The relative controls of temperature and soil moisture on the start of carbon flux phenology and net ecosystem production in two alpine meadows on the Qinghai-Tibetan Plateau

Aims Variations in vegetation spring phenology are widely attributed to temperature in temperate and cold regions.However,temperature effect on phenology remains elusive in cold and arid/semiarid ecosystems because soil water condition also plays an important role in mediating phenology.Methods We used growing degree day(GDD)model and growing season index(GSI)model,coupling minimum temperature(T_(min))with soil moisture(SM)to explore the influence of heat requirement and hydroclimatic interaction on the start of carbon uptake period(SCUP)and net ecosystem productivity(NEP)in two alpine meadows with different precipitation regimes on the Qinghai-Tibet Plateau(QTP).One is the water-limited alpine steppe-meadow,and the other is the temperature-limited alpine shrub-meadow.Important Findings We observed two clear patterns linking GDD and GSI to SCUP:SCUP was similarly sensitive to variations in preseason GDD and GSI in the humid alpine shrub-meadow,while SCUP was more sensitive to the variability in preseason GSI than GDD in the semiarid alpine steppe-meadow.The divergent patterns indicated a balance of the limiting climatic factors between temperature and water availability.In the humid meadow,higher temperature sensitivity of SCUP could maximize thermal benefit without drought stress,as evidenced by the stronger linear correlation coefficient(R2)and Akaike’s information criterion(AIC)between observed SCUPs and those of simulated by GDD model.However,greater water sensitivity of SCUP could maximize the benefit of water in semiarid steppe-meadow,which is indicated by the stronger R2 and AIC between observed SCUPs and those of simulated by GSI model.Additionally,although SCUPs were determined by GDD in the alpine shrub-meadow ecosystem,NEP was both controlled by accumulative GSI in two alpine meadows.Our study highlights the impacts of hydroclimatic interaction on spring carbon flux phenology and vegetation productivity in the humid and semiarid alpine ecosystems.The results also suggest that water,together with temperature should be included in the models of phenology and carbon budget for alpine ecosystems in semiarid regions.These fi ndings have important implications for improving vegetation phenology models,thus advancing our understanding of the interplay between vegetation phenology,productivity and climate change in future.

Interannual Variation in Terrestrial Ecosystem Carbon Fluxes in China from 1981 to 1998

A dynamic biogeochemical model was used to estimate the responses of China's terrestrial net primary productivity (NPP), soil heterotrophic respiration (HR) and net ecosystem productivity (NEP) to changes in climate and atmospheric CO2 from 1981 to 1998. Results show that China's total NPP varied between 2.89 and 3.37 Gt C/a and had an increasing trend by 0.32% per year, HR varied between 2.89 and 3.21 Gt C/a and grew by 0.40% per year, Annual NEP varied between -0.32 and 0.25 Gt C but had no statistically significant interannual trend. The positive mean NEP indicates that China's terrestrial ecosystems were taking up carbon with a total carbon sequestration of 1.22 Gt C during the analysis period. The terrestrial NEP in China related to climate and atmospheric CO2 increases accounted for about 10% of the world's total and was similar to the level of the United States in the same period. The mean annual NEP for the analysis period was near to zero for most of the regions in China, but significantly positive NEP occurred in Northeast China Plain, the southeastern Xizang (Tibet) and Huang-Huai-Hai Plain, and negative NEP occurred in the Da Hinggan Mountains, Xiao Hinggan Mountains, Loess Plateau and Yunnan-Guizhou Plateau. China's climate at the time was warm and dry relative to other periods, so the estimated NEP is probably lower than the average level. China's terrestrial NEP may increase if climate becomes wetter but is likely to continue to decrease if the present warming and drying trend sustains.

Findings through the AsiaFlux network and a view toward the future

The preliminary results of long-term CO_(2)flux measurements at forest sites in East Asia are explained and compared with each other.The features of seasonal variation of CO_(2)fluxes are different among deciduous-broadleaf,evergreen-coniferous,deciduous-coniferous and tropical forests in East Asia,and the causes of difference are discussed.The integrated yearly NEP(net ecosystem production)estimated from the CO_(2)flux by eddy covariance method in various forests of East Asia has a notable difference in the range of 2 to 8 tC ha^(-1)yr^(-1).The main factors of this difference are the annual mean temperature and tree species.Furthermore,the remaining issues are discussed,such as the quantitative estimation of the CO_(2)flux by the eddy covariance method and the synthetic analysis of the carbon budget under collaborations with biological survey.

