Seasonal variations of leaf C, N, P contents and stoichiometry of Larix principis-rupprechtii

【Objectives】Carbon (C), nitrogen (N) and phosphorus (P) play an important role in forests growth. The contents and ratios of C, N and P element reflect the nutritional status and healthy growth of forests. The objective of this research was to explore the seasonal variations of C, N, P contents in the leaves in different ages, so to provide a basis for the academic mamgement of the artificial forests. 【Methods】Three different ages, namely 5 years old (5 a), 10 years old (10 a) and 20 years old (20 a) of Larix principis-rupprechtii in Qinling Mountains were chosen as experimental materials. Leaf samples were collected in different growth phases in the middle of a month from May to October in 2012. The C, N, P contents and stoichiometry were monitored. Seasonal dynamics of leaf C, N, P contents and C∶N, C∶P, N∶P mass ratios in different ages of Larix principis-rupprechtii were analyzed. Data were also analyzed by correlation analysis between N content and C∶N and P content and C∶P. 【Results】 1) C content of these three ages increased gradually from growth stage (May to June) to the vigorous growth period (July to August),then decreased at growth anaphase (August to September), but there had been a slight pick-up in the defoliation (October). 2) The N content of three ages increased first, then decreased and went up again throughout the growing season (May to October). The increasing or reducing time among three ages was different. 5 a and 20 a maintained the upward trend basically from June to vigorous growth period, while the N content of 10 a decreased first from June to July, and then increased sharply from July to September. The N content of three ages decreased sharply to the lowest value from early defoliation to defoliation. 3) The P content of three ages kept the same trend throughout the growing season. They all decreased first, then increased and decreased again, finally they kept stable. 4) Seasonal dynamics of leaf C∶N mass ratios in three ages were different. C∶N mass ratios in 5 a, 10 a and 20 a varied in the range of 21.83-56.42, 18.43-58.49 and 22.54-87.83, respectively. The values of coefficient of variation (CV) were 37.74%, 42.05% and 68.42%, respectively. The value of CV increased gradually along with increasing age. 5) C∶P mass ratios of needles in 5 a, 10 a and 20 a varied in the range of 169.17-402.85, 180.54-395.01 and 213.02-398.75, respectively. The values of CV were 31.35%, 31.43% and 22.88%, respectively. 6) N∶P mass ratios of leaf in 5 a, 10 a and 20 a varied in the range of 6.77-15.42, 6.38-20.13 and 4.44-14.45, respectively. The values of CV were 39.65%, 50.86% and 33.03%, respectively. 7) C, N, P content and stoichiometry had relationships with the age of Larix principis-rupprechtii, but the differences among the ages in average C, N, P content and their ratios were not significant(P0.05).【Conclusions】 Correlation analysis indicated that C∶N mass ratio and C∶P mass ratio were negatively correlated with corresponding N, P content in different ages of Larix principis-rupprechtii. Furthermore, N is a restrictive factor for young and middle-aged Larix principis-rupprechtii of Qinling Mountains during its growth and development. The growth of trees was restricted by the shortage of soil nitrogen. N fertilizer should be timely supplied in this place to meet the nutritional needs of the normal growth of Larix principis rupprechtii.