梭梭对降水的响应与适应机制--生理、个体与群落水平碳水平衡的整合研究

Water and carbon balances of Haloxylon ammodendron:intergated study at physiological, plant abd community level

水是荒漠生态系统的首要限制因素。近20年来全球气候变化以及人类活动的加剧,导致古尔班通古特沙漠南缘原始盐生旱生荒漠地区的降水与地下水位正在发生显著的改变;这些改变正导致荒漠植物用水策略的适应性变化,其种间差异性影响着荒漠植物群落组成与生态系统碳水平衡。本研究以中亚荒漠关键种梭梭Haloxylon ammodendron为对象,在生理生态、个体形态与群落尺度整合研究两个优势种对自然生境水分条件改变的响应与适应。实验于2005-2006年生长期开展,在地下水位于5.2-7.9m之间波动的典型原始生境,追踪自然降水过程并设置人工降水梯度。观测生境水分条件变化时光合作用、蒸腾作用、叶水势和瞬时水分利用效率等生理活动的响应;研究根系分布和地上生物量累积等个体形态特征的适应性改变;利用涡度相关系统测定群落碳水通量,从而揭示不同尺度上梭梭的用水策略和碳同化维持机制。实验数据表明,梭梭主要利用降水形成的浅层土壤水维持生存;极为有效的形态调节和较强的气孔控制是其维持光合能力以及适应降水变化的主要机制;适量增多且在生长期内均匀分布的降水可对其产生明显正效应,这预示梭梭可能在未来种间竞争和群落演替中占有优势。本研究表明,应对环境水分胁迫与改变,荒漠植物具有高效的自我协调与适应能力,不同水平调节适应机制之间存在内在协调性;生理、个体、群落尺度整合研究是了解全球变化背景下生态系统过程与植物适应性的有效途径。

Within the current background of global climate change, significant increase precipitation has been recorded in the arid region of central Asia over the past 50 years. In addition, in some transition regions between sandy deserts and oases, the groundwater table has fallen significantly as a result of the overexploitation of groundwater. Variation in precipitation in arid ecosystems is leading to plant adaptation in water use strategies. To under- stand the effects of these widely acknowledged changes in water conditions, this integrated study investigated the re- sponse, acclimation, and adaptation of a native dominant desert shrubs in Central Asia, Haloxylon ammodendron, towards variation in precipitation, on the scales of ecophysiological activities, morphology and carbon/water balances of plant community. During the growing season in 2005 and 2006, the experiments were carried out under natural precipitation and two manipulated precipitation treatments （double and no precipitation respectively）, in its original habitat where the groundwater table fluctuated from 5.2 m to 7.9 m, on the southern periphery of Gurbantonggut Desert. Changes in photosynthesis, transpiration, leaf water potential, water-use efficiency, above-ground biomass accumulation, and root distribution were examined under the contrasting precipitation treatments, in order to reveal its water-use strategies and mechanisms of photosynthesis maintenance under drought stress and precipitation variation. Meanwhile, CO2 and H2O fluxes above the undisturbed H. ammodendron community were measured by eddy covariance method to evaluate its NEE. The experiment reveals that ecophysiological activities, morphological adjustment and community carbon assimilation of H. ammodendron responded to rainfall events and precipitation variation significantly. Its efficient morphological adjustment, combined with strong stomatal control, contributes to its maintenance of photosynthesis and acclimation to variation in water condition. On account of its positive re- sponses to increased precipitation, H. ammodendron is predicted to succeed in interspecific competition in a future, moister habitat. This integrated study of different scales is an effective approach to further the understanding of the shrub adaptation strategies and the desert ecosystem processes under global climate change.