摘要
黄土高原是我国生态系统退化和土壤侵蚀状况最严重的地区,植被的恢复与重建是控制和改善该区域水土流失现状、减轻土壤侵蚀程度的关键。种子作为种子植物的繁殖体是植被恢复的基础,早期幼苗阶段是植物生活史中对环境条件反映最为敏感的时期,种子和幼苗阶段对植被的的恢复产生着重要的影响。土壤侵蚀不仅造成土壤颗粒与养分的流失,同时通过地表径流和泥沙输移也会带走坡面上散落的种子,并且加剧土壤的干旱,恶化幼苗的生长环境。为此,本研究采用人工模拟降雨试验,研究黄土高原地区典型代表物种的种子在不同降雨强度、不同下垫面条件下的迁移与流失特征、不同形态种子的流失特征、不同雨型对刚萌发的幼苗的破坏能力、以及土壤侵蚀对幼苗建植的影响,主要得出以下结论:
1)在降雨过程中,不同下垫面条件下种子流失率随着降雨强度的增大而增加,在裸坡的流失率增加趋势最快,具草被的坡面和具草被+蹄印的坡面流失率增加趋势较小且二者相差不大。降雨强度越大,在裸坡上种子流失率越接近迁移率,而在具草被坡面和具草被+蹄印的坡面上这种规律不明显,说明草被和蹄印的存在影响着种子迁移与流失,对于缓解由降雨强度的增加而引起的流失率增加有明显的积极作用。坡面上草被和蹄印的存在对坡面上的种子具有一定的拦截效益,草被的拦截效益一般在10-20%之间,蹄印的拦截效益在10%以下,二者的拦截效益随降雨强度的变化不明显。
2) 60物种的种子在降雨过程中均发生了迁移,不同物种种子迁移率变化在3.33-100%,其中93.33%的物种种子迁移率超过50%,61.67%的物种种子迁移率达到100%。91.67%的种子发生了流失,只有黄刺枚、酸枣、扁核木、魁蓟和大针茅5个物种流失率为0,流失率达到100%的物种包括地黄、灰叶黄耆、委陵菜和猪毛菜,为最容易流失的物种。种子在坡面的迁移方式包括滚动式、悬浮式和跳跃式三种,其中大部分种子都以跳跃式迁移。
3)容易发生流失的种子特征主要包括种子重量低于10mg,种子体积低于30mm3,种子为近球形,种子表面光滑和无附属物等。具有抵抗土壤侵蚀的种子特征主要有种子重量大于150mg,种子遇水后能够分泌粘液,狭长型或扁平状的种子,种子表面结构粗糙,以及具有芒、刺、冠毛、翅等附属物的种子。
4)在微度土壤侵蚀的条件下,40天内的幼苗受降雨破坏情况明显,而40天以后的幼苗在降雨过程中均没有发生数量上的变化,说明幼苗生长到40天以后就具备了抵抗降雨侵蚀破坏的能力。狼牙刺、白羊草和猪毛蒿3个物种的幼苗在生长初期(20天)数量分别为末期(120天)的1.08倍、1.42倍和3.62倍。
5)雨量相同的条件下,短历时高雨强(150mm/h历时20min)和长历时低雨强(25mm/h历时120min)条件下产生的累积土壤侵蚀量均较大,相应的降雨条件下造成的猪毛蒿和白羊草的幼苗死亡率也较大,且长历时低雨强造成的幼苗死亡率更大。幼苗遭受降雨破坏的主要方式包括连根拔出、打倒和流失。
6)降雨过程引起的土壤侵蚀在植物发育过程中产生着持续的影响。土壤侵蚀不仅通过产生地表径流和泥沙对存在于地表的种子进行挟带和输移,造成种子的流失,减少可萌发种子的数量;部分种子还会由于淤积作用造成埋藏深度增加,不利于幼苗出土成活;同时土壤侵蚀还通过降低土壤养分,减少土壤水分含量等因素恶化种子所需的萌发条件,抑制种子的萌发;当部分种子萌发成功后,土壤侵蚀通过雨滴打击、泥沙输移等方式直接或间接的破坏幼苗及其生长环境,导致一部分幼苗死亡,减少物种的成活率;已成活的幼苗也会遭受退化侵蚀环境和种内种间的竞争等因素的影响,也会有一部分幼苗死亡;在经历土壤侵蚀这一系列的影响之后仍然在坡面存活的幼苗,最终才能够建植成功。
The Loess Plateau is a area with degraded ecosystem and serious soil erosion in China. Vegetation restoration and reconstruction is the key factor to control and reduce the extent of soil erosion in the region. As propagules of seed plants, seeds is the basis for vegetation restoration. Early seedling stage is most sensitive to environmental conditions in plant life cycle, so seed and seedling stage have important influences to vegetation restoration. Not only soil particles and nutrients can be lost by soil erosion, surface runoff and sediment transport also can carry seeds and intensify soil drought, deteriorate the seedling living environment. Thus, simulated rainfall experiments were carried out for analyzing the features of seed loss and movement under different rainfall intensities, underlying surfaces and seed morphological characteristics, the characteristics of seedlings destruction by different rainfall types, and the influence of soil erosion in seedlings establishment. Main results are as follows:
1) Under the different underlying surfaces, seed loss rate increased while the rainfall intensity increased during the rainfall events, seed loss rate increased fast on the bare slope, but slower on slopes with vegetation and hoof prints. Seed loss rate was close to seed displacement rate on the bare slope, but not the result on the slopes with vegetation and hoof prints. It suggested that vegetation and hoof print have positive impact on seed loss and movement while rainfall intensity increase. Vegetation and hoof print could intercept seed on the slopes, and the intercept benefit of vegetation and hoof print was between 10-20%, and less than 10% separately , and it was stable with rainfall intensity changes.
