The effect of grazing removal on development and species richness of nebkhas around white broom Retama raetam at Zaranik Nature Reserve, northern Sinai, Egypt

  • Published source details El-Bana M.I., Nijs I. & Khedr A.A. (2003) The importance of phyangelands in Northern Sinai. Restoration Ecology, 11, 317-324


Accumulation of wind-borne sediments within or around plants plays an important role in the ecology of many sandy coastal and desert ecosystems. The accumulation of sediment and other debris often lead to the formation of phytogenic mounds or nebkhas. In such arid and semiarid habitats, where water and nutrients are often severely limited, these vegetated hummocks cause local changes in microclimate and soil properties. In restoration of arid and semiarid environments it is important to be aware of the potential role of such features. This study tested whether the nebkhas forming around a leguminous shrub, white broom Retama raetam, promote restoration of herbaceous vegetation and soil in the degraded sandy areas of northern Sinai, Egypt. Vegetation, microclimatic and edaphic characteristics within the nebkhas and spaces between them were investigated. Ungrazed and grazed sites were compared to see if grazing influence abundance and richness of herbaceous species.

Study sites: The study was undertaken at Zaranik Nature Reserve on the Mediterranean coast of the Sinai Peninsula, Egypt. A survey located white broom Retama raetam nebkhas on two comparable sandy islands (approximately 20 m a.s.l), El Fusiyat and El Malty. El Fusiyat was heavily grazed by goats and sheep before 1983 but not after 1988 (i.e. ungrazed by livestock for 12 years at the time of study). El Malty is subject to permanent heavy goat and sheep grazing. The islands have similar topography, soil and flora, although the density and cover of perennial vegetation on El Fusiyat was much greater than that on El Malty (the grazed site).

The climate is arid with average rainfall around 82 mm, falling usually between October to May. Monthly relative humidity varies between 68 and 74%.

Sampling: Field work was carried out during the main growing season (April–May 2000). Possible vegetation and microenvironment changes of white broom nebkhas in response to exclusion of sheep and goats was examined by randomly selecting 30 widely spaced nebkhas (the distance between nebkhas 10 to 15 m) on each island. Length, width and height of each selected nebkha were measured and elliptical area calculated. The canopy diameter and height of white broom individuals inhabiting each nebkha, and area of accumulated plant litter, were recorded.

For each selected nebkha, 8 m long transects were placed radiating out in four directions from the nebkha centre into internebkha spaces. Along each, four 1 m² quadrats were randomly placed to record the number of herbaceous plant species, two on the nebkha patches and two on the internebkha spaces.

Species richness was calculated as total number of species occurring per 1 m², and the proportion of species that were found only on R.raetam nebkha patches and the proportion found only in internebkha spaces at both grazed and ungrazed sites was recorded.

To test whether microclimate and soil properties of the grazed nebkha patches were different from ungrazed ones, 10 nebkhas of comparable size (basal area from 4.48 to 6.53 m²) were selected at each site. Soil temperature and photosynthetically active radiation (PAR) was measured at 0.5, 1, 1.5, 2, and 4 m from nebkha centre between 12:00 and 14:00 hr during May 2000 under clear sky conditions.

Three soil samples from the top 30 cm were collected and mixed from each of 10 selected grazed and ungrazed nebkhas, and likewise internebkha spaces. Soil texture was determined with a hydrometer to ascertain percentage sand, silt and clay. Soil moisture content was calculated from the difference between weights before and after drying at 105°C for 48 hr. Organic matter was estimated by drying and ignition at 600°C for 3 hr. Soil electrical conductivity was also determined.

Nebkha characteristics differed significantly between grazed and ungrazed sites. Average length, width and area of ungrazed nebkhas were greater than grazed ones, but grazed nebkhas were higher. Average white broom canopy diameter of grazed nebkhas was 2.41 m (range 1.1–3.8 m) and 3.23 m (range 1.3–4.7 m) for ungrazed nebkhas. The average area of litter accumulation under ungrazed nebkhas was much greater than those grazed. Grazing had a negative effect on the abundance and the richness of herbaceous species in both internebkha spaces and nebkhas. On the grazed island an average of 42% of species were restricted to nebkhas and 7% to internebkha patches, whereas on the ungrazed island 22% of the species were found only in nebkhas and 11% internebkha spaces.

On the grazed island, three palatable grasses (foxtail brome Bromus rubens, Sahara millet Panicum turgidum and plumed stipa Stipagrostis plumose) were recorded only in the nebkhas, presumably protected to a lesser or greater extent by the broom canopy. The two short grasses (Cutandia dichotoma and Schismus aracus) and the two forbs (bird's foot trefoil Lotus halophilus and glaucous groundsel Senecio glaucus) had a high frequency in both microhabitats on both grazed and ungrazed sites. Three short forbs (desert cudweed Filago desertorum, hairy rupturewort Herniaria hirsute and zenaymeh Ifloga spicata) were common in the internebkha spaces at both sites.

Soil microclimate differed between grazed and ungrazed locations and between nebkhas and internebkha spaces. At ungrazed and grazed sites, soil temperatures and PAR were lowest at nebkha centres increasing out into internebkha spaces. Soil temperature and PAR were significantly higher at the grazed than at the ungrazed sampling points.

The proportion of finest soil particles was higher on ungrazed nebkhas than on grazed ones and in the adjacent internebkha patches. The soils of ungrazed nebkhas had significantly higher organic matter content, total nitrogen, electrical conductivity, Na+, and K+ than grazed ones. However, there was no significant difference in the total nitrogen between the soils of grazed nebkha and the adjacent internebkha spaces.

Conclusions: Abundance and richness of herbaceous plants were positively related to nebkha area. Protection from grazing, especially of nebkhas but also spaces between them, was associated with an increase in abundance and richness of herbaceous plants, improved soil microclimate, and increased soil fine particles and nutrient concentrations. The authors consider that in grazed areas, nebkhas comprising unpalatable plants have the potential to preserve plant diversity in otherwise overgrazed plant communities, as they may enhance water and soil conditions and afford protection to propagules of herbs and grasses.

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