Elevation‐richness pattern of vascular plants in wadis of the arid mountain Gebel Elba, Egypt

Abstract

Mountains provide a unique opportunity to study drivers of species richness across relatively short elevation gradients. However, few studies have reported elevational patterns for arid mountains. We studied elevation‐richness pattern along an elevational gradient at the arid mountain Gebel Elba, south‐east of Egypt, expecting a unimodal richness pattern. We sampled 133 vegetation plots (10 × 10 m) in four wadis along an elevational gradient from 130 to 680 m which represents the transition from desert to mountain wadi systems. We used generalised additive models to describe the relationship between elevation and plant species richness. We found a strong increase in species richness and Shannon diversity at low elevations followed by a plateau at mid‐ to high elevations. When we analysed each tributary as a single gradient, no pattern was found. The analysed elevational gradient seems to be a major stress gradient in terms of temperature and water availability, exhibiting a trend of increasing species richness that changes to a plateau pattern; a pattern rarely observed for wadi systems in arid mountains. We discuss the observed pattern with the climatic stress hypothesis and the environmental heterogeneity hypothesis as possible explanations for the pattern.

Plant communities and their environmental drivers on an arid mountain, Gebel Elba, Egypt

“…Because of moisture-laden north-eastern winds, the vegetation is much richer on the northern slopes of Gebel Elba than on the southern slopes (Zahran and Willis 2009). Thus, both species richness and abundance are much higher on the mountain than in the exposed open desert (Abutaha et al 2019). On the foothills of the mountain, Vachellia tortilis (synonym: Acacia tortilis) forms an extensive natural woodland landscape (Zahran and Willis 2009).…”

“…Two prominent leaf strategy types occur (deciduous and evergreen). However, these types do not occur evenly across the elevational gradient; evergreen trees are prominent in the upper altitudes, while deciduous species are more common in the lower, arid parts of the gradient (Abutaha et al 2019). The ecotone between evergreen woodland at higher elevations and deciduous woodland at lower elevations has not been studied (White 1983;Berhanu et al 2018).…”

“…Most of the previous studies on Gebel Elba have mainly focused on wadis, which are temporary waterways that collect run-off water from the surrounding slopes and contain several microhabitats (Zohary 1973;Gomaa 2014), where vegetation is rich and continuous (Ahmed 1999;Zahran and Willis 2009;Abutaha et al 2019). While the lower elevations of wadis show recognizable features of zonal communities, the vegetation on the higher slopes is more variable due to minor differences in habitat and recognizing clearly defined zonal communities is difficult (Zahran and Willis 2009).…”

“…Yahmib is located at the foothills of the mountain and receives water mainly from three mountainous tributaries: Wadis Marafai, Acow, and Kansisrob, which drain the western-northern flanks of Gebel Elba (Figure 1). The substrates of the wadis vary with an elevation gradient which increases from east to west; the substrate of Wadi Yahmib is fine sand, that of Wadi Kansisrob is coarse gravel, and Wadis Acow and Marafai, at higher elevations, have large granite boulder substrates (Abutaha et al 2019).…”

“…We sampled 169 geo-referenced vegetation relevés that were marked with a GPS device (Garmin eTrex 30x). The relevés (10 m × 10 m) were distributed along transects in the four studied wadis, i.e., Yahmib, Marafai, Acow, and Kansisrob (see Abutaha et al 2019). Locations of the relevés were selected randomly during the field work.…”