Timing of cotyledon post-emergence damage drives physiological compensation and resource allocation in oak seedling establishment

Cotyledons serve dual roles as nutrient reserves and physical protectors during early seedling establishment. However, they are vulnerable to diverse granivorous and environmental stressors. Previous studies have predominantly focused on cotyledon loss magnitude, leaving a significant knowledge gap regarding how post-emergence timing of damage interacts with environmental factors such as nutrient availability to influence seedling establishment. We conducted a controlled factorial experiment manipulating three cotyledon removal timings (1, 2, and 4 weeks post-emergence) and two nutrient supplementation levels (i.e., high and low nutrient level) treatment with a widely distributed oak species, Quercus dentata. Morphological (height, basal diameter, and biomass allocation, etc.) and physiological traits (photosynthetic rate, leaf nutrients, etc.) were measured after treatment for five months. Cotyledon removal induced severe and irreversible reductions in seedling height, basal stem diameter, and root biomass, showing that the first week after emergence is a critical vulnerability window. Nutrient supplementation exhibited time-dependent effectiveness. The compensatory effect became progressively weaker with increasingly earlier removal of cotyledons. High nutrient conditions significantly enhanced photosynthetic rates and leaf nutrient content across all timing removal treatments. Structural equation modeling revealed that nutrient influenced biomass primarily through indirect pathways, mediated by physiological traits rather than direct effects. Our model explained 90–94% of organ biomass variation. Our findings demonstrate that cotyledon damage timing determines the success of Q. dentata seedling establishment. The identification of ontogenetic windows for vulnerability and compensatory capacity challenges the traditional view of cotyledons as static nutrient pools. If cotyledon removal is anticipated in Q. dentata restoration practices, implementing protective measures during the first post-emergence week and timing nutrient interventions to coincide with periods of maximum compensatory capacity could significantly enhance seedling establishment.