Role of host identity in effects of phytogenic mounds on plant assemblages and species richness on coastal arid dunes

Question: Phytogenic mounds (nebkhas) ‐ the natural accumulation of wind‐blown sediments within or around the canopies of plants ‐ have been proposed as important structures for locally maintaining high species richness in coastal and arid ecosystems. Nebkhas are assumed to increase habitat heterogeneity, but what is the importance of the nebkha host species relative to other nebkha characteristics in determining the associated plant assemblages? Are some host species more effective in creating diversity hotspots, or does a single species‐area relationship apply to all nebkhas, regardless of host species? Can the influence of the host be ascribed to its indirect effects on abiotic attributes of the nebkha complex? Methods and location: We investigated plant species richness and composition on nebkhas around six psammophytic species on Mediterranean coastal dunes of the Sinai Peninsula. Results: Plant species richness was significantly related to nebkha size by the single power function according to the general prediction of island biogeography theory, but this relationship was modified ‐ though to a limited degree ‐ by nebkha host species identity. Canonical Correspondence Analysis revealed that nebkha host species identity and nebkha environmental and non‐environmental factors significantly explained species composition on the nebkhas, but host species identity did so to a greater extent. The latter might reflect differences in seed trapping ability or free space for colonization between host species. Conclusion: Differences in community composition and richness among nebkhas formed by different host species represent a key factor in the maintenance of plant diversity on arid coastal dunes.     

Unraveling landscapes with phytogenic mounds (nebkhas): An exploration of spatial pattern

Phytogenic mounds (nebkhas) often are symptoms of desertification in arid regions. Interactions among nebkhas and between nebkhas and their environment are however poorly examined. To this end, three main hypotheses of nebkha pattern formation were evaluated in this study. These state that nebkha patterns are either shaped by: (i) biologically induced recruitment inhibiting zones, (ii) biologically induced recruitment encouraging zones, or (iii) by the spatial distribution of abiotic factors which are not biologically driven. Contrasting nebkha landscapes were examined: a highly dense New Mexican mesquite (Prosopis glandulosa) and snakeweed (Gutierrezia sarothrae and Gutierrezia microcephala) ecosystem, and a low-density mixed Tamarix aphylla and Calligonum comosum field in central Libya. Spatial second-order statistics of strategically chosen nebkha subpatterns were compared with those of null models in which observed patches were spatially randomized without overlap. Null model deviations were assessed with goodness-of-fit tests, and interpreted in terms of hypothesized mechanisms of nebkha pattern formation. Our results suggest that biologically induced recruitment inhibiting zones surround adult mesquite nebkhas. The configuration of Calligonum and Tamarix nebkhas may be driven by spatial dynamics of abiotic microsites which are not caused by nebkha interactions. Hence we conclude that both biotic and abiotic drivers can shape nebkha spatial patterns.     

Nebkhas of Salvadora persica and their effect on the growth and survival of Prosopis cineraria,Tamarix aphylla,and Capparis decidua trees and shrubs

Few studies were published on the effect of nebkhas (phytogenic mounds) on species diversity and soil resources, but no detailed study has been conducted yet on possible specific influence of nebkhas on growth and survival of the plants associated with them. We studied the nebkhas of Salvadora persica and their effect on growth and survival of three woody species (Prosopis cineraria, Tamarix aphylla, and Capparis decidua) in the Ommanian coast of Hormozgan Province in the south of Iran. The results showed that mean height and mean canopy diameter of P. cineraria and T. aphylla trees and shrubs inhabiting nebkhas of Salvadora persica were considerably higher than those of plants of these species growing outside nebkhas. The reverse occurred in the case of C. decidua. Generally, the percentages of stems with dead parts were significantly lower in plants inhabiting the nebkha sites in comparison to comparable ones growing outside the nebkhas. Salvadora persica nebkhas are enriched with more soil nutrients in comparison to inter-nebkha sites. Soil accumulated per each hectare in the nebkhas of the study area dominated by trees of Salvadora persica amounted to 237.6 m³. This indicates the great importance of nebkhas in the protection of soil and the associating species.