2) All the seeds forn 60 species on the slopes translocated during the rainfall process, and the seed displacement rate was between 3.33% and 100%. 93.33% of all the species had seed displacement rate more than 50%, and 61.67% of all the species reached 100%. 91.67% of seeds in all the species lost in the rainfall experiments, only Rosa xanthina, Ziziphus jujube, Prinsepia utilis, Cirsium leo and Stipa grandis did not lost in any rainfall events, and the seeds of Rehmannia glutinosa, Astragalus discolor, Potentilla Chinensis and Salsola ruthenica wete all lost out of the slopes. There were three types of seed removal on slope, i.e., rolling, suspension and jumping, and most seeds moved with jumping.
3) Seed characterastics which prone to loss mainly included: seed weight less than 10mg, seed volume less than 30mm3, spherical shape, smoothly surface, and no appendages. Seed features which hard to loss mainly included: seed weight more than 150mg, segregating mucilage when contact with water, long-narrow and flat shapes, rough surface, and having appendages as hairs, wings awns and so on.
4) In the conditions of weak erosion, seedlings within 40 days damaged by the rainfall most obviously, while the amount of seedlings which was older than 40 days had no changes after the rainfall event. It indicated that the seedlings which was older than 40 days have the ability to resist erosion damage. The amount of Sophora davidii, Bothriochloa ischaemun and Artemisia scoparia in early phases(20 days)were 1.08, 1.42 and 3.62 times higher than that in the ending phases(120 days), separately.
5) Extreme rainfall with short duration and high intensity(150mm/h lasted for 20min) and rainfall with long duration and low intensity(25mm/h lasted for 120min) led to highest soil erosion amount; and high death rate of Artemisia scoparia and Bothriochloa ischaemun were occurred corresponding to the same rainfall conditions, and even higher in thr rainfall with the long duration and low rainfall intensity. There mainly were three ways of seedling destory by rainfall: pull out of roots, beat down and lost.
6) The sustained effects on the development of vegetation by soil erosion are lasted in the whole process directly and indirectly. Soil erosion can move the seeds on the slope by runoff and sediment transport and reduce the amount of germination seed; Part of the seedlings are hardly exposed to the soil surface because the seed may be buried deeper by sedimentation effects; Soil erosion can also deterioration the environment for seed germination by reducing soil nutrients and soil moisture content, leading to lower survival rate; Whenever part of the seeds which germinated successfully, the seedlings would be destroyed by raindrop hitting, sediment transport, leading to lower survival rate; Some of temporary survived seedlings would also be affected by deteriorated erosion environment conditions and intraspecific and interspecific competition factors; The seedlings which still living on the slope finally is established successful after these sustained effects by soil erosion.
引文
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