Characteristics Of Transpiration Of Picea Crassifolia Forest And Relevant Factors Analysis In The Qilian Mountains

Qilian Mountains is the source area of the Heihe River,the eco-hydrological processes in the Qilian Mountains are crucial for ecological security of the Heihe River vally and the entier Hexi Corridor.Qinghai spruce（Picea crassifolia）forest is an important type of water conservation forest in the Qilian Mountains.The transpiration of Qinghai spruce forest has crucial impacts on mountain eco-hydrological processes.Therefore,to study the characteristic of transpiration of Qinghai spruce forest and relevant factors analysis will be conductive not only to the analysis of eco-hydrological processes but also to the establishment of the distributed eco-hydrologicla model.In the present study,an experiment was conducted in the Pailugou watershed in the Qilian Mountains,located in the upper Heihe River Basin,in the arid region of Northwest China.In an 86-year-old Picea crassifolia forest stand,3-5 trees from each of dominant,subdominant,intermediate and suppressed trees were chosen as sample trees.The sap flux density for these trees was measured using the thermal dissipation probe（TDP）method from June 16 to October 14,2015.Furthermore,the related meteorological factors and the soil moisture of the main root zone（0–60 cm）were simultaneously monitored.Daily and seasonal variation of sap flow density and transpiration of per tree,variation in sap flow density and transpiration among levels of tree dominance,and forest transpiration and its relation to the changing weather conditions and soil moisture were analysed.The results showed that:（1）Variation of sap flow density for per Picea crassifolia treeDaily variation of sap flow density Daily variation of sap flow density for per Picea crassifolia tree was higher during the day than during the night.On sunny days,daily variation in sap flow density exhibited large amplitudes,whereas it exhibited little variation on cloudy and rainy days.Daily sap flow density began earlier in the morning,ended later at night,and exhibited a larger maximum of sap flow density as tree dominance class increased.Seasonal variation of sap flow density Mean daily sap flow density showed that the maximum values were achieved in July,with the next highest values in June and August,and decreasing values in September and October.In addition,sap flow density exhibited the following order: dominant > subdominant > intermediate > suppressed trees.The relationship between sap flow and environmental factors Mean daily sap flow density was influenced by meterological factors and soil moisture.Meanwhile,correlation and stepwise regression analyses indicated that the main meteorological factor was influenced by the changing season and tree dominance,and solar radiation and soil moisture were the main meteorological factor influencing mean daily sap flow density.（2）Variation of transpiration for per Picea crassifolia treeDaily variation of transpiration Daily variation of transpiration for per Picea crassifolia tree was higher during the day than during the night.Daily variation in transpiration exhibited large amplitudes for dominant and subdominant trees,whereas it showed a slight fluctuation of wide flat curve for intermediate trees and suppressed trees.Seasonal variation of transpiration The seasonal pattern of the mean daily transpiration for per tree showed an “unimodal” change during the study period,and reached the peak in July.The value of the mean daily sap flow of per dominant tree was 15.30 L·d^-1,which was 2.9 times of that of per subdominant trees and 6.3 times of that of per intermediate tree and 31.0 times of that of per suppressed tree.（3）Variation of transpiration for Picea crassifolia forestDaily variation of forest transpiration The mean daily forest transpiration varied in a wide range from 0.05 to 1.10 mm·d^-1,and the average value was 0.47 mm from June to October.Seasonal variation of forest transpiration Seasonal pattern of forest transpiration showed an “unimodal” change.The value of forest transpiration increased from 22.8 mm in June to 30.3 mm in July,and decreased significantly in August and September（17.5 mm and 9.4 mm,respectively）,and the minimum values were achieved in October（9.2 mm）.Variation of forest transpiration among levels of tree dominance The value of total transpiration of an entire stand was 60350.56,33662.99,10627.57 L and 4620.06 L for entire dominant,subdominant,intermediate and suppressed tree from June to October,respectively.The total transpiration accounted for 55.3%,30.8%,9.7% and 4.2% of the total stand transpiration（109261.2 L）for the dominant,subdominant,intermediate and suppressed trees,respectively.Meanwhile,the number of dominant and subdominant tree was low,and accounted for 40% of the total number of stand tree,but the total sapwood area was large,and the value was 15482.38 cm2,and accounted for 74.2% of the total area of the sapwood.The total transpiration of entire dominant and subdominant trees in the plot accounted for 86.1% of the total stand transpiration,while entire intermediate and suppressed trees accounted for 13.9% of the total stand transpiration.（4）forest transpiration and its relation to weather condition and soil moisturePotential evaporation and soil moisture were the main meteorological factor influencing forest transpiration Correlation and stepwise regression analyses indicated that daily stand transpiration was positively correlated with potential evaporation and soil moisture content in the Picea crassifolia stand.Soil moisture controlled forest transpiration Stepwise regression analyses indicated that transpiration declines severely when the mean soil moisture content of at depths of 0-60 cm drops below 0.14 m3·m-3.At this point,the soil relative extractable water（REW）in the root zone was 0.2.However,if the REW increases beyond the threshold（0.2）,its impact on stand transpiration is weakened and replaced by the evaporative demand,and the relationship between PET and T was confirmed as a linear equation（T=0.131PET-0.016,R²=0.941）.