## 3 Results and Analyses
### NPP Change Attributed to Geohazards  
Geohazards in gullies and valleys can further cause vegetation destruction. Geohazards are classified in terms of their causes. Landslide is attributed to the instability of rock-soil mass of a high-steep slope and overall sliding along the weak surface. Debris flow is caused by heavy rainfall, after which the slope runoff carries the loose soil with large pores and gradually converges into a flood with fast speed and large flow. Collapse \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**NPP Value (gC/m\({}^{2}\))** & & **2015** & **2016** & **2017** & **2018** & **2019** \\ \hline \multirow{4}{*}{\(\text{NPP}_{\text{max}}\)} & July & 260.3 & 239.1 & 259.3 & 255.3 & 231.3 \\  & August & 250.9 & 209.2 & 199.9 & 211.9 & 261.9 \\ \cline{1-1}  & September & 151.5 & 167.7 & 202.7 & 165.5 & 218.2 \\ \cline{1-1}  & July & 116.3 & 107.0 & 111.8 & 112.8 & 102.4 \\ \cline{1-1}  & August & 105.1 & 92.1 & 86.2 & 92.7 & 115.8 \\ \cline{1-1}  & September & 62.8 & 69.7 & 83.6 & 54.6 & 89.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The estimated values of NPP based on the CASA model in the study region in July, August and September 2015 to 2019.  
is a rolling phenomenon that often occurs on steep slopes or reverse slopes. Based on the classification above, 128 geohazards identified by field survey that caused damage to vegetation were divided into landslides, collapses and debris flows.In light of the area where geohazards occurred, landslide and collapse were integrated, and debris flows mainly occurred in Shuzheng Gully, Zharu Gully, Heye Gully, Zezhawa Gully and Xiajijiehai Gully. By combining the DEM data, the areas affected by representative geohazards are shown in Figure 7.  
The study shows that the area affected by debris flow is larger than those affected by collapse and coseismic landslide. According to the land cover database, the land cover type in five debris-flow gullies in the study region was mostly needle-leaved evergreen forest with a small proportion of alpine and sub-alpine meadow and plain grassland in Zharu Gully. The land cover type in areas affected by collapse and coseismic landslide was the needle-leaved evergreen forest. In addition, the land cover type in the central and northern areas was needle-leaved evergreen forest, where the NPP value in the same year shows a rising tendency with time. The mensurable change of NPP in geohazard zones directly manifested the influence of the earthquake on these zones. Therefore, the law of NPP\({}_{\text{mean}}\) change caused by the three representative geohazards from 2015 to 2019 was analyzed (Figure 8). Figure 7: The areas affected by geohazards, including landslides, debris flows and collapses and the scenes of typical earthquake-induced geohazards in Jiuzhaigou Valley. From Figure 8, the NPP\({}_{\text{mean}}\) values in areas hit by landslides, debris flows, and collapses were 59.7-105.9 gC/m\({}^{2}\), 56.6-100.1 gC/m\({}^{2}\), and 63.7-100.2 gC/m\({}^{2}\), respectively. Before the earthquake, the NPP\({}_{\text{mean}}\) values in geohazard zones in the same month had little difference (within 10 gC/m\({}^{2}\)). Additionally, the NPP\({}_{\text{mean}}\) values of landslide and collapse zones were generally higher than the values of debris flow zones. During the earthquake, the values sharply declined by 27.2% (landslide), 22.4% (debris flow) and 15.7% (collapse) compared with those in the same month in 2016, and the largest decline was caused by landslides. After the earthquake, the NPP\({}_{\text{mean}}\) values began to rebound and vegetation generally recovered, and the values became close again during the study period. The vegetation in debris flow zones showed a stronger recovery than the vegetation in landslide and collapse zones, with a maximum NPP\({}_{\text{mean}}\) increase of about 23.0% compared with the corresponding value in August 2017.