Nitrogen and carbon supply controls tree radial growth at an alpine treeline

Abstract It is unclear whether and how tree-ring width is associated with photosynthetic productivity and nutrient recycling in forest ecosystems. Here, we aim to demonstrate how leaf turnover and nitrogen (N) resorption are crucial in regulating the seasonality of carbon (C) availability, and how tree radial growth is controlled by C supply in an alpine treeline forest. The seasonal litterfall, N resorption, leaf N concentration, leaf and twig non-structural carbohydrate (NSC) contents across eight trees were monitored every 2 weeks during the growing season, and radial increments were recorded by automatic dendrometers at Abies georgei var. smithii treeline. Leaf N showed a positive correlation with previous bi-weekly litterfall and N-resorption in the early season before late June, but with soil temperature in the later season. Leaf NSC typically peaked in the early growing season and showed significant relationships with previous bi-weekly litterfall and N-resorption and the following bi-weekly radial increment. There was a lagged chain from previous bi-weekly litterfall and N-resorption to current NSC in leaves, and then from current NSC in leaves to the following bi-weekly radial increment. As a significant C-storage pool, twig NSC reached a maximum in mid-July, and showed no correlation with litterfall, N-resorption and radial growth rate. N resorption and C supply regulated by leaf turnover controls tree radial growth at the alpine treeline. Our results highlight the significant role of nitrogen recycling and leaf source NSC production in driving tree radial growth, as compared to stored NSC.