## 3 Results and Discussion
### Temporal Analysis of Post-Fire Vegetation Trajectory  
#### 3.2.1 Monthly Dynamics of Post-Fire Vegetation Trajectory  
The relationship between pre-fire (1986) and fire year (1987) NDVI values is illustrated in a series of scatter plots (Figure 5). The location of the cloud of points shows a large shift away from the 1:1 line when the fire occurred in May and a positive recovery trend after the fire. From January to April (Figure 5a-d), the cloud of points is close to the 1:1 line, which indicates the NDVI values were generally equal during periods without fire disturbances in both years. However, Figure 5e shows that there was a sharp decrease when the fire occurred in May 1987. Following the fire event, there was a rapid increase from June to August (Figure 5f-h), which was likely the NDVI response of the understory vegetation (e.g., herbaceous and shrubs) to temperature or precipitation variability [34, 35]. From September to November, the Great Xing'an Mountain entered autumn; NDVI values decreased in both  
Figure 4: Fire sources and fire damage classes: Very high (VH), High (H), Moderate (M), Low (L) and Slight (S). The base map image is a composition of two Landsat 5 TM images from June 1987 (false color composite bands: red-band 7; green-band 4; blue-band 1). Beryl green grids were chose to track the detail vegetation signals using Landsat images with 30 m spatial resolution.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Fire Source** & **Name** & **Longitude** & **Latitude** & **Burned Area** & **Ignition Reason** \\ \hline
**Source-1** & Gulian & 122 \(\,\)’22’ & 52 \(\,\)’26’ & 38 \(\times\,\)10\({}^{4}\) ha & Electric spark \\
**Source-2** & Hewan & 122 \(\,\)’21’ & 53 \(\,\)’11’ & 33.8 \(\times\,\)10\({}^{4}\) ha & Smoking \\
**Source-3** & Pangu & 123 \(\,\)’43’ & 52 \(\,\)’45’ & 28 \(\times\,\)10\({}^{4}\) ha & Unknown \\
**Source-4** & Xingan & 122 \(\,\)’22’ & 122 \(\,\)’22’ & 616 ha & Smoking \\
**Source-5** & Yixi & 123 \(\,\)’25’ & 53 \(\,\)’05’ & 587 ha & Electric spark \\ \hline \end{tabular}
\end{table}
Table 3: Five sources of the 1987 large fire related to Figure 4.  
years with the spread of colored foliage and the percentage of fallen leaves increasing. Regardless, Figure 5j also shows clear NDVI departures when post- and pre-fire conditions are compared. This is because colored leaves fall from grasses and shrubs while the tree leaves remain green in October. The difference in NDVI values appeared again in this month. Therefore, October was a better than the others to distinguish post- and pre-fire vegetation conditions using NDVI signals. Thereafter, this region was covered by snow from December to the following February. During this period, it is difficult to find evidence of fire effects on vegetation from NDVI values due to the values reaching a minimum for the entire seasonal cycle.  
Figure 5: Monthly scatterplots of post-fire normalized difference vegetation index (NDVI) (1987) _versus_ pre-fire NDVI (1986) ((**a**) January, (**b**) February, (**c**) March, (**d**) April, (**e**) May, (**f**) June, (**g**) July, (**h**) August, (**i**) September, (**j**) October, (**k**) November, (**l**) December) for the large fire on May 1987.  
Shortly after the fire event, the NDVI signal should decrease significantly due to the disappearance of green grasses and shrubs before rapidly increasing again in the following months because of the re-growth of understory vegetation occurs and the phenology trajectory effect. Therefore, to avoid the phenology effect we further compared monthly NDVI trajectories of the normal year (without fire) and fire year (Figure 6). The trajectory of black points (average NDVI values for each month) is expected to be distributed along the 1:1 line when there is no disturbance; the monthly differences between the two normal years are very small (Figure 6a). However, for the fire year, there was a large increase in the fire month (May); most points following the fire are below the 1:1 line, indicating a pronounced decrease in greenness due to the fire.