Size-related change in Nitraria sphaerocarpa patches shifts the shrub-annual interaction in an arid desert, northwestern China

• As patch size increasing, interactions between N. sphaerocarpa and other species shift from facilitation to interference.
• N. sphaerocarpa shrubs improved soil total N, organic C and soil texture.
• Nutrient accumulation and soil texture improvement could not explain variation in the net shrub-annual interaction.
• These negative shifts may be related to the reduction in soil moisture and available light.
• A growing niche-overlaps above- and below-ground as increased patch size can partly explain the interaction's shift.

In arid and semi-arid ecosystems, the effect of shrubs on their understory plants has been frequently reported. Many previous studies have shown that both facilitation and competition act simultaneously, and their balance changes in response to spatial or temporal variations in environmental stresses. Yet, we know little about how the interaction varies between shrubs and their understory plants in vegetation patches with different characteristics. Here, an empirical investigation was conducted in Hexi desert region of northwest China, to evaluate how the net effect of Nitraria sphaerocarpa shrubs on the herbaceous species varies across different sizes of N. sphaerocarpa patches. Our study showed that herbaceous species perform better on small shrub patches. But the magnitude of this facilitative effect decreased with increasing patch size, and finally shifted to interference in large patches. The results indicated that these negative shifts in plant interactions were not clearly related to ‘shrub-island effect’ in terms of nutrient accumulation or soil properties improvement, but may be explained by the variation in other abiotic factors, such as soil moisture and available light. Changes in root distribution and canopy structure of N. sphaerocarpa shrubs with the increase of patch size seem to also partly explain the variation in shrub-understory species interactions, as a result of increased above- and below-ground niche overlaps.