京津风沙源治理区植被固碳能力估算及归因分析

植被固碳能力是陆地生态系统碳汇的基础,可通过净生态系统生产力(NEP)这一重要指标来反映。估算植被固碳能力是生态学和地表生态系统研究领域的共同科学问题,揭示其变化特征及其因素影响对于区域生态系统具有重要意义。以京津风沙源治理区为研究区,估算了2000—2020年京津风沙源治理区NEP,并利用趋势分析法和稳定性分析法等方法分析其时空变化特征,并利用地理探测器探究了影响NEP的自然因素与人类活动因素。结果显示:(1)2020年,京津风沙源治理区的NEP均值为110.09 gC m^(-2)a^(-1),碳汇区面积约为3.591×10~5 km^(2),占总面积的78.80%;(2)2000—2020年,京津风沙源治理区多年平均NEP为77.54 gC m^(-2)a^(-1),年际变化率为4.118 gC m^(-2)a^(-1),总体上呈上升趋势,研究区以NEP明显增加区为主,占比55.16%,主要分布在研究区南部,NEP呈下降趋势地区仅占比0.15%,主要分布在北部干旱草原沙地治理区,呈斑块状分布;(3)不同生态系统NEP从高到低依次为林地、农田和草地,林地固碳作用较强,未来林地固碳潜力较大,草地面积占研究区的70%,其总固碳量远超其他类型且呈增加趋势;(4)影响京津风沙源治理区NEP的主导因素为年降水量,其次,生态工程的实施也是NEP变化的重要影响因素,不同因素对京津风沙源治理区NEP的影响表现为双因子增强或非线性增强。围绕京津风沙源植被固碳能力估算以及影响因素的空间分异性,研究也在一定程度上提出了空间恢复的建议,以期为京津风沙源加强植被固碳能力以及生态修复等决策提供科学依据。

Effects of Environmental Conditions and Aboveground Biomass on CO2 Budget in Phragmites australis Wetland of Jiaozhou Bay,China

Estuarial saline wetlands have been recognized as a vital role in CO_2 cycling.However,insufficient attention has been paid to estimating CO_2 fluxes from estuarial saline wetlands.In this study,the static chamber-gas chromatography(GC) method was used to quantify CO_2 budget of an estuarial saline reed(Phragmites australis) wetland in Jiaozhou Bay in Qingdao City of Shandong Province,China during the reed growing season(May to October) in 2014.The CO_2 budget study involved net ecosystem CO_2 exchange(NEE),ecosystem respiration(Reco) and gross primary production(GPP).Temporal variation in CO_2 budget and the impact of air/soil temperature,illumination intensity and aboveground biomass exerted on CO_2 budget were analyzed.Results indicated that the wetland was acting as a net sink of 1129.16 g/m^2 during the entire growing season.Moreover,the values of Reco and GPP were 1744.89 g/m^2 and 2874.05 g/m^2,respectively;the ratio of Reco and GPP was 0.61.Diurnal and monthly patterns of CO_2 budget varied significantly during the study period.Reco showed exponential relationships with air temperature and soil temperature at 5 cm,10 cm,20 cm depths,and soil temperature at 5 cm depth was the most crucial influence factor among them.Meanwhile,temperature sensitivity(Q10) of Reco was negatively correlated with soil temperature.Light and temperature exerted strong controls over NEE and GPP.Aboveground biomass over the whole growing season showed non-linear relationships with CO_2 budget,while those during the early and peak growing season showed significant linear relationships with CO_2 budget.This research provides valuable reference for CO_2 exchange in estuarial saline wetland ecosystem.

Evaluating and quantifying the effect of various spruce budworm intervention strategies on forest carbon dynamics in Atlantic Canada

