## 4 Discussion  
The possible reason why landslides and collapses cause more serious damage is that large plants with greater productivity are more able to grow in areas with relatively dense soil structures. As to the stronger recovery of vegetation in debris flow zones, by checking the relevant land cover database, it was found that the major vegetation types in the five debris-flow gullies are shrub and grassland [27]. It is difficult for large plants to take root in a short time in areas where the soil was loosened after debris flow [41], while herbs and shrubs with short growth cycles could grow rapidly in the debris flow regions. Previous geological and topographical studies provide a theoretical basis for the conclusion of this research. The Jiuzhaigou earthquake-induced debris flow and collapse were characterized by short duration and rapid occurrence [24]. In contrast, the coseismic landslide was related to the seismic susceptibility of ground surface and valley morphology affected by paleo-glacier and river incision [42]; the affected area was large, and the duration was long. Hence, to some extent, it could be preliminarily assumed that landslides caused greater damage to vegetation, and vegetation in debris flow zones showed a stronger recovery after the quake. In recent years, little attention was paid to the relationship between NPP and earthquakes. However, as an indicator of respiration and photosynthesis of ecological vegetation, vegetation gross primary productivity (GPP), comprised of autotrophic respiration of plants and NPP, was combined with MODIS data, and it was found that the M\({}_{8}\)8.0 Wenchuan earthquake in 2008 caused enormous damage to vegetation and GPP of the severely affected area dropped by 22% but then recovered [43]. Nine years later, the M\({}_{8}\)7.0 Jiuzhaigou \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Area Proportion (\%)**} \\ \cline{2-5}
**Seismic** & \multicolumn{2}{c}{**Dur-Quake**} & \multicolumn{2}{c}{**Post-Quake**} \\ \cline{2-5}
**Intensity** & \(\Delta\)_NPP_ **\(<\)0** & \(\Delta\)_NPP_ **\(>\)0** & \(\Delta\)_NPP_ **\(<\)0** & \(\Delta\)_NPP_ **\(>\)0** \\  & **gC/m\({}^{2}\)** & **gC/m\({}^{2}\)** & **gC/m\({}^{2}\)** & **gC/m\({}^{2}\)** \\ \hline VII-VIII & 82.79 & 17.21 & 45.74 & 54.26 \\ VIII-IX & 94.84 & 5.16 & 19.78 & 80.22 \\ IX+ & 98.98 & 1.02 & 6.88 & 93.12 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The \(\Delta\)_NPP_ area proportion corresponding to three seismic intensities one month before and after the earthquake. earthquake triggered various geohazards, which destroyed ground surface plants and decreased NPP value by around 23%. Fortunately, the vegetation in debris-flow gullies and landslide-and-collapse areas after the earthquake was restored despite the aftershocks, and the NPP value returned to a nearly normal level after one year. Furthermore, as a strong dynamic indicator of terrestrial vegetation coverage [44], NDVI also reflected the changing laws of vegetation before and after the earthquake and was similar to NPP to a certain extent. During the Wenchuan earthquake, the NDVI value showed a sharp decrease in areas with a previously high vegetation coverage, which was attributed to emergency events, natural succession or human activities [45]; GPP and leaf area index (LAI) (a crucial parameter of plant photosynthetic model) fluctuated and showed an overall upward trend, and the diversity of plant species was markedly increased correspondingly [46]. From the perspective of intuitive representation, the degree of vegetation destruction was mainly determined by the scale of geohazards. During the natural calamity, the ecosystem showed massive allogenic succession besides autogenic succession.