Aims The mid-mountain moist evergreen broad-leaved forest is a montane forest vegetation type developed in the subtropical climate zones of China. However, the community structure of the forest has never been reported based on large sized plots. The present study investigated the tree species composition and community structure within a 20 hm ² plot, serving as the fundamentals for analyzing the relationships between environmental factors and spatial distribution of tree species, and a basic platform for monitoring long-term forest dynamics in the future.

Methods We established a 20 hm² plot in a forest at Xujiaba on Mts. Ailaoshan, Jingdong County, Yunnan Province in 2014. The plot was set up in accordance to the protocols of the Center for Tropical Forest Science (CTFS). All free-standing woody plants with a diameter at breast height (DBH) ≥ 1 cm were positioned, mapped, tagged, measured and identified to species in the plot.

Important findings A total of 44 168 free-standing individuals with DBH ≥ 1 cm were recorded, belonging to 104 species, 63 genera and 36 families. Trees of Fagaceae, Theaceae and Lauraceae are dominant species. At the level of genus, the floristic composition was primarily represented by both tropical (46.0%) and temperate (38.1%) elements. The 22 individual species with importance value ≥ 1 contributed 84.6% to the total abundance. The three most abundant species were Camellia forrestii, Vaccinium duclouxii and Symplocos ramosissima. Four tree species (Lithocarpus hancei, L. xylocarpus, Castanopsis wattii and Schima noronhae) had the largest basal areas, and they were also the dominant canopy tree species in the plot. The DBH distribution of all individuals in the plot showed a reversed “J” pattern, indicating numerous recruiting populations. Such a pattern was applicable to C. forrestii and S. ramosissima as well. Forty-nine rare species (those with individual number fewer than 20 in the plot) accounted for 47.1% of the species total. Evergreen species dominated the life form spectrum of the plot, making up for 94.7%, 96.2% and 94.0% in terms of importance value, total abundance and total basal area, respectively.

Aims Anthropogenic disturbances and climate change have resulted in large scale degradation of grasslands across the landscapes in Nei Mongol. Fertilization, especially with nitrogen (N) addition, has been proposed and applied as an important management practice to promote primary production for these degraded grasslands. In this study, we examined the changes in plant diversity and biomass at three levels of degradations with N addition.

Methods Nitrogen addition experiment was installed in 2011. Six levels of N addition (0, 10, 20, 30, 40, and 50 g·m ^-2·a ^-1) were conducted at grasslands with three levels of degradations. Nitrogen was added at the beginning of each month from May to August each year. We investigated the changes in plant species richness and aboveground biomass by species in August, 2016. The total biomass of the community, as well as the biomass of each plant functional group (grasses and forbs) was calculated based on species composition.

Important findings We found that: (1) N addition decreased species richness and diversity at communities under moderate and severe degradations, but insignificant under extreme degradation. (2) N addition increased the aboveground biomass at communities under three levels of degradations. (3) N addition increased the aboveground biomass of the grasses and its proportion to the total biomass, but not on the total biomass of the forbs although it also decreased the proportion of aboveground biomass. These results indicate that the impacts of N addition on ecosystem function depended on plant function type. In addition, the fertilization effects should are examined at community level and by the degree of the degradation.

Aims The monsoon evergreen broad-leaved forest (MEBF) is the dominant vegetation type in the subtropics of Eurasian continent. It provides vital ecosystem services and supports the socioeconomic development of the societies. Recent literature indicated that the MEBF had been greatly affected by the changing climate and other disturbances such as outbreaks of insects and, consequently, shifted its species composition and structure. In this study, we aim at the long-term changes of plant species and community structure in the MEBF.

Methods Species names, diameters at breast height (DBH), and tree heights of all trees with DBH ≥ 1.0 cm were recorded in 1992, 1994, 1999, 2004, 2008, 2010 and 2015 in a 1-hm ² monitoring plot in the Dinghushan Biosphere Reserve. We quantified the long-term changes over this 24-year study period by species composition, biological diversity, aboveground biomass, DBH frequency table, and stand density by species.

