Invasion of Imperata cylindrica (L.) Beauv. by Eupatorium Species in Northern Thailand

Abstract

The pattern of invasion of native Imperata cylindrica swards by Eupatorium in the highlands of northern Thailand was studied by the technique of comparing invasion at different sites of known history. Eupatorium ground cover decreased with increasing distance from the night camp of cattle. Variations in the curves between different villages was attributed to the number of years of sustained grazing each site had undergone. A more detailed study of one area showed a high correlation (r=0.887) between mean percent Eupatorium and the number of years of grazing, but not for estimated average stocking rate at each site. The correlation coefficient between percent Eupatorium and the product of the number of years grazing and estimated average stocking rate at each site was also high (r=0.894). Some agronomic data for E. adenophorum in one area are also presented. The highlands of north Thailand are populated by seven main ethnic groups who practice shifting cultivation on the steep slopes. While their prime agricultural interest is in subsistence crops or cash crops, primarily opium, ruminant livestock in the form of cattle, buffalo, and goats are also raised. Of these, cattle are present in the greatest numbers by far, and represent a relatively high income-producing potential (Falvey 1977). Imperata cylindrica (L.) Beauv. is distributed throughout most of the tropical world, where it is usually considered to be a weed and has therefore mainly been studied in terms of control (Iven 1975). In the highlands of northern Thailand, however, it forms the basis of the present grazing industries. Imperata often populates the abandoned swiddens of shifting cultivators and persists through its fire-evading characteristics. Fires escape from the annual preparation of new swiddens and burn large proportions of the highlands each year. It is envisaged the Imperata pastures will remain important in the highland situation because, although productivity of cattle grazing these native pastures is low (Falvey et al. 1978), increases in productivity can be obtained through simple supplementation (Mikled 1976). Animal production figures from more tropical countries where Imperata is the main pasture are considerably higher (Magadan et al. 1976), a factor that is probably related to the relative vigor of the swards due to differences in soil fertility and other environmental conditions. A compounding problem of the lower vigor of Imperata swards in the highlands of north Thailand is the invasion by unpalatable broadleafed species of the Eupatoriuim genus under grazing. Eupatorium odoratum (L.) (also known as Chromolaena odorata (L)R.M. King and H. Robinson) is widely distributed in the tropics although it is believed to be a native of the West Indies and Central America. It is a perennial, diffuse, scrambAuthors are livestock specialists, Thai-Australian Highland Agricultural Project, Faculty of Agriculture, Chiang Mai University, Chiang Mai, Thailand. Manuscript received June 25, 1978. ling plant with a deep taproot (Holm et al. 1977). It can grow up to heights of 3 meters under favourable environmental conditions and its invading ability is enhanced by prolific annual seed production and a profuse branching habit (Holm et al. 1977). An allelopathic effect of Eupatorium species has also been suggested (C. Parker, pers. comm.). The species cannot usually persist in a closed canopy forest situation as the plants have light requirements (Ivens 1975), although E. adenophorum may be more tolerant to shading than E. odoratum, at least in the seedling stage (Auld and Martin 1975). Both species prefer well-drained soils (Holm et al. 1977). In the highlands of north Thailand (latitude 17?-24?N), Gibson (1976) has noted that E. adenophorum is usually encountered at altitudes greater than 1,100 metres while E. odoratum occurs at the lower altitudes although there are exceptions to the trend. The mechanism of invasion by Eupatorium into grazed swards of Imperata has not been defined. Casual observations indicate that invasion by Eupatorium increases with the length of time an area of Imperata has been grazed. The shifting cultivation pattern of the highland dwellers and the associated removal of villages at intervals of 1015 years have meant that the effects of high levels of weed invasion have been avoided. Increased population in the area and decreased land available for cultivation due to government activities such as reforestation, have significantly reduced the scope of village removal and have therefore highlighted the problem of Eupatorium invasion of Imperata pastures and, to a certain extent in cropping areas. This paper attempts to describe some of the dynamics of Eupatorium invasion, some characteristics of the species in the environment of the highlights of north Thailand, and the probable role of the species in vegetational change.