Short-term responses of Picea asperata seedlings of different ages grown in two contrasting forest ecosystems to experimental warming

Low temperatures are generally limiting factors in alpine ecosystems. Predicted global warming therefore could have profound impacts on these ecosystems in the future. This study was conducted to compare effects of experimental warming on the phenology, growth and photosynthesis of dragon spruce seedlings of two age classes (2- and 8-year-old seedlings) grown in two contrasting subalpine forest ecosystems (dragon spruce plantation versus spruce-fir dominated natural forest) using the open top chamber (OTC) method in the Eastern Tibetan Plateau of China. The OTCs enhanced daily mean air (30 cm above the soil surface) and soil temperatures (5 cm below the soil surface) by 1.2 °C and by 0.6 °C in two experimental sites, respectively, throughout the growing season. Conversely, soil volumetric moisture declined by 3.8% and 2.8% in the plantation and natural forest. Experimental warming markedly extended the growing season of two age classes in both sites. However, there were no clear differences in phenology between ages or sites. Warming often increased the growth and photosynthesis of dragon spruce seedlings. There were pronounced differences in the morphological and physiological performances between ages or sites. Nevertheless, there were no significant interactions of warming, age and site on phenology, growth and photosynthesis. Irrespective of seedling ages or experimental sites, artificial warming had significant increases in component biomass except the root. The size of warming effect on biomass depended strongly on seedling age and experimental site. Elevated temperatures remarkably increased net N mineralization rates and extractable inorganic N pools in both sites. Both net N mineralization and extractable inorganic N pool were pronouncedly greater in the natural forest than in the plantation. Taken together, our results indicate that warming generally has positive effects on the phenology, growth and photosynthesis for both seedling age classes in any site. Younger seedlings are more sensitive to warming as compared with the older seedlings. Reforestation dramatically affects the responses of soil N turnover and availability to warming. We conclude that both direct and indirect warming effects synchronously modify the seedling responses.

► Younger seedlings are more sensitive to warming than old ones.
► Seedlings grown in natural forest soil are more sensitive to warming than those grown in plantation soil.
► Warming may stimulate seedling growth through both direct and indirect effects.