Important findings Important findings From 1992 to 2015, the stand density increased by 42.7% while the total biomass decreased by 5.1%. The β diversity based on biomass and stand density indicated that the diversity increased by 37.4% and 65.6%, respectively, from 1992 to 2015. More importantly, the number of small arbor and shrubs, and smaller trees (DBH < 15 cm) also increased sharply, with major contributions from Aidia canthioides, Macaranga sampsonii and Blastus cochinchinensis. In contrast, the change in stand biomass was more from biomass decrease of Castanopsis chinensis, Schima superba and Cryptocarya concinna, but increase of Gironniera subaequalis and Pterospermum lanceifolium. We conclude that the changes on community composition and structure in the MEBF were significant, which was resulted from a combined influence of succession, changes in climate, and insect outbreaks.

Aims Water use efficiency (WUE) is an important variable for evaluating the growth adaptation of vegetation; it links carbon and water cycles of terrestrial ecosystems. Charactering the spatial and temporal variations in WUE and the driving factors not only can help understand the processes and regulations of ecosystem carbon and water cycles, but also provides scientific basis for formulating sustainable regional development policies and guiding water resources management. This study was conducted to determine the patterns of temporal changes for different vegetation types in northern China.

Methods Flux and meteorological data of nine field observation sites in northern China were used to analyze the changes of WUE under different temporal scales, including half-hourly, daily, 8-day intervals, and monthly. The effects of temperature (T_a), relative humidity (RH), vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) on WUE under different time scales were also examined.

Important findings (1) Diurnal change of WUE displayed an asymmetric “U” shape, with greater values at the sunrise than at the sunset. The seasonality of WUE in the Gobi and desert areas displayed a pattern of “U” shape, while it occurred as a single peak in other regions. The seasonal changes of WUE can be further divided into the types of gross primary production (GPP)-driven and the evapotranspiration (ET)-driven; the significance of either driven type increases with time. (2) T_a, RH, VPD and PAR were the main factors influencing the changes of WUE on a shorter temporal scale (half-hourly and daily). With increasing temporal scale, T_a and RH continue to remain the main factors affecting the changes of WUE, and their correlations with WUE were influenced by the prevailing role of GPP or ET. Moreover, the correlation coefficients became more significant with increasing temporal scales. (3) WUE increased with increases in leaf area index (LAI) until a certain value, beyond which the sensitivity of WUE to LAI decreased at the Changbai Mountain, Haibei and Zhangye stations. The relationship between precipitation and WUE was not significant in all the study areas. (4) Among the vegetation types, WUE ranked in order from high to low as forest, cropland, grassland, wetland and desert.

Aims Global climate change will increase the uncertainty of precipitation patterns and affect the growth and distribution of plants. In the sub-humid and semi-arid areas of central China, black locust (Robinia pseudoacacia) plantations are widely planted in reforestation practices. These forests are vulnerable to climate change induced water stress. This research aims to clarify the responses of black locust transpiration to rainfall and soil water conditions.

Methods To evaluate the responses of transpiration to precipitation changes, we measured and contrasted sap flow of black locust trees under throughfall exclusion treatment and the non-treated control in the Huaiping plantation forest of Yongshou County, a sub-humid area in Loess Plateau. Throughfall and soil moisture for both control and treatment plots were monitored. Waterproof panels were set in April 2015 for the treatment plot which excluded about 47.5% of the precipitation during the growing season. Stem sap flow in treatment and control plots have been measured since 2014 using Granier-type sensors. Meteorological factors including solar radiation, air temperature, and relative air humidity were monitored simultaneously outside the forest stand.

