Aims Leaf structural and functional traits have been extensively studied to explain community assembly mechanisms, species distributions, niche differentiations, and even ecosystem services functions. However, these traits are influenced by both environment and phylogeny, showing correlations or trade-offs among them. In this study, we assessed the impacts of leaf structure on drought tolerance and photosynthetic potential, and the trade-off between drought tolerance and photosynthetic capacity, to provide an explanation for species coexistence and the maintenance of high biodiversity in tropical rainforests.
Methods We chose 18 species in the Euphobiaceae family differing in distribution patterns along topographic gradients in a 20 hm² forest dynamics monitoring plot (FDP) in Xishuangbanna. We measured leaf anatomy, leaf water relations characteristics, maximum photosynthetic rate, and dark respiration, and used two different methods—the traditional Pearson correlation and phylogenetic independent contrasts—to analyze the relationships among those traits.
Important findings We found that: 1) species showed convergence in structures and functions within specific habitat; species on ridge or slope had a stronger water loss-tolerance abilities than species in the valley. 2) Correlations among some key traits (specifically, leaf density, water potential at turgor loss point, and water potential at full turgor, etc.) were found among habitats; plants adjusted leaf structure to influence simultaneously plant water loss-tolerance abilities and photosynthetic capability, which may result in a trade-off between drought tolerance (high leaf density, leaf mass per area) and photosynthetic capability (low leaf density, leaf mass per area). 3) The phylogenetic independent contrasts must be used when analyzing correlations among the traits of genetically related species due to the weakness of traditional Pearson analysis. The ecological niche differentiation to water and light gradients as revealed by the present study provides a potential explanation for the high diversity of the seasonal tropical rainforest.

Aims Our objective was to identify the degree of associations between species richness and individual densities of varying spatial scales and diameter classifications.
Methods Based on the dataset from a 60 hm² tropical montane rainforest plot in Jianfengling, Hainan, China, the relationship between species richness and stem density was analyzed at seven spatial scales (5 m × 5 m, 10 m × 10 m, 20 m × 20 m, 40 m × 40 m, 60 m × 60 m, 80 m × 80 m, and 100 m × 100 m) and three DBH (diameter at breast height) classifications (DBH ≥ 1.0 cm, DBH ≥ 2.5 cm, DBH ≥ 7.5 cm).
Important findings The relationship between species richness and stem density varies with the spatial scales and DBH classifications. For all plants with DBH ≥ 1.0 cm, there are significantly positive correlations between species richness and stem density at the spatial scales of 5 m × 5 m, 10 m × 10 m, 20 m × 20 m and 40 m × 40 m, but not at other larger scales. The patterns are similar in the relationship between species richness and stem density after grouping all stems into the three DBH classifications, except there are weak correlations between species richness and stem density for plants with DBH ≥ 2.5 cm and DBH ≥ 7.5 cm at the scale of 60 m × 60 m. The significance levels of the relationship of species richness and stem density are similar between the DBH ≥ 2.5 cm and DBH ≥ 7.5 cm classifications, which are both higher than that of DBH ≥ 1.0 cm. It is concluded that the associations between species richness and stem density are stronger for individuals with larger DBH than those with smaller DBH. It is inferred that negative dependence and self-thinning are two possible mechanisms related to the phenomenon.

