The effects of light, conspecific density and soil fungi on seedling growth of temperate tree species

Tree regeneration is an important ecological process which impacts community dynamics and determines future forest structure and functioning. Seedling growth, as a key driver of tree regeneration, is strikingly affected by multiple abiotic and biotic factors, such as light environment, conspecific density and soil fungi. However, the interactive effects of these factors remain largely unclear. In this study, we addressed this knowledge gap for seedlings of two dominant canopy tree species, i.e., ash (Fraxinus mandshurica) and linden (Tilia amurensis) in northeast China using a split-plot experiment for light availability (30 and 80% of full light) and conspecific density (2 and 8 seedlings) with soil collected from underneath conspecific adult trees. We examined the effects of light and conspecific density on seedling growth, and explored the effects of the two treatment combinations on soil fungi, and the effects of soil fungi on seedling growth. Higher light availability slightly increased ash seedling growth, but significantly decreased linden seedling growth. Conversely, higher conspecific density depressed ash seedling growth, but improved linden seedling growth. Light and conspecific density had no significant interactive effects on ash seedlings, but lower light availability and higher conspecific density tended to increase linden seedling growth. In addition, soil fungi, which were slightly impacted by light and ash seedling density, had no significant effects on ash seedling growth. However, linden seedling growth was positively affected by ectomycorrhizal fungi, which was greatly increased under lower light availability or higher seedling density. Overall, ash seedling growth may be mainly affected by available resources, controlled by light and conspecific density, while responses of linden seedlings to light and conspecific density can be regulated by soil fungi. These response differences of species to environments may be an important mechanism driving forest community dynamics in temperate forests.