J Plant Ecol ›› 2020, Vol. 13 ›› Issue (3): 281-287.

• Research Articles •

### Soil nitrogen availability intensifies negative density-dependent effects in a subtropical forest

Fengmin Huang, Minxia Liang, Yi Zheng, Xubing Liu, Yuxin Chen, Wenbin Li, Shan Luo and Shixiao Yu*

1. Department of Ecology, School of Life Sciences/State Key Laboratory of Biocontrol, Sun Yat-sen University, Guangzhou, China

*Corresponding author. E-mail: lssysx@mail.sysu.edu.cn
• Received:2019-11-12 Revised:2020-02-25 Accepted:2020-03-20 Online:2020-03-21 Published:2020-06-01

Abstract:

Aims

The importance of density-dependent mortality in maintaining tree species diversity is widely accepted. However, density-dependent effects may vary in magnitude and direction with different abiotic conditions in forests. Theoretical predictions surmise that density-dependent effects may vary with soil available nitrogen (AN), but this still needs to be tested.

Methods

We analyzed the density-dependent effects on survival of newly germinated seedlings for 18 common species based on a long-term seedling census across environmental gradients in a subtropical forest. We also conducted a root lesion detection experiment for five species to investigate the potential effects of pathogens on variation in density-dependent disease between rich and poor AN environments.

Important Findings

The seedling dynamics analysis revealed that the strength of density-dependent effects increased with AN, shifting from neutral or positive with low AN to negative with high AN. Three of the five tree species had stronger density-dependent effects on root lesions in rich AN environments than in poor AN environments, which is consistent with the results of a long-term seedling dynamics analysis. We also found higher species diversity in rich AN environments, which may be promoted by the stronger negative density-dependent effects. Both the seedling dynamic analysis and root lesion detection experiment revealed stronger negative density-dependent effects in higher AN environment, resulting from stronger disease pressure by soil pathogens. Our study emphasized the importance of considering context dependence when testing the density dependence hypotheses.