古尔班通古特沙漠南缘梭梭水分利用动态

Dynamics of water usage in Haloxylon ammodendron in the southern edge of the Gurbantünggüt Desert

植物的水分来源是荒漠地区植物水分关系研究的重要方面,有助于理解荒漠植物对干旱环境的适应。为了研究古尔班通古特沙漠主要建群种梭梭(Haloxylon ammodendron)生长季的水分利用动态,以及其对发生在不同时期相似量级降水脉冲的响应,利用稳定性同位素技术测量了梭梭小枝木质部水、降水、0–300 cm不同土层的土壤水和地下水的δ18O值。水源依据深度划分为4个:浅层土壤水(0–40 cm),中层土壤水(40–100 cm),深层土壤水(100–300 cm)和地下水。然后,应用Iso Source模型计算了梭梭对潜在水源的利用比例。结果表明:4月份,梭梭主要利用浅层土壤水,利用比例为62%–95%;5–9月份梭梭主要利用地下水,利用比例为68%–100%。梭梭对不同时期发生的两场相似量级的降水具有不同程度的响应。5月22日,6.7 mm降水后第1天,梭梭对土壤水的吸收达到最大值,由降水前的9.8%增长为降水后的40.4%,同时降低了对地下水的吸收,由降水前的83%–98%下降为42%–81%。8月31日7 mm降水后,梭梭对土壤水的吸收没有增加,仍然保持对地下水的高比例利用,达71%–98%。低的土壤含水量可能抑制了表层根系的活性,导致梭梭对降水不敏感。由冬季融雪和春季降水补给的浅层土壤水和地下水是梭梭种群可利用的两个重要水源。梭梭的水分利用动态反映了其对干旱环境的适应。

Aims Plant water use is an important aspect of plant-water relations in desert regions,and is vital in understanding the adaptation of desert species to arid environment. Haloxylon ammodendron is a dominant plant species in the Gurbantünggüt Desert,China. Its water use pattern has an important effect on water balance of the local ecosystem and plant community composition. This study aims to investigate the dynamics of water usage in H. ammodendron and its response to soil water fluctuations resulting from precipitation pulses.Methods Oxygen stable isotope ratios（δ18O） were measured for xylem water,soil water in different soil layers（0–300 cm depth）,precipitation and groundwater. Four potential water sources were classified： shallow（0–40cm）,middle（40–100 cm） and deep soil water（100–300 cm）,and groundwater. The possible ranges of potential water sources used by H. ammodendron were calculated using the Iso Source model.Important findings Main water sources for H. ammodendron shifted from the shallow soil water in April to the groundwater during May to September. In April,the contributions of shallow soil water were in the range of62%–95%,and the possible ranges of middle and deep soil water and groundwater were 0–8%,0–15% and0–38%,respectively. However,during May to September,the contribution of shallow soil water decreased drastically while that of groundwater increased rapidly. Contributions of groundwater were in the range of68%–100%. Haloxylon ammodendron responded differently to two similar precipitation pulses occurred in different months. After the 6.7 mm precipitation pulse on May 22,the usage of soil water increased from 9.8%prior to the event to maximum at 40.4% one day after rainfall（May 23）,while the usage of groundwater decreased from 83%–98% to 42%–81%. After 7 mm precipitation pulse on August 31,the usage of soil water was almost unchanged and the usage of groundwater was still up to 71%–98%,implying that roots of H.ammodendron in the upper soil layer are inactive due to the long-lasting low soil water content. Hence,H.ammodendron seemed to be insensitive to the August precipitation pulse. The shallow soil water recharged by snowmelt and precipitation in spring and groundwater are two important water sources for H. ammodendron.Dynamics of water usage in H. ammodendron reflects its adaptation to this water-limiting desert environment.