Continuous planting under a high density enhances the competition for nutrients among young Cunninghamia lanceolata saplings

Abstract

Key message: A high-density plantation inhibited growth and biomass accumulation of Cunninghamia lanceolata(Lamb.) Hook. saplings, as well as their photosynthesis. This inhibition was enhanced in a soil that had been previously planted with the same species. The main factors limiting photosynthesis and growth were leaf-level irradiance and nutrient availability, mainly of P and Mg. Context: The planting density and continuous planting greatly affect the photosynthesis and productivity of Chinese fir plantations. The effects of high density and of continuous plantations over several revolutions need be disentangled. Aims: In this study, the responses of C. lanceolata seedlings to a high planting density were tested. Two soils were compared: a soil from a secondary forest and one from a continuous Chinese fir plantation. The study focused on growth and the potential processes involved in deduced photosynthesis. Methods: C. lanceolata seedlings were planted in wooden boxes (100 × 100 × 50 cm) with high and low planting densities (16 vs 1 plant m−2) in two types of soil. Results: Under the high planting density, C. lanceolata showed less growth and biomass accumulation at the individual level and lower photosynthetic rate and instantaneous photosynthetic nutrient use efficiency (PNUE and PPUE) at the leaf level. These negative effects were larger in soils that have been continuously planted with Chinese fir. The low photosynthesis was related to low phosphorus and magnesium contents in the leaves, changes in the foliar N/P and chlorophyll a/b ratios, and the limitation of the mesophyll conductance. Conclusions: The study showed that a high planting density induced enhanced competition for nutrients (particularly for P and Mg) and that this competition is enhanced in soils from continuous plantations compared to soils from natural forests.