## 2 Materials and Methods
### Selection and Sampling of Sample Plots  
In early July 2022, samples were collected from the riparian zones of the Ertan Reservoir (26\({}^{\circ}\)48\({}^{\prime}\)39\({}^{\prime\prime}\) N, 101\({}^{\circ}\)43\({}^{\prime}\)36\({}^{\prime\prime}\) E) along the Yalong River and the Guanyinyan Reservoir (26\({}^{\circ}\)26\({}^{\prime}\)01\({}^{\prime\prime}\) N, 101\({}^{\circ}\)28\({}^{\prime}\)13\({}^{\prime\prime}\) E) along the Jinsha River, respectively (Figure 1). To minimize the impact of land use, soil type, topography, geomorphology, vegetation, and human activities on the research, dry red soil and gently sloping sample strips in a near-natural state were selected. These sample strips had similar habitat characteristics in terms of slope direction, slope gradient, and land-use history. Soil samples were then collected. Before the anthropogenic clearing of the reservoir, the land-use types in the sample zone were all dry land. After inundation, the land-use types changed to natural grassland dominated by _Cynodon dactylon_. Both Ertan and Guanyinyan Reservoir fallout zones exhibit the anti-seasonal hydrological situation of \"winter water, summer land\". Based on the historical water level data from the Yangtze River Hydrological Network of the People's Republic of China ([[http://www.cjh.com.cn/swyb_syqbg.html](http://www.cjh.com.cn/swyb_syqbg.html)]([http://www.cjh.com.cn/swyb_syqbg.html](http://www.cjh.com.cn/swyb_syqbg.html)), accessed on 16 October 2023), three sampling intervals were established for the elevations in the study area. The samples at elevation L were flooded for more than 7 months, those at elevation M were flooded for 5-6 months, and those at elevation H were flooded for 3-4 months. The horizontal distance of each elevation was divided into three sections, and three sampling points were set up to obtain the fresh roots of the plant using the total excavation method. The _Cynodon dactylon_ at different elevations was then grouped and placed into labeled gauze bags (refer to Table 1) and transported back to the laboratory for preservation and subsequent processing. This process ensured the timely completion of root morphology measurements and tensile tests on _Cynodon dactylon_.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Location** & **Geographic Coordinates** & **Elevation Gradient** & **Elevation Gradient Interval** & **Number of Samples** \\ \hline Ertan & L & 1150.579\(\cdots\)1155.963 m & 131 \\ Ertan & 26\({}^{\circ}\)48\({}^{\prime}\)39\({}^{\prime\prime}\) N, 101\({}^{\circ}\)43\({}^{\prime}\)36\({}^{\prime\prime}\) E & M & 1155.963\(\cdots\)1159.279 m & 144 \\  & H & 1159.279\(\cdots\)1163.424 m & 134 \\ \hline Guanyinyan & L & 1090.180\(\cdots\)1092.934 m & 137 \\  & 26\({}^{\circ}\)26\({}^{\prime}\)01\({}^{\prime\prime}\) N, 101\({}^{\circ}\)28\({}^{\prime}\)13\({}^{\prime\prime}\) E & M & 1092.934\(\cdots\)1095.143 m & 143 \\  & H & 1095.143\(\cdots\)1097.380 m & 129 \\ \hline \end{tabular} Note: The data of elevation gradient interval are all based on the Yellow Sea elevation.  
\end{table}
Table 1: Root sampling of _Cynodon dactylon_ roots.  
Figure 1: Sampling points of Jinsha River riparian zone.