| Abstract:The area of urban forests and green-land is expanding dramatically across China in order to face rapid urbanization. Urban green-land ecosystems, with plantations as their main vegetation type have the great potential to sequestrate atmospheric carbon. The different parts of the urban green-land ecosystem compositions have complicated physical processes and physiological and ecological processes, which precipitated the carbon flux in different time scale and had become an important content on carbon balance research in terrestrial ecosystem. Continuous measurements of CO2flux were made from2011to2012in a mixed forest in Beijing Olympic Forest Park to quantify controlling mechanisms in urban green ecosystem and its responses to environmental factors. We analyzed the dynamics the carbon and water flux components in different time scales and calculated the carbon source/sink properties and strength of urban green-land ecosystems. Meanwhile, we also analyzed the soil respiration in relation to its environment factors and its contribution to the whole ecosystem respiration. The major conclusions are summarized as follows:(1) The predicted annual totals of gross ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem productivity (NEP=-NEE) were1053.5,897.1, and156.4g C·m-2in2011, respectively, and1150.4,1040.3, and110.1g C·m-2in2012. Both years were moderate carbon sink.(2) The carbon flux was influenced by photosynthetically active radiation (PAR), water vapor pressure deficit (VPD) and air temperature (Ta). In growing season, daytime NEE increased with increasing PAR. The ecosystem quantum yield (a) and maximum photosynthesis (Amax) showed an apparent seasonal pattern, both peaking in July. VPD also affects the net ecosystem carbon exchange (NEE) through its direct effect on photosynthesis, NEE increased with the increasing PAR up to a threshold of1200μmol·m-2·s-1,but decreasing with increasing PAR above the threshold, which indicated that NEE was inhibited. The gross ecosystem productivity (GEP), ecosystem respiration (ER) and net ecosystem productivity (NEP) were all influenced by air temperature (Ta), but responded differently. ER increased exponentially with the increasing Ta, with the temperature sensitivity (Q10) of ER being2.1and2.5in2011and2012, respectively. GEP also increased with Ta. The differential response of GEP and ER determined the relationship between NEP and Ta. NEP decreased with increasing Ta when Ta<10℃, but increased when Ta>10"C.(3) In2012, the annual total evapotranspiration was627.4mm which was slightly smaller than precipitation of716.0mm. Monthly average daily water vapor flux changed almost as an inverted ’U’ pattern. The water-vapor flux was small and flat at night, but positive during daytime, indicating that the urban green-land release water vapor into the atmosphere. When Bowen ratio (the ratio between sensible heat (H) and latent heat (LE), that is X=H/LE) X<1, the underlying plants was relatively vigorous, the transpiration was stronger and the energy exchange form between ecosystems and the atmosphere was mainly latent heat. In contrast, when X>1, transpiration is small, the energy exchange form between ecosystems and the atmosphere was mainly sensible heat, indicating that the plants can’t diffuse energy to atmosphere physilogically.(4) The energy balance closure in Olympic forest park at half-hourly scale was0.72compared to daily sacle of0.76. An obvious energy balance misclosure was discovered in this urban green-land ecosystem. The ecosystem turbulence energy flux (LE+H) was underestimated by28%and26%in half-hourly and daily scale, respectively..(5) Daily mean soil respiration (SR) varied from0.17to3.75μmol CO2·m-2·s-1during2011-2012. Over the period of measurements, SR increased exponentially with rising temperature; A Q10model with5-cm soil temperature as the independent variable explained76%and62%of the variation in half-hourly SR in2011and2012, respectively. The annual total SR estimated from the Q10model was475g C·m-2and432g C·m-2in2011and2012, respectively, accounting for53.0%and42.0%of ecosystem respiration for two years. SR was hyperbolically related to VWC, increasing with increasing VWC up to a VWC threshold of0.17m3·m-3and decreasing with increasing VWC above the threshold. A bivariate Q10-hyperbolical model, which incorporated both Ts and VWC effects, improved the performance of SR simulation in summer, but not annually. These results indicated that SR was dominantly controlled by soil temperature over the annual cycle. However, VWC served as the dominant control in summer. The temperature sensitivity of respiration (Q10) varied seasonally, being greater in fall than in spring, suggesting seasonal hysteresis in the SR-Ts relationship. |
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