Aims Light-use efficiency (LUE) is an important tool for scaling up local CO₂ flux (F CO2) tower observations to regional and global carbon dynamics. Using a data set including F CO2 and environmental factors obtained from an alpine meadow on the Tibetan Plateau, we examined both diurnal and seasonal changes in LUE and the environmental factors controlling these changes. Our objectives were to (i) characterize the diurnal and daily variability of LUE in an alpine meadow, (ii) clarify the causes of this variability, and (iii) explore the possibility of applying the LUE approach to this alpine meadow by examining the relationship between daily LUE and hourly LUE at satellite visiting times.
Methods First, we obtained the LUE—the ratio of the gross primary production (GPP) to the absorbed photosynthetically active radiation (APAR)—from the flux tower and meteorological observations. We then characterized the patterns of diurnal and seasonal changes in LUE, explored the environmental controls on LUE using univariate regression analyses and evaluated the effects of diffuse radiation on LUE by assigning weights through a linear programming method to beam photosynthetically active radiation (PAR) and diffuse PAR, which were separated from meteorological observations using an existing method. Finally, we examined the relationships between noontime hourly LUE and daily LUE and those between adjusted noontime hourly and daily LUE because satellites visit the site only once or twice a day, near noon.
Important findings The results showed that (i) the LUE of the alpine meadow generally followed the diurnal and seasonal patterns of solar radiation but fluctuated with changes in cloud cover. (ii) The fraction of diffuse light played a dominant role in LUE variation. Daily minimum temperature and vapor pressure deficit also affected LUE variation. (iii) The adjusted APAR, defined as the weighted linear sum of diffuse APAR and beam APAR, was linearly correlated with GPP on different temporal scales. (iv) Midday adjusted LUE was closely related to daily adjusted LUE, regardless of the cloud cover. The results indicated the importance of considering radiation direction when developing LUE-based GPP-estimating models.

Aims Kobresia meadows, the dominant species of which differ in different habitats, cover a large area of alpine grassland on the Qinghai-Tibetan Plateau and act as potential CO₂ sinks. Kobresia meadows with different dominant species may differ in carbon sink strength. We aimed to test the hypothesis and to clarify the differences in CO₂ sink strength among three major Kobresia meadows on the plateau and the mechanisms underlying these differences.
Methods We measured the net ecosystem exchange flux (NEE), ecosystem respiration flux (ER), aboveground biomass (AGB) and environmental variables in three Kobresia meadows, dominated by K. pygmaea, K. humilis, or K. tibetica, respectively, in Haibei, Qinghai. NEE and ER were measured by a closed-chamber method. Environmental variables, including photosynthetic photon flux density (PPFD), air and soil temperature and air and soil moisture, were monitored during the above flux measurements.
Important findings The measured peak AGB increased with soil water content and was 365, 402 and 434 g dry weight m^{-2<-sup> for K. pygmaea, K. humilis and K. tibetica meadow, respectively. From the maximum ecosystem photosynthetic rate in relation to PPFD measured during the growing season, we estimated gross ecosystem photosynthetic potential (GEP max) as 22.2, 29.9 and 37.8 μmol CO₂ m-2<-sup> s-1 for K. pygmaea, K. humilis and K. tibetica meadow, respectively. We estimated the respective gross primary production (GPP) values as 799, 1-063 and 1?158 g C m-2<-sup> year-1 and ER as 722, 914 and 1-011 g C m-2<-sup> year-1. Average net ecosystem production (NEP) was estimated to be 76.9, 149.4 and 147.6 g C m-2<-sup> year-1 in K. pygmaea, K. humilis and K. tibetica meadows, respectively. The results indicate that (i) the three meadows were CO₂ sinks during the study period and (ii) Kobresia meadows dominated by different species can differ considerably in carbon sink strength even under the same climatic conditions, which suggests the importance of characterizing spatial heterogeneity of carbon dynamics in the future.}

Aims

Recent studies have recognized the alpine grasslands on the Qinghai–Tibetan plateau as a significant sink for atmospheric CO₂. The carbon-sink strength may differ among grassland ecosystems at various altitudes because of contrasting biotic and physical environments. This study aims (i) to clarify the altitudinal pattern of ecosystem CO₂ fluxes, including gross primary production (GPP), daytime ecosystem respiration (Re_daytime) and net ecosystem production (NEP), during the period with peak above-ground biomass; and (ii) to elucidate the effects of biotic and abiotic factors on the altitudinal variation of ecosystem CO₂ fluxes.

Methods

Ecosystem CO₂ fluxes and abiotic and biotic environmental factors were measured in an alpine grassland at four altitudes from 3600 to 4200 m along a slope of the Qilian Mountains on the northwestern Qinghai–Tibetan Plateau during the growing season of 2007. We used a closed-chamber method combined with shade screens and an opaque cloth to measure several carbon fluxes, GPP, Re_daytime and NEP, and factors, light-response curve for GPP and temperature sensitivity of Re_daytime. Above- and below-ground biomasses and soil C and N contents at each measurement point were also measured.

Important Findings

• (i)Altitudinal pattern of ecosystem CO₂ fluxes: The maximum net ecosystem CO₂ flux (NEP_max), i.e. the potential ecosystem CO₂ sink strength, was markedly different among the four altitudes. NEP_max was higher at the highest and lowest sites, approximately -7.4 ± 0.9 and -6.7 ± 0.6 μmol CO₂ m^-2 s^-1 (mean ± standard error), respectively, but smaller at the intermediate altitude sites (3800 and 4000 m). The altitudinal pattern of maximum gross primary production was similar to that of NEP_max. The Re_daytime, however, was significantly higher at the lowest altitude (3.4 ± 0.3 μmol CO₂ m^-2 s^-1) than at the other three altitudes.

