Determination of transpiration and water use sources of trees in arid and semiarid areas is crucial for forest management under climate change scenes. Here, seasonal variations in transpiration and water use sources of the 7– (1167 trees ha−1) and 20 (475 trees ha−1)–year–old poplar (Populus × xiaozhuanica) plantations were determined based on thermal dissipation and stable isotope methods. Results showed that transpiration per unit leaf area averaged 1.6 and 1.7 mm d−1 for the 7– and 20–year–old plantations during the measurement period, respectively, whereas stand transpiration (Es) was 1.5 and 1.4 mm d−1. Sensitivity of Es to climatic variables decreased with reduction of soil moisture for both plantations, but larger decrease for 7–year–old plantation indicated higher sensitivity to drought. Es of both plantations significantly decreased with decline of groundwater levels, but Es of 7–year–old plantation was more limited by decline of groundwater level under drought conditions. During the measurement period, accumulated Es occupied 45 % and 44 % of precipitation for 7– and 20–year–old plantations, respectively, indicating that water consumption of plantation did not increase with increasing stand age. Isotope data indicated that both 7– and 20–year–old trees switched water sources from 0 to 100 cm soil layer and groundwater in spring to 0–200 cm soil water and groundwater in summer as soil moisture and groundwater level decreased, but contribution of groundwater decreased from 47 % to 32.3 % and from 39.6 % to 37 % for 7– and 20–year–old plantation, respectively. These findings indicated that transpiration of poplar plantations did not vary with stand age, but a decline of groundwater level reduced contribution of groundwater to transpiration for 7–year–old poplar plantations and thus exacerbated negative effect of soil drought on transpiration compared with 20–year–old poplar plantation.

中文翻译：

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东北半干旱沙区7年生与20年生杨树人工林蒸腾水源


确定干旱和半干旱地区树木的蒸腾和水分利用来源对于气候变化场景下的森林管理至关重要。这里，根据热耗散和稳定同位素确定了 7 年（1167 棵树 ha−1）和 20 棵（475 棵树 ha−1）龄杨树（Populus × 小砖）人工林蒸腾量和水分利用来源的季节变化。方法。结果显示，在测量期间，7 年和 20 年树龄人工林的单位叶面积蒸腾量平均分别为 1.6 和 1.7 mm d−1，而林分蒸腾量 (Es) 分别为 1.5 和 1.4 mm d−1。两个种植园的 Es 对气候变量的敏感性均随着土壤湿度的降低而降低，但 7 年生种植园的降低幅度较大，表明对干旱的敏感性更高。两个人工林的Es都随着地下水位的下降而显着下降，但7年生人工林的Es在干旱条件下更受地下水位下降的限制。测量期间，7年生和20年生人工林的累积Es分别占降水量的45%和44%，表明人工林的耗水量并没有随着林龄的增加而增加。同位素数据表明，随着土壤湿度和地下水位的下降，7年生和20年生的树木都将水源从春季的0~100 cm土层和地下水切换到夏季的0~200 cm土层和地下水，但7 年和 20 年种植园的地下水含量分别从 47% 下降到 32.3%，从 39.6% 下降到 37%。 研究结果表明，杨树人工林的蒸腾作用并不随林龄而变化，但地下水位的下降降低了7年生杨树人工林地下水对蒸腾作用的贡献，从而与20年生杨树人工林相比，加剧了土壤干旱对蒸腾作用的负面影响。 ——老白杨种植园。