科尔沁沙地典型固沙人工林地土壤水分时空特征及其对环境因子的响应

Abstract

土壤水分时空动态特征对于干旱地区人工林的可持续经营与管理起着至关重要的作用。以位于科尔沁沙地南缘的樟子松和柠条固沙人工林为对象，于2018年11月-2019年11月连续观测了林地0-200 cm土壤剖面的含水量、温度及微气象因子，系统分析了土壤水分的时空变化特征及其对环境因子的响应。研究期内，两种林地土壤水分的季节变化可分为冻结期、补充期、消耗期和稳定期；依据土壤剖面的水分特征可分为易变层、活跃层和稳定层，但两种林地的分层深度有一定差异。在生长季内（5-10月），土壤含水量对大气降雨的响应随着土层深度的增加而减弱；降雨对樟子松人工林0-20 cm层土壤水分的影响极显著（P<0.01），对柠条人工林0-10 cm层的影响极显著（P<0.01）、20-60 cm层显著（P<0.05）。在土壤冻融周期内（2018年11月-2019年4月），两种林地的土壤均表现为"单向冻结"和"双向融化"的特点；土壤温度是影响冻融期内土壤含水量的关键因素，两者呈极显著的指数函数关系；樟子松和柠条人工林土壤的最大冻结深度分别为170 cm和190 cm，前者10 cm土层解冻时间要比后者晚11 d，可能与乔木树冠的遮阴作用有关。潜在蒸散与柠条林0-60 cm层、樟子松林0-20 cm和200 cm层的土壤水分呈极显著相关（P<0.01），而与樟子松林60 cm和160 cm层呈显著相关（P<0.05），这与树木蒸腾和土壤蒸发等综合作用有关。研究表明，由于两种人工林的树种组成、树冠大小、郁闭程度和根系分布等结构特征不同会导致林地土壤水分时空特征的异质性及其对环境因素响应的差异。;Soil moisture dynamics is essential for the sustainable management of artificial forests in dryland regions. In order to analyze the spatiotemporal characteristics of soil moisture and its responses to environmental factors, soil volumetric water content (SVWC), soil temperature (ST), and micro-meteorological factors were continuously measured in the Pinus sylvestris var. mongolica plantation and Caragana korshinskii plantation stands from November 2018 to November 2019 at the south edge of Horqin Sandy Land. The results showed that the seasonal changes of SVWC in the two stands could be divided into freezing-, replenishment-, consumption-, and stable-phases during the study period. The vertical patterns of SVWC across 0-200 cm soil profile could be classified by variable-, active-and stable-layers. But there was difference in the depth of stratification between the two stands. During the growing season (from May to October), the sensitivity of responses of SVWC to precipitation tended to decrease with increasing soil depth. The influence of rainfall on SVWC at the 0-20 cm layer of the pine stand was significant (P<0.01), while they were significant for the 0-10 cm layer (P<0.01) and for the 20-60 cm layer (P<0.05) in the C. korshinskii stand, respectively. The soil freeze-thaw cycle (from November 2018 to April 2019) was characterized by unidirectional freezing and bidirectional thawing in the two stands. The SVWC increased exponentially with ST in this period. The maximum of freezing depth of soil profile in the pine and the C. korshinskii stands were 170 cm and 190 cm, respectively; while initiative time of soil thawing at the 10 cm layer in the pine stand was 11 d later than that in the C. korshinskii stand, which may be attributed to shading effect of tree crown. The SVWC at 0-60 cm layer in the C. korshinskii plantation and at 0-20 cm and 200 cm layers in the pine stand were negatively correlated potential evapotranspiration (ET₀) (P<0.01), while SVWC at the 60 cm and 160 cm layers in the pine stand was negatively correlated to ET₀ (P<0.05), which may be a result from the combining effects of soil evaporation and tree transpiration on SVWC in the two stands. Our results suggest that the spatiotemporal characteristics of soil moisture and their responses to the environmental factors may be ascribed to the difference in the tree composition, canopy structure, and root distribution between the two stands. These findings have potential implications for a better understanding of the influences of the structure of plantations on soil moisture dynamics, and they thus benefit artificial forests management and ecological restoration in the water-limited environments.