Manipulation of vegetation communities on the Abu Dhabi rangelands. I. The effects of irrigation and release from longterm grazing

Published source details Oatham M.P., Nicholls M.K. & Swingland I.R. (1995) Manipulation of vegetation communities on the Abu Dhabi rangelands. I. The effects of irrigation and release from longterm grazing. Biodiversity and Conservation, 4, 696-709.
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Summary

Sustained intensive grazing pressure on the fragile desert rangelands may result in large scale denudation and erosion. In the Baynunah area, coastal United Arab Emirates, a large area of desert rangeland had been protected from livestock grazing for approximately 11 years to allow vegetation recovery. This study investigated the response of its xerophytic vegetation communities to release from grazing, and also low-level sprinkler irrigation in the autumn, spring and summer months, over the previous 8 years.

Study site: The study was carried out in the Baynunah area located 150 km WSW of Abu Dhabi City, United Arab Emirates. The site surveyed was 18 km by 16 km, situated on the southern edge of the coastal sabkhas (salt flats). The area is a mosaic of gravel plain and mobile sand dunes.

Exclosure and irrigation: In 1979, an exclosure was fenced to protect an area from goat and camel grazing. The exclosure was 12 km by 7 km and contained both gravel plain and sand dune substrata. Parts of the exclosure (both dune and gravel areas) had been seasonally sprinkle irrigated for at least 8 years prior to the survey. Irrigation occurred in the spring, summer and autumn months via high pressure sprinklers arranged every 50 m in lines 100 m apart. Each sprinkler sprayed an area about 40 m in diameter, but the volume of water applied was not recorded. Water was supplied for one to two hours every few days.

Vegetation sampling: Vegetation surveys were carried out during 1991 and 1992, i.e. 11-12 years after the exclosure had been established, in randomly distributed quadrats.

Substratum sampling: A brief survey of some of the physical attributes of the substratum near each vegetation quadrat was made during February 1993 in an attempt to determine factors important in influencing vegetation distribution.

Vegetation: The species richness of perennials on both sand and gravel substrata was significantly greater in quadrats inside the exclosure compared to outside it. Annual plant species richness however, was similar. Perennial percentage covers were lower outside the exclosure compared with inside, especially on sand. The species that benefited most when protected from grazing were the perennials Heliotropium kotschyi, Salsola baryosma, Panicum turgidum and the perennial grass Stipagrostis plumose (which showed the greatest difference in percentage cover), and the annuals Launaea mucronata (both sand and gravel) and Eremobium aegypticum (on sand only). Vegetation was also taller in quadrats inside the exclosure but only significantly so on sandy substrate.

Irrigation: Irrigation did not significantly affect species richness on either sand or gravel within the exclosure, but perennial percentage cover was much greater. The species which benefited most was Zygophyllum hamiense. Conversely, annual plant percentage cover was lower in irrigated quadrats.

Conclusions and discussion: The greater perennial species richness in the quadrats protected from intense grazing suggests that heavy grazing over an extended period of time will result in the creation of species-poor stands comprising plants most able to survive high grazing pressure. In this study, long term exclusion of livestock proved effective in facilitating vegetation recovery. Perennial recovery was enhanced by low intensity sprinkler irrigation. Unfortunately at Baynunah, no data were collected at the time the exclosure was established and it is not possible to interpret the results to determine whether perennial species richness unequivocally decreased as a result of grazing, but comparison with adjacent degraded areas would suggest this to be the case.

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