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The stress‐dominance hypothesis (SDH) predicts that environmental filtering will dominate in harsh habitats, while limiting similarity is more important in favourable environments. Here, we test the generality of the SDH to disentangle the relative importance of these two ecological processes along a sand desertification gradient. Data were collected from randomly selected locations in a desert steppe community in Northwest China. In this region, intense habitat fragmentation has resulted in a series of original soil habitat patches called soil islands. These soil islands typically contain Sierozem soils historically subjected to low (L), medium (M), and high (H) levels of sand desertification. Along this sand desertification gradient, soil properties and leaf traits (LDMC, leaf dry matter content; SLA, specific leaf area; LC, leaf carbon concentration; LP, leaf phosphorus concentration; and LN, leaf nitrogen concentration) were measured. We then tested for trait and phylogenetic convergence or divergence using a null model approach. Along the sand desertification gradient, trait convergence trend decreased, while trait divergence trend increased. Similarly, the phylogenetic convergence trends decreased. Traits displaying convergence included LDMC, SLA, and LCC, while LP and LN displayed divergence. The strength of LDMC convergence was negatively correlated with community weighted mean trait values (CWM) for LDMC. In contrast, the strength of LN divergence was positively correlated with CWM for LN. Our results pertaining to LDMC and LN support the SDH. Along the sand desertification gradient, the strength of environmental filtering gradually decreased, while the intensity of limiting similarity gradually increased.

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Maintaining the stability of exotic sand-binding shrub has become a large challenge in arid and semi-arid grassland ecosystems in northern China. We investigated two kinds of shrublands with different BSCs (biological soil crusts) cover in desert steppe in Northwest China to characterize the water sources of shrub (Caragana intermedia Kuang et H. C. Fu) and grass (Artemisia scoparia Waldst. et Kit.) by stable 18 O isotopic. Our results showed that both shrublands were subject to persistent soil water deficiency from 2012 to 2017, the minimum soil depth with CV (coefficient of variation) <15% and SWC (soil water content) <6% was 1.4 m in shrubland with open areas lacking obvious BSC cover, and 0.8 m in shrubland covered by mature BSCs. For C. intermedia, a considerable proportion of water sources pointed to the surface soil. Water from BSCs contributed to averages 22.9% and 17.6% of the total for C. intermedia and A. scoparia, respectively. C. intermedia might use more water from BSCs in rainy season than dry season, in contrast to A. scoparia. The relationship between shrub (or grass) and soil water by δ 18 O shown significant differences in months, which partly verified the potential trends and relations covered by the high variability of the water source at seasonal scale. More fine roots at 0-5 cm soil layer could be found in the surface soil layer covered by BSCs (8000 cm/m 3 ) than without BSCs (3200 cm/m 3 ), which ensured the possibility of using the surface soil water by C. intermedia. The result implies that even under serious soil water deficiency, C. intermedia can use the surface soil water, leading to the coexistence between C. intermedia and A. scoparia. Different with the result from BSCs in desert areas, the natural withdrawal of artificial C. intermedia from desert steppe will be a long-term process, and the highly competitive relationship between shrubs and grasses also determines that its habitat will be maintained in serious drought state for a long time.