Aims Our objective is to reveal changes of main phenophases of natural calendar and phenological seasons and the reasons for this change. Therefore, we recompiled the natural calendar for 1981-2010 at the Summer Palace in Beijing and compared it to the natural calendar for 1963-1982. Methods Based on the phenological data from Chinese Phenology Observation Network of Chinese Academy of Sciences and the meteorological data, we compiled a natural calendar (1981-2010) with phenophases of 19 plant speices, ice freeze-up and break-up dates for Kunming Lake. Correlation analysis and regression analysis were used to examine the relationships between change of phenological season and climatic drivers. Important findings In comparison with the original natural calendar, the first dates of phenological spring and summer were advanced by two days and five days, respectively, while the first dates of autumn and winter were delayed by one day and four days, respectively. The lengths of summer and autumn were prolonged by six days and three days, while the lengths of spring and winter were shortened by three days and six days, respectively. The order of spring, autumn, winter phenophases changed with different degrees. The average, earliest, latest date of phenology showed advances mainly in spring and summer and delay in autumn and winter. Temperature change before the first date of spring, summer, winter and the sunshine duration before the first date of autumn are probably the main reason for the changes of phenological season in Beijing Summer Palace. Different responses of different species and phenophase result in change of phenophase order in seasons.

Aims The temperature sensitivity and priming effect of soil respiration, or soil carbon (C) mineralization, have been important topics for global change ecology in recent decades. They can provide new evidence on the controlling mechanisms of soil respiration in future climate change. Our objective was to investigate the effect of grazing enclosure on the priming effect and temperature sensitivity of soil C mineralization in grasslands. Methods We selected three Leymus chinensis grasslands subjected to different grazing-exclusion durations in Inner Mongolia, i.e., grazing-free grassland (FG0), 11-year fenced grassland (FG11), and 31-year fenced grassland (FG31). We incubated the soils at 0, 5, 10, 15, 20, 25 °C, respectively. Important findings Enclosure duration, glucose addition, incubation temperatures and incubation time all had significant impact on soil C mineralization rate, and there were significant interaction effects among these factors (p < 0.000 1). The cumulative emission of soil C mineralization was significantly higher in FG0 than in FG11 and FG31, and showed the same trend after glucose addition. Long-term enclosure decreased the priming effect of soil C mineralization in L. chinensis grasslands. After glucose addition, the priming effect of soil C mineralization was about 2.28-9.01 times and increased with increasing temperature in 7-day incubation. In 56-day incubation, the priming effect was in the range of 2.21-5.10, with the highest value at 10 or 15 °C. The temperature sensitivity of soil C mineralization can be well depicted by classical exponential equations, and the temperature sensitivity index (Q₁₀) of soil C mineralization was higher in FG0 than in FG11 and FG31. Glucose addition significantly increased Q₁₀, which indicated that soil microbial respiration under glucose addition was more affected by temperature. In conclusion, long-term enclosure reduced soil C mineralization rates, temperature sensitivity and priming effect in Inner Mongolian grasslands, which indicated that these long-term fenced grasslands have the capacity to further sequester CO₂ from the atmosphere.

Aims Precipitation use efficiency (PUE) is a key to understanding the coupling between ecosystem carbon and water cycles. Our objective was to probe the spatial PUE pattern and its response to climate on the Qinghai- Xizang Plateau to better understand mechanisms of vegetation productivity and improve ecosystem process models. Methods GLOPEN-CEVSA model was applied to estimate net primary production (NPP) by using the Fraction of Photosynthetically Active Radiation Absorbed by Vegetation (MOD15A2), and spatially interpolated meteorological data in 2000-2008. The modeled NPP was significantly correlated with the observed above-ground net primary productivity (R ² = 0.49, p < 0.001, n = 97). The PUE was calculated as the ratio of NPP to the annual sum of precipitation. Important findings The spatial pattern of PUE showed large differences among vegetation types. Crops had the highest PUE, and alpine meadow had higher PUE than alpine steppe. These differences were related to the precipitation and temperature distribution on the plateau. The PUE was relatively stable and the lowest value of (0.026 ± 0.190) g C·m ^-2·mm ^-1 (mean ± standard deviation) with the highest coefficient of variance (CV) of 721% was where precipitation was < 90 mm. Where precipitation was 90-300 mm, PUE was relatively stable and also low ((0.029 ± 0.074) g C·m ^-2·mm ^-1) with relatively high CV (252%). Together precipitation and air temperature in this precipitation range explained 43.4% of the spatial variance of PUE, and the effect of precipitation was 1.7 times that of temperature (p < 0.001). The area with precipitation from 300-650 mm, mainly covered by alpine steppe (45%), had relatively high PUE ((0.123 ± 0.191) g C·m ^-2·mm ^-1) with a CV of 155%. The significant correlation of PUE with climate factors explained 97.8% spatial variance of PUE. Temperature had the dominant role, having 1.5 times the effect of precipitation. With increasing precipitation, PUE reached a peak of 0.26 g C·m ^-2·mm ^-1 at 650 mm of precipitation and then showed a decreasing trend. The precipitation of the mountainous Nyingchi region, Xizang, is >845 mm, and the region is mainly covered with evergreen needleleaf forest. It has relatively high PUE ((0.210 ± 0.246) g C·m ^-2·mm ^-1) with a minimum CV of 117%. Temperature and precipitation together explained 93.1% of the spatial variation of PUE for Nyingchi. Precipitation was negatively correlated with PUE and its effect was 3.5 times that of temperature.

