Different initial responses of the canopy herbivory rate in mature oak trees to experimental soil and branch warming in a soil‐freezing area

Plants and insects comprise more than 50% of known species on earth, and their interactions are of major importance in most natural ecosystems. To understand the mechanisms by which global warming affects plant–insect interactions in the canopy of mature cool‐temperate forests with a freeze–thaw cycle, we examined changes in the herbivory rate and leaf traits in oak Quercus crispula. From 2007 to 2009, we experimentally increased the temperature of the surrounding soil and canopy branches of mature oak trees by approximately 5°C using electric heating cables. Soil warming decreased the rate of herbivory in the canopy, whereas branch warming had no effect. The magnitude of the effect of soil warming on canopy herbivory varied. For the first year, the decrease was 32%, but this doubled (63%) in the third year. Branch warming did not affect canopy leaf traits; however, soil warming decreased the leaf nutritional quality by decreasing N and increasing the carbon:nitrogen (CN) ratio for three years. Additionally, soil warming increased total phenolics in the third year. Stepwise multiple regression models showed that among the leaf traits that were changed by soil warming, N explained the variation in herbivory for the first and second years, whereas total phenolics explained it for the third year. Our experimental results demonstrate that soil warming drives the rate of herbivory in the canopy of mature oak trees, and the magnitude of the soil warming effect was gradually enhanced during the initial three years. This suggests the importance of belowground temperature elevation in predicting the effect of global warming on plant–insect interactions in a forest canopy.