Aims Litter constitutes the major source of organic matter entering the soil. Different litter layers reflect different phases of decomposition. The litter originated from different plant materials and decomposition phases may have a significant impact on cellulolytic enzyme activities. Our objective was to explore the effects of vegetation types and decomposition phases on cellulolytic enzyme activities during litter decomposition process in an alpine timberline ecotone at the end of snow melting.
Methods The activities of three cellulolytic enzymes (β-1,4-endoglucanase, β-1,4-exoglucanase and β-1,4- glucosidase) and litter qualities (C, N, P and cellulose content) were measured in the fresh litter and fermentation layer (LF) and the humus layer (H) in alpine meadow, alpine shrub, and coniferous forest in the alpine timberline ecotone in western Sichuan. Two-way ANOVA was used for testing the main effects of vegetation types, decomposition phase and their interactions on cellulolytic enzyme activities and litter qualities. We used Spearman correlations to explore the relationships between cellulolytic enzyme activities and litter qualities of two decomposition phases.
Important findings Cellulolytic enzyme activities and cellulose contents in the LF layer were significantly higher than in the H layer across all vegetation types. Two-way ANOVA results showed that decomposition phase had a more significant impact on cellulolytic enzyme activities and cellulose contents than vegetation types. Cellulolytic enzyme activities were under the control of different factors between the two decomposition stages. In the early decomposition stage, the activities of β-1,4-exoglucanase and β-1,4-glucosidase appeared to be limited by N and P contents of the substrate, while β-1,4-endoglucanase activity was mainly controlled by the cellulose content of litter. In the late decomposition stage, the activities of β-1,4-endoglucanase and β-1,4-glucosidase were mainly limited by C and N contents. According to the prediction of ecological stoichiometry theory, microbial growth is considered to be nutrient-limited on substrates with C:N > 27 or C:P > 186. Overall, litter C:N and C:P were greater than 27 and 186, respectively, in the study area, indicating that cellulolytic enzyme activities were limited by litter N and P contents. In particular, the microbial biomass was limited more significantly by N and P contents in the early decomposition stage in the alpine meadow, indicating that litter quality indirectly regulates cellulolytic enzyme activities of litter decomposition process in this alpine timberline ecotone.

Aims There is an obvious seasonal freezing and thawing process (early winter freezing-thawing, late winter freezing, and early spring freezing-thawing) in the subalpine coniferous forests, where there is also a strong effect of climate change in winter. Hence, global warming will likely affect the seasonal freezing and thawing process in this area. Our objective was to determine the ecophysiological characteristics of leaves and fine roots in dominant tree species in a subalpine coniferous forest of western Sichuan during seasonal frozen soil period, in order to improve our understanding of the ecological processes in subalpine coniferous forests.
Methods We analyzed the changes in malondialdehyde (MDA) content, osmoregulation substance content, tissue water content, peroxidase (POD) activity, and nitrate reductase (NR) activity in leaves and fine roots of Picea asperata and Abies fargesii var. faxoniana seedlings, and measured specific root length, specific surface area, diameter and tissue density of fine roots over the course of the seasonal frozen soil period.
Important findings The POD activity and the contents of proline and soluble proteins in leaves were significantly lower than in fine roots, despite that the daily fluctuations of soil temperature was less than that of air temperature during the seasonal frozen soil period, suggesting that fine roots were more susceptible to the seasonal frozen soil than leaves. In comparison with the soil freezing period, a greater daily fluctuations of air and soil temperature resulted in an increase in the soluble sugar content in leaves of P. asperata and the POD activity and proline content in leaves of the two species during the soil freezing-thawing period, whereas the tissue water content was significantly decreased and the contents of proline, soluble proteins and soluble sugars were significantly increased in fine roots, indicating that the effects of soil freezing-thawing on plants were stronger than soil freezing. In soil freezing-thawing period, the POD activity and osmolyte contents were significantly increased in both A. fargesii var. faxoniana and P. asperata, but changes in the MDA content and the NR activity in the fine roots and leaves were not consistent in the two species. In A. fargesii var. faxoniana, the MDA content was significantly increased in fine roots and the NR activity was significantly reduced in both leaves and fine roots. A change in the MDA content was only observed in leaves of P. asperata, which was significantly decreased, indicating that P. asperata had more tolerance to soil freeze-thaw cycles. Moreover, no significant changes in fine root morphology and growth were observed during the seasonal frozen soil period.

