Effects of simulated nitrogen deposition on the nutritional and physiological status of beech forests at two climatic contrasting sites in Italy

Chlorophyll and topographic patterns demonstrate stress conditions drive the brightness of autumn leaf colour

Autumn leaf colour brightness is an important cultural ecosystem service. As its spatial patterns and ecophysiological mechanisms remain unclear, we analysed relationships among autumn leaf colour brightness, late summer chlorophyll content, and topographic position in both canopy-based micro-scale analysis and site-based macro-scale analysis. Multispectral drone observations were made in three Fagus crenata forests at elevations of 300, 600, and 900 m in Niigata Prefecture, Japan. In a drone-acquired digital surface model of canopy area distribution, we analysed the canopy-averaged autumn leaf colour brightness, four vegetation indices, topographic position index, and elevation. The macro-scale pattern showed brighter yellow leaves at lower elevations, suggesting the effect of temperature through productivity or evaporative stress. The micro-scale pattern showed brighter yellow leaves in canopies that had a low late summer chlorophyll content, or that grew on ridges. The negative correlation between autumn leaf colour and chlorophyll content suggests that low-chlorophyll trees experience stresses (e.g. irradiance, desiccation, senescence) that induce higher carotenoid content to defend against such stresses. Increased autumn leaf coloration on ridges is consistent with a stress gradient. Although further research is needed to reveal the underlying physiological and ecological mechanisms, autumn leaf colour brightness has different meanings at different scales and thus has potential not only as a cultural ecosystem service but also in forest management through early detection of stress.

Photosynthetic and biochemical responses of four subtropical tree seedlings to reduced dry season and increased wet season precipitation and variable N deposition

Interspecific variations in phenotypic plasticity of trees that are affected by climate change may alter the ecosystem function of forests. Seedlings of four common tree species (Castanopsis fissa, Michelia macclurei, Dalbergia odorifera and Ormosia pinnata) in subtropical plantations of southern China were grown in the field under rainout shelters and subjected to changing precipitation (48 L of water every 4 days in the dry season, 83 L of water every 1 day in the wet season; 4 g m-2 year-1 of nitrogen (N)), low N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 8 g m-2 year-1 N), high N deposition (48 L of water every 2 days in the dry season, 71 L of water every 1 day in the wet season; 10 g m-2 year-1 N) and their interactive effects. We found that the changes in seasonal precipitation reduced the light-saturated photosynthetic rate (Asat) for C. fissa due to declining area-based foliar N concentrations (Na). However, we also found that the interactive effects of changing precipitation and N deposition enhanced Asat for C. fissa by increasing foliar Na concentrations, suggesting that N deposition could alleviate N limitations associated with changing precipitation. Altered precipitation and high N deposition reduced Asat for D. odorifera by decreasing the maximum electron transport rate for RuBP regeneration (Jmax) and maximum rate of carboxylation of Rubisco (Vcmax). Ormosia pinnata under high N deposition exhibited increasing Asat due to higher stomatal conductance and Vcmax. The growth of D. odorifera might be inhibited by changes in seasonal precipitation and N deposition, while O. pinnata may benefit from increasing N deposition in future climates. Our study provides an important insight into the selection of tree species with high capacity to tolerate changing precipitation and N deposition in subtropical plantations.

Response to Hypersalinity of Four Halophytes Growing in Hydroponic Floating Systems: Prospects in the Phytomanagement of High Saline Wastewaters and Extreme Environments

Hypersaline environments occur naturally worldwide in arid and semiarid regions or in artificial areas where the discharge of highly saline wastewaters, such as produced water (PW) from oil and gas industrial setups, has concentrated salt (NaCl). Halophytes can tolerate high NaCl concentrations by adopting ion extrusion and inclusion mechanisms at cell, tissue, and organ levels; however, there is still much that is not clear in the response of these plants to salinity and completely unknown issues in hypersaline conditions. Mechanisms of tolerance to saline and hypersaline conditions of four different halophytes (Suaeda fruticosa (L.) Forssk, Halocnemum strobilaceum (Pall.) M. Bieb., Juncus maritimus Lam. and Phragmites australis (Cav.) Trin. ex Steudel) were assessed by analysing growth, chlorophyll fluorescence and photosynthetic pigment parameters, nutrients, and sodium (Na) uptake and distribution in different organs. Plants were exposed to high saline (257 mM or 15 g L-1 NaCl) and extremely high or hypersaline (514, 856, and 1712 mM or 30, 50, and 100 g L-1 NaCl) salt concentrations in a hydroponic floating culture system for 28 days. The two dicotyledonous S. fruticosa and H. strobilaceum resulted in greater tolerance to hypersaline concentrations than the two monocotyledonous species J. maritimus and P. australis. Plant biomass and major cation (K, Ca, and Mg) distributions among above- and below-ground organs evidenced the osmoprotectant roles of K in the leaves of S. fruticosa, and of Ca and Mg in the leaves and stem of H. strobilaceum. In J. maritimus and P. australis the rhizome modulated the reduced uptake and translocation of nutrients and Na to shoot with increasing salinity levels. S. fruticosa and H. strobilaceum absorbed and accumulated elevated Na amounts in the aerial parts at all the NaCl doses tested, with high bioaccumulation (from 0.5 to 8.3) and translocation (1.7-16.2) factors. In the two monocotyledons, Na increased in the root and rhizome with the increasing concentration of external NaCl, dramatically reducing the growth in J. maritimus at both 50 and 100 g L-1 NaCl and compromising the survival of P. australis at 30 g L-1 NaCl and over after two weeks of treatment.

Enhanced cadmium phytoremediation capacity of poplar is associated with increased biomass and Cd accumulation under nitrogen deposition conditions

Nitrogen (N) deposition plays a significant role in soil cadmium (Cd) phytoremediation, and poplar has been considered for the remediation of contaminated soil because of its enormous biomass and strong Cd resistance. To reveal the underlying physiological and root phenotypic mechanisms of N deposition affecting Cd phytoextraction in poplar, we assessed root phenotypic characteristics, Cd absorption and translocation, chlorophyll fluorescence performance, and antioxidant enzyme activities of a clone of Populus deltoides × P. nigra through combined greenhouse Cd and N experiments. Our results showed that Cd significantly changed the root phenotype by reducing root length, tip number, and diameter. Cd also caused the peroxidation of lipids, damaged the photosystem II (PSII) reaction centre, and reduced photosynthetic capacity, resulting in a decrease in biomass accumulation in poplar. The N60 (60 kg N·ha^-1·yr^-1) and N90 (90 kg N·ha^-1·yr^-1) treatments promoted the net photosynthetic rate of poplar by increasing the activity of antioxidant enzymes and proline content and repairing the PSII reaction centre, thus increasing the biomass accumulation of poplar exposed to Cd stress. Simultaneously, the N60 and N90 treatments might have increased Cd uptake from the soil by upregulating total root length, root tips, and fine root length. Cd mainly accumulated in roots and stems but not in leaves. The N30 (30 kg N·ha^-1·yr^-1) treatment had no obvious effects on these parameters compared with the single Cd treatment. Consequently, our study suggested that adequate N can improve biomass and Cd accumulation to enhance the phytoremediation capacity of poplar for Cd, which might be related to the improvement of leaf physiological defence and the change in root phenotypic characteristics.