**Background:** Headwater streams are highly heterogeneous, characterized by sequences of riffles and pools that differ in flow, depth, substrate, and food resources. In temperate streams, autumn defoliation creates litter patches that modify habitat structure and provide food for macroinvertebrates. However, how biological communities assemble under surface ice during winter remains poorly understood. This study tested three hypotheses: (1) riffles host higher functional diversity and lower functional redundancy than pools; (2) litter patches reduce functional diversity and increase functional redundancy compared to non-litter substrates; (3) functional diversity decreases and redundancy increases from pre-freezing to freezing periods, then reverses during thawing.
**Methods:** The study was conducted in a 500 m reach of a forest headwater stream in the Longwan Nature Reserve, Jilin Province, Northeast China. Sampling occurred across four periods: autumn (mid-October 2017), pre-freezing (early November 2017), freezing (early January 2018), and thawing (early March 2018). Four substrate types were sampled: riffle stones, pool sediments, riffle litters, and pool litters. Macroinvertebrates were identified to genus level (except Chironomidae identified to subfamily). Twenty-one functional traits in six categories were analyzed. Functional diversity was measured using Rao's diversity, and functional redundancy was calculated as FR = (D–Q)/D, where D is Simpson's diversity and Q is Rao's diversity. Statistical analyses included two-way ANOVA, ANOSIM, SIMPER, nested ANOVA, and redundancy analysis (RDA).
**Key Results:** A total of 50 taxa from 9 orders and 31 families were identified. Riffle stones had significantly higher taxonomic richness and Rao's diversity than pool sediments. Functional redundancy did not differ significantly between riffle stones and pool sediments. Litter patches had significantly lower Rao's diversity and higher functional redundancy than both riffle stones and pool sediments. In riffle litters, functional redundancy was significantly higher than in riffle stones (p < 0.05); similarly, pool litters had higher functional redundancy than pool sediments (p < 0.05). Functional trait composition differed significantly between riffle stones and pool sediments, and between stony/sediment substrates and their respective litter patches. Temporal variation was observed: in pool sediments, Simpson's diversity decreased significantly during the freezing period, and functional redundancy increased significantly during the pre-freezing period. However, functional redundancy in both riffle and pool litters showed temporal stability across all four periods. RDA showed that velocity, boulders%, and total litter abundance significantly explained community variation (3.65%, 4.99%, and 3.38% of total variation, respectively; full model adjR² = 0.120, p = 0.001).
**Clinical Implications:** This study provides evidence that litter patches in temperate streams serve as critical habitats that support high macroinvertebrate abundance and maintain functional stability through winter. The higher functional redundancy in litter patches suggests these habitats buffer against environmental fluctuations (freezing and thawing), maintaining ecosystem function even as taxonomic composition shifts. These findings have implications for stream management and conservation, particularly in understanding how riparian defoliation and leaf litter dynamics influence aquatic community structure and resilience. The temporal stability of functional redundancy in litter patches contrasts with the more variable pool sediments, highlighting the importance of maintaining natural litter inputs for stream ecosystem health.