Abstract
AbstractPhytogenic mounds or nebkhas are formed by spatially isolated burial‐tolerant plants, which have been trapping windborne sediment within their canopies. They occur in drylands all over the world, and nebkha landscapes are considered intermediate between stable grasslands and unstable deserts with mobile dunes. Depending on the previous state of the landscape, they can thus be viewed both as indicators of desertification and as signs of stabilization. While individual nebkha mounds have been studied extensively, little is known about the organization of nebkha landscapes. In this study, we collected spatiotemporal data of vegetation and topography from four self‐made remotely sensed maps, created over a three‐year period from a study site in central Saudi Arabia dominated by the nebkha‐forming shrub Rhazya stricta Decne. Using logistic regressions and t tests, we found that the nebkha shrubs grow in topographic depressions, while they shrink on topographic highs and in locations with high nebkha densities. We propose that this spatial variation in nebkha growth is driven by water availability, as the few extreme rain events that typically make up the bulk of precipitation in deserts would spatially redistribute to topographic lows, and as nebkha density would positively correlate with competition for water. Our data suggest a novel mechanism of biogeomorphological self‐organization in deserts. The observed nebkha growth in depressions would lead to increased sedimentation on and in between the nebkhas owing to reduced wind speed, which after a time lag might transform depressions into topographic highs on which nebkhas tend to shrink because of water shortage. Shrinking nebkhas, in turn, could provoke soil erosion, on as well as in between the nebkha mounds, which may ultimately transform topographic highs into depressions, thereby closing the cycle of self‐organization.
Highlights
Phytogenic mounds or nebkhas are vegetated hummocks composed of burial-tolerant desert plants, which frequently undergo windborne sediment deposition (Hesp and McLachlan 2000)
We investigate whether the nebkha host plant Rhazya stricta Decne. is subject to periodic shrinkage and growth
Despite that nebkhas tended to shrink more intensely on topographic elevations, more nebkha cover was found on these locations (Fig. 5); this paradox will be explained
Summary
Phytogenic mounds or nebkhas are vegetated hummocks composed of burial-tolerant desert plants, which frequently undergo windborne sediment deposition (Hesp and McLachlan 2000). November 2016 v Volume 7(11) v Article e01494 They restrict the eolian sediment flux above the landscape (Danin 1996), thereby reducing sand abrasion stress on plant communities and resisting desert expansion (Okin et al 2006). Nebkhas can be significant hot spots of resource concentration (soil nutrients and water) as well as hot spots of biodiversity in an otherwise species- poor desert landscape (El-B ana et al 2007). Nebkhas actively contribute to resource sustainability in drylands and are recommended in restoration projects of degraded rangelands (El-B ana et al 2003)
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