Effects of Warming on CO2 Fluxes in an Alpine Meadow Ecosystem on the Central Qinghai–Tibetan Plateau

Abstract

To analyze CO2 fluxes under conditions of climate change in an alpine meadow on the central Qinghai–Tibetan Plateau, we simulated the effect of warming using open top chambers (OTCs) from 2012 to 2014. The OTCs increased soil temperature by 1.62°C (P < 0.05), but decreased soil moisture (1.38%, P < 0.05) during the experiments. The response of ecosystem CO2 fluxes to warming was variable, and dependent on the year. Under conditions of warming, mean gross ecosystem productivity (GEP) during the growing season increased significantly in 2012 and 2014 (P < 0.05); however, ecosystem respiration (ER) increased substantially only in 2012 (P < 0.05). The net ecosystem CO2 exchange (NEE) increased marginally in 2012 (P = 0.056), did not change in 2013(P > 0.05), and increased significantly in 2014 (P = 0.034) under conditions of warming. The GEP was more sensitive to climate variations than was the ER, resulting in a large increase in net carbon uptake under warming in the alpine meadow. Under warming, the 3-year averages of GEP, ER, and NEE increased by 19.6%, 15.1%, and 21.1%, respectively. The seasonal dynamic patterns of GEP and NEE, but not ER, were significantly impacted by warming. Aboveground biomass, particularly the graminoid biomass increased significantly under conditions of warming. Soil moisture, soil temperature, and aboveground biomass were the main factors that affected the variation of the ecosystem CO2 fluxes. The effect of warming on inter- and intra-annual patterns of ecosystem CO2 fluxes and the mechanism of different sensitivities in GEP and ER to warming, require further researched.

Environmental Humidity Regulates Effects of Experimental Warming on Vegetation Index and Biomass Production in an Alpine Meadow of the Northern Tibet

“…Experimental warming also reduced average GPP and AGB in 2012 in the alpine meadow at the low elevation [52] and it was also a drier year of 2012 (312.8 mm) compared to the mean precipitation of 1963–2015 (Fig 1). These implied that the response directions of plant growth to experimental warming varied with years and experimental warming did not have a continuous negative effect on biomass production at the low elevation, which was in line with previous studies [27–29,53,54]. The variable response directions may be due to the variable environmental temperature and humidity among years [27,29] and plant growth was more sensitive to moisture than temperature change when water was lower than a certain threshold value [53,55].…”

“…These implied that the response directions of plant growth to experimental warming varied with years and experimental warming did not have a continuous negative effect on biomass production at the low elevation, which was in line with previous studies [27–29,53,54]. The variable response directions may be due to the variable environmental temperature and humidity among years [27,29] and plant growth was more sensitive to moisture than temperature change when water was lower than a certain threshold value [53,55]. The precipitation in 2014 was 38.6 mm higher compared to the mean precipitation (398.7 mm), whereas the precipitation in 2012 and 2015 was 85.88 mm and 98.48 mm lower compared to the mean precipitation, respectively.…”

“…Likewise, annual gross primary production was twice in a wet year than in a dry year in a semi-arid grassland in Hungary [48]. The significant inter-annual variations of vegetation index, AGB and GPP were also in line with previous studies conducted on the Tibetan Plateau [26,27]. …”

“…Precipitation dominated the variations of aboveground and blowground plant biomass along an elevation gradient from 4400 m to 4800 m in an alpine meadow in the Northern Tibet [42]. Soil moisture explained more variation of GPP than soil temperature under experimental warming condition in an alpine meadow in the Naqu county, Tibet [27]. …”

“…These findings imply that it is necessary to examine the response of alpine plant production to long-term (> 5 years) experimental warming on the Tibetan Plateau. Some studies indicated that the response directions of plant biomass and primary production to experimental warming varied with years [27–29], while other studies demonstrated that their response directions did not change with years [6,30] on the Tibetan Plateau. Therefore, it is unclear whether the responses of biomass production to climatic warming vary with years or whether the responses of biomass production to short-term (<5 years) experimental warming is different from those to long-term (> 5 years) experimental warming on the Tibetan Plateau.…”