Abstract: Understory light availability is a major environmental factor affecting the growth and survival of many forest species. The spatial heterogeneity of light availability most likely influences forest regeneration dynamics and determines stand-level spatial patterns of seedling regeneration. However, the implications of heterogeneity in understory light availability on regeneration patterns have not been carefully investigated. We examined spatial patterns of understory light availability and Manchurian ash (Fraxinus mandshurica) seedlings within two experimental plots in secondary hardwood forests at the Silviculture Research Station of Northeast Forestry University, northeastern China (127°30′-127°34′E，45°21′-45°25′N). Our objectives were to compare spatial patterns of understory light with seedling distribution patterns to determine whether patterns of seedling regeneration were linked to spatial patterns of understory light availability. Understory light availability was measured at 297 sample points in a regular grid-based spatial sample design. The minimum interval distance between measurements was 0.5 m and the maximum interval distance was 30 m. Light levels were measured at a height of 70 cm using an LAI-2000 Plant Canopy Analyzer at each point. Seedlings were counted at 193 sown grid subplots (20 cm×20 cm) in each plot (30 m×30 m) during the growing season. Semivariograms from geostatistical analysis methods were used to quantify the spatial variations of understory light availability and seedling distributions. We also examined the statistical relationships between light availability and seedling regeneration at different spatial scales. The results demonstrated that there was significant spatial patterning of both understory light availability and seedling regeneration. The transmittance of light to the ground within stands was 40.2%-52.3% after the growing season and only 4.2%-4.6% during the growing season. According to theoretical model parameters, understory light availability was significantly spatially autocorrelated over scales at 10.9-12.4 m, and spatially structured variance accounted for 62.5%-78.2%. Seedling density in each subplot differed significantly among microsites with significantly higher numbers of seedlings in high-light microsites than in low-light microsites. Seedling regeneration showed strong spatial patterning. Patterns of seedling regeneration were strongly linked to spatial heterogeneity of light availability. In stands with more spatially heterogeneous light availability patterns had more complex patterns of seedling distribution patterns and were spatially autocorrelated at smaller scales (2.12-6.97 m). In stands with lower complexity of light spatial distribution patterns, the patterns of seedling regeneration were relatively simple. Regeneration patterns were significantly spatially autocorrelated (>83%) at slightly larger scales (30 m) with little random variation (<17%). Our results confirmed that the spatial heterogeneity of understory light availability is a critical factor influencing the performance of seedling regeneration of Manchurian ash.