Trade-off of NSC allocation in response to the phosphorus limitation caused by increased stand age in Pinus massoniana

The nutrient limitation due to increasing stand age is one of the reasons for affecting the growth of trees. However, non-structural carbohydrate (NSC) play an important role in plant adaptation to stress environment. Yet, little is known about the adaptation mechanism of NSC when faced with nutrient limitation caused by the increasing stand age. Here, we selected a chronosequence of Pinus massoniana Lamb. plantations with increasing age including 5-, 12-, 28-, 37- and 56-year old stand. We measured the radial growth from 2016 to 2021, branch length from 2017 to 2021, N, P and NSC in each organ (including current year leaves, 1-year leaves, current year branches, 1-year branches, trunk, fine roots and coarse roots). With the increase of stand age, radial growth increased first and then decreased, with the highest value in 37-year old stand. The N: P ratio was greater than 16 and P concentrations were lower than 1 g/kg except that in 5-year old stand, and the variation of N: P along age sequence was basically consistent with the variation of radial growth, indicating that the P limitation increased along the age sequence, but it had been alleviated after reaching 37-year old. The NSC concentrations in P. massoniana were almost significantly increased in leaves, branches and fine root along the age sequence, but were only increased to 37-year old stand in coarse root and trunk, and then decreased after reaching 37-year old. This reflected that less fixed carbon had been allocated to structural biomass growth (trunk) and more had been allocated to the function organ (fine root), which played a positive effect on alleviating P limitation, but was not conducive to trunk thickening growth. The branches growth decreased with the increase of stand age, but the branch P and NSC did increase, indicating that branches increased the storage of P and NSC to supply other organs by reducing growth consumption. It was concluded that with the increase of stand age, P limitation can be alleviated by reducing trunk growth and increasing carbohydrates allocation to the functional organ (fine roots) to support absorbing the limited nutrient. The study deepens our understanding of the need to enhance nutrient management in older age P. massoniana plantations.