Grazing alters seed allocation strategies of Reaumuria soongorica in relation to soil fertility heterogeneity

Grazing is a pervasive disturbance in arid and semi-arid grasslands that can influence plant reproduction through both direct tissue removal and indirect soil-mediated pathways. Because seed production and allocation strategies determine population persistence and vegetation recovery under chronic disturbance, understanding how grazing reshapes reproductive investment is critical, particularly for long-lived desert shrubs. We investigated how different grazing intensity reshapes growth-reproduction trade-offs and seed allocation strategies of the dominant desert shrub Reaumuria soongorica in a desert steppe of northern China. Using a long-term grazing experiment with no grazing (CK), moderate grazing (MG), and heavy grazing (HG) treatments, we quantified shrub growth, reproductive traits, soil properties, and fertile island effects beneath shrub canopies and interspaces area. Grazing intensity exerted contrasting effects on shrub performance. Moderate grazing enhanced shrub biomass, seed production, and female fitness, whereas heavy grazing significantly suppressed reproductive traits, including seed number and seed mass. Across treatments, grazing induced a clear shift in seed allocation, strengthening the seed number-seed mass trade-off under increasing disturbance. Structural equation modeling revealed that grazing affected reproductive allocation through both direct negative effects and indirect soil-mediated pathways. Grazing increased soil carbon, nitrogen, and phosphorus beneath shrubs while exerting minimal effects on interspace soils, indicating spatial redistribution of nutrients and intensified fertile island effects. Soil fertility promoted reproduction, whereas altered soil stoichiometric ratios constrained growth, highlighting opposing soil controls on shrub performance. We further identified reciprocal feedback between shrub functional traits and fertile island effects, whereby trait-driven resource accumulation and grazing-enhanced soil heterogeneity jointly regulated reproductive strategies. These results demonstrate that long-term grazing reorganizes soil-plant feedbacks and seed allocation trade-offs, providing mechanistic insights into shrub persistence and informing sustainable grazing management in arid grassland ecosystems.