【作者】 王力；

【导师】 邵明安； 侯庆春；

【作者基本信息】 西北农林科技大学 ， 土壤学， 2002， 博士

【摘要】 黄土高原作为我国西部开发整体格局中的重要组成部分，恢复其脆弱的生态环境是维系这一地区可持续发展的最基本要素。黄土高原生态环境的建设，退耕还林还草是切入点，由于地处于干旱半干旱区，水分则是制约该区植被恢复和生态重建的主要限制因子。因此，研究黄土高原的土壤水分生态环境问题，对于该区的林草植被的建设和生态环境的重建工作具有十分重要的意义。 本论文主要针对黄土高原土壤水分严重亏缺现状以及因之而形成的土壤干化和林草植被衰退现象，主要对以下几方面的问题进行了研究：(1)土壤水分的意义及其在生态系统水分循环过程中的作用；(2)林草植被土壤水分亏缺状况及其对植被生长的影响；(3)土壤干层的涵义及成因分析；(4)陕北黄土高原土壤干层的分布及特征；(5)土壤干层的量化指标初步研究。 经过两年多的野外实地考察和室内分析，取得了如下主要研究结果： 1 土壤水分是指贮存在土壤中一定深度范围内能被植物吸收利用的土壤水含量，土壤水分的有效性对植物的生长发育起着最为关键的作用，因此，土壤水分应属于水资源的范畴，是水资源的重要组成部分。降水是黄土高原地区土壤水分的唯一来源，土壤水分在生态系统的水分循环过程中起着中枢的作用。 2 天然林地和人工林地土壤水分亏缺状况对比：(1)提出了“林地土壤稳定持水量”的概念，初步定义为“不会影响到林分正常生长、发育，不会导致群落产生逆向演替所需的最低土壤含水量”。(2)天然林与人工林地土壤水分状况存在明显的差异。总的来说，天然林地土壤水分条件优于人工林地。以“林地稳定持水量”为依据，天然林地水分亏缺不严重，只在浅层形成了轻度亏缺，并在雨季后能得到及时的恢复；同时这种亏缺并没有影响到了天然植被的发育与演替。人工林地土壤水分则亏缺明显，深层亏缺尤为严重，已接近凋萎湿度；同时，这种水分亏缺严重地影响到了人工林的生存和发展，部分林地已出现了明显的衰退迹象。(3)在人工林地，因水分亏缺形成了严重程度不同的干化层。(4)富县天然山杨林和天然辽东栎林的景观生态特征表明，两类森林群落没有出现衰退的迹象，这说明森林群落合理的内部结构不会造成林地土壤水分的严重亏缺，其自然气候条件可以满足森林群落生存、发展的需要。当前人工林地的土壤水分出现严重亏缺而形成土壤干化层并导致人工林衰退是人为不合理经营造成人工林群落林分结构不完善的结果，如果人为干预适当，能创造出类似于天然植被的生存条件，人工植被完全有可能得到正常的生存和发展，出现衰退的人工刺槐林完全有可能得良好的恢复。 3 土壤水分对林分生长的影响：(1)土壤水分亏缺严重制约着树高、胸径和材积的生长。具体表现为，从富县开始，人工刺槐林地下都存在不同程度的土壤干化现象，且从南到北渐趋严重。这一趋势与人工刺槐林的生长状况紧密地结合在一起，即从南到北，人工刺槐林的生长状况受土壤水分的影响也渐趋恶化。(2)降雨量不足，干旱缺水是黄土高原丘陵沟壑区低效低产林形成的主要原因。(3)土壤干化层的形成，是林木生长和气候干旱双重作用的结果，而人为高密度粗放经营，不合理的种植方式亦加剧了土壤的旱化，造成了水分长期亏损的累积，且随着亏损程丝王力博士学位论文:陕北黄土高原土壤水分亏缺状况与林木生长关系里口典口目旦口目里里里里里里里里度的加剧，干化层逐渐上移。同时，干化层的存在，严重地阻碍了植物对深层土壤水分的利用，使现有植被仅能利用当年的降雨而生存，从而使林分完全处于干旱气候的胁迫之下。(4)除了降雨量之外，栽植密度也是影响林木生长的重要因素。(5)在陕北黄土高原，除南部的宜君和富县之外，北部的吴旗、安塞、绥德、米脂等地人工刺槐林生长极为缓慢，经济效益甚微，在改善生态环境和保持水土方面所起的作用也很低，严重地影响了黄土高原地区的植被建设和可持续发展，并损害群众造林的积极性。同时表明，这些地方不适合刺槐这类耗水性较强的植物大面积生长。 4土壤干层的危害:(l)土壤干层的存在使局部小气候环境趋于旱化。(2)土壤干层导致了土地的退化。(3)土壤干层使植被生长衰退，形成大面积的低效低产林。(4)严重土壤干层使天然下种更新不良。(5)土壤干层使衰败后的林草地重新造林难度更大。 5土壤干层的成因:(l)气候干早，降水减少。(2)土壤水分物理性能。(3)严重的水土流失。(4)植被类型选择失当。(5)群落生产力过高，群落密度过大。 6土壤干层的分布及特征:(l)土壤干层在黄土高原从南到北大范围内普遍分布，根据干化程度可分为四个类型区:①以宜君为代表的高原沟壑区南部，主要包括子午岭、黄龙山的大部分地区，土壤干化程度轻微，如遇丰水年或辅以适当的整地节水措施，土壤水分亏缺可以得到有效缓解;②以富县、黄陵为代表的高原沟壑区北部，出现了轻度干化层，有的地方还相对较为严重，并有继续恶化的趋势，应引起足够的重视;③范围较广的丘陵沟壑区，出现了程度严重的土壤干化层，该区又可分为南、西、北三个小区域，南区以延安、延长为代表，土壤干化程度为中度;更多还原