Spruce budworm (SBW) outbreaks are one of the most devastating natural disturbances in spruce-balsam fir forests of eastern North America. Both early intervention strategy (EIS) and foliage protection strategy (FP) are being tested to limit forest losses, but the quantitative impact on forest carbon (C) dynamics is still unclear. In this study, we designed 19 separate scenarios of no intervention or varying success of EIS, FP, and their combination on SBW caused defoliation and mortality. We then used the TRIPLEX-Insect model to quantify their effects on forest C dynamics in the forests of the four provinces of Atlantic Canada. A scenario applying FP to 10%of the area with the greatest potential C losses of living biomass, protecting foliage in 10%of the forests is more realistic than higher proportion of FP given the high cost and large areas involved, resulted in reducing average cumulative net ecosystem productivity (NEP) from 2020 to 2039 by 56%–127%compared to a no outbreak scenario.Our results showed that FP would have to be applied everywhere to reduce tree mortality and increase NEP more than 8 years of successful EIS applied. However, if EIS can be successfully implemented for 12 years, it will maintain more forest C than FP applied everywhere during a moderate outbreak. We also found that the combination of EIS followed by FP in 10%of the areas disturbed by the SBW could maintain average cumulative NEP at similar levels to no defoliation in every province of Atlantic Canada. Black/red spruce forests younger than 60years old underwent the smallest changes in C dynamics whether using EIS, FP, or both. This highlights the importance of forest species, forest age, and their interactions on the effectiveness of a treatment during SBW outbreak. Overall, 31%–76%of the study area in Atlantic Canada could convert from a C sink to a source by 2039,if no protective measures are used under the worst-case scenarios, thus contributing to future climate warming.

Spatial and Temporal Analysis of Carbon Sequestrations in the Conterminous United States

This study develops geospatial analysis of terrestrial carbon exchange for the conterminous United State and estimates large-scale NEP （net ecosystem production） dynamic from 2008 to 2013. We apply land-use and land-cover data in order to coherently include cropland, forest, wetland and other ecologically active landscapes in the mapping. Our results show a distribution of high harvest carbon release in the Corn Belt states, in addition to hot spots around the US in areas like Southern California and Arizona. Harvest carbon is low in areas in the southern United States, and central/southern Appalachian Mountains. We identify NEP changes for coupled agricultural, forest and other high-carbon-uptake ecosystems systems, conversions to and from crop, and land in frequent conversion among forest, wetland, pasture and rangeland. Findings from this study will provide important information to support and promote the co-production of science and decision-making.

Altitudinal variation of ecosystem CO_(2) fluxes in an alpine grassland from 3600 to 4200 m

Aims Recent studies have recognized the alpine grasslands on the Qinghai-Tibetan plateau as a significant sink for atmospheric CO_(2).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 CO_(2) fluxes,including gross primary production(GPP),daytime ecosystem respiration(Redaytime)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 CO_(2) fluxes.Methods Ecosystem CO_(2) 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,Redaytime and NEP,and factors,light-response curve for GPP and temperature sensitivity of Redaytime.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 CO_(2) fluxes:The maximum net ecosystem CO_(2) flux(NEPmax),i.e.the potential ecosystem CO_(2) sink strength,was markedly different among the four altitudes.NEPmax was higher at the highest and lowest sites,ap proximately
7.460.9 and
6.760.6 lmol CO_(2) m^(-2)s^(-1)(mean 6 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 NEPmax.The Redaytime,however,was significantly higher at the lowest altitude(3.460.3 lmol CO_(2) m^(-2)s^(-1))than at the other three altitudes.(ii)Altitudinal variation of vegetation biomass:The aboveground biomass was higher at the highest altitude(154627 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 CO_(2) fluxes:Redaytime and GPP were positively correlated with above-ground biomass.The low ratio of Redaytime to GPP at either the measurement point or the site level suggests that CO_(2) uptake efficiency tends to be higher at higher altitudes,which indicates a high potential sink strength for atmospheric CO_(2) 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.

The role of shrub(Potentilla fruticosa)on ecosystem CO_(2) fluxes in an alpine shrub meadow