Aims Shrub recovery is identified as a major cause of an increase in carbon stocks in terrestrial ecosystems in China, and yet there is a great uncertainty in the contribution of shrubs to the carbon sink. Our objectives were to determine the biomass allocation pattern and carbon density in alpine shrubs.
Methods We conducted investigations in 14 shrub communities in eastern Qinghai-Xizang Plateau, at 3500 m above sea level. Plant samples were collected from each plot and measured for biomass in leaves, branches and stems, and roots in laboratory; the data were used to analyze the biomass allocation and carbon density.
Important findings The mean biomass was (5.38 ± 3.30) Mg·hm^-2 in the shrub layer. There were significant differences in biomass between different shrub types, with the mean of (7.28 ± 4.96) Mg·hm^-2 for the broadleaved deciduous shrubs and (4.32 ± 1.36) Mg·hm^-2 for the leathery-leaved shrubs. The indicators of individual feature and community structure were significantly correlated with biomass per unit land area. However, these relationships were developed based on multiple community structure factors; any single factor alone was insufficient to explain the patterns of biomass variations. The patterns of biomass allocation differed significantly between different shrub types. In this study, there was more allocation of photosynthetic products to roots. The mean total community biomass was (6.41 ± 3.86) Mg·hm^-2 and the shrub layer accounted for (83.18 ± 8.14)% of the total community biomass. There were significant correlations (p < 0.05) between shrub layer biomass and herb layer biomass, between shrub layer biomass and litter layer biomass, and between shrub layer biomass and the total community biomass. The biomass of various organs were also significantly correlated (p < 0.01) with the total community biomass. The mean biomass carbon density of the shrubs was estimated at (3.20 ± 1.93) Mg·hm^-2 across the 14 communities by using biomass conversion factor method.

Aims This paper studies the resource allocation during flowering and fruiting, as well as the relationship between variations in floral traits and seed characters in Saussurea undulata.
Methods Field sampling and measurements of dry mass were used to evaluate morphological characteristics of S. undulata distributed at different elevations in eastern Qinghai-Xizang Plateau.
Important findings Saussurea undulata invested more resources in the reproductive organs during flowering, whereas it invested more resources in seed during fruiting. The variations in floral traits during flowering led to a decline in seed number and an increase in 100-seed mass in S. undulate during fruiting with increasing elevation, because the species adopted survival strategies that produce less but larger seed to ensure survival of each seed under the extreme environment of alpine meadow in eastern Qinghai-Xizang Plateau.

Aims Several local varieties of wheat (Triticum aestivum) developed by Northwest Institute of Plateau Biology, Chinese Academy of Sciences, are widely cultivated in the agricultural regions in Qinghai-Xizang Plateau. These varieties are well adapted to multiple environmental stresses such as low temperature, strong solar radiation, and drought. The objective of this study was to determine the responses of PSII photochemical efficiency to high solar irradiance in leaves of four wheat varieties. We examined whether photo-inhibition was appeared in wheat varieties and analysed variations of quantum yield of quenching due to light-induced and non-light-induced.
Methods Field experiments were conducted on the farmland of Xiangride, which is located in the eastern side of Caidamu Basin, Qinghai Province. Four local wheat varieties were used during the heading stage in 2013. Measurements of photochemical efficiency and quantum yield were made on the abaxial surface of flag leaves facing the Sun by using a FMS-2 fluorometer, and the content of photosynthetic pigments and specific leaf weight (SLW) were concurrently determined. Pulse-modulated in-vivo chlorophyll fluorescence technique was used to obtain rapid information on photosynthetic processes. The maximum quantum efficiency of PSII photochemistry (F_v/F_m) was determined at 8:30, 12:00 and 16:30 on clear days after allowing for 20 min dark adaptation with leaf clips. The PSII maximal and actual photochemical efficiency (F_v′/F_m′ and Φ_PSII), the PSII photochemical and non-photochemical quenching coefficient (q_P and NPQ) were analyzed between morning and afternoon using inner actinic light with photosynthetically active photon flux density at 1120 μmol photons∙m^-2∙s^-1. Furthermore, along with analysis of the fraction of PSII reaction centers that are opened (q_L), the quantum yield of quenching due to light-induced processes (Φ_NPQ) and non-light-induced processes (Φ_NO) were explored.
Important findings There were significant differences in the content of photosynthetic pigments and SLW among the four wheat varieties. Under conditions of clear days, the flag leaves exhibited marked depressions in F_v/F_m at three typical times when determined after 20 min dark adaptation. At a given light intensity, the values of F_v′/F_m′ were significantly reduced in the afternoon due to influences by long-lasting high-light irradiation, and Φ_PSII showed little differences among the four wheat varieties and no difference between morning and afternoon. There were almost similar variations in q_P and NPQ among the four wheat varieties, suggesting that q_P and NPQ belong to instinct property and are influenced by the accumulative stresses of high-light intensity. The fractions of Φ_NPQ were higher than that of Φ_NO in the four wheat varieties and the up-regulatory of Φ_NPQ in the afternoon indicated that the photosynthetic apparatus in these wheat varieties had already acclimated to strong solar irradiation in agricultural regions of Qinghai-Xizang Plateau.

