Enhanced leaf turnover and nitrogen recycling sustain CO fertilization effect on tree-ring growth.

Whether increased photosynthates under elevated atmospheric CO could translate into sustained biomass accumulation in forest trees remains uncertain. Here we demonstrate how tree radial growth is closely linked to litterfall dynamics, which enhances nitrogen recycling to support a sustained effect of CO fertilization on tree-ring growth. Our ten-year observations in two alpine treeline forests indicated that annual (or seasonal) stem radial increments generally had a positive relationship with the previous year's (or season's) litterfall and its associated nitrogen return and resorption. Annual tree-ring width, annual litterfall and annual nitrogen return and resorption all showed an increasing trend during 2007-2017, and most of the variations were explained by elevated atmospheric CO rather than climate change. Similar patterns were found in the longer time series of tree-ring width index from 1986-2017. The regional representativeness of our observed patterns was confirmed by the literature data of six other tree species at 11 treeline sites over the Tibetan Plateau. Enhanced nitrogen recycling through increased litterfall under elevated atmospheric CO supports a general increasing trend of tree-ring growth in recent decades, especially in cold and nitrogen-poor environments.