【Abstract】 Loess plateau is a very important part of west development of China, and to restore its fragile ecological environment is the fundamental factor for keeping its sustainable development. As for eco-environment construction of Loess Plateau, the key point is to return farmland to forest and grassland. Since it is located at a semi-arid and arid climate region, water is main factor that refrains the vegetation recovery and eco-reconstruction. Therefore, it is very significant to study soil water eco-environment, which will be essential for eco-environment restoration.Based on the serious shortage of soil moisture, soil aridity and vegetation recession in this region, the following issues are studied in this dissertation: (1) the meaning of soil water and its role in the water circulation; (2) soil water deficit and its influence to vegetation growth; (3) the definition of soil dried layer and the causes of its formation; (4) soil dried layer distribution and characteristics in north of Shaanxi loess plateau; (5) the primary study of measurable index of soil dried layer.The main results are as follows based on field survey and analysis:1 Soil water is the water stored in the soil that can be absorbed by plants in some depth of soil, and the efficiency of soil water plays the critical role in plant growth and development. So soil water is the resource that is very crucial to vegetation. In addition, Rainfall is the only source of soil water in loess plateau that has the key function in the water circulation.2 The comparisons between soil water deficiency in natural and artificial forest land show that: (1) the concept of "forestland steady moisture capacity" is presented and defined as the lowest soil water content that can not affect the forest regular growth primarily and cause inverse succession in community. (2) There are significant differences of soil water contents between natural and artificial forestland. In general, soil moisture condition is better for the former. According to "forestland steady moisture capacity", water deficiency in the former is only in surface and recovers timely after rain season. But in artificial forestland, there is an apparent shortage especially in deep layer, so it affectsgrowth and development of vegetation. (3) In artificial forestland, soil dried layer has been formed because of water deficit. In 0-200cm light dried layer was formed and can be recovered partly after rainfall, but beneath 200cm the serious perpetual dried layer has lower water content and larger thickness and is very difficult to be recovered in short period because of the special hydrological characteristics in the Loess Plateau where infiltration depth is about 200cm and could not percolate into deeper layer. (4) The landscape ecology in Fuxian County shows that the two kinds of forest community have not declined. It points out that the rational and stable inner structure cannot lead to serious soil moisture shortage; its natural climate condition can meet the needs of forest community. It is the irrational management that causes the serious soil moisture shortage in artificial forestry land. If we intervene properly and create some growth condition similar to natural vegetation, the recessed artificial Robinniapseudoscacia can restore benignly.3 The influence of water on vegetation growth: (1) soil water shortage refrains the height, basal diameter and stock of trees. The artificial Robinnia pseudoscacia forest growth is becoming worse and worse as the soil moisture content decreases from south to north from Fuxian County. (2) Little precipitation and water shortage is the main reason of low efficiency and productivity forest in hill and gully area of Loess Plateau. (3) Soil dried layer is the dual result of forest growth and arid climate, that is high density and extensive irrational cultivation accelerate the soil aridity. The more the water shortage, the thicker the dried layer. The dried layer baffles the deeper soil water deficiency, and the vegetation can only live on ye更多还原