Abstract:

Objective: By monitoring the nutrient content, stoichiometric characteristics and nutrient reabsorption of various organs of Populus tomentosa at different stages after tending thinning, this study aims to illustrate the response of trees to light, water and nutrient resources at the organ scale under different thinning intensities, and provide theoretical support for the cultivation and management of P. tomentosa plantations in the North China Plain. Method: The 8-year-old triploid P. tomentosa S86 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, T50), and alternate row and alternate plant thinning (75% thinning, T75). By sampling the leaves, branches, trunks and roots of forest trees, the nutrient content and stoichiometric ratio of various organs, as well as leaf reabsorption efficiency and relative reabsorption rate of P. tomentosa during the growing season in August and November were analyzed, to explore the coupling relationships between nutrient content, stoichiometry and reabsorption at different stages. Result: 1) From a comprehensive perspective of the two periods, thinning intensity only had a significant impact on leaf phosphorus content and trunk carbon content. In November, thinning significantly increased the accumulation of potassium in leaves, and T50 and T75 respectively increased by 33.43% and 35.74% compared with NT. 2) C/N, C/P, and C/K represent the ability to absorb nitrogen, phosphorus, and potassium nutrients under carbon stabilization, and can be used as indicators to judge plant nutrient absorption rate and utilization efficiency. There were significant differences in leaf C/N, C/P, C/K, and N/P between August and November. In addition, compared with branches, trunks and roots, the stoichiometric characteristics of leaves were more susceptible to the influence of period, thinning and their interactions. Among them, the intensity of thinning had a significant impact on the C/P and N/P of P. tomentosa leaves. 3) The nitrogen reabsorption efficiency (NRE) of P. tomentosa leaves under different thinning intensities was 14.64%–24.55%, the phosphorus reabsorption efficiency (PRE) was 18.87%–34.97%, and the potassium reabsorption efficiency (KRE) was 7.00%–33.59%. As the thinning intensity increased, the potassium return efficiency of leaves decreased. Thinning by 50% and 75% significantly reduced the potassium reabsorption efficiency of forest trees by 53.56% and 79.16%, respectively. 4) There was a significant correlation between nutrient content and nutrient reabsorption efficiency, and the correlation was positive for mature leaves in August, and negative for litter leaves in November. The contribution rates of potassium content in mature leaves, N/K ratio in senescent leaves, and phosphorus content in senescent leaves to nutrient resorption ranked the top three, at 35.2%, 27.0%, and 19.9% respectively. Conclusion: Nutrient stoichiometric characteristics of each organ and the reabsorption of nitrogen and phosphorus in leaves are insensitive to the short-term response of thinning. However, the nutrient absorption and utilization strategies of various organs of the poplar at different periods during the growing season are different. Based on the “N/P threshold hypothesis”, the N/P range is 10.05–11.06 under different thinning intensities, indicating that the growth of P. tomentosa in this area is limited by nitrogen, and it is recommended to apply more nitrogen fertilizer in stand management after thinning. At the same time, the results show that NRE/PRE<1. Based on the “relative reabsorption hypothesis”, addition of an appropriate amount of phosphorus fertilizer to the nitrogen fertilizer can promote the physiological metabolism of forest trees and improve the growth potential of forest trees.

Key words: Populus tomentosa, thinning effect, tree organs, ecological stoichiometry, nutrient reabsorption