Aims The relationship between species diversity and productivity has been a central issue in the face of increasing species extinctions; however, this issue has resulted in many disagreements. Our objectives were to determine (1) forms of the relationships between plant species diversity and productivity in natural communities in subalpine meadow and (2) effect of the sample area on the relationship between plant diversity and productivity. Methods We evaluated the relationships between species diversity and productivity by sampling three different study sites in subalpine meadow in the Hezuo region of Gansu Province, China in 2010. These three sites have different land use histories and altered vegetation traits. Sampling areas in each site were 0.01, 0.04, 0.16 and 0.64 m ². The number of samples was 30 for each site. Sampling at each site was conducted randomly. In August 2010, we determined the aboveground, oven-dried biomass and the number of plant species in each sample quadrat. The relationship between species diversity and productivity was constructed by linearly and quadratically regressing both number of species and aboveground biomass. Important findings We found no significant relationship between number of species and aboveground biomass in two of the three sites, even though sampling areas were varied, and the relationship changed with sampling area in another experimental site that had been overgrazed. Both sampling area and study site had significant effects on number of species, while number of species in each sample increased with sampling area and aboveground biomass per unit area was constant, i.e., sampling areas had no effect on productivity. Variation of experimental site had a significant effect on aboveground biomass, and productivity of each site was not closely dependent on number of species but the site. Results suggest that there may be no fixed relationship between species diversity and productivity in natural communities of subalpine meadow.

Aims We assess the relationship among the carbon isotopic signatures of earlywood (EW), transitional wood (TW) and latewood (LW) from tree rings. Our aims were to investigate variation in the intra-annual stable carbon isotope ratio (δ ¹³C) in Pinus sylvestris var. mongolica and determine the relationship between them and homologous ring width. Methods Based on two tree discs of Pinus sylvestris var. mongolica sampled from the northern part of Daxing’an Mountains in China, the EW, TW and LW were obtained with different stripping and pooling programs. After performing ring widths measurement and cross-dating, the periods analyzed were the maximum growth periods for one sample and different growth periods for the other. The holocellulose fractions were extracted and the intra-annual δ ¹³C of samples were measured. Important findings In general, the δ ¹³C values of TW are the highest, EW come second and LW are the lowest. The intra-annual trend of δ ¹³C is fluctuateing prominently from the juvenile period to the fast-growing period and is smoother from the maturation period to the senescence period. The variation amplitude of LW is almost greater than EW at the same period. The δ ¹³C of LW is always prominently higher than EW for the juvenile period. The difference between EW and LW is indistinctive for the maturation period and is negligible for the senescence period. The intra-annual variability of δ ¹³C concentrates on the middle and later phase of the growing season. The correlation relationship between the intra-annual δ ¹³C sequences and homologous detrended ring width sequences (dRWS) decreases with the seasons, which implies that environmental factors play a dominant role in cell formation and carbon fractionation during the middle and later phase of the growing season in each year. The ring width of EW of the current year is positively correlated with LW of the previous year (pLW). Also the δ ¹³C of EW is negative correlated with the incorporative dRWS of EW + pLW. But the correlation between δ ¹³C of EW and δ ¹³C or dRWS of pLW is statistically insignificant. The growing season could be divided as: EW (from late April to middle June, with greater soil moisture and rapidly increasing temperature), TW (from late June to middle July, with lower soil moisture and maximum temperature) and LW (from late July to middle September, with greater soil moisture and decreased temperature).

