Abstract:

Objective: This study aims to clarify the leaf-fine root phenotypic plasticity of Populus tomentosa and the response mechanism of growth rhythm to thinning, so as to provide theoretical reference for the optimization of efficient structural control techniques for fast-growing tree species. Method: The 8-year-old triploid P. tomentosa plantation in the North China Plain was taken as the research object, and three thinning intensities were set: no thinning (NT), alternate row thinning (50% thinning intensity, T50), and alternate row and alternate plant thinning (75% thinning, intensity T75). By monitoring the growth indicators and leaf and fine root traits of P. tomentosa after thinning, analyzing the diameter at breast height (DBH), leaf area growth rhythm and the plasticity of leaf and fine root phenotypes during the growing season of P. tomentosa, this study explores the growth strategies of leaves and fine roots under different thinning intensities and the coupling relationship with forest growth. Result: 1) Thinning had a significant impact on specific leaf area (SLA) and leaf mass per area (LMA). As the thinning intensity increased, the leaves had smaller SLA and greater LMA. Compared with thinning intensity, the influence of soil depth on fine root traits was more significant. Fine roots showed completely opposite growth strategies in shallow soil layers and deep soil layers. 2) The growth rhythm of DBH and leaf area change pattern in thinning treatment were similar to those of NT treatment: DBH growth started in early April and stopped at the end of October, showing a “slow-fast-slow” unimodal growth pattern. In mid-to-late March, P. tomentosa started to unfold its leaves. From early April to early May, it went through a period of rapid growth and development. From early May to early August, changes in the leaf area index were comparatively steady, and after August, it started to decline. 3) Thinning was able to promote the growth of DBH of P. tomentosa and extend the period of rapid DBH growth (July–August). At the end of October, the DBH cumulative growth in T75 and T50 treatments was significantly increased by 91.57% and 56.59% compared to the NT treatment, respectively. In addition, thinning was able to regulate the tree to form a larger canopy and promote tree growth, especially in the direction of thinning. 4) Leaf traits and root traits explained 88.25% and 72.31% of forest growth variation, respectively. Leaf traits positively regulated forest growth, while fine root traits negatively regulated forest growth. Among them, leaf mass per area (LMA) and fine root biomass density (FRBD) were the most interpretable phenotypic parameters. Conclusion: After thinning, the growth rate of DBH and leaf phenotype of P. tomentosa undergo plastic changes, but the growth rhythm of DBH and leaf area do not be changed. Thinning can adjust the SLA and LAM of the vegetative organs of the forest tree, adopt a “high investment-low return”single leaf growth strategy, and promote the diameter growth of P. tomentosa by forming a larger crown and a greater number of leaves. In addition, fine roots after thinning are more prone to shallow distribution, and preferentially choose the survival strategy of “changing their biomass distribution characteristics rather than morphological characteristics” to obtain water and nutrient resources.

Key words: poplar plantation, thinning intensity, forest growth rhythm, leaf traits, fine root traits