Contrasting growth responses among plant growth forms to nitrogen fertilization in a subtropical forest in China

Abstract. Atmospheric nitrogen (N) deposition has been a noteworthy aspect of global change. Previous observational studies in temperate and tropical forests have focused on the effects of N deposition on tree growth. Here we asked how trees and other plant growth forms respond to experimental N deposition in a subtropical forest in China. We conducted a four-year N fertilization experiment in a subtropical evergreen forest in southeastern China with three treatment levels applied to 9 20 × 20 m plots and replicated in three blocks. We classified the plants to trees, saplings, shrubs(including tree seedlings) and ground-cover plants (ferns) according to the growth forms, then we measured the absolute and relative basal area increments of trees and saplings, and the aboveground biomass of understory shrubs and ferns. In addition, we grouped individuals of the dominant tree species Castanopsis eyrei into three size classes and analyzed their growth responses to N fertilization separately. Although the total tree growth on plot level did not show a significant response to the N fertilization, the small trees with DBH (diameter at breast height) values of 5–10 cm were hindered by N fertilization, while the growth of large trees with DBH > 10 cm showed neutral or weakly positive responses to N fertilization. Small trees, saplings and particularly understory shrubs and ground-cover ferns suppressed seriously by increasing N fertilization. The proportions of plant mortality in N-fertilized plots were higher than in unfertilized plots and most of the dead individuals were small trees, saplings, shrubs and ferns. N deposition potentially leads to increased growth of larger plant individuals at tree layer in the forest community and suppresses the growth and survival of other individuals at understory and ground-cover layers. Therefore, differences in the growth responses of different plant growth forms and individual sizes should be taken into account when evaluating the effects of N deposition on the functioning of these forest ecosystems, including their potential for carbon storage.