Divergent apparent temperature sensitivity of terrestrial ecosystem respiration

Aims Recent studies revealed convergent temperature sensitivity of ecosys-tem respiration(Re)within aquatic ecosystems and between terrestrial and aquatic ecosystems.We do not know yet whether various terres-trial ecosystems have consistent or divergent temperature sensitivity.Here,we synthesized 163 eddy covariance flux sites across the world and examined the global variation of the apparent activation energy(Ea),which characterizes the apparent temperature sensitivity of and its interannual variability(IAV)as well as their controlling factors.Methods We used carbon fluxes and meteorological data across FLUXNET sites to calculate mean annual temperature,tempera-ture range,precipitation,global radiation,potential radiation,gross primary productivity and Re by averaging the daily values over the years in each site.Furthermore,we analyzed the sites with>8 years data to examine the IAV of Ea and calculated the standard deviation of Ea across years at each site to character-ize IAV.Important Findings The results showed a widely global variation of Ea,with significantly lower values in the tropical and subtropical areas than in temperate and boreal areas,and significantly higher values in grasslands and wetlands than that in deciduous broadleaf forests and evergreen for-ests.Globally,spatial variations of Ea were explained by changes in temperature and an index of water availability with differing contribution of each explaining variable among climate zones and biomes.IAV and the corresponding coefficient of variation of Ea decreased with increasing latitude,but increased with radiation and corresponding mean annual temperature.The revealed patterns in the spatial and temporal variations of Ea and its controlling factors indicate divergent temperature sensitivity of Re,which could help to improve our predictive understanding of Re in response to climate change.

Temperature Responses to Infrared-Loading and Water Table Manipulations in Peatland Mesocosms

We initiated a multi-factor global change experiment to explore the effects of infrared heat loading （HT） and water table level （WL） treatment on soil temperature （T） in bog and fen peatland mesocosms. We found that the temperature varied highly by year, month, peatland type, soil depth, HT and WL manipulations. The highest effect of HT on the temperature at 25 cm depth was found in June for the bog mesocosms （3.34-4.27 ℃） but in May for the fen mesocosms （2.32-4.33 ℃） over the 2-year study period. The effects of WL in the bog mesocosms were only found between August and January, with the wet mesocosms warmer than the dry mesocosms by 0.48-2.03 ℃ over the 2-year study period. In contrast, wetter fen mesocosms were generally cooler by 0.16-3.87℃. Seasonal changes of temperatures elevated by the HT also varied by depth and ecosystem type, with temperature differences at 5 cm and 10 cm depth showing smaller seasonal fluctuations than those at 25 cm and 40 cm in the bog mesocosms. However, increased HT did not always lead to warmer soil, especially in the fen mesocosms. Both HT and WL manipulations have also changed the length of the non-frozen season.

Differential responses of carbon and water vapor fluxes to climate among evergreen needleleaf forests in the USA

Introduction:Understanding the differences in carbon and water vapor fluxes of spatially distributed evergreen needleleaf forests(ENFs)is crucial for accurately estimating regional or global carbon and water budgets and when predicting the responses of ENFs to current and future climate.Methods:We compared the fluxes of ten AmeriFlux ENF sites to investigate cross-site variability in net ecosystem exchange of carbon(NEE),gross primary production(GPP),and evapotranspiration(ET).We used wavelet cross-correlation analysis to examine responses of NEE and ET to common climatic drivers over multiple timescales and also determined optimum values of air temperature(T_(a))and vapor pressure deficit(VPD)for NEE and ET.Results:We found larger differences in the NEE spectra than in the ET spectra across sites,demonstrating that spatial(site-to-site)variability was larger for NEE than for ET.The NEE and ET were decoupled differently across ENF sites because the wavelet cospectra between ET and climate variables were similar at all sites,while the wavelet cospectra between NEE and climate variables were higher(i.e.,closer coupling between NEE and climatic drivers)in semi-arid and Mediterranean sites than in other sites.Ecosystem water use efficiency(EWUE)based on annual GPP/ET ranged from 1.3±0.18 to 4.08±0.62 g C mm^(−1) ET,while EWUE based on annual net ecosystem production(NEP)/ET ranged from 0.06±0.04 to 1.02±0.16 g C mm^(−1) ET)among ENFs.Responses of NEE and ET to T_(a) varied across climatic zones.In particular,for ENF sites in semi-arid and Mediterranean climates,the maximum NEE and ET occurred at lower ranges of T_(a) than in sites with warm and humid summers.The optimum T_(a) and VPD values were higher for ET than for NEE,and ET was less sensitive to high values of T_(a) and VPD.Conclusions:Large spatial variability in carbon and water vapor fluxes among ENFs and large variations in responses of NEE and ET to major climate variables among climatic zones necessitate sub-plant functional type parameterization based on climatic zones to better represent climate sensitivity of ENFs and to reduce uncertainty in model predictions.