Evaluation and regulation strategies for stand vigour based on crown structure and tree rings of dominant Cunninghamia lanceolata

Abstract

Cunninghamia lanceolata (Lamb.) Hook, a key species for forest plantations in subtropical China, is experiencing a critical decline in productivity due to management practices like long-term successive rotation. Within the C. lanceolata ecosystems, the vigour of the dominant trees reflects their growth potential under the prevailing site conditions. This is crucial for informing management strategies aimed at optimizing plantation productivity. This study focuses on dominant individuals of C. lanceolata, employing their basal area increment in the final year as a quantitative indicator of stand vigour. A dual-dimensional evaluation framework integrating crown structure and tree rings was developed to investigate the underlying mechanisms by which crown structural parameters, stand density and tree age influence stand vigour. This was based on stem analysis data from 76 dominant trees sampled across six southern Chinese provinces. Tree ring data were combined with crown structural parameters including length, width, ratio, volume, shape ratio, and crown projection ratio. A multi-method analytical framework incorporating correlation analysis, difference testing, subgroup analysis, and linear threshold regression modeling was employed to systematically examine these interactions. The results demonstrated that: (1) crown length exhibited a significant positive correlation with basal area increment, while crown shape and projection ratios had significant negative effects. (2) Middle-aged stands (11–20 years) and low-density stands (≤ 1000 ind. ha^− 1) exhibited the highest vigour, with significantly greater basal area increment compared to other age classes and density gradients. (3) Linear threshold regression analysis identified a critical threshold for the clear bole ratio at approximately 0.5. Staying below this value optimizes crown morphology and boosts vigour. Therefore, silvicultural management of C. lanceolata plantations should prioritize density regulation to alleviate inter-tree competition, complemented by precision pruning during the critical 11−20-year phase. Strategic control of the clear bole ratio is recommended to enhance stand vigour.

Keywords

Dominant trees / Crown structure / Tree rings / Stand vigour

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