (1997). GRAZPLAN: Decision support systems for Australian grazing enterprises. III Pasture growth and soil moisture submodels, and the GrassGro DSS. Agricultural Systems 55, 535-582.

A. Malcolm Gill
13 May 1999