永定河沿河沙地杨树人工林蒸腾耗水特征及其环境响应

杨树是我国北方最常见的人工造林树种之一。一直以来在干旱、半干旱地区,速生杨树用材林和生态防护林的耗水问题备受关注。研究不同生长发育阶段杨树人工林蒸腾耗水及其对各环境因子的响应对于实现杨树人工林可持续经营具有重要价值。采用树干液流法结合微气象观测系统和土壤水分观测,在2010—2011年对位于北京南郊大兴林场、林龄为13a的杨树人工林林分蒸腾耗水和环境因子进行了同步观测,以期能够探究该林分的蒸腾耗水及其对环境因子的响应。结果表明,树干液流密度(Js)日变化呈明显的单峰曲线,单株样木耗水量随着胸径的增加而增大。在半小时尺度上,单株树木Js与浄辐射(Rn)、饱和水气压差(VPD)存在时滞,这种时滞现象随土壤水分条件不同而变化。林分蒸腾耗水总量在2010和2011年生长季内分别为113.7 mm和174.8 mm,占同期降雨的30.2%和36.9%,与该杨树人工林前期研究相比,随着林龄的增长2010—2011年的蒸腾量呈减小趋势。日尺度上,该人工林蒸腾耗水与净辐射(Rn)、饱和水汽压差(VPD)和土壤体积含水率(SWC)显著相关,在不同土壤水分条件下Rn与林分蒸腾的相关关系发生变化,而VPD过高会对林分蒸腾产生抑制。林分月蒸腾和年总蒸腾主要取决于同期降雨量,因此,降雨年际差异较大时,蒸腾的年际变化也相应较大。

Transpiration responses of a poplar plantation to the environmental conditions on a floodplain in Northern China

Poplar plantations are the most prevalent and the fastest expanding plantations in Northern China. However, the high water use of poplars necessitates careful consideration of their suitability for water-stressed regions. It is essential to find out the relationships between the water transpired by poplar plantation and environmental factors during different growing periods, which is very important in poplar plantation sustainable management. Many researchers have done a lot of job on this subject, but few referred to a mature fast growing poplar stand. In the current study the water use and response to environmental factors was evaluated by simultaneous measurements of sap flow using the TDP (Thermal Dissipation Probe) method, micrometeorological conditions and soil moisture in a 13-year old poplar (Populus euramericana cv. "74/76") plantation located on the floodplain of Yongding River in Daxing district, the southern suburb of Beijing, China from 2010 to 2011. Our objectives were to illustrate the water used for transpiration by this 13-year old fast growing poplar plantation and explore how the tree/stand transpiration responses to the different environmental conditions. The results showed that the daily pattern of sap flow was a bell curve. The daily water consumption of individual trees ranged from 6.4 to 59.7 kg/d, varying by season and sapwood which was calculated through the trunk diameter at breast height. On a ground area basis, the growing season transpiration was 114 and 175 mm in 2010 and 2011, respectively. It accounted for 30%-37% of precipitation over the same periods. Compared to earlier reports for poplars in literature, the observed water use was relatively low, likely attributable to growing age and no available irrigation during the study period. A diurnal hysteresis was observed between sap flow and vapor pressure deficit (VPD). Similarly, this situation was also true between sap flow and net radiation (Rn). The hysteresis was greater with high than low soil moisture, suggesting that transpiration did response to the change of soil moisture though indirectly. On daily scale, significant correlations were observed between daily stand transpiration and Rn, VPD and soil moisture. Besides, the relationship between transpiration and soil moisture was related to the depth of soil. The soil moisture under the surface 30-90cm affected transpiration most effectively. When the soil moisture changed, the relationship between stand transpiration and Rn or VPD varied. The growth of VPD stimulated the stand transpiration when it was not greater than 1 kPa. The over great value of VPD would slow down the growth of stand transpiration. Compared to the data of earlier research at this site, we found that the transpiration of the plantation declined with the stand age. On a monthly and annual scale, transpiration was correlated with rainfall, suggesting limited soil water storage. Simultaneously, we found that during dry months when transpiration could not be satisfied by rainfall, soil water storage offered the water to be transpired. However, the source of soil water storage was also come from rainfall thus the transpiration between 2010 and 2011 were quite different because of great rainfall difference in these two years.