Research On Carbon Sink Capacity Of Several Green Trees In Northern Cities

With the acceleration of the urbanization process, the increasing emission of thegreenhouse gas such as CO₂, the increasingly serious of urban heat island effect, how to dealwith the negative impact of global warming to human has been increasingly concerned. Urbangreen land is in special environment and has inhibition function to urban heat island effect,thus research on carbon sink capacity of urban green trees has great practical significance tobuild ecological and low-carbon city. In this paper,6species of common green trees in Tianjinincluding pear, hawthorn, pomegranate, chinese ash, sophora japonica, and ginkgo as theresearch subjects, through researching on the change situation of photosynthetic rate ofvarious trees in different time, the carbon sink capacity of green trees in northern cities wasanalyzed. The results show as following:1. The diurnal change curves of photosynthetic rate of every monitoring trees are typicaldouble-peaked trend, and have obvious midday depression. Trees obtain the first peak in10:00-12:00in every day, obtain the second peak in14:00-16:00, and obtain the bottom valuein12:00to14:00. The first peak value is larger than the second. The monthly change curvesof photosynthetic rate of every trees appear regularity, and the maximums of monthlyphotosynthetic rate of every trees are in May. The minimums of monthly photosynthetic ratehave some differences, because of the influence of temperature and plants growth habits,which excepted the photosynthetic rate minimum of pomegranate is in June, the pear,hawthorn, sophora japonica, gingko, and white wax's are in November.2. The photosynthetic rate of every monitoring trees is correlated with the temperature(T), photosynthetic active radiation (PAR), air CO₂concentration, vapor pressure deficit(VPD) and air relative humidity (RH). First, with the increase of T and PAR, photosyntheticrate is increased firstly and then decreased; when T and PAR reach the maximum,photosynthetic rate reaches the bottom value; when T and PAR turn from up to down,photosynthetic rate increases again, and appears double-peaked trend at the end. Secondly,photosynthetic rate have the same change-decreased firstly and then increased-with thechange of CO₂concentration and RH. Finally, the variation of photosynthetic rate with thechanges of VPD is increased firstly and then decreased, and appears single-peaked trend.3. The change situation of photosynthetic rate of every monitoring trees as following: themaximums of photosynthetic rate of every trees appear at about daily11:00in May, and thebottom value appear at about daily14:00in June. The photosynthetic rate maximum of pear is5.33μmol/m²s, and the bottom value is1.65μmol/m²s; the photosynthetic rate maximum ofhawthorn is8.46μmol/m²s, and the bottom value is1.7μmol/m²s; the photosynthetic ratemaximum of pomegranate is8.5μmol/m²s, and the bottom value is1.4μmol/m²s; thephotosynthetic rate maximum of white wax is9.1μmol/m²s, and the bottom value is 1.92μmol/m²s; the photosynthetic rate maximum of sophora japonica is8.84μmol/m²s, andthe bottom value is1.92μmol/m²s; the photosynthetic rate maximum of gingko is9.05μmol/m²s, and the bottom value is1.78μmol/m²s.4. The carbon sink capacity of every monitoring trees as following: the carbonsequestration of unit leaf area is largest in May; the carbon sink capacity of white wax is thelargest in every months, and the maximum can reach9.63g/m²d at daily8:00to19:00, and theminimum is7.64g/m²d; the carbon sink capacity of pear is relatively low, and the maximum is4.85g/m²d, the minimum is only3.95g/m²d.5. The soil physical condition of park, district, campus and street tree-hole area in cityhave been analyzed, and the results show that the soil physical properties of the green land inTianjin didn't appear regularity, which the soil bulk density between1.259g/cm3and1.437g/cm3, the total soil porosity between41.17%and62.94%, the soil capillary porosity isrelatively low, between5.16%and14.12%, and the soil water content between12.84%and21.70%. In Qing Longwan sand region, the soil bulk density between1.202g/cm3and1.504g/cm3, the total soil porosity between39.37%and47.87%, the soil capillary porosity islow, between5.18%and8.80%, and the soil water content between7.35%and20.77%.Therefore, excepted the street tree-hole soil is poor, the physical properties of other green landsoil in Tianjin are more superior than the natural soil in Qing Longwan, and suitable for plantgrowth.