Contribution of the synergistic interaction between topography and climate variables to pine caterpillar (Dendrolimus spp.) outbreaks in Shandong Province, China

Outbreaks of the pine caterpillar (Dendrolimus spp.) have substantial impacts on coniferous forest ecosystem structure, distribution, and productivity, as well as the economy. Furthermore, there is emerging evidence that increasing frequencies of fires and windstorms driven by climate change are contributing to more severe pine caterpillar outbreaks. Therefore, there is a need to identify areas susceptible to pine caterpillar outbreaks to apply prevention measures efficiently and facilitate sustainable forest management. This study constructed an integrated index for assessing the susceptibility of forest ecosystems in Shandong Province, China to pine caterpillar outbreaks based on the indicators of mainly forest structure, climate, topography, landform, and soil conditions. This index suggests the risk and susceptibility of a forest area to a pine caterpillar outbreak. In addition, a geodetector model was used to identify the synergistic or antagonistic effects of environmental factors on risk of pine caterpillar outbreak. The results showed that: (1) mountain and hilly areas are more susceptible to pine caterpillar outbreaks, whereas surrounding areas show a medium susceptibility in central and southeastern part of Shandong Province; (2) there was an increasing susceptibility of the study area to pine caterpillar outbreak in the 1990s, 2000s and 2010s; (3) synergism between certain topography and climate variables enhanced the susceptibility of the study area to pine caterpillar outbreaks, including the interaction between elevation and sunshine hours and between slope and humidity. The integrated susceptibility index was shown to perform well in indicating the risk and susceptibility of forest ecosystems to pine caterpillar outbreaks. The present study can act as a reference for forest management within the identification of areas with high susceptibility to pine caterpillar outbreak.