Abstract:

Objective: The purpose of this study was to investigate the contents of organic carbon (OC), total nitrogen (TN), and total phosphorus (TP) and their stoichiometric characteristics in leaves of four typical plantation species (coniferous and broad-leaved life forms) in the alpine zone of the Loess Plateau in Qinghai, and their changes with the stages of the growing season (Ⅰ: May 15 to June 15; Ⅱ: June 15 to July 15; Ⅲ: July 15 to August 15) and the slope positions, and to provide a scientific basis for the management of forest in Qinghai. Method: The OC, TN, and TP in leaf samples of four typical plantation species (Larix principis-rupprechtii, Picea crassifolia, Betula platyphylla, and Populus cathayana) were measured by field manual sampling and indoor experimental analysis. The nutrient contents and stoichiometric characteristics were analyzed by normality test, one-way analysis of variance, Duncan multiple comparison, correlation analysis, detrended correspondence analysis, redundancy analysis, and Monte Carlo permutation test. Result: The mean values and variation range of the OC, TN, and TP contents in leaves of the four typical plantation species in the alpine zone of the Loess Plateau in Qinghai were 591.84 (571.19-605.67), 17.76 (7.14-27.62), and 1.47 (0.81-2.65) g·kg^-1, respectively. The coefficients of variation were ranked TP> TN> OC. The leaf TN content of the broad-leaved trees was higher than that of the coniferous trees. The leaf TN content and TP content of the broad-leaved trees were more affected by the stages of the growing season than that of the coniferous trees. The mean values and variation range of leaf C ∶N (OC ∶TN), C ∶P (OC ∶TP), and N ∶P (TN ∶TP) were 38.87 (26.04-97.73), 1 042.97 (667.53-1 971.00), and 28.32 (18.87-51.50), respectively. The coefficients of variation were ranked C ∶N < N ∶P < C ∶P. The leaf C ∶N and C ∶P of the broad-leaved trees were more affected by the stages of the growing season than that of the coniferous trees. Except for P. crassifolia, the contents of TN and TP in leaves of the other three tree species showed a very significant positive correlation (P < 0.01). The effects of stages of the growing season and life forms on leaf OC, TN, and TP contents and stoichiometric ratios of the four typical plantation species reached extreme levels of significance (P < 0.01), while the effect of slope positions was not significant (P>0.05). Conclusion: In the alpine zone of the Loess Plateau in Qinghai, the mean values of C ∶N and C ∶P in leaves of P. crassifolia were at a high level, while N ∶P was at a low level, indicating that the growth rate of P. crassifolia was relatively slow and the major reason was probably the limitation of TN. Attention should be paid to the supply of TN nutrient. Since the stages of growing season and the life form were two significant factors affecting leaf nutrient contents and stoichiometric ratios of the four typical plantation species (P < 0.01), both of them should be concerned carefully in the nutrient management in the alpine zone of the Loess Plateau in Qinghai.

Key words: leaf nutrient, ecological stoichiometry ratio, life form, stage of the growing season, slope position