| There are various types of ecosystems and complex landform in alpine area ofsouthwestern China, which make it an ideal place to research regional response to globalchanges in climate. Therefore, it is of great importance to investigate the response of soilorganic carbon（SOC）， soil heterotrophic respiration（HR）， net ecosystem production（NEP），water use efficiency（WUE） to climate in this region for evaluating the contribution of regionalcarbon cycle to global climate changes. A process-based biogeochemical modelCEVSA（Carbon Exchange between Vegetation, Soil, and the Atmosphere）was used to estimatetemporal and spatial variations of SOC, HR, NEP, WUE of terrestrial ecosystems in alpine areaof southwestern China during the period from1954to2010. We conducted CEVSA model withobservation-based data sets of climate （ten days mean value of temperature, precipitation,relative humidity, and cloudiness）, atmospheric CO2concentration, and soil and vegetationdistribution at a spatial resolution of0.1°. Firstly, we ran the model with the average climatedata from1954to2010until an ecological equilibrium was reached, then conducted dynamicsimulations with climate data at a time-step of ten days from1954to2010. Also, thecorrelation coefficients between SOC, HR, NEP, WUE and climate variables were calculated toanalyze their response to climate change. To achieve the results, various kinds of computersoftware were used, such as ANUSPLIN4.1, Fortran90, Arcgis9.3, SPSS18.0. The resultsshowed that:1. Temporal-spatial variations of soil organic carbon of terrestrial ecosystems in alpinearea of southwestern China（1） The mean values of SOC density were14.16kg C·m-2during the period of1954-2010in alpine area of southwestern China. In spatial distribution, SOC density increased fromsoutheast to northwest and was significantly negatively correlated with annual meantemperature（r=-0.447, P<0.01）, but had no significant correlation with annual totalprecipitation.（2） Total SOC storage ranged from6.95to7.64Pg C and showed a statistically significant increasing trendwith a growth rate of0.013Pg C（P<0.05）during the period1954-2010.（3） The increasing trend of SOC density was highly significant （P<0.01） inherbaceous cover（closed-open）, evergreen needle-leaved tree cover and evergreen broadleavedtree cover which was the three main vegetation types. SOC density of herbaceous cover（closed-open）and evergreen needle-leaved tree cover in study area were both correlatedpositively with annual mean temperature（r=0.527, P<0.01; r=0.501, P<0.01）, but SOC densityof evergreen broadleaved tree cover had no correlation with annual mean temperature. Therewere no significant correlation between SOC density and annual total precipitation in all of thethree vegetation types.（4） The growth of soil organic carbon（SOC）storage may slow down orreverse by ongoing climate change, for that litter production which acted as the carbon input ofsoil ecosystem was less sensitive to temperature than soil heterotrophic respiration.2. Temporal-spatial variations of soil heterotrophic respiration of terrestrial ecosystems inalpine area of southwestern China（1） The mean values of soil HR density was422g C·m-2·a-1during the period of1954-2010in alpine area of southwestern China. The soil HR showed a decreasing trend fromsoutheast to northwest and had significant positive correlation with both annual meantemperature（r=0.721，P<0.01）and annual total precipitation（r=0.564，P<0.01）.（2） Total annualsoil HR showed an increasing trend with the rate of0.710Tg C/a（P<0.05）, and varied between197and251Tg C/a. The increasing rate was1.621,1.496and1.055g C m-2 a-2in herbaceouscover（closed-open）, evergreen needle-leaved tree cover and evergreen broadleaved treecover（P<0.01）, which were three main vegetation types in alpine area of southwestern China,respectively.