Nitrogen (N) deposition has profound impacts on the phosphorus (P) cycling in forest ecosystems. Especially, the aggravated P limitation on tree growth under N addition has caused much attention to researchers. This article reviews the effects of N addition on plant P content in forest ecosystems. The result showed that N addition increased soil available P and facilitated the absorption of P by plants by promoting soil phosphatase activity, thereby increasing plant P content. Furthermore, changes in tree P content following N addition were also affected by species, life forms as well as experimental duration. Due to the inconsistency, the underlying mechanisms of changes in P content under N addition were further summarized as follows: 1) changes in soil available P content induced by exogenous N input affected the source of plant P; 2) N input affected the P uptake capacity of plants by affecting plant root exudates, mycorrhizal symbiosis and root morphological structure; 3) plant P utilization efficiency was also influenced with changes of P re-distribution and P re-absorption. Overall, for the changes in plant P under increasing exogenous N inputs, alterations of soil available P under N addition was the primary factor, while changes in plant P uptake capacity and P utilization efficiency ulteriorly regulated plant P content.

Aims Warm-temperate deciduous broad-leaved forest plays an important role in maintaining regional ecosystem function and balance. To explore the growth and eco-physiological response to nitrogen (N) addition will deepen our understanding of its dynamic development under the scenario of global N deposition.

Methods A simulated N deposition experiment was established with four N addition plots (100 kg·hm ^-2·a ^-1) and four control plots in a deciduous broad-leaved forest in Dongling Mountain, Beijing. The responses of branch growth, photosynthesis, chlorophyll fluorescence and seed mass of dominant species Quercus wutaishanica and associate species Acer pictum subsp. mono to N addition were investigated.

Important findings Net photosynthetic rate, transpiration rate, and chlorophyll content were enhanced by the N addition. Furthermore, the N addition also enlarged the electron transport pool in photosystem II reaction center and increased the turnover number of the primary quinone acceptor (QA) reduction expressed by fast chlorophyll fluorescence method of JIP test in both species. The N addition also increased the length and biomass of the current year branches of the two tree species and improved the seed quality of Q. wutaishanica to a certain extent. In general, we observed more pronounced responses of photosynthetic nitrogen use efficiency, and branch biomass formation to N addition in Q. wutaishanica compared with A. pictum subsp. mono. Our results suggested that the dominant position of Q. wutaishanica should be further highlighted under increased N deposition scenario in the warm-temperate deciduous broad-leaved forest in Dongling Mountain.

Aims The community assembly mechanisms are among the focal topics in ecological studies. In Changbai Mountains Nature Reserve, there is an intact primary broadleaved-Korean pine forest ecosystem. With increasing loss of species diversity in recent years, study that explores the community assembly mechanisms in this region is particularly important.

Methods This study was conducted in three large permanent plots, each of the size 5.2 hm ², along suessional stages (secondary poplar and birch mixed forest, PBF; secondary mixed conifer and broad-leaved forest, CBF; and primary Tilia amurensis-Pinus koraiensis mixed forest, TKF) in Changbai Mountains. Six functional traits of major tree species were measured, including leaf area, specific leaf area, leaf thickness, leaf nitrogen content, leaf phosphorus content, and maximum tree height. Changes in the spatial values of community traints were analyzed at different spatial scales (5 m × 5 m, 10 m × 10 m, 20 m × 20 m, 30 m × 30 m, 40 m × 40 m, 50 m × 50 m and 60 m × 60 m). By comparing the observed values with expected values of null models, the community assembly mechanisms in temperate forests of Changbai Mountains were explored.

Important findings Results show that the size of species pool has an important impact on the outcome; in a larger species pool, the environmental filtration has a significant impact. At the plot level and for early and intermediate stages of succession, the observed spatial values of community traits do not significantly differ from the expected values. At the late successional stage, the observed spatial values of community traits were greater than the expected values. The analysis of multiple community functional diversity indices shows that the combined processes of habitat filtring and competitive exclusion are the main determinants of the species composition of the climax community in this region. In the early successional stage, large numbers of species are immigrated, and there are strong resource competitions among the species within a community. With progressing succession, some species are excluded, species maintained in the community show significant niche differentiations, and competition is the main mechanism species coexistence.

Aims Phylogenetic and functional diversity are important components of species biodiversity, and are thought to play key roles in the mechanisms of community assembly. In this study, we explore the mechanisms of community assembly in tropical cloud forest plant community in Hainan Island, in southern China, using phylogenetic and functional diversity based methods.

