黄土丘陵区油松和沙棘人工林的水分来源变化及其生理响应研究

Plantation forests account for 59.8% of forested area in Loess Plateau hilly region, and approximately 70.4% of plantation forests are vulnerable to seasonal drought stress. The water use stability of plantation forest is essential to assess the sustainability of soil and water conservation and ecological recovery in this region, which can be evaluated from both water sources variation and physiological response of plantation species. This study mainly focuses on Pinus tabulaeformis, Hippophae rhammoides and P. tabulaeformis-H. rhammoides plantations during vigorous growth period in Ansai National Ecological Experimental Station in Loess Plateau hilly region. Based on isotopic ratio infrared spectroscopy systems, stable hydrogen and oxygen isotopes of water in plant xylem, soil, precipitation and groundwater for P. tabulaeformis and H. rhammoides in these plantations can be analyzed. Then, the fractions of water sources for P. tabulaeformis and H. rhammoides can be quantified through a MixSIR model. The mixed plantation effect to water sources for these two forest species can also be evaluated. Meanwhile, physiological response for P. tabulaeformis and H. rhammoides can be analyzed through stomatal conductance, leaf photosynthesis rate and leaf water potential observation. Then, combined with drought stress and precipitation events, water conditions that may alter plant water sources variation and physiological response can be investigated. The water use strategies for P. tabulaeformis, H. rhammoides and P. tabulaeformis-H. rhammoides plantations can be revealed based on plant water sources variation and physiological response analysis. In further, it would be useful to provide scientific basis for evaluate the water use stability and sustainability management of plantations in terms of ecological recovery in this region.

人工林占黄土丘陵区森林面积的59.8%，其中70.4%的人工林易受季节性干旱的影响。人工林水分利用的稳定性是评价该地区水土流失治理和生态恢复可持续性的重要指标，且需要通过植物水分来源的变化和植物的生理响应进行判断。本项目拟以黄土丘陵区安塞国家生态试验站旺盛生长时期的油松、沙棘人工纯林和油松-沙棘混交林为研究对象，以稳定同位素红外光谱技术为基础，开展植物茎秆水、土壤水、降水和地下水氢氧稳定同位素的连续采样与分析，结合MixSIR模型量化油松和沙棘的水分来源，阐明混交造林对油松和沙棘水分来源的影响。同时，对油松和沙棘的气孔导度、叶片光合速率以及叶片水势进行观测，并结合该地区季节性干旱和降水事件，探讨改变油松和沙棘水分来源及其生理响应的水分环境条件，进而揭示油松、沙棘人工纯林和油松-沙棘混交林的水分利用策略，为黄土丘陵区生态恢复过程中人工林水分利用稳定性的评估和可持续性管理提供科学依据。

人工林水分利用的稳定性是评价黄土丘陵区水土流失治理和生态恢复可持续性的重要指标，且需要通过植物水分来源的变化和植物的生理响应进行判断。本研究以黄土丘陵区2002年栽植的油松、沙棘人工纯林和油松-沙棘混交林为研究对象，使用氢氧稳定同位素技术，结合植物树干液流以及叶片生理参数的观测,从植物水分来源、蒸腾耗水特征、以及植物的水分利用策略等方面进行分析，最终结合该地区降水特征初步评价了油松和沙棘人工纯林和混交林水分利用的可持续性，以及混交造林的影响。主要结果如下：(1) 纯林中油松的水分来源以浅层(0-40cm)和中层(40-100cm)土壤水为主, 对降水响应不明显。纯林中沙棘的水分来源则随着干旱程度和降水的补充而改变，当降水量大于20mm时，纯林中沙棘的水分来源以浅层为主，季节性干旱(2017年5-7月份)和降水小于20mm时，以深层(100-200cm)土壤水为主。混交造林，导致油松和沙棘具有相似的水分来源变化特征。从定性和定量两方面分析，都表明混交造林导致油松和沙棘存在水分竞争。(2) 油松蒸腾耗水的变化幅度大于沙棘，且在出现季节性干旱的2017年，纯林和混交林中油松的相对树干液流存在显著性差异 (p < 0.05)，而混交造林并不显著影响沙棘的相对树干液流。纯林和混交林中，能够影响油松树干液流最小有效降雨量高于沙棘，而对降雨的滞后时间则小于沙棘；混交造林导致油松对水分(降水)的需求增加，而对沙棘的影响不大。(3) 沙棘以牺牲水势为代价维持相对恒定的气孔导度和光合速率，而油松以维持叶片的水势为目的而以明显降低的气孔导度和光合能力为代价。因此，将沙棘和油松分别定义为变水型(anisohydric)和恒水型(isohydric)植物。混交造林并没有显著影响沙棘的干旱敏感性，却显著增加油松的干旱敏感性。(4) 结合当地长期(1998-2015年)降水的分布特征，如果以后当地0-5cm的降水频率持续增加，混交林中两个树种的水分竞争会增加；如果>30mm的频率明显增加，混交林中两个树种的水分竞争会减弱。研究结果能够为当地后续“退耕还林”工程混交林中树种的选择以及群落的管理提供科学依据。