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To fully understand the forest transpiration processes and evaluate the contribution of nocturnal sap flow to total transpiration in dryland region,the sap flow velocity in the stems of 4 sample trees of a 30m×30m Larix principis-rupprechtii plantation was continually monitored with thermal dissipation probes(TDP) during the growing season(May to October) of 2011,at the semi-arid area(with an annual precipitation of 449 mm,but mostly concentrated in the summer months from July to September) of the north part of Liupan Mountain,NW China.Then,the sap flow density data measured in individual sample trees were used to calculate the sap flow amount in daytime,nighttime and whole day,and up-scaled to the stand level by using the index of sapwood area.Then,the dynamics of nocturnal sap flow and stem water recharge as well as their relations with environmental factors were analyzed.The daily variation of sap flow demonstrated a typical wide single-peak curve.The nocturnal sap flow existed in the whole growing season.The sap flow velocity in nighttime was notably lower than that in daytime and always fluctuated above zero.Furthermore,the nocturnal sap flow velocity decreased with time,and was significantly higher in the first half of night than that in the second half of night,especially for sunny days with higher transpiration and stem water deficit.Thus,the variation of nocturnal sap flow was significantly larger in sunny days than that in rainy days.The total amount of nocturnal water recharge in the stand studied was 11.03 mm during the whole growing season,which accounted for 7.22% of the total transpiration.The monthly stem water recharge and its ratio to total transpiration differed obviously in the months observed.The absolute stem water recharge was highest in May,with a value of 4.19 mm;but obviously decreased in other months and fluctuated in the range of 0.9—1.7 mm.However,the ratio of nocturnal water recharge to total transpiration was higher in the later period(September and October) compared in the earlier period(May) and middle period(June to August) of the growing season.The correlation analysis showed a less dependency of daily nocturnal water recharge on the daily weather parameters,but it was significantly correlated with the precipitation(P0.05) and soil moisture(P 0.01) as it indicates the soil water availability,and with the diurnal and daily total transpiration(P0.01) as they indicate the water demand pressure.The monthly ratio of stem water recharge to total transpiration was significantly correlated with the monthly averages of soil moisture,air temperature,diurnal and total transpiration(P 0.05).But the ratio of stem water recharge to total transpiration in daily scale was influenced by more factors than in the month scale,it was significantly correlated with the daily parameters of maximum and minimum as well as mean air temperature,diurnal and total transpiration,mean air saturation vapor pressure deficit,mean solar radiation intensity,mean air humidity,daily precipitation,daily soil moisture(P 0.01).Finally,the regression equation to relate the daily nocturnal water recharge′s contribution to total transpiration with environmental parameters was established,for quantitatively predicting the behavior of nocturnal water recharge in Larix principis-rupprechtii plantation under changing environment. |