• (ii)Altitudinal variation of vegetation biomass: The above-ground biomass was higher at the highest altitude (154 ± 27 g DW m^-2) than at the other altitudes, which we attribute mainly to the large biomass in cushion plants at the highest altitude. The small above-ground biomass at the lower altitudes was probably due to heavy grazing during the growing season.

• (iii)Features of ecosystem CO₂ fluxes: Re_daytime and GPP were positively correlated with above-ground biomass. The low ratio of Re_daytime to GPP at either the measurement point or the site level suggests that CO₂ uptake efficiency tends to be higher at higher altitudes, which indicates a high potential sink strength for atmospheric CO₂ despite the low temperature at high altitudes. The results suggest that the effect of grazing intensity on ecosystem carbon dynamics, partly by decreasing vegetation biomass, should be clarified further.

Aims Alpine ecosystems may experience larger temperature increases due to global warming as compared with lowland ecosystems. Information on physiological adjustment of alpine plants to temperature changes can provide insights into our understanding how these plants are responding to current and future warming. We tested the hypothesis that alpine plants would exhibit acclimation in photosynthesis and respiration under long-term elevated temperature, and the acclimation may relatively increase leaf carbon gain under warming conditions.
Methods Open-top chambers (OTCs) were set up for a period of 11 years to artificially increase the temperature in an alpine meadow ecosystem. We measured leaf photosynthesis and dark respiration under different light, temperature and ambient CO₂ concentrations for Gentiana straminea, a species widely distributed on the Tibetan Plateau. Maximum rates of the photosynthetic electron transport (J max), RuBP carboxylation (V c max) and temperature sensitivity of respiration Q 10 were obtained from the measurements. We further estimated the leaf carbon budget of G. straminea using the physiological parameters and environmental variables obtained in the study.
Important findings1)?The OTCs consistently elevated the daily mean air temperature by ~1.6°C and soil temperature by ~0.5°C during the growing season. 2)?Despite the small difference in the temperature environment, there was strong tendency in the temperature acclimation of photosynthesis. The estimated temperature optimum of light-saturated photosynthetic CO₂ uptake (A max) shifted ~1°C higher from the plants under the ambient regime to those under the OTCs warming regime, and the A max was significantly lower in the warming-acclimated leaves than the leaves outside the OTCs. 3)?Temperature acclimation of respiration was large and significant: the dark respiration rates of leaves developed in the warming regime were significantly lower than leaves from the ambient environments. 4)?The simulated net leaf carbon gain was significantly lower in the in situ leaves under the OTCs warming regime than under the ambient open regime. However, in comparison with the assumed non-acclimation leaves, the in situ warming-acclimated leaves exhibited significantly higher net leaf carbon gain. 5)?The results suggest that there was a strong and significant temperature acclimation in physiology of G. straminea in response to long-term warming, and the physiological acclimation can reduce the decrease of leaf carbon gain, i.e. increase relatively leaf carbon gain under the warming condition in the alpine species.

Aims The aims of this study were to compare the fungal communities developing on cotton strips at three different altitudes on the Tibetan Plateau and to assess the environmental variables influencing them.
Methods Cotton strips that had been buried in soil for a year were sampled at three sites at different altitudes (4500, 4950 and 5200 m) located on a southeast-facing slope on the Nyainqentanglha Mountains near Damxung. The fungi on the cotton strips were isolated using a modified washing method. The decomposition abilities and colony growth properties of the major species cultured in pure-culture conditions were investigated and compared. Canonical correspondence analysis (CCA) was used to evaluate the relationships between fungal community composition and environmental variables (altitude, soil depth, soil water content [SWC], plant root mass and gravel content).
Important findings A total of 24 species were isolated from the cotton strips, and 12 species occurred frequently and were regarded as major species. The number of fungal species was lower at the 4950-m altitude site than at the other two sites, indicating that not only altitude but also other factors affected the number of species present. All of the major species were able to decompose the cotton strips. In the CCA ordination, automatic forward selection revealed that altitude, SWC and plant root mass significantly affected fungal species composition. Our results suggest that species number and the composition of cellulolytic fungal communities are highly correlated with environmental variables as well as altitude in the alpine meadow on the Tibetan Plateau.

Aims This study was conducted to (i) determine if soil CO₂ efflux is more sensitive to temperature changes in alpine areas than in lowland grasslands, (ii) examine the effects of temperature and moisture on soil respiration, and (iii) evaluate the potential for change in soil carbon storage in response to global warming in different grasslands in East Asia.
Methods We collected soil samples from two different temperate grasslands, an alpine meadow on the Qinghai-Tibetan plateau, China, and a lowland grassland in Tsukuba, Japan. The CO₂ emission rate was then measured for these soil samples after they were incubated at 25°C and 60% of the water holding capacity for 7 days.
Important findings (i)?The soil respiration rate was more sensitive to temperature change in the alpine soil than in the lowland soil. The average Q 10 was 7.6 for the alpine meadow soil but only 5.9 for the lowland soil. The increased sensitivity appears to be due, at least in part, to the soil organic carbon content and/or soil carbon to nitrogen ratio, especially in the surface layer. (ii) The relationship between the CO₂ emission rate and the soil moisture content revealed that the alpine meadow had a more clear response than the lowland soil. (iii) This study suggests that changes in soil moisture and soil temperature may have larger impacts on soil CO₂ efflux in the alpine meadow than in the lowland grassland evaluated here.