Important findings Difference in soil moisture developed between treatment and control plots, the mean soil moisture content in treatment plot was 23.76%, lower than 22.59% (p < 0.001, n = 31) in the control plot during the middle growing season. Sap flux densities reduced following the treatment. The mean sap flux density under general weather conditions for the treatment plot was 1.64 mL·m ^-2·s ^-1, not only lower than 2.42 mL·m ^-2·s ^-1 of the previous year (before treatment), but also lower than 3.38 mL·m ^-2·s ^-1 for control plot during the same period. The response patterns of sap flux density to solar radiation and vapor pressure deficit were also different, with the trees in treatment plot showing lower sensitivities to meteorological factors than those in the control plot. Our results show that transpiration of planted black locust trees was suppressed by the throughfall exclusion. The responses of transpiration to meteorological factors decreased compared with a pretreatment period or control plot. These results suggest that decrease in precipitation will not only reduce soil moisture, but may cause decreased transpiration, decreased responses to climatic variables, and lowered productivity as well.

Aims How plant species interaction changes along environmental gradients has remained a hot issue in studies of species coexistence and biodiversity maintenance. This study was conducted to determine the responses of root systems to drought stress in desert Reaumuria soongarica-Salsola passerina communities, in order to better understand the interspecific relationships of plant communities in response to environmental stresses.

Methods Monocultural and mixed R. soongarica and S. passerina communities were selected in Lingwu, Zhangye and Jiuquan, representing a drought stress gradient varying from desert steppe habitat to extreme desert habitat. Measurements were made on the vertical structures of root traits including specific root length (SRL) and specific root surface area (SRA). Root samples were collected by trench method by layers.

Important findings Both SRL and SRA had greater values in mixed R. soongarica-S. passerina communities than in monocultural communities of either species within given habitat, suggesting a mutually beneficial relationship between the two species and the enhanced ability of roots to absorb soil nutrients and water when in mixture. Reaumuria soongarica had deeper roots and a greater root extinction coefficient in vertical distribution than S. passerina in either monocultural or mixed communities, indicating the belowground niche segregation between the two species. We also found that greater the environmental stress, the higher values of SRL and SRA in both R. soongarica and S. passerina. The root extinction coefficient was about the same between the monocultural and mixed communities for R. soongarica, but changed a little for S. passerina. With increasing drought stress, the belowground niche segregation became more apparent, conforming to the stress gradient hypothesis. The observed patterns of aggregation aboveground and segregation belowground between R. soongarica and S. passerina in the mixed communities might be an adaptive strategy to drought environment of these two desert plant species.

Aims The effects of freeze-thaw cycles on seasonal snow thickness may play a significant role in the decomposition process of forest litter in arid areas, whereas the understanding on this issue remains poor. Therefore, our objective was to understand the effects of snow cover on the decomposition and the carbon, nitrogen and phosphorus release of Picea schrenkiana leaf litter, the representative species in arid areas in northwest China.

Methods A field experiment was conducted in Mt. Tianshan of Xinjiang from October 2015 to October 2016 using litterbag method. Air-dried leaf litter of P. schrenkiana was put into nylon litterbags and the litterbags were placed on the forest floor along the gradient of snow cover depth from forest gap to full canopy. Mass loss rates and carbon, nitrogen and phosphorus release of P. schrenkiana leaf litter were measured at three critical stages (freeze-thaw period, deep-freeze period, thawing period) under snow cover and the growing seasons (early growing season and late growing season) during one year of decomposition.

Important findings The results showed that (1) after one year’s decomposition, the decomposition rates of the P. schrenkiana leaf litter under different snow depths were 24.6%-29.2%, and there were significant difference (p < 0.05) between the decomposition rates under different snow depths. The decomposition constant (k) was highest under thick snow cover and lowest under no snow cover. (2) The decomposition during the winter snow cover period contributed 46.0%-48.5% of total decomposition of P. schrenkiana leaf litter in the whole year, and the litter decomposition was the fastest during the freeze-thaw cycles. (3) With the decomposition of leaf litter, the nitrogen content of P. schrenkiana leaf litter increased while the content of carbon and C:N decreased roughly. There was a significant difference (p < 0.05) in carbon content between different snow treatments in the deep freezing period and late growing season. The phosphorus content in leaf litter is irregular with the decomposition of leaf litter. Snow thickness significantly influenced the phosphorus content in leaf litter during freeze-thaw period and thawing period (p < 0.05). (4) Net N immobilization during leaf litter decomposition was observed in the whole snow cover season, C and P were mainly released. Among them, thin and medium snow patches showed higher carbon enrichment rates in the thawing period. Thin, medium and thick snow treatments in the freeze-thaw period, no and thick snow treatments in the thawing period and medium and thick snow patches in the late growing season showed higher nitrogen enrichment rates. In contrast, the effect of snow cover on the release of leaf litter phosphorus was not significant (p > 0.05).

