Abstract
Abstract. Reactive nitrogen (N) increase in the biosphere has been a noteworthy aspect of global change, producing considerable ecological effects on the functioning and dynamics of the terrestrial ecosystems. A number of observational studies have explored responses of plants to experimentally simulated N enrichment in boreal and temperate forests. Here we investigate how the dominant trees and different understory plants respond to experimental N enrichment in a subtropical forest in China. We conducted a 3.4-year N fertilization experiment in an old-aged subtropical evergreen broad-leaved forest in eastern China with three treatment levels applied to nine 20 m × 20 m plots and replicated in three blocks. We divided the plants into trees, saplings, shrubs (including tree seedlings), and ground-cover plants (ferns) according to the growth forms, and then measured the absolute and relative basal area increments of trees and saplings and the aboveground biomass of understory shrubs and ferns. We further grouped individuals of the dominant tree species, Castanopsis eyrei, into three size classes to investigate their respective growth responses to the N fertilization. Our results showed that the plot-averaged absolute and relative growth rates of basal area and aboveground biomass of trees were not affected by N fertilization. Across the individuals of C. eyrei, the small trees with a DBH (diameter at breast height) of 5–10 cm declined by 66.4 and 59.5 %, respectively, in N50 (50 kg N ha−1 yr−1) and N100 fertilized plots (100 kg N ha−1 yr−1), while the growth of median and large trees with a DBH of > 10 cm did not significantly change with the N fertilization. The growth rate of small trees, saplings, and the aboveground biomass of understory shrubs and ground-cover ferns decreased significantly in the N-fertilized plots. Our findings suggested that N might not be a limiting nutrient in this mature subtropical forest, and that the limitation of other nutrients in the forest ecosystem might be aggravated by the enhanced N availability, potentially resulting in an adverse effect on the development of natural subtropical forest.
Highlights
Reactive nitrogen (N) increase in the biosphere, especially atmospheric N deposition, is a globally prevalent phenomenon (Galloway et al, 2004)
The small trees with a DBH of 5–10 cm growing under unfertilized plots showed greater basal area increments than those growing under N-fertilized plots (Fig. 3a, p = 0.02)
Contrasting growth responses among plant growth forms to N fertilization were present in the mature subtropical evergreen forest in this study
Summary
Reactive nitrogen (N) increase in the biosphere, especially atmospheric N deposition, is a globally prevalent phenomenon (Galloway et al, 2004). It has become a serious environmental issue in China, especially in the southeastern regions, with drastic increase of N entering terrestrial ecosystems which produces considerable ecological effects on the functioning and dynamics of the terrestrial ecosystems (Liu et al, 2013; Gu et al, 2015). Since the 1990s, the simulated Nfertilization experiments have been conducted in various forest ecosystems to explore the responses of plants and other organisms to the potential high N enrichment and changes of soil N availability Tian et al.: Growth responses of trees and understory plants to nitrogen fertilization (e.g. Aber et al, 1998; Högberg et al, 2006; Gilliam, 2006; Thomas et al, 2010), specific responses of plants appeared to be highly growth form-dependent and ecosystem-dependent (LeBauer and Treseder, 2008; Bedison and McNeil, 2009; Dirnböck et al, 2014)
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