El Ninosouthern Oscillation And Vegetation Changes

Since the native Australian vegetation was adapted to the climate rhythms and variability induced by the ENSO, it is not surprising that the introduction of plants and animals not so adapted led to rapid changes in vegetation (Nicholls, I99l). The best known of these changes is probably the area now known as the Pilliga Scrub in northern New South Wales (Rolls, 1981; Austin and Williams, 1988). Much of this area of 400,000 ha was open grassy country with only about eight large trees per hectare when Europeans arrived in the 1830s. Frequent burning by Aboriginals, and grazing by indigenous marsupials, restricted the opportunities for trees and shrubs to establish. Fire germinated the seed of the trees and shrubs, but rat kangaroos ate many of the resulting seedlings before they could establish.

The introduction of sheep reduced the numbers of rat kangaroos, by destroying their cover and their food. A severe drought during the major El Niño of 1877-1978 further reduced the numbers of indigenous marsupials. The following year, a major La Niña event, was very wet. The few large trees seeded well and when stock owners burnt to destroy grasses with seeds that got into their sheep's wool, seedlings came up thickly, unhindered by the grasses that would usually compete with them for space. This time there were no rat kangaroos to eat the seedlings either and the trees grew unchecked.

Over the next decade there were several further periods of establishment, again synchronized with El Niño-La Niña oscillations. The European rabbit, also an enthusiastic eater of seedlings, arrived in the area in the late 1880s and prevented further establishment until myxomatosis in 1951 reduced the rabbit population. The first successful release of myxomatosis occurred in 1950. Earlier releases of the disease had not led to widespread establishment. The extensive rains and flooding in 1950, associated with a major La Niña, contributed to the successful establishment of the disease by providing ideal breeding conditions for the insects that spread it.

In 1917 the Forestry Commission stopped burning in the Pilliga and by 1950 large amounts of forest litter had accumulated. So had decades of seed production. The forest dried in El Niño event of 1951, following good growth during La Niña of 1950, and a major fire started in November 1951. In the absence of rat kangaroos and rabbits, the new growth induced by the fire bad nothing to stop it.

In less than a century Europeans had unintentionally transformed the area from grazing land into the dense Pilliga Scrub supporting sustained timber harvesting. The ENSO phenomenon played a critical role in this transformation. McKeon et al. (1990) cite other examples where the extreme climate events associated with both extremes of the ENSO resulted in major long-term vegetation degradation. In western Queensland there was a rapid increase in the sheep population during the above-average rainfall years of the early 1890s. Major El Niño events between 1899

Rimmington and Nicholls (1993) demonstrated that wheat yields in all states were correlated with values of the SOI from before and near the sowing date, which therefore can provide skillful yield forecasts of Australia's major crop. These forecasts would be available several months before harvest starts, require little data, and are quick and simple to prepare. Strong negative relationships also exist with the SOI in the year before the crop is planted, i.e., an El Niño episode is often followed by good crops the following year. This partly reflects the biennial nature of the ENSO, but may also reflect a tendency for a drop in pests in the droughts associated with negative SOI values. This would amplify any response of the crops to good rains in the following year.

Hammer et al. (1996) examined the value of ENSO-based forecasting methodologies to wheat crop management in northern Australia, by examining decisions on nitrogen fertilizer and cultivar maturity using simulation analyses of specific production scenarios. The average profit and risk of making a loss were calculated for the possible range of fixed (i.e., the same each year) and tactical (i.e., varying depending on the ENSO-based seasonal forecast) strategies. Significant increases in profit (up to 20%) and/or reduction in risk (up to 35%) of making a loss were associated with the tactical (forecast-based) strategies. The skill in seasonal rainfall and frost predictions, based on the ENSO, generated the value from using tactical management. This study demonstrated that the skill obtainable in Australia was sufficient to justify, on economic grounds, their use in crop management. Presumably, these forecasts could also be useful in drought management decision making, for instance, in determination of appropriate stocking rates on pastoral properties (McKeon et al., 1990).

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