Aims The non-rectangular hyperbolic model (termed as model I) is the main submodel of the FvCB biochemical model, which is used to estimate the maximum electron transport rate (J_max) of plant leaves. The submodel is widely applied to fit the light-response curves of electron transport rate (J-I curves), and obtain J_max. However, it has not been strictly verified whether J_max calculated by model I is consistent with the measured values.

Methods Light-response curves of electron transport rate and of photosynthesis rate of soybean (Glycine max) (under shading and full sunlight) were simultaneously measured by LI-6400-40, then these data were simulated by model I and the mechanistic model of light-response of electron transport rate (termed as model II).

Important findings The results showed that there was the significant differences between J_max estimated by model I and the observation data irrespective of shading and full sunny leaves of soybean. However, there was no significant difference between J_max calculated by model II and the measured value. Because J_max was overestimated by model I, it must lead to overestimate the amount of photosynthetic electron flow to allocate to photorespiration pathway, and magnify the photoprotection of photorespiration on plants. On the contrary, the J_max and saturation light intensity (PAR_sat) obtained by the model II were in very close agreement with the observations. It can be concluded that the model II was superior to the model I in estimates of J_max and PAR_sat. Therefore, we recommend model II to be used as an operational model for fitting J-I curves and accurately assess the role of photorespiration on plant photo-protection.

Aims Forest conversion is an important factor affecting the ecosystem organic matter cycle, and has an impact on the productivity of forest ecosystems, carbon sequestration and nutrient conservation. This study aims to provide more scientific evidence for better understanding the mechanism of different forest types regulating forest soil carbon and nitrogen cycling in the context of forest conversion.

Methods The study site is located in Sanming City, Fujian Province, in subtropical China. Soil samples in the A horizon from an artificial-assisted natural regeneration forest of Castanopsis carlesii (AR), a natural secondary forest of C. carlesii (SF) and a plantation of Pinus massoniana (PM) sites were collected in November, 2016. We investigated the contents of soil organic carbon, soil organic nitrogen, soil dissolved organic matter (DOM), NH₄ ⁺-N and NO₃ ^--N. The spectroscopic characteristics of soil DOM were also measured by means of ultraviolet absorbance and fluorescence emission spectroscopic techniques. The activity of five kinds of enzymes related to carbon and nitrogen cycle were determined to decipher their relationships with soil properties.

Important findings The results showed that, due to different tree species and man-made disturbance, the contents of dissolved organic carbon (DOC), DON, humification index of fluorescence emission spectrum were all in the order SF > AR > PM, whereas the aromatization index was in the order PM > AR > SF. NH₄ ⁺-N were significantly richer for SF and AR than for PM, while NO₃ ^--N content was low and similar across the three stands. The β-glucosidase activity of PM was significantly lower than that of SF and AR. The activities of cellulolytic enzyme were in sequence of AR > SF > PM. The activities of polyphenol oxidase enzyme in PM was significantly higher than in SF and AR. There was no significant difference in the type of forest peroxidase. The activity of β-N-acetylglucosaminidase of AR was significantly higher than those of the other two kinds of stands. The redundancy analysis indicates that total nitrogen (TN) and DON are the major environmental factors driving soil enzyme activity. Soil total nitrogen content and NAG activity were positively correlated, and DON may be an important component of the N cycle. Soil microorganisms prefer to use readily decomposable carbon; and there is a certain coupling relationship between carbon and nitrogen cycles. Higher soil N contents would increase the C-related hydrolytic enzyme activity, thereby promoting carbon turnover.