Aims The branch size-number trade-off among woody plants has implications for both the formation of plant architecture and biomass allocation in response to environment stresses. Our objective was to examine how the twig size-number relationship varies among woody plants in subtropical broad-leaved evergreen forests in southeastern China.Methods The study site is located in Tiantong National Forest Park (29°52′ N, 121°39′ E), Zhejiang Province, in Eastern China. We measured twig length, twig diameter, number of twigs and length and diameter of branches from which the twigs were sampled for 76 woody species in a 1-hm ^-2 plot. Standardized major axis (SMA) analysis was conducted to examine the quantitative relationship between twig size (cross-sectional area) and the number of twigs at a given twig size (twig intensity). Important findings A significantly negative allometric scaling relationship was found between twig intensity and its cross-sectional area. Under level-II light exposure (LE-II, 40%-80% of the plant under direct light), evergreen species had a much higher twig intensity than deciduous species. However, there was no difference in twig intensity between these two life forms under LE-I (<40% exposure) or LE-III (>80% exposure). Higher twig intensity was found in evergreen species under LI-III than under both LE-I and LE-II. In contrast, twig intensity of deciduous species was not different among any of the light exposure levels. Shrubs <4 m height had higher twig intensity than sub-trees and trees >4 m at a given twig size. We conclude that the twig size-number trade-off across woody plants in Tiantong was consistent with the Corner’s rule (that describes the relationship between twig size and the number of twigs), but might differ among different life forms. Both evergreen species and shrubs are inclined to develop higher twig intensity, which indicates their adaptation to environment stress caused by limitations in light availability.

Aims Our primary aim was to compare the differences and accuracies of light-response curves between C₃ and C₄ plants as described by five light-response curve models: modified exponential (a new model), rectangular hyperbola, modified rectangular hyperbola , non-rectangular hyperbola and exponential. Methods Light-response curves of Xizang hulless barley (Hordeum vulgare), grain sorghum (Sorghum bicolor), amaranth (Amarantus mangestbus) and Pinellia ternate were measured by a portable photosynthetic gas analysis system with a LED radiation source (LI-6400). The collected data were used to fit and test the five light-response curve models, and photosynthetic parameters, such as light saturation point and maximum net photosynthetic rate, were compared with the measured values. Important findings The results show that the light-response curves of the four plants described by the modified exponential and modified rectangular hyperbola models are better than the rectangular hyperbola, non-rectangular hyperbola and exponential models, especially when light intensity is beyond the light saturation point. The results also indicate that the modified exponential and modified rectangular hyperbola models also describe the light-response curves characteristics of C₄ plants, although the two models were established based on C₃ plants. Therefore, these two models provide a reference for the application of light-response curve models in plants with different photosynthetic pathways including C₃ and C₄.

Aims The effects of fractionated nitrogen (N) fertilization (once, twice and thrice) on leaf senescence and nitrogen uptake and utilization of potted Malus hupenhensis were explored using the ¹⁵N labeling technique. Methods We measured growth parameters such as plant height, stem diameter, leaf area and chlorophyll content (SPAD), enzyme parameters such as superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in leaves and ¹⁵N parameters such as uptake and utilization for potted M. hupehensis under different fractionated N applications. Important findings Plant height, stem diameter, leaf area and SPAD were highest at early growth stage for once fractionated application and lower for twice fractionated application, and lowest for thrice fractionated application. At middle growth stage, twice fractionated application showed the highest values, while once fractionated application exhibited the lowest values. At the late growth stage, thrice fractionated application was the highest, while once fractionated application was the lowest. The leaf SOD, POD and CAT activities were highest for once fractionated application at early growth stage and lowest for thrice fractionated application. At the middle growth stage, twice fractionated application was the highest, whereas once fractionated application was lowest. At the late growth stage, thrice fractionated application showed the highest values, while once fractionated application had the lowest values. The ¹⁵N derived from fertilizer (N_dff) value in different organs was significantly different. Thrice fractionated application demonstrated higher values than either of once and twice fractionated application at the late growth stage. Plant total N, ¹⁵N uptake and N-usage efficiency were highest for thrice fractionated application at late growth stage. This indicates that thrice fractionated application not only can enhance the total N content of leaves and delay leaf senescence, but also can increase the N-usage efficiency.