Fertile Island Soils Promote the Restoration of Shrub Patches in Burned Areas in Arid Saline Land

Shrub encroachment and expansion have been widely reported globally and are particularly severe in arid saline land. Shrubs in harsh habitats have fertile island effects, but the promoting effect of fertile island soil on shrub patch restoration remains unclear. To clarify the role of fertile island soils in shrub patch recovery, we took single Tamarix ramosissima shrubs with different volume sizes (3.62–80.21 m3) as experimental subjects. The fertile island effect was quantified, and the 5-year natural recovery of shrub patches in the burned area was measured. The results strongly support that shrubs formed a fertile island soil in unburned areas; soil nutrient content beneath the canopy was 1.34–3.09 times higher than those outside the shrubs, while the soil salinity was 0.03–0.48 times lower than that of intercanopy spaces. The diversity of herbaceous plants beneath shrubs was significantly lower than that of outside shrubs, while the herbage biomass first increased and then decreased with the increase in the volume of shrubs. The maximum biomass of herbage was found when the shrub volume was 30.22 m3, but oversized shrubs could inhibit the growth of herbage. In terms of burned area, the recovery of burned area mainly depends on resprouts and seedlings. The mean values of seedling density, height, coverage, and biomass beneath the canopies were 0.47, 2.53, 2.11, and 5.74 times higher, respectively, than those of the intercanopy spaces. The results of the structural equation models showed the weight coefficient of the fertile island soils for the vegetation recovery in burned shrubland was 0.45; low salinity contributed more to vegetation recovery than high nutrient and moisture contents. Thus, compared with intercanopy spaces, shrub patches reinforce fertile island effects through direct and indirect effects and enhance the recovery of shrubland vegetation in the burned area. Our results demonstrate the positive implications of shrub expansion in the context of global climate change and also deepen the understanding of the sustainable development of burned shrubland.