Understory vegetation dynamics of Chinese fir plantations and natural secondary forests in subtropical China

Understory vegetation is a vital component of forest ecosystems and contributes to terrestrial plant diversity, nutrient cycling and energy flows. Plantations and natural secondary forests are two dominant types that emerge following stand-replacing disturbances in managed landscapes. However, to date, the long-term dynamics of the cover, diversity, and composition of understory vegetation in plantations and secondary forests remain poorly understood. We sampled understory vegetation of Chinese fir plantations and natural secondary forests with their ages spanning 4 to 40 years and used primary forests as a reference in subtropical China. The cover and species diversity of the shrub layer were significantly higher in the natural secondary forests than in Chinese fir plantations in two younger age classes, but they converged in older stands. In the herb layer, Chinese fir plantations possessed a higher total and vascular species diversity across all age classes, but their covers did not differ significantly between the two forest types, except for a higher cover of vascular plants in the 8-12-year-old Chinese fir plantations. The bryophyte cover did not differ with forest type nor stand age, while bryophyte species diversity was significantly higher in Chinese fir plantations. The cover and species diversity of understory vegetation in the primary forests were similar to those in the 35-40-year-old natural secondary forests. Across all forest types and age classes, the cover and diversity were positively correlated within each plant group, and the shrub cover and diversity were negatively correlated with vascular diversity in the herb layer, but neither the cover nor the diversity of bryophytes was correlated with those of the shrub layer or herb layer. Species composition differed significantly with forest type and stand age. Natural secondary forests had more similar understory species composition to primary forests than Chinese fir plantations did. Our results suggest that forest management may increase the diversity of the herb layer, but at the expense of the abundance and diversity of the shrub layer. The shrub layer may not regenerate, or its regeneration will be slow due to exhausted seed and propagulum banks after multiple management rotations. Moreover, the high cover of the shade-intolerant and nutrient-demanding herbaceous plants in Chinese fir plantations may have adverse effects on ecosystem service functions.