Aims Photorespiration and non-photochemical quenching of chlorophyll fluorescence in photosystem II (PSII) were studied in bracts and leaves of cotton (Gossypium hirsutum) plants, in order to investigate the photoprotective mechanisms and drought tolerance in cotton bracts under field conditions of water-saving drip irrigations.
Methods Gas exchange and chlorophyll fluorescence parameters of PSII were analyzed in bracts and leaves of cotton plants after anthesis. The study was conducted with two treatments comprising normal drip irrigation (5228.5 m³·hm^-2) and water-saving drip irrigation (3874.1 m³·hm^-2) under field conditions.
Important findings The actual photochemical efficiency of PSII (Φ_PSII) decreased in both bracts and leaves of cotton plants after anthesis under water-saving drip irrigation, but the magnitude of decrease was less in bracts than in leaves. Results showed the bracts experienced less severe photoinhibition than leaves. The rate of net photosynthesis (P_n), Φ_PSII, net photorespiration (P_r), photochemical quenching (q_P), and non-photochemical quenching (NPQ) decreased in the leaves of cotton plants under water-saving drip irrigation, but no significant difference was observed in the bracts. With decreasing water supply, the P_r/P_n in bracts was much higher than that in leaves and water deficit had no significant effect on the P_r/P_n in bracts. The results of chlorophyll fluorescence parameters showed that the quantum yield of regulated energy dissipation (Y(NPQ)) was higher in bracts than in leaves under high irradiance and temperature conditions, and that the thermal dissipation in bracts was not susceptible to water deficit, thus protecting the photosynthetic apparatus against photodamage. Overall, both photorespiration and energy dissipation in bracts were found to alleviate photoinhibition and played important roles in protecting PSII in cotton plants.

Aims Soil respiration plays a critical role in the process of carbon cycling in terrestrial ecosystems, and it often shows spatio-temporal variations in response to diverse abiotic and biotic factors. Our objective was to examine the seasonal and spatial variations of soil respiration under five typical plant communities in the meadow steppe of western Songnen Plain.
Methods Using a LI-6400 soil CO₂ flux system, we investigated soil respiration and environmental factors under five vegetation types (Suaeda glauca, Chloris virgata, Puccinellia distans, Phragmites australis and Leymus chinensis) in the meadow steppe of Songnen Plain during the growing seasons of 2011 and 2012.
Important findings Soil temperature was the dominant controlling factor of soil respiration, which explained approximately 64% of the changes in soil CO₂ effluxes. Soil water content was not the limiting factor of the seasonal variations in soil respiration. The sensitivities of soil respiration to temperature (Q₁₀) ranged from 2.0 to 6.7, showing significant differences among vegetation types. The cumulative CO₂ emission averaged 316.6 g C·m^-2 during the growing season. The magnitude of soil CO₂ emission during the growing season was positively correlated with aboveground plant biomass, soil organic carbon content, and mean soil water content, and negatively linked to mean soil temperature, pH, electrical conductivity, and percentage of exchangeable sodium. The spatial variations of soil CO₂ emission were mainly caused by changes in soil microclimate, plant biomass, and soil properties.