Methods We constructed a species pool of 186 woody plant species from three tropical cloud forest sites in Hainan Island. For these species, we measured 13 functional traits and assessed their phylogenetic signals. In addition, we measured seven environmental factors and assessed their relationships using Principal component analysis (PCA). Then we chose Rao’s quadratic entropy (RaoQ) and mean pairwise distance (MPD) indices to examine the effects of functional diversity and phylogenetic diversity on tropical cloud forest community assembly. To do this we compared these indices to expectations under null models that assume neutral community assembly. We used standard effect sizes to evaluate the influence of environmental factors on community assembly.

Important findings Canopy openness, soil total phosphorus content and slope were significant environmental predictors in tropical cloud forest. The phylogenetic signals of most functional traits were very low and not significant, indicating that the phylogenetic relationship and functional traits were not consistent with the change of historical process. The observed values of RaoQ and MPD were significantly lower than expected, and their standard effect sizes were significantly correlated with soil phosphorus content, which suggested that habitat filtering driven by soil phosphorus was the key factor driving the community assembly in tropical cloud forest.

Aims Tropical seasonal rain forest of the karst landscape in southwestern Guangxi is one of the global biodiversity hotspots, with its habitats characterized with large exposed boulders, shallow and patchy soils, and cascading streams. As a result, plants in the karst landscapes experience frequent drought due to limited soil water holding capacity. In this study, we investigate the hydraulic characteristics of woody species in this drought-prone environment for sound conservation of plants in the fragile forests as well as through restoration actions.

Methods We measured xylem vulnerability curves, minimum xylem water potential (Ψ_stem-min), sapwood density and leaf turgor loss point of 17 typical woody plant species that co-occur in a tropical seasonal rain forest. We compared cavitation-resistance and hydraulic safety margins among the species and/or by plant groups. In addition, we analyzed the relationships between hydraulic safety margins and other hydraulic characteristics.

Important findings We found large inter-species variations in xylem water potential at 50% of hydraulic conductivity (P₅₀ ranged from -0.51 to -2.51 MPa), with evergreen species showing more negative P₅₀ than deciduous species. The hydraulic safety margin (HSM), calculated as the difference between Ψ_stem-min and P₅₀ (Ψ_stem-min - P₅₀), has been widely used to quantify the degree of hydraulic risk under droughts. The average HSM of the 17 woody species was 0.36 MPa, indicating a high hydraulic risk in this forest during the dry season. However, there existed significant differences in HSM among species. Interestingly, there was no significant correlation between HSM and other key hydraulic characteristics, likely because of different hydraulic strategies under drought stress. This study provides evidences of different hydraulic risks of the common species in response to droughts and highlights the importance of long-term monitoring of water potential for conservation of the plants, especially in extreme drought events.

Aims Axial parenchyma (AP) and ray parenchyma (RP) in secondary xylem have many important ecological functions, such as storage and translocation. Quantifying the compositions of parenchyma in secondary xylem will benefit the further research about their functions. However, the understanding of parenchyma compositions in current-year branches is still lacking.

Methods Eighteen woody saplings in the Tianma National Nature Reserve were selected for sampling. The proportions of cross-sectional area occupied by AP and RP were measured. The variations in parenchyma among species and their phylogenetic signals were analyzed. Compared with the related dataset, the differences between twigs and trunks were tested.

Important findings (1) The proportions of total parenchyma in twigs of the 18 woody saplings were 9.96%-18.56%, with the average of 14.80%; the proportions of RP (7.74%-15.45%) were higher than that of AP (1.13%-7.49%). (2) The total parenchyma (RAP) in twigs was lower than that in trunks, of which, RP in twigs was lower than that in trunks while AP showed an opposite pattern. The differences between twigs and trunks may be caused by the differences between different organs and different life history periods. (3) Significant phylogenetic signal was detected in the amount of AP in secondary xylem of twigs. This study primitively verified the phylogenetic signal of the secondary xylem parenchyma, and suggested that the difference between organs, and the difference between life history periods, had important effects on the variations of parenchyma.

Aims The objectives of this study were to characterize the carbon (C) : nitrogen (N) : phosphorus (P) stoichiometry of the “host branches-haustorias-parasitic branches-parasitic leaves” continuum and to better understand nutrient relationship between hemiparasite plants and their hosts.

