Abstract: To better understand the effects of forest gap on the herbaceous species community in a mixed birch-fir forest of Taibai Mountain in Qinling, CCA ordination and random permutation test were employed to analyze the distribution pattern of the species composition across a gradient of gap size, and the relationships between the distribution of 55 herbaceous species with ≥5 individuals and the habitat variables (convexity, slope, and soil total N, total P, available N, available P, pH, and organic matter). In this forest, gap area occupied 19.8% of the total land area, gap density was 20.7 per hm², and gap size varied from 25.6 to 279.1 m², with a mean of 93.7 m². The species richness in herbaceous layer in gaps was significantly positively correlated with gap size, but of the 69 herbaceous species identified in the gaps, most species were found across all gap sizes, and only eight species were found in larger gaps (>120 m²). No successional change was observed in the herbaceous species distribution with gap size. The CCA ordination and random permutation test also showed that 27.3% of the 55 species with abundance ≥5 had significant association with the eight habitat variables. It was concluded that gap size contributed to the species richness, but determined the diversity constitution in random.

Key words: forest gap, herbaceous species, gap-partitioning, birch-fir mixed forest, water coupling with nitrogen, dry matter accumulation, drip irrigation, N fertilizer use efficiency, maize.