Crown pruning and understory removal did not change the tree growth rate in a Chinese fir (Cunninghamia lanceolata) plantation

• Published in: Forest ecology and management 2020-05, Vol.464, p.118056, Article 118056
• Main Authors: Li, Renshan, Han, Jianming, Guan, Xin, Chi, Yonggang, Zhang, Weidong, Chen, Longchi, Wang, Qingkui, Xu, Ming, Yang, Qingpeng, Wang, Silong
• Format: Article
• Language: English
• Subjects: Carbon sequestration; Foliar nitrogen; Needle age; Nitrogen trade-off; Stable isotopic signature; Timber yield
• ISSN: 0378-1127; 1872-7042
• DOI: 10.1016/j.foreco.2020.118056

•Pruning caused an up-regulation in the net photosynthetic rates of remaining foliage.•The photosynthetic increase upon pruning occurred mainly in two youngest needles.•Pruning enhanced the investment of needle nitrogen to CO2 fixing enzyme.•Pruning favored stomatal conductance and carboxylation rate of current-year-needle.•Understory removal had no effect on tree growth and needle photosynthesis. How tree growth and the underlying photosynthesis of leaves, especially those multi-aged leaves, change after the application of both pruning and understory removal remains unclear. In this study, the tree growth and photosynthetic responses of various-aged needles to pruning, understory removal and their interactions were investigated in a Chinese fir (Cunninghamia lanceolata) plantation. The biochemical and stomatal limitations to photosynthesis was separated by the combined measurements of stable carbon (δ 13C) and oxygen (δ 18O) isotopes in needles. Our results showed that the tree growth rates did not respond to pruning, understory removal, and their interactions. Pruning significantly stimulated the net photosynthetic rates (PA) and stomatal conductance (gs) of remaining foliage (especially the current- and one-year-needles). Although pruning had no effect on needle total nitrogen (N) concentration, the concentrations of water-soluble N (NS), the ratio of water-soluble N to sodium dodecyl sulfate (SDS)-insoluble N (NS/ND), and the photosynthetic N use efficiency (PNUE) were higher in needles of pruned trees compared with the unpruned trees. A significant and positive correlation between PNUE and NS/ND was also detected. The constant δ 13C and declined δ 18O in the current-year-needles from pruned trees suggested that both the photosynthetic capacity and the gs were responsible for the enhancement in PA of the youngest needles. Conversely, in the previous year needles, δ 13C and δ 18O were not significantly different between the control and pruned trees. Consistent with the response of tree growth rate, the foliar photosynthesis also did not exhibit changes following understory removal in both pruned and unpruned stands. We highlighted that pruning caused an up-regulation in PA of remaining foliage, thereby mitigating the negative effects of canopy loss on carbon assimilation.