Methods The study site is located in the Xujiaba area of Ailao Mountain, Yunnan Province. Two common hemiparasite plants Loranthus delavayi and Taxillus delavayi were selected, and the C, N and P concentrations of host branches, haustorias, parasitic branches and parasitic leaves were measured.

Important findings The results showed that, the tendency of C, N, P stoichiometry characteristics of host branches-haustorias-parasitic branches-parasitic leaves were species specific, and were not identical between the two hemiparasites. The host branches of the same parasitic plant have similar C, N, and P stoichiometry characteristics, and the host species have no significant effect on the stoichiometry of hemiparasites. There was a close coupling relationship between the C, N, P stoichiometry characteristics in the host branches, and the haustorias was weaker than the host branch, the parasitic branch was weaker than the haustorias, and there was no significant correlation between the N and P concentrations in the parasitic leaf. There was a significant negative correlation between the host branches and the parasitic leaves of C concentration. The C, N, P stoichiometry characteristics of the haustorias were more similar to the parasitic branches, and it had a very significant positive correlation with the host branches. As a key part of the host and parasitic plants, the haustorias had a significant correlation with the host branches, which reflected the importance of the host branch nutrients to the parasitic plants. The element stoichiometry and their relationship of the haustorias were more similar to those of the parasitic branches, which embodied that haustorias as a parasitic plant organ had physiological functions similar to those of the parasitic branches. These results provided important data for in-depth study of nutrient utilization strategies and ecological adaptability of hemiparasitic plants.

Aims The objective was to explore the stoichiometry of rhizosphere soil enzymes under major understory vegetation and their responses to plantation types and seasons.

Methods Rhizosphere soils of understory shrubs (Loropetalum chinense, Adinandra millettii and Eurya muricata) and herbs (Woodwardia japonica and Dryopteris atrata) were sampled in the early growth stage (April) and the vigorous growth stage (July) in Cunninghamia lanceolata, Pinus massoniana and Pinus elliottii plantations at Qianyanzhou Ecological Research Station, Taihe, Jiangxi. Potential activities of β-1,4-glucosidase (BG, carbon (C) acquiring enzyme), β-1,4-N-acetylglucosaminidase (NAG, nitrogen (N) acquiring enzyme) and leucine aminopeptidase (LAP, N-acquiring enzyme), acid phosphatase (AP, phosphorus (P) acquiring enzyme) and their stoichiometric ratios were measured. Soil physical and chemical properties were also analyzed.

Important findings The results found that (1) rhizosphere soil extracellular enzyme activities associated with C and N acquisition and BG:AP (enzyme C:P) were significantly different among understory species, but P acquisition were not. Both forest stand types and sampling seasons influenced BG:(NAG+LAP) (enzyme C:N). Interactions of understory species, forest stand types and seasons observably affected enzyme C:P. Principal component analysis showed that rhizosphere soil enzyme activities and ecoenzymatic stoichiometry differed significantly among different understory species (Loropetalum chinense was obviously different from Eurya muricata, and both of them were evidently different from other understory species), forest stand types (Cunninghamia lanceolata was different from Pinus massoniana and Pinus elliottii plantations) and sampling seasons. Soil NO₃ ^--N, NH₄ ⁺-N, DOC content and C:N were the main edaphic abiotic factors influencing the rhizosphere soil enzyme activities and ecoenzymatic stoichiometry. (2) Standardized major axis analysis showed that there were significantly linear relationship among lg(BG), lg(NAG+LAP) and lg(AP) of rhizosphere soils of understory species. lgBG:lg(NAG+LAP):lgAP(enzyme C:N:P) was approximately 1:1:1.3. Rhizosphere soil enzyme C:P and (NAG+LAP):AP (enzyme N:P) of understory species were 0.14 and 0.15, respectively. The regression slopes of lg(BG), lg(NAG+LAP) and lg(AP) deviated significantly from 1 because AP activities were much higher than BG activities and NAG+LAP activities. This study found that rhizosphere soil enzyme activities and ecoenzymatic stoichiometry were affected by understory species, forest stand types and sampling seasons in which substrate availability played an important role. Compared with C- and N-acquiring enzymes, microorganisms allocated more resources to the production of P-acquiring enzymes, which implied that the growth and activity of soil microorganisms were much more limited by P in rhizosphere soil of understory vegetation in subtropical plantations.