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Reactive nitrogen (N) increase in the biosphere has been a\udnoteworthy aspect of global change, producing considerable ecological effects\udon the functioning and dynamics of the terrestrial ecosystems. A number of\udobservational studies have explored responses of plants to experimentally\udsimulated N enrichment in boreal and temperate forests. Here we investigate how the\uddominant trees and different understory plants respond to experimental N\udenrichment in a subtropical forest in China. We conducted a 3.4-year N\udfertilization experiment in an old-aged subtropical evergreen broad-leaved\udforest in eastern China with three treatment levels applied to nine\ud20 m  ×  20 m plots and replicated in three blocks. We divided the\udplants into trees, saplings, shrubs (including tree seedlings), and\udground-cover plants (ferns) according to the growth forms, and then measured\udthe absolute and relative basal area increments of trees and saplings and the\udaboveground biomass of understory shrubs and ferns. We further grouped\udindividuals of the dominant tree species, Castanopsis eyrei, into\udthree size classes to investigate their respective growth responses to the N\udfertilization. Our results showed that the plot-averaged absolute and\udrelative growth rates of basal area and aboveground biomass of trees were not\udaffected by N fertilization. Across the individuals of C. eyrei, the\udsmall trees with a DBH (diameter at breast height) of 5–10 cm declined\udby 66.4 and 59.5 %, respectively, in N50 (50 kg N ha−1 yr−1)\udand N100 fertilized plots (100 kg N ha−1 yr−1), while the growth\udof median and large trees with a DBH of  >  10 cm did not significantly\udchange with the N fertilization. The growth rate of small trees, saplings,\udand the aboveground biomass of understory shrubs and ground-cover ferns\uddecreased significantly in the N-fertilized plots. Our findings suggested\udthat N might not be a limiting nutrient in this mature subtropical forest,\udand that the limitation of other nutrients in the forest ecosystem might be\udaggravated by the enhanced N availability, potentially resulting in an\udadverse effect on the development of natural subtropical forest

Growth responses of trees and understory plants to nitrogen fertilization in a subtropical forest in China