Soil seed bank and soil property dynamics create regeneration islands during dryland ecosystem degradation

Grassland to shrubland state transitions are associated with ecosystem‐level degradation. In dry grassland systems, shrub encroachment often creates islands of fertility, where vegetation and critical resources concentrate under shrubs and are depleted from surrounding interspaces, reducing above‐ground plant biodiversity and ecosystem resilience. However, soil seed bank and soil nutrient spatiotemporal dynamics are often disparate from above‐ground vegetation patterns. Thus, potential shrub encroachment trajectories and overall system dynamics can be better understood by assessing seed bank and nutrient differences between shrub and interspace microsites across a shrub encroachment degradation gradient. We used a large‐scale, three‐year field study in a semi‐arid dryland to assess seed bank composition as it relates to the above‐ground vegetation, soil macronutrients, and soil pH across a degradation gradient. We evaluate the effect of the degradation gradient, microsite and year to indicate how degradation impacts ecosystem regenerative potential. Above‐ground vegetation species richness fluctuated year‐to‐year in a nonlinear fashion across the degradation gradient while seed bank species richness was consistent. Along the degradation gradient, the seed bank had greater interspace depletion while staying relatively constant under shrubs. Perennial grass seeds were an order of magnitude less abundant than annual forbs yet followed the same pattern of having a higher seed density under shrubs than interspaces across the gradient. Soil macronutrients followed the seed bank pattern, being concentrated under shrubs relative to interspaces, but concentrations of soil phosphorus, potassium and organic matter under shrubs decreased as degradation increased. In interspaces, soil nitrogen was depleted across the gradient, and pH increased to moderate to strongly alkaline levels at high levels of degradation. Synthesis and applications . Our findings suggest soil resource depletion from interspaces and concentration underneath shrubs during grassland to shrubland degradation are an amplifying feedback maintaining alternate shrubland states. Regeneration islands likely influence shrubland state persistence after above‐ground thresholds have been crossed and pose a continued barrier to grassland restoration efficacy. However, our work suggests that given resources are not lost from the system, a redistribution of seeds and nutrients with active shrub removal interventions may in fact enhance restoration success.