Nutrient status of Populus euphratica growing in desert riparian forests of northwestern China

Seasonal and microhabitat variations of chemical constituents of foliar organic carbon (C), total nitrogen (N), total phosphorus (P), and total potassium (K), in Populus euphratica growing in desert riparian forests in northwestern China and their correlations were studied. Results show that ranges of C, N, P and K contents in the leaves of P. euphratica were 39.08%?46.16%, 0.28%?2.81%, 0.05%?0.18% and 0.35%?2.03%, with means of 43.51%, 1.49%, 0.102% and 1.17%, respectively. The ratio of C/N, C/P and N/P changed from 16.26 to 146.61, from 258.08 to 908.67 and from 2.89 to 26.67; the mean was 37.24, 466.27 and 15.14, respectively. The mean N content was significantly lower than of deciduous trees in China, but the mean P content was nearly equivalent. The ratio of C/N was remarkably higher than of global land plants. The ratio of N/P indicated that growth of P. euphratica was jointly limited by N and P nutrient deficiency. During the growth season, total trends of leaf C, N, P and K contents decreased. The maximum appeared in May, and the minimum in September. Among microhabitats, C, N and K contents gradually increased from riparian lowland, flatland, sandpile, Gobi and dune, but C/N ratio was opposite, and P content was not apparent. Foliar C content was extremely, significantly and positively correlated with N and K contents, respectively. The relationships of N-K and P-K were both significantly positive.     

极端干旱区荒漠河岸林胡杨生长季树干液流变化

应用热脉冲技术和自动气象站对极端干旱区荒漠河岸林建群种胡杨(Populus euphratica Oliv.)的树干的液流流速及其环境因子进行了为期2 a的连续观测,对胡杨树干液流日、季变化及其与环境因子相关性进行了深入的研究。结果表明:胡杨树干液流速率日变化具有明显的昼夜节律性。白天树干液流流速变化曲线呈多峰型。夜间,胡杨同样存在明显的树干液流现象,这主要是由根压引起的。胡杨树干液流速率7月最高,6月、8月次之,9月液流速率高于5月,10月最小,主要生长期6—8月胡杨日均蒸腾量占整个生长季的70％以上。不同林龄的胡杨树干液流速率不同,表现为15龄＞25龄＞50龄。树干液流量变化与环境因子中气象因子的变化密切相关。逐步回归结果表明,液流速率与净辐射、空气温度、空气相对湿度呈显著线性相关, 同时给出了依据常规气象因子估算液流速率的统计模型。     

Characteristics of the stem sap flux of Populus euphratica in the lower reaches of the Heihe River Basin, Northwest China

Populus euphratica trees are the sole natural perennial riparian woodlands native to the river oases in the lower reaches of Heihe River Basin in northwestern China.This study investigated characteristics of the stem sap flux of Populus euphratica and its rela-tionship to environmental factors using the thermal dissipation probe(TDP) method.The results showed that(1) daily variation of sap flow of P.euphratica on clear days exhibited an obvious unimodal curve;sap flow rates in June,July,August,and September were 13.39,12.07,12.69,and 5.10 L/d,respectively;(2) the average transpiration of the Populus euphratica from June through September amounted to 1,309.84 L;(3) stem sap flow can be affected by a number of environmental factors that,in terms of the influential degree,can be arranged in the descending order of air temperature,soil moisture,relative humidity,total solar radiation,soil temperature,and wind velocity.     

Influence of environmental, root, and stand parameters on soil surface CO2 efflux in a Populus euphratica of desert forest in extreme arid region

Spatial variation in soil surface CO2 efflux was measured in a stand of Populus euphratica in the Ejina Oasis of desert riparian forest in the extreme arid region in northwestern China from April 2007 through October 2007.Measurements were taken with a gas-exchange analyzer linked to a soil-respiration chamber.The mean soil CO2 efflux in the stand was 2.71 μmol/(m2·s) during the growing season and 1.38 μmol/(m2·s) in the nongrowing season.The seasonal maximum (end of May through early June) andminimum (October) CO2 efflux were 3.38 and 0.69 μmol/(m2·s),respectively.The diurnal fluctuation of CO2 efflux was relatively small (< 20 percent),with theminimum appearing around 05:00 and the maximum around 15:00.Linear regression analysis showed soil-surface CO2 efflux to be most highly correlated with soil temperature (R2=0.435) and soil moisture (R2=0.213).When all variables were considered simultaneously,only soil temperature (R2=0.378),soil moisture (R2=0.147),and root volume density (R2=0.021) explained a significant amount of variance in soil surface CO2 efflux.Stand volumes were not correlated with soil CO2 efflux on our sites.