Live and dead shrubs and grasses have different facilitative and interfering effects on associated plants in arid Arabian deserts Ali El-Keblawy a, b, * , Tamer Kafhaga c , Teresa Navarro d a Applied Biology Department, Sharjah University and Sharjah Research Academy, Sharjah, United Arab Emirates b Department of Biology, Faculty of Education in Al-Arish, Suez Canal University, Egypt c Desert Conservation Reserve, Dubai, United Arab Emirates d Departmento de Biología Vegetal, Universidad de M alaga, P. O. Box 59, 29080, M alaga, Spain article info Article history: Received 9 February 2015 Received in revised form 3 October 2015 Accepted 7 October 2015 Available online 6 November 2015 Keywords: Arid deserts Competition Facilitation Grasses Inhibition Shrubs abstract The Arab Gulf desert region is dominated by few shrubs and grasses, although mostly devoid of vege- tation. The impact of both live and dead shrubs and grasses on plant diversity and community compo- sition on sand dunes of the United Arab Emirates was assessed. Species richness, diversity indices (Simpson, ShannoneWiener, and Brillouin), and plant abundance were significantly greater under dead grasses than in the surrounding open areas. However, the opposite was true for live grasses. Dead and live shrubs did not differ significantly in species richness and abundance. The relative interaction index indicated that live nurse grasses inhibited 13 species and facilitated only one species, whereas dead grasses facilitated 13 species and did not inhibit any species. Live shrubs facilitated four species and inhibited two, but dead shrubs facilitated 10 species and inhibited none. Organic matter and most of the assessed soil nutrients were significantly higher under both shrubs and grasses than in the barren spaces in-between. The facilitative effect of dead grasses on soil characteristics was more obvious. The results support the feasibility of growing nurse shrubs and grasses to restore degraded arid desert environment. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Plants can exert influence on their neighbors in myriad ways, resulting in a broad range of harmful or beneficial outcomes. The abundance, performance, and spatial distribution of plant species are markedly related to the strength and sign of their interactions in communities (Roughgarden and Diamond, 1986; Brown et al., 2001). Understory plants can exert both facilitative and competi- tive effects on larger neighboring “nurse” plants. Their benefits include reduced thermal stress or evapotranspiration (Valiente- Banuet and Ezcurra, 1991; Greenlee and Callaway, 1996); improved soil texture, nutrient content, and water availability (Nobel, 1989; Moro et al., 1997; Barnes and Archer, 1999; Pugnaire et al., 2004); and protection from herbivory (Haase et al., 1997; Brown and Ewel, 1987). Conversely, nurse plants can also have negative effects on the survival and establishment of the associated understory plant community. These plants may interact competi- tively through light deprivation, competition for water and nutri- ents, or leaching of allelopathic compounds (Nobel, 1989; Barnes and Archer, 1999; Holmgren et al., 1997; Kitajima and Tilman, 1996; Moro et al., 1997). In general, the net direction and strength of these interactions are considered to depend on the severity of the physical environment (Bertness and Leonard, 1997) and site productivity (Bruno et al., 2003). It has been hypothesized that facilitative interactions may be more prevalent in harsh environ- ments, such as those occurring in arid and semiarid environments (Callaway, 1995; Callaway and Walker, 1997; G omez-Aparicio et al., 2004). In terms of depth, grass roots are generally distributed nearer to the surface than shrub roots. In their global analysis of root distri- bution, Jackson et al. (1996) indicated that 44% of grass roots were found in the top 10 cm of soil, whereas only 21% of shrub roots were found at the same depth. In addition, Schenk and Jackson (2002) collected a data set of >1300 records of root system sizes for indi- vidual plants from various water-limited ecosystems, including deserts. They concluded that root system sizes differed among growth forms: annuals < perennial forbs ¼ grasses < semi- shrubs < shrubs < trees. Furthermore, in the arid Patagonian * Corresponding author. Applied Biology Department, Sharjah University and Sharjah Research Academy, Sharjah, United Arab Emirates. E-mail addresses: akeblawy@sharjah.ac.ae, akeblawy@gmail.com (A. El- Keblawy). Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv http://dx.doi.org/10.1016/j.jaridenv.2015.10.007 0140-1963/© 2015 Elsevier Ltd. All rights reserved. Journal of Arid Environments 125 (2016) 127e135