Studies Of Carbon And Water Fluxes Over Larch Forests At Huoditang Forest Region In The Qinling Mountains

The dynamic characters of carbon and water fluxes of forest ecosystem, resulting fromthe interreactions of physical and physiological processes of different components, playvital roles in carbon cycle and water balance analysis. Based on the continuous CO2andH2O exchange observation during the growing season over larch forest ecosystem by eddycovariance method in the Qinling Mountains, the diurnal and seasonal dynamics of soilrespiration, the characters of carbon and water fluxes at different stages during the growingseason were analyzed. The controlling mechanisms of environmental factors to netecosystem carbon exchange (NEE) and water fluxes were also discussed. Major conclusionsare as follows:(1) Soil respiration had significant exponential relationship with10cm soiltemperature, Rs=0.28201e0.12662T(R2=0.9087，P＜0.001). Soil respiration rates of the wholeyear ranged from1.09to23.91g CO2·m-2·d-1, and the soil respiration rates in the growingseason were higher than those in non-growing season. On monthly scale, the minimum soilrespiration rate occurred in February (31.99g CO2·m-2·mon-1), and the maximum rate inAugust, reaching to544.29g CO2·m-2·mon-1. The diurnal average soil respiration of thewhole year was6.76g CO2·m-2·d-1, and the total amount of soil respiration in a year2473.19g CO2·m-2.(2) The diurnal patterns of NEE could be expressed as one-peak curves with themaximum NEE appearing at about11:30. The maximum NEE in a day at different stageswere-0.339mg CO2·m-2·s-1,-0.960mg CO2·m-2·s-1and-0.339mg CO2·m-2·s-1,respectively. The diurnal average NEE at different stages were different. The diurnalaverage NEE at three stage were-6.4956g CO2·m-2·d1,-11.7072g CO2·m-2·d-1and-2.6698g CO2·m-2·d-1, respectively. The NEE values during the growing season werenegative, which means the larch forest ecosystem was carbon sink. On half hour scale, therewere significant relationship between NEE and the main environmental factors (PAR, Ta,RH, VPD and WS). The relative coefficients in the relationship between NEE and PAR at three stages were0.702,0.613and0.587, which were higher than those between NEE andother factors.(3) The larch forest ecosystem acted as a water source during the growing season. Thediurnal pattern of water fluxes could be expressed as one-humped curves. The water fluxesvalues were positive during daytime, while the values were close to zero after sunset. Themaximum water fluxes value in a day at the beginning stage was0.185g H2O·m-2·s-1,appearing at11:30. The maximum value at the mid-stage was0.273g H2O·m-2·s-1while0.063g H2O·m-2·s-1at the end-stage. The diurnal average water fluxes values weredifferent. The diurnal average water fluxes at three stages were2.4624kg H2O·m-2·d-1,2.7993kg H2O·m-2·d-1and1.0540kg H2O·m-2·d-1, respectively. On half hour scale, waterfluxes and the main environmental factors (PAR, Ta, RH, VPD and WS) showed significantrelationships.(4) There were significant relationships between NEE and water fluxes at three stages,showing as triple polynomials with high relative coefficients. The most significantrelationship of these two item appeared at mid-stage(y=-73.512x3+51.673x2-12.388x+0.168, R2=0.682, P＜0.001). The lowest relativecoefficient was observed at end-stage.(5) The water use efficiency (WUE) had a similar diurnal pattern with the NEE in thelarch forest ecosystem, which means that most WUE values were negative. The highestaverage WUE absolute value (0.0046) appeared at the beginning stage, and the value atmid-stage (0.0039) was lower than that at end-stage (0.0041).