（3） Annual variation of soil HR was mainly influenced by temperature. Thetemperature sensitivity of soil HR （Q10） in the high-elevation northwestern areas was higherthan that in the low-elevation southeastern areas, and it is2.35,2.34and1.93in herbaceouscover （closed-open）, evergreen needle-leaved tree cover and evergreen broadleaved tree coverin this area, respectively.3. Temporal-spatial variations of net ecosystem production of terrestrial ecosystems inalpine area of southwestern China （1） The mean value of NEP was29.7g C m-2 a-1during the period1954-2010in alpinearea of southwestern China, with higher NEP in the evergreen needle-leaved and evergreenbroadleaved tree cover at low altitude and lower NEP in herbaceous cover at high altitude ofstudy area.（2） Annual total NEP ranged from-8.36to29.4Tg C/a with a mean value of15.4TgC/a. NEP showed a statistically significant decreasing trend with a reduction rate of0.187Tg C/a （P<0.05） during the period1954-2010and the significant decreasing area of NEP accountedfor35.2%（P<0.05）. The decreasing trend was highly significant in herbaceous cover(-0.526gC·m-2·a-2, P<0.01) and evergreen needle-leaved tree cover(-0.691g C·m-2·a-2, P<0.01).（3）Annual total NEP was significantly negatively correlated with annual mean temperature（r=-0.454, P<0.01） and positively correlated with annual total precipitation（r=0.708, P<0.01）.The negative correlation（P<0.05）of annual NEP and temperature existed in60.3%of the studyregion while the positive correlation（P<0.05）of annual NEP with precipitation existed in52.1%of the study region, respectively. Herbaceous cover and evergreen needle-leaved tree coverwere both correlated negatively with annual mean temperature（r=-0.603, P<0.01. r=-0.485,P<0.01）and positively with precipitation（r=0.554, P<0.01. r=0.749, P<0.01）, reaching anextremely remarkable level.（4） alpine area of southwestern China acted as a carbon sink,however, a small fraction of terrestrial ecosystems here shifted from carbon sinks into carbonsources in recent20years, for that the net primary production growth rate was less than that ofthe soil heterotrophic respiration. These estimates indicate that the carbon sequestrationcapacity in alpine area of southwestern China was weakened by the ongoing climate change.4. Temporal-spatial variations of water use efficiency of terrestrial ecosystems in alpinearea of southwestern China（1） The average WUE in alpine area of southwestern China was1.13g C·mm-1·m-2during1954-2010. In spatial distribution, significant negative correlations were found betweenthe annual WUE and annual mean temperature（r=-0.386, P<0.01）, and significant positivecorrelations were shown（r=0.100, P<0.01） between the annual WUE and annual totalprecipitation. The mean WUE of three main vegetation types followed an order of: herbaceouscover（closed-open）(1.35g C mm-1m-2)>evergreen needle-leaved tree cover(1.14g C mm-1m-2)>evergreen broadleaved tree cover(0.99g C mm-1m-2)in alpine area ofsouthwestern China.（2） The annual WUE showed a statistically significant decreasing trendwith a rate of0.006g C mm-1m-2 a-1（P<0.01）. The decreasing trend was highly significant inall the three main biome types and herbaceous cover（closed-open）decreased the fastest(1.37×10-3g C mm-1m-2 a-1), followed by evergreen needle-leaved tree cover (6.17×10-3gC mm-1m-2 a-1) and evergreen broadleaved tree cover(1.03×10-2g C mm-1m-2 a-1).（3） Theannual WUE showed significant negative correlations with temperature in most part of studyareas（76.3%，P<0.05）. Herbaceous cover and evergreen needle-leaved tree cover in study areaswere both correlated negatively with temperature（r=-0.889, P<0.01; r=-0.863, P<0.01） andherbaceous cover showed the higher response of WUE to climate changes than evergreenneedle-leaved tree cover.SOC、HR、NEP and WUE are extremely complex process involving a variety of biologicaland non-biological factors. Climate and vegetation changed with topography and high spaceheterogeneity existed in alpine area of southwestern China. For these reasons, we suggest thatthe model resolution should be improved and validation of the simulation result should bestrengthened in future to reduce uncertainty. |
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