南方红壤区植被结构类型与降雨模式对林下水土流失的影响

植被和降雨是水土流失的关键因素,探究二者对水土流失的影响对开展水土保持具有重要意义。该研究基于鹰潭红壤生态试验站5种植被结构类型的径流小区2016-2018年93次降雨、径流、泥沙观测资料以及各小区植被结构参数,利用自组织映射(self-organizing maps,SOM)方法,根据雨量、历时、60 min最大雨强、平均雨强、降雨集中性等特征指标划分降雨模式,研究了不同降雨模式和植被结构类型的水土流失特征,并采用冗余分析(RDA)定量研究降雨与植被对林下水土流失的影响。结果表明,SOM方法能客观识别红壤区4种典型侵蚀降雨模式,R_Ⅲ模式(短历时、大雨强、雨量集中)是造成水土流失的主要降雨模式,R_Ⅳ模式(多雨量、大雨强、长历时)最具侵蚀性破坏力;植被结构类型显著影响水土流失,水土保持功能从大到小依次为:灌草混交林、草地、低灌林、乔木林、高灌林。RDA分析表明,降雨模式与植被结构类型能够改变降雨、植被对水土流失的影响,随着降雨模式由弱到强转变,植被的水土保持功能逐渐减小,降雨影响增强,水土流失由植被主控演变为平衡控制、降雨主控;随着植被结构类型由近地表植被向灌木、乔木的变化,植被的调节能力减小,降雨影响增强。

Effects of vegetation structure types and rainfall patterns on soil and water loss of understory vegetation in red soil areas of South China

Vegetation and rainfall are the key factors that affect the soil and soil erosion. Exploring the surface runoff and soil loss process associated with different rainfall patterns and vegetation structure types is of great significance to the development of soil and water conservation. In this study, we carried out a series of experiments on surface runoffs and sediments monitoring at different runoff plots with five structures types of vegetation(i.e., shrub-grass mixed forest, grassland, low shrub, arbor forest, high shrub). There are 8 runoff test plots with length × width(10 m×2.6 m) on the gentle slope of 8° in Red soil Ecological Experimental Station, Yingtan city. The experiment lasted three years from 2016 to 2018. During the study period, 93 individual natural erosive rainfall events, surface runoffs and sediments data were collected, as well as the vegetation structure parameters were obtained. The self-organizing mapping(SOM) method was applied to classify rainfall patterns according to rainfall characteristic indicators such as rainfall amounts, rainfall durations, 60 min maximum rainfall intensity, average rainfall intensity and rainfall amount concentration index. Then the characteristics of soil and water loss under different rainfall patterns and vegetation structure types were studied, and the effect of rainfall and vegetation on soil and water loss were quantitative analysis by redundancy analysis(RDA). The results showed that SOM method could identify the four typical erosion rainfall patterns in red soil region, the rainfall patterns were as follows: RI(low intensity, long duration, minor amount of rainfall and low rainfall concentration index), R_Ⅱ(medium amount of rainfall, moderate intensity and long duration), R_Ⅲ(short duration, strong rainfall intensity and high rainfall concentration index), R_Ⅳ(a great amount of rainfall, strong rainfall intensity and long duration). Among all of the erosion rainfall events, the least rainfall was pattern R_Ⅳ, which was the most erosive and destructive, so it was easy to cause serious soil erosion. R_Ⅲ was the main pattern that made the highest cumulative contribution rate to soil and water loss. The erosion ability of pattern R_Ⅱ was moderate, but it was also easy to cause soil and water loss due to its moderate intensity and long duration. The pattern RI could hardly cause soil erosion. Vegetation structure types significantly affected the soil erosion and runoff generation, and the soil and water conservation function of different vegetation in turn was as followed the order of shrub-grass mixed forest, grassland, low shrub, arbor forest, and high shrub. RDA results shown that comprehensive interaction effects on soil erosion exist between rainfall patterns and vegetation structure types. The ability of soil and water conservation of vegetation decreased gradually and the effects of rainfall became greater in the later stage of erosion process, when rainfall patterns changed from weak to strong(R_Ⅰ→R_Ⅱ→R_Ⅲ→R_Ⅳ), meanwhile the dominant factors of soil and water loss changed from vegetation to the interaction of vegetation and precipitation, and then to rainfalls. The influences of canopy structure on soil and water loss reduced, while the ones of rainfall increased when vegetation structure types changed from near surface vegetation to shrub and tree.