Effects of Light Environment on Understory Herbaceous Diversity and Regeneration Across Degradation Gradients in Robinia pseudoacacia L. Stands

Light environments within plantation forests vary significantly with stand degradation. This study investigated how light-related factors change along degradation gradients in Robinia pseudoacacia L. (black locust) plantations and how these changes influence understory herbaceous vegetation and regeneration. An R. pseudoacacia plantation at the Zhongmu State-owned Forest Farm, ZhengZhou, China was studied across three degradation levels (least degraded, moderately degraded, and severely degraded). Integrated analyses were employed to assess light–vegetation relationships under different stand densities. The results indicated that canopy openness (CO), photosynthetically active radiation (PAR), and light transmittance increased significantly with increasing degradation severity, whereas the leaf area index (LAI) declined. Specifically, differences in LAI among degradation levels were observed in all density stands. CO, TDR, and PAR showed degradation-related differences in medium- and high-density stands, while other light variables varied under specific density–degradation combinations. Furthermore, herbaceous biomass declined, canopy cover showed a fluctuation trend, and species diversity increased. Significant correlations were observed between multiple light parameters and herbaceous attributes. Overall, variations in the light environment were closely associated with understory vegetation dynamics. Moderate degradation was linked to higher herbaceous diversity, whereas regeneration density exhibited a non-monotonic response across degradation levels, with the lowest value under moderate degradation rather than a continuous decline under severe degradation.