Aims Recent studies have shown that alpine meadows on the Qinghai-Tibetan plateau act as significant CO_(2)sinks.On the plateau,alpine shrub meadow is one of typical grassland ecosystems.The major alpine shrub on the plateau is Potentilla fruticosa L.(Rosaceae),which is distributed widely from 3200 to 4000 m.Shrub species play an important role on carbon sequestration in grassland ecosystems.In addition,alpine shrubs are sensitive to climate change such as global warming.Considering global warming,the biomass and productivity of P.fruticosa will increase on Qinghai-Tibetan Plateau.Thus,understanding the carbon dynamics in alpine shrub meadow and the role of shrubs around the upper distribution limit at present is essential to predict the change in carbon sequestration on the plateau.However,the role of shrubs on the carbon dynamics in alpine shrub meadow remains unclear.The objectives of the present study were to evaluate the magnitude of CO_(2)exchange of P.fruticosa shrub patches around the upper distribution limit and to elucidate the role of P.fruticosa on ecosystem CO_(2)fluxes in an alpine meadow.Methods We used the static acrylic chamber technique to measure and estimate the net ecosystem productivity(NEP),ecosystem respiration(Re),and gross primary productivity(GPP)of P.fruticosa shrub patches at three elevations around the species’upper distribution limit.Ecosystem CO_(2)fluxes and environmental factors were measured from 17 to 20 July 2008 at 3400,3600,and 3800 m a.s.l.We examined the maximum GPP at infinite light(GPPmax)and maximum Re(Remax)during the experimental time at each elevation in relation to aboveground biomass and environmental factors,including air and soil temperature,and soil water content.Important Findings Patches of P.fruticosa around the species’upper distribution limit absorbed CO_(2),at least during the daytime.Maximum NEP at infinite light(NEPmax)and GPPmax of shrub patches in the alpine meadow varied among the three elevations,with the highest values at 3400 m and the lowest at 3800 m.GPPmax was positively correlated with the green biomass of P.fruticosa more strongly than with total green biomass,suggesting that P.fruticosa is the major contributor to CO_(2)uptake in the alpine shrub meadow.Air temperature influenced the potential GPPat the shrub-patch scale.Remax was correlated with aboveground biomass and Remax normalized by aboveground biomass was influenced by soil water content.Potentilla fruticosa height(biomass)and frequency increased clearly as elevation decreased,which promotes the large-scale spatial variation of carbon uptake and the strength of the carbon sink at lower elevations.

Carbon and nitrogen dynamics in a Pinus densiflora forest with low and high stand densities

Aims Understanding carbon(C)and nitrogen(N)dynamics and their dependence on the stand density of an even-aged,mature forest provides knowledge that is important for forest management.This study investigated the differences in ecosystem total C and N storage and flux between a low-density stand(LD)and a high-density stand(HD)and examined the effects of stand density on aboveground net primary productivity(ANPP),total belowground C allocation(TBCA)and net ecosystem production(NEP)in a naturally regenerated,65-to 75-year-old Pinus densiflora S.et Z.forest.Methods LD(450 trees ha^(−1))and HD(842 trees ha^(−1))were established in an even-aged,mature P.densiflora forest in September 2006.The forest had been naturally regenerated following harvesting,and the stand density was naturally maintained without any artificial management such as thinning.The diameter at breast height(DBH≥5.0cm)of all live stems within the stands was measured yearly from 2007 to 2011.To compare C and N storage and fluxes in LD and HD,C and N pools in aboveground and belowground biomass,the forest floor,coarse woody debris(CWD)and soil;soil CO_(2) efflux(R_(S));autotrophic respiration(R_(A));litter production;and soil N availability were measured.Further,ANPP,TBCA and NEP were estimated from plot-based measurement data.Important Findings Ecosystem C(Mg C ha^(−1))and N(Mg N ha^(−1))storage was,respectively,173.0±7.3(mean±SE)and 4.69±0.30 for LD and 162±11.8 and 4.08±0.18 for HD.There were no significant differences in C and N storage in the ecosystem components,except for soils,between the two stands.In contrast,there were significant differences in aboveground ANPP and TBCA between the two stands(P<0.05).Litterfall,biomass increment and R_(S) were major C flux components with values of,respectively,3.89,3.74 and 9.07 Mg C ha^(−1) year^(−1) in LD and 3.15,2.94 and 7.06 Mg C ha^(−1) year^(−1) in HD.Biometric-based NEP(Mg C ha^(−1) year^(−1))was 4.18 in LD and 5.50 in HD.Although the even-aged,mature P.densiflora forest had similar C and N allocation patterns,it showed different C and N dynamics depending on stand density.The results of the current study will be useful for elucidating the effects of stand density on C and N storage and fluxes,which are important issues in managing natural mature forest ecosystems.

