Drivers of understory shrub and herb biomass in subtropical Chinese fir forests: a cluster analysis and SEM approach

Understory vegetation plays a key role in forest ecosystem material cycling and energy flow, and studying its biomass dynamics and underlying driving mechanisms is vital for understanding forest ecosystem functions and achieving precise management. In order to elucidate the complex pathways through which stand structure and environmental factors influence understory vegetation biomass, a study was conducted on 180 plots of Chinese fir forests at different developmental stages in the subtropical region of China. The study included 36 plots each of young forest (6 years), middle-age forest (12 years), near-mature forest (20 years), mature forest (25 years), and over-mature forest (30 years) stands. Based on the quantification of shrub and herb biomass in Chinese fir forests and functional clustering, structural equation modeling (SEM) was employed to systematically analyze the direct and indirect effects of environmental factors and stand structure on understory vegetation biomass. The results indicate the following: First, K-means clustering analysis revealed that shrub can be classified into two categories: Tall, dense shrubs (TD-SB) and Low-Sparse Shrubs (LS-SB), and herb can be classified into three categories: Tall, dense herbs (TD-HB), Low-Sparse herbs (LS-HB), and Medium-density herbs (MD-HB); Second, stand structure and environmental factors have significant effects on understory shrub and herb biomass. The Mingling Index of tree species within stand structure exerts the greatest negative effect on shrub biomass and the greatest positive effect on herb biomass. Among environmental factors, slope position and slope gradient have been found to exert a relatively strong positive influence on understory vegetation biomass. The results revealed that all stand structure indicators had a negative effect on the shrub biomass of TD-SB and LS-SB, while environmental factors had a negative impact on MD-HB. This study establishes a theoretical framework for the sustainable management of subtropical Chinese fir forest ecosystems and demonstrates its practical application.