# How Environmental Factors Affect Forest Fire Occurrence in Yunnan Forest Region
## 4 Discussion  
Based on previous studies, two main driving mechanisms are responsible for igniting general forest fires [18; 51]: one is drought, which generally triggers forest fires in areas with a copious and spatially continuous accumulation of combustible substance, and the other is the fire-source mechanism, which involves ignition of a combustible substance by other means, thus triggering a forest fire [39; 52; 53]. The latter mainly occurs in ecosystems with relatively little accumulation of combustible substance and relatively low productivity [54; 55]. The climatic characteristics of Yunnan are mainly tropical and subtropical, which are more consistent with the drought-driven mechanism. The results of this study show that RH\({}_{\text{min}}\) is the most important factor causing forest fire, followed by maximum temperature, whereas other factors, such as GPP and altitude, make limited contributions (Figure 3). On an interannual scale, RH\({}_{\text{min}}\) is also an important factor that causes forest fires throughout the year; the second most important interannual factor is GPP (Figure 4). In addition, the importance of RH\({}_{\text{min}}\) undergoes a significant interannual variation, which indicates that other factors, especially vegetation cover and the accumulation of combustible substance, were relatively more variable over the time interval studied [56]. According to previous studies, most of the forest fires in the study were anthropogenic [57], suggesting that most of the farmland and forest in Southern China coexist in the form of a mosaic, with this mixed landscape being particularly common in mountainous areas. Forest fires due to agricultural activities were very common [58], and agricultural activities in Yunnan were also the main cause of forest fires during this study interval, especially at the peak of the farming season (April to June). GPP becomes the second-most-important factor affecting the occurrence of forest fire in April and May and surpasses RH\({}_{\text{min}}\) as the most important factor in June. April to June is the growth period of most vegetation, so the combustible load increases during this period. The significantly higher summertime maximum temperature then makes forest fires easy to trigger. Traditional festivals, such as the Qingming Festival in China, which involves burning incense at graves, can then lead to numerous forest fires. In months other than June (i.e., January to May), RH\({}_{\text{min}}\) is the main factor affecting the occurrence of forest fire, which indicates that the combustible substance in the region was suffering from a prolonged drought, thus establishing environmental conditions ideal for forest fires. Figure 7: **(a)** Ranking of RH\({}_{\text{min}}\) thresholds by county. **(b)** Ranking of partial derivatives of RH\({}_{\text{min}}\) by county. **(c)** Ranking of T\({}_{\text{max}}\) thresholds by county. **(d)** Ranking of partial derivatives of T\({}_{\text{max}}\) by county. **(e)** Ranking of altitude thresholds by county. **(f)** Ranking of partial derivatives of altitude by county. At the county level, \(\mathrm{RH_{min}}\) and \(\mathrm{T_{max}}\) may play a spatially complementary role in determining the exact location of forest fires (Figure 6). With the influx into Yunnan of monsoons from the Western Pacific Ocean and the Indian Ocean, \(\mathrm{RH_{min}}\) increases significantly [59; 60], and the relative importance of \(\mathrm{T_{max}}\) becomes more pronounced in Central and Southwestern Yunnan, where frequent droughts occur [61; 62]. Meanwhile, the threshold of \(\mathrm{RH_{min}}\) decreases upon going from Northwest Yunnan to Southeast Yunnan, corresponding to the path of monsoons. Given the change in the threshold gradient, the load of vegetation and other combustible substance in this area is relatively low, therefore, drier climatic conditions are required to initiate forest fires, and the initiation of forest fire depends more on the combustibility level of the combustible substance.