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Nitrogen deposition and uncertainty of precipitation are currently two of the most important ecological issues of global climate change in subtropical areas.To study the effects of nitrogen and precipitation on the eco-physiological traits and growth of subtropical trees,we set up a two-factorial experiment controlling the factors of precipitation and soil fertilization for the seedlings of Elaeocarpus glabripetalus,Liquidambar formosana,Schima superba and Cyclobalanopsis glauca for three years on Gutian Mountain in Zhejiang Province,China.Precipitation was set at three levels: natural precipitation(CK),increase of 30% natural precipitation and decrease of 30% natural precipitation.Soil fertilization was set at two levels: natural nitrogen and nitrogen addition of 10 g · m-2 · a-1NH4NO3.In general,the pH values in subtropical soil are low and the growth of most subtropical tree species may be limited by phosphorus deficiency in the soil not nitrogen availability,so we need to test if addition of nitrogen in soil will promote the eco-physiological traits and growth of the subtropical tree species.We also want to test how the interactive soil fertilization and precipitation changes affect the growth and photosynthetic traits in the four subtropical tree species.We periodically measured the seedlings' photosynthetic eco-physiological traits and their environmental growth factors for three years.The results showed that precipitation variation and nitrogen addition did have some effects on eco-physiological characteristics and total biomass across four tree species.We found that 10 g · m-2 · a-1 NH4NO3 nitrogen addition significantly improved the leaf chlorophyll synthesis(P0.05)and photosynthesis rate(P0.05)across the four subtropical tree seedlings.The addition of nitrogen also positively stimulated PSⅡ photochemical efficiency(Fv/Fm,PIABS)(P0.01)in the four subtropical tree seedlings.Addition of nitrogen significantly increased total biomass formation by more than 30%(P0.01).Hence,nitrogen addition would increase tree seedling photosynthesis ability and promote their growth in subtropical evergreen forest of Gutian Mountain.In contrast,we found that the effects of precipitation changes were less significant on the eco-physiological traits and total biomass for the four subtropical tree seedlings,Nevertheless,precipitation variations significantly influenced photosynthetic rate(P0.05),stomatal conduction and transpiration(P0.001) for some tree seedlings during the dry season of Gutian Mountain.Meanwhile,stomatal conduction was more sensitive to precipitation variations.The interactive effects of precipitation and nitrogen addition on the eco-physiological characteristics(excluding PIABS) and total biomass among the four tree species were not significant(P0.05).The differences in eco-physiological characteristics among the four tree species should attribute to their natural biological properties,while the difference in biomass among different tree species was due to the variations in precipitation change and nitrogen addition(P0.01)from this experiment.

Spiraea pubescens, a common shrub in the warm-temperate deciduous forest zone which is distributed in the Dongling Mountain area of Beijing, was exposed to ambient and enhanced ultraviolet-B (UV-B, 280–320 nm) radiation by artificially supplying a daily dose of 9.4 kJ/m2 for three growing seasons, a level that simulated a 17% depletion in stratospheric ozone. The objective of this study was to explore the effects of long-term UV-B enhancement on stomatal conductance, leaf tissue δ13C, leaf water content, and leaf area. Particular attention was paid to the effects of UV-B radiation on water use efficiency (WUE) and leaf total nitrogen content. Enhanced UV-B radiation significantly reduced leaf area (50.1%) but increased leaf total nitrogen content (102%). These changes were associated with a decrease in stomatal conductance (16.1%) and intercellular CO2 concentration/ air CO2 concentration (Ci/Ca) (4.0%), and an increase in leaf tissue δ13C (20.5‰), leaf water content (3.1%), specific leaf weight (SLW) (5.2%) and WUE (4.1%). The effects of UV-B on the plant were greatly affected by the water content of the deep soil (30–40 cm). During the dry season, differences in the stomatal conductance, δ13C, and WUE between the control and UV-B treated shrubs were very small; whereas, differences became much greater when soil water stress disappeared. Furthermore, the effects of UV-B became much less significant as the treatment period progressed over the three growing seasons. Correlation analysis showed that enhanced UV-B radiation decreased the strength of the correlation between soil water content and leaf water content, δ13C, Ci/Ca, stomatal conductance, with the exception of WUE that had a significant correlation coefficient with soil water content. These results suggest that WUE would become more sensitive to soil water variation due to UV-B radiation. Based on this experiment, it was found that enhanced UV-B radiation had much more significant effects on morphological traits and growth of S. pubescens than hydro-physiological characteristics.