Evaluation of the carbon sequestration of Zhalong Wetland under climate change

Wetland ecosystems are crucial to the global carbon cycle.In this study,the Zhalong Wetland was investigated.Based on remote sensing and meteorological observation data from 1975-2018 and the downscaled fifth phase of the coupled model intercomparison project(CMIP5)climate projection dataset from 1961-2100,the parameters of a net primary productivity(NPP)climatic potential productivity model were adjusted,and the simulation ability of the CMIP5 coupled models was evaluated.On this basis,we analysed the spatial and temporal variations of land cover types and landscape transformation processes in the Zhalong Nature Reserve over the past 44 years.We also evaluated the influence of climate change on the NPP of the vegetation,microbial heterotrophic respiration(Rh),and net ecosystem productivity(NEP)of the Zhalong Wetland and predicted the carbon sequestration potential of the Zhalong Wetland from 2019-2029 under the representative concentration pathways(RCP)4.5 and RCP 8.5 scenarios.Our results indicate the following:(1)Herbaceous bog was the primary land cover type of the Zhalong Nature Reserve,occupying an average area of 1168.02±224.05 km^(2),equivalent to 51.84% of the total reserve area.(2)Since 1975,the Zhalong Nature Reserve has undergone a dry-wet-dry transformation process.Excluding several wet periods during the mid-1980s to early 1990s,the reserve has remained a dry habitat,with particularly severe conditions from 2000 onwards.(3)The 1975-2018 mean NPP,Rh,and NEP values of the Zhalong Wetland were 500.21±52.76,337.59±10.80,and 162.62±45.56 gC·m^(2)·a^(-1),respectively,and an evaluation of the carbon balance indicated that the reserve served as a carbon sink.(4)From 1975-2018,NPP showed a significant linear increase,Rh showed a highly significant linear increase,while the increase in the carbon absorption rate was smaller than the increase in the carbon release rate.(5)Variations in NPP and NEP were precipitation-driven,with the correlations of NPP and NEP with annual precipitation and summer precipitation being highly significantly positive(P<0.001);variations in Rh were temperature-driven,with the correlations of Rh with the average annual,summer,and autumn temperatures being highly significantly positive(P<0.001).The interaction of precipitation and temperature enhances the impact on NPP,Rh and NEP.(6)Under the RCP 4.5 and RCP 8.5 scenarios,the predicted carbon sequestration by the Zhalong Wetland from 2019-2029 was 2.421(±0.225)× 10^(11) gC·a^(-1) and 2.407(±0.382)× 10^(11)gC·a^(-1),respectively,which were both lower than the mean carbon sequestration during the last 44 years(2.467(±0.950)× 10^(11) gC·a^(-1)).Future climate change may negatively contribute to the carbon sequestration potential of the Zhalong Wetland.The results of the present study are significant for enhancing the abilities of integrated eco-meteorological moni-toring,evaluation,and early warning systems for wetlands.

Heat and drought impact on carbon exchange in an age-sequence of temperate pine forests

Background:Most North American temperate forests are plantation or regrowth forests,which are actively managed.These forests are in different stages of their growth cycles and their ability to sequester atmospheric carbon is affected by extreme weather events.In this study,the impact of heat and drought events on carbon sequestration in an age‑sequence(80,45,and 17 years as of 2019)of eastern white pine(Pinus strobus L.)forests in southern Ontario,Canada was examined using eddy covariance flux measurements from 2003 to 2019.Results:Over the 17‑year study period,the mean annual values of net ecosystem productivity(NEP)were 180±96,538±177 and 64±165 g C m^(–2)yr^(–1)in the 80‑,45‑and 17‑year‑old stands,respectively,with the highest annual carbon sequestration rate observed in the 45‑year‑old stand.We found that air temperature(Ta)was the dominant control on NEP in all three different‑aged stands and drought,which was a limiting factor for both gross ecosystem productivity(GEP)and ecosystems respiration(RE),had a smaller impact on NEP.However,the simultaneous occurrence of heat and drought events during the early growing seasons or over the consecutive years had a significant negative impact on annual NEP in all three forests.We observed a similar trend of NEP decline in all three stands over three consecutive years that experienced extreme weather events,with 2016 being a hot and dry,2017 being a dry,and 2018 being a hot year.The youngest stand became a net source of carbon for all three of these years and the oldest stand became a small source of carbon for the first time in 2018 since observations started in 2003.However,in 2019,all three stands reverted to annual net carbon sinks.Conclusions:Our study results indicate that the timing,frequency and concurrent or consecutive occurrence of extreme weather events may have significant implications for carbon sequestration in temperate conifer forests in Eastern North America.This study is one of few globally available to provide long‑term observational data on carbon exchanges in different‑aged temperate plantation forests.It highlights interannual variability in carbon fluxes and enhances our understanding of the responses of these forest ecosystems to extreme weather events.Study results will help in developing climate resilient and sustainable forestry practices to offset atmospheric greenhouse gas emissions and improving simulation of carbon exchange processes in terrestrial ecosystem models.